required condenser fan power. Table 3 shows the energy saving that can be achieved
by utilizing TD-based floating condensing pressure.
Table 3: Effect of condenser control method on total annual energy usage.
The first column shows the total (compressor + condenser fan) annual
refrigeration condensing system energy consumption for an example refrigeration
system when condensing pressure is controlled at a fixed pressure equal to a
condensing temperature of 90 F. The example results shown here are based on weather
data for Atlanta, Georgia. The second column shows total annual refrigeration system
energy consumption when the condenser fan control method is changed to floating
condensing pressure (TD control method). It can be seen that floating condensing
pressure provides savings of 7% over fixed pressure. Note that higher energy savings
are obtained in colder weather locations and lower energy savings are obtained on
warmer weather locations. Therefore, it is expected that stores in northern state such as
Michigan will provide higher savings than southern states such as Florida. Nevertheless,
the energy savings will always be substantial when floating condensing pressure is
compared with conventional fixed pressure control.
Fixed Pc Float Pc
Float Pc +
VFD
Jan 10445 8467 7640
Feb 9572 7934 7203
Mar 10814 9828 9022
Apr 10717 10252 9472
May 11582 11326 10534
Jun 11889 11716 10968
Jul 12716 12493 11730
Aug 12676 12449 11685
Sep 11916 11706 10958
Oct 11308 10952 10153
Nov 10421 9344 8553
Dec 10535 8707 7904
Full Year 134591 125174 115822
% savings over fixed Pc 7.00% 7.47%
overall % savings 13.95%
Calculations done using refrigeration model with following conditions:
- TMY-2 weather data for Atlanta, GA.
- R-22, Fixed Condenser Temp=90 F, Case load = 200,000 Btu/hr
kW (comp. + cond. Fan)
Month
VARIABLE SPEED CONTROL FOR CONDENSER FANS & COMPRESSORS
Traditionally, supermarket refrigeration system condenser fan and compressor
controllers cycle fans and compressors “on” and “off” in an attempt to match system
refrigeration capacity to current system refrigeration load levels. Unfortunately, given the
discrete capacity steps available from this control methodology, it is rarely possible to
deliver the exact amount of compressor and condenser capacity required for a particular
level of compressor and condenser loading. Varying degrees of uneven or unstable
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