Symptomanalysis of common failures in refrigeration system

    When a failure occurs in a refrigeration system, it is generally impossible to see directly where the failure site occurs, nor is it possible to dissect and dissect the parts of the refrigeration system one by one. Only by looking at the surface can abnormal phenomena be found in operation, for a comprehensive analysis. In the inspection generally through the look, listen, touch to understand the running state of the system. When the operating pressure and temperature of the system exceed the normal range, except indoor and outdoor environment temperature deterioration, otherwise there will be problems, which is an important basis for judging the source of failure.
1. Measurement of pressure and temperature of refrigeration system
(1) pressure concept of refrigeration system: The refrigeration system can be divided into two parts: high pressure and low pressure. The high pressure section from the compressor outlet to the throttle front, this section is called the evaporation pressure. The suction pressure of the compressor is called suction pressure, suction pressure is close to the evaporation pressure, the difference is the flow resistance of the pipeline.
 For convenience, the evaporating pressure and condensing pressure of the refrigeration system are detected at the suction and exhaust ports of the compressor. That is often called the compressor suction, exhaust pressure. The purpose of measuring the suction and exhaust pressure of refrigeration system is to obtain the evaporating temperature and condensing temperature of refrigeration system, so as to obtain the running status of refrigeration system.
(2) the concept of temperature in refrigeration system: the temperature in refrigeration system involves a wide range, including evaporation temperature te, suction temperature ts, condensation temperature, exhaust temperature, etc. . The evaporating temperature te and condensing temperature TC are the decisive factors to the operating condition of the refrigeration system.
 A. Evaporation temperature te: refers to the liquid refrigerant in the evaporator boiling vaporization temperature.
 B. Condensing temperature TC: is the temperature at which the superheated vapor of a refrigerant condenses into a liquid after exothermic action in the condenser. The condensation temperature can not be measured directly either, only by detecting the corresponding condensation pressure, and then by consulting the thermodynamic properties of the refrigerant table. The higher the condensing temperature is, the higher the condensing pressure is. The condensing temperature is too high, the load of the unit is too heavy, the motor is over-loaded, and the operation is unfavorable.
C. Exhaust temperature td: refers to the temperature of the compressor exhaust port (including the temperature of the exhaust port nozzle) , testing exhaust temperature must have a temperature measuring device, general minicomputer is not set up, temporary measurement can be used semiconductor point thermometer detection, but the error is larger. The exhaust temperature is affected by the suction temperature and the condensation temperature, the suction temperature or the condensation temperature rise, the exhaust temperature rise accordingly, so it is necessary to control the suction temperature and the condensation temperature to stabilize the exhaust temperature.
D. Inspiratory temperature ts: refers to the gas temperature of the connecting pipe of the compressor suction, the temperature measurement device is needed to detect the suction temperature, generally, the suction temperature of air conditioning unit should be controlled around TS = 15 °C. Above this value has certain influence to the refrigeration effect.
2. Influence of suction pressure variation on refrigeration system
The suction pressure of a refrigeration system is closely related to the evaporation temperature and the flux of the refrigerant. For Systems using expansion valves, suction pressure is related to the opening of expansion valves, refrigerant charge, compressor cooling efficiency, and load. In a capillary system, suction pressure is related to condensing pressure, refrigerating capacity, compressor refrigerating efficiency, and load. Therefore, when inspecting the refrigeration system, the pressure gauge should be installed on the suction pipe. It is very important to detect suction pressure for fault analysis.
(1) factors of low suction pressure: The suction pressure is lower than the normal value. The factors include insufficient refrigeration capacity, small cooling load, small expansion valve opening, low condensing pressure (refers to the capillary system) , and unblocked filter.
(2) factors of high suction pressure: The suction pressure is higher than the normal value, such as too much refrigerant, large refrigeration load, expansion valve opening, high condensing pressure (capillary system) and low efficiency of compressor, etc. .
3. The influence of exhaust (condensation) pressure change on refrigeration system
When the refrigeration system is running, the exhaust pressure corresponds to the condensing temperature, and the condensing temperature is related to the flow rate of the cooling medium, temperature, refrigerant inflow, cooling load and so on. When the refrigeration system is inspected, an exhaust pressure gauge should be installed at the exhaust pipe to detect the exhaust pressure as the data for analysis of faults.
(1) factors of high exhaust pressure: When the exhaust pressure is higher than the normal value, the flow rate of the cooling medium is small or the temperature of the cooling medium is high, the refrigerant charge is excessive, the cooling load is large and the expansion opening is large, etc. .
The above factors will cause the increase of circulating flow and the condensation heat load. As the heat can not be all out in time, causing condensation temperature rise, and can be detected is the exhaust (condensation) pressure rise. When the flow rate of the cooling medium is low or the temperature of the cooling medium is high, the cooling efficiency of the condenser decreases and the condensing temperature increases. When the flow rate of the cooling medium is low or the temperature of the cooling medium is high, the cooling efficiency of the condenser decreases and the condensing temperature increases. The reason of excessive refrigerant charge is that the surplus refrigerant liquid occupies part of the condenser tube, which reduces the condensing area and causes the condensing temperature to rise.
(2) factors of low exhaust pressure: The exhaust pressure is lower than the normal value. The factors include low efficiency of the compressor, insufficient refrigerating dose, Small Cooling Load, small expansion valve opening, and unblocked filter, including expansion valve filter screen and cooling medium temperature low.
Above several kinds of factors all can cause the system the refrigeration flow to drop, the condensation load is small, causes the condensation temperature to drop.
 From the above suction pressure and exhaust pressure and exhaust pressure changes, there is a close relationship between the two. In general, when the suction pressure rises, the exhaust pressure rises, and when the suction pressure drops, the exhaust pressure drops. Also may estimate from the suction pressure gauge change the exhaust pressure approximate situation.
4. The relationship between suction temperature and exhaust temperature
In fact, the exhaust temperature of the system is closely related to the suction temperature. Inspiratory temperature rises, exhaust temperature also rises relatively, otherwise is low. By understanding their relationship, you can control them and make the refrigeration system work better.
5. Effect of temperature change on refrigeration system of condensing unit
Unit components have normal temperature range, beyond this range is not normal state. These abnormal factors may be the fault, may be adjusted incorrectly, but it is necessary to analyze the cause, and timely processing or inspection. These temperature points are difficult to measure with a thermometer, generally can only be estimated by hand, and then judge whether normal.
(1) exhaust temperature effect: In summer, the exhaust temperature of the compressor is relatively high, hands can not touch. According to national standards, R22 refrigeration system exhaust temperature should not exceed 150OC, more than this temperature line is abnormal. The cause of super high exhaust temperature is due to super high suction temperature of compressor or super high condensing temperature, which must be noticed. Exhaust temperature is too low, hand-touch exhaust pipe is not hot, which means that the suction temperature is particularly low, compressor may run wet stroke or system working fluid is relatively small operating state. The compressor wet stroke is easy to damage the valve structure, the refrigerant is very few circumstances operation, will affect the motor winding heat dissipation, accelerate the aging of insulation material.
(2) the influence of casing temperature change on compressor and refrigeration system: The temperature field on the casing surface of a fully enclosed reciprocating piston compressor can be divided into two parts: A. The upper casing is affected by the inhalation of steam, the temperature is relatively low, in a slightly hot or cool range, it is estimated at about 30 °C, in the vicinity of the suction pipe local casing surface dew. B. The heat generated by the motor in the lower casing and the friction heat brought out by the frozen oil are mainly brought out of the casing by steam.
 A. The influence and reason of too high temperature of Chassis: the surface temperature of chassis exceeds the normal range, mainly because the suction temperature of refrigeration system is too high (higher than 15 °c) . Too high hot steam into the compressor, absorbing heat in the casing, so that the steam temperature is higher, so that the temperature of the casing rise. Superheated steam temperature rise is very high, chassis temperature rise is very high, to the oil cooling, this will affect the lubrication of moving parts, accelerate wear, serious so that the bearing holding shaft (Bite) . In addition, it will cause the exhaust temperature to rise.
B. Chassis temperature is too low and the reasons: Chassis surface temperature below the normal range, the reason is too low suction temperature (less than 15 °C) . It is beneficial to the cooling of the oil and the motor windings, but the cooling capacity is decreased. When the inspiratory temperature is particularly low, will make most of the shell dew, there is the danger of hydraulic blow, this is the fatal blow to the compressor, should pay special attention. At the same time, a large amount of refrigerant is dissolved in the frozen oil, which is disadvantageous to the lubrication of moving parts.
 (3) temperature condition of condenser
A. Condenser temperature status: normally, the first half of the heat pipe is very hot, and its temperature has a slow gradual decline in the balance. Compared with the front part, the heat-sensing degree of the latter part of the heat sink pipe is greatly reduced, which is due to the fact that the refrigerant in the latter part of the pipe has been liquefied gradually and has reached the condensing temperature and the supercooling temperature. When abnormal conditions occur, one is the front half is not too hot, the latter half near normal temperature (ambient temperature) , the reason is compressor hygroscopic steam refrigerant or refrigerant dose is insufficient. The other is the whole condenser tube is very hot, its reason is too much refrigerant or small ventilation, or high ambient temperature.
B. WATER-COOLED CONDENSER: the Shell of a shell-and-tube condenser is normally relatively hot in the upper half and lukewarm in the lower half. The abnormal condition is that the whole Shell is not too hot, the reason is not enough refrigerant. Another situation is that the whole Shell is very hot, which is due to insufficient cooling water or poor cooling effect (scaling in the pipe) . Casing condenser in normal circumstances, casing surface is very hot, its reason is too small cooling water or poor cooling effect; the other is the whole casing surface is not too hot, its reason is insufficient refrigerant. (4) temperature profile of the reservoir: under normal conditions, the suction tube feels cool to the touch of the hand and is covered with dew. The reason is the condenser cooling difference, high condensation temperature or refrigerant charge too much.
(5) the liquid tube temperature condition: under normal circumstances, the liquid tube is warm. Under abnormal circumstances, the liquid tube is relatively hot. The reason is the condenser cooling difference, high condensation temperature or refrigerant flow too much.
(6) filter temperature condition: The basic condition is the same as the infusion tube, but it has a prominent abnormal phenomenon, that is, the filter may be cool, the reason is that the filter hole is blocked by sludge, making the filter blocked, when the refrigerant flows through the filter screen, throttling occurs, that is, a part of the liquid gasification endothermic, so that the filter cool, serious condensation. Another abnormal phenomenon is the filter is not hot, and the ambient temperature, the reason is completely blocked filter, refrigerant can not flow.
(7) the temperature of the suction tube, under normal circumstances, the suction tube feel very cold by hand, and with dew. Abnormal circumstances, one is the inhalation tube cold, Dew too much, resulting in large-scale Shell dew. The reason is that the refrigerant flow is too large, the liquid can not be vaporized in the evaporator, there are liquid reflux phenomenon. Its harmfulness is the compressor may run wet stroke, serious will produce hydraulic blow, the valve plate is threatened. Second, the suction tube is not cold, not dew, the Shell is very hot. The reason is that the refrigerant flow is too small or the amount of refrigerant is insufficient. The result is an increase in exhaust temperature and a decrease in cooling capacity.