Catalogue

UNT-PRC001-EN 23
General Data
Selecting the Correct Modulating Valve Size
Modulating valves are available in any of four port sizes. The port sizes relate to a Cv of 0.6, 1.1, 2.3,
2.7, 3.3, and 3.8, which is the coefficient of flow. The coefficient of flow is defined as the volume of
water flow through a control valve in the fully open position with a 1 psig differential across the
valve. It is calculated using the following formula:
Cv = Q/Square root ΔP where:
Cv = flow coefficient
Q = flow rate (Gpm)
ΔP = pressure drop across the valve or coil (psig).
For good control, the valve Cv should be approximately equal to the Cv of the water coil.
Modulating Valve Selection Example
Assume a size 06 fan-coil is selected to operate at the following conditions:
Vertical Cabinet Fan-Coil
Entering water temperature = 45°F
Leaving water temperature = 55°F
EAT conditions = 80/67.
The coil is selected as a four-row coil.
Select the best modulating valve size for this unit.
1. Fi nd th e ΔP across the water coil. Refer to the AHRI performance table to determine the ΔP
across the water coil (or use TOPSS™ selection program). The water pressure drop is found to
be 7.0’ of water at a flow rate of 3.74 gpm. This converts to a pressure drop of 3.03 psig (1.0 feet
of water = 0.4328 psig.)
2. Calculate the Cv of the water coil.
Cv = Gpm/Square root ΔP
Cv =
3.74/Square root 3.03
Cv = 2.15
Therefore, the valve with the Cv of 2.7 should be selected since it has the Cv which is closest
to the Cv of the water coil. The following tables illustrate possible valve selections at AHRI
conditions for horizontal concealed units with a high static motor and vertical cabinet units with
a free discharge motor.
Note: Do not use these tables for any applications other than vertical cabinet or horizontal
concealed units at AHRI conditions.
UNT-PRC001-EN.book Page 23 Tuesday, May 8, 2012 10:54 AM