In general, compressor energy use increases about 2% per degree that suction temperature is reduced. In addition, there is more room thermal loss, infiltration and defrost penalty. When you include all factors, one could assume about 2.5% penalty per degree reduction in room temperature set point.
So, reducing a room from 0°F to -10°F would increase the energy use of THAT ROOM (and any others served by the same suction) by 20% to 25%.
Refrigerated facility owners, managers and operators are faced with what would seem to be a simple task of keeping things cold and moving product in and out to meet their customer’s needs. The principle may sound simple but in reality it is a complicated orchestra of multiple disciplines that all need to be focused on the dollars at the bottom line.
In today’s modern cold chain environment correct temperature control is essential to maintain food quality, nutrient content, and control of bacterial growth. Daily monitoring of temperatures is necessary to ensure adequate storage conditions. Refrigeration increases shelf life of most products. Most importantly, refrigeration slows bacterial growth. Optimal refrigerated storage conditions can be achieved when Refrigerated Storage temperature areas are maintained usually between +32°F and +40°F (0°C to 4.4°C). Optimal Frozen Storage conditions can be achieved by maintaining freezer storage spaces at 0°F (-17.78°C) or below. Nevertheless, one should always first obtain proper storage and handling criteria of the different products handled in your warehouse. Proper storage and handling of many perishable food products can be obtained from the Global Cold Chain Alliance, (GCCA) Commodity Storage Manual at www.gcca.org.
In the real world of refrigerated storage and warehousing you are sometimes confronted with the need to provide Freezer Storage temperatures lower than 0°F to -5°F (-17.78°C to -20.56°C). For example, the storage of ice cream will usually require storage temperatures ranging between -15°F to -20°F (-20.11°C to -28.89°C). If your warehouse or a storage room within your warehouse was not designed to provide these lower storage temperatures the added cost impact can be substantial.
Reducing a room from 0°F to -10°F (-17.78°C to -23.33°C) would increase the energy use of that room and any others served by the same suction by 20% to 25%.
I would like to point out that this article is not intended as a clinical dissection of the heat transfer problems of a cold storage facility, but intended to point out that:
- As temperatures inside a cold storage space are lowered, the amount of heat to be removed increases; and,
- As the temperatures inside a cold storage space are lowered, the cost of removing a unit of heat increases.
I would further like to point out a few basic concepts which are involved in the problem.
- There is no unit of “COLD” rather we deal with units of “HEAT” referred to as British Thermal Unit (BTU). BTU is a traditional unit of heat; it is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. And “COLD” is recognized as the absence of heat.
- Temperature within an area or space is a measure of the concentration of heat units and does not necessarily relate to the total amount of heat units present.
- To maintain a constant temperature in a refrigerated space, we must maintain an equilibrium in the concentration of heat units. In other words, we must remove heat units at the same rate at which they enter, or are created in the refrigerated space.
- Heat always flows from a higher temperature to a lower temperature.
- The rate at which heat flows is dependent upon:
- The difference in temperatures.
- The resistance to heat flow caused by the material or substance through which the flow takes place; in the case of a refrigerated warehouse, insulation.
- The four primary sources of heat in a refrigerated space are:
- Transmission Losses. The heat flowing through walls, roof and floors.
- Infiltration Losses. The heat which enters with warm air through door openings, leaks in vapor barrier (retarders) and normal flow through vapor barriers or retarders.
- Product Losses. Heat generated by product within the space, or heat which must be removed from incoming product because its temperature is above that of the room temperature.
- Internally Created Load. Heat generated by electrical lights, motors, people and handling equipment operating within the space.
Of the four principal sources of heat, three are directly related to differences in temperature. Therefore, as temperatures change, the heat flow changes. For the most part, cold storage warehouses operate their refrigerated space at temperatures which are lower than those of the surrounding outside space or ambient temperature, and heat must be removed by refrigeration. Since the temperature of the air that surrounds the refrigerated space varies from hour to hour, from day to day, and from month to month, the heat flow into the space is constantly changing; therefore, to illustrate a point, three conditions will be shown.
Case One will show the heat flow and resulting annualized tons of refrigeration required when the space is maintained at 0°F.
Case Two will show the heat flow and resulting annualized tons of refrigeration required when the space is maintained at -10°F; and,
Case Three will show the heat flow and resulting annualized tons of refrigeration required when the space is maintained at -20°F.
For the purpose of this article let’s assume that we have a cold storage warehouse facility with a storage freezer measuring 300 feet long by 200 feet wide and 45 feet high. The freezer storage is serviced from an enclosed and refrigerated loading dock. The walls of the storage consist of 5 inch thick insulated metal panels, the roof is insulated with 8 inches of urethane foam insulation with a white single membrane roofing material. The freezer floor has been provided with an under floor warming system and 6 inches of insulation.
Warehouse lighting is accomplished with LED fixtures at a rate of approximately ½ watt per square foot. It is estimated that four lift trucks will be operating within the refrigerated space at any given time during an 8 to 10 hour day. The room air handling units have the capacity to hold the room at 0°F with a refrigerant temperature of -10°F. There is approximately 36 horsepower of fan motors.
The estimated frozen product receipts are 600,000 pounds per day received at a temperature of approximately +5°F and to equilibrate to room temperature in 24 hours.
Air infiltration into the storage freezer has been assumed to be one air change per day, but to occur in the 8 to 10 hours of operation.
Table 1 below shows the Average Annualized Load (Tons Required) for the three case studies examined; i.e., 0°F, -10°F and -20°F for the typical 60,000 square feet freezer storage room. In studying these three cases one can readily see the changes in refrigeration requirements as the temperature in the refrigerated space is lowered from 0°F to -10°F and then to -20°F.
Assuming that there are no changes in the room refrigeration units but restrict the alterations to the compressor capacity, the following occurs as the room temperatures are lowered.
- To lower the room temperature, we must lower the temperature of the refrigerant in the coils; i.e., increase the split. This is accomplished by lowering the pressure of the refrigerant in the coils.
- When the pressure of the refrigerant in the coils is lowered, we also lower its specific density; i.e., the gas becomes lighter, and it will be necessary to pump more gas to have the amount of refrigeration required.
- As we lower the pressure in the coils, the range through which we compress the refrigerant (its compression ratio) is increased and this increases the amount of work that is required to pump the gas through its cycle.
- As the compression ratio increases, the efficiency of the compressor decreases.
- In summary, as the temperatures are lowered, it is necessary to handle more gas with decreased efficiency at increased power requirements.
In conclusion, it should be mentioned that some very important areas of increased costs have been circumvented because they are somewhat difficult to pin point. These areas are:
- In most cases where temperatures are lowered an investment must be made in additional low side compressor capacity; and, in some cases in intermediate compressor capacity.
- In some cases the problems are aggravated by undersized suction lines resulting in excessive pressure drop and further inefficiencies.
- As room temperatures are lowered the general efficiency of the warehouse labor force decreases.
- Building maintenance increases particularly in the realm of insulation and doors. With increased vapor pressure due to temperature differences, air leaks become more critical and require more attention.
- Defrosting costs increase due to increased air infiltration, and the corresponding amount of moisture that enters the room in the warm air.
In a final analysis, the above five items could and probably do exceed the increased cost of refrigeration.
This article was written by Chuck Toogood, VP of Business Development at M&M Refrigeration.