An orifice or a capillary tube (used in smaller home type refrigerators) and thermal expansion valves are all calibrated restrictions to cause a pressure differential between the high side and the low side. They meter the amount of liquid refrigerant to flood the evaporator so it can boil off and absorb heat with out flooding back to the compressor.

On your car there is a large aluminum can (accumulator) mounted on the firewall between the evaporator and the compressor to allow excess liquid refrigerant to be captured and allowed to boil off.
Because the orifice is a fixed or variable opening device (governed by pressures) and cannot adjust as it does not sense anything like a thermal expansion device can. Both orifice and capillary systems are charge specific. It must be charged with an exact amount in order to balance how much is kept on both sides of the expansion device.

Your system depends on the proper charge and the low side pressure switch mounted on the accumulator.


The orifice device is sized for the type refrigerant and the systems (both coils) capability to move X amounts of BTU’s per hour with a fixed compressor size.
The orifice system is a very closely engineered system done in testing labs under controlled conditions.

Take refrigerators, they are sold geographically. There are units for tropical areas versus the other hemisphere changes. This is done with the amount of charge and the device tube lengths. If you have bought a recent refrigerator you can (now) read the temperature range limits in the manuals. They watch every watt and disclose it, at least in the United States, that is. They really do not want you to put one in a hot garage anymore!

The fixed charge system is cheaper to manufacture when compared to the thermal expansion valves systems that allowed for more adjustments for humid climates and variances in initial charge in a commercial setting.

Earlier systems had sight glasses and a liquid receiver than held a small amount excess charge for fluctuations caused by wide swings in evaporator heat/head pressure loads (cars have those big time) without disrupting compressor operation.
Especially those with electric motors, the run of thumb, it is not nice to stop and start any electric motor more than 15 to 20 times an hour as the windings can overheat, let alone relays wear out quicker.

This should answer a thought that it is a “systems design perimeters,” that govern the expansion device used. Each has a specific purpose.

Phil