Calculate the heat loads.

When cooling or freezing a product, we must calculate the heat loads involved and need to know general information on the products we are processing.
3 main items:

  • Specific heat above freezing
  • Specific heat below freezing
  • Latent Heat

Specific Heat
The specific heat of a product is the amount of heat required, BTU to raise or lower the temperature of one pound of the product by 1 degree F.
Latent Heat
The latent heat of a product refers to a state change in a product from one phase to another. An example is changing water from a liquid to a solid.

Numerous published data pertaining to each product’s specific heats and latent properties exist. A.S.H.R.A.E. has published much information on this subject. Krack Corporation has also published a manual that contains very helpful information.
Finding heat loads on a product
We will use water as a basis of comparison since most products contain water; these numbers are the maximum that applies in refrigeration.
The Water of Your Product Bread.

  • Specific Heat above freezing: 1.0 .74
  • Specific heat below
  • freezing: .44 .35Latent heat: 144 54

If you have the real numbers for your particular product, use them. If not, try to develop these numbers or get them from your system supplier if possible. You can use water, and you will be safe, but you will also be overstating the loads if you are freezing.

An example:
You want to know what an estimated refrigeration load is of the product only of taking 10,000 lbs per hour production rate of cookie dough, from 65F down to 15F. Using a freezing point of 26 degrees, a specific heat of .75 above freezing, .35 below freezing, and 59 as latent, we have the following:

10,000 x Sp Heat above (.75) x temperature differential from 65 to freezing.
= 292,500 BTU
10,000 x Sp Heat below (.35) x temperature differential from 26 down to 15.
=38,500 BTU
10,000 x Latent (59).
=590,000 BTU

We can see where the high loads are in freezing a product, which is latent.

Adding to the above, we have 921,000 BTU (energy), or 76.75 tons of refrigeration needed for the product load only. Therefore, it is to your benefit to identify these numbers accurately and not guess; if we were to use water in the above example, the latent load would only have been 120 tons of refrigeration.

Other considerations, including infiltration loads, motors, and lights, must also be considered when determining a system's full design load.