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View Subscription Details: Letter ( 03/14/2011 ) - HTML Format

How to optimize hot gas defrosting

 

I think that we should have two approaches to optimizing hot gas defrosting.  Evaporators of the production (spiral, tunnel) freezers operate nonstop during production time unless they have defrosting. A criterion for these freezers is maximum average capacity at given suction pressure (temperature). During production evaporator fans run nonstop and we do not have a reserve of refrigeration capacity. Frost formation reduces evaporator capacity. To restore this capacity, evaporator should be defrosted. However, during defrosting evaporators do not produce refrigeration and it adds heat to the refrigerated space. Typically, after 40 min. of hot gas defrosting this evaporator should run 60 min. in cooling mode to remove the added heat. It means that 100 min. of cooling cycle have been lost. Rate of frost formation and defrosting frequency are two major factors to optimize hot gas defrosting of the evaporators of production freezers. Optimized hot gas defrosting can help to increase production or to increase suction pressure and save energy.

Operating time of the cold storage evaporators depends on the refrigeration load. 99% of the operating time refrigeration load is lower than design refrigeration load. It means that we have reserve of the refrigeration capacity and evaporator capacity can be reduced by fan cycling or VFDs. If, due to frost formation, capacity of the evaporator is 90% of nominal capacity, to make up lost capacity additional evaporator should run 10% of operating time. Additional evaporator fan energy will be used and this energy will be released in refrigerated room as parasitic refrigeration load. It means that evaporator fan power is an additional factor to consider during optimization of the hot gas defrosting. My research has shown that cold storage evaporators should be defrosted at capacity of 80 - 90%. However, you should know how to determine real life capacity of the evaporators.

I think that optimization of hot gas defrosting can improve efficiencies of many refrigeration plants. However, this optimization is a complicated issue and it should be done by the experts.

 

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