What does the expansion valve do in an air conditioner?
The Thermal Expansion Valve is responsible for metering the correct amount of refrigerant to the evaporator. The thermostatic expansion valve (TXV) meters the refrigerant to the evaporator by using a thermal sensing element to monitor the superheat. This valve opens or closes in response to a thermal element. The thermal expansion valve maintains a constant superheat in the evaporator. The evaporator performs best when it is as full of liquid refrigerant as possible with none left in the suction line. Liquid refrigerant that enters the suction line may reach the compressor because only small amount of heat is added in the suction line, and cause catastrophic damage.
The TXV responds to a change in a load in the following manner:
When the load is increased, the TXV opens, allowing more refrigerant into the coil. The evaporator needs more refrigerant at this time, because the increased load is evaporating the refrigerant in the evaporator faster. The suction pressure increases.
When there is a load decrease, the liquid in the evaporator evaporates more slowly and the suction pressure decreases. At this time the TXV will throttle back by slightly closing the needle and seat to maintain the correct superheat.
What are the components of a Thermal Expansion Valve?
The TXV consists of the valve body, diaphragm, needle and seat, spring, sensing bulb and transmission tube.
The VALVE BODY: The valve body is an accurately machined piece of solid brass or stainless steel that holds the rest of the components and fastens the valve to the refrigerant piping circuit.
NEEDLE AND SEAT: The needle and seat control the flow of refrigerant through the valve. They are used in a metering device to maintain close control of the refrigerant, and normally made of some type of very hard metal, such as stainless steel, to prevent the passing refrigerant from eroding the seat.
The size of the needle and seat determines how much liquid refrigerant will pass through the valve under a specific pressure drop.
THE SPRING: The spring is one of the three forces that act on the diaphragm. It raises the diaphragm and closes the valve by pushing the needle into the seat.
THE SENSING BULB AND TRANSMISSION TUBE: The sensing bulb and transmission line are extensions of the valve diaphragm. When the suction line temperature goes up, the temperature change occurs inside the bulb as well. As the temperature of the taxís sensing bulb increases, the valve will gradually open ,letting more refrigerant into the evaporator. This increases both the evaporating pressure and the compressorís suction pressure, while attempting to reduce the amount of superheat in the evaporator. When the pressure changes the transmission line allows the pressure between he bulb and diaphragm to equalize back and forth.
One side of the diaphragm gets its pressure from the bulb, and the other side gets its pressure from the evaporator and the spring.
The diaphragm moves up and down in response to three different pressures, which act at the proper time to open, close, or modulate the valve needle between open and close. The three pressures are:
The bulb pressure.
The evaporator pressure.
The spring pressure.
What are the Types of bulb charge?
The fluid inside the expansion valve bulb is known as the charge of the valve. Four types of charge can be used in the TXV:
Cross liquid charge
Cross vapor charge
The liquid charge bulb: The liquid charge bulb is charged with a fluid characteristic of the refrigerant in the system. The diaphragm and bulb are not actually full of liquid but they have enough so as to always have some liquid inside them. The liquid will not entirely boil away.
The cross liquid charge bulb: The cross liquid charge bulb is filled with a fluid that is different from the system fluid. It does not follow the systemís temperature/pressure relationship but has a flatter curve and will close the valve faster on a rise in evaporator pressure.
The vapor charge bulb, or gas-charged bulb, has only a small amount of liquid refrigerant in the bulb. It is sometimes called a critical charge bulb. When the bulb temperature rises, more and more of the liquid will boil to a vapor until there is no more liquid. When this point is reached, an increase in temperature will no longer bring an increase in pressure.
The cross vapor charge bulb: The cross vapor charge bulb is similar to the vapor charge bulb, but is charged with a fluid different from the refrigerant in the system. This creates a different pressure and temperature relationship under different conditions. These special valves are applied to special systems.
What are the symptoms of a bad expansion valve?
A defective capillary tube would most likely cause the TXV to remain closed, causing poor cooling at all times.
An overcharged system will cause higher suction pressures and poor cooling at all times.
Moisture in the ac system causes the TXV to freeze. When the Expansion Valve freezes, refrigerant flow is reduced and cooling is also reduced.
How to determine what type of metering device is installed in your car?
When trying to determine if an A/C system uses a fixed orifice tube or TXV,look for an accumulator(FOT systems) or receiver drier(TXV systems).
Where is the air conditioning expansion valve?
Thermal Expansion valve is mounted at the inlet to the evaporator and separates the high-pressure side of the system from the low-pressure side of the system. In most cases you could get access to TXV on the firewall, facing the engine compartment. In some cases the TXV is inside the hvac box connected to the evaporator inlet making it really hard to reach for service or repair.
How much does it cost to replace an AC expansion valve?
The cost for replacing the Thermal Expansion valve will depend on the Make, Model and Year of the vehicle and also the location of the thermal expansion valve: accessible from the engine compartment or deep underneath the dashboard inside the hvac box. The price range is from about $150.00 to $800.00.In case we need to service a faulty TXV, and is the less accessible design is always a good idea to replace the evaporator core too.
How to properly install the Thermal expansion valve sensing bulb ?
Particular care should be taken when installing the expansion valve sensing element. The valve sensing bulb has to be mounted at the end of the evaporator on the suction line. The recommended location for the thermal bulb on smaller suction lines is near the top of the line.
The best location for larger suction lines is near the bottom of the line on horizontal run where the bulb can be mounted flat and not be raised by a fitting.
The bulb should no be located at the bottom of the line because returning oil will act as an insulator to the sensing element. The objective of the sensing element is to sense the temperature of the refrigerant gas in the suction line. To do this, the line should be very clean and the bulb should be fastened to this line very securely. Normally, the manufacturer suggests that the bulb be insulated from the ambient temperature if the ambient temperature is much warmer than the suction-line temperature, because the bulb will be influenced by the ambient temperature as well as by the line temperature.