The amount of heat dissipated at the end of the control loop is generally nonlinear with respect to the amount of water. This non-linearity of the end device can be compensated by a control valve with the opposite characteristics. This forms a system (valve + end device) that gives a linear relationship between heat dissipation and valve opening and therefore a linear relationship with the control signal. However, the characteristics of the valve are based on a constant differential pressure across the valve, but the differential pressure will vary with changes in water volume, thus causing deviations in the actual characteristics. Therefore, the amount of heat dissipated by the terminal device tends to be more or less nonlinear with respect to the degree of opening of the valve, and the degree of non-linearity depends on the valve characteristics and the valve authority of the valve. If the degree of non-linearity is severe, it is difficult to control at medium loads and light loads, because small flow changes at this time can cause large variations in heat output. If the loop is in excess of water, control becomes poor. Under the design load conditions, the control valve can only work close to the closed position, which leads to unstable and inaccurate control. Overflow is the number one obstacle to effective control, the danger is not only the flow rate is too large loop control valve will run most of the time close to the closed position caused by shock, and part of the loop flow is too large for the other loop flow , Can not even provide design effort. Therefore, you should avoid excessive traffic at any cost. Balance valve and line balance are necessary for all control loops in order to achieve stable and precise control. Cold (Heat) Source Keeping the hot (cold) source unit flow within the limits specified by the manufacturer can protect the equipment from damage, but this does not guarantee good operation, a satisfactory indoor thermal environment, and low energy consumption. If the flow rate is lower than the unit design flow, it will not reach the installed capacity. When the load exceeds a certain value, which depends on the actual flow rate as a percentage of the design flow, the safety device will stop the unit so that when the load is higher than this value the output will be lower than required power. Even installed capacity in the case of several times the design load, there will still be unable to provide sufficient output. In the system with this problem, the number of units actually put into operation exceeds the number of units actually required. Some of the units are repeatedly turned on and off shortly afterwards, which results in low productivity and high energy consumption. If the flow rate exceeds the design value in a certain unit, then the flow of other units will be lower, resulting in the above problem. If the unit flow is in line with the design flow, but the flow of the distribution system is too large, the temperature of the water supply in the heating area will decrease, while the temperature of the water supply in the air-conditioning area will increase, thus failing to design the power. In order to ensure good operation anyway, it is necessary to avoid over or under traffic. In order to be able to measure the flow rate and to adjust the flow rate to the design value, it is reasonable and appropriate to install a balancing valve at each unit. The balance valve is not only used to adjust the flow rate to the designed value, but also to check the coordination between the hot (cold) source side water volume and the water distribution system side water volume. Installation of balancing valve at the unit has the following advantages: (1) The problem can be easily detected and corrected. (2) The water quantity of the boiler and chiller can be checked and kept within the range specified by the manufacturer, so as to avoid the loss of the generator set. (3) Boiler and chiller can be adjusted to achieve the design flow value respectively, so as to minimize the number of units put into operation under various loads and minimize the number of units to be opened or stopped. (4) Adjustable (cold) The heat source side is coordinated with the delivery side water volume to ensure that the required power is always provided. Transmission system Pump head should be the most adverse system loop access to design flow to select. However, this will result in excessive differential pressure at all remaining end devices, resulting in uneven flow distribution. Balance is a simple way to get the right traffic distribution. The principle is to use a counterbalance valve to eliminate the residual pressure differential of the loop to achieve design flow in all loops. Eliminating excessive flow means that the control valve will not operate in a close-to-close position at medium and light loads so that there will be no unstable control and room temperature fluctuations. Eliminating too little flow means that all end devices provide their design output under any operating conditions. Uniform flow distribution will reduce the difference between the room temperature, it is because the temperature throughout the building is more consistent. This not only greatly improves the comfort level, but also saves energy by reducing the average room temperature of the heating system and increasing the average room temperature of the air-conditioning system under the condition that residents in the building can not complain. Balance the benefits are as follows: (1) room temperature difference between different rooms to reduce, improve comfort. (2) In the absence of residents complain about the average room temperature in heating can be reduced, air conditioning can be increased, thus reducing energy consumption. (3) The convergence of the system and the singleness allows the centralized controller to be effectively controlled and to apply the same control curve to the entire building. (4) zone controller, or thermostatic valve can be effectively controlled. This is because they operate in an ideal or near-ideal working condition, with no room temperature fluctuations and improved comfort. Therefore, it is of vital importance to install a balancing valve at each stem, riser, branch and end unit. Once the counterbalance valve is installed, the flow can be measured and adjusted, and there is a very handy tool to find and solve problems in the system. The characteristics of the end device have one thing in common with all types of end devices in a hydraulic system. That is, when the temperature of the water supply on one side of the loop is constant, the amount of heat released is nonlinearly proportional to the amount of water. As shown in the figure convex curve suitable for the air coil, heating radiator, convector and radiant floor heating and air coil design percentage of the end device heat dissipation depends on the following factors: 1. Under different loads, the average temperature of logarithm of primary and secondary loop water. The heat transfer coefficient, which varies depending on factors such as flow pattern (laminar or floc) and the condensate on the coil surface. 2. Control valve characteristics When the valve ends to maintain a constant pressure, the valve opening and the relationship between the amount of water is defined as the static characteristics of the control valve. Valve opening and water are expressed as a percentage of the corresponding maximum. For linear characteristics of the valve, the water is proportional to the opening. At light loads and loads, a small change in control signal causes a large change in heat output, resulting in instability of the control loop due to the fact that heat dissipation is much more sensitive than water. Our goal is to have a linear relationship between the output of the end unit and the opening of the control valve. In this way, the amount of heat dissipation is proportional to the controller's signal so that the stability of the control loop does not depend on the load, so that the proportional band can be set to a minimum value. In order to achieve this goal, we need a valve that compensates for the non-linear nature of the coil. If the coil dissipates 50% of the design flow at 20% then our goal is to provide 20% of the flow when the valve is 50% open, which results in a 50% valve opening and a coil heat loss of 50% %. 3. Overflow of Control Valves To achieve stable and precise control, the following three requirements must be met: 1 All control valves have Kvs values ​​calculated by the system designer. 2 All pumps should provide accurate pump head. 3 With the control valve and pump selected by the system designer, the pressure drop across all end devices should be exactly as designed. Pump due to model reasons, for the selection of pump flow may differ from the required value of 10 to 40%. In order to ensure adequate flow, the pump is usually used 10% higher than the required flow, and sometimes up to 25%. Air Cooler, Radiator and Other Final Equipment: These devices are also available only on the market. In fact, no one will choose a smaller model of the air coil. The problem is not only the large size of the device itself, but in fact the variation of the device to a wide range of devices varies in practice. In practice, the amount of water in the end-of-loop device tends to be 0% to 200% above the designed water volume. This means: 1 Many control valves are still operating in a small section of their range of operation, even if the distribution system is indeed balanced. The larger the size, the smaller the running section. 2 The valve characteristics as determined by the manufacturer may deviate to a greater or lesser extent due to the degree of valve power in the system. The smaller the valve weight, the greater the deviation of the valve characteristics, making the control worse. Transmission and distribution system building uneven temperature, continuous fluctuations in room temperature, flow distribution caused by uneven regional controller following two major problems, then the residents is a headache. 1 inconsistent room temperature. Because of the excessive flow of some end devices, this alone comes at the cost of low flow rates at other end devices. 2 high flow loop at medium and light loads at room temperature in a fluctuating state, because the flow of high pressure to force the control valve is close to the closed position. Balance and energy consumption 1 makes it possible to reduce the average room temperature in buildings during heating and to increase the average room temperature in air conditioners. 2 makes the controller work effectively. 3 reduce the pump energy consumption.