MECHANICAL ENGINEERING ME.2017 FUNDAMENTAL OF REFRIGERATION AND AIR CONDITIONING Sample Questions and Answers
CHAPTER 5 EVAPORATORS 1. What is Evaporator? Classify the various types of evaporator. Evaporator The evaporator in the vapor compression cycle is a heat exchanger which absorbs heat from the substance to be cooled and transfers it to boiling refrigerant. Types of evaporators The evaporators may be classified as Forced convection and Free convection Type depending upon whether the substance to be cooled is forced by pump or fan through the heat transfer surfaces of the evaporator, or it flows naturally by density difference or warmer and cooler fluid. Some evaporators have refrigerant in the tubes and substance to be cooled surrounding the tubes, but in other cases, the refrigerant is in the shell with substance to be cooled passing through the tubes. Evaporators are also classified as Flooded type and Dry type depending upon whether liquid refrigerant covers all heat transfer surfaces or some portion is having gas being superheated. The evaporators with thermostatic expansion valve will have some portion of heat transfer surface where superheating is taking place and can be designated as dry evaporator where as evaporators with float valve will be flooded type. 2. Discuss the flooded type evaporator Flooded evaporators A typical flooded evaporator with float control is shown in fig. The liquid on its flow passage upwards through the tubes, boils, due to absorption of heat from the warmer substance, which is to be cooled The vapor so formed on boiling bubbles up in flash chamber The flash chamber separates vapor from liquid, which t1ov back to the evaporator whereas vapors re sucked by the compressor. The flash chamber collects the
flash or vapor obtained in the expansion device, plus the vapor formed by refrigerant liquid boiling in the evaporator. In a flooded type evaporator refrigerant liquid level is maintained Float valve is used as throttling device. The heat transfer efficiency increases because the entire surface is in contact with the liquid refrigerant. But the refrigerant charge is relatively large as compared to dry expansion type. To prevent liquid Cray over to compressor, accumulator or flash chamber is used. The evaporator coil is contacted to accumulator and the liquid flow from the accumulator to the evaporator coil is generally by gravity The vapor formed by the vaporizing of the liquid in the coil being lighter rises up and passes on to the top of the accumulator from where it enters the suction line In some cases liquid eliminators are provided in the accumulator top to prevent the possible carry over of liquid to suction line. Further a liquid suction heat exchanger is used on the suction line to superheat the suction vapor.
3. Discuss the liquid chillers. Figure - Flooded Liquid Chiller Figure Dry Liquid Chiller Two types of liquid chillers are shown in fig. The former has refrigerant in the shell and liquid to be chilled in the tubes whereas the latter has refrigerant in the tubes and liquid to be chilled in the shell. When the refrigerant is in the shell, the refrigerant liquid level is so kept that there is enough space on the top portion of the shell for the liquid and vapor to separate. Vapors are drawn from the top portion by the compressor. Liquid level must be maintained constant as the chilled tubes are also immersed in the refrigerant liquid. Thus float control is preferred. But, when the liquid to be chilled is in the shell and the refrigerant is in the tubes as shown in fig, thermostatic expansion valve is preferred. The refrigerant gets superheat in the last portion of the set of tubes and is collected in the end chamber from where it is sucked by the compressor. In order to facilitate proper contact of water with the refrigerant tubes, baffles are provided to ensure larger circuit up and down for the water, resulting in increased turbulence and hence better overall heat transfer co-efficient Thus, the first fig. can be termed as flooded whereas second evaporator to be dry.
4. Explain about direct expansion coil evaporator with neat and clear diagram In the liquid chiller, the chilled liquid is fed to the coils, which are used for cooling air. But, if the coils of the evaporator with refrigerant passing through them are used directly to cool air by natural or forced convection, the evaporator is called direct expansion evaporator. The refrigerant feed comes through the thermostatic expansion valve more often located at the top particularly for F-12, F22 to improve the lubricating oil return to the compressor. Air is blown over the outside of the firmed tubes. For air conditioning purposes, the direct expansion coil is preferred where the evaporator is very near to the compressors. It is direct method of cooling the substance and, therefore, quite efficient. But when the coil has to be located very far away from the compressor, it is preferred to chill the water and pump it to the air-cooling coil. For long distances, there is possibility of refrigerant leakage and the cost of the refrigerant would be also high. Besides, the pressure drop in the line would impair evaporator efficiency and co-efficient of performance. The pressure effect becomes very significant if the direct expansion coil is located at a great height from the compressor and condenser unit. The expansion valve controls the rate of the refrigerant to evaporator in such a way that all the liquid is vaporized and the vapor is also superheated to a limited extent. The inside of evaporator is far from dry but weed with liquid. All the same, this type is called dry expansion to distinguish it from flooded system and also probably because by the time
the refrigerant reaches the evaporator outlet it from flooded system and also probably because by the refrigerant reaches the evaporator outlet it is no more wet but dry superheated vapor. 5. What type of evaporator can be used where the evaporator is very near to the compressor? Explain about it. In the liquid chiller, the chilled liquid is fed to the coils, which are used for cooling air. But, if the coils of the evaporator with refrigerant passing through them are used directly to cool air by natural or forced convection, the evaporator is called direct expansion evaporator. The refrigerant feed comes through the thermostatic expansion valve more often located at the top particularly for F-12, F22 to improve the lubricating oil return to the compressor. Air is blown over the outside of the firmed tubes. For air conditioning purposes, the direct expansion coil is preferred where the evaporator is very near to the compressors. It is direct method of cooling the substance and, therefore, quite efficient. But when the coil has to be located very far away from the compressor, it is preferred to chill the water and pump it to the air-cooling coil. For long distances, there is possibility of refrigerant leakage and the cost of the refrigerant would be also high. Besides, the pressure drop in the line would impair evaporator efficiency and co-efficient of performance. The pressure effect becomes very significant if the direct expansion coil is located at a great height from the compressor and condenser unit. The expansion valve controls the rate of the refrigerant to evaporator in such a way that all the liquid is vaporized and the vapor is also superheated to a limited extent. The inside of evaporator is far from dry but weed with liquid. All the same, this type is called dry expansion to distinguish it from flooded system and also probably because by the time the refrigerant reaches the evaporator outlet it from flooded system and also probably because by the refrigerant reaches the evaporator outlet it is no more wet but dry superheated vapor.
CHAPTER 6 CONDENSER 1. Discuss on the following topics concerning with condensers used in refrigeration and air conditioning. (a) Function (b) General types (c) Heat rejection rate (d) Heat rejection ratio (a) Function: The function of condenser in a refrigeration system is to de - superheat and condense the compressor discharge vapor and frequently to sub cool the resultant liquid while introducing a minimum pressure drop. It is a heat rejection component in the refrigeration cycle. (b) Condensers are of three general types. (1) Water- cooled condensers (2) Air cooled condensers (3) Evaporative condensers (c) For proper selection and design of a condenser. It is essential to know the heat transfer rate from the condenser. The heat rejected by the condenser per ton of refrigeration is a function of the evaporator and condenser temperature. The condenser has to reject heat absorbed at the evaporator and also the heat of compression added to the system by the compressor. Thus, for each unit of heat absorbed at the evaporator, the work of compression, and hence heat of compression depends upon the conditions at suction and discharge from the compressor. (d) The heat rejection ratio for a condenser is defined as the ratio of heat rejected at the condenser to the refrigerating effect.
2. Explain about water cooled condenser. Water- cooled Condensers These are used for all sizes and types of refrigeration machines. When abundant clean water is cheaply available, these condensers are the first choice from the an economic stand point. When water is costly it is usually cooled in cooling towers and fed again to the condensers. Water-cooled condensers require less surface area than air cooled or evaporative condensers, because of high water film co-efficient. Shell and Tube Condenser Fig. Horizontal Shall and Tube Type Condenser In horizontal shell and tube condenser, in order to obtain high water film heat transfer, the cooling water always flows through the tubes and the refrigerant condenses on the outside of the tubes. The tubes are circuited to give the desired water velocity and pressure drop. The following figure shows the arrangement for a two-pass condenser. Tubes are sometimes finned and high velocities employed to increase heat transfer, especially for Freon group, which has low film co-efficient Copper tubes are used for Freon group and steel tubes for Ammonia. Since, even small quantities of air or non-condensable affect the performance adversely, a purge connection is provided for air removal. Shell tube type condenser can also serve the purpose of a receiver, mainly when all the refrigerant has to be pumped down because there is enough space in the shell.
Shell and Coil Condenser: Fig. Helical Shell and Coil Condenser It uses a helical water coil in the shell instead of straight tubes. The connection A for taking out the condensed liquid refrigerant is so made that its dips in the liquid refrigerant at the bottom position which acts as the receiver. The refrigerant vapor s from the compressor enters at higher level, i.e., B. The cooling water is fed at C and leaves D after passing through the helical coil. Also the cooling water enters at the lowest level in the condenser, the liquid refrigerant accumulated gets sub-cooled. Thus lower portion acts as a sub cooler. In fact a sub cooler section is now a-days built up in the condenser lower portion and the condenser cooling water is made to enter through bundle of tubes first, The vertical shell and tube condenser is essentially the same as the horizontal condenser except for the position in which it is installed resulting in a small space required. 3. What is the water cooled condenser and explain any one. Water- cooled Condensers These are used for all sizes and types of refrigeration machines. When abundant clean water is cheaply available, these condensers are the first choice from the an economic stand point. When water is costly it is usually cooled in cooling towers and fed again to the condensers. Water-cooled condensers require less surface area than air cooled or evaporative condensers, because of high water film co-efficient.
Shell and Tube Condenser Fig. Horizontal Shall and Tube Type Condenser In horizontal shell and tube condenser, in order to obtain high water film heat transfer,the cooling water always flows through the tubes and the refrigerant condenses on the outside of the tubes. The tubes are circuited to give the desired water velocity and pressure drop. The following figure shows the arrangement for a two-pass condenser. Tubes are sometimes finned and high velocities employed to increase heat transfer, especially for Freon group, which has low film co-efficient Copper tubes are used for Freon group and steel tubes for Ammonia. Since, even small quantities of air or non-condensable affect the performance adversely, a purge connection is provided for air removal. Shell tube type condenser can also serve the purpose of a receiver, mainly when all the refrigerant has to be pumped down because there is enough space in the shell. 4. With the help of neat sketches, explain the working principles of a double tube condenser. Double- Pipe Condenser: It consists of concentric tubes with the refrigerant condensing in the annular space and the water flowing in the inner tubes. This is popular because it can be easily made to fit the size of the unit to be cooled.
The outside tube is also cooled by air in the room providing an efficient operation. Heat transfer rates are low in this type of condenser if the tubes are long because of poor drainage of the condensing refrigerant keeps the vapor from coming in contact with the water tube. Fig. A Double Pipe Condenser Coil 5. Write down about the two typical shape of air cooled condensers. AIR COOLED CONDENSERS (a) Spiral Coil Finned Type (b) Flat Coil Finned type Fig. Two typical shapes of air-cooled condenser
The refrigerant circulates through a coil and air flows across the outside of the tubing. The air motion may be caused by natural convection effects when the air is heated. Thus it can be assumed as a fan to increase the airflow rate resulting greater capacity. This type of condenser is usually made of copper or steel tubing with fins attached which increase the effective area of heat dissipation surface. For domestic use, the condenser is usually air-cooled by natural convection. Air surrounding the condenser will be warmer than the air in the room. This warm air will rise and cooler air will flow into take its place. Some condensing units use motor driven fan to force air over the condenser tubing and to increase the cooling effects on the condenser. Two typical shapes of aircooled condensers namely flat coil finned type and spiral coil finned type are shown in figures. They are usually used for small capacity application, which are seldom more than 7 TR because of high head pressure and comparatively high kw power/ton and the fan vibration and noise. The Capacity of an air-cooled condenser may be calculated using one of the two basic methods. 1. Using the total external area of the condenser to compute its heat dissipating capacity. 2. Computations based upon what is called the frontal area of the condenser. Using the total external area of the condenser for dissipating heat depends upon the following variables: (i) External area (ii) Air velocity (iii)time (iv)temperature difference Using these values, the capacity of an air- cooled condenser vanes between 20 kj/hrm 2 C. The effect of air velocity is to increase the condenser capacity with increase in air velocity. A single tube condenser has a total area approximately 20 times its frontal area.
6. How do you determine the capacity of an air cooled condenser? The Capacity of an air-cooled condenser may be calculated using one of the two basic methods. 1. Using the total external area of the condenser to compute its heat dissipating capacity. 2. Computations based upon what is called the frontal area of the condenser. Using the total external area of the condenser for dissipating heat depends upon the following variables: (i) External area (ii) Air velocity (iii)time (iv)temperature difference Using these values, the capacity of an air- cooled condenser vanes between 20 kj/hrm 2 C. The effect of air velocity is to increase the condenser capacity with increase in air velocity. A single tube condenser has a total area approximately 20 times its frontal area. 7. With the help of neat sketch, explain two type of shell and tube type water cooled refrigeration condenser. Shell and Tube Condenser Fig. Horizontal Shall and Tube Type Condenser
In horizontal shell and tube condenser, in order to obtain high water film heat transfer. The cooling water always flows through the tubes and the refrigerant condenses on the outside of the tubes. The tubes are circuited to give the desired water velocity and pressure drop. The following figure shows the arrangement for a two-pass condenser. Tubes are sometimes finned and high velocities employed to increase heat transfer, especially for Freon group, which has low film co-efficient Copper tubes are used for Freon group and steel tubes for Ammonia. Since, even small quantities of air or non-condensable affect the performance adversely, a purge connection is provided for air removal. Shell tube type condenser can also serve the purpose of a receiver, mainly when all the refrigerant has to be pumped down because there is enough space in the shell. Shell and Coil Condenser: Fig. Helical Shell and Coil Condenser It uses a helical water coil in the shell instead of straight tubes. The connection A for taking out the condensed liquid refrigerant is so made that its dips in the liquid refrigerant at the bottom position which acts as the receiver. The refrigerant vapor s from the compressor enters at higher level, i.e., B. The cooling water is fed at C and leaves D after passing through the helical coil. Also the cooling water enters at the lowest level in the condenser, the liquid refrigerant accumulated gets sub-cooled. Thus lower portion acts as a sub cooler. In fact a sub cooler section is now a-days built up in the condenser lower portion and the condenser cooling water is made to enter through bundle of tubes first,
The vertical shell and tube condenser is essentially the same as the horizontal condenser except for the position in which it is installed resulting in a small space required. 8. Explain about evaporative condenser with neat and clear diagram. Evaporative Condenser: It is defined as comprising a coil in which the refrigerant is flowing and condensing inside and its outer surface is wetted with water and exposed to stream of air to which heat is rejected principally by evaporation of water. Fig. shows a typical arrangement of the components of an evaporative condenser. Fig. Evaporative Condenser Description. The coils are generally made of copper or steel in multiple circuits and passes. The external surfaces are sometimes finned to increase heat transfer surface. The coil should have arrangements for cleaning under fouling water condition.
The wetting of coil is done by re-circulating system comprising water-pan, a pump and water distribution system. The water distribution system mainly comprises nozzles for spray of atomized water on the coils. The pan catches the drainage of all coils. There is a float valve to admit make up water and maintain the correct level in the pan. Centrifugal pumps of moderate head are necessary. Evaporative condensers reject heat to the atmosphere as do air cooled condensers, hut by spraying water on the coils some heat is transferred to the water as well as the air, increasing the capacity of the condenser. Most evaporative condensers employ forced circulation of air with a fan to either blow or to draw air through the unit. Effective elimination of moisture from the leaving air stream by eliminators is essential to prevent projection of mist, which can deposit moisture on the surrounding surfaces. The eliminator plates work on the simple principle of abrupt changes in flow- direction. Moisture particles being heavier get deposited on these eliminator plates and get drained back to the sump or the pan. 9. Discuss control and maintenance of condenser. Control and maintenance of Condensers: The capacity of condensers must be controlled to maintain the condensing pressure within certain limits. Higher condensing pressures result in more power use and extremely high pressures can damage the equipment On the other hand, if the pressure is too low, the flow control device will not operate satisfactorily. An automatic valve regulating water flow rate is a convenient way of controlling capacity of water-cooled condensers. Air-cooled condensers are often controlled by reducing airflow across the coils, through use of dampers or cycling the fan. The control is usually in response to a change in condensing pressure. Proper water treatment is important for maintaining the capacity of water-cooled condensers. 10. Discuss on the following topics. (a) Heat rejection rate of condenser (b) Control and maintenance of condenser
(a) Heat rejection rate of the condenser For proper selection and design of a condenser,it is essential to know the heat transfer rate from the condenser. The heat rejected by the condenser per ton of refrigeration is a function of the evaporator and condenser temperature. The condenser has to reject heat absorbed at the evaporator and also the heat of compression added to the system by the compressor. Thus, for each unit of heat absorbed at the evaporator, the work of compression, and hence heat of compression depends upon the conditions at suction and discharge from the compressor. Heat Rejection Ratio: The heat rejection ratio for a condenser is defined as the ratio of heat rejected at the condenser to the refrigerating effect. (b) Control and maintenance of Condensers: The capacity of condensers must be controlled to maintain the condensing pressure within certain limits. Higher condensing pressures result in more power use and extremely high pressures can damage the equipment On the other hand, if the pressure is too low, the flow control device will not operate satisfactorily. An automatic valve regulating water flow rate is a convenient way of controlling capacity of water-cooled condensers. Air-cooled condensers are often controlled by reducing airflow across the coils, through use of dampers or cycling the fan. The control is usually in response to a change in condensing pressure. Proper water treatment is important for maintaining the capacity of water-cooled condensers.
CHAPTER 7 EXPANSION DEVICES 1. What is expansion devices? Write down the various types of expansion device. Expansion Device Expansion device is the restricting device that causes the pressure drop of the refrigerant also regulates the refrigerant flow according to the load. Common expansion Devices are: 1 Capillary tube 2. Thermostatic expansion valves 3. Low and high side float valves 4. Constant pressure expansion valve. 2. Explain the working principle of capillary tube. Capillary Tube circuit Fig. Schematic arrangement of capillary tube in a refrigerating plant
Almost all fractional horsepower, vapor compression refrigeration units employ capillary tube. The unit is being extended presently to larger units up to about 3.5kW. The capillary tube when used as a liquid refrigerant expanding device usually consists of an extremely small bore tube from 0.5 mm to 2.5 mm of about 0.5 m to 5 m long. Numerous combinations of bore and length are available to get the desired restriction. It is extreme simplicity and very low cost makes it very popular. In its operation, liquid refrigerant enters the capillary tube and due to flow, there is pressure drop due to friction. Some of the liquid flashes into vapor as the refrigerant flows through the tube. Once the sizing and length of the tube is selected, no modifications are possible to adjust itself to variation in discharge pressure, suction pressure and load. Care must be taken to prevent plugging of the tube by any dirt, ice, or any other decomposed material. The capillary tube is substituted for the conventional liquid line from the condenser and soldered to a length of the Suction line from a simple heat exchanger. 3. Explain the most popular and very efficient type of expansion device in use at present. Thermostatic Expansion Valve: This is the most popular and very efficient type of expansion device in use at present. The operation of the thermostatic expansion valve is based on the principle of constant degree of superheat for the evaporator exit. This ensures the evaporator completely tilled with refrigerant irrespective of the load and also no liquid can spill over to the suction line to the compressor. Because of its adoption to load changes, it is specially suitable for variable Load systems The following is the schematic diagram of a thermostatic expansion valve. Operation The remote bulb changed with fluid which is open on one side of the diaphragm through a capillary tube is clamped firmly to the evaporator outlet. The temperature of the saturated liquid vapor mixture is the same as the temperature of the superheated gas
leaving the evaporator at this location. The pressure of the fluid in the bulb tends to open the valve. This pressure is balanced by pressure due to spring plus pressure in the evaporator. There is thus, interaction of three independent forces namely force due to evaporator pressure, force due to spring compression and the force due to saturated liquid-vapor in the bulb. If the diaphragm is on balance point and in equilibrium condition, any change in degree of superheat of the suction gas will alter the valve position. If degree of superheat becomes less than the evaporator temperature, the sum of the evaporator and spring pressure will exceed the pressure exerted by the bulb. This will tend to close the valve and throttle the flow to the evaporator till again the same degree of superheat is obtained. Reverse action will take place for increase of the super heat. It may however be noted that the equilibrium condition degree of superheat can be changed by the adjusting screw. Thus spring adjustment is also called superheat adjustment. Fig. Schematic diagram of a thermostatic expansion valve 4. Write down about the thermostatic expansion valve. Thermostatic Expansion Valve: This is the most popular and very efficient type of expansion device in use at present. The operation of the thermostatic expansion valve is based on the principle of
constant degree of superheat for the evaporator exit. This ensures the evaporator completely tilled with refrigerant irrespective of the load and also no liquid can spill over to the suction line to the compressor. Because of its adoption to load changes, it is specially suitable for variable Load systems The following is the schematic diagram of a thermostatic expansion valve. Operation The remote bulb changed with fluid which is open on one side of the diaphragm through a capillary tube is clamped firmly to the evaporator outlet. The temperature of the saturated liquid vapor mixture is the same as the temperature of the superheated gas leaving the evaporator at this location. The pressure of the fluid in the bulb tends to open the valve. This pressure is balanced by pressure due to spring plus pressure in the evaporator. There is thus, interaction of three independent forces namely force due to evaporator pressure, force due to spring compression and the force due to saturated liquid-vapor in the bulb. Fig. Schematic diagram of a thermostatic expansion valve If the diaphragm is on balance point and in equilibrium condition, any change in degree of superheat of the suction gas will alter the valve position. If degree of superheat
becomes less than the evaporator temperature, the sum of the evaporator and spring pressure will exceed the pressure exerted by the bulb. This will tend to close the valve and throttle the flow to the evaporator till again the same degree of superheat is obtained. Reverse action will take place for increase of the super heat. It may however be noted that the equilibrium condition degree of superheat can be changed by the adjusting screw. Thus spring adjustment is also called superheat adjustment. 5. Explain the operation of thermostatic expansion valve. Operation The remote bulb changed with fluid which is open on one side of the diaphragm through a capillary tube is clamped firmly to the evaporator outlet. The temperature of the saturated liquid vapor mixture is the same as the temperature of the superheated gas leaving the evaporator at this location. The pressure of the fluid in the bulb tends to open the valve. This pressure is balanced by pressure due to spring plus pressure in the evaporator. There is thus, interaction of three independent forces namely force due to evaporator pressure, force due to spring compression and the force due to saturated liquid-vapor in the bulb. Fig. Schematic diagram of a thermostatic expansion valve
If the diaphragm is on balance point and in equilibrium condition, any change in degree of superheat of the suction gas will alter the valve position. If degree of superheat becomes less than the evaporator temperature, the sum of the evaporator and spring pressure will exceed the pressure exerted by the bulb. This will tend to close the valve and throttle the flow to the evaporator till again the same degree of superheat is obtained. Reverse action will take place for increase of the super heat. It may however be noted that the equilibrium condition degree of superheat can be changed by the adjusting screw. Thus spring adjustment is also called superheat adjustment. 6. Draw and labeled the schematic diagram of the internally and externally thermostatic expansion valves. Fig. Schematic diagram of a thermostatic expansion valve
Fig. An Externally Equalized Thermostatic Expansion Valve 7. Write down about the low pressure float valves. Low and high pressure float valves The low pressure float control maintains the liquid at constant level in the evaporator by regulating the flow into the evaporator in accordance with the supply from the evaporator to the compressor or the rate of vapor compression in the evaporator. If the refrigeration load increases, the evaporator temperature and the pressure rise, which temporarily allows the compressor to pump a greater mass rate of flow than the valve is feeding. The valve reacts to keep the level constant by opening more. If the refrigeration load decreases, the evaporator pressure falls and the compressor now pumps less mass rate temporarily and the level in float chamber rises resulting in the tendency to close the valve. It may be noted that this device is responding to the level of liquid in the evaporator and keeps the evaporator always filled to desired level without regard for evaporator temperature or pressure. This device may be incorporated directly in the evaporator or accumulator in which it has to control the level as shown in fig. It can also be installed external to the unit in a separate float chamber.
Fig. Float valve. 8. Explain about the externally equalized valves. Externally Equalized Valves In actual practice, there is a drop in pressure as the refrigerant flows through the evaporator. If this drop is very small and negligible the internally equalized valve shown in fig. Fig. An Externally Equalized Thermostatic Expansion Valve
But, if the drop is appreciable, the thermostatic expansion valve of the externally equalized type is preferred. Due to drop in pressure, the saturation temperature also drops. Thus evaporation pressure and temperature at inlet to the evaporation is higher than the evaporative pressure and temperature at the end. The situation will require a higher degree of superheat for the balanced point of the diaphragm. Also more evaporator surface will be utilized for superheat purposes adversely affecting the effective cooling. Thus, in these situations, an externally equalized valve is used. 9. Write down about the high pressure float valves. High pressure Valve: The high pressure float valve also maintains the flow to the evaporator by actuating the level in the float chamber in the same manner as the low pressure float valve except that the high pressure float valve is located on the high pressure side and controls the amount of liquid by maintaining level in the float chamber. The condensation rate and evaporation rate are matched by the high-pressure float valve by actuating the level and thus altering the opening and closing of the needle valve. To have further control on the expansion of liquid refrigerant, a pressure valve is also used in the circuit as shown in figure.
Pressure reducing valve which is located as close to the evaporator as possible also performs the function of throttling. Most popular arrangement is the high pressure side float valve arrangement. 10 Explain the automotive expansion valve. Automotive Expansion Valve An explanatory sketch of an automatic expansion valve is shown m fig. This valve is simply woks on the principle of maintaining a set pressure in the evaporator by adjusting the spring. The constant pressure in the evaporator is maintained by two opposing forces namely evaporator pressure and the spring pressure. The evaporator pressure tries to close the valve whereas the spring pressure tries to open the valve. Suppose the evaporator pressure falls below the set value for which spring is set. The valve will open more and increase the liquid flow to the evaporator resulting in flooding more evaporator surface. When more evaporator surface becomes effective, the vaporization rate increase resulting in rise of evaporator pressure till again it balances the spring pressure. If the evaporator pressure rises above the set valve, the valve tends to close and therefore less refrigerant liquid flows to the evaporator resulting in reducing the effective surface area of the evaporator. This leads to reducing to pressure in the evaporator till again it is equal to the spring pressure. When the plant is shut off, the evaporator pressure will build and keep the valve also firmly shut. On starting the plant, the suction from the evaporator attars resulting in the fall in pressure in the evaporator below the springs pressure to assume the flow of refrigerant liquid through the valve. The main disadvantage of automatic expansion valve is relatively low efficiency as compared to thermostatic expansion valve. In view of the matching of evaporator and compressor, it is imperative that rate of vaporization should be kept constant to maintain the evaporator pressure unaltered.
Thus varying loads cannot be adequately taken care of without sacrificing efficiency. When the load on the evaporator is very heavy and heat transfer capacity per unit surface area of the evaporator is high, there will be severe throttling to limit the effective surface area as shown in fig (a). The decrease of load and consequently less heat transfer capacity per unit area is achieved by flooding more surface of the evaporator as shown in fig (b). Due to low efficiency at heavy load, the automatic expansion valve is used for small equipment having relatively constant load. For varying load operation thermostatic expansion valve has completely replaced constant pressure valve. Fig. Automotive Expansion Valve or Constant Pressure Valve 11. Explain the constant pressure valve.
Constant Pressure Valve An explanatory sketch of an automatic expansion valve is shown m fig. This valve is simply woks on the principle of maintaining a set pressure in the evaporator by adjusting the spring. The constant pressure in the evaporator is maintained by two opposing forces namely evaporator pressure and the spring pressure. The evaporator pressure tries to close the valve whereas the spring pressure tries to open the valve. Suppose the evaporator pressure falls below the set value for which spring is set. The valve will open more and increase the liquid flow to the evaporator resulting in flooding more evaporator surface. When more evaporator surface becomes effective, the vaporization rate increase resulting in rise of evaporator pressure till again it balances the spring pressure. If the evaporator pressure rises above the set valve, the valve tends to close and therefore less refrigerant liquid flows to the evaporator resulting in reducing the effective surface area of the evaporator. This leads to reducing to pressure in the evaporator till again it is equal to the spring pressure. When the plant is shut off, the evaporator pressure will build and keep the valve also firmly shut. On starting the plant, the suction from the evaporator attars resulting in the fall in pressure in the evaporator below the springs pressure to assume the flow of refrigerant liquid through the valve. The main disadvantage of automatic expansion valve is relatively low efficiency as compared to thermostatic expansion valve. In view of the matching of evaporator and compressor, it is imperative that rate of vaporization should be kept constant to maintain the evaporator pressure unaltered. Thus varying loads cannot be adequately taken care of without sacrificing efficiency. When the load on the evaporator is very heavy and heat transfer capacity per unit surface area of the evaporator is high, there will be severe throttling to limit the effective surface area as shown in fig (a).
The decrease of load and consequently less heat transfer capacity per unit area is achieved by flooding more surface of the evaporator as shown in fig (b). Due to low efficiency at heavy load, the automatic expansion valve is used for small equipment having relatively constant load. For varying load operation thermostatic expansion valve has completely replaced constant pressure valve. Fig. Automotive Expansion Valve or Constant Pressure Valve
Maintenance and Repair 1.Write down the possible faults in electrical system and compressor. Possible Faults: (1) Fault in electrical system. (a) Switches (b) Low line voltage (c) Thermostat (d) LP and HP cut-outs (e) Starting relay (f) Starting/Running Capacity (g) Motor windings-shorted, grounded or open-circuit (2) Fault in compressor (a) Valves (b) Piston rings (c) Bearings (d) Belt-loose, slipping or broken (e) Oil separator-faulty 2. Write down the possible faults in expansion valve lines and joints (1) Fault in expansion valve (a) Clogged or stuck needle valve or orifice Due to dirt; wax, etc, (b) Leaky diaphragm (c) Leakey remote control bulb (d) Incorrect valve spring tension adjustment (2) Faulty lines and joints (a) Leaks in lines and joints (b) Faulty solenoid valves (c) Incorrect amount of charge (d) Air or moisture in system (e) Faulty drier, strainer, filter 3. Write down the possible cause of the refrigeration and air conditioning system. Explain any two. Possible causes; 1. Shortage of refrigerant 2. Poor flow due to obstruction 3. Faulty compressor 4. Air in system 5. Too much refrigerant
CAUSES OBSERVATION/STYMPTONS 1. Shortage of refrigerant (i) Low discharge pressure (ii) Hissing sound at expansion valve (iii) Cool compressor head 2. Faulty compressor (i) Low discharge pressure (Laky valves, rings, gasket) (ii) Cool compressor head and cool liquid line.