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View Subscription Details: Letter ( 08/18/2006 ) - HTML Format

Load management/reduction for cold storages

Load management and/or load reduction can be a very effective approach to minimize energy consumption by cold storages.

The major contributors to refrigeration loads are: the heat transmission through the roof, floor and walls; infiltration through open doorways; internal load from lights, people, motors and lift trucks; defrost heat; incoming product load.

Several actions to reduce the load are obvious. We can minimize infiltration load by using timers and different sensors for freezer doors. Bi-level lighting will reduce refrigeration load as well as VFDs for evaporator fan motors. Optimum defrosting (January, February 2006 newsletters) will minimize the penalties related to frost and defrosting.

How about incoming product refrigeration load? Is this load significant? According to 2006 ASHRAE Handbook, Refrigeration, this load can very from 3% to 11% of total refrigeration load. However, my real life experience have shown a higher percentage of incoming product refrigeration load.

Example . Initial (Monday morning) refrigeration load for holding freezer is 200 TR. This freezer operates 24 Hrs per day, 5 days per week (Monday to Friday). During the weekend (Saturday, Sunday), light in the freezer is turned off. Temperature in the freezer is constant at -2°F. Dock temperature is 40°F.

Refrigeration load will increase from 200 TR to 220 TR as soon as light is turned on and operating begins (infiltration load). During the week this load gradually increases and by Friday evening it will be around 300 TR.

I discussed this issue with several chief engineers and managers. Mistakenly, they believe that this additional 300 TR - 220 TR = 80 TR refrigeration load is from the lighting and infiltration. But these two types of load are instantaneous. On Friday evening, as soon as we close the freezer doors and shut the light off, these loads will be eliminated. However, total refrigeration load will go down from 300 TR to 280 TR. During the weekend, this load will gradually decrease to 200 TR. This means that incoming product refrigeration load will be 80 TR on Friday evening. I found that this type of refrigeration load can be very significant and sometimes it can reach up to 50% of total refrigeration load.

What can we do to reduce or to eliminate this unwanted refrigeration load?

1.     Check the temperature of incoming product. This temperature should be equal to or lower than the temperature in your storage freezer.

2.     Minimize the time of transferring incoming product from the trailer to the holding freezer.

 

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