When a refrigeration system fails, it is usually impossible to directly see where the fault occurs, nor is it possible to disassemble and dissect the components of the refrigeration system one by one. The only way is to check the appearance, find out the abnormal phenomena in operation, and conduct a comprehensive analysis.
During the inspection, the operating status of the system is generally understood by looking, listening, and touching. When the operating pressure and temperature of the system exceed the normal range, in addition to the deterioration of the indoor and outdoor ambient temperatures, there must be a problem. This is an important basis for determining the root cause of the fault.
1. Detection of refrigeration system pressure and temperature
(1) Concept of refrigeration system pressure
During operation, the refrigeration system can be divided into high and low pressure parts; the high pressure section is from the exhaust port of the compressor to the front of the throttle valve, which is called high pressure pressure; the suction port pressure of the compressor is called suction pressure, which is close to the evaporation pressure. The difference between the two is the flow resistance of the pipeline; the pressure loss is generally limited to less than 0.018Mpa. The evaporation pressure and condensation pressure of the refrigeration system are detected at the suction and exhaust ports of the compressor. That is, it is usually called the suction and exhaust pressure of the compressor.
The purpose of detecting the suction and exhaust pressure of the refrigeration system is to obtain the evaporation temperature and condensation temperature of the refrigeration system, so as to obtain the operating status of the refrigeration system.
(2) Temperature concept in refrigeration system
Temperature concept: The temperature in the refrigeration system covers a wide range, including evaporation temperature te, suction temperature ts, condensation temperature, exhaust temperature, etc.; the evaporation temperature te and condensation temperature tc play a decisive role in the operating conditions of the refrigeration system.
Evaporation temperature te: Evaporation temperature te refers to the temperature at which the liquid refrigerant boils and vaporizes in the evaporator.
For example, the te of the air-conditioning unit. 5~7℃ is the optimal evaporation temperature of the air-conditioning unit, that is, the design te of the air-conditioning unit is between 5~7℃. When the air-conditioning unit is debugged after maintenance, if te does not reach between 5~7℃, the expansion valve should be adjusted or the refrigerant filling amount should be adjusted. The purpose of detecting the suction pressure of the compressor is to understand the evaporation temperature of the unit during operation. However, te cannot be directly detected, and its evaporation temperature can only be obtained by detecting the corresponding evaporation pressure.
Condensation temperature tc: Condensation temperature tc is the temperature when the superheated vapor of the refrigerant condenses into liquid after releasing heat in the condenser. The condensation temperature cannot be detected directly either. It can only be obtained by detecting its corresponding condensation pressure and then consulting the refrigerant thermodynamic property table;
When the condensation temperature is high, its condensation pressure is relatively increased, and they correspond to each other; when the condensation temperature is too high, the unit load is heavy, the motor is overloaded, which is unfavorable for operation, and its cooling capacity decreases accordingly, and the power consumption increases, which should be avoided as much as possible.
Exhaust temperature td: Exhaust temperature td refers to the temperature of the compressor exhaust port, including the temperature of the exhaust port pipe.
There must be a temperature measuring device to detect the exhaust temperature. Generally, small units do not have a temperature measuring device. Temporary measurement can be detected by a semiconductor point thermometer, but the error is large.
The exhaust temperature is affected by the suction temperature and the condensation temperature. When the suction temperature or the condensation temperature increases, the exhaust temperature also increases accordingly. Therefore, the suction temperature and the condensation temperature must be controlled to stabilize the exhaust temperature.
Intake temperature ts: The suction temperature ts refers to the gas temperature of the compressor suction connecting pipe. There must be a temperature measuring device to detect the suction temperature. Generally, small units do not have a temperature measuring device. During maintenance and debugging, it is generally estimated by hand touch. The suction temperature of the air conditioning unit is generally required to be controlled at about ts=15℃. Exceeding this value will have a certain impact on the refrigeration effect.
2. The impact of suction pressure changes on the refrigeration system:
When the refrigeration system is running, its suction pressure is closely related to the evaporation temperature and the flow rate of its refrigerant.
For systems using expansion valves, the suction pressure is related to the opening degree of the expansion valve, the amount of refrigerant charged, the cooling efficiency of the compressor, and the load size.
For systems using capillary tubes, the suction pressure is related to the condensing pressure, cooling capacity, compressor cooling efficiency, and load size.
For this reason, when checking the refrigeration system, a pressure gauge should be installed on the suction pipe. Detecting the suction pressure plays an important role in fault analysis.
Low suction pressure: The suction pressure is lower than the normal value. The factors include insufficient cooling capacity, small cooling load, small expansion valve opening, low condensing pressure, and blocked filters.
High suction pressure: The suction pressure is higher than the normal value. The factors include too much refrigerant, large cooling load, large expansion valve opening, high condensing pressure, and poor compressor efficiency.
3. The impact of exhaust pressure changes on the refrigeration system:
When the refrigeration system is running, its exhaust pressure corresponds to the condensation temperature, and the condensation temperature is related to the flow and temperature of its cooling medium, the inflow of refrigerant, the cooling load, etc. When checking the refrigeration system, an exhaust pressure gauge should be installed at the exhaust pipe to detect the exhaust pressure as fault analysis data.
High exhaust pressure: When the exhaust pressure is higher than the normal value, there are generally small flow of cooling medium or high temperature of cooling medium, excessive refrigerant filling, large cooling load and large expansion opening.
Low exhaust pressure: The exhaust pressure is lower than the normal value. The factors include low compressor efficiency, insufficient refrigerant, small cooling load, small expansion valve opening, unobstructed filter, including expansion valve filter and low cooling medium temperature.
The above factors will cause the system's refrigeration flow to decrease, the condensation load to be small, and the condensation temperature to drop. From the above changes in suction pressure and exhaust pressure, the two are closely related. Under normal circumstances, the suction pressure increases, the exhaust pressure also increases accordingly; the suction pressure decreases, and the exhaust pressure also decreases accordingly. The general situation of the exhaust pressure can also be estimated from the changes in the suction pressure gauge.
4. Relationship between suction temperature and exhaust temperature:
In fact, the exhaust temperature of the system is closely related to the suction temperature. When the suction temperature rises, the exhaust temperature also rises relatively, and vice versa.
5. Impact of condensing temperature changes on the refrigeration system:
The temperature of the unit components has a normal temperature range. Exceeding this range is an abnormal state. The factors causing these abnormalities may be faults or incorrect adjustments, but the causes must be analyzed and handled or checked in time. These temperature points are difficult to measure with a thermometer. Generally, they can only be estimated by hand and then judged whether they are normal.
(1) Impact of exhaust temperature
Note: In summer, the exhaust temperature of the compressor is relatively high and cannot be touched by hand.
According to national standards, the exhaust temperature of the R22 refrigeration system should not exceed 150℃. Exceeding this temperature line is an abnormal condition.
Exhaust temperature is too high: The suction temperature of the compressor is too high, or the condensing temperature is too high, which must be paid attention to. Exhaust temperature is too low: The exhaust pipe is not hot to the touch, which means that the suction temperature is particularly low. The compressor may be running in a wet stroke or the system is running with very little working fluid.
(2) The impact of casing temperature changes on the compressor and refrigeration system The temperature field on the outer surface of the casing of a fully enclosed reciprocating piston compressor can be divided into two parts:
Upper casing: Affected by the inhaled steam, the temperature is relatively low, in the slightly hot or slightly cool range, estimated to be around 30°C, and there is a possibility of condensation on the local casing surface around the suction pipe. Lower casing: The heat generated by the motor and the friction heat brought out by the refrigeration oil are mainly brought out of the casing by the steam.
1) The impact and cause of excessive casing temperature The casing surface temperature exceeds the normal range
The suction temperature of the refrigeration system is too high (above 15°C); excessively hot steam enters the compressor and absorbs the heat in the casing, making the steam temperature higher, thereby raising the casing temperature; the temperature of the superheated steam rises very high, and the casing temperature also rises very high, which is not conducive to the cooling of the oil, which will affect the lubrication of the moving parts and accelerate wear. In severe cases, the bearings will seize the shaft, and the exhaust temperature will also rise. 2) The impact and causes of low casing temperature The casing surface temperature is lower than the normal range and the suction temperature is too low (lower than 15°C); it is beneficial to the cooling of the refrigeration oil and the motor windings, but the cooling capacity is reduced; when the suction temperature is particularly low, most of the casing will condense, and there is a risk of liquid hammer, which is a fatal blow to the compressor and should be paid special attention to; at the same time, a large amount of refrigerant is dissolved in the refrigeration oil, which is not conducive to the lubrication of moving parts.
(3) Temperature condition of the condenser
Normal situation: The front half of the heat dissipation pipe is very hot, and its temperature has a slow and gradual decline. The heat sensation of the rear half of the heat dissipation pipe is much lower than that of the front half. This is because the refrigerant in the rear half of the pipe has gradually liquefied and has reached the condensation temperature and supercooling temperature.
Abnormal situation: The front half is not too hot, and the rear half is close to normal temperature (ambient temperature). The reason is that the compressor absorbs wet steam refrigerant or the amount of refrigerant is insufficient. Another is that the entire condenser pipe is very hot. The reason is that the amount of refrigerant is too much or the ventilation volume is small, or the ambient temperature is high.
Under normal circumstances, the upper half is relatively hot and the lower half is warm. Abnormally, the entire shell is not too hot, the reason is that the amount of refrigerant is insufficient; another situation is that the entire shell is very hot, the reason is insufficient cooling water or poor heat dissipation effect.
(4) Temperature condition of the liquid receiver Under normal circumstances, the suction pipe feels very cool to the touch and has condensation. The condenser has poor heat dissipation, the condensation temperature is high, or the amount of refrigerant is overcharged.
(5) Liquid pipe temperature condition is warm; abnormal: relatively hot. The condenser has poor heat dissipation, the condensation temperature is high, or the refrigerant flow is too much.
(6) Filter temperature condition
It has a prominent abnormal phenomenon, that is, the filter may be cold, the reason is that the filter mesh holes are blocked by sludge, making the filter unobstructed. When the refrigerant flows through the filter, throttling occurs, that is, part of the liquid vaporizes and absorbs heat, making the filter cold, and severe condensation occurs. Another abnormal phenomenon is that the filter is not hot and is equivalent to the ambient temperature. The reason is that the filter is completely blocked and the refrigerant cannot flow.
(7) Temperature of the suction pipe
Under normal conditions, the suction pipe feels very cool to the touch and has condensation. Under abnormal conditions, one is that the suction pipe is relatively cold and has too much condensation, resulting in condensation on a large area of the casing. The reason is that the refrigerant flow is too large, the liquid cannot be completely vaporized in the evaporator, and there is liquid reflux.
VI. The impact of evaporation temperature changes on the refrigeration system:
(1) Surface temperature of the thermal expansion valve
Normal condition: The lower half of the expansion valve body is very cold and has condensation, and the sound of refrigerant flow is very dull.
Abnormal condition: The valve body is relatively cold, there is a lot of condensation on the surface, and even frost, and the sound of refrigerant flow is loud.
Reason: The filter is blocked, or the refrigerant in the power box leaks, and the valve hole is closed.
(2) Temperature of the capillary tube
Normal condition: The capillary tube is cold and has condensation, and there is a sound of liquid flow.
Abnormal condition: The surface is very cold and condensation occurs, but the sound of flow is louder, which is gas flow.
Cause: Insufficient refrigerant.
(3) Evaporator temperature condition
Normal condition: The outer surface of the evaporator is very cold, and the condensation drops are constantly dripping. The inlet and outlet air temperatures are relatively high, and Δt is usually between 12 and 14°C.
Abnormal condition: The evaporator surface is not too cold, there is not much dew, or there is no condensation. The sound of refrigerant flowing can be heard, and the inlet and outlet air temperature difference is small.
Cause: Insufficient refrigerant or small expansion valve opening.
VII. Influence of ambient temperature:
(1) Ambient temperature requirements for outdoor units
According to national standards, when the ambient temperature of the outdoor unit is below 35°C, the air conditioning unit should ensure normal operation and achieve the cooling capacity and other indicators marked on the product nameplate.
When the ambient temperature is between 35 and 43°C, the air conditioning unit can operate, but the cooling capacity marked on the nameplate cannot be guaranteed. Temperature: within the range of 35 to 43°C.
If the indoor unit heats up too much, the electronic control protector may be activated, cutting off the power supply and stopping operation.
When the outdoor temperature exceeds 43℃, the air conditioner unit is in overload operation, which will cause the electric control protection device to operate, cut off the power supply, and stop operation.
(2) Indoor air conditioning temperature requirements
The maximum normal indoor constant temperature value should not exceed 30℃. If the air conditioner unit is operated at a temperature exceeding 30℃, it may be operated under overload conditions, and the condensing temperature and exhaust temperature of the refrigeration system will rise. It may also cause the electric protector to operate and cut off the power supply, which is not good for the operating life of the air conditioner unit.
(3) Heat pump system
Whether the heat pump is operating normally, mainly check the working condition of the four-way reversing valve. When the reversing valve is reversing, you can hear a relatively loud sound of gas flow and the impact sound of the solenoid valve pin (the electromagnetic field attracts the valve core). If the above two sounds are not heard during the reversing process of the solenoid valve, the solenoid valve may be faulty.
