Absorption Refrigeration Systems PDF - 4th Class Edition PDF

4th Class Edition 3 Part B Unit 9 Chapter 5 Absorption Refrigeration Systems Learning Outcome When you complete this chapter you should be able to: Describe the operating principle, maintenance, and operati...

4th Class Edition 3 Part B Unit 9 Chapter 5 Absorption Refrigeration Systems Learning Outcome When you complete this chapter you should be able to: Describe the operating principle, maintenance, and operation of absorption refrigeration systems. Learning Objectives Here is what you should be able to do when you complete each objective: 1. Describe the basic absorption system, comparing the differences to the compression system. 2. Describe the theory and operation of an ammonia absorption refrigeration system. 3. Describe the theory and operation of a lithium bromide absorption refrigeration system. 4. Explain the operation of absorption refrigeration systems with respect to crystallization and dilution. 5. Describe the major parts and systems of an absorption system, including: heat exchanger bypass system, pump motor lubrication and cooling system, and purging system. 6. Describe the startup and shutdown procedures for an absorption refrigeration system. 7. Describe the preventive maintenance that should be performed on an absorption refrigeration system. 8. Explain typical problems and resolutions related to an absorption refrigeration system. 5-1  Absorption Refrigeration Systems Chapter 5 Chapter Introduction The energy used to drive a refrigeration compressor is mechanical energy, supplied by an electric motor or steam turbine. In an absorption refrigeration system, heat energy is used instead of mechanical energy. These systems use low cost energy sources (such as waste heat) to provide cooling. Other sources of heat can also be used, including steam, hot water, and fuel-firing. Because heat energy is used instead of mechanical energy, absorption systems do not require compressors, thus reducing the electrical energy consumption. This chapter describes the operation and maintenance of two types of absorption systems: ammonia and lithium bromide. Large-scale ammonia absorption plants are used in industrial settings. Lithium bromide systems are used for large HVAC chillers. 5-3 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Objective 1 Describe the basic absorption system, comparing the differences to the compression system. Basic Absorption Refrigeration Systems In the compression refrigeration system, one job of the compressor is to maintain a low evaporator pressure by withdrawing refrigerant vapour as it is produced. The next job is to raise the vapour pressure (and corresponding saturation temperature) of the refrigerant to the point where it can be condensed at ambient temperature in a condenser. Finally, the compressor causes refrigerant to flow from the low side to high side. Absorption systems also require a condenser, a liquid receiver, an expansion valve, and an evaporator. A compressor is not required. Its functions are performed by: a) A low pressure absorber containing a liquid (the absorbent) that withdraws the refrigerant vapour from the evaporator. b) A high pressure generator that contains and heats up a concentrated absorbent/refrigerant solution. This drives the refrigerant vapour out of solution, and raises its vapour pressure. c) A heat source that adds energy to the system. d) A pump that moves the concentrated solution from the low-pressure absorber to the high pressure generator. Comparison of Absorption and Compression Systems Economy of Operation The decision to use an absorption system in industrial or air conditioning applications is often based on the availability of waste heat when cooling is required. Many absorption systems are designed to use waste heat from boiler flue gas or gas turbine exhaust. To supplement their operation when waste heat is unavailable, the same units may use direct firing, steam, or hot water. This ability to use a variety of energy sources permits flexibility in choosing what source is most economical, and at what times. If a steam or hot water boiler needs to be kept in operation for other purposes, it may be highly economical to use steam heat, hot water, or flue gas waste heat for an absorption system generator, rather than to use electrical power for a compression refrigeration system. A critical factor is the simultaneous availability of waste heat and the requirement for cooling. When these alternative heat sources are unavailable, conventional mechanical refrigeration systems can be operated. The electrical energy used by an absorption system is less than a similar capacity compression refrigeration system. This is because the pumps in an absorption system require less electrical power than a refrigeration compressor. Starting an absorption chiller does not create a huge electrical demand, so that demand charges are avoided. Because absorption systems use less electricity, smaller emergency generators can be installed. 5-4 4th Class Edition 3 Part B  Absorption Refrigeration Systems Chapter 5 Environmental Impact Absorption refrigeration systems use only natural refrigerants: R-717 (ammonia) and R-718 (water). This reduces the contribution to global warming should a refrigerant leak occur. Absorption systems also minimize global warming potential (GWP) by greatly reducing electricity consumption and production of greenhouse gases. Finally, unlike CFCs and HCFCs, R-717 and R-718 are not ozone-depleting substances (ODS), and are not being phased-out. Safety in Operation If slugs of liquid refrigerant are carried over in a compression refrigeration system, compressor damage may result. In an absorption system, carryover from the evaporator will not cause damage. This permits continuous operation of the absorption system without changing set points or continuously monitoring of loading, because slugging and evaporator freezing are not a problem. Size Packaged absorption systems for HVAC service do not require much more space than compression systems, despite the additional number of vessels required. The footprint of a packaged absorption system is not much more than that of a packaged HVAC chiller of the same capacity. Capacity Absorption chillers for HVAC service are available in capacities up to around 6000 kW (1545 TR). Centrifugal chillers are available in up to double that capacity. Cooling Water Requirements Absorption systems need more cooling water than compression systems. This is because the absorption system condenser must reject the heat added in the generator plus the heat absorbed in the evaporator, which is significantly more than the heat of compression added by a compressor. Maintenance More equipment and piping are needed for an absorption system. It takes about the same amount of labour hours to maintain the pumps as those to service a refrigeration compressor. Sound Absorption systems are very quiet in operation. They are well suited for concert halls, theatres, and other applications where the noise of a compression system would affect human comfort. 5-5 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Objective 2 Describe the theory and operation of an ammonia absorption refrigeration system. Ammonia Absorption System During the early development of absorption refrigeration, ammonia was the most commonly used refrigerant. Water is used as the liquid absorbent in ammonia absorption systems because it has the ability to absorb ammonia vapours in large quantities, and at a rate equaling the rate of vapour production in the evaporator. A basic ammonia absorption system is shown in Figure 1. A controlled amount of high-pressure liquid ammonia passes through the expansion valve into the low-pressure evaporator. The refrigerant absorbs heat from the chilled water, lowering the chilled water temperature. The heat absorbed by the refrigerant causes it to evaporate into low pressure, low-temperature ammonia vapour. Figure 1 – Basic Ammonia Absorption System H.P. Ammonia Vapour Generator Condenser Steam or Cooling Hot Water Strong Water Aqua Weak Receiver Aqua High Side Low Side Pump Expansion Valve Absorber Evaporator Building Cooling Chilled Water Water L.P. Ammonia Vapour The low-temperature, low-pressure vapour is then drawn into the absorber where it dissolves into a spray of water. The rapid absorption of ammonia by the absorber maintains the low pressure in the evaporator. This solution of refrigerant vapour and water forms a concentrated aqueous solution, called strong aqua. A pump transfers the strong aqua from the low-pressure absorber to the high-pressure generator. In the generator, a heat source (steam, hot water, direct fired burner, or waste heat) raises the temperature of the strong aqua. This boils off the ammonia absorbed by the water in the absorber, producing high-pressure, high-temperature ammonia vapour. The water left in the generator is called weak aqua, which flows back to the absorber. The generator must produce a pressure and temperature high enough to raise the ammonia temperature above that of the cooling medium used in the condenser. The high-pressure, high-temperature ammonia vapour leaves the generator and enters the condenser, where cooling coils convert it back to liquid. The rejected heat is transferred to a cooling tower. The liquid ammonia drains from the condenser to the receiver, where it is stored until re-introduced to the evaporator. 5-6 4th Class Edition 3 Part B  Absorption Refrigeration Systems Chapter 5 Ammonia absorption systems are used in cold storage, food processing, or other applications where extremely low temperature cooling is required. Ammonia absorption systems can cool well into the -30°C range, which cannot be done with lithium bromide systems. However, ammonia absorption is seldom used for air conditioning purposes because of the toxic properties of ammonia. The lithium bromide absorption system is more suitable for HVAC use. 5-7 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Objective 3 Describe the theory and operation of a lithium bromide absorption refrigeration system. Lithium Bromide Absorption (Water Vapour) System Lithium bromide (LiBr) absorption systems are large-capacity packaged water chillers, used in HVAC service. They use R-718 (water) as a refrigerant, and a lithium bromide solution as the absorbent. On Track When this objective refers to water acting as a refrigerant, it will be called either by its ASHRAE designation (R-718) or it will be called “the refrigerant.” This will reduce confusion when referencing chilled water, cooling water, and refrigerant water. The boiling point of R-718 is reduced by lowering the pressure in the evaporator below atmospheric. To lower the saturation temperature of R-718 to a temperature useful for cooling, the system must operate under an extremely high vacuum. For example, when the pressure is 1.7 kPa absolute (0.247 psia), R-718 boils at around 15°C (59°F). If the pressure is reduced to 0.87 kPa absolute (0.127 psia), the boiling point drops to 5°C (41°F). These numbers can be confirmed with steam tables. Since very low temperatures are not required for air conditioning purposes, lithium bromide absorption systems maintain an evaporator temperature of 4°C to 7°C (40°F to 45°F). This requires an absolute evaporator pressure of 0.84 to 1.01 kPa (0.12 to 0.15 psi). These pressure conditions create a sufficient temperature differential between the chilled water and the refrigerant for rapid heat transfer to take place. The basic operating cycle of a lithium bromide absorption unit is based on two factors: 1. The lithium bromide solution has the ability to readily absorb water vapour. 2. R-718 boils at low temperatures when under a high vacuum. R-718 enters the evaporator, and removes about 2490 kJ/kg from the chilled water loop as it evaporates. 5-8 4th Class Edition 3 Part B  Absorption Refrigeration Systems Chapter 5 Since R-718 vaporizes easily and quickly when it is in a finely divided state, the evaporator is equipped with a pump that forces the refrigerant through a spray header. The evaporator is fitted with a coil through which chilled water is circulated. The chilled water is cooled by evaporation of the refrigerant sprayed on the outside of the tubes. See Figure 2. Figure 2 – Low Side Components of an LiBr Absorption System Water vapour Evaporator Absorber “Warm” Water in Lithium Bromide Chilled Water out Absorber Pump Evaporator Pump Similarly, to speed up the absorption of refrigerant vapour by the lithium bromide, the absorber is equipped with a pump, which circulates the aqueous lithium bromide solution through a spray header. The spray provides much greater contact with the refrigerant vapour, increasing the absorption rate. When the lithium bromide solution absorbs R-718, it becomes dilute (weak), and its ability to absorb more refrigerant vapour decreases. The weak solution is pumped to a generator (also called a concentrator) where heat is applied by a steam or hot water coil, or by a burner. R-718 is boiled off and the concentrated solution is returned to the absorber where it absorbs more refrigerant vapour. This principle is shown in Figure 3. Since the weak solution going to the generator must be heated and the strong solution coming from the concentrator must be cooled, a heat exchanger is used in the solution circuit to conserve energy. It transfers heat from the concentrated solution to the weak solution. Figure 3 – LiBr System, with Generator Generator Steam Heat Exchanger. Evaporator Absorber Chilled Water Generator Pump Absorber Pump Evaporator Pump 5-9 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration The refrigerant boiled from the weak solution raises the pressure in the generator to about 10 kPa (1.45 psia) absolute. At this pressure, R-718 is at around 46°C (114°F). This refrigerant vapour is condensed back to liquid with cooling water coils in the condenser. It then flows from the condenser through an expansion orifice (ie. a restriction) into the evaporator, repeating the cycle. This cycle is shown in Figure 4. Figure 4 – Complete LiBr Absorption Refrigeration System Condenser Generator Steam Cooling Water to Tower Exchanger Expansion Orifice Building Chilled Absorber Water Evaporator System Cooling Evaporator Generator Absorber Water from Pump Pump Pump Tower The absorption reaction liberates heat. This heat must be removed to enable the absorption process to continue. The cooling water first passes through a tube bundle located in the absorber, where the cooling water picks up the heat generated during the absorption process. The cooling water then passes through the condenser where it picks up additional heat as it condenses the refrigerant. In review, the high-pressure, high-temperature refrigerant enters the low-pressure, low temperature evaporator after passing through the expansion orifice. As the refrigerant enters the evaporator, it vaporizes at low temperature and cools the chilled water. An evaporator pump is used to spray the refrigerant over the chilled water coils to speed up the heat transfer process. The low-pressure, low-temperature refrigerant vapour is absorbed in the absorber section by the lithium bromide solution flowing from the generator through a heat exchanger. The absorber pump continually withdraws some of this solution from the bottom of the absorber section and recirculates it back to the absorber through spray nozzles. This increases the absorption of the refrigerant vapour. This absorption of refrigerant vapour increases the temperature of the lithium bromide solution. The absorption of the refrigerant vapour produces and maintains the low pressure in the evaporator and the absorber. A cooling water coil is used to cool the absorber chamber. The spray eventually absorbs enough refrigerant to become a weak lithium bromide solution. The low-temperature, low-pressure water vapour is drawn into the absorber where a spray of strong lithium bromide solution mixes with the vapour. The water vapour gives up its latent heat and changes back to a liquid. The lithium bromide uses this heat as sensible heat, causing the solution temperature to increase. Cooling water is used to remove the sensible heat from the solution so that the absorption process can continue. As the water vapour condenses into the lithium bromide solution, this produces and maintains the low pressure in the evaporator and the absorber. A pump is used to spray the solution over the vapour so that the absorption process is quicker. The spray eventually absorbs enough water to become a weak lithium bromide solution. 5-10 4th Class Edition 3 Part B  Absorption Refrigeration Systems Chapter 5 A generator pump is used to pump the weak solution from the low-pressure absorber through the heat exchanger into the high-pressure generator. In the heat exchanger, the weak lithium bromide solution is heated up so less heat needs to be added in the generator. At the same time, the exchanger cools the concentrated solution going to the absorber so less cooling water is required in the absorber. In the generator heat is added to boil the weak solution, in order to drive off the absorbed refrigerant. The resulting high-pressure, high-temperature refrigerant vapour is hot enough to be condensed in the condenser. The pressure created in the generator must be high enough so that the condensing temperature of the refrigerant vapour is above the temperature of the cooling water. The concentrated lithium bromide then returns to the absorber through the heat exchanger. The refrigerant vapour leaves the generator and enters the condenser where it is condensed back into liquid. The liquid then returns to the evaporator. The absorption cycle described above is divided into a high-pressure side of 10 kPa (1.45 psia) and a low-pressure side of 0.84 kPa (0.12 psia). The high side consists of the generator and condenser. The low side consists of the evaporator and absorber. Figure 5 – Lithium Bromide Absorption System Cooling Water to Tower Condenser Generator Steam Evaporator Building Chilled Water System Absorber Cooling Water from Heat Tower Exchanger Evaporator Generator Absorber Pump Pump Pump (Courtesy of Carrier Corporation) In actual absorption machines, the high side units are often combined in one vessel and the low side units in another vessel. This is shown in Figure 5. 5-11 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration In another absorption system design, the four main components (evaporator, absorber, generator or concentrator, and condenser) are enclosed in a single shell. Figure 6 shows this arrangement. Lithium bromide absorption systems must operate under an extremely high vacuum. Maintenance of this vacuum is vital for proper operation. Air infiltration must be prevented. For this reason, these absorption units are all built on the “hermetic” principle. This means that the parts of the units which are under vacuum are completely seal welded. The pumps are also of hermetic design, so no external seals are used. All lithium bromide absorption units are equipped with purge systems to remove non-condensable gases that may infiltrate the system, despite hermetic construction. Figure 6 – Hermetic Absorption Chiller Condenser 46°C High Pressure Cooling 10 kPa Evaporator Water 4°C Out Concentrator 35°C 99°C Steam 118°C Condensate Evaporator Pump Refrigerant Heat Pump Exchanger Cooling Water In Flash 30°C Chamber Absorber 7°C 12°C 40°C Out In Chilled Water Absorber Low Pressure Pump 0.83 kPa Temperatures shown are typical Concentrator Pump (Generator Pump) 5-12 4th Class Edition 3 Part B  Absorption Refrigeration Systems Chapter 5 Objective 4 Explain the operation of absorption refrigeration systems with respect to crystallization and dilution. Concentration and Crystallization of A Solution When a salt is dissolved in water, the two substances form a solution. At a given temperature, only a specific amount of salt can dissolve in water. When the solution contains as much salt as it possibly can, it is said to be saturated. The solution has then reached its maximum salt concentration. Adding more salt will not increase the concentration. The additional salt will not dissolve; rather, it will remain as crystals. On Track The solubility of a solid in a liquid, and the maximum concentration of a solution, depends on its temperature. This is a key concept for understanding crystallization in a lithium bromide chiller. Most salt solutions, including that of lithium bromide, follow the rule that the higher the solution temperature, the more salt can be dissolved. From this principle it follows that when a saturated solution is heated, it becomes “unsaturated”, because it will be able to contain a higher salt concentration at the higher temperature. Conversely, when a saturated solution is cooled, it will not be able to hold all the salt in solution. Some salt will precipitate out of solution. This process is called crystallization. Crystallization occurs when the temperature of a solution is lowered so far that its salt concentration exceeds the maximum concentration the solution can possibly hold at the lower temperature. Crystallization of a solution is a problem in lithium bromide systems. When it occurs, the precipitated salt can restrict or even block circulation. Temperature, Pressure, and Concentration Changes during Lithium Bromide Absorption Cycle With basic knowledge of the behaviour of a salt solution, it becomes much easier to understand the operating principle of a lithium bromide absorption refrigeration unit. Refer to Figure 6. The unit is divided into two pressure sections: the low-pressure section consisting of an evaporator and an absorber, and the high-pressure section consisting of a concentrator (generator) and a condenser. Since there is no air in the unit, the pressures in the high and low-pressure sections are the same as the vapour pressure in their respective sections. The vapour pressures, in turn, depends on the temperature and concentration of the solution in each section. As described previously, the low temperature and high concentration of the lithium bromide solution in the absorber reduces the vapour pressure in the low-pressure section to about 0.83 kPa absolute. The solution temperature in the concentrator is approximately 99°C (210°F). Here, the lithium bromide solution is diluted with refrigerant. Therefore, the vapour pressure in the high-pressure section is around 10 kPa absolute. 5-13 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Observe how the concentration of the solution changes in the unit. Start at the point in the cycle where the low temperature, dilute solution is drawn from the absorber by the concentrator pump. This solution is pumped through the heat exchanger to the concentrator. In the heat exchanger the dilute solution picks up heat from the strong solution leaving the concentrator. This raises the temperature of the dilute solution, thus increasing the vapour pressure. The concentration, however, remains constant. In the concentrator, additional heat is supplied and the temperature of the solution increases to its boiling point. The vapour pressure rises to about 10 kPa. The vapour condenses on the surfaces of the cooling coils in the condenser, causing a flow of vapour from the concentrator to the condenser. This disrupts the equilibrium of the solution in the concentrator. As more heat is added to this solution, more water is evaporated, and the solution becomes more concentrated. If the amount of heat added is controlled, it follows that the final concentration of the solution leaving the concentrator can be controlled. The concentrated solution leaving the concentrator flows back through the heat exchanger to the absorber, and transfers heat to the dilute solution entering the concentrator. At the same time, the vapour pressure of the concentrated solution drops considerably. The low temperature concentrated solution flows to the flash chamber connected to the absorber. Here the pressure of the solution is equalized with that in the absorber and any excess sensible heat is removed by evaporation (flashing) of some of the water in the solution. The concentrated solution is mixed with some of the dilute solution drawn from the absorber by the absorber pump. This mixed or intermediate solution is sprayed over the absorber tube bundle, where it is cooled and absorbs refrigerant vapour drawn from the evaporator. The solution in the absorber is not in equilibrium since more molecules of water vapour are absorbed than leave the solution. As a result, the solution becomes increasingly dilute. Concentration and Dilution It may seem strange that the solution, after having been concentrated, should be mixed again with dilute solution, since the concentrated solution is more able to absorb water vapour than the intermediate solution. There are two reasons for mixing dilute and concentrated solutions together to form an intermediate solution. First, the concentrated solution, after having been cooled in the heat exchanger, will be very close to its saturation point. If sprayed directly on the absorber cooling coils, its temperature would drop below that required to keep the salt in solution, and salt crystals would precipitate. Dilution keeps this from occurring. Secondly, a higher solution flow rate is required in the absorber than in the concentrator. Some recirculation of the dilute solution is required to obtain this higher rate. Action of the Refrigerant The refrigerant collected in the lower part of the condenser is at a pressure of 10 kPa and a corresponding condensing temperature of 46°C (115°F). It passes through several small orifices as it enters the evaporator section. Since the pressure in this section is only 0.83 kPa corresponding to a boiling point of 4°C (39°F), the enthalpy of the refrigerant entering the evaporator is considerably higher than that of saturated liquid at low pressure in the evaporator. This causes part of the refrigerant to flash. The remaining refrigerant, cooled to about 4°C, together with the liquid refrigerant recirculated by the evaporator pump, is distributed over the chilled water cooling coil in the evaporator. It picks up sensible heat from the chilled water, lowering its temperature. From the perspective of the refrigerant, this sensible heat is latent heat, and it causes the refrigerant to continually evaporate. The refrigerant vapour formed in the evaporator is drawn into the absorber section where it is absorbed by the lithium bromide solution. 5-14 4th Class Edition 3 Part B  Absorption Refrigeration Systems Chapter 5 Crystallization in an Absorption Unit The concentrated lithium bromide solution, after giving up part of its heat in the heat exchanger, approaches its saturation point. Crystallization could occur if the temperature of the solution continues to drop before mixing with the dilute solution. For this reason, a lithium bromide absorption system, before it is shut down, has to go through a dilution cycle. During this cycle, the dilute and concentrated solutions are mixed so that crystallization is prevented when the temperature drops during the shutdown period. Crystallization is of particular concern when a power failure occurs and the unit cannot operate through its normal dilution cycle. If the unscheduled shutdown were to last for any length of time, the temperature of the concentrated solution could drop so much that crystallization would occur. This would block passages, especially in the heat exchanger. 5-15 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Objective 5 Describe the major parts and systems of an absorption system, including: heat exchanger bypass system, pump motor lubrication and cooling system, and purging system. Heat Exchanger Bypass Figure 7 shows a pipe that leads directly from the concentrator to the absorber, called the heat exchanger bypass. Figure 7 – Lithium Bromide Absorption Unit Concentration Flow 70 mm Hg Control Valve 118°C 132°C Condenser 45°C 39.5°C Cooling Water Out Hot Water In Steam or 35°C Condenser Bypass 110°C 114°C Valve Out Water Cond 99°C Evaporator 4.5°C Refrig. 99°C Concentrator Sump Evaporator Absorber Pump 47°C 35°C 29.5°C Cooling Water In Flash 6.3 mm Hg Chamber 7°C 13°C Out In Absorber Chilled Water System Heat Exchanger Pump Bypass Concentrated Solution Economizer Dilute Solution Concentrator Valve Bypass Valve Intermediate Solution 57° Chilled Water C Refrigerant Heat Exchanger Cooling Water Concentrator Pump Steam (Courtesy of Trane, a Division of American Standard) 5-16 4th Class Edition 3 Part B  Absorption Refrigeration Systems Chapter 5 The purpose of this bypass is twofold: 1. It limits the level of the solution in the concentrator by bypassing excess solution directly back to the absorber. During start-up, the level in the concentrator has a tendency to rise since the proper pressure difference between concentrator and absorber is not yet established. The bypass holds the solution at the design level. 2. The bypass conducts the full flow of hot concentrated solution directly back to the absorber, in case the regular return through the heat exchanger is blocked by crystallization. This direct return increases the temperature of the diluted solution pumped through the heat exchanger to the concentrator. The extra heat may dissolve the crystals blocking the return passages. This also guarantees a supply of fluid for the concentrator and absorber pumps so that circulation can be maintained if the return line is restricted. The lower loop of the bypass tube is filled with solution at all times. This forms a loop seal between the high and low-pressure sides of the system. Pump Motor Lubrication and Cooling The concentrator, absorber, and evaporator circulating pumps used on an absorption unit are of the hermetic type to prevent infiltration of air into this high vacuum unit. The motors are specially constructed with passages for liquid cooling, and are equipped with plastic or carbon bearings, which are usually lubricated by the liquid circulated by the pump. With hermetically sealed motors, the stator windings are protected from direct contact with the fluid by being sealed in a material that is impervious to moisture. Some motors may have a very thin stainless steel liner between the rotor compartment and the stator laminations and windings. Since the same fluid which cools the motor is also used as the lubricant for the motor bearings, it is very important that no solid particles such as scale or metallic slag be allowed to enter the motor assembly. To protect the motor from the possibility of bearing scoring or cutting of the liner by stray particles, a strainer with a screen and a magnetic core is installed on the coolant inlet of each fluid cooled motor. These strainers must be cleaned whenever the system is opened for maintenance. On older absorption systems, the evaporator, absorber, and concentrator pumps were generally installed as separate units. On some newer models the concentrator and absorber pumps are often combined in one housing and are driven by a single motor. Some manufacturers combine all three pumps into a tandem housing, driving them with one motor. Motor cooling and lubrication follow the same principles, regardless of the layout. 5-17 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Figure 8 shows a three-motor cooling and lubrication arrangement. Note that the coolant supply originates at the discharge of the evaporator pump. It is then distributed to the three motors through a common supply line. The return from each motor is led to a common return line, which discharges the warm coolant into the refrigerant sump. Also note the HTC bulb. This is a high temperature cut-off device that trips the motor circuit if the coolant return from any one of the motors has a high temperature. Figure 8 – Pump Lubrication and Cooling Circuits Access HTC Bulb Shutoff Valve Valves Refrigerant Sump e Return Lin e Supply Lin Magnetic Strainers Magnetic Strainer tor entra Shutoff Conc ump P Valve rber Abso p P u m r orato Evapump P (Courtesy of Trane) System Purging There are always some non-condensable gases in an absorption unit. The most common cause of non-condensable gases is an air leak somewhere in the unit. Accumulation of non-condensable gases has the following detrimental effects on the operation of the unit: Reduced capacity and higher chilled water outlet temperature - The increased vapour pressure in the absorber caused by the non-condensable gases raises the evaporation temperature of the refrigerant in the evaporator. This results in a higher than normal temperature of the chilled water leaving the evaporator coils and a decrease in system cooling capacity. Crystallization - Normally, a reduction in load results in a decreased evaporator and absorber temperature. The heat supply to the concentrator is then cut back and the unit continues operation, but with a more dilute solution. If the reduction in capacity is the result of non-condensable gases, the heat supply to the concentrator is not cut back since the chilled water outlet temperature is higher than normal. The concentration of the solution leaving the concentrator may become so high that crystallization occurs in the heat exchanger when the solution is cooled by the dilute solution coming from the absorber, which is at a lower than normal temperature due to the reduced load. 5-18 4th Class Edition 3 Part B  Absorption Refrigeration Systems Chapter 5 Corrosion - Oxygen in the non-condensable gases, combined with the lithium bromide solution, produces a corrosive solution, which attacks the metal components of the system. It is vitally important that non-condensable gases be purged from the absorption unit. The purge systems used are divided into two types: 1. Systems with a mechanical vacuum purge pump 2. Systems with non-mechanical automatic continuous purge Mechanical Purge System Figure 9 illustrates the basic principle of a mechanical purge system. It consists of a purge pickup tube, purge chamber, manual shutoff valve, safety solenoid valve, oil trap, and vacuum pump. Figure 9 – Basic Mechanical Purge System Manual Solenoid Shutoff Valve Valve Absorbent Solution Oil Trap Cooling Water Purge Chamber Pick-up Tube Vacuum Water Vapour Plus Pump Non-Condensables Motor Solution to Absorber (Courtesy of Trane) The purge chamber is basically a small absorber. It is mounted externally on smaller units, and internally on larger ones. It is fitted with a cooling coil that maintains the temperature (and therefore the pressure) at a lower level than that in the absorber. A small amount of lithium bromide solution, taken off the line leading to the absorber spray headers, is sprayed over the cooling coil. The non-condensable gases, and some refrigerant vapour, collect in the absorber, since it has the lowest pressure in the entire unit. These gases are drawn from the absorber into the purge chamber, via a pickup tube. Here, they come in contact with the lithium bromide solution. The refrigerant vapour is absorbed by the solution on the surfaces of the coil. The dilute solution falls to the bottom of the purge chamber and drains back to the absorber. The non-condensable gases collect near the top of the purge chamber and are drawn off by the vacuum pump, which compresses these gases to a pressure high enough to be discharged to the atmosphere. The vacuum pump is a two-stage, oil sealed, rotary vane pump. It is a low volume pump, able to operate with very low suction pressure. The pump suction line contains an oil trap and a solenoid valve. The oil trap keeps oil from being drawn into the absorption unit. The solenoid valve is a fail-safe device: if the power fails or the vacuum pump drive belt breaks, the valve automatically closes to prevent air leaking back into the system. 5-19 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Automatic Purge System Figure 10 shows a purge unit that automatically keeps the absorption unit free of non-condensable gases, without using a vacuum pump. It consists of a hermetically sealed, welded assembly of three cylindrical vessels. The suction chamber (A) functions as a low-pressure vessel which draws non-condensable gases from the absorber. The return chamber (B) is where the non-condensable gases are separated from the lithium bromide solution. The storage chamber (C) collects the non-condensable gases before they are discharged to atmosphere. The operation of the unit is as follows. A measured amount of weak lithium bromide solution is supplied to the suction chamber (A). Since the chamber is under low pressure, non-condensable gases are drawn in. Some of the solution also flows into the storage chamber (C). The solution and non-condensable gases flow from the suction chamber (A) to the return chamber (B) via a check valve. Here, the non-condensable gases separate out of the lithium bromide solution and bubble up into the storage chamber (C). The solution returns to the absorber through the purge return valve. The non-condensable gases trapped in the storage chamber (C) cannot return to the system. As the storage chamber fills with non-condensable gases, it displaces the solution into the return chamber, and then to the absorber. When the solution in storage chamber (C) is at a predetermined low level, it triggers a switch that turns on an indicator light on the control panel signaling the need to exhaust the purge unit. Figure 10 – Automatic Purge Unit Non-condensibles Liquid Suction Non- Storage cond. Chamber Chamber Purge C A Exhaust Valve Liquid Non- free of cond. non-cond. Purge line from Absorber (solution plus Return non-cond’s) Chamber Check Valve B Non-cond’s Purge Return Transfer Valve Tube Return (Courtesy of Carrier Corporation) To start the purge, the purge return valve is closed. The solution supply now compresses the non-condensable gases above atmospheric pressure and refills part of the storage chamber. The purge exhaust valve is now cracked open and the non-condensable gases trapped in the upper part of the storage chamber are bled to atmosphere, usually via a hose with the end submerged in a water filled bottle so the escaping gases can be observed. After all gases have been purged, the purge exhaust valve is closed, and the purge return valve is opened to start a new cycle. 5-20 4th Class Edition 3 Part B  Absorption Refrigeration Systems Chapter 5 Objective 6 Describe the startup and shutdown procedures for an absorption refrigeration system. Start-Up Sequence for an Absorption Chiller This is a typical start-up sequence for an absorption unit. Automated controls ensure that each step in the sequence is complete before the next step is begins. 1. The sequence is initiated by a manual pushbutton, or a control point on an HMI. 2. The chilled water pump motor is started. 3. Chilled water flow is detected by a flow switch. 4. The chilled water temperature rises to the point where cooling is required, as detected by a temperature switch. 5. The condenser cooling water pump motor is started. 6. The cooling water temperature controller starts and assumes control of the cooling tower fan motor. 7. The absorption unit is placed in operation by an automatic start switch. 8. After a preset time delay, the absorber and generator pump motor(s) are started. 9. The chilled water temperature controller assumes control of the steam or hot water flow to the generator. 10. If the chilled water temperature drops below its control range, a dilution cycle and system shutdown is initiated. Operation The start-up and shutdown procedures for various absorption units differ, depending on the make and model of these units. Since it is not possible to discuss each procedure separately, only general guidelines can be given. Many of these units operate only on a seasonal basis, so the following guidelines are for seasonal start-up and shutdown. Seasonal Prestart Service 1. Lock out all sources of energy. 2. Clean all cooling and chilled water strainers, and the cooling tower sump. 3. Check the lubricant in circulating pumps and cooling tower fans. Check them for free rotation. 4. Open the necessary valves in the cooling and chilled water systems. If the systems were drained during shutdown, fill them with clean water. Vent all the air from the systems. It may take one or two days of circulation before all the air is removed. 5. Add the required water treatment chemicals. 6. Check and clean the magnetic strainers in the absorption unit pump motor cooling circuit. 5-21 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration 7. If the absorption unit is equipped with a mechanical purge system, check the purge pump. Follow the manufacturer’s recommended procedure. 8. If the refrigerant sump is empty, connect a temporary clean water supply for pump motor lubrication and cooling. 9. Start all pumps briefly (for a few seconds) to ensure they will start. 10. Ensure that there is a sufficient charge of lithium bromide in the system. Seasonal Start-Up 1. Open the air supply valve to the pneumatic control system. Check the air supply pressure. It should not exceed 140 kPa (20 psig). 2. Make sure starting switches are in the “off ” position, and then close the main breakers. 3. Place the condenser water pump and cooling tower fan switches in the “automatic” position. 4. Open the manual shutoff valve in the steam or hot water supply line to the unit. If a temporary water supply is used for the unit pump motor circuit, open the supply valve. Limit water pressure to 35 kPa (5 psig). 5. Start the auxiliaries and the absorption unit following the procedure recommended by the manufacturer. 6. When the refrigerant sump has filled, stop the unit. Disconnect the temporary water supply to the pump lubrication and cooling circuit, and open the valves in the regular supply circuit. Restart the unit. 7. After the absorption unit has been operating for approximately 30 minutes, start the purge unit. Check all temperatures, pressures, and flows. Enter the required data on the log sheet. 8. Add octyl alcohol to the unit as recommended. Seasonal Shutdown 1. Turn the unit switch on the control panel to “off ” and allow the machine to complete the dilution cycle. 2. Stop the chilled water pump. This stops the cooling water pump and cooling tower fan. 3. Close the manual steam or hot water supply valve. 4. Open all breakers 5. Turn off the air supply to the pneumatic control system. 6. Drain the cooling water circuit. 7. Service all the auxiliary pumps, the cooling tower and the fan, etc. Follow the manufacturer’s instructions. 8. Service the purge pump (if so equipped). Start-Up after Short Shutdown (Weekend or Less) 1. Open the manual shutoff valve in the steam or hot water supply line. 2. Start up the unit according to the manufacturer’s recommended procedure. 3. Start the purge unit after operating for one half-hour. 5-22 4th Class Edition 3 Part B  Absorption Refrigeration Systems Chapter 5 Shutdown for Short Period (Weekend or Less) 1. Turn the unit switch on the control panel to “off ” and allow the machine to complete the dilution cycle. 2. Stop the chilled water pump. This stops the cooling water pump and cooling tower fan. 3. Close the manual steam or hot water supply valve. 5-23 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Objective 7 Describe the preventive maintenance that should be performed on an absorption refrigeration system. Maintenance Mechanical Purge System The following checks should be performed on a monthly basis: 1. Check pulley alignment and V-belt tension. The belt should move inward midway between the pulleys about 15 mm (5/8 inch) under light thumb pressure. 2. Clean the belts of dust, dirt, and lint. 3. Change oil in the vacuum pump. Run the pump until the oil is hot, after which the oil will drain freely. Follow the manufacturer’s instructions closely to avoid damaging a shaft seal. On a yearly basis, the pump motor must be lubricated according to the manufacturer’s instructions. Strainers Clean all strainers and traps in the steam or hot water supply, condensate return, and cooling water circuits two weeks after seasonal start-up, again at midseason, and at seasonal shutdown. Clean the magnetic strainer in the cooling and lubrication line to the absorption unit pump motors monthly. Reclaiming the Solution During normal operation some lithium bromide solution may be carried over into the refrigerant. A refrigerant sample should be taken weekly and its relative density should be measured. If contamination exists, the solution should be reclaimed. This is done by draining part of the refrigerant into the solution circuit through either an automatically or manually operated valve. This refrigerant evaporates out of the solution again in the concentrator, and returns via the condenser to the evaporator. This process results in lithium bromide-free refrigerant in the evaporator. Circulating Pumps, Cooling Tower Fan Once a week, check for proper oil level in oil-lubricated bearings, leakage from stuffing boxes and seals, bearing temperatures, unusual sounds, and vibration. Grease the bearings as recommended by the manufacturer. Water Treatment For the cooling water, perform daily checks of the pH and TDS (total dissolved solids) of the water. Add chemicals and adjust the bleed off as required. Check for algae growth in the cooling tower. Add biocide to control bacterial growth. For the chilled water, check the strength of the anti-corrosion agent monthly. 5-24 4th Class Edition 3 Part B  Absorption Refrigeration Systems Chapter 5 Octyl Alcohol Charge Octyl alcohol is a wetting agent that helps the lithium bromide solution absorb water vapour, increasing the efficiency of the system. It should be added to the system at intervals as recommended by the manufacturer. To add octyl alcohol to a system: 1. Connect a charging tube to the access valve located on top of the concentrator sump, as shown in Figure 11. 2. Raise the open end of the tube and fill it with distilled water to remove all the air. 3. Place the open end of the tube in a flask containing the recommended amount of alcohol. Do not spill the alcohol on clothing as the smell is long lasting. 4. With the unit in operation, slowly open the access valve, allowing the alcohol to be drawn into the system. Quickly close the valve the instant that the last of the alcohol leaves the flask. Figure 11 – Charging Alcohol into the System Access Valve Octyl Alcohol Charge Control System The control system of an absorption refrigeration unit is set and adjusted by the manufacturer’s service representative at the time that the unit is placed in service. This is one of the benefits of purchasing a packaged unit. Unless the operator is fully familiar with the controls and their servicing, repairs should not be attempted when malfunctions occur. It is recommended that a service company be called. Tinkering with controls often aggravates the problem. Log Sheets The best way to spot gradual changes in the performance of the absorption unit is by the use of log sheets or computer printouts on which periodic readings of pressure and temperature conditions are recorded. They help the operator recognize operating conditions and their trends, diagnose unit troubles, and plan maintenance requirements. These log book entries hold legal status if they are ever presented in a court of law. 5-25 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Objective 8 Explain typical problems and resolutions related to an absorption refrigeration system. The following guide may help determine the cause of several common problems encountered when operating an absorption refrigeration system. ABSORPTION REFRIGERATION SYSTEM TROUBLESHOOTING GUIDE TROUBLE POSSIBLE CAUSE CORRECTIVE ACTION Non-condensable gases in machine Search for and stop leaks. Purge. Purging ineffective Check purge equipment and valves. Lithium bromide requires octyl alcohol Add octyl alcohol charge. Fouled condenser Clean condenser tubes. Check water Decreased treatment. Capacity Cooling water too warm Check cooling tower, pumps, strainers, and valves. Faulty capacity control setting Check for loose connections, re-adjust setting. Low solution temperature in generator Check control valves, strainers, steam traps, steam pressure or hot water temperature. Non-condensable gases in machine Purge the machine. Crystallization Following Faulty purging Check the purge system. Startup Condenser cooling water too cold Adjust cooling tower control. Non-condensable gases in machine Purge the machine. Faulty purging Check the purge system. Crystallization During Condenser cooling water too cold Adjust cooling tower control. Normal Lithium bromide solution requires octyl Add octyl alcohol charge. Operation alcohol Steam temperature too high Reduce steam pressure. Low refrigerant temperature Reduce flow rate of condensing water. Chilled water pump inoperative during Check pump operation. dilution cycle Crystallization Dilution cycle is too short Adjust control to recommended setting. at Shutdown Improper operation of capacity control Check chilled water thermostat and steam or hot water control valve. Check low limit control. Motor temperature control trips out Reset. Determine reason for trip. Safety or Chilled water flow switch opens Reset. Determine reason for opening. Interlock Shutdown Low temperature control switch opens Reset. Determine reason for opening. Chilled water pump trips out Reset. Determine reason for trip. 5-26 4th Class Edition 3 Part B  Absorption Refrigeration Systems Chapter 5 Chapter Summary This chapter introduced absorption refrigeration systems. It discussed reasons for choosing this type of system over a compression system, including several economic and environmental advantages. Absorption systems do not require compressors. Rather, they have absorbers, pumps, heat sources, and concentrators that perform the functions of the compressor. Like compression refrigeration systems, absorption systems require condensers, evaporators, and metering devices. Ammonia systems also have liquid receivers. Ammonia absorption is used in large industrial facilities, such as cold storage and packing houses. Lithium bromide systems are only used in HVAC service, because they cannot achieve low temperatures. Both use natural refrigerants, with zero ODS and GWP. In a lithium bromide system, water (R-718) is the refrigerant. To boil water at low temperature, these systems must operate under a deep vacuum. Because of this, the associated pumps and vessels must be hermetically sealed units, and purgers are necessary to keep air out of the system. Because lithium bromide systems use a hygroscopic salt solution to absorb the refrigerant vapour, solubility issues may arise, including the precipitation of lithium bromide salt crystals that interfere with chiller operation. Special shut down procedures must be used to prevent crystallization. System temperatures must be carefully controlled, and non-condensable gases continually purged, to prevent crystal formation. As with all refrigeration equipment, manufacturer’s operating and maintenance procedures must be followed. The best source of operating and troubleshooting information comes directly from the manufacturer, though troubleshooting guides from other sources may be helpful as well. 5-27 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration 5-28 4th Class Edition 3 Part B  Absorption Refrigeration Systems Chapter 5 Knowledge Exercises – Chapter 5 Name: _____________________________ Date: _______________________________ Instructor: __________________________ Course: _____________________________ Objective 1 1. List the common heat sources for absorption refrigeration systems. 2. Describe the environmental impact of absorption refrigeration systems. 3. Describe the machinery in an absorption refrigeration system used to perform the functions of a compressor. Objective 2 4. When would an ammonia absorption refrigeration system be preferred to a lithium bromide system? 5-29 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Chapter 5 (Cont.) 5. Draw a schematic of an ammonia absorption refrigeration system. Label all components. Show the direction of flows. Identify the high-side from the low-side. 6. Describe the purpose of the absorber in an ammonia absorption refrigeration system. 7. Describe the purpose of the generator in an ammonia absorption refrigeration system. 5-30 4th Class Edition 3 Part B  Absorption Refrigeration Systems Chapter 5 Chapter 5 (Cont.) Objective 3 8. What is the refrigerant used in a lithium bromide absorption refrigeration system? Give its common name and its ASHRAE designation. 9. List the low side components of a lithium bromide absorption refrigeration system. 10. List the high-side components of a lithium bromide absorption refrigeration system. 11. Why is lithium bromide used as the absorbent in a lithium bromide absorption refrigeration system? Objective 4 12. Explain the relationship of temperature and the solubility of lithium bromide in water. 13. What is the purpose of the dilution cycle that occurs before a lithium bromide absorption system shuts down. 5-31 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Chapter 5 (Cont.) 14. Why is concentrated solution from the concentrator mixed with dilute solution to form an intermediate strength solution? Objective 5 15. State two purposes of the heat exchanger bypass line. 16. What are the four main effects of non-condensable gases in a lithium bromide absorption system? 5-32 4th Class Edition 3 Part B  Absorption Refrigeration Systems Chapter 5 Chapter 5 (Cont.) 17. Sketch a simple mechanical purge system used in a lithium bromide absorption system, and explain its operation. 5-33 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Chapter 5 (Cont.) Objective 6 18. What items should be on a seasonal prestart service checklist for a lithium bromide absorption system? 19. List the items to complete on a seasonal shut down for a lithium bromide absorption system. Objective 7 20. What chemical is usually added to lithium bromide absorption systems to help the salt solution absorb water? 5-34 4th Class Edition 3 Part B  Absorption Refrigeration Systems Chapter 5 Chapter 5 (Cont.) 21. Describe the steps to add octyl alcohol to a lithium bromide absorption system. Objective 8 22. What may be the cause of crystallization on lithium bromide absorption system shutdown? 23. A lithium bromide absorption system contains insufficient octyl alcohol. What problems may arise? 24. What condition causes decreased capacity, crystallization following startup, and crystallization during normal operation of a lithium bromide absorption system? 5-35 4th Class Edition 3 Part B  Unit B-9 Basic Concepts of Compression and Absorption Refrigeration Chapter Glossary Term Definition Absorber A component of an absorption refrigeration system that contains fluid with a high affinity for the refrigerant in use. The fluid dissolves the refrigerant at the rate it is produced by the evaporator. Concentrator A component of an absorption refrigeration system that separates dissolved refrigerant from an absorbent solution, through the application of heat. Also called a generator. Generator A component of an absorption refrigeration system that separates dissolved (Refrigeration) refrigerant from an absorbent solution, through the application of heat. Also called a concentrator. Lithium Bromide A salt (LiBr) with a high affinity for water, used to develop low pressure in (LiBr) the evaporator of an absorption refrigeration system. Octyl Alcohol An alcohol derived from octane. Octyl alcohol is used in lithium bromide absorption refrigeration to enhance the affinity of lithium bromide solution for water vapour, and to reduce the likelihood of crystallization. Strong Aqua In an ammonia absorption refrigeration system, a concentrated solution of water and ammonia produced in the absorber. Weak Aqua In an ammonia absorption refrigeration system, a weak solution of water and ammonia produced in the generator. 5-36 4th Class Edition 3 Part B