Variable Speed Control For Condenser Fans & Compressors; Effect Of Condenser Control Method On Total Annual Energy Usage - Emerson E2 User Manual

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.
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
kW (comp. + cond. Fan)
Month
Fixed Pc
Jan 10445
Feb 9572
Mar 10814
Apr 10717
May 11582
Jun 11889
Jul 12716
Aug 12676
Sep 11916
Oct 11308
Nov 10421
Dec 10535
Full Year 134591
% savings over fixed Pc
overall % savings
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
Float Pc +
Float Pc
VFD
8467 7640
7934 7203
9828 9022
10252 9472
11326 10534
11716 10968
12493 11730
12449 11685
11706 10958
10952 10153
9344 8553
8707 7904
125174 115822
7.00% 7.47%
13.95%
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