expansion valve is an important component in the refrigeration system, usually installed between the liquid storage cylinder and the evaporator. The expansion valve throttles the medium temperature and high pressure liquid refrigerant into low temperature and low pressure wet vapor, and then the refrigerant absorbs heat in the evaporator to achieve refrigeration effect. The expansion valve controls the valve flow rate through the change in superheat at the end of the evaporator, preventing insufficient utilization of evaporator area and cylinder knocking.
The commonly used expansion valves in refrigeration systems include thermal expansion valves and electronic expansion valves. The thermal expansion valve is divided into: internal balance thermal expansion valve and external balance thermal expansion valve.
Ⅰ. Adjustment techniques and taboos for thermal expansion valves
To enable the automatic adjustment function of the thermal expansion valve under operating conditions, adjustments must be made during the debugging of the refrigeration system to ensure that the thermal expansion valve can correctly perform automatic adjustment during system operation.
The adjustment of the thermal expansion valve is achieved through the adjustment rod. The rotation or withdrawal of the regulating rod essentially means the compression or relaxation of the spring, which means adjusting the magnitude of the static assembly superheat of the thermal expansion valve to meet the needs of refrigeration conditions. Generally, clockwise rotation represents advance, while counterclockwise rotation represents retreat. When adjusting the evaporation pressure, remove the cap and use a wrench to clockwise rotate the adjusting rod to increase the compression of the spring, forcing the diaphragm to move up and close the valve, causing the evaporation pressure to gradually decrease. Similarly, the reverse rotation adjustment lever will open the valve wider and increase the evaporation pressure.
When adjusting the thermal expansion valve, a low-pressure gauge should be installed on the suction stop valve of the compressor to observe the changes in evaporation pressure. The normal evaporation pressure is white frost or condensation that forms in the suction pipeline (white frost on medium and low temperature equipment; condensation on air conditioning equipment). If white frost or condensation forms on the suction shut-off valve or even half of the compressor, it indicates that the valve is too large and should be adjusted down. If white frost or condensation only forms on the outlet of the evaporator or cannot form on the outlet, it indicates that the valve is too small and should be adjusted up.
The debugging of thermal expansion valves can generally be carried out in two steps. At the beginning, it is coarse adjustment, that is, it can rotate about one turn each time it is adjusted. When the equipment approaches its operating conditions, fine adjustments should be made, rotating 1/4-1/2 turns each time. After each adjustment, the system should be operated for a few minutes or more, and the changes in the low-pressure gauge should be observed before deciding on the next adjustment. At the end of the expansion valve debugging, the cap should be screwed on and tightened with a wrench to prevent refrigerant leakage.
Adjusting the thermal expansion valve is a meticulous task, and it is important to avoid being hasty during the adjustment process. The adjusted thermal expansion valve shall not be adjusted for other reasons, unless the operating conditions of the refrigerator are changed. The thermal expansion valve of a unit that is generally delivered from the factory has been adjusted before leaving the factory, and should not be adjusted during on-site debugging of the unit.
Ⅱ.Maintenance operation of thermal expansion valve
Common faults of thermal expansion valves include blockage, leakage of working fluid filled in the temperature sensing bag, etc.
1. There are two types of valve hole blockage: ice blockage and oil blockage. If there is water in the refrigeration system, it will dissolve in the refrigerant, and its solubility is related to the temperature of the refrigerant. When the temperature is high, its solubility is large, while when the temperature is low, its solubility is small. When the refrigerant liquid with a condensation temperature of about 30t flows into the expansion valve orifice for throttling, it immediately cools down to the saturation temperature (evaporation temperature) under the evaporation pressure. A part of the water is precipitated and attached to the valve orifice. If the evaporation temperature is below 0 ℃, it will form an ice layer. When the ice layer increases, the valve orifice will be blocked.
Refrigerating oil and refrigerant also dissolve with each other, and their solubility is also related to temperature. At high temperatures, the solubility is greater, while at low temperatures, the solubility is smaller. When the liquid enters the expansion valve hole and its temperature drops rapidly after throttling, some of the refrigerant oil is separated and stuck around the valve hole. When the evaporation temperature is lower than the freezing point of the refrigerant oil, the refrigerant oil will condense into a paste. As the oil gradually increases, the valve hole is blocked because the lower the evaporation temperature, the easier it is for water and oil to separate. Therefore, ice blockage and oil blockage are easy to occur in low-temperature refrigeration equipment with lower evaporation temperatures.
A. Judgment of valve hole ice blockage or oil blockage. When the refrigeration unit starts to operate normally for a period of time, the suction pressure rapidly drops to negative pressure (vacuum), the temperature of the warehouse (room) rises, the frost layer melts (first of all, the frost layer of the suction pipe melts), and the flow sound of the expansion valve cannot be heard in the ear. At this point, it can be basically determined as ice blockage or oil blockage in the valve hole. To further confirm that the valve hole is blocked, an alcohol lamp can be used to heat the expansion valve body (without stopping the machine). After heating for one or two minutes, if you hear the sound of air flow, followed by a creaking sound, the suction pressure also rises, and the outlet of the expansion valve begins to form white letters indicating that the valve hole is indeed blocked by ice or oil.
B.Method of troubleshooting. Replace the desiccant in the drying filter. When there is a lot of water in it, reagent grade anhydrous calcium chloride can be used first. After cooling the pepper for one or two days, the silicone can be replaced. If there is no drying filter in the system, a drying filter can be temporarily connected.
2.The inlet filter screen of the expansion valve is blocked. The filter screen of the expansion valve is filled with impurities and dirt, so as not to block the valve hole. When there are too many impurities in the filter screen, the mesh is not very smooth or even completely blocked. The phenomenon is also that the suction month pulls the vacuum, and the temperature of the reservoir (chamber) does not decrease, while half or the entire expansion valve is connected. Song carefully listens to the weak creaking and intermittent sound of the expansion valve, indicating that the filter screen is not smooth and completely blocked. When tapping the inlet joint of the valve with a wrench handle, if the sound is heard louder than the original sound, it is confirmed that the majority of the mesh holes of the filter screen are severely blocked, and the sound of the refrigerant flow inside the valve cannot be heard. The valve body is also: generally, tapping the inlet joint with a wrench handle is ineffective. But the filter screens are all blocked to a lesser extent.
The obvious difference between filter clogging and valve hole clogging is that the valve hole is clogged when running normally during startup. If the unit experiences suction or vacuum pulling at the beginning, it is considered filter clogging. The phenomenon that occurs when the refrigerant is insufficient should be distinguished from the phenomenon when the filter screen is blocked. When the refrigeration unit is running, if you hear the expansion valve hissing and hissing loudly, the suction pressure is also low, the suction pressure gauge point fluctuates, the temperature of the warehouse (room) is not low enough, some evaporation pipes do not frost, and the refrigerant is insufficient, do not mistake it for the expansion valve malfunction. The method to eliminate clogging of the filter is to remove the filter and clean it with gasoline.
3.When the working fluid injected into the sensing mechanism leaks, the sensing pressure of the temperature sensor disappears, causing the valve to close and not open. At the beginning of operation, the tear refrigeration unit pulls the vacuum, and the temperature inside the warehouse (room) cannot drop at all. There is no sound of airflow from the expansion valve. Remove the expansion valve, remove the filter interface, and use your mouth to suck on the outlet connector of the expansion valve. If it cannot be sucked and can be lifted with your lips, it indicates that the valve hole is completely blocked. It is indeed that the valve cannot be closed and replaced with an expansion valve of the same model.