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Air-conditioning System Failure Analysis
Some of these common failures can be read on the display, while others need to analyze and judge to find.


High-pressure protector alarms (condensing pressure is too high)
(1) The possible reason is cooling water tailed off, or water temperature rose, and for air-cooled condenser, it may be the airflow became smaller. The flow of water (air) is the main reason which causes the changes of condensing pressure. In addition, air slip in the circuit, will also cause the condensing pressure rise.

(2) One of the possible reasons is refrigerant charging too much, when the condenser is also used as liquid accumulator, the excessive refrigerant will result in the decrease of effective condensing area. Another reason is that the fluid valve of condenser do not open fully, it can also cause the condensing pressure rise.

The scale on the surface of heating exchanger can also cause condensing pressure increases after using for a long time. For shell and tube condenser, it can be found by removing the head, or analyzing the temperature difference between inlet and outlet water. When there is scale, the thermal resistance will be larger, and the temperature difference between inlet and oulet water will be smaller. Scale may seriously affect the condensing pressure; it will be 0.1-0.2Mpa higher than the normal pressurewhen there is too much scale.
 

Low-pressure protector alarms (evaporating pressure is too low)

Too low evaporating pressure will seriously affect cooling capacity of refrigeration unit. Cooling slowly, frosting unevenly on the surface of evaporating cooling coil, and no frost in evaporator outlet will appear. The possible reasons of too low evaporating pressure are below.

 (1) The possible reason is airflow insufficiency, check and cleans the return air filter (primary filter, secondary filter), check and clean the evaporator surface if there is dust and other debris blocking.
(2)There are moisture and dirt in the system, check whether there is large temperature difference between inlet and outlet pipe of dry filter, check the color of sight glass (becoming yellow means water in the system).
(3）The dry filter plugs, solenoid valve does not work, or expansion valve with ice jam, etc.
(4) The opening of throttle or expansion valve is too small, which cause flow of refrigerant inadequate , most space of evaporator is used for steam overheating, so there is decline of cooling capacity, the evaporating pressure is drop ,too.

 
Low-pressure protector alarms (solenoid valve failure)

The results are the same, that the solenoid valve won’t open or the expansion valve won’t.The main reasons are following:

(1) Voltage is too low, so that solenoid valve can not work
(2) Circuit wiring is bad or the coil is short-circuited, which can be measured by multimeter.
(3) Solenoid valve installing position is not suitable, or the iron core is dirty, which cause iron core stuck, and no sound with dropping when power is off.
In short, the basic reason of solenoid valve failure is that the iron core can’t be sucked up, we can’t hear the swill when power is on and the sound with dropping when power is off.

Low-pressure protector alarms(leakage of refrigerant)

Leakage of refrigerant is also one of the possible reasons. Leaking fluoride (R22) is one of the most common failures. Refrigerant leakage not only causes money lost, but also seriously affects the cooling effect of cooling system. Therefore, equipment daily maintenance should be strengthened, and once leakage is found, deal with it in time, let the system no leakage or few leakage as far as possible

Because there are a lot of sealing surface and connections, and fluoride is colorless, tasteless, non-irritating and strong penetration, fluoride leakage is difficult to be discerned.

(1) The valve stem (filling) is one of the most easily leaking points in the refrigerating unit. The valve which is normally open or closed for regular maintenance operations, filling in these valves become loose or dry and shrink, or due to stem rust, sealing is damaged and refrigerant will leak along the stem. So during normal maintenance, the stem should be protected by butter.

The filling of valve should be in the condition of good seal, strong acid and alkali resistance, large temperature adaptation range, and wear-resistance. Currently domestic production of Teflon tape, with a ideal sealing performance, especially at low temperature, its sealing performance is better than the general one.

(2) Other parts leakage. Such as connectors, flanges, etc., though the leakage is not common, it should be checked frequently. For the one-way valves, solenoid valves, filters, etc., soldering is used in some parts; it is easily brittle at low temperatures, so these sites should also be checked.

The methods of dealing with the leakage of the system pipe: a. tools: a set of common tools, freon pressure gauge (used to measure the system pressure), current clamp-on meter(used to measure the current of compressor and motor), several phosphor copper welding rods, a set of gas welding fittings(including oxygen, acetylene, the hose of welding torch). b. Firstly, cut off power supply of the air-conditioning system, discharge the refrigerant; Secondly, while the pressure measurement of system is zero, you may use gas welding (oxygen, acetylene) to repair, reducing valve of oxygen bottle regulated to 0.5Mpa, reducing valve of acetylene bottle regulated to 0.05Mpa, use oxidizing flame while welding, the process of welding should be as soon as possible, completed within 15 minutes. After welding is completed, charge 1.5 Mpa nitrogen for leakage checking, if there is no leakage, let the nitrogen release, vacuum to -76cmHg(with freon pressure gauge connected to refrigerant charging valve of the high-pressure pipes and low-pressure pipe, the middle gauge pipe connected to vacuum pump), finally, charge the refrigerant until the freon pressure gauge displays about 0.7Mpa in the state of shutdown, and then start air-conditioning unit,charge the refrigerant from the side of low-pressure,until the normal parameters appears.

 
Expansion valve failure

Low-pressure protection device alarms may be caused by expansion valve as well. The main reason of the expansion valve is that it cannot open or cannot close tightly，it’s pinhole is plugged by ice or oil, filter is blocked and etc.. All these troubles  can make the refrigerant flow be unfree, then evaporating pressure drop off and cooling effect poor.According to the different problems，the solutions shall be different too.

（1）Expansion valve can’t be opened, you can straightening by using the wooden hammer. If it doesn’t work, then replace the expansion valve.

（2）Expansion valve can’t be turned down. There are two reasons : first,Temperature sensing bulb is far from the evaporator outlet or as the expansion valve doesn’t insulate with air suction pipe together，disturbed by outside high temperature。Second，preload force of regulating spring isn’t enough，valve pin can’t be turned down。After finding above two reasons，then separately to deal and adjust.

（3）Freezing and oil plugging, during the construction, maintenance, filling fluorine, refueling process, it’s possible that there is some water in the refrigerant system. When there is moisture in the refrigerant system，it’s easy to freeze under the low temperature. The effect solution is to replace desiccant, strongly absorb moisture. While maintaining and repairing, we should avoid the air slip into the system and use refrigeration oil which has low freezing point。

In addition to the above, sometimes expansion valve will sound "sisi", this means the system lacks of refrigerant，or jams of desiccant, leading the refrigerant in liquid pipe evaporates ahead. When the gasified refrigerants go through the expansion valve hole, it sounds noise because of its high speed.
 

The temperature of return air is too high（  for constant temperature ＆ humidity air conditioning unit）

The reasons are as follow:
(1) System of return-air conditioning work abnormally, check the system pressure, compressor, fan motor operating current,and etc., to find the true failure and deal with it. (2) When there are some additional thermal things (such as adding equipments, etc.), the thermal load increases, and the temperature will not decrease.

 
The temperature of return water is too high（for water chiller）

The reasons are as follow:(1) compressor of water chiller failed，so it need to check the current and system pressure。（2）When there are some additional thermal things (such as adding equipments, etc.), the thermal load increases, and the terminal temperature will not decrease, the return water temperature will also increase.

 
water flow switch alarms, temperature of return water is too low or antifreezing switch alarms（for water chiller）

The reasons are as follow:
（1）Air handling unit doesn't start up,check and start.（2）cooling water pump failed, or the cooling water system valve didnot open enough or closed，check and deal with it.


Compressor overload switch alarms

The possible reasons include the following:
（1）The thermorelay protector value of compressor sets lower than normal, to check the thermorelay protect value is right or not. If the protect value set lower than normal，then it needs to be readjusted.
（2）The system doesn’t work normally. Step A. to check whether the ventilation system (including fan, air filter ect.) running normally, handle the abnormal components, immediately. Step B, to check the water flow (including water pump and inlet water filter), handle the abnormal components immediately.
（3）After exclude the above two reasons, if the thermal relay protector of compressor still alarms, and to check whether three-phase resistance value of compressor is normal (In general, if the resistance value under 10 ohmand the three-phase resistance value is equal, otherwise it is abnormal).

 
Cooling volume is not enough, and temperature drops slowly

After running for a period, the air-conditioners will be insufficient in cooling volume, thus the temperature drop is slowed down.This is a common problem offen happens, especially when maintenance of the equipments is not done well or right. In addition, the equipments fray as they are used, so cooling volume will become insufficient. But the change of processing that increases the heat load is another reason.

There are many ressons for the slow down of temperature drop and the insufficient cooling volume, here are the main causes.

（1）cooling volume losses a lot
As the thermal retardation material of the equipment and pipes is not thick enough, or the unti-vapor layer of the heat preservation construction has been broken, the performance of the thermal retardation material changes,thus the equipment, pipes,and valves that transmit the cold medium get a harmful tempetature rise which causes the loss of cooling colume and the decrease of temperature drop.Another reason for the loss of cooling volume is that the seal of the air-handling room is broken, and the pipes, windows and doors are leaking.
（2）compressors are low efficient
Due to the long running time, the running parts of the equipments frayed,and the cooperatig clearance enlarged or the seal became intight, thus the actual discharge rate of the compressor declined, so the cooling volume or the temperature drop rate decreased.
（3）air existed in the system
Due to the existng of air, the pressure and power consumption increases, and the relative cooling volume decreases. If air was found, it should be released according to the air-releasing steps.
（4）the evaporating temperature is too high or too low
When the evaporating temperature is too high, the temperature drops fast during the high temperature period, While during the low temperature period, the temperatue difference between the inside and outside of the heat transfering side of the evaporator is minor,and the temperature drop slowes down. The cooling volume will decreas if the evaporating temperature is too low.
（5）The temperature drop slowly because of the inadequate cooling volume which was caused by the excessive refrigerant oil in the evaporator.As R22 and refrigerant oil can dissolve together; it is difficult to release the oil in fluorin eveporator only. So when fixing the fluorin pipes, it would be better to return the oil to the crankcase of the refrigerating machine.
 

Overload alarm of the blower
（1）Maybe the protection value of the fan genrator’s thermal relay electrical equipment is small. Check the value, if it is small, adjusted it.
（2）Maybe the running of the system is abnormal; check the components (fan belt and belt pully) of the ventilation system, and handle the abnormal components.
（3）After exclude the above two reasons, check wether the three-phase resistance value of the ventilation motoris normal( generally, the resistance value is below 10Ω, and the value of the three phases are the same, if they are not, it is abnormal).