Study for maintenance the water flow switch of chiller
(the copy rights belongs to A.YITE TECH --- ZHOU ENSI LTD, if reproduce, must marked the original web address)
Abstract: In view of the annual maintenance of chillers, it is very necessary to check or replace the target flow switch. We propose the method of replacing the target flow switch with a differential pressure flow switch in the field, which completely avoids the problem of the target flow switch.
Central air-conditioning have a large cooling capacity, using large amounts of screw compressors or centrifugal compressors. The maintenance and repair costs of these two compressors are very high, and the price of compressors accounts for more than half of the price of the entire mainframe. It is especially important to ensure reliable operation of the compressor. The condenser and evaporator of the chiller use water as the medium for secondary heat exchange. The proper water flow is the necessary guarantee for the reliable operation of the main engine. The improper water flow may cause the chiller evaporator to freeze and the condensing pressure is high. Compressor "jumping machine" and other faults, so the appropriate water flow detection method and detection components are to ensure that the unit only works when the system water flow is greater than the allowable * small water flow, to avoid failure of the air conditioning host. In view of the importance of water flow detection, many project tenders have begun to require the type of water flow switch to be differential pressure, taking into account the user's future maintenance costs.
Necessity of water flow monitor in air conditioning system
In normal operation of the refrigeration system, the compressor stops running when the water flow is reduced to the disadvantage of the refrigeration system. Reliable heat exchange between evaporator and condenser. The flowing water takes away the cold and heat.
The detection of water pump operation, the normal operation of water pump is the precondition of reliable operation of air conditioning system.
When there is no water flow , or there is little water flow, the load of the evaporator is reduced and the evaporation temperature is reduced. If the compressor continues to operate, it will cause the evaporator to freeze. If the evaporator cannot be protected from freezing in time, the evaporator will and may burst, causing the water side and the refrigerant side of the refrigeration system to collude, and the compressor will pay an expensive overhaul cost. In addition, if the water flow of the water system is too low for a long time, the return pressure will be too low for a long time, and the lubricating oil discharged from the compressor will not return to the compressor smoothly, which may lead to compressor failure.
When there is no water flow or there is little water flow, the load of the condenser is reduced, and the temperature and pressure of the condenser are increased, which causes the flow of the refrigerant at the outlet of the condenser through the expansion valve to be greatly reduced, and the cooling capacity to be reduced. If the compressor continues to operate under the high refrigerant pressure, the "trip" fault will occur.
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Common problems of target type flow switch in water chiller
At present, there are two low-cost water flow detection methods in air conditioning system, one is target flow switch, the other is differential pressure flow switch. In the past, a considerable part of the emergency maintenance calls we received from users were related to the failure of the target flow switch to work normally. According to our observation, the following situations are common:
Because the target flow plate of the target water flow switch is in the bending deformation state under the pressure of water flow for a long time in normal use, which is easy to be damaged by fatigue, we stipulate that the target flow plate for the maintenance of the water chiller must be replaced after 2 years of use. It is listed in the maintenance record as a vulnerable product and its condition, but some users think it is not necessary to replace it in order to save money. As a result, it fails during operation.
The target flow switch installed in the vertical pipe has a great impact during the start-up and shut-down process, and the target flow plate's service life does not exceed one year, or the bellows inside the target flow switch is broken, which results in water flowing out of the target flow switch. Since the installation of target type flow switch requires a certain amount of straight pipe before and after, if it is installed near the water pipe with a right angle elbow rising, the above situation still occurs.
In addition, when the water system is mixed with air, the air in the water impacts the target flow plate and causes the flow velocity to drop. The target flow plate resets instantly and sends the wrong flow information to the unit, which causes the chiller to alarm and shut down.
Application of differential pressure Type flow switch
The differential pressure type water flow switch, produced by "A.YITE TECH in Hong Kong http://www.ayite.net", is designed according to the curve of resistance and flow of HVAC equipment. We know that the heat exchanger, water filter, water pump and valve of HVAC equipment all have their performance curve of resistance and flow. By detecting the difference of inlet and outlet water pressure at both ends and comparing with the preset value of the device, the flow can be accurately controlled 。 Compared with the target type flow switch, the differential pressure type is a * flow control mode, which has accurate flow control value. From the pressure difference switch connecting two copper pipes to the inlet and outlet of the heat exchanger, measure the pressure difference between the inlet and outlet, that is, reflect the flow. The installation is very simple, which avoids the hidden trouble of unit failure caused by inaccurate installation of target water flow switch.
In view of many problems of target type flow switch, we started to replace the original target type flow switch of water chiller with differential pressure flow switch as early as 2002, and adjusted the user's water flow, which has received very obvious economic benefits. The replacement steps are as follows: we take out the original target type flow switch and seal it with a plug, and connect the pressure gauge at the inlet and outlet of the heat exchanger The position is connected with a three-way connector, and the extra pressure tap is provided to the differential pressure type flow switch. The additional pressure tap on the inlet and outlet of the heat exchanger is respectively connected to the "high" and "low" pressure ports of the differential pressure switch. In order to be fully responsible for the users, we use ultrasonic flow meter to measure the flow protection value, adjust the set value of differential pressure flow switch to make it reset to 70% of the rated flow, that is, when the flow of water through the heat exchanger is less than 70%, the water flow switch will not reset, avoiding the performance degradation or failure of the water pump, insufficient opening of the field valve or closing of the end coil and pressure When the differential bypass is abnormal, the water flow drops, which leads to the hidden trouble of the main engine failure. When the water flow suddenly drops to 60% of the rated flow when the air conditioner is running, the differential pressure water flow switch is immediately protected. The function of differential pressure flow switch * water flow protection is incomparable to target flow switch.
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Add: Building 17 &22, No.2928, Chuan Zhou Highway, Shanghai, 201319, China