Last month I mentioned some basics on cooling and asked some questions as to how your systems were. At this point lets start to get into the calculations of how much cooling do you need.
Taking data that has been previously published in various works we can state the following: 1 BTU is the amount of heat required to change the temperature of 1 lb. of water 1 degree F.
The unit of refrigeration is a commercial ton of refrigeration which is defined as the removal of heat at the rate of 200 BTU/minute, or 12,000 BTU/hr.
Thus the standard ton of refrigeration is 288,000 BTU or the amount of BTU’s removed by a commercial ton of refrigeration in one day.
Last month I stated that the mold is basically a heat exchanger, we inject hot plastic into the mold and take the part out after we have removed enough heat and it has solidified. Each plastic has a certain heat content in BTU/lb., we can calculate by how many pounds of material we are processing per hour and then how much heat we must remove or more accurately how many tons of capacity we need from our cooling system.
The equation to calculate out what you may need is as follows:
(pounds per hour) X (BTU per pound) X (24 hours)
288,000 BTU/DAY
= TONS cooling
Thus if you do this calculation for each machine that is processing and add them together it will give you the necessary tons of cooling that is required.
An example would be that we are processing 1 pound of polystyrene per minute and polystyrene has a heat content of 160 BTU/lb.
60 X 160 X 24
288,000 BTU/DAY
= 0.8 tons of cooling capacity
note this is only one machine.
Thanks for the time. More on the subject next time.
Steven L Silvey
Sr. Technical Service
General Polymers Division/
Ashland Chemical
Steven L Silvey