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View Subscription Details: Letter ( 12/11/2007 ) - HTML Format

Wet bulb approach (2)

Today, many companies sell PLCs with the wet bulb approach feature. This feature can help to balance the capacities of condensers and compressors, minimizing the power use of the refrigeration plant. Unfortunately, this feature is useless for many refrigeration plants. I believe that many cold storages will not benefit from this feature.

Example. Cold storage has a design refrigeration load of 300 TR. Condenser capacities of cold storages are usually undersized(less than optimum), in the range of 50%-70% of optimum capacity. In this example, the condenser capacity is 60% of the optimum one. Design condensing pressure is 166 psig. The PLC will operate the wet bulb approach, as soon as relative capacity (relative to refrigeration load) of the condenser increases over 100%. This means that, when refrigeration load decreases, relative capacity of the condenser increases, and at a certain point the condenser will become oversized. As soon as the capacity of the condenser becomes oversized (more that 100%), PLC will initiate the wet bulb approach feature and the capacity of the condenser will be reduced (pumps and/or fans off) to 100%.

For our refrigeration plant, as soon as the outside temperature decreases to 50°F, refrigeration load will reduce to 200 TR. The condenser will operate at full capacity and the relative capacity of this condenser will increase to 60% * 1.5 = 90%, because the refrigeration load was reduced 1.5 times (from 300 TR to 200 TR). Condensing pressure will decrease to 120 psig. The wet bulb approach will not be initiated by PLC, because relative condenser capacity is lower than 100%.

We can predict that at 45°F of ambient temperature, condensing pressure will be 110 psig and relative condenser capacity will be 100%. This is the time for the wet bulb approach to be turned on. However, 110 psig is minimum allowable condensing pressure for this refrigeration plant due to the oil cooling (hot gas defrosting, etc.). This means that, the refrigeration plant will operate at 110 psig condensing pressure, as soon as the outside temperature is 45°F or lower, and the wet bulb approach will never be turned on. This feature is useless for the mentioned refrigeration plant.

Usually, the wet bulb approach feature can be a useful tool to save energy during periods of hot and warm weather for the following refrigeration plants:

1.     Refrigeration plants with compressor power more than 1000 HP.

2.     Refrigeration plants with 3 or more condensers designed for the wet bulb approach less than 15°F.

3.     Refrigeration plants with significant and frequent fluctuation of refrigeration loads.


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