Flow coefficient of the valve

The valve flow coefficient flow coefficient concept, for the valve, the choice of flow coefficient and the choice of aperture corresponds, the valve flow coefficient is the coefficient used when the flow is calculated, the symbols used are very mixed, in fact, the United States and Japan use Cv this symbol And concept, Europe uses Kvs symbol and concept, Britain uses fp, international unit should be Kv, Kv is also the traditional flow coefficient code of China's regulating valve. In the international standard, the Kv value is defined as the hourly cubic meter flowing through the regulating valve when the pressure drop is 1 Bar. The calculation of the flow coefficient has the following formula:

Where: Q - maximum flow m3/h

G—specific gravity (usually 1)

P1—inlet pressure bar

P2 - outlet pressure bar

â–³P=P1-P2 bar

And the relationship between Cv and Kv is as follows:

Cv=1.17Kv, in fact it is accurate to say that Cv=1.167Kv, and Kv and Kvs are equivalent.

Theoretically speaking, in different air-conditioning loops, the ΔP value is different and it is a dynamically changing value, and the impact on the Cv/Kv calculation is still relatively large. When the valve is in the same kilogram level, the larger the ΔP is, the smaller the corresponding caliber is, and the greater the controllability of the medium is, but the smaller the flow capacity is, the smaller the valve size is, the less the system is. Capacity requirements, on the other hand, the valve will need to provide a large pressure difference through the system to maintain sufficient flow, increase the load of the pump, the valve is vulnerable; valve diameter is too large to make the control performance deteriorate, easy to shock and oscillation system , And the investment will increase. If the valve is too big or too small, the life of the control valve will be shortened and the maintenance inconvenience will be caused.

Therefore, we choose the valve pressure drop as much as possible, and the size of the pressure drop in the system operation is best to be constant, this can also ensure the constant flow characteristics of the valve, to ensure that the PI regulation has a good effect, when the pressure The larger the proportion of the drop in the pressure drop of the total supply and return water pressure, the smaller the influence of pressure fluctuation on the magnitude of the pressure drop. However, the pressure drop should not be too large. Consider the maximum allowable pressure drop and allowable pump pressure. Therefore, experience has shown that it should generally be chosen such that the pressure drop at full valve opening is equal to or close to 50% of the total pressure drop between the supply and return water. The pressure difference for the general supply and return system is 2-4 Bar. This pressure drop on the air conditioning valve is generally selected as 1-2Bar.

After the pressure drop, we must also know the rated flow of the valve, sometimes the design institute will directly give, or we can calculate the cooling capacity and the flow rate based on the cooling capacity according to the following formula:

The amount of cold (or calories) of a cold or calorie calculating device (or device) is calculated as follows:

Q=G×C×△t/3600

G=L×ρ

In the formula:

Q---- cold or heat, KW;

L--- volume flow of the fluid, m3/h;

G--fluid mass flow rate, T/h;

ρ --- the bulk density of the fluid, T/m3;

C---- specific heat of the fluid, J/Kg;

△ t ---- Import and export fluid temperature difference, °C.

The specific heat of water is 4184 J/Kg.°C

So we can choose the valve after calculating the Cv/Kv value. The valve's rated value is slightly larger than the calculated value.