EVAPORATIVE CONDENSERS FOR INDUSTRIAL APPLICATIONS
BASIC EVAPORATIVE CONDENSER SIZING
Evaporative condenser basic design and performance can be numerically examined using Newton’s equation.
Q = U * A * LMTD
where
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