Chemical Engineering June 2022 - 29

a variety of different heat sources,
such as those available on industrial
facilities. The temperature, quantity,
and availability of different sources of
waste heat will dictate the configuration,
type of evaporator (for example,
gas or liquid hot fluid), and working
fluid to be used in the system.
Hybrid-source heat pumps combine
different sources for either single
or multiple applications.
FIGURE 7. This chart shows the typical operating ranges for heat pumps (adapted from Ref. 2)
ergy, with a typically low consumption
of electricity for pumping. Figure
4 shows combustion (for example,
from natural gas) as a heat input to
the system, but other suitable heat
streams, such as waste heat, can be
utilized. Also, depending on the configuration
and integration with the
surroundings, heat from the pump
can be utilized for different temperature
services: this includes heat from
the evaporator, heat removed from
the absorber, and heat losses (for example
as fluegas) from the generator.
Figure 5 shows a thermal vaporrecompression
(steam-jet) heat
pump. This technology also uses a
combination of thermal energy (in the
boiler) and work (at the pump). In this
case, the system is open as some of
the steam is vented and make-up is
added to the evaporator.
Mechanical
vapor-recompression
heat pumps differ from thermal vapor-recompression
heat pumps in
that they use a mechanical driver to
move the vapor to a higher pressure.
Based on driving energy. The driving
energy refers to the energy that
generates the motion of the working
fluid from the low to the high-pressure
side of the cycle. This energy
can be sourced from process heat,
steam, combustion, engines, or motors,
depending on the technology
and model.
Heat sources
Typical heat pump sources include
air, ground (geothermal), water, solar-assisted,
waste heat or a hybrid.
Air-source heat pumps (ASHPs)
are subject to seasonal variation of
ambient temperature. Their advantages
are relatively low costs and
ease of installation.
Ground (geothermal) source heat
pumps (GSHPs) are subject to lower
seasonal temperature variations, but
at a usually higher initial cost than air
source heat pumps due to the excavation
required to reach the desired
depths and install the piping needed
to exchange heat with the ground.
Water-source heat pumps
(WSHPs) allow for a higher temperature
difference between the source
and the sink, provided a significant
water reservoir is nearby. Permitting
requirements, freezing during winter,
and limited geographical access to
large volumes of water limit the use
of these types of sources. However,
their configuration can use hot water
from other sources, such as process
water, as described below.
Solar-assisted heat pumps combine
the heat from another source
with that from solar thermal collectors
for applications such as water
heating that require moderately
higher temperatures.
Waste-heat heat pumps may use
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Other configurations
Figures 3, 4 and 5 show singlestage
configurations with one evaporator
and one condenser. However,
heat pumps can be configured in
many different manners to improve
performance, capture heat from different
sources, deliver heat to different
sinks, or deliver heat to highertemperature
differentials. Figure 6
shows some examples of different
configurations of vapor-compression
heat pumps:
* A multiple-stage heat pump
where intermediate flash tanks are
placed after each expansion stage
to separate vapor going to compression
from liquid going to the
evaporator (Figure 6a)
* A cascade arrangement, where
a low-temperature heat pump using
one working fluid delivers heat to a
high-temperature heat pump using
another working fluid, which delivers
heat to the sink. In this arrangement,
the first loop's condenser acts as the
second loop's evaporator (Figure 6b)
* Systems involving different evaporators
to capture heat from different
sources, such as geothermal, solar
collectors and waste heat (Figure 6c)
* Systems involving different condensers
to deliver heat to different
sinks (Figure 6d)
* A reversible heat pump used
for space heating during winter and
cooling during summer, where the
direction of the flow is reversed so
that the evaporator and condenser
switch roles depending on the ambient
temperature (Figure 6e)
Operating ranges & refrigerants
Figure 7 shows typical operating
ranges for commercially available
vapor compression heat pumps
29

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Chemical Engineering June 2022

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