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View Subscription Details: Letter ( 02/12/2009 ) - HTML Format

Winter operation of evaporative condensers.

 

Many refrigeration plants operate in geographic regions where ambient temperatures fall below freezing. There are two measures to prevent the spray water from freezing in such instances:

1. To locate the sump in a warm area.

2. To drain the water from the condensers and operate the condensers dry.

 

The indoor sump must be able to accommodate all of the water normally in suspension in the condenser during operation. The cost of pumping spray water will be slightly higher than experienced when the sump is integrated to the condenser because of the additional head attributable to the differences in elevation between the condenser and the sump. However, water pump operation during the cold weather will improve the efficiency of a refrigeration plant. Condenser power (pumps, fans) of wet operation will be lower than the condenser power of dry operation. This means that water pumps should be operated as long as possible, though at subzero temperatures fan blades can freeze.

Centrifugal fans use more energy than axial fans, but they are less sensitive to subzero temperatures. If your refrigeration plant has condensers with centrifugal and axial fans, condenser sequence should be different during summer and winter operation. During summer operation the condensers with axial fans should run first in order to save energy. During winter operation the condensers with centrifugal fans should run first to prevent the fan from freezing and to maximize the efficiency of the refrigeration plant.

During winter, some people may prefer the dry (water pumps off) operation of evaporative condensers. One incentive for a plant to shift to dry operation is that the costs of water treatment can be eliminated. I have found that energy costs of dry operation are typically greater than those of water treatment savings. A refrigeration plant should be switched to dry operation when ambient temperature is 30 degC or lower.

Some plants can have low head pressure even when condenser pumps and fans are off. The reason for this low head pressure is low refrigeration load and cold strong wind. The valve off of the condensers can help to increase the head pressure. Nonetheless, I wound not recommend this action, as many evaporative condensers do not have the safety relief valves. Cold valved off condenser can be filled up with liquid ammonia. During a sudden weather warm up, a condenser can be damaged if the valves are not opened. I recommend wind shields to prevent low head pressure during winter operation.

 

 

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