8/12/2019 Proposal Project Thermo
2/3
UNIVERSITI TUN HUSSEIN ONN MALAYSIA
EDUCATION CENTRE OF DIPLOMA
Title: Water Temperature Increase Due To Mechanical
Efficiency
Problem Statement
The pump of the water distribution system is powered by a
0.75-kWelectric motor whose efficiency
us 90 percent. The water flow rate though the pump is 50 L/s.
The diameter of the inlet and outlet
pipe is the same, and the elevation difference across the pump
is negligible. If the pressure at the inlet
and outlet of the pump are measured to be 50 kPa and 150 kPa
(absolute), respectively, determine the
(a) the mechanical efficiency of the pump and (b) the
temperature rise of water as its flow though the
pump due to the mechanical efficiency
Assumption
1) The flow are steady and incompressible2) The pump is driven
by external motor so heat generated by motor is dissipated to
the
atmosphere
3) Pipe elevation is negligible, z1=z24) Inlet and outlet
diameter are same thus velocity 1 and velocity 2 are equal
Assumptions 1 The flow is steady and incompressible. 2 The pump
is driven by an external motorso that the heat generated by the
motor is dissipated to the atmosphere. 3 The elevation
difference
between the inlet and outlet of the pump is negligible, z1 = z2.
4 The inlet and outlet diameters are
the same and thus the inlet and exit velocities are equal, V1 =
V2.
Physic law and thermodynamic law
1) 2) 3)
4) 5) 6) ( ) 7)
8/12/2019 Proposal Project Thermo
3/3
UNIVERSITI TUN HUSSEIN ONN MALAYSIA
EDUCATION CENTRE OF DIPLOMA
CALCULATION
i. The mass flow rate of water through the pump is , , = 50
kg/s
ii. The motor draws 0.75 kW of power and is 90 percent
efficiency, thus the mechanical (shaft)power is
, ()( ), =0.675 kWTo determine the mechanical efficiency of the
pump, we need to know the increase in the
mechanical energy of the fluid as it flows through the pump,
which is
Simplifying it for this case and substituting the given
values,
, =( ) () , = 5 kW
iii. Then the mechanical efficiency of the pump becomes
,
, = 7.4 %
iv. Of the 0.675 kW mechanical power supplied by the pump, only
5 kW is imparted to the fluidas mechanical energy. The remaining
3.5 kW is converted to thermal energy due to frictional
effects, and this lost mechanical energy manifests itself as a
heating effect in the fluid,
, = 0.675 kW5 kW , = 4.325 kWThe temperature rise of water due
tp this mechanical efficiency us determined from the thermal
energy balance
( ) , , ()(), = 0.021
Therefore, the water will experience a temperature rise of
0.017C, which is very small, as it
flows through the pump.
Discussion In an actual application, the temperature rise of
water will probably be less since
part of the heat generated will be transferred to the casing of
the pump and from the casing
to the surrounding air. If the entire pump motor were submerged
in water, then the 1.5 kW
dissipated to the air due to motor inefficiency would also be
transferred to the surrounding
water as heat. This would cause the water temperature to rise
more.
