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Optimized suction pressure (1)

A majority of storage coolers and storage freezers have single speed evaporator fans. In the following 2 newsletters I will show you the opportunity to optimize suction pressure for these coolers and freezers.


It is common knowledge in the industry that raising the suction pressure improves efficiency of the compressor (BHP/TR). Typical improvement might be 1% to 2% increase in saturated suction pressure per degree F. If all space temperatures are satisfied and evaporator coils are in a low load mode, the suction pressure usually increases to the maximum until some limiting temperatures are approached; thus, the efficiency of the compressors is improving.

However, in order to operate the entire refrigeration plant efficiently, we have to pay attention to evaporator fans.


There are 4 steps to optimized suction pressure.


Step 1. Compressor efficiency.

Example. Freezer temperature is 0 °F; single stage screw compressor with economizer Frick RWB - II 134E;

condensing temperature is 75°F at 125.8 psig head pressure; saturated suction temperatures are – 5 °F, –10 °F, –15 °F, – 20 °F. Compressor efficiency at suction temperature of –10 °F

Ecomp = 221.1 BHP / 163.9 TR = 1.349 BHP/TR

   Table 1. Compressor efficiency at different suction temperatures

Suction temperature, °F

Compressor efficiency, BHP/TR

–  5

1.237

– 10

1.349

– 15

1.469

– 20

1.599

                              

If we increase the suction temperature, our compressors are using less energy (BHP) per unit of refrigeration (TR) and their efficiency is improving.

Step 2. System efficiency.

To estimate efficiency of the system (compressors + evaporators) we have to add efficiency of compressors and efficiency of evaporators.


We have compressor efficiencies in the Table 1.


Efficiency of evaporators can be estimated as follows.

Example.  Evaporator coil fan power is 15 HP; capacity of the coil is 20 TR at temperature difference (TD) of 10 °F.

Evaporator coil efficiency at TD of 10 °F (suction temperature –10 °F)

Eevap (10) = 15 HP/ 20 TR = 0.75 HP/TR

Capacity of the coil is proportional to TD. At TD of 5 °F capacity of our coil is 10 TR. Evaporator coil efficiency at TD of 5 °F (suction temperature is – 5 °F) 

Eevap (5) = 15 HP/ 10 TR = 1.5 BHP/TR.

Table 2. System efficiency at different suction temperatures.



Suction temperature, °F

Compressor efficiency, BHP/TR

Evaporator efficiency, BHP/TR

System efficiency, BHP/TR

– 5

1.237

1.500

2.737

– 10

1.349

0.750

2.099

– 15

1.469

0.500

1.969

– 20

1.599

0.375

1.974

From this table we can see that efficiencies of the system are better (less BHP per TR) at suction temperatures –15 °F and – 20 °F, than efficiencies at suction temperatures –5 °F and –10 °F. This means that the highest suction temperature (pressure) is not the most efficient for this refrigeration plant.

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