Page 348 - HVAC Pump Handbook

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Open Cooling Tower Pumps

Open Cooling Tower Pumps 345

Partial indirect cooling can be accomplished by placing the free
cooling heat exchanger in series with a chiller. This reduces the
return water temperature to the chiller and enables it to operate at a
lower load. Like the heat exchanger above, the friction loss in both
loops must be evaluated.
All these free cooling methods require careful energy and cost eval-
uations by the system designer to determine the optimal procedure
for a particular system.

11.7 Understanding Legionnaires’ Disease
Although cooling tower water pumping in itself has little effect on the
prevention of Legionnaires’ disease, it does not hurt to have some spe-
cific information included in this book on this disease. Legionnella,
the bacteria that causes it, thrives in water of temperatures from 70 to
140°F. This temperature range is the same as that for operation of
most HVAC cooling towers. Therefore, it is urgent that all cooling tow-
ers be equipped with a comprehensive water treatment system that
not only controls corrosion and scale but also limits biologic con-
tamination. The system operator should become familiar with the rec-
ommendations of various authorities as contained in publications
such as those of the Occupational Safety and Health Administration.
Another source for this information is the 1995 ASHRAE Handbook,
Heating, Ventilating, and Air-Conditioning Applications, Chap. 44,
Corrosion Control and Water Treatment. Spills of water around cool-
ing tower pumps and leaks from cooling tower pumps should be elimi-
nated to aid in the control of this disease.

11.8 Summary

Cooling towers remain the best method of rejecting heat in HVAC and
industrial process water systems. It should be emphasized how much
heat is passed out through a cooling tower. Equation 11.3 develops
this as total heat per year:

Btu/year heat/ton tons annual hours (11.3)
For example, if a 1000-ton chiller has an average heat rejection of
15,000 Btu/ton and operates an equivalent of 2000 hours per year at
full load, the annual heat rejection is

Btu/year 15,000 1000 2000 30,000,000,000

This is 30 billion British thermal units per year! With an overall
3
boiler efficiency of 80 percent and a cost of $3 per 1000 ft of natural

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