SiBE811001EB TroubleshootingCONVENI-PACK 3189. Basic Control9.1 Mounting of Feeler Bulb9.1.1 Functions of Thermostatic Expansion Valve1. Functions of thermostatic expansion valve2. Operation principle of thermostatic expansion valveThe thermostatic expansion valve reduces thepressure of high pressure liquid refrigerant fromthe condenser to turn it to low pressure liquidrefrigerant (mixed with a small amount of vapor),thus feeding this refrigerant to theevaporator.The feeler bulb and the upper part of diaphragm onthe main unit of expansion valve is connected andsealed with the capillary tube.Connect the externalequalizer pipe on thedownstream side of feelerbulb of the suction pipe.The feeler bulb part is charged with the samerefrigerant as that used for the refrigeratingunit, in general.The feeler bulb is mounted in the evaporatoroutlet piping, thus sensing the evaporator outlettemperature and exerting a force to open theexpansion valve.Capillary tubeExternal equalizer pipeFeeler bulbLow pressure liquid refrigerantEvaporatorDiaphragmEvaporatingpressureTo evaporatorRefrigerant inletCondenserP1P2P3Main unit ofexpansion valveSpringpressureHigh pressureliquid refrigerantInternal pressure offeeler bulbDiaphragmEvaporatingpressureTo evaporatorRefrigerant inletThis expansion valve introduces the evaporatingpressure (P2 ) to the lower part of the diaphragm as avalve closing force as opposed to the valve openingforce (P1 ) exerted on the upper part of the diaphragm.Furthermore, this valve incorporates a spring, whichexerts the valve closing force (P3 ).If the upper and lower forces of the diaphragm keepequilibrium,P1 = P2 + P3P1 : Charged fluid pressure (Upper part of diaphragm +Inside of feeler bulb)As the temperature becomes higher, the valveopening force becomes larger.P 2 : In the case of external equalizer pipe·················· Evaporator outlet pressureIn the case of internal equalizer pipe········· Expansion valve outlet pressureAs the pressure becomes higher, the valve closingforce becomes larger.P 3 : Spring force (superheated degree setting spring)ConstantSpringpressureP 1P 2P 3
