Page 103 - HVAC Pump Handbook

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Basics of Pump Design

100 HVAC Pumps and Their Performance

4.6 Critical Speed of a Pump

Every rotating device has a natural frequency at which vibrations
become pronounced; the vibrations can increase until noise becomes
objectionable, and there is danger in a pump that the rotating equip-
ment will become damaged. The speed at which this occurs is called
the critical speed of that rotating element. Some pumps will actually
have more than one critical speed. Fortunately, for HVAC pumps, the
impeller diameters are relatively small and the rotational energy low.
All pump designers must take critical speed into consideration in the
structural design of their pumps, but it is of little concern to the
HVAC system designer in the selection of pumps.
The diameter or stiffness of the impeller shaft determines the criti-
cal speed of a pump. Therefore, the pump designer ensures that the
critical speed of the pump is out of the normal range of pump opera-
tion. If a pump has a normal operating range of up to 1800 rev/min,
the pump designer will ensure that the critical speed of that pump is
at some speed such as 2500 rev/min. Therefore, in most cases, critical
speed of HVAC pumps is not part of the HVAC pump selection process.

4.7 Minimum Speed for a Variable-Speed Pump
Minimum speed for a variable-speed HVAC pump will be discussed
here, where information is also included on specific and critical
speeds. There is no minimum speed for these HVAC pumps. Misinfor-
mation about specific, critical, and minimum speeds for HVAC pumps
has caused misapplication of them along with the installation of
unnecessary speed controls. As will be reviewed in Chap. 6, variable-
speed centrifugal pumps can be operated down to any speed required
by the water system. Since they are variable-torque machines, very
little energy is required to turn them at low speeds. Information on
this subject is included in Chap. 7 on pump drivers and drives as to
the minimum speed required by motors for their cooling.
The HVAC water system designer really has no need to be con-
cerned with the specific, critical, or minimum speeds of these pumps.
Only on extremely low speed operations where variable-speed pumps
are connected in series does the designer need to verify pump motor
performance with respect to heating.

4.8 Minimum Flow for HVAC Pumps
The question often arises as to the minimum flow that is required
through a pump to prevent it from overheating. The following equa-
tion provides this flow:

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