An internally equalized TXV senses its outlet pressure, using it as a closing force on the underside of the powerhead’s diaphragm to offset the opening force of the pressure from the temperature sensing bulb on the top side of the powerhead. These valves are typically used on evaporators with a relatively low pressure drop…ie, the outlet pressure is nearly the same as the inlet pressure.
An externally equalized valve senses the pressure at the evaporator outlet through an external equalizer tube, which runs to the same location as the valves temperature sensing bulb, compensating for any pressure drops of the evaporator, the refrigerant distributor and distributor tubes.
An internally equalized valve used on an evaporator with a high pressure drop, the valve will tend to control at a higher superheat, starving the coil
An externally equalized valve should be used on any evaporator with a multiple refrigerant circuits and a distributor because the typical refrigerant distributor and distributor tubes will have approximately a 35 psi pressure drop.
On evaporators without a distributor, if the pressure drop through the evaporator is greater than about 3 psi, then an externally equalized valve should be selected.
A good rule of thumb for the field service tech is you can always use an externally equalized valve, so long as an equalizer tube is installed and connected as well. Never cap off the external equalized port of a TXV.
Never replace an externally equalized TXV with an internally equalized valve.
It’s a commonly thought that an externally equalized valve will allow a system to equalize the high and low side pressure during the off-cycle.
This is totally false. The external equalizer function is solely to sense evaporator outlet pressure. If equal system pressures are required, then some TXV models may be be ordered with an internal bleed port.
On commercial refrigeration evaporator coils, an external equalizer tube will usually be provided on coils requiring an externally equalized TXV. They are often connected to the suction header or near the suction or outlet connection. If an equalizer tube is needed but not provided, locate it within 6″ upstream of the sensing bulb location, on the top side of the tubing, and away from oil traps.
An external equalizer tube should have no flow though it because it’s only there to sense pressure, so it should be neither hot nor cold. If it is found to be sweating or frosted, this would be an indication that the valve is worn around the push rods, allowing high pressure liquid to pass. This valve body should be replaced.