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Q)
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How is a T.E.V. selected?
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A)
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The selection depends on the refrigeration duty, the
evaporator pressure and the pressure drop across the
T.E.V. Should the discharge (head) pressure fall this
will reduce the pressure drop and the flow rate through
the valve. This will result in a reduction in evaporator
pressure and refrigeration duty. This is why on some
systems head pressure control is used to maintain the
pressure drop. This prevents the evaporator pressure
falling below 0 C and so causing the coil to freeze.
Refer to FAQ's -general topics
& FAQ's - pressure
regulators
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Q)
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If the valve has an external equalising connection
must it be connected?
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A)
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Yes, or the valve will not regulate.
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Q)
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Why use a liquid receiver in a system fitted with
a TEV?
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A)
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When set up correctly the TEV allows sufficient refrigerant
into the evaporator, such that it will all boil off
and only allow superheated gas to return to the compressor.
Under high load conditions there would be a smaller
amount of refrigerant in the evaporator so to store
the excess requires a liquid receiver. If the receiver
was not used the liquid would backup in the condenser
making it inefficient during this period.
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Q)
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Why is a liquid receiver not fitted with a capillary
tube application?
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A)
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Normally a capillary tube is used where a relatively
constant load is being cooled, e.g in a domestic refrigerator,
ice cream freezer, bottle cooler and many display cases.
To allow for when a sudden high load occurs and liquid
is carried over into the suction line, a suction line
accumulator is fitted to catch any liquid before it
enters the compressor. The refrigerant charge of a capillary
tube system is critical and so it must be carefully
weighed in during manufacture or servicing.
Note:
It is advisable to position the drier pointing downwards
with the capillary outlet at the bottom. This prevents
the drier acting as a liquid receiver and the possibility
of overcharging the system.
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Q)
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Are Parker T.E.V.'s factory set?
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A)
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Yes, Parker
valves are set with a static superheat of 8 F ( 4.4
C ). The static superheat is the amount to just start
opening the valve. Should it be necessary to alter the
superheat setting then 1 full turn clockwise will give
an increase of approx. 3 F (1.7 C). A normal operating
superheat for most systems should be approx. 15 F (
8.3 C ) when measured close to the suction connection
on the compressor.
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