Thermostatic Expansion Valve (Txv); Txv Operation - Carrier WeatherMaker 50TC 17-30 Series Service And Maintenance Instructions

THERMOSTATIC EXPANSION VALVE (TXV)

All two-stage 50TC units with Humidi-Mizer 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 or R-22 refrigerant, use only factory authorized
TXVs. See Fig. 21.
DIAPHRAGM
PUSHRODS
FEEDER TUBES
INLET
OUTLET
NEEDLE
VALVE
SPRING
DISTRIBUTOR
Fig. 21 — Thermostatic Expansion Valve (TXV)
Operation

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 de-
pendent upon the following:
1. Superheat temperature is sensed by the cap tube sensing bulb
on suction the tube at the outlet of 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.
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, exert-
ing pressure on the underside of the diaphragm. Therefore,
the bulb pressure equals the evaporator pressure (at the out-
let of the coil) plus the spring pressure. If the evaporator
load increases, the temperature increases at the bulb, which
increases the pressure on the topside of the diaphragm,
pushing the carrier away from the seat, opening the valve
and increasing the flow of refrigerant. The increased refrig-
erant flow causes increased leaving evaporator pressure
which is transferred through the equalizer tube to the under-
side of the diaphragm. This causes the 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.
CAPILLARY TUBE
COIL
BULB
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. Disconnect all AC power to the unit. Use approved lockout/
tag-out procedures.
2. Using the gage set approved for use with Puron (R-410A)
refrigerant, recover all refrigerant from the system.
3. Remove the TXV support clamp.
4. Disconnect the liquid line at the TXV inlet.
5. Remove the liquid line connection at the TXV inlet.
6. Remove the equalizer tube from the suction line of the coil.
Use a tubing cutter to cut the brazed equalizer line approxi-
mately 2-in. (50 mm) above the suction tube.
7. Remove the bulb from the vapor tube above the evaporator
coil header outlet.
8. Install the new TXV; avoid damage to the tubing or the valve
when attaching the TXV to the distributor. Protect the TXV
against over-temperature conditions by using wet rags and
directing the torch flame tip away from the TXV body. Con-
nect the liquid line to the TXV inlet by repeating the above
process.
9. Attach the equalizer tube to the suction line. If the replace-
ment TXV has a flare nut on its equalizer line, use a tubing
cutter to remove the mechanical flare nut from the equalizer.
Then use a coupling to braze the equalizer line to the stub
(previous equalizer line) in the suction line.
10. Attach the TXV bulb in the same location as the original (in
the sensing bulb indent), wrap the bulb in protective insula-
tion and secure using the supplied bulb clamp. See Fig. 22
and 23.
11. Route the equalizer tube through the suction connection
opening (large hole) in the fitting panel and install the fitting
panel in place.
12. Sweat the inlet of the TXV marked "IN" to the liquid line.
Avoid excessive heat which could damage the valve.
13. Check for leaks.
14. Evacuate the system completely and then recharge.
15. Remove the lockout/tag-out on the main power switch and
restore power to the unit.
16. Complete the charging procedure.
16
CAUTION