13
the suction line, use a commercial metering/expansion
device at the gauge manifold; remove liquid from the
cylinder, pass it through the metering device at the gauge
set and then pass it into the suction line as a vapor. Do not
remove Puron (R−410A) refrigerant from the cylinder as a
vapor.
Refrigerant Charge
The amount of refrigerant charge is listed on the unit’s
nameplate. Refer to Carrier Publication, “GTAC2−5
Charging, Recovery, Recycling and Reclamation Training
Manual” and the following procedures:
Unit panels must be in place when unit is operating during
the charging procedure. If unit is equipped with a head
pressure control device, bypass it to ensure full fan
operation during charging.
Charge checking and adjustments must be made while the
system is operating in Cooling only.
No Charge:
Use standard evacuation techniques for Puron (R−410A)
refrigerant.. After evacuating system, weigh the specified
amount of refrigerant.
THERMOSTATIC EXPANSION
VALVE (TXV)
All 50TCQ’s have a factory installed nonadjustable
thermostatic expansion valve (TXV). The TXV will be a
bi-flow, bleed port expansion valve with an external
equalizer. TXVs are specifically designed to operate with
Puron refrigerant, use only factory authorized TXVs.
See Fig. 15 for a typical piping schematic.
TXV Operation
The TXV is a metering device that is used in air
conditioning and heat pump systems to adjust to changing
load conditions by maintaining a preset superheat
temperature at the outlet of the evaporator coil. The
volume of refrigerant metered through the valve seat is
dependent upon the following (see Fig. 17):
1. Superheat temperature is sensed by the cap tube
sensing bulb on the suction tube at outlet of the
evaporator coil. This temperature is converted into
pressure by refrigerant in the bulb pushing downward
on the diaphragm which opens the valve using the
push rods. As long as this bulb and cap tube contain
any liquid refrigerant, this temperature is converted
into suction pressure pushing downward on the
diaphragm, which tends to open the TXV valve
through the push rods.
2. The suction pressure at the outlet of the evaporator
coil is transferred through the external equalizer tube
to the underside of the diaphragm.
3. The needle valve on the pin carrier is spring loaded,
exerting pressure on the underside of the diaphragm.
Therefore, the bulb pressure equals evaporator
pressure (at outlet of coil) plus spring pressure. If the
load increases, the temperature increases at the bulb,
which increases the pressure on the top side of the
diaphragm, pushing the carrier away from the seat,
opening the valve and increasing the flow of
refrigerant. The increased refrigerant flow causes
increased leaving evaporator pressure which is
transferred through the equalizer tube to the underside
of the diaphragm. This causes pin carrier spring
pressure to close the TXV valve. The refrigerant flow
is effectively stabilized to the load demand with a
negligible change in superheat.
OUTLET
INLET
PUSHRODS
DIAPHRAGM
NEEDLE
VALVE
SPRING
DISTRIBUTOR
FEEDER TUBES
CAPILLARY TUBE
COIL
BULB
EXTERNAL EQUALIZER TUBE
C12046
Fig. 17 − Thermostatic Expansion Valve (TXV) Operation
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