Chemical Engineering June 2022 - 30

utilizing different compressor types
and working fluids. These ranges
are under continuous improvement,
given research efforts into increasing
the efficiency of the components,
utilizing new solvents, or enhancing
the system configuration. As an example,
efforts are underway to increase
temperatures, in order to enable
heat pumps to reach industrial
heating (for example, low-pressure
steam) ranges.
When selecting refrigerants, aside
from the pressure, temperature and
efficiency considerations inherent to
the system, other criteria to consider
include toxicity, flammability, environmental
considerations and cost.
The International Institute of Refrigeration
estimates that R-22 and R410A
are currently " the main refrigerants
used for ASHPs " [3].
R-22, along with other Hydrochlorofluorocarbon
(HCFC) refrigerants,
are being phased out due to ozone
depleting potential. For instance,
in the U.S., the production and import
of R-22 and R-142b has been
banned for new units since 2010,
and for servicing of existing units
since 2020 [4].
Hydrofluorocarbon (HFC) refrigerants
are replacing HCFCs, but some
of the options available also possess
environmental concerns due to their
Global Warming Potential (GWP).
For instance, R-410A has a GWP of
1,924 [5].
Refrigerants such as ammonia or
CO2 are mostly used in industrial settings
due to safety and pressure concerns.
Propane is utilized in some refrigeration
applications, especially in
industrial settings where electrical installations
comply with electrical area
classification requirements.
There is wide research into developing
non-toxic, low-GWP refrigerants,
but many of these are flammable
and thus subject to safety
considerations. Different tables are
available to compare their performance
and characteristics, subdividing
them into low, medium or high
pressure, or into low, medium and
high GWP.
Some of these newer refrigerants
are marketed as direct replacements
to their phased-out counterparts,
which is especially important
30
to maintain usage of equipment designed
for the latter. When replacing
the working fluid, it is always recommended
to consult with the heatpump
supplier or service technician.
Heat-pump process design
Heat-pump design typically involves
the following steps: 1) Defining the
service heat load and temperature;
2) Assessing the sources for the heat
pump; 3) Defining the design capacity
of the heat pump; 4) Specifying
and selecting the heat pump.
Step 1. The first step in designing a
heat-pump system consists of identifying
the required service (heating or
cooling), heat load and temperature.
The temperature is defined in
accordance with the required application.
For instance, for space
heating, the temperature is typically
selected around the comfort temperature.
Water heating or industrial
applications typically require higher
temperatures.
Step 2. The required heat pump
load is calculated to maintain a net
zero energy balance around the volume
of the space or system receiving
the service:
0 = Qheat pump + Qheat sources +
Qtransfers to/from environment + Qprocess
(7)
Heat sources may include heat
rejection from equipment, machinery
or appliances, or heat emanated
from occupants.
Heat transfers to and from the environment
may include heat radiated
from the environment (for example,
solar radiation), heat losses to the environment
(when the system is at a
higher temperature than the environment),
or heat gains from the environment
(when the system is at a lower
temperature than the environment).
Process heat encompasses other
services used for process streams
(for example, water heating or cooling,
process cooling and so on).
These heat-pump requirements
typically involve heating, ventilation
and air conditioning (HVAC)
calculations that consider space
distribution, types of equipment or
appliances, insulation, and other
contributors to the heat-balance
equation, as well as seasonal variations
in heat loads and environmental
conditions. Commercial or opensource
tools are available to assess
heating or cooling requirements.
In Equation (7), especially in space
heating or cooling service, passive
design features directly affect
the heat requirements of the heat
pump. Measures such as solar passive
design and increased insulation
reduce the heat pump requirement
and are typically evaluated in an
economic optimization study that
seeks to reduce the overall cost of
the service throughout the project
life horizon.
The second step is to assess the
available source(s) for the heat pump
and determine its temperature and
operating conditions. As discussed
previously, heat sources may come
from air, ground, water, waste heat or
combinations. The evaluation of the
heat source includes temperature
profile (daily and seasonal), hot fluid
phase (liquid, gas) and available heat
(especially when considering waste
heat, water, or ground sources,
which may have flowrate or regeneration
constraints).
Step 3. The next step is to define the
capacity of the heat pump. The capacity
is typically defined as a margin
above the calculated heat load
considering seasonal variations and
is set at the worst condition for the
expected service (for example, for an
ASHP used for space heating, the
heat pump capacity is defined by the
minimum expected ambient air temperature
and the maximum desired
room temperature, and vice versa for
space cooling).
Step 4. The heat-pump specification
then comprises the required capacity,
heat load evaluation, source type
and temperatures. The specification
may also include the heat-pump
type, a desired minimum coefficient
of performance, some technical,
service and guarantee considerations,
and a preference for working
fluid (for example, a non-toxic,
non-flammable, low-GWP refrigerant).
There may be some tradeoff
between the desired specifications
(for instance, a low-GWP refrigerant
may lead to a lower coefficient of
performance or a higher cost than
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JUNE 2022
http://WWW.CHEMENGONLINE.COM
Chemical Engineering June 2022

Table of Contents for the Digital Edition of Chemical Engineering June 2022
