Page 295 - HVAC Pump Handbook

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Basics of Pump Application for HVAC Systems

292 The HVAC World

often to where the fourth pump is required, higher operating effi-
ciency is achieved by making it a variable-speed pump.
A careful evaluation of the constant-speed pumps must be made
when they are used, including calculating where they will operate on
their head-flow curves, to ensure that they do not operate at an ineffi-
cient or high radial thrust point.

10.8 Efficiency of a Pumping System
The efficiency of a pumping system is more than just the efficiency of
the individual pumps. There are three different energy losses in a
constant-speed electric motor–driven pumping system:
1. Loss in the motor
2. Loss in the pump
3. Friction loss in the pump and header fittings
The variable-speed pumping system has a fourth loss, that of the
variable-speed drive.
There may be some concern as to why the pump fitting and header
losses are included in the efficiency evaluation for a pumping system.
These losses must be computed separately because they are part of
the pumping system, not part of the water distribution system. Also,
on multiple-pump systems, they must be separated from the water
system loss because they do not vary with the total flow on the sys-
tem. Rather, they vary with the flow through the pump to which they
are connected. The flow rate through these fittings affects appreciably
the overall efficiency of such a pumping system. The friction loss for
these fittings varies closely with the square of the flow through them.
The points at which pumps are programmed on and off on a multiple-
pump system have a definite impact on the efficiency of the overall
pumping system; these points are affected by the variation in friction
in the pump fittings and valves.
There can be six to ten specific losses in the fittings and valves on a
pumping system:
1. Entrance loss from suction header
2. Suction shutoff valve
3. Suction strainer
4. Suction piping
5. Pump suction reducer
6. Pump discharge increaser
7. Check valve

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