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Evaporative Condensers

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Evaporative condenser basic design and performance can be numerically examined using Newton's equation.

Q = U * A * LMTD
Q = heat reject, btu/hr
U = transfer coefficient
A = surface area, e.g. st.ft.
LMTD = mean logarithmic temperature

Let's work out an hypothetical design for an ammonia evaporative condenser using said formula using U = 100, and a common condensing pressure target.

Ammonia Condenser Sizing
Q 756049 kca/h 3000002 btu/h 880 kW
Twb 25 °C 77.0 °F
Tcondensing 35.72 °C 96.3 °F
chiller tons 200.0 tons (15k) 250 tons (12k) IF at 2.1 U.S. GPM per chiller ton
water flow 600.0 U.S. GPM at 3 GPM per chiller ton 486.0 U.S. GPM at 2.43
37.9 lps 30.7 lps
face area sq.ft. 100.0 sq.ft. at 6 gpm/sq.ft. 81.0 sq.ft. area (alt.)
tentative air flow 60000.0 CFM at typ. 600 fpm 48600.0 CFM (alt.)
l/g = 1.2
delta enthalpy 11.7 btu/lb
Hin 45.0 btu/lb
Hout = Hin + delta 56.7 btu/lb
saturated air out at 88.7 °F
LMTD 12.6 °F 7.0 °C
U 100 cu.ft. 3/4" coil
required area 2388.4 sq.ft. 222.0 m2 2388.4 sq.ft. 37.3
+ 20% safety factor 2866.0 sq.ft. 266.4 m2 2866.0 sq.ft. 44.8
condenser Q thickness Thermal DeRating % Capacity
Original Capacity 756049 kcal/h 0 200 chiller tons 100
0.8mm scale 551916 kcal/h 0.8 146 chiller tons 73
1.0mm scale 415827 kcal/h 1 110 chiller tons 55
2.0mm scale 287299 kcal/h 2 76 chiller tons 38
2.0mm & recirculation 244204 kcal/h 65 chiller tons 32



This unassuming graph unfortunately accurately depicts what one can frequently see in actual plants and real world reports:  

"Evaporative condensing is still by far the most economical means to remove latent heat. However, this holds true as long as the heat transfer surfaces on both sides of the tubes are kept clean and free of thermal insulating films such as oil, scale, algae growth." (James Dodds - Argentina/Brazil/USA; manufacturer; awarded United States Patent 4,693,302).   

"Because of these problems, the potential for lower capital and operating costs from evaporative condensers may not be realizeddue to poor design, poor installation or poor operating practices." (Brake, Australia) 

You should pay extreme attention to water treatment of the condensers [evaporative], as this aspect is one of the vulnerabilities of these condensers [evaporative]. If water is not properly treated it will lead to rapid deposition of salts of calcium and magnesium on the surface of the exchanger, thereby significantly reducing the transfer and the efficiency of the condenser, canceling advantages over other types of condensers. "(Wong et al., Cuba)

We feel that installing twice the required EC capacity may not be a commendable path to pursue.  Probably more energy would be saved/generated in the industrial  refrigeration field alone (versus wind, solar, other) by sticking to said subject fundamentals (low HPs everywhere) and simply selecting instead easy to maintain cooling tower and shell and tube condensers, matching condensing targets with lower HPs as well.  In many locales, actual EC practice (energy savings not a priority) runs against the true evaporative cooling/condenser rationale.   Machines cannot be more efficient than the standard set by installers and vendors





Need Some Help? - James C. Young Environmental - Balestie, Irwin & Balestie