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Tech Tip November 98

Cooling continued:

In the previous few articles I have covered various aspects of the cooling system. Tower, chillers and rate of cooling or demands on the systems and how to do an average cooling rate calculation. I have left out the rate of water flow in the system and also the pressure.

In a chilled system the standard flow rate per ton is 2.4 GPM. This rate allows for a 10 degree F delta T. The recommended rate for molds is 4.8 GPM per ton which equates to a 5 degree F delta T. The tower system flow rate is 3.0 GPM per ton with a 10 degree delta T as a standard.

What does it mean? Simply put, if you have a 50 ton chiller the standard pump would be a 120 GPM pump unit allowing for a 10 degree Delta T in the water temperature. If someone put in a pump based on the needs for a mold, the GPM would than equal 240 GPM. The basic math is 2.4 GPM per ton times 50 ton which than equals the 120 GPM. To cut the delta T in half we double the flow and get a 5 degree delta T ( 4.8 GPM per ton). I f the part is critical it has been stated that the delta T should equal 2 degrees F which requires 9.6 GPM per ton. This than equates to a pump with 580 GPM capacity on a 50 ton chiller (9.6 X 50). The point being what is your flow rate to the molds? What is your delta T through the mold circuit when measured individually. The simplest test is to measure the flow of fluid through the mold with flow meters. The other is to measure the temperature difference between the in and the out of the same circuit. For critical parts 2 degrees is allowed and 4-5 degrees for non- critical parts.

With the tower we stated 3 GPM with a 10 degree variation from the supply to the return. With a 50 ton tower this than equates to a 150 GPM pump. Again if the variation is too great we can always increase the pump size, to minimize the variation.

In looking at your systems you have in place, what are the pump flows? Whatis the temperature variation from the supply to the return? Do you have the proper equipment in place to check the variation? Does it fall within theexpected results.

Next month I shall try to wrap it all up when we look at the pipe size and
maximum flow rate and pressure lost that you may have.

Thanks for the time. More on the subject next time.

Steven L Silvey
Sr. Technical Service
General Polymers Division
Ashland Chemical