Abstract: Based on the mathematical equation of the flow characteristics of the control valve, the formulas for calculating the flow cross-sectional area of ​​the linear and equal percentage type control valves are derived. The problems existing in the design of the valve shape of the existing control valve are pointed out. Thermal system control valve selection principle, that should be based on the ability to flow through the computer to choose the diameter of the control valve, rather than by the pipe diameter to determine, and that the control valve installed at different locations, the formula in the coefficient of difference . Keywords: spool-type line flow characteristics of the circulation capacity of circulation 1 Introduction In recent years, in order to solve the heating, air conditioning system, hydraulic imbalance, uneven heating and cooling problems, automatic control system is applied more and more, so the control valve A wide range of applications, while regulating valve performance, especially the flow characteristics of the requirements are also getting higher and higher. Control valve flow characteristics, that is, the flow rate changes with the valve opening relationship, depending on the spool of the profile and its position in the system. At present, in the valve design, even if the flow characteristics of the percentage of the valve, in practice will become a quick opening characteristics, the system can not be normal adjustment. The reason is many, but there is an important reason is that there are some problems in the design of the existing control valve spool, which makes the working parameters of the control valve inconsistent with the design parameters and in many cases can not meet the requirements of operation regulation. In order to do this, the spool design needs to be corrected. In addition, the control valve size or caliber choice is also very important, it has a direct impact on the quality of the control system is good or bad, so the need to adjust the valve according to the characteristics of the object, the use of regulating valve occasions and the ability to circulate the correct choice. 2 spool line calculation formula Valve spool shape can basically be divided into three categories: plunger, open and sleeve. But no matter what spool, can have the same flow characteristics, each flow characteristics have the same mathematical model and mathematical equations. The most widely used linear equations with linear flow characteristics and logarithmic equations of equal percentage flow characteristics. Linear flow characteristics and equal percentage flow characteristics refer to the ideal flow characteristics of the control valve, the ideal flow characteristics are obtained in the case of constant pressure across the valve. Obviously, the actual pressure drop before and after the valve is not constant. Therefore, the study of regulating valve is the most important study of its working characteristics, that is, the actual conditions of flow and valve opening changes in the relationship. From the literature [1] shows: linear flow characteristics of the mathematical equation: (1) Where: G / Gmax - the relative flow, that is, the control valve at a certain opening of the flow and full open flow ratio; L / Lmax- - the relative opening, that is, the control valve opening under a certain degree of opening and the ratio of the total open stroke; k - constant, that is, the amplification factor valve. (2) where R is the adjustable ratio, that is, the ratio of the maximum flow to the minimum flow that can be controlled is adjusted, R = Gmax / Gmin. Equivalent percentage flow characteristics of the mathematical equation (3) (4) (2), (4) only applies to the computer flow characteristics for the linear and equal percentage of the flow control valve opening under each opening can not be used Calculate the flow of each opening cross-sectional area, and this is exactly the design of the existing control valve spool where the literature [2], etc. that is: G / Gmax = F / Fmax. In order to calculate the flow cross-sectional area of ​​each opening, available from the literature [3]: (5) Where: G - flow, kg / s; α - flow coefficient; F - flow control valve cross-sectional area, m2; Ï - medium density, kg / m3; Δp - before and after the control valve pressure, Pa. By equation (5) available: (6) under the superscript max is the maximum stroke of the control valve of the various parameters. (7), (8), respectively, the flow characteristic curve is linear and equal percentage adjustment Valve flow cross-sectional area formula. The flow coefficient α can be approximated by literature [3], and can be calibrated according to the principle. 3 Δpmax, Δp value 3.1 In order to better use (7), (8) two equations, to determine the Δpmax, Δp value. Δpmax valve when the valve is fully open, the pressure difference between it and the total pressure regulating system is called the valve power PV [1], also known as valve capacity. (9) Where PV is the control valve valve authority; Δpx for the system resistance pressure drop, Δpmax + Δpx = Δps. See Figure 1. Figure 1 PV value will be related to the quality of the system regulation, as shown in Figure 2 [1]. In actual work, the ideal straight line characteristics tend to open characteristics, the ideal percentage characteristics tend to linear characteristics, the smaller the PV value, the worse the quality of the system adjustment. Therefore, in actual use, it is generally desirable that the PV value be no less than 0.3 [1]. Figure 2 regulating valve operating characteristics 3.2 Δp calculation Δp is the control valve at a certain opening under pressure. The value of Δpmax and Δpmin (Δpx for the minimum adjustable valve flow rate when the pressure difference, preferably 0.95Δps) between the fluctuations can be used interpolation method to estimate that the pressure regulator valve with the flow rate proportional to the proportion (10) Based on the above formula, we can get: (11) where Φ is the coefficient related to the PV value of the valve authority. When PV = 0.3, Φ = 0.217; PV = 0.5, Φ = 0.9. By substituting (11) into (7) and (8), the flow cross-sectional area at each opening can be calculated, so that the spool profile can be designed. For the proportional control plunger valve, assuming that PV = 0.3, L / Lmax = 0.5 and R = 30, Φ = 2.17, G / Gmax = 0.138 and α / αmax = ), (11): F / Fmax = 1.4 G / Gmax. Obviously, F / Fmax ≠G / Gmax, and the deviation is very large (with the valve weight PV value has a great relationship). This shows that: In the ideal case of the design is not the percentage of equal percentage type control valve, but the linear type, or even quick open type (relative opening from 0 to very small range, the relative flow from 0 to 80%), not to mention in the actual work? This is a "congenital" flaw in the design of the control valve spool, which further results in poor performance of the control valve in actual operation. 4 heating system selection of the control valve 4.1 Selection Principle The ultimate goal of the heating system is the balance of thermal conditions, requiring the flow change, while the heat sink (or heat exchanger) to adapt to changes in heat load. In other words, changes in the opening of the control valve and radiator heat dissipation into a linear relationship, which is the best regulation of the heating system. In other words, the product of the amplification factor of the control valve and the amplification factor of the adjustment object is unchanged in the adjustment process as described in reference [1]. From the literature [4] can draw the relationship between the radiator flow and heat dissipation, as shown in Figure 3. Q is the relative heat capacity, refers to the heat sink at a given flow rate and the rated flow (design flow) the ratio of heat, G is the relative flow of the radiator, curves 1,2,3,4, respectively, for the return water The temperature difference is 10,20,30,40 ℃ when the radiator flow and the relationship between heat. It can be seen from the figure that: when the flow rate is small, the change of the flow rate has a great influence on the heat dissipation of the radiator; when the flow rate is large, the influence is small, that is, the amplification coefficient of the radiator heat dissipation decreases with the flow rate. Figure 3 Analysis Figure 2, Figure 3: In order to get the radiator relative heat dissipation Q / Qmax and the relative opening of the valve opening L / Lmax linear relationship, you must select the percentage performance of the control valve. This is the case for radiators and heat exchangers, as long as the medium is hot water, which is the case, while the linear control valve will not reach the linear relationship requirements. For different temperature difference between supply and return, the rate of change of radiator amplification coefficient (slope of curve) is different. In different installation positions of valve, the PV value of valve authority is different, and the rate of change of amplification coefficient is different. In order to ensure that the product of the two amplification factors is a constant, the resistance of the valve when it is fully open should not be the same. It can be drawn: In the current heating system, high-flow, small temperature difference mode of operation, the quality of regulation system worsened. Through the above analysis, I believe that the hot water heating system should be used such as the percentage of control valve, in addition, valve resistance should also be considered, which is very important for the control valve used in different occasions. In general, the system of resistance in the heat source points, collectors (Note: For the heat source points, the collector at the control valve, the regulation of the entire heating system, the heat and flow of the relationship is also similar The shape of Figure 3 [5]), the thermal station is the largest, trunk branch and the user's thermal entrance second. For the plunger, open and sleeve valve control valve, they all use the body of the valve, the valve is open state. In the same test conditions, in general, the largest sleeve valve resistance, open type, followed by the smallest plunger. Therefore, it can be considered in the selection of other percentage control valve, when the object of regulation of resistance larger, should use sleeve or open-style control valve; resistance is small, these three control valve can be used. When the regulation object of the control valve is a heating system, such as the heat source, the collector, the trunk branch and the user's thermal entrance, the regulating valve plays a role of distributing the flow, that is, the relation between the opening and the flow, Did not relate to the final relationship between valve opening and device heat dissipation. Figure 2b can be drawn when the valve power PV = 0.1, the operating characteristics of the valve opening relative to the relative flow and the basic linear relationship, but also can play a good role in the distribution of traffic. In this case, if the valve power PV≥0.3 as mentioned before is also required, the resistance of the selected control valve is too large, causing the system resistance to be excessive and the pump power consumption to be too large. If the regulating valve regulating object for the heat exchanger or radiator, etc., in order to meet the valve adjustment and equipment heat or heat dissipation, the final requirement is the relative heat sink heat or heat exchanger relative heat exchange with the control valve The relative opening into a linear relationship, then it is necessary to require the selected valve in the work of the valve power PV ≥ 0.3. In addition, the user heat inlet control valve is installed in the water supply pipe or return pipe, should be determined according to the hydraulic diagram. If the high-rise buildings installed in the water supply pipe may result in the valve pressure is too small (valve resistance large), part of the pipeline in the negative pressure zone, resulting in emptying, this time should be considered to install the control valve on the return pipe; On the contrary, low-rise buildings should be installed in the water supply pipe, you can also use the valve to decompression. If the heat medium is steam, under certain pressure, the latent heat of vaporization is a fixed value, and the amount of heat dissipation or heat exchange is proportional to the amount of steam. In order to ensure that the relative heat output or the relative heat exchange capacity is linear with the relative opening degree of the regulating valve, Need to ask the working characteristics of the valve into a linear relationship. Therefore, when the system or equipment resistance is small, the use of linear control valve; the other hand, the selection of equal percentage valve, but this time the valve weight PV should be about 0.1. Also note: If the heat medium is steam, to prevent the vapor pressure from decreasing too much, the resistance of the selected regulator should be small. 4.2 Caliper valve diameter selection calculation At present, some people in the heating system design often do not carry out the calculation of the caliber of the control valve is generally considered the size of the diameter of how many caliber valve, this concept is wrong. For the control valve must be in accordance with its ability to choose the flow of caliber. It has been seen from the foregoing that the pressure drop across the regulator valve is very important for spool profile design as well as for caliber selection. Select the calculation steps are as follows: To determine the control valve pressure Δp (12) where η - coefficient, when the control valve for the heat source points, collectors, trunk branch and the user thermal entrance, η = 0.15 ~ 0.5; η = 0.5 to 0.7 when used to adjust radiators or heat exchangers; pg - system or equipment water supply pressure, Pa; ph - system or equipment return pressure, Pa. Calculate the maximum flow capacity of the control valve Gmax flow capacity C refers to the valve in the control valve fully open, the pressure across the valve 105Pa, fluid density Ï 1g / cm3, flow through the regulating valve every h Gmax, m3 / h As follows: (13) In the regulating valve product specification, select the value or value of the regulating valve according to the Cmax value and select the C value greater than Cmax and the latest gear. Follow the steps above to correctly select the caliber of the regulator. Literature [4] also pointed out: According to the diameter smaller than the diameter of the pipe to choose user thermal inlet valve, from the flow capacity considerations. Specific caliber than the diameter of the pipe should be reduced by a few, according to the design conditions of the heating system or the design of hydraulic pressure chart may be. If you choose the same diameter and pipe diameter control valve, in accordance with the above analysis, it may cause excess capacity, the adjustment range decreases, the actual adjustable ratio R decreases, the adjustment is not good, sometimes causing the system or equipment can not Make normal adjustments. 5 Conclusion Whether it is the design of spool valve line, or spool shape and the choice of valve diameter, the purpose is to make the heating system has good adjustability. However, the heat dissipation characteristics of the radiator under different conditions are different (mainly refers to the difference between supply and return water temperature), the amplification factor is also different; control valve installed in different locations, the valve value of the PV value is different, so its Magnification factor is also different. Therefore, to make the product of the two amplification factors unchanged, it is very difficult for the serialization of industrial products to require serialization. Only hope that the heat dissipation of the radiator can basically maintain a linear relationship with the change of the opening degree of the regulating valve. Based on the above analysis, the following conclusions can be drawn: The calculation formula of the flow cross-sectional area of ​​the regulating valve should be calculated according to (7) and (8), and the relative flow cross-sectional area should not be simply considered as the relative flow rate; The same percentage of different spool shape control valve at the same time, if the regulation of the control valve is a system, PV ≥ 0.1, if a device, PV ≥ 0.3; on the regulating valve size or diameter should be based on the flow of the valve Ability to choose to calculate the control valve installed at different locations, the formula in the calculation of the coefficient differences; user inlet control valve is installed in the supply and return pipe, should be based on hydraulic pressure chart.
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