Root Causes and Prevention Methods of Coil Freezing Cracks of Air Conditioning Unit

Root Causes and Prevention Methods of Coil Freezing Cracks of Air Conditioning Unit

In general, the heat medium in the heat exchanger of the air conditioning unit is water (surface cooling: 7-12°C; heating 70-90°C or steam heating). In winter in the north, when the temperature drops below zero degrees Celsius, whether it is a commercial air conditioner or a fresh air unit, the operation and management of the computer room must be carefully monitored. A little carelessness will cause the heating coil of the air conditioner, the cold coil, etc. Because the fan coil unit is mostly installed in the ceiling, once water leaks, it will cause great damage to the ceiling and indoor equipment. Therefore, the antifreeze of the fan coil unit is very important. This article mainly discusses the root causes and prevention methods of the coils of air conditioning units.

One. The root cause of freeze cracking:

　　 In order to prevent the occurrence of freezing and cracking of the fan coil, we usually heat it up, and even add partial anti-freeze heating. However, freezing and cracking incidents still occur from time to time, causing serious losses to many individuals or enterprises. The reasons are roughly as follows

　　 1. Dirty blockage. Dirty blocking is a relatively common problem. Generally, the substances that cause dirty blocking are iron filings, dust, sand, and weeds. Usually in order to maintain the stability of the water quality, an appropriate amount of sodium hexametaphosphate is required to be added to the system, but a small amount of oxygen is still easily dissolved in the hot water, which will cause corrosion of metal pipes and equipment over time and produce rust. Rust and the substances precipitated from the water after the system has been recycled for a period of time, plus accumulated dust, microorganisms, etc., are retained in certain parts of the pipes and equipment as the hot water is circulated, and they accumulate to form a dirty block and block some fan disks. Pipe exhaust valve automatic device passage.

　　 2. Air blockage. Air blockage is also a relatively common problem. The reasons for air blockage usually include the following: part of the air in the cleaning and water filling stage is not exhausted; the air overflowing in the water during the heating stage and evaporating water vapor; and the water quality is changed many times. This leads to an increase in the gas content in the system; some automatic parts of the exhaust valve are damaged, and the exhaust system is not ideal.

　　 3. The influence of outdoor temperature. In different parts of the building, the corresponding temperature is different, and the north side lacks sunlight, so the relative temperature is low and it is easy to freeze and crack.

　　 4. Management omissions. Do not ignore the sound of water flowing in the pipeline; pay attention to the occasional large fluctuations of the pressure gauge; each valve should be in the correct open (close) state.

　　 5. The control system has a large lag. When the temperature drops to the set value, the electric hot water regulating valve does not act, until the temperature further decreases, the hot water valve increases the hot water flow, but it is too late, and z* eventually causes the coil to freeze and crack.

　　 6. The installation position of the antifreeze sensor is wrong. For example, the antifreeze sensor is tied to the return water on the windward side of the hot water coil; only the antifreeze alarm is displayed remotely without interlocking with the fresh air fan and electric fresh air valve.

　　 In addition to the above points, there is another common problem in the ERV system. The following is a detailed analysis:

When residential buildings in northern areas use full heat exchangers and the fresh air volume is greater than the exhaust air volume, the fresh air temperature rises slightly. When the outdoor temperature drops sharply and the initial room temperature is low, the air outlet temperature of the ERV device will be lower than 0℃; In the initial stage, the water flow in the hot water system is not smooth, and fresh air is directly introduced into the return air box of the fan coil to reheat. The fresh air blows directly on the surface of the coil, which may cause the coil to freeze and crack. If the conditions are met at the same time, the coil will freeze and crack.

　　 For residential buildings in northern areas, in the initial stage of heating, if the outdoor temperature drops sharply and the initial room temperature is low, the air outlet temperature of the ERV device will be below 0°C. In order to prevent the hot water coil from freezing and cracking and to ensure the quality of fresh air, when the ERV fresh air supply is used in conjunction with the fan coil system, the supply air should be directly sent into the room with a separate air outlet instead of being sent through the coil after reheating. The new and exhaust air volume through the ERV device should be kept equal.

　　 For the hot water coil, if the water in the coil is smooth, the flow is constant, and the hot water temperature is high enough, the cold air carried by the cold air is not enough to reduce the water temperature in the coil to below 0°C, there will be no problem of coil freezing. The problem is that with the improvement of building energy saving requirements, the heating load of northern residential heating is reduced. The fan coil unit is only used to meet the needs of residential heating load in winter. The cooling adopts direct evaporative air conditioner, and the hot water coil is configured The previous 3 rows became 2 rows; at the same time, a single tube stroke was adopted. The longer the stroke is, the air blockage is easy to occur, and the heat storage is low. If the water pipe installation adopts the top-in-bottom-out method, the air cannot be vented well, and the property cannot vent air to each household. The cold air carried by the cold air reduces the water temperature in the coil to below 0°C, which causes the coil to freeze in the initial heating stage. crack.

　　 For residential buildings in northern areas, in the initial heating stage, if the outdoor temperature drops sharply and the initial room temperature is low, the air outlet temperature of the ERV device will be lower than 0℃, which will easily cause the coil to freeze and crack. In order to prevent the hot water coil from freezing and cracking and to ensure the quality of fresh air, when the ERV fresh air supply is used in conjunction with the fan coil system, the supply air should be directly sent into the room with a separate air outlet instead of being sent through the coil after reheating. At the same time, the new and exhaust air volume through the ERV device should be kept equal.

　　 Two. Prevention method:

　　 There are also many prevention methods for the phenomenon of coil freeze cracking:

　　 1. Prevent dirty blocking.

　　 Clean the system pipeline regularly to discharge rust, dust and other debris; regularly add chemicals to the system to prevent rust.

　　 2. Prevent air blockage.

　　 When the system is filled with water, drain the air; regularly exhaust the system; regularly check whether the exhaust valve is intact.

　　 3. The influence of outdoor temperature.

　　 Operators need to pay more attention to whether the temperature of the instrument is normal, and if any abnormalities are found, they must be dealt with in time. For locations where the temperature is relatively low, the opening of the hot water valve should be increased appropriately.

　　4. Household centralized air-conditioning system with constant pressure of tap water is easy to cause air lock due to unstable constant pressure, water flow is blocked, and the coil is frozen and cracked. Pay attention to check the air outlet temperature.

　　 5. Improve and change the control system

The plan is implemented in two steps. First, correct the original antifreeze sensor installation position, change the sensor originally installed on the windward side of the hot water coil to the return end of the leeward side of the hot water coil, and connect it with the fresh air electric air valve and fresh air unit. The fan and the electric valve of the hot water coil are interlocked. When the temperature is lower than or equal to 5°C, the fan and the electric valve of the fresh air are forced to turn off, and the electric valve of the hot water coil is opened to . Secondly, add a temperature sensor at the fresh air inlet, which is also interlocked with the fresh air electric damper, fresh air blower and hot water coil electric valve, and set the small opening of the electric hot water valve according to the outdoor fresh air temperature, namely z *The small opening is 15 minus the outdoor fresh air temperature, but it should be greater than or equal to zero; in addition, when the outdoor fresh air temperature is less than -12℃ and the fresh air unit stops running, turn off the fresh air unit fan and close the fresh air electric air valve, and turn off the hot water The coil electric valve is opened to . The above transformation is equipped with double insurance for the hot water coil, and the workload of modification on the original basis is small.

　　 6. Partition method

This method is to install a heat-preserving electric air volume control valve interlocked with the fresh air processor at the fresh air inlet. When the fresh air processor stops working (no longer need to introduce fresh air in the room), the heat-preserving electric air volume control interlocked with the fresh air processor The valve is automatically closed, and the surface cooler of the fresh air processor is no longer affected by the outdoor cold air, so the interlocking thermal insulation electric air volume regulating valve plays a role in protecting the surface cooler. This design requires the air conditioner operator to ensure that the heat The fresh air processor can only be turned on under the premise of water circulation. Otherwise close. If the hot water in the copper pipes of the surface cooler of the fresh air processor stops circulating and the fresh air blower fails to stop in time, the copper pipes of the surface cooler will be frozen and cracked and a water soaking accident will occur. Therefore, although this method is simple to install, it also has obvious drawbacks.

　　7. Preheating method

　　 This method is to install a preheater between the fresh air and the surface cooler in addition to the interlocking electric heat-preserving air volume regulating valve at the fresh air inlet. Increase the outdoor fresh air temperature to above zero in winter before contacting the surface cooler, so that in the event of other unexpected situations such as power failure, failure of the circulating water pump or blockage of the surface cooler, the fresh air will not pose a threat to the surface cooler.

　　 There are the following preheating methods;

　　 (1) Hot water, steam preheating

Install a set of steel pipes with a larger diameter (usually on the right of DN32mnuZE) between the inlet of the fresh air processor and the surface cooler, so that when the fresh air passes through this group of steel pipes heated by hot water or steam, the fresh air temperature will increase before entering Table cold section. Using this method z* It is good that the heat source for preheating and the heat source for the surface cold section are not the same heat source. 　　 (2) Electric heating preheating

Renovate the functional section of the existing fresh air unit, add an electric heating preheating section in front of the heating section of each fresh air unit, and add a control system to achieve hierarchical adjustment of heating and interlock with the fans for no wind, low wind, etc. Protect it so that the temperature of the air outlet after preheating is controlled within the range of 5℃～10℃. However, this method is limited by two factors: one is that there must be sufficient space at the fresh air unit, otherwise installation and construction will be more difficult; the other is that the fresh air volume of each fresh air unit cannot be too large, otherwise the preheating to the required temperature will consume too much power. As a result, the power configuration of each computer room is insufficient, which cannot meet the large-load power demand of the preheating section. If you use this method of heating, you should focus on how to prevent fire.

　　 (3) Natural preheating

　　 This method is to use the characteristics of the building structure to preheat the fresh air before entering the fresh air processor surface cooler. ① Install enough radiators in the fresh air fan room, and fresh air enters the computer room through the valve. The heat provided by the radiator increases the temperature of the fresh air before entering the surface cooler (there is no duct connection between the fresh air valve and the fresh air processor). ②The fresh air passes through the building air duct formed by the wall with heat to preheat the fresh air.

　　 (4) Total heat exchange heat recovery device

　　 A set of full heat exchange heat recovery system is added to each air-conditioning machine room, and the exhaust air and fresh air in the air-conditioning machine room are used for total heat exchange to realize the preheating of outdoor fresh air and also realize energy saving. However, this method is also limited by the installation space of the air-conditioning machine room, and when the actual exhaust air volume is much smaller than the fresh air heat recovery demand, this scheme cannot be realized.

　　 8. No surface cooler method

　　 This method is to turn the fresh air unit into a blower box without a surface cooler. Does not bear the cold and heat load. In summer, the heat load of the fresh air sent into the room by the blower box is borne by air conditioning equipment such as enlarged fan coils. In winter, the fresh air cooling load from the air supply box into the room is borne by the enlarged fan coil, radiator, etc.

9. If the coil is not used in winter, it is recommended: (1) Drain all the water in the coil, (2) add blind plates to the water supply and return water, (3) use compressed air to blow (depending on the size of the coil) Compressed air blowing time) (4) Add antifreeze to the coil, not necessarily full, but ensure that the liquid in the coil is fully diluted by the antifreeze. (5) A small-capacity circulating pump is connected in parallel to the chilled water system during design. ①If the system is out of service in winter but there is a heat source, the heat source can be switched to the fan-coil water system to make the small circulating pump run. ②Without heat source, an electric heater (shell and tube type parallel to the water system) can be installed in combination with a small circulating pump, so that the temperature of the water in the fan-coil water system pipe is not lower than 5°C to ensure no freezing. To ensure that the water in the system is 5°C, the electric heating power will not be too large, and the operating cost will not be high. It is more reliable than methods such as water discharge and antifreeze. What should be considered when using this method is the power outage. Once the power outage, the electric heating and the circulating pump will lose their effect, and it is easy to freeze and crack. Therefore, it is recommended not to use it in places without backup power generation equipment.

　　 Conclusion:

　　 In the design of the central air conditioner, anti-freezing in winter should be considered. The anti-freezing method recommends using hot water instead of steam and electric heating. Steam antifreeze is also possible, but it is prone to uneven temperature. The upper part of the coil has a relatively high temperature and the bottom has begun to freeze. Electric heating is also not supported. first electric heating is prone to danger, and then electric heating consumes more electricity and does not save energy. In the control, it is also necessary to pay attention to the single coil switch, which should be controlled by all.

　　 In short, there are many ways to prevent freezing of fresh air processors in winter, and they are not limited to those listed above. Designers can flexibly apply according to specific conditions in actual design. Freezing and cracking of coils is common in the north, especially in the northeast. Therefore, practical protective measures are very important. Only by conscientiously formulating and implementing the operating procedures and continuous improvement to improve the anti-freezing technical solutions and facilities, two-pronged approach, is the effective protection to avoid the freezing and cracking of the coil.