The low exhaust pressure is mainly caused by the refrigerant flow rate in the refrigeration system pipeline being too small or even stopping. The exhaust pressure is too low, although it is manifested at the high pressure end, the reason is mostly due to the low pressure end.
1. Low exhaust temperature of refrigeration system:
The low exhaust pressure is mainly caused by the refrigerant flow rate in the refrigeration system pipeline being too small or even stopping. The exhaust pressure is too low, although it is manifested at the high pressure end, the reason is mostly due to the low pressure end.
Common reasons for low exhaust pressure:
(1) The expansion valve hole is blocked, reducing or even stopping the liquid supply. At this time, both the suction and exhaust pressures decrease.
(2) Expansion valve ice blockage or dirty blockage, as well as filter blockage, will inevitably cause a decrease in suction and exhaust pressure;
2. Refrigeration system return fluid:
(1) For small refrigeration systems using capillary tubes: excessive liquid addition can cause liquid reflux. When the evaporator is severely frosted or the fan malfunctions, the heat transfer deteriorates, and the unevaporated liquid can cause backflow. Frequent temperature fluctuations can also cause the expansion valve to malfunction and lead to liquid return.
(2) For refrigeration systems using expansion valves, the selection and improper use of return liquid and expansion valves are closely related. Excessive selection of expansion valve, too small superheat setting, incorrect installation method of temperature sensing package or damaged insulation wrapping, and failure of expansion valve may all cause liquid return.
For refrigeration systems where liquid return is difficult to avoid, installing gas-liquid separators can effectively prevent or reduce the hazards of liquid return.
3. High suction temperature of refrigeration system:
(1) Poor insulation of the return air pipeline or excessively long pipelines can cause high suction temperatures. Under normal circumstances, the compressor cylinder head should be half cool and half hot.
(2) Insufficient refrigerant charge in the system. Or the expansion valve opening is too small, resulting in insufficient circulation of refrigerant in the system, less refrigerant entering the evaporator, high superheat, and thus high suction temperature.
(3) The expansion valve port filter is clogged. The supply of liquid in the evaporator is insufficient, and the amount of refrigerant liquid decreases. A part of the evaporator is occupied by superheated steam, resulting in an increase in the suction temperature.
4. Liquid impact:
(1) Avoid inhaling temperatures that are too high or too low. If the suction temperature is too low, it indicates that the refrigerant has not evaporated completely in the evaporator, which not only reduces the heat transfer efficiency of the evaporator, but also causes the suction of wet steam and the formation of compressor liquid hammer. Under normal circumstances, the suction temperature should be 5-10 ℃ higher than the evaporation temperature.
(2) In order to ensure the safe operation of the compressor and prevent liquid hammer, it is required that the suction temperature be slightly higher than the evaporation temperature, that is, it should have a certain degree of superheat.
5. Starting the refrigeration system with liquid:
(1) The foaming phenomenon during liquid start can be clearly observed on the oil sight glass. The fundamental reason is that a large amount of refrigerant dissolved in the lubricating oil and sinking under the lubricating oil suddenly boils when the pressure drops, causing foaming of the lubricating oil, which can easily lead to liquid hammer.
(2) Installing a crankcase heater (electric heater) on the compressor can effectively prevent refrigerant migration. Short term shutdown, keep the crankcase heater powered on. After a long shutdown, heat the lubricating oil for a few or ten hours before starting up. Installing a gas-liquid separator on the return gas pipeline can increase the resistance of refrigerant migration and reduce the migration amount.
6. Oil return in the refrigeration system:
(1) Lack of oil can cause serious lubrication problems. The fundamental reason for oil shortage is not the amount and speed of oil flow from the compressor, but the poor oil return of the system. Installing an oil separator can quickly return oil and prolong the compressor's non oil return operation time.
(2) When the position of the compressor is higher than that of the evaporator, there is no oil return bend on the vertical return pipe. The oil return bend should be as compact as possible to reduce oil storage. The spacing between the return bends should be appropriate, and when there are many return bends, some lubricating oil should be added.
(3) The compressor starts frequently. Frequent starting of the compressor is not conducive to oil return. Due to the short continuous operation time, the compressor stops and there is no time to form a stable high-speed airflow in the return pipe, so the lubricating oil can only remain in the pipeline. If the return oil is less than the running oil, the compressor will run out of oil. The shorter the operating time, the longer the pipeline, the more complex the system, and the more prominent the oil return problem.
7. Low evaporation temperature of refrigeration system:
The evaporation temperature has a significant impact on refrigeration efficiency. For every 1 degree decrease, an increase of 4% in power is required to produce the same cooling capacity. So, if conditions permit, increasing the evaporation temperature appropriately is beneficial for improving refrigeration efficiency. The evaporation temperature is generally 5-10 degrees lower than the outlet temperature.
Although blindly lowering the evaporation temperature can cool the temperature difference, the cooling capacity of the compressor is reduced, so the cooling speed may not be fast. Moreover, the lower the evaporation temperature, the lower the refrigeration coefficient, while the load increases, the operating time prolongs, and the power consumption increases.
8. Overheating of exhaust from refrigeration system:
The main reasons for exhaust overheating are as follows: high return air temperature, motor heating, high compression ratio, back expansion and gas mixing, compression temperature rise and refrigerant type, and high condensation pressure.
Regarding the specific causes and analysis of exhaust overheating in refrigeration systems, our Refrigeration Encyclopedia has previously published an article specifically detailing it. Interested friends can refer to the previously published article.
9. Low suction temperature of refrigeration system:
(1) The expansion valve is too open. Due to loose binding of the temperature sensing element, small contact area with the return pipe, or failure to wrap the temperature sensing element with insulation material and incorrect wrapping position, the temperature measured by the temperature sensing element is inaccurate and close to the ambient temperature, which increases the opening degree of the expansion valve and leads to excessive liquid supply.
(2) Too much refrigerant is being charged. Excessive refrigerant charge occupies a portion of the condenser's internal volume, leading to an increase in condensation pressure and an increase in liquid entering the evaporator. The liquid in the evaporator cannot be completely vaporized, resulting in liquid droplets in the gas sucked in by the compressor. In this way, the temperature of the return gas pipeline decreases, but the evaporation temperature remains unchanged due to the pressure not decreasing, and the degree of superheat decreases. Even if the expansion valve is turned down, there is no significant improvement.
10. Refrigeration system but fluorine:
(1) When the fluorine content is low or the regulating pressure is low (or partially blocked), the valve cover (bellows) and even the inlet of the expansion valve will frost; When the fluorine content is too low or almost no fluorine, there is no reaction on the surface of the expansion valve, and only a slight sound of airflow can be heard.
(2) Check from which end the icing starts, from the liquid separator or the compressor return pipe. If it starts from the liquid separator, it means there is a lack of fluorine, and if it starts from the compressor, it means there is too much fluorine.
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