Page 313 - Sustainable On-Site CHP Systems Design, Construction, and Operations

286 Op erations

Pumps
Pumps use rotational mechanical energy, usually provided by an electric motor, to
create the pressure differences that drive the flow of fluids.

Efficiency of Pumps
The efficiency of a pump (η ) can be expressed as
Pump
W
η = Pump (17-43)
Pump W
Pump,elec
where W is the mechanical power output imparted by the pump to the liquid, and
Pump
W is the electric power input to the pump motor. The mechanical power imparted
Pump,elec
by the pump to the liquid is equal to the product of the volumetric flow rate through the
pump and the pressure difference across the pump, that is,
W = v P ( − P ) (17-44)
Pump Pump, w discharge suction
where v is the volumetric flow rate through the pump, P represents pressure, and
Pump
the subscripts “discharge” and “suction” identify variables at the pump suction port
(inlet) and discharge port (outlet). The difference between the discharge and suction
pressures on an operating pump is sometimes called the dynamic head.
Combining Eqs. (17-43) and (17-44), yields the relation for pump efficiency, which
incorporates motor efficiency.
v P ( − P )
η = Pump discharge suction (17-45)
Pump
W
Pump,elec
Fans
Similar to pumps, fans use rotational mechanical energy, usually provided by an electric
motor, to create the pressure differences that drive the flow of gases, often air.

Efficiency of Fans
The efficiency of a fan (η ) can be expressed as
Fan
W
η = Fan (17-46)
Fan
W
Fan,elec
where W is the mechanical power output imparted by the fan to the gas, and W
Fan Fan,elec
is the electric power input to the fan motor. The mechanical power imparted by the fan
to the gas is equal to the product of the volumetric flow rate ∗ through the fan and the
pressure difference across the fan, that is,
W = v P ( − P ) (17-47)
Fan Fan Fan, o Fan, i
where v is the volumetric flow rate through the fan, and P and P represent the
Fan Fan,i Fan,o
pressure immediately upstream and downstream of the fan.

∗ The work of compressing the air is assumed negligible, which is reasonable for fans operating at or
below about 4 in w.c. (= 0.145 psig = 996 Pa).