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148 VAV-PRC011M-EN

Application Considerations

Linearity

With the increase in DDC controls over pneumatic controls, the issue of linearity is not as great as

it once was. The important aspect of flow measurement versus valve position is the accuracy of

the controller in determining and controlling the flow. Our units are tested for linearity and that

position versus airflow curve is downloaded and commissioned in the factory to insure proper

control of the unit.

Reheat Options

Hot water heating coils are generally applied on VAV terminal units as reheat devices. When

applying these coils it is important to make sure that they are operating in the proper air flow and

water flow range. Either a two-way or a three-way valve controls the coils.

The most important factor when sizing valves is the coefficient of velocity or C

. The C

is defined

as the flow rate, in gallons of 60°F (15.56°C) water, that will pass through the valve in one minute

with a one pound pressure drop. The coefficient of velocity, which is commonly called the flow

coefficient, is an industry standard rating. Valves having the same flow coefficient rating,

regardless of manufacturer, will have the same waterside performance characteristics.

The equation that governs valve sizing is:

Where

=Flow coefficient

GPM=The maximum water flow rate through the valve in gallons per minute

DP=The maximum allowable differential pressure across the valve in psi

The flow and differential pressure are generally the known quantities. The equation is solved for

the flow coefficient. The flow coefficient is then compared to the published C

values for the control

valves that are available. The control valve with the C

that is the closest, but greater than, the

calculated flow coefficient is the correct choice for the control valve. This choice will keep the valve

pressure drop below the maximum allowable valve pressure drop. The valve pressure drop should

then be checked against the coil pressure drop. If the coil pressure drop is appreciably larger than

the valve pressure drop, a valve with a smaller C

should be selected to produce a larger control

valve pressure drop. If this new valve has a pressure drop that is much larger than the maximum

allowable pressure drop for valves, the system designer should be consulted to make sure that the

system hot water pumps can deliver the water at the new conditions.

Figure 15. Hot water coil (L) & hot water valves (R)

GPM

ΔP

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