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Wet bulb approach

Modern refrigeration plant control PLCs have wet bulb control feature. This is a relatively new method to control head pressure of refrigeration plant. However, many operating engineers prefer to operate plants at fixed head pressures. Do we really need wet bulb approach control?

Wet bulb approach is the temperature difference between condensing temperature and wet bulb temperature. Condensing pressure is 150 psig, corresponding condensing temperature is 85°F, wet bulb temperature is 70°F. Wet bulb approach will be equal to 85°F - 70°F = 15°F. The idea of wet bulb approach control is to balance capacities of condensers and compressors to minimize total (condensers + compressors) power use.

Example. Refrigeration plant has 5000 HP of compressor power and 500 HP of condenser (pumps and fans) power. Total refrigeration capacity of this plant is 2000 TR. This is a production facility. It has production 24 Hrs per day and 5 days per week. During weekends only 200 TR (cold storage) or 500 HP of compressor power is required. Set point of condensing pressure is 120 psig.

One weekend wet bulb temperature was 70°F. 500 HP of compressors were running to keep temperature in the cold storage. Condensers were running at full capacity, but head pressure has reached only 125 psig. Total power use was 500 HP (compressors) + 500 HP (condensers) = 1000 HP.

Assume that we installed wet bulb approach feature for refrigeration plant PLC. This approach was set to 15 degF. New condensing temperature will be 70°F + 15°F = 85°F. Condensing pressure will increase from 125 psig to 150 psig (85°F). Compressor power will increase from 500 HP to 550 HP. Condenser power will reduce from 500 HP to 50 HP. Total power use will be 550 HP (compressors) + 50 HP (condensers) = 600 HP. Our savings of using wet bulb approach feature is 1000 HP - 600 HP = 400 HP.

Certainly, 400 HP imbalance is an extreme. However, I saw very often an imbalance of 50 - 100 HP, usually when set point of wet bulb approach is not optimal. Every 10 HP of energy savings at energy rate $ 0.1KWh/Hr will save you from $ 3000 to $ 5000.

To maximize energy savings, refrigeration plants with wet bulb approach should have the optimum settings. Every refrigeration plant has its own optimum setting. My research has shown that this optimum can vary from 8°F to 30°F . Refrigeration plant runs inefficiently, if optimum wet bulb approach is 20°F, but your setting is 10°F.

Assume that we have found the optimum wet bulb approach. However, this approach should be changed from time to time because properties of wet air are different at different level of wet bulb temperature.

Wet bulb approach optimization is not an easy task. Therefore, if anybody is interested in improvement of wet bulb approach setting, please contact me and I will help you to optimize this setting.

 

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