TAILIEUCHUNG - HVAC Systems Design Handbook part 11

By definition, air conditioning involves control of the air temperature, humidity, cleanliness, and distribution. It follows that an air-handling unit (AHU) of some kind is an essential part of an air conditioning system, though not necessarily of a heating-only system. The function of the AHU is to provide air at a quantity, temperature, and humidity to offset the sensible and latent heat gains to the space (in the cooling mode) and the heat losses (in the heating mode), while maintaining the required temperature and humidity in the space | Source HVAC Systems Design Handbook Chapter 11 Equipment Part 3 Air-Handling Systems Introduction By definition air conditioning involves control of the air temperature humidity cleanliness and distribution. It follows that an air-handling unit AHU of some kind is an essential part of an air conditioning system though not necessarily of a heating-only system. The function of the AHU is to provide air at a quantity temperature and humidity to offset the sensible and latent heat gains to the space in the cooling mode and the heat losses in the heating mode while maintaining the required temperature and humidity in the space. This can be most clearly shown on a psychrometric chart Fig. . A typical cooling design room condition is 78OF dry-bulb db temperature and 50 percent RH. For illustration a load of 120 000 Btu h sensible and 30 000 Btu h latent cooling is assumed. Then for an assumed 20OF temperature difference between the room and supply air temperatures 58OF supply air the design flow rate of air designated CFM in cubic feet per minute cfm will be CFM 5555 ft3 min cfm 20 X where is the air factor in Btu h cfm F. The change in specific humidity Aw may be calculated as follows . 1 min 1 ft3 1 h 1 lbw W tu X 5555 ft3 X lbo X 60 min X 1059 Btu lbw lbo 367 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 3 368 Chapter Eleven Figure Psychrometric chart for draw-through air conditioning process. The point defined by these two differential values can be plotted on the chart as shown. The validity of this point must be verified based on the cooling coil capability and the AHU arrangement as discussed in Sec. . For a draw-through arrangement . with the supply fan downstream of the cooling coil the supply air temperature will be greater

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