I am trying to determine chiller power draw at reduced condensing temperatures. The load is constant (100%) for all condensing temps. I have manufacturer data for 100% 90% 80% ... 20% at both constant condensing temperature (85F) and with ARI relief (but a partial loads).
Any suggestions to get full load kW/ton at different condensing temperatures?
Thanks.
Kevin L. Wyman, P.E.
Assuming this is a fixed-speed centrifugal chiller, a reasonable first cut
is that the power is proportional to the lift (compressor pressure out-
pressure in). Look at the refrigerant pressures at the expected condensing
and evaporating temperatures, and ratio the lift at the new condensing temp
to the lift at the old condensing temp.
This approach neglects the change in compressor efficiency that probably
occurs, but I've done this in the past prior to getting a complete chiller
map, and the approach works out pretty well. It also matches with the
common rule of thumb that the power changes 1 to 2% per deg F change in
condensing temp.
With nagging, it may be possible to get a complete chiller map from the
manufacturer. You can almost certainly get it in a competitive procurement
situation.
William E. Koran, P.E.