Page 294 - HVAC Pump Handbook

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Basics of Pump Application for HVAC Systems

Basics of Pump Application for HVAC Systems 291

6. Since the constant-speed pump is handling most of the load, the
variable-speed pump is forced to operate at point D . This is a very
1
inefficient point with the pump running with a high radial thrust.
7. If the system load subsides slightly, the variable-speed pump will
be forced to run at zero flow, with resulting heating inside the
pump casing. If the controls are of the wrong configuration or are
set improperly, the variable-speed pump may run at this condition
until it is destroyed by heat.

This demonstrates that mixing a variable-speed pump with a con-
stant-speed pump can have disastrous results. This unacceptable
operation can be contrasted with the proper operation of two variable-
speed pumps. Using the preceding procedure, assume that steps 1
through 4 are the same. Then
5. Let the standby pump be a variable-speed pump instead of a con-
stant-speed pump. The resulting satisfactory operation is demon-
strated in Fig. 10.11b.
6. The standby pump will ramp up, and the controls will reduce the
speed of the lead pump until both pumps are operating at point D .
3
Both pumps are operating at a high efficiency and a low radial thrust.
7. If the system load subsides slightly, both pumps will slow down, and
the controls can stop the standby pump when it is efficient to do so.
It is not recommended that a pumping system with four or more
pumps consist of constant- and variable-speed pumps operating in
parallel. Usually, more energy is consumed with a mixture of con-
stant- and variable-speed pumps. If it is imperative that some of the
pumps be constant-speed pumps, the two lead pumps must be variable-
speed pumps. Since one of these pumps may be out of service, it is rec-
ommended that at least three pumps be variable-speed pumps.
Systems with four or more pumps can consist of constant-speed
pumps, as demonstrated in Table 10.2, but this forces all the normal
operation on the variable-speed pumps. Also, if the load does increase

TABLE 10.2 Number of Variable- and Constant-Speed Pumps in Parallel
Operation
Number of pumps Variable-speed pumps Constant-speed pumps
1 1 None
2 2 None
3 3 None
4 3 1
5 3 2
6 3 3

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