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View Subscription Details: Letter ( 01/17/2011 ) - HTML Format

Two Approaches to Energy Savings

 

Recently, I reviewed operation of one refrigeration plant. Chief engineer had concerns about raising suction pressure during hot gas defrosting. Typically, this two stage refrigeration plant operates at vacuum - 5 in of Hg due to requirement of blast freezers. During summer operation suction pressure can increase up to 0 psig if one evaporator goes on defrost. The reason for suction pressure increase is that defrost ammonia condensate and defrost blow by gas go to the suction line due to old design of the refrigeration plant. Local contractor suggested installing separate ammonia condensate line from the evaporators to the intercooler. This installation costs $20,000.

For efficient hot gas defrosting, demand and supply should be balanced. Demand is the heating requirement of the evaporator. This demand is unchanged all year-round. We have the same evaporator, the same temperature around evaporator, and probably, the same amount of frost. Supply is hot gas supply to the evaporator. During summer operation condensing pressure of this refrigeration plant goes up to 180 psig and it is significantly greater than winter condensing pressure of 120 psig. At 180 psig hot gas will be oversupplied to the defrosting evaporator. Uncondensed blow by gas will create significant parasitic load and suction pressure will increase to 0 psig.

To solve this issue, my suggestion was to install outlet pressure regulator for the defrosting hot gas line. This regulator will keep pressure of 120 psig all year round and summer oversupply of defrost hot gas will be prevented. The cost of this installation is $1500. Certainly, additional ammonia condensate pipe will save some energy, but these savings are not significant (around $1500).

Similarly, we can find two different approaches in energy savings. Usually, contractor suggests to end user energy saving measures that require capital investments. Definitely, investments in VFDs, PLCs, repiping, additional condensers will save you some energy, but it is a very expensive approach. Optimization of the refrigeration plant operation does not require significant investments and it has payback 10 - 20 times shorter than capital investments. Many end users have done initial optimization, but a lot of the refrigeration plants have an opportunity for final optimization. Do not miss this opportunity.

 

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