tailieunhanh - Ebook Air conditioning and refrigeration: Part 2

(BQ) Part 2 book "Air conditioning and refrigeration" has contents: Evaporators, servicing and safety, freezers, comfort air conditioning, commercial air conditioning systems, various types of air conditioners and heat pumps, various types of air conditioners and heat pumps, estimating load and insulating pipes,. and other contents. | 10 CHAPTER Evaporators Copyright © 2006 by The McGraw-Hill Companies, Inc. Click here for terms of use. PERFORMANCE OBJECTIVES After studying this chapter you should: 1. Be able to identify various types of evaporators. 2. Know how a shell-and-tube chiller operates. 3. Know how hot-gas defrost of ammonia evaporators controls operate. 4. Know how various types of evaporators operate. 5. Know the value of control valves in the proper operation of evaporators. 6. Know how to troubleshoot a differential pressurerelief regulator. 7. Know how the differential pressure-relief regulator works. The evaporator removes heat from the space being cooled. As the air is cooled, it condenses water vapor. This must be drained. If the water condensing on the evaporator coil freezes when the temperature is below 32°F (0°C), the refrigerator or freezer must work harder. Frozen water or ice acts as an insulator. It reduces the efficiency of the evaporator. When evaporators are operated below 32°F, they must be defrosted periodically. This eliminates frost buildup on the coils or the evaporator plates. There are several types of evaporators. The coiled evaporator is used in warehouses for refrigerating large areas. The fin evaporator is used in the air-conditioning system that is part of the furnace in a house. See Fig. 10-1. The finned evaporator has a fan that blows air over its thin metal surfaces. Plate evaporators use flat surfaces for their cooling surface. See Fig. 10-2. They are commonly used in freezers. If the object to be cooled or frozen is placed directly in contact with the evaporator plate, the cold is transferred more efficiently. Figure 10-3 shows a home refrigerator cooling system. Note the evaporator. COILED EVAPORATOR Evaporator coils on air-conditioning units fall into two categories: • Finned-tube coil. The finned-tube coil is placed in the air stream of the unit. Refrigerant vaporizes in it. The refrigerant in the tubes and the air flowing around the fins .