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The compilation of heat loads in regard to designing a walk-in refrigerator can be complex. For the design of a cooler which only maintains the temperature of a pre-cooled product, you need only figure the heat gain through every square foot of exposed panel based on the temperature differential between the outside (Ambient) air and what you want to maintain within the structure. However, many people want to reduce the temperature of a warm product (such as a side of beef) several degrees or even to a state of being frozen.
This brings us to the study of specific heat and latent heat. Every substance has a specific heat factor above freezing, another below freezing, and a latent (hidden) factor in between to change into a frozen state. This latent factor which changes the state of a product from fresh to frozen is considerably greater than the factor required to lower the same product just one degree either above or below freezing. These heat loads all must be combined by the engineer to determine the power necessary to accomplish the desired result in the hours of time desired for accomplishment.
The engineer knows that one British Thermal Unit (BTU) is the energy required to raise or lower the temperature of one pound of pure liquid water one degree Fahrenheit. If you are familiar with the terms used in relation to window air conditioners (half-ton, three quarter ton, one ton), the terms developed about the time cooling went from ice melting to mechanical refrigeration; it means the cooling capacity which is derived by melting one ton of ice within 24 hours, or 12,000 BTU’s/hour.