Handbook of air conditioning and refrigeration / Shan K

29.28 CHAPTER TWENTY-NINE
System Description
Space Conditioning Line
In many supermarkets, 50 percent of the electric energy used annually is consumed by frozen
food refrigerators, and another 15 percent is used for the air conditioning system to maintain the required
space condition.
Consider a supermarket with an area of 30,000 ft 2 (2788 m 2 ) and an outdoor ventilation air requirement
of 3000 cfm (1416 L/s). The condition of outdoor air is dry-bulb temperature of 95°F, relative
humidity of 40 percent, and humidity ratio of 0.0142 lb/lb (100 gr/lb or 0.0142 kg/kg). The space
cooling load is about 83,000 Btu/h (24,320 W). If a desiccant-based air conditioning system with
an impregnated lithium chloride dehumidifier is used instead of a conventional vapor compression
refrigeration system, it is possible to maintain an indoor space temperature of 75°F (23.9°C) and a
relative humidity of 45 percent (0.0085 lb/lb or kg/kg, 60 gs/lb). Meanwhile, the supply air volume
flow rate can be reduced from the 1 cfm/ft 2 (5 L/s�m 2 ) of a conventional system to 0.5
cfm/ft 2 (2.5 L/s�m 2 ) for a desiccant-based system. Thus, there is a significant reduction in refrigeration
load of the frozen food refrigerators because of lower space relative humidity as well as the
fan power consumption in the air conditioning system.
Figure 29.4a is schematic diagram of a desiccant-based air conditioning system for a 30,000 ft 2
(2788 m 2 ) supermarket. The process airstream of the desiccant-based air conditioning cycle is indicated
on the psychrometric chart by the solid line in Fig. 29.4b. The state points of the process air at
the exit of various components are as follows:
Outdoor air o
Filter o
Rotary desiccant dehumidifier dl
Heat-pipe heat exchanger pl
Mixing with recirculating air m
Indirect evaporative cooler or refrigeration s
Supply air after supply fan s
Space air r
The state points at the exits of the various components of the regeneration airstream are as follows:
Outdoor air o
Filter o
Heat-pipe heat exchanger xl
Gas heater rg
Rotary desiccant dehumidifier ro
Exhaust fan to atmosphere ro
For a supermarket of 30,000 ft 2 (2788 m 2 ), if 50 percent of the total space cooling load Q rc is latent
load Q rl, then
Q rl � 0.5 � 83,000 � 41,500 Btu/h (12,160 W)
If the supply air is at a temperature of 72.5°F (22.5°C), a relative humidity of 48 percent, and a humidity
ratio of 0.0079 lb/lb (55 gr/lb or 0.0079 kg/kg), from Eq. (20.72), the supply air volume