TACLB004469C

Houston Air Conditioning & Heating Service News

SUPERHEAT

Houston Air Conditioning & Heating Service
08-26-2006

The Superheat of Metering Devices

 The two most common metering devices used in air conditioning and refrigeration systems are the thermostatic expansion valve and fixed orifice.  Each of these metering devices is charged differently.  The thermostatic expansion valve will open and close with the superheat of the evaporator and the fixed orifice will never change in size by itself. 

 The superheat of a thermostatic expansion valve is normally set between 8º to 12º and can vary with the design of a system.  To measure superheat for a thermostatic expansion valve, the measurement must be obtained at the leaving refrigerant line of the evaporator, where the thermostatic expansion valve bulb is located.

 An air conditioning and refrigeration system with a fixed orifice is charged to the superheat of the suction line leaving the evaporator. A fix orifice system has a critical charge.  The smaller the system is the more critical is the charge of refrigerant.  A fix orifice itself has no means to adjust the flow of refrigerant to maintain superheat.  The only adjustment for superheat is the charge of refrigerant.  There are two reasons that cause a change in superheat in fix orifice system: 1) the amount of refrigerant entering the evaporator and 2) the total heat of the air entering the evaporator. Force and load set the superheat.

 Force is the pressure of the high side, forcing the refrigerant into the fix orifice so it can be measured by the outside air temperature at the condenser.  Load is the total heat of the air entering the evaporator and can be measured by the wet bulb temperature.  Wet bulb temperature is an indication of the total enthalpy of the air.  When the outside air temperature changes or the heat (enthalpy) of entering air to the evaporator change, the superheat changes.  Superheat is the gas temperature above the saturated temperature.