Unit 2.5,expansion Valve 4u2u71

Regulator or expansion valves For Freon systems the simplest regulator is found on Domestic Refrigerator cabinets. This consists of a length of capillary tubing. No adjustments are possible but at design stage the length and bore of the tube are calculated to give the pressure drop required for normal working conditions. Thermostatic expansion valve

These valves have the space above the bellows or diaphragm filled with the liquid refrigerant used in the main system and connected by capillary tube to a remote bulb. This remote bulb is fixed in close with the

gas line at the outlet from the evaporator and is responsive to changes in refrigerant vapour temperature at this point. These valves are the most commonly employed and are suitable for the control of systems where changes in the loading are frequent. The thermostatic expansion valve is based on a constant degree of superheat in the vapour at the evaporator outlet, so enabling the evaporator at any load to be kept correctly supplied with liquid refrigerant without any danger of liquid carry over to the suction line and thence to the compressor. The aperture in the expansion valve is controlled by pressure variation on the top of a bellows. This is effective through the push pins and tends to open the valve against the spring. Spring pressure is set during manufacture of the valve and should not be adjusted. The pressure on the bellows is from a closed system of heat sensitive fluid in a bulb and capillary connected to the top of the bellows casing. The bulb is fastened to the outside of the evaporator outlet so that temperature changes in the gas leaving the evaporator are sensed by expansion or contraction of the fluid. Ideally the gas should leave with 6° or 7°C of superheat. This ensures that the refrigerant is being used efficiently and that no liquid reaches the compressor. A starved condition in the evaporator will result in a greater superheat which through expansion of the liquid in the bulb and capillary, will cause the valve to open further and increase the flow of refrigerant. A flooded evaporator will result in lower superheat and the valve will decrease the flow of refrigerant by closing in as pressure on the top of the bellows reduces. Saturation temperature is related to pressure but the addition of superheat to a gas or vapour occurs after the latent heat transaction has ended. The actual pressure at the end of an evaporator coil is produced inside the bellows by the equalizing line and this is in effect more than balanced by the pressure in the bulb and the outside of the bellows is the result of saturation temperature plus superheat. The additional pressure on the outside of the bellows resulting from superheat overcomes the spring loading which tends to close the valve. For larger systems where a significant pressure drop exists across the evaporator it is necessary to fit a ‘Equalizing line or Balance line’. This is a small bore tube which feeds the outlet pressure back to the thermostatic valve ‘motor’ element. Therefore the measured temperature is directly related to the superheat temperature at outlet pressure.

Superheat is measured at the point where the sensing bulb is located on the evaporator outlet; superheat is the difference between the temperature at the bulb and the evaporating pressure/evaporating temperature at the same point.