Refrigerant plays a major role in the successful transfer
of energy from one place, where it is available,
to another place that needs or wants it...
taking energy from the sun and using it to heat the water
for the hydronic systems. During warmer weather, the
ground and the air absorb this heat, and as the weather
gets colder, some of this heat dissipates to the outside air.
However, the heat absorbed into the soil can take a
long time to transfer back to the atmosphere, even in the
dead of winter. The soil temperature in the earth is much
warmer than the outside air temperature. This condition
generally favors water source heat pumps because their
efficiency, or thermal performance, remains high due to
the warmer source water temperature. Conversely, the
Air-to-Water heat pump’s efficiency dips as the outside
air temperature gets very cold (-5°F to10°F). However,
manufacturers are continuing to improve the efficiency of
the refrigerant cycle, enabling them to extract heat from
these very cold temperatures while maintaining their
rated efficiencies and capacities.
How the Heat Pump Does What It Does…
Refrigerant plays a major role in the successful transfer
of energy from one place, where it is available, to
another place that needs or wants it. The refrigerant is
a chemical that has unique properties that allows it to
absorb heat from low temperatures and transfer that
energy to a medium operating at a higher temperature.
For this to happen, the refrigerant has to change its
state from liquid to vapor and then back to liquid, and
in doing so, has to undergo some pressure changes. This
whole process can be referred to as the “Refrigeration
Cycle” and is the starting point for the operation of all
vapor-compression heat pumps. There are four components
that play a major role in this cycle. Their respective
names indicate their function and how they play
their part in the process.
• Evaporator
• Compressor
• Condenser
• Thermal expansion valve
The Evaporator
Here we have cold liquid refrigerant enter into the
evaporator. Now the pressure of this cold liquid 410A
refrigerant is low, and there is a direct relationship
between the refrigerant’s pressure and its corresponding
temperature—the lower the pressure the lower the
refrigerant’s temperature. The refrigerant’s pressure/
temperature will adjust as the “source” (air) temperature
changes.
As it gets colder outside, the temperature of the liquid
has to become colder so it can absorb heat from the cold
air outside. As it absorbs heat from the air outside, the
refrigerant evaporates or changes its state from liquid
into a vapor or gas. Its temperature is still low, but
warmer than when it was a liquid. This is important
because the next component in line is the compressor.
Liquids are not compressible, so if the refrigerant didn’t
evaporate into a vapor, the liquid entering the compressor
would severely damage it.
Compressor
The compressor’s job is just as it sounds—it compresses
the low temperature vapor. This creates a large increase
in both its pressure as well as its corresponding temperature
as the refrigerant leaves the compressor. Another
factor to consider is that the electrical energy used to
compress the refrigerant by the compressor is added to
the refrigerant. Now we have a high temperature vapor
that contains a lot of energy ready to be utilized.
Condensor
This high-pressure, high-temperature vapor then enters
the condenser and the cooler return water from the
hydronic system is pumped across the exchanger. The
refrigerant is hotter than the water; it transfers its energy
to the cooler water, elevating the water temperature
as the water goes back out to the heating system. This
transfer of energy causes the refrigerant to change its
state and condense back to a high pressure liquid.
Thermal Expansion Valve
The last step in this process is for the high pressure/high
temperature liquid refrigerant to flow through an expansion
valve (either thermal or electronic). The expansion
valve controls the flow of the liquid refrigerant through
its orifice, drastically reducing its pressure, and thus its
temperature, so that the refrigerant is back to the cold
temperature it was at the beginning of this process.
Air-to-Water Heat Pumps are starting to play a major
role in renewable energy applications. Questions or comments?
e-mail me at gcarey@fiainc.com, call me at 800-
423-7187 or follow me on Twitter at @Ask_Gcarey. ICM
ICM/July/August 2019 17