Parker Hannifin
Thermostatic
Expansion Valves
Function
The thermostatic expansion valve is a metering device.
It regulates the flow of high pressure liquid refrigerant
to the low pressure evaporator in response to the heat
load on the evaporator.
The valve's primary function is to maintain maximum
evaporator efficiency and to protect the compressor by
preventing liquid refrigerant from passing through the
suction line to the compressor. It does this by
controlling th flow of liquid refrigerant into the
evaporator so that it equals the rate at which the liquid
is completely vaporized by the heat load on the
evaporator. The resulting suction gas is superheated. The
thermostatic expansion valve controls flow to maintain a
predetermined level of superheat.
Superheat
Superheat is the difference between the refrigerant
temperature and the saturation temperature corresponding
to the refrigerant pressure. Superheat can be broken down
into three categories:
Static Superheat is the amount of superheat
required to move the ball of the valve so that it is
just cracked of the seat.
Opening Superheat is the difference between
static superheat and full open.
Operating Superheat is the superheat at
which the valve operates at normal running conditions
or nominal capacity.
The reserve capacity part of the curve in the chart
below shows the increased flexibility of Parker's
balanced port, multi-range design. Many valve designs
attempt to overcome the lack of a balanced port by
increasing the diaphragm area. However, the balanced port
provides considerably more flexibility. The Parker Power
Piston design allows the user to reach high indoor load
conditions despite reduced pressure drop across the
orifice because of decreased ambient and/or lower head
pressures.
Charges
Power elements may be system charged (charged with the
same refrigerant used in the system) or cross charged
(refrigerant different from that used in the system).
The Parker "W" liquid cross charge can be
used with evaporator temperatures from -40' to +50' F
(-40* to +10' C). Unlike conventional cross charges, the
"W" charge maintains a nearly constant
superheat throughout this range of evaporator
temperatures. A liquid charged bulb maintains control
even when the power element is colder than the bulb.
MOP Charges - The Parker X gas cross charge is
available with pressure limiting characteristics (MOP).
Maximum Operating Pressure (pressure limit) charges are
used to limit the flow on start up to prevent flooding
and/or compressor overload. Valves with a MOP charge
should not be used where the power element could get
colder than the thermal bulb. Migration of the charge to
the power element can occur causing loss of control.
All "W" and "X" charges will
control a constant superheat down to -40' F (-40* C)
evaporator temperature. However, if MOP charges are
employed, the table below lists the oneratinn evanorator
temperature range.
Recommended thermostatic valve chages
are listed in the table below:
Equalizers
The equalizer passageway is the link from the
evaporator to the under side of the diaphragm (Force 2,
see page 5). Internally equalized valves are effective in
applications where the pressure drop between the valve
outlet and the evaporator outlet is negligible. When
there is a high pressure drop (over 3 psi) or a
distributor is used, externally equalized valves should
be used.
Off-Cycle Unloading (Bleed Type)
Low starting torque or permanent split capacitor
motors require expansion valves with off-cycle unloading
capability. This reduces high to low side pressure
differential. Parker valves with off-cycle unloading
capability are identified by the letter B (bleed
orifice). This type of valve should be employed only when
the original equipment manufacturer has specified their
use. For bleed type valves, contact the factory.
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