Valve flow coefficient The concept of flow coefficient for the valve, the choice of flow coefficient and the diameter of the choice is corresponding to the valve flow coefficient is used when calculating the flow coefficient, the symbol is now mixed, in fact, the United States, Japan and more use of Cv this symbol And the concept of multi-purpose European symbols and concepts Kvs, the United Kingdom fp, the international unit should be Kv, Kv is also the traditional flow coefficient valve code. In the international standard, Kv value is defined as: When the pressure drops to 1Bar flow through the control valve cubic meters per hour, the flow coefficient is calculated as follows: Where: Q-maximum flow m3 / hG-specific gravity General 1) P1 - inlet pressure barP2 - outlet pressure bar â–³ P = P1-P2 bar And Cv and Kv relationship as follows: Cv = 1.17Kv, in fact, accurate point that Cv = 1.167Kv, and Kv and Kvs is equivalent . In theory, in different air-conditioning loop, ΔP values ​​are different, is a dynamically changing value, the impact of Cv / Kv calculation is still relatively large. When the valve kg unchanged, the larger the choice of ΔP, the corresponding caliber is small, the greater the control of the media capacity, but the smaller the flow capacity, the valve is too small on the one hand, the system does not reach the Capacity requirements, on the other hand the valve will need to provide a larger pressure differential through the system to maintain adequate flow, increase the pump load, the valve vulnerable; over-large valve control performance deterioration, easy to system shock and vibration And investment will increase. The valve is too small will bring about the shortened life of the control valve and maintenance inconvenience consequences. Therefore, we choose the valve pressure drop, as much as possible choose a larger number, and the size of the pressure drop in the system operation is best to be constant, this can also ensure that the valve flow characteristics of a constant, to ensure good PI regulator effect, when the pressure When the proportion of descending pressure in the total supply and return pressure is larger, the effect of pressure fluctuation on the pressure drop size is smaller. However, the pressure drop can not be too large, taking into account the maximum allowable pressure drop and allowable pump pressure. Therefore, experience has shown that the general should be so chosen: the pressure drop when the valve is fully open equal to or close to 50% of the total pressure drop between supply and return. General supply and return system pressure in the 2-4Bar. This air-conditioning valve on the pressure drop generally choose 1-2Bar. After the pressure drop, we also need to know the rated flow of the valve, sometimes the design institute will give it directly, or we can calculate it according to the cooling capacity. The cooling capacity and the flow rate can be calculated according to the following formula: Equipment (or device) of the cold (or heat) according to the following formula: Q = G × C × â–³ t / 3600G = L × Ï Where: Q ---- cold or heat, KW; L --- - fluid volume flow, m3 / h; G ---- fluid mass flow, T / h; Ï --- fluid bulk density, T / m3; C ---- fluid specific heat, J / Kg; â–³ t ---- import and export of fluid temperature, ℃. The specific heat of water is 4184 J / Kg. ° C so that we can choose the valve after calculating the Cv / Kv value, the valve rated value slightly larger than the calculated value can be.
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