In the operation of the chiller, the low-pressure protection mechanism plays a vital role. It is the key line of defense to ensure the stable operation of the evaporator and the compressor. The low-pressure protector is usually connected to the starting circuit of the compressor. When the compressor is running, once the low-pressure protector detects that the low-pressure signal is too low, it will automatically cut off the power supply and prevent the compressor from continuing to run, thereby preventing the liquid from entering the compression chamber under low-pressure conditions and causing damage to the machine. So, what exactly causes the low-pressure protection of the chiller? Let Yuxin Air Conditioning analyze it in detail for everyone.
1. Common causes of low-pressure protection
The factors that cause low-pressure failures of chillers (refrigeration) are complex and can be roughly divided into several categories such as system leakage, system blockage, and evaporator blockage (sorted from easy to difficult). After handling each fault point, be sure to press the "reset button" on the pressure controller. It should be noted that the common low-pressure fault analysis and treatment methods are also applicable to other types of refrigeration units. (The pressure reset button is a small red button on the white plastic box on the left side of the control box. For screw-type freezers, the pressure controller is located outside the control box, and the left side is a green plastic box)
2. System leakage failure and treatment
There are leakage points in the connecting pipes of the refrigeration system, which may be caused by welding problems, corrosion problems caused by the environment, fractures caused by mechanical vibration, and human factors. During the installation and operation of the refrigeration system, if the pipe joints are not welded firmly, gaps may appear over time and due to the vibration of the unit, causing refrigerant leakage. In addition, corrosive substances in the environment may also corrode the pipes, causing them to become thinner and perforated, thereby causing leakage.
Treatment method:
It is relatively simple to deal with such problems. First, you can use a leak detector (soapy water or detergent water) or a halogen leak detector to find the leak point. When the leak point is found, use welding equipment to repair it by welding. After that, after the necessary vacuum treatment, refrigerant is filled in the amount specified by the manufacturer, and the operation of the equipment can be restored. For example, in a small commercial refrigeration system, a small leak at a pipe weld was found by leak detector. After welding repair and refrigerant filling, the chiller resumed normal operation.
3. System blockage failure and treatment
(I) Blockage by debris
Since the refrigeration system has strict requirements in every link of production and manufacturing, it is necessary to ensure the cleanliness of every link. However, most chiller (refrigeration) machine manufacturers purchase parts and assemble them, which will inevitably lead to some impure systems. In severe cases, the impurities in the system will accumulate to a certain extent, causing the filter to be blocked, affecting the flow of refrigerant in the system, and then causing low-pressure failure. Generally, blockage occurs at the inlet of the filter or expansion valve.
If the filter is blocked due to a dirty system, the cooling capacity of the equipment will decrease, or even may not be affected. When the "filter" is slightly blocked, there is a temperature difference between the inlet and outlet of the "filter", which can be felt by hand. In severe cases, the filter may condense or frost. (Normal condensation during machine operation does not count)
Handling method:
Replace the filter of the same model. For example, the cooling effect of a chiller in a factory deteriorated due to filter blockage, and it returned to normal after replacing the filter.
(II) Blockage by ice
Ice blockage is mainly caused by the presence of moisture in the system pipeline. The water in the system flows to the throttle valve (expansion valve) after throttling expansion with the refrigerant, forming a low temperature and low pressure environment at the outlet of the throttle valve. When the evaporation temperature (throttle) valve is below the freezing point, the water in the refrigerant will form ice cubes here, hindering the normal operation of the throttle valve (expansion valve), thus forming a low pressure fault. Ice blockage usually occurs at the outlet of the throttle valve (expansion valve).
Treatment method:
First, use a removable filter with "calcium chloride" (strong water-absorbing material) to thoroughly filter the water in the system, and then replace it with a disposable filter of the same model. For example, in the refrigeration system in some cold areas, due to the high humidity of the environment, ice blockage is prone to occur, which can be effectively solved by this method.
(III) Solenoid valve failure
The solenoid valve is an indispensable accessory in the refrigeration system and has the function of protecting the system safety. When the compressor stops working, the solenoid valve can cut off the flow of refrigerant to the evaporator to prevent a large amount of refrigerant in the compressor evaporator. If the solenoid valve is damaged, the refrigerant circulation is blocked, which will cause a low pressure fault. At present, some manufacturers omit the solenoid valve to reduce costs, leaving hidden dangers for the equipment.
There are two types of solenoid valve damage: one is that the solenoid valve coil is burned and the valve membrane cannot be opened. The solenoid valve coil can be measured by a multimeter to determine whether it is burned; the other is that the solenoid valve body itself is damaged, causing the valve inner membrane to be unable to open. When there is a temperature difference between the inlet and outlet of the solenoid valve, it means that the solenoid valve is faulty.
Treatment method:
If the solenoid valve fails, a solenoid valve of the same specification needs to be replaced. For example, a chiller in a shopping mall caused low-pressure protection due to a solenoid valve failure. After replacing the solenoid valve, it resumed normal operation.
(IV) Damage to the expansion valve
The expansion valve is one of the four major components of the refrigeration system. After the condensed low-temperature and high-pressure refrigerant flows through the expansion valve, it is affected by its throttling effect and expands and evaporates at the outlet of the expansion valve to form a low-temperature and low-pressure gas-liquid mixed refrigerant, thereby taking away the heat in the system and forming a refrigeration process. There are mechanical expansion valves and electronic expansion valves. Mechanical expansion valves are affected by the environment during use. For example, corrosive gases or liquids in some environments may cause corrosion of the expansion valve, mostly due to leakage of the expansion valve temperature sensing package (corrosion of the connecting pipe connecting the temperature sensing package and the valve body). Although the electronic expansion valve has the advantages of large temperature control range and high precision, its independent control system also increases the failure rate.
Treatment method:
When the expansion valve is damaged, it can only be replaced. For example, the chiller of a hotel had abnormal refrigeration due to damage to the expansion valve. The problem was solved after replacing the expansion valve.
IV. Failure and treatment of insufficient heat exchange capacity of the evaporator
(I) Insufficient water flow in the evaporator
The main factors causing this phenomenon are water pump failure or foreign matter entering the pump impeller, or air leakage in the water inlet pipe (difficult to check and requires careful analysis). The water pump is the power source of the water system. If it fails or the impeller is blocked by foreign matter, the water flow will be reduced. Air leakage in the water inlet pipe will also affect the water delivery volume, resulting in insufficient water flow in the evaporator.
Treatment method:
Replace the pump, or disassemble the pump to remove foreign matter in the impeller and eliminate the fault. For example, the chiller of an office building had insufficient water flow in the evaporator due to foreign matter in the water pump impeller. After cleaning, it returned to normal.
(II) Evaporator blockage or scaling
To determine whether the evaporator is blocked or the evaporation tube is scaled, first eliminate the pump problem and ensure that the pump and inlet pipeline are normal. Evaporator blockage or structure has a common and very obvious feature (only for medium temperature units): when the equipment is normal, there will be no large amount of condensed water or frost on the compressor surface. When you see a large amount of condensed water or frost on the compressor surface, it can be basically determined that the evaporator is blocked.
Treatment method:
If the evaporator is blocked or the evaporation tube is scaled, the evaporator should be disassembled and the evaporation tube should be rinsed clean with a high-pressure water gun or soaked in a special liquid. For some evaporators with special chemical liquids, such as the refrigerator evaporator of the aluminum oxidation (anodizing) processing plant contains sulfuric acid solution. When sulfuric acid crystals are blocked, hot water above 50 degrees can be used to circulate and dissolve in the evaporator. For evaporators in electroplating plants, such as acid zinc plating evaporators, the "potassium chloride" in the liquid may precipitate crystals on the surface of the evaporation tube, causing the evaporator to lose its heat exchange capacity, and needs to be handled according to the specific situation. For example, sulfuric acid crystals in a factory blocked the evaporator, and the problem was solved by circulating hot water to dissolve it.