Summer energy savings

In industrial refrigeration we save energy when opportunity exists. During periods of cool weather, mother nature gives us many opportunities to improve energy efficiency of the refrigeration plants. Unfortunately, during summer operation limited energy saving opportunities can be implemented. One of these opportunities is optimized hot gas defrosting. Defrosting frequency should be optimum. Uderdefrosting or overdefrosting will reduce energy efficiency of the refrigeration plant. Typically, evaporators should be defrosted at capacity of 80 - 90% compared to capacity of clean coils. Reduced air flow is a major factor that reduces capacity of the frosted evaporator. Usually, freezer evaporators have 3 fins per inch and cooler evaporators have 4 fins per inch. Due to reduced fin spacing, cooler evaporators are more sensitive to frost formation and should be defrosted more often than freezer evaporators.

Recently, I found that some people defrost blast freezer evaporators during the freezing cycle. Should we do that or not?

Example.

Blast freezer evaporator has capacity of 20 TR. Freezing cycle of blast freezing is 48 hours. After 24 hours of operation, capacity of frosted evaporator was reduced to 95%. If we continue freezing cycle of 48 hours without defrosting, evaporator capacity at the end of the cycle will be 90%. Average capacity of the evaporator will be 95% or 19 TR. Heat removed from the product during this freezing cycle will be 19 x 48 = 912 TR.

Assume that we will do defrosting after 24 hours of operation. Defrost cycle takes 60 min including 40 min of hot gas supply. Efficiency of hot gas defrosting is low and a lot of heat will be released into the blast freezer. After 40 min of hot gas supply we should run defrosted evaporator for 60 min to remove parasitic load of the defrost. It means that during the freezing cycle with hot gas defrost actual cooling period of the product will be 46 hours. Average evaporator capacity will be 98% or 19.6 TR. Heat removed from the product during the cycle with defrost will be 19.6 x 46 = 901.6 TR.

As we can see, no defrost freezing cycle produce 1.15% more of useful refrigeration compared to the freezing cycle with defrost. These numbers are very close. However, for the longer cycles or for the cycles with greater rates of frost formation, mid - cycle defrosting can improve energy efficiency of the freezing cycle.  

In May 2013, International Institute Refrigeration conference was held in Ohrid, Macedonia. At this conference, I have presented technical paper Operating Ammonia Refrigeration Systems at Peak Efficiency. If anybody is interested in this paper, let me know and I will send it to you.