How to adjust the hydraulic balance of air conditioning system
In air conditioning water systems, hydraulic imbalance is the most common problem. Due to the hydraulic imbalance, the system flow distribution is unreasonable and the heat is unreasonable, which causes a waste of energy, or to solve this problem, increase the pump head, but still produce uneven heat (cold) and greater electrical waste. This article explains the reasons for the selection of hydraulic balance valves in air conditioning water systems, and introduces the characteristics of hydraulic balance valves, as well as the steps and methods of applying hydraulic balance valves to hydraulic balance adjustment of the water system, and the requirements, process and evaluation of system joint adjustment. As an important part of the air conditioning system, the rationality of its design not only determines whether the entire air conditioning system can achieve high efficiency and energy saving, but also determines whether the air conditioning system can operate normally and stably. After nearly a hundred years of development, the overall theoretical development of the air-conditioning water system is relatively complete, but there is less research on its adjustability and balance. In actual projects, the water system is not well adjusted and the water power is out of balance. At present, a large amount of work on energy saving of air-conditioning is mainly aimed at the research and development of high-efficiency refrigeration equipment and the matching of building and air-conditioning forms. However, insufficient attention has been paid to the adjustment and optimization of water system control and design, resulting in good design schemes. Poor adjustability of the water system and hydraulic imbalance appear. With the emergence of various new technologies and new equipment, people have increased requirements for the control accuracy of the air conditioning system and higher requirements for energy saving, making the air conditioning water system the focus of recent air conditioning technology research. In order to achieve the rationality of system energy utilization, the system needs to have good adjustability and balance, which is also the z big problem faced by the current water system. 1. Common methods of hydraulic balance To ensure the good operation of the air conditioning chilled water system, the hydraulic balance of the system should be met first. At present, as the scale of the system expands and the complexity of the system increases, hydraulic balance becomes more and more important. Many groups and scholars have done a lot of research on hydraulic balance. 1.1 Hydraulic balance of constant flow system The constant flow water system is a common hydraulic system in central air conditioning. The system does not contain any dynamic valves. After the system is debugged, the valve opening generally does not change anymore. The flow of each branch loop of the system basically remains unchanged during operation. change. Constant flow systems are mainly used for systems where terminal equipment does not need to be adjusted by flow, such as terminal equipment with three-way regulating valves, fan coils regulated by three-speed switches, and air conditioning systems with variable air volume air handling units. The constant flow system only has static hydraulic imbalance, and there is no dynamic hydraulic imbalance, so it is only necessary to install static hydraulic balance equipment in the corresponding position. Commonly used hydraulic balancing equipment for constant flow systems are throttle orifices, manual control valves, static balancing valves, and dynamic flow balancing valves. When the water volume of the terminal equipment does not change, a throttle orifice, a manual regulating valve, a static balance valve, and a dynamic flow balance valve can be installed on the return pipe of each loop. 1.2 Hydraulic balance of variable flow system In order to save energy, variable flow water systems are increasingly used in air conditioning projects. During the operation of the variable flow system, the flow of each branch loop changes with the load. Since the air-conditioning system operates under part-load conditions for most of the year, and the system water flow is lower than the design flow most of the time, the variable flow system is highly efficient and energy-saving. But the variable flow water system has a big shortcoming in that the parallel loops have a strong cohesion, and the hydraulics will cause mutual influence and dynamic hydraulic imbalance. To achieve dynamic hydraulic balance, it is necessary to satisfy that the flow rate of each terminal device in the water system reaches the actual instantaneous load requirement flow rate. At the same time, the flow rate change is only affected by the change of the equipment load, and not interfered by system pressure fluctuations. The purpose of the dynamic hydraulic balance of the variable flow system is to ensure the instantaneous consistency of the system supply and demand water (this function is realized by various regulating valves), avoid the mutual interference of the flow changes of each terminal equipment, so as to ensure the efficient and stable flow of the system Accurately convey to each terminal device. Two, the choice of control valve The common method of adjusting the cold and heat output of the air conditioning system is to throttle the chilled water of the air conditioning system. Throttling is accomplished through various regulating valves. The regulating valve is an element in the piping system and at the same time an important link in the automatic regulating system. Therefore, the selection of the regulating valve in the air conditioning water system is very important. When the pressure difference across the regulating valve changes, the regulating characteristics of the regulating valve will also change. In order to ensure the reliability of the regulating valve during pressure changes, the valve authority of the regulating valve must be considered. The valve authority of the regulating valve is defined as the ratio of the pressure difference on the valve to the total pressure difference of the system when the regulating valve is fully opened, as shown in Figure 1.
When the control valve is fully open, the pressure difference ΔPmin is equal to the total service pressure difference minus the pressure drop of the terminal device, pipeline and its accessories. When the control valve is closed, since the flow is zero, there is no pressure drop on other components, and the available pressure difference ΔPmax all acts on the control valve. The ΔPmin refers to the pressure difference to obtain the design flow when the valve is opened. When the valve is close to closing, the pressure difference between the two ends of the valve will increase and cause the deviation of the valve characteristics. The degree of deviation depends on the valve authority. At the same time, it should be noted that when the system pressure changes, ΔPmin and ΔPmax will change at the same time at the same ratio, and the valve authority SV remains constant. Valve authority is only related to the initial selection of the control valve. The function of the control valve is to generate a supplementary pressure drop in the hydraulic circuit to limit the water volume to the required value. The choice of the control valve is to enable the valve operating under the design conditions to provide the required flow under the applied pressure difference. When the control valve is selected too large, the control valve often has to work in a position close to closed, resulting in unstable control. In the system startup phase, if the valve is too large, the flow will exceed the design flow, while other devices will under flow. In the selection process of the control valve, the key is to estimate the pressure difference across the valve. In practice, the pressure drop produced by the valve should be equal to the total available pressure minus the pressure drop of the circuit it controls, and these pressure drops are estimated according to the design flow. In the actual valve selection, it is generally required that the selected regulating valve is usually smaller than the pipe diameter, mainly because the small diameter can improve the adjustment accuracy and save investment. 3. System hydraulic balance adjustment The mission of the central air conditioning system is to provide users with a comfortable indoor environment at a low cost (operating cost). Among them, the water system has a large capacity to transport cold and heat per unit energy consumption, so it is the main method of long-distance transport of cold and heat often used in large-scale air-conditioning projects. According to current statistics, in actual operation of the central air-conditioning system, there is a general hydraulic imbalance problem. Therefore, it is necessary to analyze the hydraulic balance of the air conditioning water system. For most of the current HVAC water systems, the system should be adjusted so that all hydraulic balance valves reach the design flow at the same time. The specific steps of system hydraulic balance joint adjustment are as follows: 1) Adjust the shutoff valve and hydraulic balance valve in the system to the full open position, and adjust them to the z position for other dynamic valves. For example, the temperature control head must be removed for the radiator temperature control valve Or set it to zlarge opening position; 2)Grouping and numbering the hydraulic balance valves: follow the sequence of the first-level parallel valve group 1~6, the second-level parallel valve group I, and the system main valve G, as shown in Figure 2;
Figure 2 Schematic diagram of series-parallel system
3) Measure the actual flow Q real of the hydraulic balance valve V1~V18, and calculate the flow ratio q=Q real/Q design;
4) Analyze the flow ratio of the hydraulic balance valve in each parallel valve group. For example, analyze the flow ratio of the hydraulic balance valve V1~V3 of the first-level parallel valve group 1. If q1
