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 10°F (suction temperature is -10°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

From this table we can see that useful system efficiency at suction temperature of -20°F (T.D. is 20 °F) is the best and almost twice more efficient than system efficiency at -5 °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 -10°F (T.D. is 10°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 2°F - 4°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.