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
The specific heat of a product is the amount of heat required (BTU’s) to raise or lower the temperature of one pound of the product by 1 degree F.
The latent heat of a product refers to a change of state in a product from one phase to another. An example of this would be changing water from a liquid to a solid.
There is numerous published data pertaining to what each product’s specific heats and latent properties are. A.S.H.R.A.E. has published much information on this subject. Krack Corporation has also published a manual which 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 apply in refrigeration.
Water Your Product Baked 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 supplier of the system if possible. You can use water and you will be safe, but you will also be overstating the loads if you are freezing.
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’s
10,000 x Sp Heat below (.35) x temperature differential from 26 down to 15.
10,000 x Latent (59).
We can see where the high loads are in freezing a product, latent.
Adding up the above, we have 921,000 BTU’s (energy), or 76.75 tons of refrigeration needed, just for the product load only. It is therefore to your benefit to identify these numbers accurately, and not guess; as if we were to use water in the above example the latent load only would have been 120 tons of refrigeration.
There are other considerations in determining the full design load of a system, including infiltration loads, motors, lights and others.