Compression Refrigeration Systems PDF Chapter 2 (4th Class)

4th Class Edition 3 Part B Unit 9 Chapter 2 Compression Refrigeration Systems Learning Outcome When you complete this chapte...

4th Class Edition 3 Part B Unit 9 Chapter 2 Compression Refrigeration Systems Learning Outcome When you complete this chapter you should be able to: Describe the operating principles of compression refrigeration systems. Learning Objectives Here is what you should be able to do when you complete each objective: 1. Describe the basic layout of compression refrigeration systems. 2. Distinguish between direct and indirect refrigeration systems. 3. Describe the layout of packaged refrigeration systems and the role of a refrigeration economizer. 4. Describe the special types of refrigeration compressors, and how they are similar to and different from air compressors. 5. Describe the special designs of refrigeration system evaporators and condensers. 2-1  Compression Refrigeration Systems Chapter 2 Chapter Introduction As previously defined refrigeration may be broadly described as a controlled process of cooling or removing heat from a substance and then maintaining the temperature of that substance below that of the surrounding atmosphere. One of the first methods of refrigeration made use of the fact that in order to evaporate a liquid latent heat must be supplied to bring about the change of state from liquid to a gas. It was found that water could be cooled by placing it in porous jars. The moisture seeping through the jars evaporated, cooling the water remaining inside the jar. Another early method of refrigeration involved the use of ice kept in insulated storehouses. When placed in an ice chest, the melting ice absorbed heat from the substances to be cooled, lowering the ice box temperature to near 0°C. Ice is still used to some extent for refrigeration, but has the disadvantage of not being able to cool below 0°C as well, it must be replenished after melting. Modern refrigeration systems make use of thermodynamic principles: Heat moves naturally from hot to cold. There is a direct relationship between pressure and boiling point. By decreasing the pressure exerted on a refrigerant, its boiling point decreases, and it will absorb latent heat of evaporation from its surroundings. After evaporating, the refrigerant is pressurized, and returns to its liquid state by discarding latent heat. Because this process involves the use of machinery, it is called mechanical refrigeration. Most refrigeration systems work on the closed cycle principle. In the closed cycle system, the vapour from the evaporator is collected continuously. This vapour is then compressed, condensed, and returned to the evaporator so that the same refrigerant is used over and over again. Closed cycle refrigerating systems can be divided into two classes according to the method used to raise the pressure of the refrigerant after it leaves the evaporator. The two classes are: Compression system Absorption system This chapter will cover only the compression system. Prior to studying this material, it is advisable to study or review Part B, Unit 2, on Pumps and Compressors, especially Chapter 3 Introduction to Compressors. 2-3 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Objective 1 Describe the basic layout of compression refrigeration systems. Operators must learn the components of the refrigeration systems they operate. The manufacturer design operating parameters must be observed and followed, for the plant to meet performance expectations. This objective introduces the equipment common to most mechanical compression systems. Closed Cycle Compression Refrigeration System A closed-cycle compression refrigeration system, such as that shown in Figure 1, consists of the following principal parts: Evaporator Compressor Condenser Metering device (Liquid refrigerant control or regulating valve) Liquid receiver The centre line divides the system into two sections: a high and a low side. The high side contains refrigerant at high pressure and temperature. The refrigerant leaving the compressor is high-pressure superheated gas. The refrigerant in the condenser is in both states: liquid and gas. In the condenser, the refrigerant is at saturation temperature and pressure. The refrigerant leaving the condenser is high pressure, high temperature liquid. The refrigerant entering the metering device is high pressure and high temperature, but sub-cooled a few degrees. The low side contains refrigerant at low pressure and temperature. The refrigerant leaving the metering device is low pressure saturated liquid and flash gas. The refrigerant in the evaporator is in both states: liquid and gas, at saturation temperature and pressure. The refrigerant leaving the evaporator is low pressure, low temperature gas. The refrigerant entering the compressor device is low pressure and low temperature, but superheated to a certain extent. Figure 1 – Simple Compression Refrigeration System Low Pressure High Pressure Superheated Vapour Superheated Vapour Liquid/Vapour Mixture Liquid/Vapour Mixture Evaporator Condenser Compressor Liquid/Vapour Mixture Flow Control Liquid/Vapour Mixture HP Liquid Low Pressure/ High Pressure/ Receiver Low Temperature Side High Temperature Side 2-4 4th Class Edition 3 Part B  Compression Refrigeration Systems Chapter 2 Therefore, the low side of the system includes all equipment downstream of the metering device: The evaporator The suction side of the compressor All interconnected tubing and piping The design pressure of the low side is determined by the temperature requirements for the cooled medium. For example, a deep-freeze would be designed for relatively low refrigerant gas pressure. The high side consists of: The compressor discharge The condenser The liquid receiver The piping upstream side of the metering device Connected tubing and piping The high side design pressure is determined by the required condensing temperature of the refrigerant vapour, which depends on the temperature of the available condensing medium. It is also important to remember that the heat removed from the refrigerant by the condensing medium is equal to the heat added to the refrigerant in the evaporator plus the work done on the refrigerant by the compressor. This is the basic energy flow in the compression refrigeration system. Industrial refrigeration systems are developments of this simple system. Additional equipment found in industrial systems may include all or some of the following: Multiple evaporators. Some plants have more than one cooling requirement. For example, one room may flash-freeze product to -40°C. Another room may be used for long-term storage at -10°C. Another room may be used for storage of fresh produce at 3°C. Each of these rooms would be equipped with its own evaporator, operating at different pressures. Multiple compressors. Many plants have multiple compressors, for the same reason that many plants have multiple boilers. Plants need redundant compressors should one compressor fail. As well, multiple compressors may be required to meet the entire load range of the plant. For example, at the beginning of the skating season, additional compressor capacity is required to make ice. In fall and spring, the required refrigerating capacity is greater, due to the warmer weather. In mid-January, most ice plants have adequate cooling capacity running a single compressor. Other plants use booster compressors to achieve the low temperatures and pressures for the coldest evaporators in the system. Multiple condensers. Many plants have multiple condensers for redundancy. If one condenser fails, the plant can continue to operate. Depending on the required condenser tonnage, the plant may need to operate at a lower capacity, though. Isolation and servicing valves. Refrigeration plants need valves to isolate evaporators, condensers, receivers, and compressors for servicing and repairing system components. Emergency discharge valves release all the refrigerant from the system in the event of an emergency. Other valves are used for adding or reclaiming refrigerant from a system. Safety valves and safety limit controls. Refrigerant systems are comprised of pressure piping and pressure vessels. As such, they require overpressure protection. Operating temperature and temperature limit controls. Temperature controls may be used to control the temperature of a refrigerated space, a cooled medium, or compressor discharge pressure. Temperature limit controls may be used to prevent the freezing of cooled medium, such as chilled water. 2-5 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Pressure regulating valves. Pressure regulating valves can be installed to maintain an evaporator temperature setpoint in systems with multiple evaporators. Compressor protective devices. Compressor protective devices include oil pressure cut-off switches, cooling water temperature controls, high discharge temperature and pressure cut-off switches, and safety valves. Also, for systems with oil-miscible refrigerants, compressors are equipped with crankcase heaters. These heaters drive dissolved refrigerant from the oil. This keeps the oil from foaming, which will starve the compressor lube oil pump, causing bearing failure. Heat exchangers, intercoolers, and economizers to improve cycle efficiency. 2-6 4th Class Edition 3 Part B  Compression Refrigeration Systems Chapter 2 Objective 2 Distinguish between direct and indirect refrigeration systems. Mechanical refrigeration systems can be categorizes as direct and indirect. Each of these systems is in common use. The CSA B52 Mechanical Refrigeration Code describes these systems and A E their variations. Direct Systems The direct system is one in which the evaporator surface is in direct contact with the material or space being refrigerated. Household refrigerators and air conditioners are examples of direct systems. These have evaporators that transfer heat directly from the cooled medium or refrigerated space. If an evaporator, piping joint, or fitting developed a leak, refrigerant could enter the occupied space. If the refrigerant was toxic or flammable, an unsafe situation would arise. Direct refrigeration systems are often called direct expansion (or DX) systems. Evaporators used in DX systems are often called DX coils. Indirect System An indirect system is one in which a liquid, such as brine, glycol or water is cooled by the refrigerant and then circulated by means of a pump to the material or space being refrigerated. One advantage of this type of system is that hazardous refrigerants can be used to cool the brine. The refrigerant containing parts are kept in a machinery room separate from the occupied space. If a refrigerant leak occurs, it cannot readily proceed from the machinery room into the occupied space. Indirect systems are also cost-saving designs. Rather than filling the entire cooling system with expensive refrigerant, most of the system is filled with chilled water or brine, which is far less costly. The chilled water or brine transfers heat from the occupied space, thus reducing the amount of refrigerant required for an initial charge. 2-7 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Figure 2 shows the arrangement of an indirect system. The evaporator coils are located within a tank of brine. The refrigerant evaporates within the coils, cooling the brine. The cold brine is then pumped through coils located within the refrigerated space. The brine absorbs the heat from this space, and then circulates back to the brine tank to be cooled once again. Figure 2 – Indirect Refrigeration System Refrigerated Cooling Coil Space Cold Brine Warm Brine Expansion Valve Evaporating Coil Liquid Refrigerant from Receiver Refrigerant Vapour to Compressor Brine Pump On Track Evaporators used to cool water or brine are called “chillers.” This method is commonly used for ice rinks. Chillers cool the brine (a salt or glycol solution) to about -11°C (12°F) to freeze the ice surface. The most common brine used for this service is a solution of calcium chloride. Depending on the concentration, calcium chloride brine can be cooled as low as -51°C (-60°F) without freezing. Indirect refrigeration systems are frequently used in larger air conditioning systems. Water is often used instead of brine as the heat transfer medium in these applications. The water is cooled in a chiller and then circulated through cooling coils over which air passes. Since the air is not cooled any lower than 13°C (55°F), the temperature of the chilled water can be kept high enough to prevent freezing. Normally, the chilled water temperature is kept at or above 5°C. 2-8 4th Class Edition 3 Part B  Compression Refrigeration Systems Chapter 2 Objective 3 Describe the layout of packaged refrigeration systems and the role of a refrigeration economizer. Packaged Ice Rink Plant The packaged rink plant has all the ice rink equipment on a skid except for the externally mounted condenser. Figure 3 – Ice Rink Packaged Plant (Courtesy of Whitelaw Agricultural Society, Curling Club) The plant shown in Figure 3 has the compressor, evaporator, liquid receiver, and necessary controls mounted on a skid. The compressor and its electric drive motor can be seen in the foreground. A control panel can be seen on the left hand side of the skid. The control panel contains the motor starter for the compressor, the start and stop pressure controls, the high pressure cut out, and the compressor low oil pressure cut-off. The two large tanks at the rear of the skid contain brine, which is pumped through the concrete slab beneath the ice surface, by the brine pump. The uninsulated pressure vessel at the rear of the skid is the liquid receiver. An uninsulated pipe carries refrigerant liquid through a solenoid liquid shut-off valve and a metering device (a thermostatic expansion valve), and into the chiller. The chiller (evaporator) is the black insulated vessel behind the compressor, and below the receiver. 2-9 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration The system brine is kept in constant circulation by the brine pump. When the return brine temperature exceeds the brine set point temperature, the solenoid valve opens. This permits liquid refrigerant to enter the evaporator to cool off the brine. When the evaporator pressure rises, the compressor starts. The compressor draws the refrigerant vapour off the top of the evaporator through the suction line (the black insulated pipe that leads to the compressor. The compressor discharges high pressure, high temperature vapour to the condenser, which is located outside. The condenser converts the vapour to liquid. The liquid refrigerant then flows to eh liquid receiver. Different activities require different ice temperatures. Table 1 shows different ice surface temperatures for different activities. Table 1 – Ideal Ice Rink Temperatures Activity Ideal Temperature °C Hockey -6 to -4 Public Skating -4 to -3 Figure Skating -4 to -3 Ice Maintenance -3 to -2 Curling -4 to -3 Packaged Centrifugal Water Chillers The main components of a refrigerating plant (compressor, evaporator, condenser, and liquid receiver) can be ordered and installed separately. Common practice, though, is to use factory assembled packaged units which contain as many of the components of a refrigerating system placed on a single skid. This is especially true for chilled water systems with water-cooled condensers used for air conditioning systems. There are many advantages to packaged refrigeration units. The components of such a system are designed to match each other in order to obtain the greatest operating efficiency. The unit is very compact, with all the components mounted on a single frame so space requirements are kept to a minimum. Units are equipped with all the required auxiliary equipment such as gauges and controls. Because of this, they are easily and quickly installed. The unit is factory tested and the manufacturer takes full responsibility for design and performance, provided it is installed according to their recommendations. The basic flow diagram of a centrifugal unit is shown in Figure 4. Of particular interest is the economizer installed between the condenser and chiller (evaporator). The economizer serves several purposes: It produces flash gas to cool the compressor motor. It increases the net refrigerating effect (NRE) of the evaporator. It reduces the power consumption of the compressor. Liquid refrigerant flows from the condenser, through the economizer, and into the chiller. The economizer is a chamber with two float valves in series. Pressure and temperature drop across each valve. The flash gas produced after the first float valve is at the pressure and temperature of the refrigerant at the second stage compressor inlet. The flash gas flows through the compressor motor windings, cooling the motor. The flash gas is recompressed and enters the condenser, where it is converted to liquid with the rest of the circulating refrigerant. This mass of flash gas takes less power to recompress, because it is compressed from an intermediate pressure (not low side pressure) to high side pressure. 2-10 4th Class Edition 3 Part B  Compression Refrigeration Systems Chapter 2 The liquid that remains in the economizer after the first pressure drop flows to the evaporator. Some of this refrigerant flashes in the evaporator. However, because of the relatively small pressure drop from the economizer to the evaporator, the amount of flash gas produced is considerably less than in similar refrigeration systems, where the reduction in pressure from condenser to evaporator takes place in a single step. When evaporator flash gas is reduced, net refrigerating effect (kJ per kg of liquid entering the evaporator) increases. This increases the cooling capacity of the system. Only the refrigerant vapour produced by the evaporator goes through two compression stages. Because some refrigerant is diverted directly to the second compression stage through the motor windings, less compressor work is required. Figure 4 – Water Chiller with Economizer Using R-134a Refrigerant Condenser 20°C 862 kPa/38°C Condenser Water 33°C Discharge 2nd Stage Impeller 1st Stage Impeller Compressor Inlet Guide Vanes Motor Condenser Float Chamber Suction Economizer Float Liquid Refrigerant Valve Refrigerant Vapour Economizer Vapour Eliminator Economizer Damper Economizer Float 6°C 261 kPa Chamber 8°C Chilled Water 15°C Chiller/Evaporator (Courtesy of Carrier Corporation) The economizer also provides an intercooling effect for the compressor. The cool vapour from the intermediate chamber cools the first stage compressor discharge gas, reducing the power required for compression in the second stage. 2-11 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Objective 4 Describe the special types of refrigeration compressors, and how they are similar to and different from air compressors. Compressor types and applications were covered in general in Part B, Unit 2, Chapter 3 Introduction to Compressors. Refrigeration compressors, though, have some unique features. Construction materials and strength of components vary with the type of refrigerant used. Refrigeration compressors are cooled without fins. Refrigeration compressors are designed specially to prevent or inhibit refrigerant leakage. Refrigeration Compressors Refrigeration compressors have three main functions: 1. They draw refrigerant gas from the evaporator as it is produced. This prevents the evaporator pressure (and therefore temperature) from varying from the desired set point. 2. They raise the refrigerant gas pressure so that refrigerant can flow from the high side to the low side 3. They raise the saturation temperature of the gas to above the temperature of the condensing medium (usually air, water, or both). This is done by adding work (kJ) to the gas, which is converted to heat and pressure. When exposed to the condensing medium, the refrigerant returns to its original liquid state, for recycling in the refrigerant circuit. Refrigeration compressors can be classified as: Reciprocating compressors, in which a piston travels back and forth in a cylinder, drawing in and compressing the vapour. Rotary compressors, which use helical rotors (screws) or an eccentric rotor with vanes to compress the vapour. Centrifugal compressors, which use rapidly revolving impellers to draw in the vapour and discharge it at high velocity by centrifugal force. The high velocity, low pressure vapour is converted to low velocity, high pressure vapour before it leaves the compressor. Reciprocating Compressor Types Compressor housings for reciprocating compressors are divided into three types according to their design: 1. Open 2. Hermetic 3. Serviceable or semi-hermetic These are discussed in Part B, Unit 2, Chapter 3 Introduction to Compressors. Safety Head If a drop in refrigeration load occurs, it may be possible for liquid refrigerant to enter the compressor through the suction line. This is because when cooling requirements decrease, some refrigerant in the evaporator may not evaporate. In other words, it is possible for an evaporator to contain more refrigerant than necessary for the load. To prevent damage to the compressor by the hydraulic pressure resulting from trapped liquid at the end of the compression stroke, reciprocating compressors are often fitted with safety heads. 2-12 4th Class Edition 3 Part B  Compression Refrigeration Systems Chapter 2 During normal operation, safety heads are kept firmly in place with a heavy spring, as shown in Figure 5. The entire head lifts when the pressure in the cylinder becomes too high, due to liquid refrigerant in the cylinder. This lifting action allows the liquid to pass into the discharge line without doing serious damage. The same principle of construction is often applied in compressors equipped with ring plate valves. Only the valve assembly, instead of the entire cylinder head, is held down by the spring. The valve assembly lifts when excessive pressure occurs in the cylinder. Figure 5 – Compressor Showing Valve Assembly with Safety Spring Compressor Bearings Compressor bearings are identical in type and function to those used in air compressors and pumps. Bearing materials must be compatible with the type of refrigerant used. Bearings with copper or copper alloys are not used in ammonia compressors, because ammonia attacks these materials. Rotary Compressors Rotary compressors compress refrigerant vapour with rotary motion instead of reciprocating motion. Rotary compressors have several different designs, including: Stationary single-blade Rotating sliding vane Helical rotor or screw Scroll Rotary compressors are often used in compound refrigeration systems. In this type of plant, the compressor is called a booster. The rotary compressor produces very low evaporator pressure for deep freeze applications. It discharges intermediate pressure refrigerant into the suction of the main compressor. An intercooler is used to lower the density of the intermediate pressure refrigerant, and therefore reduce the work and power requirements of the main compressor. The intermediate pressure refrigerant vapour is then raised to condensing pressure and temperature by a reciprocating or other high-pressure compressor. Figure 6 shows a rotary style of booster compressor, equipped with an intercooler. Frost can be seen on the large-diameter compressor suction line. Several smaller frosted lines can be seen at the bottom of the picture. These are the distributor pipes of the intercooler. They feed liquid ammonia into the booster compressor discharge, to lower the discharge temperature of the intermediate pressure refrigerant. 2-13 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Figure 6 – Booster Compressor and Intercooler Centrifugal Compressors In both the reciprocating compressor and the rotary compressor, the vapour was compressed by the direct action of pistons, vanes, rollers, or gears. These components forced the vapour into a decreasing space, thus compressing it. That is, these compressors use positive displacement to force the vapour into a smaller volume. Centrifugal compressors are dynamic compressors. They increase the velocity of the refrigerant with a rapidly rotating impeller. This high velocity vapour travels through specially shaped passages of increasing cross-sectional area. Here, the high velocity is converted to high pressure. In terms of energy transformation, the kinetic energy of the low pressure, high velocity vapour is transformed into potential energy in the high pressure, low velocity vapour. Helical Rotor (or Screw) Design The application of the helical rotor design is primarily in medium and high-capacity refrigeration compressors and is identical to those used for air compression. 2-14 4th Class Edition 3 Part B  Compression Refrigeration Systems Chapter 2 Scroll Compressors Scroll compressors are increasing applied to commercial chillers, due to their efficiency and relatively quiet, vibration free operation. Figure 7 shows four hermetic scroll compressors as part of a packaged water chiller used for HVAC purposes. Each compressor operates successively, with changes in cooling load. The black insulated lines are refrigerant suction lines. The compressors are also well insulated. Figure 7 – Scroll Compressors for a Packaged Water Chiller Developments in Compressors With the advent of new refrigerants and refrigerant blends, the refrigeration industry requires new compressor designs. These may be units designed to more efficiently overcome higher discharge pressures, such as when CO2 (R-744) is used as a refrigerant. They may be compressors better designed to handle refrigerant blends. These evaporate over a range of temperatures rather than at a single temperature. New compressors must be able to efficiently operate over a pressure range rather than a single evaporator or condenser pressure. One compressor design that is proving quite capable of handling refrigerant blends is the scroll compressor, whose use is becoming more commonplace. 2-15 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Objective 5 Describe the special designs of refrigeration system evaporators and condensers. Heat exchangers are equipment in which heat is transferred from one fluid to another without mixing the fluids. Heat exchange results in one process fluid gaining heat, and the other losing heat. In a refrigeration system, two main heat exchangers are used: the evaporator and the condenser. This objective explains the principle functions, varieties, and applications of evaporators and condensers. Evaporators An evaporator is that part of the refrigerating system in which the liquid refrigerant is vaporized by the absorption of heat from the medium to be cooled. Operating Classification Evaporators can be divided into three basic types, based on how they operate: 1. Direct expansion (or “dry”) evaporator 2. Flooded evaporator 3. Liquid Recirculating (or liquid overfeed) evaporator The three types differ in the method of refrigerant circulation. Dry Evaporator In a dry evaporator, the expansion valve admits only enough liquid refrigerant to maintain the desired temperature. As the liquid enters and flows through the evaporator, it absorbs heat, evaporates, and leaves again as a vapour. The amount of liquid refrigerant entering the evaporator balances the amount of refrigerant leaving the evaporator as a vapour, with no recirculation of liquid or gas taking place. The evaporator contains only a small amount of liquid refrigerant at any time. The percentage of liquid in the evaporator varies with the load demand. Flooded Evaporator The flooded evaporator is kept almost completely filled (or flooded) with liquid refrigerant regardless of load demand. The vapour formed by the boiling of the liquid is drawn off by the compressor and fresh liquid is automatically supplied to maintain the level. Since the heat exchange surface of the flooded evaporator is always completely wetted with liquid, it has a higher heat transfer rate than the dry evaporator. It also responds rapidly to changes in load as refrigerant has already gone through a pressure reduction and is awaiting exposure to heat. This makes it applicable for pasteurization processes where on start-up, fluid must not pass through without being cooled. The disadvantages of the flooded evaporator, however, are that it requires a relatively large refrigerant charge and is often quite bulky. 2-16 4th Class Edition 3 Part B  Compression Refrigeration Systems Chapter 2 Figure 8 – Flooded Bare Tube Evaporator Suction to Compressor Baffle Flash Chamber Accumulator Liquid Liquid Level from Receiver Float Control Liquid-Vapour Mixture Liquid Recirculating Evaporator The liquid recirculating evaporator is used in liquid overfeed refrigeration systems. The liquid recirculating evaporator has a constant flow of liquid refrigerant regardless of load demand. The refrigerant is supplied to the evaporators with a liquid ammonia pump that draws from a low-pressure liquid receiver. The pump feeds around three time the amount of liquid to the evaporator than it requires. The evaporators return a mixture of liquid and vapour to a low-pressure receiver (also called a surge drum or an accumulator), to be pumped back again through the evaporators. The vapour formed by the boiling of the liquid is drawn off by the compressor from the low pressure receiver. Fresh liquid is automatically supplied to maintain the level. The low-pressure receiver is a critical element of the system. It separates the liquid and vapour returning from the evaporators, so that the vapour can be recompressed and condensed. Without the low-pressure receiver, liquid would enter and damage the compressor. Figure 9 shows a simplified liquid overfeed system. Low-pressure liquid piping is shown in green. Low-pressure vapour piping is blue. High-pressure vapour and liquid is shown in red. Figure 9 – Simple Liquid Overfeed System with Liquid Recirculating Evaporators 2-17 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Since the heat exchange surface of the liquid recirculating evaporator is completely wetted with liquid, it has a higher heat transfer rate than the dry evaporator. It also responds rapidly to changes in load as refrigerant has already gone through a pressure reduction and is awaiting exposure to heat. This makes it applicable for quick freeze and loads that swing form almost no load to full load quickly. The liquid recirculating evaporator, however, has disadvantages: The system requires a relatively large and costly refrigerant charge. Additional equipment (surge drum, pumps, level controller) adds to the initial cost of the system. Construction Classification According to their construction, evaporators may be divided into the following classes: Bare tube Plate surface Finned tube Shell-and-tube Bare Tube Evaporator This type of evaporator consists of either a single coil bent in various shapes such as the flat serpentine coil illustrated in Figure 10, or a number of coils placed in parallel and connected to common headers. They are constructed of either steel pipe or copper tubing. The bare tube evaporator can be used as a direct expansion evaporator (Figure 10), as a flooded evaporator (Figure 8), or as a liquid recirculation evaporator. If in flooded or liquid recirculation service, the outlet of the coil goes to an accumulator. Figure 10 – Direct Expansion Bare Tube Evaporator Liquid from Refrigerant Receiver Flow Control Vapour to Compressor Feeler Bulb Bare tube evaporators are used where the space temperature is maintained below 1°C and frost accumulation cannot be readily prevented, such as in coolers and freezers. They can also be submerged in liquids that are to be cooled. 2-18 4th Class Edition 3 Part B  Compression Refrigeration Systems Chapter 2 Figure 11 shows heavily frosted, bare-tube, liquid recirculation evaporator piping on the ceiling and walls of a cold-storage facility. Figure 11 – Bare Tube Evaporator Plate Surface Evaporator These evaporators are manufactured in a variety of ways. One method employs two flat steel plates embossed or stamped so that a serpentine coil for the refrigerant is formed when the plates are fused together (Figure 12). Another method uses a regular coil, which is attached between two plates. Figure 12 – Plate Surface Evaporator Refrigerant Connections The plate surface evaporator can be formed into various shapes. It is used individually or in banks. It is widely used in refrigerators, freezers, display cases and locker plants because it can be easily cleaned and defrosted by manual scraping without interrupting the cooling process. It is usually used as a direct-expansion evaporator. 2-19 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Finned Tube Evaporator The finned tube evaporator (Figure 13) is basically a bare tube evaporator coil with fins attached to it by crimping or bonding to form a large surface area for heat exchange. The most common method of attaching fins to tubing is by placing the fins over the tubing and running a mandrel or expander down the tube, causing it to expand. This is sometimes called a “pressed fin” design, because of the method of manufacturing. Figure 13 – Finned Tube Evaporator A finned tube evaporator is used as a direct-expansion evaporator in many applications, including walk-in coolers and display cases. The finned tube evaporator with forced air circulation is the most widely used in air conditioning applications. The air to be cooled is forced between the fins and gives up its heat to the vaporizing refrigerant inside the coils. In applications where low air velocities and minimum dehydration are required, the air flows over the evaporator by natural convection. In other applications, however, the evaporator is equipped with one or more fans to increase the airflow and cooling capacity. Figure 14 shows the different fin spacing requirements for finned tube evaporators used for medium and low temperature applications. The lower the temperature of the space (and coil), the greater the rate of frost buildup. At low temperatures, frost accumulates faster and becomes thicker. For this reason, the lower the temperature, the greater the fin spacing. Finned evaporators have a large number of finned serpentine coils mounted in a frame. Each coil is connected at one end to a liquid refrigerant distributor and at the other to a common compressor suction header. Figure 14 – Finned Evaporator Spacing (a) Medium Temperature (b) Low Temperature Shell-and-Tube Evaporator This evaporator (also called a chiller) is used for almost any type of liquid cooling application. It consists of a cylindrical steel shell in which a number of straight bare tubes are arranged in parallel. The tubes are held in place at the ends by tube sheets, which close the shell. In the case of the chiller shown in Figure 15, each head has baffles to redirect the flow of refrigerant through multiple passes. Other designs admit the liquid refrigerant to the shell side of the heat exchanger. 2-20 4th Class Edition 3 Part B  Compression Refrigeration Systems Chapter 2 Chillers may be direct-expansion or flooded. In the direct-expansion evaporator, the liquid refrigerant enters the lower tubes and, while traveling back and forth through a number of passes, evaporates by absorption of heat from the liquid surrounding the tubes. The vapour is drawn from the evaporator by the compressor through the suction connection after it leaves the final, upper pass. The liquid to be chilled is admitted to one end of the shell. It is then forced by baffles to follow a serpentine course across the tubes to the outlet at the other end of the shell where it leaves at a reduced temperature. A direct-expansion shell-and-tube evaporator, used as a water chiller, is illustrated in Figure 15. Figure 15 – Direct-Expansion Shell-and-Tube Chiller Water Inlet Water Outlet Vaporized Refrigerant Out Liquid Refrigerant In Drain In the flooded type evaporator, the liquid to be chilled passes through the tubes arranged in single or multi-pass configuration. The shell is filled with liquid refrigerant which surrounds the tubes. The level of the liquid in the shell is maintained by a float-controlled valve so that at least seventy-five percent of the tubes are submerged at all times. The vapour, boiled off by the heat of the liquid circulating through the tubes, is then drawn off through nozzles in the upper part of the shell. At high loads, the refrigerant in a flooded chiller may boil quite violently and cause some liquid refrigerant to be carried over with the vapour leaving the evaporator. This liquid carry-over will damage the compressors. Two methods are used to separate this liquid from the vapour before it enters the compressor suction line. In the first method, shown in Figure 16, a surge drum is mounted on top of the evaporator. The surge drum and evaporator are connected to each other by two or more large diameter pipes. The diameter of the evaporator is kept relatively small and the tubes fill nearly all of the shell, thus the liquid level has to be kept quite high. Any liquid refrigerant carried over with the vapour into the surge drum will drop out of the gas flow before it reaches the suction line on top of the surge drum due to the change of direction and the length of the gas travel. This liquid will run back into the evaporator. 2-21 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Figure 16 – Flooded Chiller with Surge Drum Mounted on Top In the second method, the liquid is separated from the vapour before it leaves the evaporator. The same number of tubes are used for a given heat exchange capacity as are used in an evaporator with a surge drum. However, a large diameter shell is used and all the tubes are mounted in the lower half of the shell. 2-22 4th Class Edition 3 Part B  Compression Refrigeration Systems Chapter 2 The tubes are submerged in the liquid refrigerant. The space above the tubes (the upper half of the shell) is then used for separating the liquid from the vapour. To aid in this process, most evaporators are also equipped with a bank of chevron shaped mist eliminator plates as shown in Figure 17. Figure 17 – Flooded Chiller with Eliminators Charging Valve Eliminators Thermometer Sight Glass Drain Condensers The function of the refrigeration condenser is to remove heat from compressed refrigerant vapour until it changes state and becomes a liquid. To do this, the condenser must remove both the heat absorbed in the evaporator by the vapour, and the heat of compression added to it in the compressor. Condenser Types There are three general types of condensers: 1. Air-cooled 2. Water-cooled 3. Evaporative Air-Cooled Condensers The air-cooled condenser uses air to remove heat from the refrigerant vapour. The hot refrigerant vapour flows through finned tubes. Cooling air is circulated by fans or blowers around the outside of the tubes. Units with larger capacities are usually equipped with a remotely mounted condenser installed either indoors or outdoors. When the remote air-cooled condenser is mounted indoors in a relatively warm location, provisions must be made for an adequate supply of outdoor air to the condenser. For small units, the opening of a window may suffice, but for larger units, ducts must be used to carry outdoor air to the condenser and back to the outside. Because large condensers mounted indoors would require extensive space-consuming ductwork, they are commonly mounted outdoors with a roof location being the most popular. 2-23 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration One of the disadvantages of air-cooled condensers is that as cooling load increases in hot weather, the condenser capacity is reduced as the temperature difference between the cooling medium and the refrigerant decreases. However, air cooled condensers do not require cooling water or water treatment chemicals, and do not breed harmful bacteria. Therefore, air cooled condensers are preferred when environmental considerations are of primary importance. Figure 18 shows a horizontal air-cooled condenser for outdoor use. Cooling air is drawn through the condenser by a belt-driven propeller fan. Condensers of this type are also manufactured as vertical units. In either case, fans are used to force or draw air through the condenser. Figure 18 – Outdoor Air-Cooled Condenser Water-Cooled Condensers This type of condenser uses cooling water to remove heat from the refrigerant vapour. Three basic designs of water-cooled condensers are commonly used: 1. Double tube 2. Shell-and-coil 3. Shell-and-tube Double Tube Condenser A double tube condenser consists of a small tube contained concentrically within a larger tube, as shown in Figure 19. Cooling water flows through the inner tube of the double tube condenser while refrigerant flows in the opposite direction in the annular space between the inner and outer tubes. This type has the lowest efficiency of the three designs, resulting in the need of large condenser for a given capacity. It is sometimes used as a booster condenser with air-cooled condensers during periods of peak loading. Notice this is a counter-flow design. The water flows through the inside tube to keep cooling water from absorbing heat from the surroundings. All heat absorbed should be directly from the refrigerant gas. 2-24 4th Class Edition 3 Part B  Compression Refrigeration Systems Chapter 2 Figure 19 – Double Tube Condenser Refrigerant Vapour In Refrigerant Water Out Water Water In Condensed Refrigerant Out Shell-and-Coil Condenser The shell-and-coil condenser consists of a horizontal or vertical steel shell that contains a coiled bare or finned tube, through which cooling water circulates. The hot refrigerant vapour enters near the top of the shell, gives up its heat to the cool surface of the coil, and collects in the bottom of the shell as a liquid. The bottom portion of the shell often serves as a receiver. A cross-sectional view of a horizontal shell-and-coil condenser is shown in Figure 20. This condenser is very efficient and compact, making it easily adaptable to packaged refrigeration units. Its main disadvantages are that it is difficult to clean. When leaks develop, the repair costs may exceed the cost of a new condenser. Tubes can be cleaned with an acid solution and then neutralized. Figure 20 – Shell-and-Coil Condenser A A - Refrigerant Vapour In B - Liquid Refrigerant Out C - Water In D C D - Water Out B Shell-and-Tube Condenser The shell-and-tube condenser consists of a welded steel shell containing a number of straight tubes fastened in the tube sheets which close the shell. The condenser is fitted with water boxes to which the cooling water supply and return pipes are connected. The water boxes direct the cooling water through two or more passes. The hot refrigerant vapour enters at the top of the shell and condenses by coming in contact with the cool outer surface of the tubes. As the vapour condenses, the refrigerant drains and collects in the bottom of the shell as a liquid. Figure 21 shows a cross-sectional view of a shell-and-tube condenser with two-pass cooling water flow. This type of condenser is used extensively in ammonia installations of all sizes, as well as in medium and large sized air conditioning installations using other types of refrigerants. The horizontal shell-and-tube condenser provides high cooling efficiency at reasonable cost. It can be cleaned easily so routine maintenance costs are low as well. 2-25 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Figure 21 – Shell-and-Tube Condenser Liquid Line Hot Gas Condenser Inlet Tubes Water Out Water In Condenser Shell Evaporative Condensers Evaporative condensers use both air and water to provide cooling in order to condense the refrigerant vapour. A cross-sectional view of a typical evaporative condenser is shown in Figure 22, while Figure 23 shows a basic schematic diagram of a similar unit. The evaporative condenser contains condensing coils mounted in a cabinet. The hot refrigerant vapour from the compressor enters at the top of the condenser, while the refrigerant liquid leaves at the bottom, and flows to a receiver Figure 22 – Evaporative Condenser Fan Cooling Water Cooling Spray Water Nozzles Vapour Vapour Inlet Coils Liquid Air Outlet Inlet Receiver Sump Sump Drain Connection Make-up Water Connection 2-26 4th Class Edition 3 Part B  Compression Refrigeration Systems Chapter 2 The refrigerant vapour flows through a set of coils over which water is sprayed from nozzles. Air is drawn counter flow to the water. A portion of the water evaporates, producing a cooling effect on the remaining water and on the refrigerant in the tubes. The water is continuously circulated by a sump pump, located at the bottom of the condenser, back to the spray nozzles (Figure 23). The fan may be the induced or forced draft type. As the air exits the condenser, it flows through drift eliminators where entrained water droplets are trapped and fall back over the cooling coil. Figure 23 – Schematic Diagram of an Evaporative Condenser Fan Air Out Eliminators Spray Nozzles Refrigerant Vapour In Refrigerant Liquid Out Condensing Coil Air In Make-Up Water Water Tank Since the amount of heat absorbed by the evaporating water is so large (approximately 2257 kJ per kg of water evaporated), the amount of water used by an evaporative condenser is only a small percentage of that required in a water-cooled condenser of the same capacity. The amount of air circulated through this condenser is also only a small percentage of that required in an air-cooled condenser of the same capacity. Evaporative condensers are favoured in locations where it is necessary to conserve water. A saving of 80 to 90 percent in water consumption may be achieved over a conventional water-cooled condenser that discharges the water to waste. Since part of the spray water evaporates and is carried off by the air leaving at the top, it is necessary to supply makeup water to the condenser sump. A float valve is used to maintain the water at a constant level. Evaporative condensers may be located either indoors or outdoors. When the condenser is placed indoors, the air inlet and discharge are connected to the outside of the building by ducts which are closed by louvres when the condenser is not in operation. When the condenser is placed outdoors, care should be taken to prevent freezing of the water during cold weather. This prevention is usually done by placing a heating coil in the sump of the condenser. The coil keeps the temperature of the water above the freezing point when the condenser is not in operation. Alternatively, the condenser sump drains to an indoor tank. The spray water pump (also placed indoors) takes its suction from this tank, so no water stays in the sump at any time. As can be seen in Figure 22, the liquid refrigerant receiver is sometimes placed in the sump of the evaporative condenser. This subcools the liquid refrigerant before it enters the evaporator. This reduces flash gas and increases the net refrigerating effect. The exterior of the tank, however, should be protected against the corrosive action of the sump water and the air. 2-27 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration The air intake should be provided with a filter to prevent dust or sludge formation in the sump or water tank. Due to the continual evaporation of water and its replenishment from the makeup supply, the concentration of dissolved solids steadily increases in the water which, if uncontrolled, would eventually precipitate out as scale. Sumps are usually provided with a bleed valve to continuously bleed off some of the water. The makeup water has a low solids concentration, thus reducing the dissolved solids concentration of the condenser water. Figure 24 shows a large evaporative condenser used in an industrial refrigeration plant. It has four condensers combined in a single large block. Each cell has two cooling fans. High-pressure refrigerant vapour enters the top of each condenser through the orange coloured pipes. High-pressure liquid refrigerant leaves the condenser coils at the bottom of each cell, where it is collected and returned to the liquid receiver. The large black pipes deliver condenser water that sprays above each condenser coil. Figure 24 – Large Industrial Evaporative Condenser Water-cooled condensers, if discharging coolant to waste, waste a tremendous amount of water. Local water-use regulations may prohibit the installation or limit the capacity such open-loop cooling systems. Therefore, cooling towers are often used with water-cooled condensers to limit water consumption. These use the same evaporative principles as evaporative condensers to cool condenser water. A condenser water pump circulates cooled water from the tower through the water-cooled condenser. The warm water flows from the condenser to the cooling tower, where evaporation again cools the condenser water. Cooling towers are designed so water circulates falls over wood or plastic slats (called “fill”). The fill breaks the water stream to droplets, which then fall by gravity towards the water sump. Air flows in the opposite direction. A certain portion of the water evaporates and cools the remaining water, which collects in the cooling tower sump. A condenser water pump draws from the sump and supplies the cooling water to the water-cooled condenser. 2-28 4th Class Edition 3 Part B  Compression Refrigeration Systems Chapter 2 Figure 25 – Cooling Tower Showing Fill and Air Fan Air Outlet Fan Drift Eliminators Water Inlet Distribution System Louvres Fill Air Air Inlet Inlet Cool Water Out Cool Water Basin Cooling tower location is important because in winter conditions humid air leaving the cooling tower can cause ice buildup on surrounding equipment due to drift. Drift can cause corrosion, and create slip hazards. In the winter, drift can increase the weight loading on structure, causing it to collapse. Often these towers are elevated so that drift is carried away by the prevailing winds. 2-29 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Chapter Summary This chapter examined many of the main components of direct and indirect compression refrigeration systems. This included a description of high and low side components, such as compressors, metering devices, evaporators, and condensers. Reciprocating, rotary, and centrifugal types of compressors were described, as were their hermetic, semi-hermetic, and open frame variants. Packaged refrigeration systems are commonly used for ice arenas and HVAC service. These indirect systems were examined, and their characteristics noted. They may use direct expansion evaporators or flooded evaporators. Most of these systems use water-cooled or evaporative condensers, though larger air-cooled condensers are becoming more common because they have less environmental impact. If water-cooled condensers are used, they are usually coupled with cooling towers to conserve water. Liquid overfeed systems use a type of flooded evaporator supplied with refrigerant by a circulating pump. A low-pressure receiver prevents liquid refrigerant from damaging compressors. These systems are commonly used in industry, in large direct systems. 2-30 4th Class Edition 3 Part B  Compression Refrigeration Systems Chapter 2 Knowledge Exercises – Chapter 2 Name: _____________________________ Date: _______________________________ Instructor: __________________________ Course: _____________________________ Objective 1 1. Identify the components found on the high pressure and low pressure sides of a compression refrigeration system. 2. Sketch a simple refrigeration system. Label all parts. Identify the components as high-side or low side. Indicate the state of the refrigerant (vapour, liquid, or both) in each part of the refrigerant circuit. 2-31 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Chapter 2 (Cont.) 3. Explain why large refrigeration systems use multiple compressors. 4. Name six devices used to protect refrigeration compressors. 5. What is the purpose of a refrigeration compressor crankcase heater? Objective 2 6. What is the difference between direct and indirect refrigeration? 7. The most common brine used in ice arenas is a solution of _______________________ ______________________. Depending on the concentration, this brine can be cooled as low as ________ degrees Celsius without freezing. 2-32 4th Class Edition 3 Part B  Compression Refrigeration Systems Chapter 2 Chapter 2 (Cont.) 8. In a direct expansion system, what could happen if an evaporator developed a leak? Objective 3 9. Describe the advantages of purchasing a packaged refrigeration unit over installing individual components. 10. Name three reasons why economizers are installed on packaged water chillers. 11. How do economizers reduce centrifugal compressor power requirements? 2-33 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Chapter 2 (Cont.) Objective 4 12. Describe the three main functions of refrigeration compressors. 13. List four types of rotary compressors used in refrigeration service. 14. What bearing materials should not be used in ammonia refrigeration compressors? Why? 15. What is the function of a booster compressor? Objective 5 16. Name the three basic types of evaporators. 2-34 4th Class Edition 3 Part B  Compression Refrigeration Systems Chapter 2 Chapter 2 (Cont.) 17. The __________ evaporator has no recirculation of liquid or gas, and contains only a small amount of liquid refrigerant at any time. 18. Regarding evaporators, what is meant by the term “flooded”? 19. Why do liquid overfeed systems require low pressure receivers? 20. Why are some chillers equipped with surge drums? 21. Describe the advantages and disadvantages of air-cooled condensers. 2-35 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Chapter 2 (Cont.) 22. Make a simple diagram of a double-tube condenser. Label the direction of refrigerant and cooling water flow. 2-36 4th Class Edition 3 Part B  Compression Refrigeration Systems Chapter 2 Chapter Glossary Term Definition Accumulator A low-pressure vessel, located between the evaporator and the compressor, used to prevent liquid refrigerant carryover to the compressor suction. Brine In refrigeration, a liquid solution cooled by a refrigerant and used for heat transmission without changing its state. Also called a secondary coolant. Direct System A refrigeration system with an evaporator arrangement whereby liquid refrigerant is fed through a metering device and evaporates due to heat absorbed directly from the cooled medium. Indirect System A refrigeration system whereby a secondary coolant or “brine” is circulated through the substance to be cooled. Mechanical A term describing a refrigeration system that uses mechanical means to Refrigeration transfer energy to and from a cooling medium. 2-37 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration 2-38 4th Class Edition 3 Part B