Abstract: A thermostatic expansion valve adapted for refrigerant flow at low flow rates includes a body (12) with an inlet (14) and an outlet (16). The body includes an orifice (24) and a cylindrical chamber (20). A movable metering pin (26) includes a needle head portion (28) which is movable into said orifice. A power element assembly (46) is operative to move the metering pin to enable refrigerant flow through the orifice. The metering pin also includes a cylindrical body portion (30) positioned in the cylindrical chamber of the body. An outer wall of the cylindrical body portion of the metering pin is disposed from an inner wall of the cylindrical chamber to form an accurately sized yet very small opening. The valve enables accurate superheat control at low flow rates through the valve.

Abstract: When the valve is closed, the pressure differential across the valve port area acts to hold the valve on its seat and must be overcome to open the valve. At this time the gap between the armature and the core of the solenoid is at its greatest and, therefore, the magnetic force available upon energization of the coil is at its lowest. Due to the lost motion connection between the armature and the valve, the magnetic force available is required to only lift the armature and compress the armature return spring and the valve spring and, therefore, need not overcome the pressure differential acting across the area of the port. When the armature has almost reached the core, the armature impacts on the head of the valve and the kinetic energy of the armature plus the now increased magnetic force (by reason of decreased gap) are available to unseat the valve. After the valve is unseated and the armature seats on the core, the valve spring continues the valve opening motion to fully open the valve.