As summer approaches I thought it would be a good idea to revisit one of our previous blog postings from a few years back…
How many of you have “scale” on your evaporative condenser? [I am guessing your hand might be in the air]. How many of you feel that this is “normal” and something that should be treated as a “side effect” from running an evaporative condenser? [I am guessing your hand is still up].
Although scale buildup on evaporative condensers is common, it should not be considered “normal”. Operators should be aware of the performance and life expectancy issues caused by the formation of scale on their condenser tubes.
Please note: Calculations and values below have not been checked for accuracy. The performance reductions have been gathered over the years from various refrigeration newsletters and publications.
One might expect that if you factor in a powdery “white rust” buildup on a heat exchange surface then the heat exchanger’s performance will suffer. Since condensers are heat exchangers, they too are subject to performance decreases due to unclean heat exchange surfaces. A consequence of operating a “scaled” condenser would be to experience a higher discharge pressure than with no scale. Increasing the condensing temperature will cause a decrease in compressor capacity, an increase in compressor b.h.p. required, and a decrease in condenser performance.
• With 1/64” of scale, Condenser performance decreased by 15% (non-linear with respect to scale thickness)
• For every 1/64” of scale, Compressor BHP increases by 3.5%
• For every 1/64” of scale, Compressor capacity decreases by 0.5%
Another inherent performance issue that results from excessive condenser scale is a reduction in compressor capacity. As brake horsepower increases, compressor ratings decrease (holding suction pressure constant)
CSR has installed and serviced many evaporative condensers over the years. The water treatment practices of the facility have a direct impact on how long the condensers can safely be in service. CSR has seen condensers last only two (2) years before springing a leak due to poor water treatment practices. Vice-a-versa, CSR has seen evaporative condensers be in service for 15+ years that have little to no scale because the facility took great care in the water treatment.
EXAMPLE (values are calculated with an Excel Spreadsheet and with some assumptions):
Let us look at a facility located on 123 Fake Street. They operate a 1,000TR central ammonia refrigeration system running at +12°F Saturated Suction Temperature. This Fake Street facility is in the upper Midwest where on a sticky hot/humid day, you might see a wet bulb temperature of 78°F. The facility has negotiated a pretty good electrical rate of $0.07/kWh (assume no demand charge). Assuming this 1,000TR system runs “24/365,” and with just 1/64” of scale on the condenser tube bundle, this facility might expect an additional $28,850.00 in electrical costs over the year vs a system with no scale. Similarly, for a system with 1/32” of scale, the additional electrical costs for the year are estimated to be $56,740.00 vs as system with no scale.
The effects of condenser scale on system performance (specifically your pocketbook) are obvious. The question now is…what are YOU doing about keeping scale off your condensers?