《制冷原理与技术》讲 Istllauonoperaues, a Thermal Expansion Valve In a nutshell, the role ofthe TE\ refrigerant to flow into the 陈江干 上海交通大学制冷研究所 and protects th sure that Parts matA varying pressare on the diaphragm: alphagan: the can be fined or adjustable. Whatis superheat Howto easure Superheat most accurale readings, place inkt thermocoupl o the Irem the semine1 Thermal Expansion Valve 陈江平 上海交通大学制冷研究所 《制冷原理与技术》讲义 Installation, Operation, and Troubleshooting of TEVs In a nutshell, the role of the TEV is to control liquid injection into an evaporator as a function of the load. The controlling parameter is superheat at the evaporator. As the load on the evaporator increases, the valve responds to an increase in superheat and opens to allow more liquid refrigerant to flow into the evaporator. In so doing, the TEV maximizes the usable evaporator heat transfer surface and protects the compressor by making sure that only vapor returns to it. Parts Of a TEV · The power head assembly, enclosing a diaphragm; · The capillary tube and bulb connected to the power head containing a charge which, as it expands and contracts, exerts a varying pressure on the diaphragm; and · The valve body, with one or more pushpins operated by the diaphragm; the pushpins regulate the opening of an orifice through which the refrigerant is metered into the evaporator. The movement of the pushpins depends on the pressure on the diaphragm, which is opposed by the force of a spring. Spring force, which determines static superheat, can be fixed or adjustable. How TEVs Work The function of a TEV depends on the relationship between three fundamental pressures. Bulb charge pressure acts on the upper surface of the diaphragm, moving it in the valve-opening direction. Two pressures oppose bulb pressure. Evaporating pressure is introduced by either internal or external equalization. This equalization pressure acts on the underside of the diaphragm in the valve closing direction. Note: Evaporat-ing and equalization pressures should always be the same. Spring pressure also acts on the underside of the diaphragm in the closing direction. In a valve with adjustable superheat, the spring pressure can be adjusted manually. As the expansion valve regulates, there is balance between bulb pressure on one side of the diaphragm and equalization pressure plus spring pressure on the other side. This balance can be upset in either of two ways: 1. When spring force is adjusted manually, there is a proportional change in the TEV’s static superheat. 2. A change in the cooling load will change the evaporating pressure of the refrigerant and hence the equalization pressure under the diaphragm. This change occurs in proportion to the change in temperature at the evaporator outlet tube where the bulb is strapped. Any change in pressure is transmitted from the bulb through the capillary tube to the diaphragm. The balance of forces is disturbed until a new equilibrium is reached as more refrigerant is injected into the evaporator and the cooling load demand is met. Static superheat + opening superheat = operating superheat. What is Superheat? Physically, superheat is the temperature difference between the external pipe wall temperature and the evaporating pressure converted to temperature (saturation temperature) measured in °C. The level of superheat equals the temperature increase above the saturation temperature at the existing pressure. A vapor is superheated when its temperature is higher than the saturation temperature corresponding to its pressure. For example, R-22 at 70 psig has a saturation temperature of 41? and if its temperature actually is 51? it is said to be superheated by 10? With respect to valve operation, superheat has two distinct components: 1. Static superheat is the superheat at which spring force is met and the valve is ready to open. 2. Opening superheat is the amount of superheat above static superheat that opens the valve to allow refrigerant flow. The superheat measured at the outlet of the evaporator is the sum of the two and is called operating superheat. On valves with adjustable superheat, we are only changing spring force, and therefore only the static superheat. By adjusting the static superheat, however, we are effectively adjusting operating superheat. The opening superheat cannot be adjusted and is dependent on the system load or operating pressures as transmitted from the sensing bulb. Taking the measurements for superheat calculation; for the most accurate readings, place your gauge and thermometer in the positions shown. How to Measure Superheat 1. Measure the suction pressure at the evaporator outlet (or, if there is no fitting there, at the compressor inlet service valve). 2. Clean an area of the suction line near the bulb. 3. Tape your thermocouple to the cleaned area and insulate it; connect the thermocouple to a calibrated electronic thermometer and read the temperature. 4. Convert the suction pressure to a temperature using a refrigerant slide rule or chart, and subtract the temperature measured near the bulb. The difference is the superheat. A common but inaccurate method for determining superheat in the field uses evaporator inlet temperature instead of the saturated suction temperature equivalent to the evaporator outlet pressure. The problem with this method is its inaccuracy, which is most often due to misplacement of the inlet thermocouple or the inability to access the inlet at all. Make sure the sensing bulb is mounted in the corre t position, according to the tubing size