Environmental Engineering Reference
In-Depth Information
High temperature (T
H
)
Q
H
Direction heat
flows with a
heat pump
Direction heat
flows naturally
Work (
W
)
HP
Q
L
Low temperature (T
L
)
Figure 11.9
Diagram of a heat pump that takes heat from a lower temperature source and discharges
the heat to a higher temperature by using mechanical energy.
heat from a low temperature source to a higher temperature sink using mechanical energy, and
therefore is applicable for the recovery of low grade heat.
A heat pump uses mechanical energy instead of burning a fuel to produce heat and that
mechanical energy may come from an electric motor or an engine operated with any fuel. The
advantage of using an engine is that heat can be recovered from the exhaust, thus making the
whole system more efficient.
The efficiency of heat pumps are measured according to the coefficient of performance
(
COP
HP
), which is the ratio of heat output (
Q
L
) divided by the work (
W
) spent in the process
(Fig. 11.9) (Dinçer, 2003):
+
==
QQWQ
[11.9]
COP
H
L
=+
L
1
HP
WWW
It is clear from Equation 11.9 that COP
HP
for a heat pump is always greater than 1 (Dinçer,
2003), and that the COP
HP
increases with the temperature of the heat source.
The COP of a heat pump (COP
HP
) should not be confounded with the COP for refrigeration
equipment (COP
R
), which is defined as:
Q
[11.10]
COP
=
L
R
W
And therefore, they are both related by the following equation (Dinçer, 2003):
COP
=
COP
+
1
[11.11]
HP
R
Low-pressure steam by vapor recompression
Low-pressure steam from evaporators or cookers can be recovered and upgraded by mechani-
cal or thermal recompression. Low-pressure steam is compressed with a centrifugal compres-
sor, or a steam ejector, to boost the pressure to what is required by the process at a fraction of
the cost of producing steam in the boiler. For instance, a waste stream of steam at 15 psig can
be boosted to 40 psig with mechanical recompression with an expense of energy of 63.5 Btu/
lb instead of the 1,387 Btu/lb that would be required by a boiler (assuming a feed water tem-
perature of 21°C [70°F] and a boiler 82 percent efficient) (DOE, 2001).
