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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 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.
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 position of the needle is determined by the difference between two pressures:.
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. There are two kinds of metering devices, thermal expansion valves TXV and capillary tubes.
Either one does the same thing; they lower the high-pressure liquid pressure by forcing it through a small hole or nozzle. To adjust the static superheat, turn the valve's setting stem. Turning clockwise increases static superheat and effectively reduces refrigerant flow through the valve. Turning counterclockwise reduces static superheat and increases refrigerant flow.
What is the primary function of a metering device? To provide refrigerant to the evaporator in a condition essential for efficient heat absorption. What are two ways the metering device accomplishes its primary function?
Changes incoming liquid to a spray, and lowers its temperature. To provide low- temperature refrigerant to the evaporator. Changes incoming liquid to a spray , and lowers its temperature.
The piston fixed orifice and TXV Thermostatic Expansion Valve are the two most common metering devices in use today, with some modern systems utilizing an electronically controlled metering device called an EEV Electronic Expansion Valve.
The term subcooling also called undercooling refers to a liquid existing at a temperature below its normal boiling point. A subcooled liquid is the convenient state in which, say, refrigerants may undergo the remaining stages of a refrigeration cycle. If the load on the system is constant and the ambient temperature remains steady, then a simple capillary tube or piston would be preferable to an adjustable orifice device like a TXV.
Unless the system is in a laboratory and only tested under one condition, a TXV will always be the better choice. A thermal expansion valve or thermostatic expansion valve often abbreviated as TEV , TXV , or TX valve is a component in refrigeration and air conditioning systems that controls the amount of refrigerant released into the evaporator thereby keeping superheat, that is, the difference between the current refrigerant.
Let's start with superheat : Boiling is when a liquid gains heat and transforms into a vapor. Superheat occurs when that vapor is heated above its boiling point. Superheat is critical in HVAC because it ensures the liquid refrigerant is boiled off before it leaves the evaporator and heads to the compressor.
There are basically four types of valves that are in used. These valves are also refer to as metering devices. Automatic Expansion Valve regulates the flow of refrigerant from the liquid line to the evaporator by using a pressure-actuated diaphragm. The metering device is located after the condenser coil. There are two kinds of metering devices, thermal expansion valves TXV and capillary tubes. Either one does the same thing; they lower the high-pressure liquid pressure by forcing it through a small hole or nozzle.
The metering device acts like your garden hose nozzle, turning a steady stream of liquid into sprayed droplets.
Lowering the pressure of the refrigerant and spaying it into droplets lets the refrigerant boil at a much lower temperature. As refrigerant saturation point is relevant to pressure, the lower the pressure the lower the saturation point. Refrigerant droplets are more easily boiled than a steady stream. That is the purpose of the metering device. The main purpose of the TXV is to maintain superheat.
It does this by delivering refrigerant to the evaporator in an amount that is necessary for efficient heat absorption. The txv is always fighting to maintain balance. The pressure from inside the evaporator is constantly pushing the needle and seat inside the valve closed.
While the pressure inside of the bulb and capillary tube, are pushing open the TVX. Always feeding the right amount of flow.
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