Carrier 50TC*17 Series Service And Maintenance Instructions

17THERMOSTATIC EXPANSIONVALVE (TXV)All two−stage 50TC units with Humidi−Mizer have afactory installed nonadjustable thermostatic expansionvalve (TXV). The TXV will be a bi-flow, bleed portexpansion valve with an external equalizer. TXVs arespecifically designed to operate with Puron or R-22refrigerant, use only factory authorized TXVs. See Fig.20.TXV OperationThe TXV is a metering device that is used in airconditioning and heat pump systems to adjust to changingload conditions by maintaining a preset superheattemperature at the outlet of the evaporator coil.The volume of refrigerant metered through the valve seatis dependent upon the following:1. Superheat temperature is sensed by the cap tube sens-ing bulb on suction the tube at the outlet of evapora-tor coil. This temperature is converted into pressureby refrigerant in the bulb pushing downward on thediaphragm which opens the valve using the push rods.2. The suction pressure at the outlet of the evaporatorcoil is transferred through the external equalizer tubeto 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 the evaporatorpressure (at the outlet of the coil) plus the spring pres-sure. If the evaporator load increases, the temperatureincreases at the bulb, which increases the pressure onthe topside of the diaphragm, pushing the carrieraway from the seat, opening the valve and increasingthe flow of refrigerant. The increased refrigerant flowcauses increased leaving evaporator pressure whichis transferred through the equalizer tube to the under-side of the diaphragm. This causes the pin carrierspring pressure to close the TXV valve. The refriger-ant flow is effectively stabilized to the load demandwith a negligible change in superheat.Replacing TXVPERSONAL INJURY HAZARDFailure to follow this CAUTION can result in injuryto personnel and damage to components.Always wear approved safety glasses, work gloves,and other recommended Personal ProtectiveEquipment (PPE) when working with refrigerants.CAUTION1. Disconnect all AC power to the unit. Use approvedlockout/tagout procedures.2. Using the gauge set approved for use with Puron (R−410A) refrigerant, recover all refrigerant from thesystem.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 ofthe coil. Use a tubing cutter to cut the brazed equaliz-er line approximately 2 inches (50 mm) above thesuction tube.7. Remove the bulb from the vapor tube above the evap-orator coil header outlet.8. Install the new TXV; avoid damage to the tubing orthe valve when attaching the TXV to the distributor.Protect the TXV against over−temperature conditionsby using wet rags and directing the torch flame tipaway from the TXV body. Connect the liquid line tothe TXV inlet by repeating the above process.9. Attach the equalizer tube to the suction line. If thereplacement TXV has a flare nut on its equalizer line,use a tubing cutter to remove the mechanical flare nutfrom the equalizer. Then use a coupling to braze theequalizer line to the stub (previous equalizer line) inthe suction line.10. Attach the TXV bulb in the same location as the orig-inal (in the sensing bulb indent), wrap the bulb in pro-tective insulation and secure using the supplied bulbclamp. See Figs. 19 and 21.DIRECTION OFREFRIGERANT FLOWEQUALIZER TUBEFROM TXV VALVECAPILLARY TUBETO TXV15.9 mm(REF)TXV SENSOR BULBSENSOR BULBINDENTSENSOR BULBINSULATIONC10372Fig. 19 − TXV Sensor Valve Insulation11. Route the equalizer tube through the suction connec-tion opening (large hole) in the fitting panel and in-stall the fitting panel in place.12. Sweat the inlet of the TXV marked “IN” to the liquidline. Avoid excessive heat which could damage thevalve.13. Check for leaks.14. Evacuate the system completely and then recharge.15. Remove the lockout/tagout on the main power switchand restore power to the unit.16. Complete the charging procedure.Refrigerant System Pressure Access PortsThere are two access ports in the system: on the suctiontube near the compressor, and on the discharge tube nearthe compressor. These are brass fittings with black plasticcaps. The hose connection fittings are standard 1/4 SAEmale flare couplings. See Fig. 15.The brass fittings are two-piece High Flow valves, with areceptacle base brazed to the tubing and an integralspring-closed check valve core screwed into the base. Thischeck valve is permanently assembled into this core body