TAILIEUCHUNG - HVAC Systems Design Handbook part 9

Cooling means the removal of heat. In HVAC, a cooling process is usually identified as one which lowers the temperature or humidity (or both) of the ambient air. The effective temperature includes not only the temperature and humidity of the ambient air but also radiant effects and air movement. Some adiabatic cooling processes, ., evaporative cooling, do not actually remove any heat, but create a sensation of cooling by lowering the sensible temperature of the air. | Source HVAC Systems Design Handbook Chapter 9 Equipment Part 1 Cooling Introduction Cooling means the removal of heat. In HVAC a cooling process is usually identified as one which lowers the temperature or humidity or both of the ambient air. The effective temperature includes not only the temperature and humidity of the ambient air but also radiant effects and air movement. Some adiabatic cooling processes . evaporative cooling do not actually remove any heat but create a sensation of cooling by lowering the sensible temperature of the air. Refrigeration Cycles A refrigeration cycle is a means of transferring heat from some place where it is not wanted heat source to another place where it can be used or disposed of heat sink . The necessary components are 1 two or more heat exchangers one each at source and sink 2 a refrigerant 3 a conduit for conveying the refrigerant 4 mechanical and or heat energy to move the refrigerant through the system and 5 devices to control the rate of flow to control temperature and pressure gradients and to prevent damage to the system. There are several basic refrigeration cycles. The two most common two-phase vapor compression mechanical and single- and double-effect absorption are discussed below. Steam-jet refrigeration has historical importance but is not used in modern practice. Noncondensing one-phase mechanical cycles are used primarily in aircraft where light weight and simplicity are important. Thermoelectric re- 287 Downloaded from Digital Engineering Library @ McGraw-Hill Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Equipment Part 1 288 Chapter Nine frigeration utilizes thermocouples working in reverse When an electric current is impressed on a thermocouple a cooling effect is obtained. These are small systems for specialized applications and are comparatively expensive to install and operate. .

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