Optimized suction pressure
(2)
Evaporator fan motors not only
use up electricity, but all the energy used by the motors must be removed from
the space as refrigeration load. This load is parasitic. In order to obtain an
estimate of the useful refrigeration load we have to subtract this parasitic
load from total gross refrigeration load.
Qnet = Qgross 
Qfan
Useful efficiency of the
system (compressors + evaporators) can be estimated as
follows
Esystem useful = Ntotal /
Qnet
Where: Ntotal = Ncomp +
Nfan
Esystem  useful system
efficiency,BHP/TR
Ncomp  compressor power,
BHP
Nfan  fan power,
BHP
Qgross  total refrigeration
load on evaporator coil, TR
Qfan  heat added to
refrigerated space by evaporator fans, TR
Step 3. Useful system
efficiency.
Example. Gross refrigeration load is 100 TR; T.D.
is 10Â°F (suction temperature is 10Â°F);
Eevap (10) and Ecomp (10) from Table 2 (October 2005
newsletter)
Nfan = Eevap
(10) x 100 TR = 0.75 * 100 =75 HP
Ncomp = Ecomp
(10) * 100 TR = 1.349 x 100 = 134.9 HP
Ntotal = Ncomp +
Nfan = 134.9 + 75 = 209.9 HP
Qfan = 75
HP * 0.212 TR/HP = 15.9 TR
Qnet =
Qgross  Qfan = 100  15.9 = 84.1 TR
Esystem useful =
Ntotal / Qnet = 209.9
BHP / 84.1
TR = 2.496
BHP
/TR
Table 3. Useful system efficiency
Suction temperature, Â°F 
Temperature difference, Â°F

Useful system efficiency,
BHP/TR 
 5 
5 
4.030 
10 
10 
2.496 
 15 
15 
2.203 
 20 
20 
2.144 
From this table we can see that
useful system efficiency at suction temperature of 20Â°F (T.D. is 20 Â°F) is the
best and almost twice more efficient than system efficiency at 5
Â°F.
Optimum suction temperature depends of
the evaporator fan power. If our coil would have fan power of 4 HP instead of 15
HP, optimum suction pressure will be 10Â°F (T.D. is
10Â°F).
Step 4. Real life optimum
T.D.
In real life optimum T.D. can be
increased by the follow factors:
 suction
pressure losses
 frost on
the coils
 actual
fan power usage. Real fan power can increase up to 25% due to increase of cold
air density
 static
pressure penalty
To get real life optimum T.D. we have to
take the best T.D. from Table 3 and add the real life penalties, usually
2Â°F  4Â°F.
This estimation has shown that a
refrigeration plant has to run at optimized suction pressure or at optimum T.D.
regardless of the refrigeration load.