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Tech Tip October 98

Cooling, continued:

In the last two months I have brought up some generalities and specifics about cooling. This month let us talk about the tower and basic calculations that you can make.

The main difference between a tower and a chiller is the difference in the efficiencies or how we rate the unit. Last time I stated that the chiller removed 12,000 BTU/hr or 288,000 BTU/ 24 hr. per ton rating (commercial ton). When we get to the tower the calculation is a bit more. The heat removal rate is 15,000 BTU/hr or 360,000 BTU/ 24 hr. per ton of cooling tower.

The other factor to look at since we are now letting nature help us here, is how dry is the air in which we place the tower. A towers' efficiency depends on the rate of evaporation of water taking place in the tower cell. Dry ambient air means a higher rate of evaporation. Cold, saturated air may not cause as much cooling as warm dry air. Tower manufacturers should be able to supply efficiencies and charts for the various temperatures and climates.

Given the above, it now becomes apparent that there is some variation if we run the molds off a tower. But if as was suggested we use the tower for machine, thermulator and chiller cooling only, we may be okay if we have sized it correctly. This again brings up the pointof knowing actual water temperature, pressure and flow rates.

The following are recommendations for sizing:
(based on Tower Ton)

Machinery:
	Hydraulics	1 Hp = 0.1 ton
	Feed throats	0.5 tons each
	Gear Boxes	1 ton each
	Temp. Control unit pumps	1 Hp = 0.2 ton
	Screws and barrels	1 ton per inch of screw diameter
General:
	Water cooled chiller	1 Ton per Ton of Capacity
	Air compressors	1 Hp = 0.1 ton
Examples:
	Machine has a 25 hp motor on the hydraulics; 25 hp X 0.1 = 2.5 tons for the hydraulics plus that for the feed throat 0.5 tons equals 3 tons of cooling. The machine has 3 pumps with a total hp rating of 55 and we have a water cooled chiller rated at 2.5 tons. We have the 55 hp x 0.1 ton/1 hp = 5.5 tons plus the chiller at 2.5 tons (1=1) for a total of 8 tonsí usage.
Go through your plant and add everything up to evaluate your needs. The one factor I left out is how much water flow is need for the delta T, more on that next time.

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

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