Latent Heat
Latent heat is different from sensible heat because it is not able to be measured on a thermometer. Latent heat is a hidden heat that causes a substance to change states. For example, when water changes from a liquid to a gas, the latent heat of evaporation takes place. When water vapor or steam is cooled down and changes from a gas back to a liquid, the latent heat of condensation takes place.
These two processes are continually happening with the refrigerant in a functioning AC system. The last type of latent heat is when water turns into a solid and this is called the latent heat of fusion. This is not supposed to happen in a normally functioning AC system. However, if the AC system malfunctions, then ice may be seen forming on some of the low side components while in operation.
Heat quantity is a unit of measure for heat and there are two terms that are commonly used to measure this: the British thermal unit (BTU) and the calorie. The BTU is the amount of heat it takes to change 1 pound of water 1 degree Fahrenheit. The calorie is the amount of heat it takes to change 1 gram of water 1 degree Celsius.
As stated above, the refrigerant is continually moving between the liquid and vaporous states when the AC system is functioning. A key point to remember is that large quantities of heat are transferred during the changes of state of the refrigerant. It takes 180 BTU of heat to change 1 pound of water from 32°F to 212°F. It then takes 970 BTU to change the 212°F water into a vapor. On the contrary, 970 BTU of heat must be released from the water vapor to change it back to a liquid. The processes of great heat absorption during evaporation and great heat released during condensation is why the AC system is so efficient at moving heat out of the cabin.
Orifice Tube Systems
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 doors.
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 from.
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 line.
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 operates.
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 tube.
| Orifice Tube Sizes | ||
| Color | English (inches) | Metric (mm) |
| Green | 0.047 | 1.19 |
| Brown | 0.053 | 1.35 |
| Oranqe | 0.057 | 1.45 |
| Red | 0.062 | 1.57 |
| Black/Blue | 0.067 | 1.70 |
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.
Normal Operating Characteristics
Orifice tube AC systems are very effective designs that have been used for many years. The one drawback of this system design is that the orifice tube does not change the flow rate of refrigerant into the evaporator core, so the compressor needs to be cycled periodically to keep the evaporator from getting too cold and becoming covered with ice. There are two ways to control the output of the compressor: turn it off for short periods of time or change the internal displacement of the compressor.
The first of these strategies is typically called the cycling clutch orifice tube (CCOT) design. CCOT systems use pressure switches or temperature switches to monitor the conditions of the AC system, which then causes the AC compressor to be turned off to prevent evaporator freeze up. The cycle rate of these systems will vary depending on the ambient temperature and the humidity level. It is common to notice the compressor turning on and off during operation. A clicking sound will be heard in the engine area as the compressor is cycled, which is often accompanied by a slight change in engine RPM.
In recent years, vehicles that use orifice tubes have used compressors that are a variable-displacement style. Many variable-displacement compressors use mechanical operation to change the internal capacity of the compressor. This action was accomplished by using a valve that sensed the temperature of the suction side of the compressor and changing the angle of the wobble plate, which caused the internal pistons to change the length of the stroke.
Flow of Refrigerant Through the AC System
It is helpful to understand the refrigerant flow through an operable AC system. The refrigerant is continuously moving and changing pressures and states while the system is running. Here is a step-by-step description of what is happening to the refrigerant as it moves through the components of the AC system.
1. The compressor pulls in low-pressure refrigerant