One design for automotive AC systems uses an orifice tube as the metering device. This system has been used for a long time by many manufacturers and is still being used on modern cars and trucks. As stated previously, the main purpose of the AC system is to remove heat from the cabin, which leaves cool and dry air that cools the cabin and passengers.
It takes 180 BTU to raise a pound of water from 32 to 212°F, but it takes 970 BTU to change 212°F water into a gas because of the large amount of heat absorbed during the latent heat of evaporation. This concept is why the warm air that is pushed through the evaporator core loses its heat and the result is very cool and dry air that is then directed to the passenger compartment through the mode
Orifice tubes are devices that are used to meter a fixed amount of refrigerant into the evaporator core. They can be located in the liquid line at some point after the condenser and before the evaporator core. These devices are available in several different sizes, which is why they are made to be different colors. The color of the orifice tube assists the individual who is servicing the system in choosing the correct size for replacing the original device. It is not advisable to change the color/size of the original orifice tube when replacing it during service procedures.
Orifice tubes are available in a variety of sizes that are designed by engineers to meter the correct amount of refrigerant into the evaporator core. The different colors represent the different sizes of the internal passage that the refrigerant must pass through during the operation of the AC system. Orifice tubes also have a screen that prevents contaminants from passing into the evaporator core. If the orifice tube screen is covered by a large number of foreign particles, then the system needs to be closely checked to determine where it came
The following chart gives the size of the opening for each color of orifice tube.
Orifice tubes can be located at any point between the exit side of the condenser and the entry point of the evaporator. Ease of assembly is the main factor when manufacturers choose the location to mount the orifice tube. Most orifice tubes are in a location where the line can be opened up to allow the orifice tube to be serviced. To locate the orifice tube, look for a spot on the liquid line that is a little larger in diameter than the rest of the
Variable orifice valves can be used in place of standard fixed orifice tubes because they are the same size and will fit in the same space. These devices vary the flow rate of refrigerant as the heat load changes in a similar way that a thermal expansion valve
The orifice tube is often located near the entry point of the evaporator. A pair of needle-nose pliers can usually be used to remove the orifice tube from the mounting location. Remember that the orifice tube is made of plastic and can be easily broken if it is stuck in place. A good penetrant or lubricant can be sprayed into the area to free up a stuck orifice
While most of the vehicles that use orifice tubes are like the ones described above, there are a couple of variations that need to be discussed. The variable orifice valve and the electronic orifice tube have been used in certain applications. Both of these types allow for some variable operation.
The variable orifice valve can be used in place of a standard orifice tube and is recommended for vehicles that idle for long periods of time, such as police cars and taxis. Electric orifice tubes are used on a few late-model vehicles, and they have the capacity to increase the flow rate under certain conditions when commanded by the vehicle’s electronic control module.
