Refrigeration Basics - 4th Class Edition 3 - PDF

4th Class Edition 3 Part B Unit 9 Chapter 1 Refrigeration Basics Learning Outcome When you complete this chapter you should be able to: Explain the basic concept of refri...

4th Class Edition 3 Part B Unit 9 Chapter 1 Refrigeration Basics Learning Outcome When you complete this chapter you should be able to: Explain the basic concept of refrigeration and refrigerants. Learning Objectives Here is what you should be able to do when you complete each objective: 1. Explain the fundamentals of refrigeration. 2. Describe the cycle of operations in a vapour compression refrigeration system. 3. Explain how the operating temperatures and pressures are selected and related for a vapour compression refrigeration system. 4. State how the capacity of a refrigeration system is described and how refrigeration tables are used to calculate system performance. 5. Describe how refrigerants are classified. 6. Describe the thermodynamic properties of refrigerants. 7. Describe the properties of refrigerants relating to miscibility, leakage tendency, odour, moisture reaction, toxicity, and flammability. 1-1  Refrigeration Basics Chapter 1 Chapter Introduction One source describes refrigeration as “The branch of science that deals with the process of reducing and maintaining the temperature of a space or material below the temperature of the surroundings.” Another source describes refrigeration as “The process of moving heat from one location to another by use of refrigerant in a closed cycle.” Combining these two results in a more comprehensive definition: Refrigeration is the thermodynamic process of moving heat from one location to another, for the purpose of reducing and maintaining the temperature of a material below the temperature of its surroundings, through the circulation of refrigerant in a closed cycle. This material will deal with the science and theory of refrigeration, by introducing the principle components of a refrigeration system, and describing the role and interaction of these components. Prior to studying this material, it is advisable to review Part B Unit 2 Introduction to Pumps and Compressors, especially Chapter 3 Introduction to Compressors. 1-3 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Objective 1 Explain the fundamentals of refrigeration. Evaporation From thermodynamics, recall that one kilogram of pure boiling water, at 101.3 kPa absolute pressure, absorbs 2257 kJ of latent heat. For water, this is the amount of energy needed, per kg, to change from its liquid to gaseous state. For water to evaporate, heat must be transferred to it. The heat source, when giving off heat energy, may: Undergo a temperature drop (sensible heat), or Undergo a change of state (latent heat). On Track Another word for “state” is “phase.” Change of state is also called “phase change.” This chapter uses the terms “phase change” and “change of state” interchangeably. Consider the first point. Most everyone has stepped out of a shower, pool, or bathtub, and shivered vigorously until dry. While shivering, every kilogram of surface water that evaporated withdrew 2257 kJ of heat, resulting in a cold feeling. The evaporative process that occurred reduced the body temperature. Therefore, the first fundamental principle of refrigeration is that a substance must absorb or reject latent heat in order to change state. When latent heat is added to a liquid, it evaporates. When latent heat is removed from a gas, it condenses. The evaporative process removes heat, and causes a cooling effect. Consider the second point. It is possible for a liquid refrigerant to boil below the freezing point of water. If a body of water has its heat continually drawn away by this refrigerant, the water will first drop in temperature, and then change in state from liquid to solid. First, the refrigerant removes sensible heat from the water, and then it removes latent heat of fusion. This occurs each time an ice-cube tray is placed into a freezer. Therefore, the evaporation of a liquid can lower a body’s temperature by the extraction of sensible heat, and the evaporation process can cause a change in state (if the liquid evaporation occurs at a low enough temperature). Let us now look at the effect of pressure on a boiling liquid. Pure water, at 101.3 kPa absolute pressure (zero gauge pressure), will boil at 100°C. In a boiler though, due to the higher pressure exerted on the surface of the water, boiling occurs at a higher temperature. For example, at 200 kPa absolute, pure water boils at 120.21°C. At higher altitudes, where the atmospheric pressure is lower, pure water boils at temperatures below 100°C. For example, in Lake Louise, Alberta, the elevation is 1731 metres above sea level. At this elevation, the air pressure is around 83 kPa absolute, and water boils at 94.48°C. Therefore, the second fundamental principle of refrigeration is that the pressure exerted on the surface of a boiling liquid affects the temperature at which the liquid boils. 1-4 4th Class Edition 3 Part B  Refrigeration Basics Chapter 1 When a pure liquid is heated, its temperature increases until the substance is heat-saturated. Additional heat causes a change in state, but no temperature change. This state-change occurs at the temperature where heat saturation occurs. This temperature is called the saturation temperature. Because saturation temperature depends on the applied pressure, the pressure applied to the boiling liquid surface when saturation temperature is reached is called the saturation pressure. When saturation pressure increases, saturation temperature increases. The opposite is also true. Therefore, with regard to pure liquids, we can state a third principle of refrigeration: for every saturation pressure, there is one corresponding saturation temperature. Lithium bromide refrigeration systems, which will be discussed in a later chapter, use water as a refrigerant. The American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) identifies all refrigerants with the capital letter “R” followed by a number. Water has the refrigerant designation “Refrigerant 718” (or simply “R-718”). To work as a refrigerant, water must be boiled in a deep vacuum. For example, the saturation pressure of pure water, boiling at 4°C, is 0.814 kPa (absolute)! Many commonly used refrigerants though, boil at pressures well above atmospheric pressure. The common refrigerant ammonia (R-717), at atmospheric pressure, boils at -33.3°C. Carbon Dioxide (R-744), at atmospheric pressure, boils at -93.7°C. CH2FCF3, 1,1,1,2-tetrafluoroethane (R-134a), at atmospheric pressure, boils at -26.2°C. Therefore, a fourth principle of refrigeration is that one of the physical properties of every pure liquid is a characteristic set of saturation pressures and temperatures. Table 1 compares saturation temperatures of a variety of refrigerants, at a saturation pressure of 101.3 kPa (normal atmospheric pressure at sea level). Note the wide range of saturation temperatures. A refrigerant will boil if the temperature of its surroundings exceeds the saturation temperature (in the far right column). Below the saturation temperature, a refrigerant will remain liquid. Note that R-123 remains a liquid at normal room temperature. Side Track Propane, which is also used to fuel forklifts and other machinery, will not evaporate below -41°C. So, if a propane powered vehicle is left outside on a cold winter night, the vehicle may not start until the propane warms up. Table 1 – Saturation Temperature of Various Refrigerants, at 101.3 kPa Absolute Saturation Common or ASHRAE Chemical Name and Formula Temperature Trade Name Designation (°C) at 101.3 kPa Suva 123 2,2-dichloro-1,1,1-trifluoroethane, CHCl2CF3 R-123 27.7 Suva 134a 1,1,1,2-tetrafluoroethane, CH2FCF3 R-134a -26.2 Ammonia Ammonia, NH3 R-717 -33.3 Freon 22 Chlorodifluoromethane, CHClF2 R-22 -40.8 Propane Propane, CH3CH2CH3 R-290 -41.9 1-5 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Table 2 shows the saturation pressures of these same refrigerants, at a saturation temperature of 0°C. Note that each of these refrigerants, except R-123, boils well above 103 kPag (204.3 kPa) at 0°C. Therefore, refrigeration systems are designed with high-pressure components, according to ASME BPVC VIII Rules for Construction of Pressure Vessels and ASME B31.5 Refrigeration Piping and Heat Transfer Components. Table 2 – Saturation Pressure of Various Refrigerants, at 0°C Saturation Common or ASHRAE Chemical Name and Formula Pressure (kPa Trade Name Designation absolute) at 0 °C Suva 123 2,2-dichloro-1,1,1-trifluoroethane, CHCl2CF3 R-123 32.9 Suva 134a 1,1,1,2-tetrafluoroethane, CH2FCF3 R-134a 292.8 Ammonia Ammonia, NH3 R-717 429.4 Propane Propane, CH3CH2CH3 R-290 471 Freon 22 Chlorodifluoromethane, CHClF2 R-22 497.6 To summarize, the main principles involved in mechanical refrigeration are: 1. A substance must absorb latent heat in order to change state. 2. The pressure exerted on the surface of a boiling liquid affects the temperature at which a liquid boils. 3. For every saturation pressure, there is one corresponding saturation temperature. 4. A physical property of every pure liquid is a characteristic set of saturation pressures and temperatures. Basic Refrigeration System A refrigerant is a liquid that is capable of boiling at a low temperature. Consider a refrigerant liquid, at saturation temperature and pressure, contained inside a vessel. According to principle 2 (above), the temperature of this refrigerant can be controlled by varying the pressure in the vessel. Figure 1 shows such a vessel, containing liquid ammonia (R-717), and located inside an insulated room (the refrigerated space). The air in the refrigerated space is the refrigerated medium: in other words, air is the substance being cooled. The vessel in Figure 1 is called the evaporator; it is within this vessel that the liquid boils. The evaporator in a refrigeration system is in physical contact with the refrigerated medium, so that heat may transfer to the refrigerant. The small arrows show the direction of heat transfer, from the refrigerated medium to the boiling ammonia liquid. Note that the refrigerated space is about four degrees warmer than the boiling ammonia. So, even though both the ammonia and the air in the room are “cold,” heat continues to transfer from the warmer material to the colder material, in accordance with the second law of thermodynamics. 1-6 4th Class Edition 3 Part B  Refrigeration Basics Chapter 1 Figure 1 – A Simple “Refrigerator” Operating at Atmospheric Pressure This simple “refrigerator” is very impractical. Consider that most refrigeration systems are used for freezing food products, or preserving food freshness. First of all, the temperature of the refrigerated space (-29°C) is appropriate for freezing food, but not for preserving fresh produce. Secondly, there is no means of controlling the temperature of the refrigerated space, because the refrigerant can only boil at atmospheric pressure. Finally, the refrigerant vapour vents directly to the atmosphere. Such a refrigerator would waste costly refrigerant and contaminate the natural environment (depending on the toxicity or environmental impact of the refrigerant). Figure 2 addresses the issue of temperature control. A backpressure regulator, in the form of a throttling valve, is installed at the evaporator outlet. By adjusting the valve, refrigerant vapour flow can be adjusted, which in turn changes the saturation pressure and saturation temperature in the vessel. In the case of Figure 2, the valve is partially closed, raising the saturation pressure to 413.6 kPa, and causing the ammonia to boil at 1°C. Now, the refrigerated space can be kept at a temperature suitable for fresh produce (5°C). Figure 2 – A Simple “Refrigerator” Operating Above Atmospheric Pressure 1-7 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration With the vent valve closed, vapour cannot escape the evaporator. If this should occur, the evaporator pressure would increase until the corresponding saturation temperature equals the temperature of the refrigerated space. When this happens, heat transfer ceases, and boiling stops. In the case of Figure 2, if the space temperature remained at 5°C, the evaporator pressure would increase to at 515.8 kPa if the throttling valve was shut. Sometimes, an evaporator must operate below atmospheric pressure in order to develop a low enough temperature. Figure 3 addresses this situation. For ammonia to boil at -40°C, for example, the evaporator pressure must be 71.7 kPa absolute, which is approximately 30 kPa below atmospheric pressure. This low pressure can be achieved by installing a vacuum pump at the evaporator outlet. The vacuum pump can withdraw refrigerant vapour faster than the rate at which it boils. In this way, the vacuum pump can lower the evaporator pressure to below atmospheric pressure. Figure 3 – A Simple “Refrigerator” Operating Below Atmospheric Pressure The simple refrigerators shown in Figures 1 to 3 show how refrigerant evaporation, relying on the four fundamental principles mentioned earlier, can be utilized to artificially create various cold temperatures. The next objective examines the components needed to create an actual, practical refrigeration system. 1-8 4th Class Edition 3 Part B  Refrigeration Basics Chapter 1 Objective 2 Describe the cycle of operations in a vapour compression refrigeration system. Actual Refrigeration Systems Actual refrigeration systems are not like those described in Objective 1. Additional equipment must be added to address the significant shortcomings of the simple systems of the previous objective. First, in order to run continuously, the vaporized refrigerant must be continuously fed to replace the refrigerant boiled in the evaporator. The rate of replacement must be the same as the rate of evaporation. To do this, larger refrigeration systems have a reservoir of refrigerant (the liquid receiver) to supply liquid to the evaporator. A metering device (such as an adjustable valve, or non-adjustable restriction) must be installed between the liquid receiver and the evaporator, to control the refrigerant flow. If too much refrigerant enters the evaporator, both vapour and un-boiled liquid will leave the evaporator. This liquid will cause damage to the compressor (which will be discussed later). If fed too little refrigerant, the evaporator will “starve” or “run dry.” With insufficient boiling liquid, heat flow to the evaporator will be reduced, thus reducing the refrigerating effect. Secondly refrigerant vapour must be recovered and re-used. In actual refrigeration systems, the refrigerant vapour that leaves the evaporator is compressed, and then cooled in a heat exchanger. In this exchanger, called the condenser, the vapour gives off latent heat to a coolant (or cooling medium) and returns to its liquid state. The liquefied refrigerant then flows from the condenser to the liquid receiver where it resides until it is re-used in the evaporator. Finally, a refrigeration system needs a compressor. To get refrigerant vapour to release its latent heat, the vapour must be hotter than the cooling medium. Consider Figure 1. The refrigerant vapour leaving the evaporator is at its saturation temperature (-33.3°C). To condense the vapour, it would need to be exposed to a cooling medium colder than -33.3°C. However, most condensers use relatively warm cooling media (between 0 and 40°C, depending on the source). Therefore: The temperature of the refrigerant vapour must be made higher than the temperature of the cooling media (typically, higher than 40°C); and The pressure of the refrigerant vapour must be raised to a point where the condensed liquid refrigerant – now at high temperature – will remain a liquid. In other words, the saturation pressure in the condenser must correspond with the high saturation temperature of the condensed liquid. The compressor accomplishes both of these tasks. The compressor in a refrigeration system takes the place of the vacuum pump shown in Figure 3. A compressor draws refrigerant vapour at a low pressure and discharges it at a higher pressure. Compression is work, and requires the expenditure of mechanical energy (W = F × d). The mechanical energy of compression is converted to potential energy (pressure) and internal energy (vapour temperature). Therefore, in compressing the low-pressure refrigerant vapour, the compressor also raises the refrigerant vapour temperature. The pressurized, high temperature vapour then enters the condenser, transfers heat to the surrounding coolant, and condenses. The refrigerant liquid drains to the receiver, and eventually returns to the evaporator, where the cycle repeats. 1-9 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Figure 4 shows an actual refrigeration system with the following mandatory components: Refrigerant An evaporator A refrigerant metering device (control valve, orifice or capillary tube) A condenser A compressor Optional components shown in Figure 4 include: A liquid receiver A condenser cooling fan An evaporator fan This simple but complete system is typical of a household refrigerator, freezer, or air-conditioning system. Such a system transfers heat from a low temperature region (such as the inside of a refrigerator) to higher temperature condensing medium, such as air or water. This system is called a vapour compression refrigeration system. The evaporator and condenser pressures, maintained by the compressor and metering device, are essential for achieving proper system temperatures. Figure 4 – Vapour Compression Refrigeration System 1-10 4th Class Edition 3 Part B  Refrigeration Basics Chapter 1 Fundamental Cycle of Operation and Operating Principles The following is a theoretical description of a refrigeration system cycle of operations. It is presented here as a starting point on which to develop an understanding of “real life” refrigeration systems. Refer to the arrows on Figure 4. The liquid receiver contains warm liquid refrigerant under high pressure. The temperature of the liquid is at the saturation temperature associated with the saturation pressure inside the liquid receiver (refer to mechanical refrigeration principles #2 and #3). The refrigerant, then, is saturated liquid, and ready to evaporate, given additional heat. The refrigerant liquid flows from the liquid receiver to the metering device through a pipe. The difference in pressure between the liquid receiver and the evaporator causes the refrigerant liquid to flow into the evaporator. The metering device controls the flow rate. In the evaporator (which has low internal pressure), the liquid immediately boils and becomes vapour. Because the evaporator saturation pressure is low, the saturation temperature is also low (again, refer to mechanical refrigeration principles #2 and #3). The boiling low-pressure refrigerant absorbs heat from the interior of the refrigerated space, because the evaporator saturation temperature is less than the temperature of the refrigerated space. In order to maintain a constant evaporator pressure, and therefore a constant evaporator temperature, refrigerant vapour must be drawn from the evaporator at the same rate at which it develops. This is one of several functions of the compressor. The compressor also raises the pressure of the vapour high enough that its saturation temperature is higher than the temperature of the condenser cooling medium. In Figure 4, the condenser cooling medium is air, supplied by a fan. The vapour discharging from the compressor, and entering the condenser, is at high temperature and pressure. In the condenser, the vapour gives off latent heat, and returns to its liquid state. The liquid refrigerant flows by gravity to the receiver, where it will again flow to the evaporator in a continuous cycle. A refrigeration cycle, like a steam-water cycle, can be represented by a pressure-enthalpy (p-h) diagram. Figure 5 shows a p-h diagram for R-717, ammonia refrigerant. On Track Review the thermodynamics of steam to help understand the concepts related to refrigeration. The Academic Supplement has additional refrigeration p-h diagrams and tables for reference. The y-axis (or “ordinate”) of the p-h diagram represents pressure, in kPa. Lines may be drawn perpendicular to the ordinate to connect points with the same pressure. In other words, a horizontal line on the diagram is a line of constant pressure. Note that condenser pressure is higher than the evaporator pressure, and therefore condenser temperature is higher than evaporator temperature. Because the condenser pressure is higher than the evaporator pressure, the condenser, and any part of the refrigeration system that is under high pressure, are said to be on the high side of the system. Any part of the system that is not under condenser pressure, then, is said to be part of the system’s low side. 1-11 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Figure 5 – Pressure-Enthalpy Diagram for Ammonia at Standard Conditions A S The high side and low side are shown as opposite sides of a trapezoid, drawn on the p-h diagram. At each corner of the trapezoid are points labelled “1” through “4.” Each point represents a different component of a compression refrigeration system: Point 1 Metering device inlet (or Condenser outlet) Point 2 Evaporator inlet (or Metering device outlet) Point 3 Compressor inlet (or Evaporator outlet) Point 4 Condenser inlet (or Compressor outlet) Note that the points discussed above are also shown on the refrigeration cycle shown in Figure 6. Figure 6 also shows pressures and temperatures of an ammonia refrigeration system operating at an evaporator temperature of -15°C and a condenser temperature of +30°C. Side Track Refrigerant cycles are compared using a set of “standard” conditions: evaporator temperature of -15°C and condenser temperature of +30°C. However, there are few refrigeration plants that actually operate under these conditions. Therefore, Figure 5 is a graphical representation of the refrigeration cycle shown in Figure 6. This chapter will continue to use the p-h diagram to illustrate various refrigeration system operating conditions. 1-12 4th Class Edition 3 Part B  Refrigeration Basics Chapter 1 On Track Note that the evaporator and condenser pressure readings in Figure 6 are in kPag. These are the pressures that standard pressure gauges show, assuming an atmospheric pressure of 101.3 kPa. The p-h diagram (Figure 5) shows the same pressures in kPa absolute (kPa). Be aware the refrigeration tables and p-h diagrams in the Academic Supplement use absolute pressure, not gauge pressure. Other publishers may show pressures in kPag in their tables and diagrams. Therefore, to know the thermodynamic properties of a refrigerant in a system, the learner MUST know the units of pressure measurement. Figure 6 – Diagram of Ammonia Refrigeration Cycle at Standard Conditions Point 1 to Point 2 Following the process already discussed, consider the four points on the p-h diagram and on Figure 6. From point 1 to point 2, liquid refrigerant at its saturation pressure and temperature passes through the metering device from the high side to the low side. On the p-h diagram, this process is shown with a vertical straight line. On the p-h diagram, a vertical line (perpendicular to the x-axis or “abscissa”) connects points with the same enthalpy. Therefore, the enthalpy of the high pressure liquid is the same as the enthalpy of the boiling low-pressure two-phase refrigerant just as it passes through the metering device. Point 2 to Point 3 From point 2 to point 3, the liquid refrigerant boils at low temperature and pressure in the evaporator. Though the temperature and pressure remain constant, the enthalpy of the refrigerant increases, as it gains latent heat from the refrigerated space. At point 3, the refrigerant is completely vapourized, and is 100% dry and saturated. Point 3 to Point 4 At point 3, the dry low-pressure vapour enters the compressor. Compression takes place between points 3 and 4. Note that during compression: The vapour increases in enthalpy The vapour increases in pressure The vapour increases in temperature Energy is consumed in order to compress the refrigerant. The energy consumed by the compressor results in these increases in enthalpy, pressure, and temperature. 1-13 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Point 4 to Point 1 After compression, the refrigerant vapour enters the condenser. In the condenser, the refrigerant first gives off the energy added by the compressor. Then, the saturated refrigerant vapour gives off the latent heat it absorbed in the evaporator, thus condensing back to saturated liquid. The cycle described is not a “batch” process, where a discrete mass of refrigerant circulates through the system, and repeatedly undergoes thermodynamic processes. Rather, each part of the system is simultaneously filled with refrigerant, in various states of expansion, evaporation, compression and condensation. Heat is constantly absorbed from the refrigerated space and rejected to the condenser cooling medium, whenever the compressor is operating. Because the heat transfer is continuous, the refrigeration cycle is called a “constant flow” cycle. Heat Absorbed by the Evaporator Notice that the enthalpy of the refrigerant at point 1 is the same as the enthalpy at point 2. The heat absorbed by the evaporator from the refrigerated space is the difference between the enthalpy at point 3 minus the enthalpy at point 2. Because point 2 equals point 1, we can also say that the heat absorbed by the evaporator is the enthalpy at point 3 minus the enthalpy at point 1: Qe = h3 − h1 Energy Added by The Compressor The compressor takes refrigerant from the evaporator at point 3, and compresses it to point 4. Therefore, the energy added by the compressor is: Qcompressor = h4 − h3 Heat Rejected by the Condenser The condenser rejects the heat absorbed by the evaporator plus the energy added by the compressor. Therefore, the energy rejected by the condenser is: Qc = (h3 − h1)+ (h4 − h3) = h4 − h1 A More Accurate System Model To complete our analysis of the compression refrigeration cycle, we must augment the information already presented in order to develop a more accurate model. Observe Figures 7 and 8. These show an ammonia refrigeration system operating: At standard evaporator and condenser conditions. With 10 degrees of evaporator superheat. With 10 degrees of condenser subcooling. 1-14 4th Class Edition 3 Part B  Refrigeration Basics Chapter 1 Figure 7 – Diagram of Ammonia Refrigeration Cycle at Standard Conditions with Subcooling and Superheat Figure 8 – Pressure-Enthalpy Diagram for Ammonia at Standard Conditions with Sub-Cooling and Superheat Note that in Figures 7 and 8, the refrigerant entering the metering device is not saturated liquid. In reality, the liquid piping to the receiver and the metering device, as well as the liquid stored in the receiver continues to lose sensible heat to the surrounding environment. In the example given in Figures 7 and 8, this sensible heat loss produced a 10°C temperature drop in the liquid refrigerant. Thus, the refrigerant became subcooled – that is, it is cooled to below its saturation temperature. Note, then, that point 1 on Figure 8 is to the left of the saturated liquid line, and is located in the “subcooled liquid” region of the chart. Subcooling can also occur in the condenser, if the cooling medium is quite cool. 1-15 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Figures 7 and 8 also show evaporator superheat. It is unsafe for the refrigerant to be dry and saturated at the compressor inlet. If the cooling load on the evaporator was to decrease, un-boiled liquid would enter the compressor, causing considerable damage (liquids are incompressible). Therefore, refrigeration systems must be able to accommodate either decreasing evaporator load or increasing evaporator load, by regulating the evaporator’s cooling capacity (“evaporator capacity control”). This may be accomplished by using a metering device that continuously monitors the evaporator superheat, and regulates the refrigerant flow to the evaporator to keep the superheat constant. In this way, the compressor will not ingest slugs of liquid. Other systems may use low-pressure receivers (“accumulators”) in the suction line of the compressor, to separate liquid and vapour if evaporator loads change suddenly. On Track The metering device in Figure 7 has a small bulb located at the evaporator outlet, which monitors evaporator superheat and controls the opening of the metering device. This type of metering device is called a thermostatic expansion valve. System Pressure Drops In Figures 5 and 8, it is assumed that the pressure of the refrigerant in the low side of the system remains constant from the moment it enters the evaporator until it passes through the suction inlet of the compressor as vapour. In a realistic system, this will not be true. Figures 5 and 8 also assume that the pressure of the refrigerant in the high side remains constant, from the time it leaves the compressor until the time it leaves the high side as liquid. Again, in a realistic system, this will not be true. In actual refrigeration systems, due to the length of the piping, internal roughness of the pipe and the numerous bends in the evaporator and condenser, the pressure of the refrigerant drops from the metering device outlet to the compressor inlet, and from the condenser inlet to the metering device inlet. Figure 9 shows how refrigerant pressure drops in an actual refrigeration system. In both the high and low sides of the system, pressures drop in the direction of refrigerant flow. The evaporator pressure drop reduces the compressor suction pressure. Because of this, the pressure ratio increases, making the compressor work harder for every kilogram of refrigerant circulated. Other effects of system pressure drop are beyond the scope of Fourth Class Power Engineering. 1-16 4th Class Edition 3 Part B  Refrigeration Basics Chapter 1 Figure 9 – p-h Diagram with Subcooling, Superheat, and System Pressure Drops 1-17 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Objective 3 Explain how the operating temperatures and pressures are selected and related for a vapour compression refrigeration system. Figure 10 shows a compression refrigeration system using R-134a as the refrigerant. This system chills water to cool the air in a central air conditioning system. The evaporator is a shell-and-tube heat exchanger. The chilled water flows through the tubes and the liquid refrigerant surrounds the tubes. The condenser is water-cooled. Figure 10 – Indirect Refrigeration System (R-134a) The pressures and temperatures in this refrigeration system are determined by the temperature required for the air leaving the cooling coil in the air duct - in this case 12°C. The average temperature of the air passing through the coil is approximately 18°C. Assume that the cooling coil is designed for a temperature difference of 10°C between the average air temperature and the average chilled water temperature. The average temperature of the chilled water must be 8°C. Since the water leaves the cooling coil at 10°C, it should be therefore be supplied at 6°C to provide an average temperature of 8°C. From the above, it follows that the water enters the evaporator at 10°C and is cooled to 6°C. Again, assume that the evaporator is designed for a temperature difference of 10°C between the average chilled water temperature (8°C) and the temperature of the boiling refrigerant. It is not practical to operate the evaporator below freezing, so 0°C is the lowest temperature at which the evaporator can operate. This requires an evaporator pressure of 292.8 kPa absolute pressure (394.1 kPag). The cooling water enters the condenser at 18°C and leaves at 26°C, giving an average cooling water temperature of 22°C. Assuming the condenser is designed for a temperature drop of 8°C, the condensing temperature of the refrigerant vapour must be 8°C higher than the average cooling water temperature. Therefore, the condensing temperature will be 30°C. This requires a condenser pressure of 770 kPa absolute (871.3 kPag). 1-18 4th Class Edition 3 Part B  Refrigeration Basics Chapter 1 On Track It is important to know that converting temperatures between units is done differently than converting temperature changes. When a given temperature is converted from °C to °F the equation, °F = 9/5 x °C + 32, is used. For example, 18.5°C = 9/5 x 18.5 + 32 = 65.3°F. However, when considering a temperature change or a temperature difference, an increment of 1°C equals an increment of 1.8°F. Therefore, when a temperature difference is converted from °C to °F the equation, °F = 1.8 x °C, is used. For example, a temperature difference of 10°C equals a temperature difference of 1.8 x 10 = 18°F. 1-19 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Objective 4 State how the capacity of a refrigeration system is described and how refrigeration tables are used to calculate system performance. Refrigerant Tables Certain properties of refrigerants such as the saturation temperatures and pressures, volume, density and enthalpy are called thermodynamic properties of refrigerants. These values must be known in order to solve capacity and performance problems. These thermodynamic properties have been obtained by careful experiments. The resulting values have been tabulated in the refrigerant tables. Refer to Table 1 - Refrigerant R-717 (Ammonia): Saturation Properties (Temperature) in the PanGlobal Academic Supplement. This table lists the thermodynamic properties of ammonia at various saturation temperatures. Part of this table is shown below in Table 3. Table 3 – Refrigerant R-717 (Ammonia): Saturation Properties (Temperature) Density Saturation Saturation Volume Liquid Enthalpy, kJ/kg Entropy, kJ/kg Temperature Pressure Vapour Vg 1/Vf °C kPa m3/kg kg/m3 hf hfg hg sf sg -5 354.76 0.34664 645.37 157.77 1279.73 1437.5 0.62848 5.4009 -4 368.8 0.33414 644.02 162.37 1276.23 1438.6 0.64553 5.3874 -3 383.27 0.32218 642.66 166.98 1272.82 1439.8 0.66253 5.374 -2 398.19 0.31074 641.3 171.59 1269.31 1440.9 0.67949 5.3607 -1 413.56 0.29979 639.94 176.21 1265.79 1442 0.6964 5.3474 Consider the columns as numbered from left to right. Column 1 lists the saturation temperature in °C for each absolute pressure. Column 2 lists the corresponding saturation pressure in kPa (absolute). It is obvious, by examining the values in columns 1 and 2 that the temperature increases as the pressure increases. Column 3 lists the specific volume (Vg) in m3/kg of refrigerant vapour. Note that the specific volume of the vapour decreases as the pressure increases. Column 4 lists the density (1/Vf) in kg/m3 of refrigerant liquid. Column 5 lists the enthalpy (hf) in kJ/kg of saturated liquid refrigerant, for each temperature. Column 6 lists the enthalpy of evaporation (hfg) in kJ/kg of refrigerant. Column 7 lists the enthalpy (hg) in kJ/kg of saturated refrigerant gas, for each temperature. Columns 8 and 9 list values of entropy, which are used for calculations beyond the scope of this chapter. 1-20 4th Class Edition 3 Part B  Refrigeration Basics Chapter 1 Refrigeration System Capacity Internal combustion engines are compared according to kilowatt output. Boilers are compared according to boiler horsepower. Refrigeration plants are compared according to tonnes of refrigeration (or in United States Customary System (USCS) units, tons of refrigeration). On Track This text abbreviates the SI unit “tonne of refrigeration” with the letters “TR.” This must not be confused with the USCS “ton of refrigeration,” which, if used in the learning materials, will not be abbreviated. A boiler horsepower is the amount of heat required to produce 15.68 kg (34.5 lbs.) of dry and saturated steam at 100°C (212°F) from pure feedwater at 100°C (212°F), over a period of one hour. This calculates to a heat output rate of 35 394 kJ per hour (33 475 Btu/hr). However, no boilers actually produce steam under such conditions. Despite this, the concept of boiler horsepower remains a useful way to compare the heat output of various boilers. A similar situation applies to the concept of “tonnes of refrigeration.” A tonne of refrigeration is the amount of heat required to produce 1 tonne of ice at 0°C from pure water at 0°C, over a 24-hour period. In other words, one tonne of refrigeration is the amount of latent heat of fusion that must be extracted from 1 tonne of water at 0°C to turn it into ice at 0°C. No refrigeration systems operate under such conditions; however, the concept of tonnes of refrigeration is a useful way to compare the heat transfer rate of various refrigeration plants. SI Tonne of Refrigeration (TR) A tonne of refrigeration is a heat rate based on the latent heat of fusion of pure water, which is 335 kJ/kg. Consequently, to produce 1 tonne of ice at 0°C from water at 0°C would require: 1 tonne _______1000 kg ______ 335 kJ 1 TR = _______ ​​   ​​ × ​​   ​​ × ​​   ​​ = 335 000 kJ/day day tonne kg As a heat rate, a tonne of refrigeration can be expressed over different time spans. So, one TR can also be expressed as a heat rate per hour, a heat rate per minute, or a heat rate per second. 1 day 335 000 kJ ________ 1 TR = _________ ​​   ​​ × ​​   ​​ = 13 958 kJ/hour day 24 hours 1 day 335 000 kJ ________ 1 hour 1 TR = _________ ​​   ​​ × ​​   ​​ × ______ ​​   ​​ = 232.6 kJ/min day 24 hours 60 min 1 day 335 000 kJ ________ 1 hour _____ 1 min 1 TR = _________ ​​   ​​ × ​​   ​​ × ______ ​​   ​​ × ​​   ​​ = 3.877 kJ/s day 24 hours 60 min 60 s Note that the last conversion to kJ/s can be expressed as kW. So, one TR is equal to 3.877 kW. 1-21 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration United States Customary System (USCS) Ton of Refrigeration USCS units are still commonly referred to in Canada. It is useful, then, to review the USCS definition “ton of refrigeration.” A weight of one ton equals 2000 pounds (lbs.). The latent heat of fusion of water, in USCS, is 144 Btu/lb (British Thermal Units per pound). Therefore, to produce one ton of ice at 32°F from water at 32°F would require: 1 ton ______ 2000 lb _______ 144 Btu 1 ton of refrigeration = _____ ​​   ​​ × ​​   ​​ × ​​   ​​ = 288,000 Btu/day day ton lb One ton of refrigeration can also be expressed as a heat rate per hour, a heat rate per minute, or a heat rate per second. 1 day 288,000 Btu ________ 1 ton of refrigeration = __________ ​​   ​​ × ​​   ​​ = 12,000 Btu/hour day 24 hours 1 day 288,000 Btu ________ 1 hour 1 ton of refrigeration = __________ ​​   ​​ × ​​   ​​ × ______ ​​   ​​ = 200 Btu/min day 24 hours 60 min 1 day 288,000 Btu ________ 1 hour _____ 1 min 1 1 ton of refrigeration = __________ ​​   ​​ × ​​   ​​ × ______ ​​   ​​ × ​​   ​​ = 3​​ __ ​​ Btu/s day 24 hours 60 min 60 s 3 Because one Btu equals 1.055 kJ, a direct comparison can be made between one tonne of refrigeration and one ton of refrigeration: 1 3​ __ ​ Btu 1.055 kJ 3 1 ton of refrigeration = ______ ​​  s ​​ × _______ ​​   ​​= 3.517 kJ/s or 3.517 kW Btu It can be seen, then, that a USCS ton of refrigeration is a slightly smaller heat transfer rate than an SI tonne of refrigeration. To convert directly from ton of refrigeration to TR, the conversion factor is 0.907, as shown below: 3.517 kW TR 1 ton of refrigeration = ________________    ​​ × ________ ​​    ​​   ​​= 0.907 TR ton of refrigeration 3.877 kW To convert directly from TR to ton of refrigeration, the conversion factor is 1.1, as shown below: ton of refrigeration 3.877 kW ________________ 1 TR × ________ ​​   ​​ ×    ​​      ​​ = 1.1 TR TR 3.517 kW Example 1 A household air conditioning system is rated at 1.5 TR. Calculate its heat transfer rate in: a) kJ/min b) kW Solution 1 233 kJ/min a) 1.5 TR × _________ ​​   ​​ = 349.5 kJ/min (Ans.) TR 3.877 kW b) 1.5 TR × ________ ​​   ​​ = 5.816 kW (Ans.) TR Note that this kW rating is not the energy consumed by the compressor. Rather, it is the amount of heat a system can transfer from one location to another, in a given period of time. 1-22 4th Class Edition 3 Part B  Refrigeration Basics Chapter 1 Example 2 Calculate the heat transfer rate of a 2-ton commercial icemaker in: a) Btu/hr b) kW Solution 2 12 000 Btu/hr a) 2 tons of refrigeration × ________________    ​​ = 24 000 Btu/hr (Ans.) ​​    ton of refrigeration 3.517 kW b) 2 tons of refrigeration × ________________    ​​ = 7.03 kW (Ans.) ​​    ton of refrigeration Example 3 a) Convert the capacity of the refrigeration system in Example 1 to USCS units. b) Convert the capacity of the refrigeration system in Example 2 to SI units. Solution 3 1.1 tons of refrigeration a) 1.5 TR ×____________________    ​​      ​​ = 1.65 tons of refrigeration (Ans.) TR 0.907 TR b) 2 tons of refrigeration × _________________ ​​    ​​ = 1.814 TR (Ans.)    ton of refrigeration Self-Test 1 An industrial refrigeration plant is rated at 800 tons of refrigeration. How many kg of ice at 0°C can it make from pure water at 0°C, over one week? 5 079 200 kg (Ans.) Evaporator Capacity Evaporator capacity refers to the cooling capacity of the evaporator. Evaporator capacity is expressed in tonnes of refrigeration (or simply evaporator tonnage). When referring to a system’s capacity in TR, as in the previous section, we are referring to the evaporator capacity. The evaporator transfers less heat than the condenser, because it does not need to handle the energy added to the refrigerant by the compressor. Therefore, evaporators often have smaller capacities than condensers. 1-23 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Condenser Capacity Condenser capacity refers to the heat rejection capability of the condenser. Once again, this can be expressed in TR. A system’s condenser must have a greater heat transfer capability than the system evaporator. Net Refrigerating Effect The net refrigerating effect (NRE) is the heat absorbed per kilogram of refrigerant circulated through the evaporator, expressed in kJ/kg. This was described earlier in the chapter as the heat absorbed by the evaporator. Consider a boiler. In a boiler, feedwater enters, removes heat from the furnace, and then leaves as vapour. The feedwater enters with some amount of heat. The boiler adds heat to the water and produces steam. The steam leaves the boiler with more heat per kilogram than when it entered as feedwater. The heat added by the boiler, per kg of water, is the difference between the specific enthalpy of the steam minus and the specific enthalpy of the feedwater. This is a fundamental part of the boiler efficiency calculation. In an evaporator, the same is true. The refrigerant entering the evaporator is like boiler feedwater. It enters the evaporator with heat (hf at high side pressure). The evaporator adds heat to the refrigerant, turning it into vapour. The vapour then leaves the evaporator with more heat (hg at low side pressure) than when it entered as liquid. The NRE is the difference between the enthalpy of the vapour leaving the evaporator at low pressure and the liquid entering the evaporator at high pressure. Calculating NRE Figure 5 shows the refrigeration cycle at standard operating conditions, with no superheat or subcooling. The heat absorbed by the evaporator from the refrigerated space, per kilogram of refrigerant, is the difference between the enthalpy at point 3 minus the enthalpy at point 1. This is also called the NRE, which is calculated as follows: NRE = Qe = h3 − h1 The enthalpy at points 3 and 1 can be found in two ways: using a p-h diagram or refrigerant tables. Both are in the PanGlobal Academic Supplement. Using the p-h Diagram 1. Draw a horizontal line at the -15°C isotherm. Draw another horizontal line at the 30°C isotherm. These lines also represent the high side and low side pressures. 2. Draw a vertical line from the intersection of the high-side line and the saturated liquid line, until it intersects the x-axis. Read the enthalpy where this line intersects the x axis. This, then, is the enthalpy at points 1 and 2 (the liquid enthalpy entering the evaporator). From Figure 5, the enthalpy is approximately 320 kJ/kg. 3. Finally, draw a vertical line from the intersection of the low-side line and the saturated liquid line so it intersects the x-axis. From this intersection, read the enthalpy at point 3. From Figure 5, the enthalpy of the saturated refrigerant vapour at point 3, leaving the evaporator, is approximately 1425 kJ/kg. 4. Subtract the value of the enthalpy at point 1 from the enthalpy at point 3. 1425 – 320 = 1105 kJ/kg. 1-24 4th Class Edition 3 Part B  Refrigeration Basics Chapter 1 Using the Refrigeration Tables It is far more accurate to use refrigeration tables for refrigeration calculations. The Academic Supplement has tables for four different refrigerants. The correct tables must be used. Also, recall that pressures in the Academic Supplement refrigeration tables are absolute. Each set of tables are like steam tables, and have data organized according to temperature or to pressure. Depending on the information given, it may be easier to use the temperature table or the pressure table. The following example refers once again to the ammonia refrigeration system shown in Figure 5. 1. Locate hf at +30°C (enthalpy of saturated liquid), from the ammonia refrigerant temperature tables (Table 1) in the Academic Supplement. The value of the specific enthalpy is shown as 322.59 kJ/kg. This is the specific enthalpy at points 1 and 2. In other words, each kilogram of refrigerant liquid enters the evaporator with an enthalpy of 322.59 kJ. 2. Locate hg (enthalpy of saturated vapour) at -15°C. The value of the specific enthalpy is shown as 1425.2 kJ/kg. This is the specific enthalpy at point 3. In other words, each kilogram of refrigerant liquid leaves the evaporator with an enthalpy of 1425.2 kJ. 3. Subtract the value of the enthalpy at point 1 from the enthalpy at point 3. Now the NRE can be calculated. The NRE of the system shown in Figures 5 and 6 is: NRE = h3 − h1 = 1425.2 kJ/kg − 322.59 kJ/kg = 1102.61 kJ/kg (Ans.) Self-Test 2 An ammonia refrigeration plant operates with a 35°C condenser temperature and a -35°C evaporator temperature. Determine the following, using ammonia refrigeration tables. a) The specific enthalpy of the refrigerant entering the evaporator. b) The specific enthalpy of the refrigerant leaving the evaporator. c) The NRE. a) 346.9 kJ.kg (Ans.) b) 1396.5 kJ/kg (Ans.) c) 1049.6 kJ/kg (Ans.) 1-25 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Flash Gas In actual refrigerating systems, the pressure and temperature of the liquid refrigerant supplied from the high side of the system to the metering device are considerably higher than the pressure and temperature in the evaporator. Consequently, part of the liquid entering the evaporator will immediately become vapour, as soon as its pressure drops in the evaporator. The portion of the refrigerant that evaporates is called flash gas. Flashing occurs because the liquid has higher enthalpy in the high side than it can have on the low side. This excess enthalpy is converted to latent heat in the evaporator, producing flash gas. Subcooling reduces the amount of flash gas produced. The refrigerant that flashes into vapour will not take part in the actual refrigerating process. Only the remaining liquid will absorb heat from the surrounding medium for evaporation. This means that the NRE is considerably less when the liquid refrigerant entering the evaporator is at a temperature higher than the boiling point in the evaporator. The larger the difference between the temperature of the liquid refrigerant entering the evaporator and the actual evaporator temperature, the smaller the NRE will be. In other words, the refrigerating effect will be less than the latent heat of vaporization of the refrigerant. The amount of liquid that may flash into vapour can be as high as 30%, depending on the difference between the temperature of the liquid refrigerant supplied and the evaporator temperature. Coefficient of Performance (COP) The coefficient of performance (COP) is the ratio of the amount of heat absorbed from the refrigerated medium by the evaporator, to the amount of energy used to drive the compressor. A higher COP means a more effective refrigeration system. To find the COP, calculate the net refrigerating effect of the evaporator per second. Then, divide the NRE by the compressor power in kW. Pressure Ratio Pressure ratio is defined as the absolute compressor discharge pressure divided by the absolute suction pressure. Gauge pressures must not be used. In everyday plant terminology, pressure ratio is often called compression ratio, although they are technically different. Compression ratio is defined as the clearance volume plus the swept volume (between top and bottom dead centers) divided by the clearance volume. Compression ratios are used in more advanced compression calculations, beyond the scope of 4th Class. 1-26 4th Class Edition 3 Part B  Refrigeration Basics Chapter 1 Objective 5 Describe how refrigerants are classified. Refrigerant Identification and Classification Of the hundreds of refrigerants available, only a few are widely used in modern residential, commercial, industrial, and institutional refrigerating and air conditioning systems. It is important to know the chemical and physical properties of refrigerants to permit their comparison. As well, the proper refrigerant must be selected, ordered, and used for particular applications. Therefore, it is important to be able to refer to refrigerants consistently, so they may be properly identified. These named refrigerants are categorized according to flammability and toxicity, and then placed in a safety group. Refrigerants are also classified according to their impact on the environment, their chemical origins, and their operating temperature suitability. ASHRAE Designation Table 4 shows the various ways to identify refrigerants, including: ASHRAE designation Chemical formula Chemical name Trade name Table 4 lists common refrigerants designated by ASHRAE. ASHRAE denotes refrigerants with the capital letter “R” followed by a dash and a number. There are well over 300 ASHRAE designated refrigerants and refrigerant blends. Many of these refrigerants are used only in very small amounts, in special laboratory equipment. 1-27 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Table 4 – Common Refrigerants and Their Use Common or ASHRAE Chemical Name Temperature Common Uses Trade Name Number Chemical Formula class High 2,2-dichloro-1,1,1-trifluoroethane Temperature Large scale Suva 123 R-123 (Evaporator low-pressure CHCl2CF3 Temperature chillers (HVAC) Above 0°C) High to Medium Domestic, 1,1,1,2-tetrafluoroethane Temperature Automotive and Suva 134a R-134a (Evaporator Large scale CH2FCF3 Temperature high-pressure above 0°C to chillers (HVAC) -25°C) Low to Very Low Ammonia Cold storage, Temperature Ammonia R-717 ice-making, NH3 (Evaporator flash-freezing Temperature Below -25°C) High to Domestic, Medium Automotive, Chlorodifluoromethane Temperature Freon 22 R-22 Large scale (Evaporator high-pressure CHClF2 Temperature chillers (HVAC), above 0°C to ice-making -25°C) High to Very Low Carbon Dioxide Temperature Industrial, Carbon Dioxide R-744 (Evaporator commercial, CO2 Temperature automotive above 0°C to below -25°C) Safety Group Refrigerants are classified in the Canadian Standards Association (CSA) B52 Mechanical Refrigeration Code into six groups, according to their toxicity and flammability. This designation is based upon ASHRAE Standard 34. Table 5 shows the ASHRAE 34 eight-group designation. CSA B52 combines A2L with A2, and B2L with B2. On Track Due to timing issues, the refrigerant groups A2L and B2L in ASHRAE Standard 34 were not captured in the 2013 CSA B52 code. Later editions of the CSA B52 code will include groups A2L and B2L. 1-28 4th Class Edition 3 Part B  Refrigeration Basics Chapter 1 Table 5 – Refrigerant Safety Classification, according to ASHRAE 34 Safety Group (example) Higher A3 B3 Flammability (R-290 propane) (R-1140 vinyl chloride) Lower A2 B2 Flammability (R-406a) (R-40 Chloromethane) Lower Flammability, with A2L B2L a maximum burning velocity (R-32 Difluoromethane) (R-717 ammonia) of ≤ 10 cm/s No Flame A1 B1 Propagation (R-134a) (R-123) Lower Toxicity Higher Toxicity Group A1 refrigerants are refrigerants that are nontoxic and nonflammable. Other examples of refrigerants in this group are R-22 (chlorodifluoromethane) and R-744 (carbon dioxide). ASHRAE separates Group A2 refrigerants into two categories: A2 and A2L. Both categories are considered lower flammability refrigerants. However, A2L refrigerants have a lower burning velocity, and so present a somewhat lower flammability hazard. Both A2 and A2L refrigerants have low toxicity. Other examples of A2 refrigerants are R-142b (1-chloro-1,1-difluoroethane) and R-152a (1,1-difluoroethane). Other examples of A2L refrigerants are R-1234ze (1,3,3,3-Tetrafluoropropene) and R-1234yf (2,3,3,3-Tetrafluoropropene). Group A3 refrigerants are refrigerants that are highly flammable with low toxicity. Other examples of refrigerants in this group are R-600 (butane) and R-170 (ethane). Group B1 refrigerants are nonflammable and highly toxic. Other examples of refrigerants in this group are R-764 (sulfur dioxide) and R-245fa (1,1,1,3,3-Pentafluoropropane). Group B2 refrigerants are separated into two categories: B2 and B2L. Both categories are considered lower flammability refrigerants. However, like A2L refrigerants, B2L refrigerants have a lower burning velocity, and so present a somewhat lower flammability hazard. Both B2 and B2L refrigerants are highly toxic. Examples of refrigerants in this group are R-717 (ammonia) and R-611 (methyl formate). Group B3 refrigerants are highly flammable and highly toxic. R-1140 (vinyl chloride) is the example given in the table above. Environmental Impact Perfectly sealed refrigeration systems should allow no ingress of air into the system, and (more importantly) no leakage of refrigerant from the system. However, leaks are inevitable. Leaking refrigerant may have considerable environmental impact on the Earth’s ozone layer and on global warming. Ozone Depleting Potential (ODP) Refrigerants may have ozone depleting potential because their chemical compositions include chlorine. Chlorine, and chlorine-containing refrigerants, have been implicated in the destruction of the Earth’s atmospheric ozone, which protects the natural environment from the harmful effects of ultraviolet light. Certain ASHRAE-designated refrigerants are no longer in production, due to their high ODP. Phased-out refrigerants include R-11 and R-12 (commonly called by their trade names: Freon 11 and Freon 12). Refrigerants are ranked according to their ozone depleting potential. Some refrigerants with medium ozone-depleting potential, such as R-22, are still being produced but will be phased out. 1-29 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Global Warming Potential (GWP) Some refrigerants have been shown to be greenhouse gases. Refrigerants are therefore rated according to their global warming potential (GWP). Refrigerants with a high GWP, such as R-12, are being phased out. Oddly enough, CO2 (R-744) is considered to have zero GWP. This is because unlike combustion equipment, refrigeration systems do not produce CO2. They merely store CO2 that originated in the atmosphere. Releases of CO2 refrigerant to the environment are therefore neutral in their effect on global warming. Natural Refrigerants Natural refrigerants are refrigerants that occur naturally in the environment. They include R-717 (ammonia), R-744 (CO2) and R-718 (water). These refrigerants are increasing in popularity because they have less environmental impact. Ammonia is particularly desirable from an environmental standpoint because it is an energy efficient refrigerant with zero ODP and zero GWP. Chemical Origins Some hydrocarbon compounds are excellent refrigerants, though highly flammable. An example is R-290 (propane). Propane, butane, ethane, and other hydrocarbons are called hydrocarbon refrigerants. Therefore, propane is also called HC-290. Many refrigerants begin as hydrocarbon compounds, but are chemically modified to achieve certain physical properties. Some refrigerants are based on the methane molecule (CH4). Others are based on ethane (C2H6). These methane and ethane molecules are chlorinated, fluorinated, or chlorinated and fluorinated. The resulting refrigerants are called CFCs, HFCs, and HCFCs. CFCs are chlorinated fluorocarbons, and include the phased-out refrigerants R-11 and R-12. Because they are chlorinated, they have high ODP. HCFCs are hydrochlorofluorocarbons. They have less environmental impact than CFCs. HCFCs include R-22 and R-123, which are both designated for phase-out, due to their ODP. HFCs are hydrofluorocarbons. They are not chlorinated, and so have no ODP. However, they may still have GWP. An example of an HFC in common use is R-134a. The elements chlorine and fluorine are in an elemental group called halogens. Collectively, CFCs, HFCs, and HCFCs are therefore called halocarbons. Operating Temperature Due to their physical properties, refrigerants may be more suited to one application than another. One application consideration involves how easily a refrigerant may achieve the desired system evaporator temperature. Some refrigerants, such as R-717, can easily achieve the low temperatures required for a deep-freeze. Others, such as R-123, cannot, even operating at a deep vacuum. However, R-123 is quite well suited for chilling water to a few degrees Celsius, for use in HVAC cooling commercial buildings. Table 4 shows typical temperature ranges and applications for the listed refrigerants. 1-30 4th Class Edition 3 Part B  Refrigeration Basics Chapter 1 Physical and Chemical Properties of Refrigerants Refrigerants must possess physical and chemical properties that make them suitable for use in refrigerating systems. Refrigerants vary widely in physical and chemical properties. The ideal refrigerant should have: a) A low boiling point at atmospheric pressure. b) A high latent heat capacity: that is, it should require a large amount of heat to convert it from a liquid to a gas after its saturation temperature has been reached. c) A fairly low condensing (high-side) pressure. d) An inoffensive odour, yet easy to detect. e) A non-toxic nature. f) A noncorrosive action on metals. g) A non-flammable and non-explosive nature when mixed with air. h) A low vapour specific volume. i) A low liquid density. As well, the refrigerant should be inexpensive to purchase. However, no single refrigerant meets all of these criteria. Any refrigerant selection, therefore, involves compromise. 1-31 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Objective 6 Describe the thermodynamic properties of refrigerants. Thermodynamic Properties of Refrigerants Thermodynamic properties are the physical properties that directly affect the movement of heat. These properties are pressure, temperature, volume, density, enthalpy, and entropy. The PanGlobal Academic Supplement has tables and charts for the thermodynamic properties A S of R-717, R-134a, R-123 and R-744. Refer to the Academic Supplement while studying this objective. A discussion of the various properties of refrigerants follows. As each property is discussed and a comparison is made between the properties of different refrigerants, it will become obvious how much refrigerants differ. Because of these differences, no single refrigerant is ideal. The differences in properties, however, make one refrigerant more suitable for an application than another. Refrigerant Properties at Standard Operating Conditions Table 6 shows the boiling point at atmospheric pressure (101.3 kPa) of the refrigerants listed in Table 1. Note that some refrigerants boil well below 0°C, the freezing point of water, while others boil above 0°C. Table 6 also compares the thermodynamic properties of these common refrigerants, at standard operating conditions of -15°C evaporator temperature and +30°C condenser temperature. Table 6 – Comparison of Thermodynamic Properties of Common Refrigerants at Standard Conditions R-134a R-123 R-717 Suva 134a Suva 123 (ammonia) Boiling point at 101.3 kPa absolute -26.1 27.7 -33.3 Evaporating pressure (kPa abs.) at -15°C 163.9 15.7 236.2 Condensing Pressure (kPa abs.) at +30°C 770 110 1167 NRE at standard conditions (kJ/kg) 147.9 142.2 1102.6 Specific volume of vapour (m3/kg) at -15°C 0.1207 0.8848 0.5087 Density of liquid (kg/m3) at +30°C 1187.5 1451 595.2 Careful review of this table reveals significant differences in the properties of these refrigerants. Pressure-Temperature Relationship Note that R-123 boils at 27.7°C at atmospheric pressure. An R-123 chiller, then, must operate with its evaporator well below atmospheric pressure. In fact, at -15°C, the evaporator is at 15.7 kPa absolute pressure. This is the reason R-123 is termed a high temperature refrigerant: it is suitable for HVAC use, but not well suited for industrial cold storage freezers or ice making. Because of the low evaporator pressure, air may leak into the refrigeration circuit; therefore, air purgers must be installed to continuously remove air from R-123 systems. 1-32 4th Class Edition 3 Part B  Refrigeration Basics Chapter 1 Also, note the condensing pressures. R-123 has a low pressure at +30°C, so that the refrigeration equipment is considered low pressure by many jurisdictions, and may not require operation by Power Engineers. R-717 and R-134a both have high condensing pressures at 30°C: therefore, large plants using these refrigerants generally require full-time operation by Power Engineers. Note also that R-717 has the highest condensing pressure of the three listed. The high side equipment of R-717 systems (and R-134a systems) must be rugged and built according to ASME BPVC and ASME B31.5. Table 6 compares how different refrigerants’ operating pressures vary with temperature. This is important since it determines the strength of the equipment required, cost of construction, and operator staffing requirements, for a particular refrigerant. One application consideration when choosing a refrigerant is that it should have a low condensing pressure. As well, evaporators should preferably operate above atmospheric pressure to prevent air and moisture infiltration, which causes operational problems. Specific Volume Table 6 gives the specific volumes (in m3/kg) of refrigerant vapour at -15°C. R-717 vapour has the lowest specific volume. This means that a refrigeration compressor in an R-717 system can have a smaller displacement than a compressor for R-123 or R-134a, in a system of similar capacity. As well, suction lines for an R-717 system can be made smaller in diameter, for a given mass of refrigerant circulated, than an R-123 or R-134a system. Density Refrigerant liquid density (in kg/m3) informs designers about how heavy liquid lines will be. The liquid refrigerant density must also be known to calculate of the size of control valves and piping. It can be seen from Table 5 that R-717 is not a very dense liquid in comparison to R-123 and R-134a. In fact, liquid ammonia is less dense than water. Refrigerating Effect Finally, examine the NRE of each refrigerant listed on Table 6. The NRE is the heat absorbed in the evaporator, under standard operating conditions. R-717 is the most effective at transferring heat in the evaporator. In fact, R-717 removes over 7 times the heat, per kg of refrigerant, than R-123 or R-134a. Table 7 – Comparison of Thermodynamic Properties of Common Refrigerants at Standard Conditions SI Units USCS Units Temp. Pressure (kPa absolute) Temp. Pressure (psia) (°C) R-134a R-123 R-717 (°F) R-134a R-123 R-717 -15 163.9 15.7 236.2 5 23.8 2.3 34.3 -10 200.6 20.2 290.7 15 29.7 3.0 43.1 -5 243.3 25.8 354.8 25 36.8 4.0 53.7 0 292.8 32.6 429.4 35 45.1 5.1 66.3 5 349.7 40.8 515.8 45 54.7 6.5 81.0 10 414.6 50.6 615.1 55 65.9 8.2 98.1 15 488.4 62.1 728.5 65 78.7 10.3 117.9 20 571.7 75.6 857.5 75 93.4 12.7 140.6 25 665.4 91.4 1003.2 85 109.9 15.6 166.5 30 770.2 109.6 1167.2 95 128.7 18.9 195.9 1-33 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Enthalpy The PanGlobal Academic Supplement refrigeration tables use the same abbreviations for enthalpy at various states as the Academic Supplement Steam Tables. “hf” is the enthalpy of saturated liquid. “hg” is the enthalpy of saturated vapour. “hfg” is the latent heat of evaporation of the refrigerant, which is equal to hg minus hf. Remember that these three properties are thermodynamic characteristics specific to every refrigerant, and vary with the refrigerant’s pressure and temperature. The values in the refrigeration tables are used to determine net refrigerating effect, pressure ratio, and coefficient of performance. As well, the refrigeration tables provide the information necessary to calculate the work done in compression, heat rejected by the condenser, mass of refrigerant circulated per tonne of refrigeration, compressor displacement and others. Many more advanced calculations may be performed. These, however, are beyond the scope of Fourth Class Power Engineering. 1-34 4th Class Edition 3 Part B  Refrigeration Basics Chapter 1 Objective 7 Describe the properties of refrigerants relating to miscibility, leakage tendency, odour, moisture reaction, toxicity, and flammability. Physical properties of refrigerants The important physical properties of a refrigerant are as follows: Miscibility Leakage tendency Odour Toxicity Flammability/explosiveness Moisture reaction Table 8 – Comparison of Physical Properties of Common Refrigerants R-134a R-123 R-717 Suva 134a Suva 123 Ammonia Flammable no no slightly Explosive when mixed with no no yes air Toxic no yes yes Corrosive to Corrosive to metals in the no no copper and copper presence of water alloys Spoils taste, may Effect of contact with food none none cause toxicity Incompatible with Non-miscible. Moderately miscible Effect on lubricating oil mineral oil. Miscible Compatible with with mineral oils with Polyolester oil. mineral oil. slight, ether-like Strong and Odour slightly sweet odour odour offensive Miscibility Miscibility refers to when two or more liquids are soluble in all proportions. Some refrigerants are entirely miscible with lube oil. Some are partially miscible. Some are not miscible at all. Refrigerants come in contact with lube oil in the compressor crankcase, cylinder walls, and screws. Refrigerants carry some of this oil into other parts of the system. Miscibility is dependent on the type of refrigerant and the type of lube oil. Lube oils are categorized as conventional mineral-based oils and synthetic polyolester oils. Most halocarbon refrigerants are miscible with one or the other. Ammonia, however, is not miscible with lube oil. Oil-miscible refrigerants dilute compressor crankcase oil, lowering its viscosity and lubricating ability. Allowance should be made for this when selecting a lubricating oil. 1-35 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Oil carried into the condenser and evaporator coats the heat transfer surfaces. This tends to be an insulating coating, reducing evaporator and condenser capacity. When oil-miscible refrigerants are used, the refrigerant will dissolve most of the oil in the system components and return it to the compressor, thus avoiding problems with heat exchange or oil blockage. However, when non-miscible refrigerants are used, the oil tends to build up in the evaporator and the condenser. This reduces system capacity, and eventually blocks the evaporator tubing. Therefore, ammonia systems require oil separators and oil return systems. These remove oil from the piping and other low points in the system, and return the oil to the compressor. Figure 11 shows oil separators installed on the compressor discharge piping in an ammonia refrigeration plant. Figure 11 – Oil Separators The miscibility of the refrigerant used in the refrigerating system plays an important role in system design, regarding the need for oil separators, and oil return lines. As well, miscibility affects the sizing of piping. Pipes are sized to maintain sufficiently high refrigerant velocity to prevent oil from settling in evaporator tubing. Odour In large concentrations, the refrigerants in the halocarbon group have a slight odour resembling ether. They are practically odourless in low concentrations. Other refrigerants such as ammonia have a strong, pungent smell. A slight odour may be advantageous since even small leaks are easily and quickly detected, so that repairs can be made before all refrigerant is lost. A strong, pungent odour, even when escaping from a minor leak, will also be noticed quickly. However, when even a small leak of ammonia is noticed in a place of public assembly, such as an arena, its smell may induce panic-like behaviour. 1-36 4th Class Edition 3 Part B  Refrigeration Basics Chapter 1 Flammability/Explosiveness Flammable and explosive refrigerants require special consideration as to how and where they are employed. Typically, flammable and explosive refrigerants, such as R-290 (propane), are limited by CSA B52 code to industrial sites. These locations would likely employ full-time A E system supervision, and would have specialized explosion-proof electrical equipment (including explosion-proof light switches, lamps, heaters, transformers and motors). Of the refrigerants listed in Table 8, only ammonia supports combustion. It is explosive in concentrations between 15% and 28% by volume in air. Toxicity As previously mentioned, refrigerants are divided into groups according to toxicity. Although refrigerants in the chlorofluorocarbon group are not toxic except in high concentrations, it should be mentioned that, when exposed to a flame, they will react and decompose, forming highly toxic products. When a halide torch is used for leak detection, ample ventilation should be provided. CAUTION Even non-toxic refrigerants can kill. If a substantial leak occurs, the refrigerant vapour displaces air. Cold refrigerant vapour is usually denser than air. Pockets of refrigerant gas accumulate in low-lying areas. If these areas are not properly ventilated, persons entering these areas could die of asphyxiation. Therefore, all refrigeration plants must have appropriate leak detection systems, ventilation systems, and breathing apparatus. Leakage Tendency Many factors determine the leakage tendency of a refrigerant. These factors include operating pressure, viscosity, density, and the chemical effects on seals and gaskets. The leakage tendency also depends on the molecular mass of the refrigerant. The greater the molecular mass, the larger the molecule and the less able it is to escape through tiny openings. Moisture Reaction Moisture will combine in varying degrees with most of the commonly used refrigerants. CFCs, HFCs, and HCFCs absorb only small amounts of moisture. Ammonia, however, readily absorbs moisture. At high temperatures, refrigerants are able to absorb a greater percentage of moisture than at low temperatures. This means that when a warm moisture-saturated refrigerant is cooled to a lower temperature (for instance, in the evaporator), it will produce free water. Moisture in the refrigerant should be avoided for three reasons: 1. When present in the liquid refrigerant, moisture may cause ice to form between the valve and valve seat of the metering device when the liquid is reduced in pressure and temperature as it enters the evaporator. This causes improper operation of the metering device. 2. When present in the system, moisture may also cause acid formation, resulting in corrosion, sludge forming in the compressor crankcase, and deterioration of the electric motor insulation in hermetic compressors. 3. In ammonia refrigeration systems, ammonia combines with water to form ammonium hydroxide, which is highly corrosive to copper and its alloys. For this reason, ammonia systems must NEVER use components made of copper or copper alloys. 1-37 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Chapter Summary This chapter introduced basic refrigeration terminology and discussed mechanical refrigeration, from its fundamental principles. From these principles, an understanding of the vapour compress