Page 104 - HVAC Pump Handbook

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Basics of Pump Design

Basics of Pump Design 101

pump bhp 2544
Minimum flow Q
m T 60( /7.48)

317.2 pump bhp
(4.3)
T

where pump bhp brake horsepower of the pump near the shutoff or
no-flow condition
T allowable rise in temperature in °F of the water
specific weight of the water at the inlet water
temperature

The difficulty with this equation is getting the actual brake horse-
power consumed by the pump at very low flows. Some pump compa-
nies do publish pump brake horsepower curves down to zero flow.
Other equations that use pump efficiency have the same problem,
since the efficiency of a centrifugal pump approaches zero at very low
flows. If this information is not available, the pump company should
provide the minimum flow that will hold the temperature rise to the
desired maximum. The obvious advantage of the variable-speed pump
over the constant-speed pump appears here because there is much
less energy imparted to the water at minimum flows and speeds.
For most HVAC applications, there should not be temperature rises
in a pump higher than 10°F. Do not bypass water to a pump suction!
This continues to elevate the suction temperature. Bypass water
should be returned to a boiler feed system or deaerator on boiler feed
systems or to the cooling tower, boiler, or chiller on condenser, hot, or
chilled water systems. Since it is desired to maintain supply tempera-
ture in hot and chilled water mains, flow-control valves should be
located at the far ends of these mains to maintain their water tem-
perature. Usually, the heat loss or heat gain for these mains requires
a flow that is greater than that required to maintain temperature in
the pump itself.

4.9 Two Types of Centrifugal Pumps for
HVAC Service
As indicated earlier, there are two basic types of centrifugal pumps for
HVAC water systems. These are volute and axial-flow pumps. Volute
pumps are found in most of the low-flow applications, while axial-flow
pumps are used in the very high volume or low-flow, high-head appli-
cations. Chapter 5 will describe the various applications in detail.

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