Nominal Sizes And Rates For Evaporative Condensers

The design of an efficient evaporative condenser requires optimizing a number of factors, including tube size, tube length, tube spacing, refrigerant circuiting, air-flow rate, casing size, and spray-water flow rate. The condenser manufacturer/designer must draw on knowledge of refrigerant heat transfer, wetted-surface heat transfer (such as described in References 8 and 9), and a thorough understanding of fabrication economics and of the end-user operation. The condenser user is spared the responsibility of making most of these decisions and is best served with an understanding of how three variables affect the performance of an evaporative condenser: the wet-bulb temperature, air-flow rate, and spray-water flow rate. The next several sections will focus on those factors, but first a few nominal magnitudes applicable to many commercial evaporative condensers will be presented:

Heat-transfer area:
0.25 m2 per kW of heat rejection (0.8 ft2 per 1000 Btu/hr)
Spray water circulating rate:
0.018 L/s per kW of heat rejection (5 gph per 1000 Btu/hr)
Air volume flow rate:
0.03 m3/s per kW of heat rejection (18 cfm per 1000 Btu/hr)
Air pressure drop through the condenser:
250–375 Pa (1 to 1–1/2 inches of water)
Rate of water evaporated:10
1.5 L/hr per kW of heat rejection (0.12 gph per 1000 Btu/hr)
Total rate of water consumption:
with good quality makeup water the bleed rate may be as low as 50% of the evaporation rate, so the total rate evaporated and blown down may be about 2.2 L/hr per kW of heat rejection (0.18 gph per 1000 Btu/hr).

Many years ago the typical flow rate of spray water was quite low, perhaps 0.68 L/s per m2 (1 gpm/ft2), but this rate has gradually climbed so that it may run as high as 4.1 L/s per m2 (6 gpm/ft2) to achieve favorable capacity. The practical limit is reached when the spray water flow rate is so high that it restricts the air flow rate.

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