How Metering Devices Work
Air conditioners use a closed cycle that repeats itself. For the cycle to repeat itself, what ever is done must later be undone. If pressure is increased, it must be decreased. If heat is absorbed it must be rejected. Just as the compressor increased the temperature and pressure of the refrigerant entering it, another component is responsible for reducing the pressure and temperature of the refrigerant in the system. This component is the metering device, also referred to as the expansion device.
The metering device is responsible for feeding the proper amount of refrigerant to the evaporator coil. The refrigerant that enters the metering devices is a high temperature, high pressure, subcooled liquid that leaves the devices as a low-temperature, low-pressure saturated liquid. The saturated liquid is roughly 80% liquid and 20% vapor, and, since the refrigerant is saturated, there is a pressure/temperature relationship for the refrigerant. The metering devices creates a restriction for the flow of the refrigerant. The reduction in pressure after refrigerant passes the expansion device is created by the combined effects of the metering device and the compressors suction.
There are three types of expansion devices: capillary tubes, automatic expansion valves, and thermostatic expansion valves.
The capillary tube is a fixed-bore device, meaning that the opening the refrigerant flows through does not change in size. Capillary tubes are found on critically charged systems. Critically charged systems have an exact quantity of refrigerant and all of the refrigerant is moving through the system at all times.
Capillary tube systems operate differently from other systems that store refrigerant until it is needed by the evaporator. So the amount of refrigerant in the system must be to the manufacturer’s specifications because there needs to be an exact amount of refrigerant. During operation as the high-temperature-high-pressure-subcooled-liquid-refrigerant from the liquid line enters the capillary tube its flow is restricted through the device. As the refrigerant flows through the capillary tube, its pressure begins to drop and some of the liquid boils into a vapor. When the refrigerant reaches the end of the capillary tube, the pressure of the refrigerant has dropped significantly. When it leaves the capillary tube as a saturated liquid, it expands because the tubing size of the evaporator is larger, and the pressure drops down to the desired evaporator saturation pressure.
More liquid refrigerant boils into a vapor upon leaving the capillary tube and helps cool the remaining vapor by absorbing heat from it. The boiling refrigerant is referred as flash gas because it immediately flashes into a vapor. This flashing of the liquid to a vapor occurs within the capillary tube as well as its outlet. Although flash gas helps the system operate efficiently, too much is an indication of a system problem.
The automatic expansion valve modulates the flow of the refrigerant into the evaporator to keep the evaporator pressure constant. Unlike the capillary tube, which cannot adjust the flow of refrigerant, the automatic expansion valve opens and closes to either increase or decrease the amount of refrigerant feeding into the evaporator in response to the pressure of the refrigerant in the evaporator.
The valve operates on a “needled and seat” mechanism that changes the amount of refrigerant that is able to pass through the valve. The position of the needle is determined by the difference between two pressures:
* The spring pressure
* The evaporator pressure
The spring pressure is the pressure that opens the valve. This pressure is adjustable and is set at the desired evaporator pressure. The higher the spring pressure, the higher the evaporator pressure will need to be for refrigerant to flow. The evaporator pressure is the pressure that closes the valve. The spring and evaporator pressures push against each other in opposite directions. Between the spring pressure and evaporator pressure is the “needle and seat assembly”. When there’s higher evaporator pressure than the spring pressure then the needle will “close in to the seat” allowing less refrigerant through the system. The lower the evaporator pressure the more refrigerant will flow. The hotter the space is that needs to be cooled the higher the evaporator pressure will be which will allow less refrigerant through and of course if its cooler then the pressure will be lower, and will allow more refrigerant through. The automatic expansion valve is ideal for systems that experience a constant heat load as opposed to systems that operate under a wide range of conditions.
The thermostat expansion valve is a modulating valve that opens and closes in order to feed the proper amount of refrigerant to the evaporator. Unlike the capillary tube, which does not modulate fluid flow, and the automatic expansion valve, which is designed to maintain a constant pressure in the evaporator, the thermostatic expansion valve is designed to maintain a constant evaporator superheat. The device modulates so that it can change for external conditions to regulate the flow of refrigerant. The thermostatic expansion valve operates on a “seat and needle” concept that is very similar to the automatic expansion valve. The main difference is that the TXV closes as the system load is reduced while the AEV closes as the system load is increased.
To understand how the thermostatic expansion valve operates, it’s important to understand how evaporator superheat is measured. Superheat is the amount of sensible heat (sensible heat is just measurable heat) that is added to the refrigerant after it has boiled off into a vapor. Superheat is calculated by subtracting the evaporator saturation temperature from the evaporator outlet temperature.
Refrigerant entering the compressor needs to be 100% vapor to prevent component damage. The thermostatic expansion valve ensures that this happens. The amount of superheat that will be maintained by the valve depends on the setting of the superheat spring and the size of the valve.
The thermostatic expansion valve’s “needle and seat” is controlled by three pressures that position the needle properly to feed the correct amount of refrigerant to the evaporator. The pressures push on a diaphragm, which is a thin and very flexible piece of steel, whose position determines the position of the needle in the seat. The three pressures are:
*Evaporator pressure
*Spring Pressure
*Bulb Pressure
Evaporator Pressure
The evaporator pressure is one of the pressures that helps close the valve. It attempts to push the needle into the seat to reduce the flow of refrigerant in to the evaporator. The evaporator pressure can be taken from either the inlet or the outlet of the coil. If the pressures drop (difference from the inlet and outlet pressure) across the evaporator coil is small, the inlet and outlet pressures are relatively close to each other. A Thermostatic expansion valve that senses pressure at the inlet of the evaporator is called an internally equalized valve; a thermostat expansion valve that senses the evaporator outlet pressure is called an externally equalized valve.
Spring Pressure
The spring pressure, also known as the superheat spring pressure, determines how much superheat the evaporator will open with. The higher the spring pressure, the higher the amount of superheat. The spring comes factory set and should only be adjusted by trained professionals, because improperly adjusted superheat springs can cause major system damage, including compressor failure. The spring pressure is the other pressure that closes the valve, reducing the amount of refrigerant flowing into the evaporator.
The evaporator pressure and the spring pressure both deliver the closing pressure for the valve.
Bulb Pressure
The bulb pressure is the only pressure that opens the valve. This pressure is generated inside a thermal bulb that is mounted at the outlet of the evaporator. The line that connects the thermal bulb to the thermostatic expansion valve is called the transmission line. The thermal bulb is refrigerant filled and for the most part follows a pressure/temperature relationship. It is the thermal bulb that senses the evaporator outlet temperature. The refrigerant in the thermal bulb is isolated from that of the system so no mixing takes place. The refrigerant in the bulb exerts a specific amount of pressure depending on its temperature that pushes down on the diaphragm, opposing the evaporator and spring pressures.