Catalogue

20 CAB-PRC001-EN
General Data
Selecting the Correct Modulating Valve Size
Modulating valves are available in any of four port sizes: 0.7, 1.5, 2.5 or 4.0 Cv (coefficient of flow
values). 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. Calculate the coefficient of flow
using the 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 vertical cabinet heater is selected to operate at the following conditions:
EWT = 180°F
LWT = 15 0°F
EAT = 70°F
The coil selection is a four-row coil. Select the best modulating valve size for this unit.
1. Find the ΔP across the water coil. Refer to the ARI performance table to determine the ΔP across
the water coil or use the Trane Official Product Selection System, TOPSS™, selection program.
The water pressure drop is found to be 5.7’ of water at a flow rate of 3.59 gpm. This converts
to a pressure drop of 2.47 psig (1.0 feet of water = 0.4328 psig.)
2. Calculate the Cv of the water coil.
Cv = gpm/Square root ΔP.
Cv =
3.59/Square root 2.47
Cv = 2.29
Therefore, select the 2.5 Cv valve because it is closest to the water coil.
Ta b l e 3 and Table 4, p. 21 illustrate possible valve selections at ARI conditions for horizontal
concealed units with a high static motor and vertical cabinet units with a free discharge motor. For
other applications, use TOPSS to determine flowrate and make calculations using the formulas
above.
Table 3. Modulating valve selections for horizontal concealed units, high static motor, 70°F EAT,
180°F EWT, 30°F ΔT
Unit Size Coil gpm Coil WPD Coil Cv Valve Cv
02 2-Row 1.19 6.0 0.74 0.7
3-Row 1.52 13.8 0.62 0.7
4-Row 1.59 3.8 1.24 1.5
03 2-Row 1.53 10.3 0.72 0.7
3-Row 1.82 4.3 1.33 1.5
4-Row 1.98 6.2 1.21 1.5
04 2-Row 1.73 3.3 1.45 1.5
3-Row 2.57 9.1 1.29 1.5
4-Row 2.81 13.4 1.17 1.5
06 2-Row 2.87 9.9 1.39 1.5
3-Row 3.96 5.9 2.48 2.5
4-Row 4.37 8.2 2.32 2.5
CAB-PRC001-EN.book Page 20 Tuesday, May 8, 2012 10:11 AM