Page 345 - HVAC Pump Handbook

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Open Cooling Tower Pumps

342 Pumps for Open HVAC Cooling Systems

11.6.4 Free cooling with cooling tower water
Free cooling is the name applied to processes that use the tower water for
cooling chilled water systems instead of the chiller. Free cooling can be
developed through a number of piping and equipment arrangements.
Basically, these systems break down into two classes, direct and indirect.
Direct free cooling. Direct free cooling defines the fact that the cooling
tower water circulates directly in the chilled water system instead of
through the chiller condenser. Two-way valves are provided for this
change. The water passes into the chilled water system at the bypass,
as shown in Fig. 11.8a. It circulates water through the chiller bypass
and acts like the primary chiller pump. The secondary or distribution
pumps pick up the cooling tower water and move it through the chilled
water system as with the chillers in operation. The condenser pump
head may be less in the free cooling mode than during normal opera-
tion with the chiller condenser. Both modes of operation must be eval-
uated to ensure that the condenser pump performance is adequate.
The principal drawback of direct free cooling is mixing of the cooling
tower water and its chemistry with the relatively clean chilled water.
Cooling tower water has to have a number of airborne contaminants,
since cooling towers are efficient air washers. Cleaning cooling tower
water can be accomplished with either full filtration such as sand fil-
ters or sidestream filters mounted at the cooling tower. Neither of
these systems can guarantee complete removal of all the objectionable
material that may be in the cooling tower water. The sidestream filter
should not affect the condenser pump performance; the full sand filter
may have a pressure drop greater than that for the chiller condenser.
This must be included in the condenser pump evaluation.
One of the advantages of direct free cooling is the lower water tem-
peratures that are available from the cooling tower compared with
the higher temperature emanating from the heat exchanger on indi-
rect free cooling systems.
Indirect free cooling. Indirect free cooling can be supplied in several
forms. The simplest and most common is through the use of a heat
exchanger that separates the cooling tower water from the chilled
water (Fig. 11.8b). As indicated earlier, some temperature difference is
lost across the heat exchanger. The designer must determine the dif-
ference in head between the friction loss through the chiller con-
denser and the loss through the heat exchange and size the condenser
pump to accommodate both the friction losses.
An alternative method of indirect free cooling is described in Fig. 11.9
where the heat exchanger is located in tandem with the chiller evapo-
rator. This installation is used where chilled water is required with

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