| Meier
Supply
Company, Inc.
HVACR Specialists
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What are expansion valves and how will they improve my system?
Expansion Valves are the most commonly used
metering device in refrigeration systems, and can be found on a wide range of applications.
Properly applied expansion valves can be a great asset to a refrigeration system.
They are much more forgiving to pressure changes than fixed metering devices
such as a capillary tube.
The expansion valve is available in many types
of configurations and capacities. Regardless of the type selected, there are certain
guidelines to be followed when selecting the correct expansion valve.
The first variable to consider is sizing the capacity of the valve.
It has always been simple enough to match the valve capacity to
system capacity, in many cases this will work, in others it won't. Different types
of head pressure controls and fluctuating loads make this sizing practice useless and can cause
many system problems. Expansion valves should be correctly sized by pressure drop and
mass flow; this is accomplished quite easily using sizing charts provided by the manufacturer.
The second consideration is the refrigerant used in the system where the valve
will be applied. Even though an expansion valve may be tagged
for use with a specific type of refrigerant, there are often other refrigerants that may be used with
that same valve. For example, a valve designed for R-502 may accommodate the following: R-507,
R-402a, and R-404a. All are acceptable replacements for R-502. Pressures, densities,
and enthalpies are all very similar and when necessary, any minor adjustments can be made on the valve.
The third item to consider is whether to use an external or internal equalized
expansion valve. In all applications the system evaporator will
determine this. If the evaporator uses a distributor, or if there is an excessive pressure drop
through the evaporator, an externally equalized valve will be needed. If needed, typically the
evaporator manufacturer will supply an equalizer connection at the end of the evaporator.
Something to remember: An expansion valve has
just one objective, to control superheat, making sure that all latent heat has been absorbed just before
exiting the evaporator.
Properly sized, properly installed, and properly adjusted, the expansion
valve can be the balance point for an efficient and reliable refrigeration system.
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