Discussion on ills of central air-conditioning water system

Discussion on ills of central air-conditioning water system

The central air-conditioning water system is a relatively complex system, which is very important to the operation effect of the central air-conditioning system. The article summarized several common ills of central air-conditioning water system, discussed their causes, and proposed corresponding improvement measures.

　　1. The problem of hydraulic imbalance in the central air-conditioning water system

　　 One of the more prominent problems in the central air conditioning water system is the hydraulic imbalance. For some large-scale and more complex systems, there are usually many control loops. Due to the different sizes of loops and different lengths of pipelines, hydraulic imbalance will occur if you are not careful.

　　1.1 The impact of hydraulic imbalance on cold and heat source units

　　 Keeping the flow rate of the cold and heat source unit within the limit specified by the unit can protect the equipment from damage. When the flow rate is lower than the design flow rate of the unit, the safety device will stop the unit from running. Turning on and off at all times will make the output of the unit lower than the power required by the indoor load. At the same time, if the water volume suddenly decreases, the controller has no time to react, and it is too late to adjust the output of the unit, and the water may freeze in the pipe. Is quite serious. If multiple units are used in parallel, as the load decreases, the design unit capacity will be several times the capacity required by the load. When more units are actually put into operation than actually needed, some units will repeatedly start and stop for a long time, and the start-stop cycle is very short. In this way, the efficiency of the unit will decrease and energy consumption will increase, and the service life of the unit will be shortened.

　　 In order to ensure the good operation of the unit, a reasonable method is to install a balance valve at each unit so that the flow can be adjusted to the design value. For cooling towers installed in parallel, a balance pipe should be installed on the outlet pipe to ensure the balance of the water volume of each cooling tower.

　　1.2 The impact of hydraulic imbalance on the transmission and distribution system

　　In the transmission and distribution system, the loop farthest from the water pump has the smallest differential pressure due to its large resistance, while the loop closest to the pump has the largest differential pressure value. If there is no measure to make up for this difference, then the water flow will be much higher than the design flow near the pump section or the loop with low system loop resistance; otherwise, it will be much lower than the design value, and the water flow in the entire system will be unevenly distributed status. This uneven amount of water will make the room temperature uneven in the building, and the room temperature will continue to fluctuate; the room near the chiller is too cold, and the room at a distance is higher; in addition, the room with a large flow rate will reach it relatively quickly The required room temperature, the room with a small flow rate loop takes a long time to reach the required room temperature.

　　 A better way to solve the hydraulic imbalance caused by the difference in loop pressure is to install a balance valve on the return water main pipe of each loop to adjust the flow rate of each loop to the design requirement value.

　　1.3 A problem worthy of attention

For a variable flow system using a two-way control valve, when the room temperature reaches the required value, the opening of the control valve will be closed to reduce the output of the terminal device. However, as the control valve is closed, it will act on the control valve. The differential pressure will increase. In this way, even if the valve is closed again, the flow rate will be higher than the theoretical value due to the further increase of the differential pressure. Eventually, it may appear that all the valves operating at low load will operate in the on/off mode, causing oscillation. As the total water volume in the system decreases, the pressure drop in the pipeline also decreases, which increases the pressure drop of all end devices in the system. The pump operating point moves upward along the pump characteristic curve, and the pump lift increases, making these control valves bear close to The differential pressure value of the highest head causes the control valve to malfunction and produce noise.

　　The solution is to short-circuit a set of self-acting differential pressure control valves on the water supply (return) pipe to keep the total water volume of the system unchanged, so that the branch pipe loop maintains a constant differential pressure value.

　　2. The problem of blockage of air conditioning water system

　　 It is also a common problem that the air-conditioning water system cannot work normally due to the blockage of the pipe. In the process of commissioning and trial operation, it is often found that the effect of air conditioning in some rooms is poor or even ineffective. After searching, it was found that many cases were caused by the construction of hemp wire, iron powder and other debris blocking the pipeline.

　　 Therefore, the cleaning of the piping system is directly related to whether the air conditioning system can work normally, and attention should be paid to the following aspects.

　　(1) First of all, in the design, a large-diameter blow-off valve should be considered at the lowest part of the pipe network to facilitate the blowdown during cleaning. If the design adopts an automatic exhaust valve on the top of the pipe network, the construction unit should not rush to install the exhaust valve before the pipe network is cleaned, so that the water in the pipe can be drained as soon as possible when filling water and draining sewage.

　　 (2) Set up a "Y" filter on the main equipment and terminal equipment inlet pipes to prevent debris in the pipe network from entering the equipment and terminal equipment, causing blockage and scrapping. For the filter of the fan coil in the ceiling, it is best to install it in the water collecting pan for easy disassembly and washing. If the water collecting pan is not large enough, the manufacturer can be required to increase it appropriately; if this is not possible, the blocking part of the filter should be made into a detachable heat preservation during the heat preservation, so that the filter can be removed and washed in the future maintenance.

　　(3) A short-circuit valve can be installed between the inlet and outlet pipes of the main equipment. When flushing, close the inlet and outlet valves, and open the short-circuit valve to flush the entire pipe network. For the end device is a fan coil, it will be difficult to install the short-circuit valve due to the installation space, so you can consider measures to facilitate flushing on the water supply and return mains. If flushing valves are installed at the end of the water supply main pipe and the head end of the return water main pipe, the horizontal main pipes for the water supply and return water are flushed separately, and the flushing direction should be the same as the water flow direction when the system is running.

　　(4) An emptying valve is installed on each layer of horizontal main pipe to facilitate layered flushing and emptying and subsequent daily maintenance.

　　(5) The water quality should be monitored regularly to avoid blockage of terminal devices and corrosion of pipelines due to hot and cold water pollution. For the chilled water system, it is best to consider installing an electronic descaling device from the design, and to stabilize the water quality every year, and the cooling water system should also be treated every year.

　　3. Condensation problem of air conditioning water system

　　 (1) Water leakage due to the condensate drain pipe with a small slope or no slope at all, or due to uneven installation of the fan coil's collection pan or blockage of the drainage port and drain pipe in the fan coil is the most common malpractice.

Solution: First, the condensate horizontal pipe must be guaranteed to have a slope of no less than eight thousandths, and the section of the condensate pipe should be large enough; it is recommended that the branch pipe be no less than DN20, and there should be no longitudinal bending in the middle to avoid air resistance and make the water flow; The water collecting pan of the fan coil should be inclined about 1% to the drain hole. For some self-made water collecting pans, the flange of the pan should be compensated for the level height so that the water capacity is not reduced due to the inclination. It is not appropriate to tilt the disk too large or too small. PVC pipe or ABS pipe is recommended for the selection of condensate drainage pipe. The inner wall of the pipe is smooth and pollutants in the water are not easy to sag.

　　 (2) Poor thermal insulation at the interface between the chilled water pipe and the valve can easily cause condensation on the surface and cause condensation to leak. During the heat preservation operation, the construction should be carried out in strict accordance with the standard drawing, the heat preservation layer should be close to the pipe wall, and the gas barrier should be tight and sealed. In addition, it is impossible to keep the valve stem from dripping water during operation, and the exposed valve stem and handwheel will still be condensed, so a waterproof layer must be provided between the valve insulation and the pipeline insulation. It is strictly forbidden to use inferior thermal insulation materials with different densities and thicknesses in engineering.

(3) If the drainage port on the water collecting pan of the terminal device is very small, when the fan is started, the water collecting pan will be under negative pressure, which will cause the gravity drainage to be unsmooth, not only may cause the water collecting pan to overflow, but also from the drain pipe Inward air leakage, if the condensate pipe is directly introduced into the manhole, bacterial contamination will also occur. It is recommended that the manufacturer change the fan's water collection pan to the positive pressure area of ​​the fan. If there is any difficulty, enlarge the drainage port of the water collection pan or set a water seal. If it is a floor fan, its foundation can be increased to 150mm higher than the ground, and a water seal is added at the lower part of the drain pipe, or a water seal is installed under the condensate standpipe.

　　4. The choice of water pump and flow limitation

(1) For high-rise buildings, the pressure strength of the pump must not be ignored when choosing a water pump. In the case of no vertical partitions in the system, the water pump installed at the bottom of the building bears a lot of hydrostatic pressure, and the design often does not provide the pump resistance. Pressure requirements, coupled with water pump samples also do not have this parameter. If the construction unit ignores this problem when purchasing, it will often cause the water pump shell to break after a period of operation. Therefore, when purchasing the water pump, it must be based on the actual needs of the project. Put forward the pressure requirements of the pump.

　　(2) In the centralized cold and heat source system of the air conditioner, whether it is a chilled water system or a cooling water system, a design with multiple pumps in parallel is common.

However, in actual operation, most of the time is often part-load operation, sometimes only one pump is needed. At this time, the friction of the system is greatly reduced and the water volume rises, which may cause the pump motor to be overloaded. , Affecting the normal operation of the refrigeration system.