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Components of ref and ac u5
- 1. Components of ref. and air conditioning systems By Prof. P.W.Ingle Sanjivani College of Engineering Kopargaon
- 2. Components of ref. and air conditioning systems: Working of reciprocating, screw and scroll compressor Working of air cooled, water cooled and evaporative condensers DX, flooded, force feed evaporators Expansion devices, capillary tube, TXV, EXV, operation and safety controls
- 3. Working of reciprocating, screw and scroll compressor
- 4. Working of screw compressor
- 5. A typical rotary screw air compressor has two interlocking helical rotors contained in a housing. Air comes in through a valve, typically called the inlet valve and is taken into the space between the rotors. As the screws turn, they reduce the volume of the air, thus increasing the pressure.
- 6. Working of Scroll compressor A scroll compressor has one fixed scroll which remains stationary and another moving or orbiting scroll that rotates through the use of a swing link. When this happens, the pockets of refrigerant between the two scrolls are slowly pushed to the center of the two scrolls, causing the reduction of the volume of the gas. It is then discharged though the center port to the condenser.
- 8. Working of air cooled condenser Function of Condenser In a cooling cycle of a refrigeration system, heat is absorbed by the vapor refrigerant in the evaporator followed by the compression of the refrigerant by the compressor. The high pressure and high temperature state of the vapor refrigerant is then converted to liquid at the cond. It is designed to condense effectively the compressed refrigerant vapor. •Air-Cooled types are usually used in the residential and small offices applications. They are used in small capacity systems below 20 tons. The advantages of using this design include not having to do water piping, not necessary to have water disposal system, saving in water costs and not much scaling problems caused by the mineral content of the water. It is also easier to install and has lower initial cost. There isn't much maintenance problems. The disadvantages are that it requires higher power per ton of refrigeration, has shorter compressor life and on days when most cooling is required, the least is available.
- 10. Most all water-cooled condensers (Fig.b) condense refrigerant in the shell and on the outside of tubes through which water passes. The condenser cooling water picks up heat in passing through the condenser and this warm water is cooled by circulating through a cooling tower. While the shell-and-tube construction predominates for water- cooled condensers, plate-type condensers, sister of the plate- type evaporator explained , are now appearing. The evaporative condenser of Fig.c might be considered a cooling tower, with the condenser tubes washed by the water spray. Ultimately, the heat rejected from the refrigeration plant is discharged to ambient air, except where the condenser is cooled...
- 12. DX, flooded, force feed evaporators An evaporator, like condenser is also a heat exchanger. In an evaporator, the refrigerant boils or evaporates and in doing so absorbs heat from the substance being refrigerated. The name evaporator refers to the evaporation process occurring in the heat exchanger. Flooded and Dry Type :- flooded type and dry type. Evaporator is said to be flooded type if liquid refrigerant covers the entire heat transfer surface. This type of evaporator uses a float type of expansion valve. An evaporator is called dry type when a portion of the evaporator is used for superheating the refrigerant vapor after its evaporation.
- 13. Natural and Forced Convection Type The evaporator may be classified as natural convection type or forced convection type. In forced convection type, a fan or a pump is used to circulate the fluid being refrigerated and make it flow over the heat transfer surface, which is cooled by evaporation of refrigerant. In natural convection type, the fluid being cooled flows due to natural convection currents arising out of density difference caused by temperature difference. The refrigerant boils inside tubes and evaporator is located at the top. The temperature of fluid, which is cooled by it, decreases and its density increases. It moves downwards due to its higher density and the warm fluid rises up to replace it.
- 14. Natural Convection type evaporator coils These are mainly used in domestic refrigerators and cold storages. When used in cold storages, long lengths of bare or finned pipes are mounted near the ceiling or along the high sidewalls of the cold storages. The refrigerant from expansion valve is fed to these tubes. The liquid refrigerant evaporates inside the tubes and cools the air whose density increases. The high-density air flows downwards through the product in the cold storage. The air becomes warm by the time it reaches the floor as heat is transferred from the product to air. Some free area like a passage is provided for warm air to rise up. The same passage is used for loading and unloading the product into the cold storage
- 15. Direct expansion type, Shell-and-Tube Evaporator a liquid chiller with refrigerant flowing through the tubes and water flowing through the shell. A thermostatic expansion valve feeds the refrigerant into the tubes through the cover on the left. It may flow in several passes through the dividers in the covers of the shell on either side. The liquid to be chilled flows through the shell around the baffles. The presence of baffles turns the flow around creating some turbulence thereby increasing the heat transfer coefficient. Baffles also prevent the short-circuiting of the fluid flowing in the shell. This evaporator is of dry type since some of the tubes superheat the vapour.
- 16. To maintain the chilled liquid velocity so as to obtain good heat transfer coefficient, the length and the spacing of segmental baffles is varied. Widely spaced baffles are used when the flow rate is high or the liquid viscosity is high. The number of passes on the refrigerant side are decided by the partitions on the heads on the two sides of the heat exchanger. Some times more than one circuit is also provided. Changing the heads can change the number of passes. It depends upon the chiller load and the refrigerant velocity to be maintained in the heat exchanger. 23.5. Shell-and-Coil type evaporator
- 17. Direct expansion type, Shell-and-Tube Evaporator
- 18. DX(Direct Expansion Fin and Tube Type
- 19. Flooded type Evaporator a flooded type of shell and tube type liquid chiller where the liquid (usually brine or water) to be chilled flows through the tubes in double pass just like that in shell and tube condenser. The refrigerant is fed through a float valve, which maintains a constant level of liquid refrigerant in the shell. The shell is not filled entirely with tubes as shown in the end view of Fig. This is done to maintain liquid refrigerant level below the top of the shell so that liquid droplets settle down due to gravity and are not carried by the vapor leaving the shell. If the shell is completely filled with tubes, then a surge drum is provided after the evaporator to collect the liquid refrigerant.
- 20. Shell-and-tube evaporators can be either single pass type or multipass type. In multipass type, the chilled liquid changes direction in the heads. Shelland-tube evaporators are available in vertical design also. Compared to horizontal type, vertical shell-and-tube type evaporators require less floor area. The chilled water enters from the top and flows downwards due to gravity and is then taken to a pump, which circulates it to the refrigeration load. At the inlet to tubes at the top a special arrangement introduces swirling action to increase the heat transfer coefficient.
- 22. Forced feed type of evaporator In forced convection evaporator , the air is forced over the refrigerant cooled coils and fins. This is done by an electric motor. The fins are provided to increase the heat transfer rate. The forced convection evaporators are more efficient than natural convection evaporators. Forced circulation cooling unit may be divided as 1. Low velocity cooling units:- these units have a discharge air rate from 60 m/ min to 90 m/ min. The low velocity cooling units are used in comfort air conditioning where low noise and low air velocity rates needed. 2. Medium velocity:- 150 m/min to 240 m/ min used in freezers 3. High velocity:- above 240 m/min used in blast freezer.
- 23. Expansion devices, capillary tube An expansion device is another basic component of a refrigeration system. The basic functions of an expansion device used in refrigeration systems are to: 1. Reduce pressure from condenser pressure to evaporator pressure, and 2. Regulate the refrigerant flow from the high-pressure liquid line into the evaporator at a rate equal to the evaporation rate in the evaporator The expansion devices used in refrigeration systems can be divided into fixed opening type or variable opening type. As the name implies, in fixed opening type the flow area remains fixed, while in variable opening type the flow area changes with changing mass flow rates. There are basically seven types of refrigerant expansion devices. These are: 1. Hand (manual) expansion valves 2. Capillary Tubes 3. Orifice 4. Constant pressure or Automatic Expansion Valve (AEV) 5. Thermostatic Expansion Valve (TEV) 6. Float type Expansion Valve a) High Side Float Valve b) Low Side Float Valve 7. Electronic Expansion Valve
- 24. Capillary Tube A capillary tube is a long, narrow tube of constant diameter. The word “capillary” is a misnomer since surface tension is not important in refrigeration application of capillary tubes. Typical tube diameters of refrigerant capillary tubes range from 0.5 mm to 3 mm and the length ranges from 1.0 m to 6 m. The pressure reduction in a capillary tube occurs due to the following two factors: 1. The refrigerant has to overcome the frictional resistance offered by tube walls. This leads to some pressure drop, and 2. The liquid refrigerant flashes (evaporates) into mixture of liquid and vapour as its pressure reduces. The density of vapor is less than that of the liquid. Hence, the average density of refrigerant decreases as it flows in the tube. The mass flow rate and tube diameter (hence area) being constant, the velocity of refrigerant increases since = ρVA. The increase in velocity or acceleration of the refrigerant also requires pressure drop.
- 26. Hand operated expansion valve It is most simple type of expansion valve but it requires an operator to regulate the flow of refrigerant to the evaporator manually. The conical shaped needle valve extends down into the valve port and restrict the flow area through the port. When closed, the valve rests on its conical seat The use of hand operated valve is limited systems operating under nearly constant loads for long period of time, such as in ice making plants and cold storages. It is not suitable for installations where the load varies and the compressor runs intermittently to maintain a constant temperature
- 27. Hand operated Expansion valve
- 28. Automatic (or constant pressure) expansion valve It is known as constant pressure expansion valve, because it maintains constant evaporator pressure regardless of the load on the evaporator. Its main moving force is the evaporator pressure. It is used with dry expansion evaporators where the load is relatively constant. The automatic expansion valve, as shown in fig. consist of a needle valve and a seat which forms an orifice, a metallic diaphragm or bellows, spring and an adjusting screw. The opening and closing of the valve with respect to the seat depends upon the following two opposing forces acting on the diaphragm, 1. the spring pressure acting on top of diaphragm. And 2. the evaporator pressure acting below the diaphragm.
- 29. Automatic (or constant pressure) expansion valve