Page 331 - HVAC Pump Handbook

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Open Cooling Tower Pumps

328 Pumps for Open HVAC Cooling Systems

The first information that must be obtained in pumping a tower is
to determine the minimum and maximum flow rates required by the
using equipment such as chiller condensers. In the past, there were
simple rules such as 3 gal/min per ton of cooling for an electric chiller.
This assumed that the heat of rejection included the 12,000 Btu/ton
plus 3000 Btu/ton for the chiller compressor and auxiliaries for a total
of 15,000 Btu/ton of cooling. Today, with improved chiller perfor-
mance, the water rates are calculated more carefully. The flow rates
should be checked for the actual chiller or other equipment under con-
sideration at specific design water temperatures such as 95°F return
and 85°F sump water temperatures. If the heat of rejection from the
chiller is expressed in British thermal units per hour at a specific
temperature difference, the following equation can be used to com-
pute the water rate.

total heat of rejection (Btu/h)
Cooling tower flow (gal/min) (11.1)
500 T (°F)
If the cooling load is expressed in tons of cooling, this formula becomes

heat of rejection/ton load in tons
Cooling tower flow (gal/min) (11.2)
500 T (°F)
3
These formulas assume a specific weight for water of 62.33 lb/ft .
3
The specific weight of 85°F water is 62.15 lb/ft , so these standard for-
mula can be used except on special applications that may require
exact calculations in gallons per minute or pounds of water per hour.
In those cases, Table 2.3 should be used, taking in consideration the
actual specific gravity of the water at operating conditions.
Many chiller manufacturers specify an actual flow rate in gallons
per minute for their machines. They do not provide a heat rejection
rate, so these figures can be used in lieu of the preceding formulas.
Absorption-type steam or hot-water–heated chillers have a higher
heat of rejection per ton than electric chillers. The cooling tower flow
rate is usually specified by the manufacturers of these machines at a
specific temperature difference.
The use of a direct-fired chiller, where natural gas is the source of
heat, requires special sizing of cooling towers. The heat of rejection
must also include the combustion efficiency, so the total heat rejection
per ton of cooling may be in excess of 20,000 Btu/ton. The allowable
temperature difference may be much greater than the traditional
10°F, so the flow rate in gallons per minute through the tower will be
different than that for the more traditional electric chiller.

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