Thermostatic Expansion Valve (Txv) - Carrier 50HCQA05 Service And Maintenance Instructions

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
PuronR 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
DIAPHRAGM
INLET
PUSHRODS
FEEDER TUBES
OUTLET
NEEDLE
VALVE
SPRING
DISTRIBUTOR
Fig. 17 - - Thermostatic Expansion Valve (TXV) Operation
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 sens-
ing bulb on the suction tube at outlet of the evaporat-
or coil. This temperature is converted into pressure by
refrigerant in the bulb pushing downward on the dia-
phragm 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 pres-
sure (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 refriger-
ant. The increased refrigerant flow causes increased
leaving evaporator pressure which is transferred
through the equalizer tube to the underside of the dia-
phragm. This causes pin carrier spring pressure to
close the TXV valve. The refrigerant flow is effect-
ively stabilized to the load demand with a negligible
change in superheat.
CAPILLARY TUBE
COIL
EXTERNAL EQUALIZER TUBE
13
BULB
C12046