Page 343 - HVAC Pump Handbook

Rishel_CH11.qxd 20/4/06 6:37 PM Page 340

Open Cooling Tower Pumps

340 Pumps for Open HVAC Cooling Systems

TABLE 11.2 Pumping System Analysis for Chillers in Fig. 11.6b

Condenser water No. of pumps Wire-to-wire
flow, gal/min running System bhp Input kW efficiency, %
1500 1 20.0 17.8 56.6*
3000 1 36.6 31.0 68.6*
4500 1 66.1 54.1 63.5
4500 2 60.8 52.1 66.0*
6000 2 86.3 72.0 69.8*
7500 2 121.3 99.4 70.0*
9000 2 167.8 135.7 68.7
7500 3 122.9 102.5 67.9
9000 3 161.3 132.7 70.3*
*Optimal number of pumps.

number of chillers (Fig. 11.7b), the variable-speed pumps also can be
controlled by the pressure drop across the condensers. As a chiller is
started or stopped, the control valve on the chiller condenser water
circuit opens and closes. The optimal number and size of pumps
appear to be three 50 percent system capacity pumps on most of the
multiple-chiller applications. Table 11.2 is a tabulated analysis of the
pumps for the chillers in Fig. 11.7b.
This wire-to-water efficiency analysis indicates the number of
pumps that should be run with various chillers in operation. Even
though two pumps are required to handle the maximum load of 9000
gal/min, this evaluation indicates that it is more efficient to operate
all three pumps at full load.

11.6.1 Operating absorption and
centrifugal chillers together
A problem that the variable-speed pump is solving is the operation of
an absorption and an electric chiller from the same cooling tower
installation. In the past, this was unacceptable because of the differ-
ence in condenser supply water temperatures. The absorption
machine required fairly constant supply water temperature, while the
electric chiller preferred colder condenser water that improved its
performance. With variable-speed condenser pumps, the inlet water
temperature to the condenser no longer need be a constant. With vari-
able flow through the condenser, the control procedure now is to
maintain the heat balance in the condenser like any other heat
exchanger. The variable-speed pump for the electric machine is con-
trolled by the lift pressure of the chiller, while the absorption machine
is controlled by leaving water temperature and minimum flow. If the
cooling tower water drops as low as 60°F, it may be necessary to reset

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