Thermostatic Expansion Valve (Txv); Txv Operation; Replacing Txv - Carrier WeatherMaker 50TCQ 17-24 Series Service And Maintenance Instructions

DIAPHRAGM
INLET

THERMOSTATIC EXPANSION VALVE (TXV)

All 50TCQ units have a factory installed nonadjustable thermo-
static expansion valve (TXV). The TXV will be a bi-flow, bleed
port expansion valve with an external equalizer. TXVs are specifi-
cally designed to operate with Puron
authorized TXVs. See Fig. 19.

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 main-
taining a preset superheat temperature at the outlet of the evapora-
tor coil. The volume of refrigerant metered through the valve seat
is dependent upon the following.
1. Superheat temperature is sensed by the cap tube sensing bulb
on the suction tube at outlet of the evaporator coil. This tem-
perature 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 suc-
tion 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 under-
side 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 evapora-
tor 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.
PUSHRODS
FEEDER TUBES
OUTLET
NEEDLE
VALVE
SPRING
DISTRIBUTOR
Fig. 19 — Thermostatic Expansion Valve (TXV) Operation
refrigerant; use only factory
®
CAPILLARY TUBE
COIL
EXTERNAL EQUALIZER TUBE

Replacing TXV

PERSONAL INJURY HAZARD
Failure to follow this caution can result in injury to personnel
and damage to components.
Always wear approved safety glasses, work gloves and other
recommended Personal Protective Equipment (PPE) when
working with refrigerants.
1. Recover refrigerant.
2. Remove TXV support clamp using a
3. Remove TXV using a backup wrench on connections to pre-
vent damage to tubing.
4. Remove equalizer tube from suction line of coil. Use file or
tubing cutter to cut brazed equalizer line approximately
2 inches above suction tube.
5. Remove bulb from vapor tube inside cabinet.
6. Install the new TXV and avoid damaging the tubing or the
valve when attaching the TXV to the distributor.
7. Attach equalizer tube to suction line. If coil has mechanical
connection, then use wrench and back up wrench to attach. If
coil has brazed connection, use file or tubing cutters to
remove mechanical flare nut from equalizer line. Then use
coupling to braze the equalizer line to stub (previous equal-
izer line) in suction line.
8. Attach TXV bulb in the same location as original (in the sens-
ing bulb indent), wrap bulb in protective insulation and
secure using the supplied bulb clamp. See Fig. 20.
9. Route equalizer tube through suction connection opening
(large hole) in fitting panel and install fitting panel in place.
10. Sweat inlet of TXV marked "IN" to liquid line. Avoid exces-
sive heat which could damage valve.
11
BULB
CAUTION
/ -in. nut driver.
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