General Data
Selecting the Correct
Modulating Valve Size
Modulating valves are available in any of four port sizes. These four port sizes relate to a Cv of 0.7, 1.5, 2.5 or 4.0, 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 (6.895 kPa) differential across the valve. It is calcu- lated using the following formula:
Cv = Q/Square root ∆P where: Cv = flow coefficient
Q = flow rate (GPM)
∆P = 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 cabinet heater is selected to operate at the following conditions:
Vertical cabinet cabinet heater
Entering water temperature = 180 F (82 C) Leaving water temperature = 150 F ( C) EAT = 70 F (21 C)
The coil is selected as a
1
Find the ∆P across the water coil. Refer to the ARI performance table to determine the ∆P 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’ (17.0 kPa) 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 ∆P.
Cv = 3.59/Square root 2.47 Cv = 2.29
Therefore, select the valve with the Cv of
2.5since it is closest to the Cv of the water coil. The following tables illustrate possible valve selections at ARI condi- tions 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 the flowrate and make calculations using the formulas above.
Table
Unit |
|
| Coil | Coil | Valve | |
Size | Coil | GPM (L/s) | WPD (kPa) | Cv | Cv | |
| 1.19 (0.08) | 6.0 | (17.8) | 0.74 | 0.7 | |
02 | 1.52 (.10) | 13.8 | (41.2) | 0.62 | 0.7 | |
| 1.59 (.10) | 3.8 | (11.5) | 1.24 | 1.5 | |
| 1.53 (.10) | 10.3 | (30.8) | 0.72 | 0.7 | |
03 | 1.82 (.11) | 4.3 | (12.8) | 1.33 | 1.5 | |
| 1.98 (.12) | 6.2 | (18.6) | 1.21 | 1.5 | |
| 1.73 (.11) | 3.3 (9.8) | 1.45 | 1.5 | ||
04 | 2.57 (.16) | 9.1 | (27.0) | 1.29 | 1.5 | |
| 2.81 (.18) | 13.4 | (39.9) | 1.17 | 1.5 | |
| 2.87 (.18) | 9.9 | (29.5) | 1.39 | 1.5 | |
06 | 3.96 (.25) | 5.9 | (17.7) | 2.48 | 2.5 | |
| 4.37 (.28) | 8.2 | (24.6) | 2.32 | 2.5 | |
| 3.71 (.23) | 4.7 | (14.2) | 2.60 | 2.5 | |
08 | 4.74 (.30) | 9.1 | (27.1) | 2.39 | 2.5 | |
| 5.22 (.33) | 12.7 | (37.8) | 2.23 | 2.5 | |
| 4.71 (.30) | 8.1 | (24.1) | 2.52 | 2.5 | |
10 | 6.50 (.41) | 18.1 | (54.0) | 2.32 | 2.5 | |
| 7.13 (.45) | 25.3 | (75.6) | 2.15 | 2.5 | |
| 5.48 (.35) | 11.4 | (34.0) | 2.47 | 2.5 | |
12 | 7.19 (.45) | 14.5 | (43.2) | 2.87 | 2.5 | |
| 7.83 (.51) | 10.5 | (31.4) | 3.67 | 4.0 | |
Table
Unit |
|
| Coil | Coil | Valve | |
Size | Coil | GPM (L/s) | WPD (kPa) | Cv | Cv | |
| 1.06 (.07) | 4.8 | (14.4) | 0.74 | 0.7 | |
02 | 1.31 (.08) | 10.5 (31.4) | 0.61 | 0.7 | ||
| 1.34 (.08) | 2.8 (8.4) | 1.22 | 1.5 | ||
| 1.40 (.09) | 8.8 | (26.3) | 0.72 | 0.7 | |
03 | 1.70 (.11) | 3.8 | (11.3) | 1.33 | 1.5 | |
| 1.81 (.11) | 5.3 | (15.7) | 1.20 | 1.5 | |
| 1.71 (.11) | 3.2 (9.5) | 1.45 | 1.5 | ||
04 | 2.12 (.13) | 6.4 | (19.0) | 1.27 | 1.5 | |
| 2.28 (.14) | 9.1 | (27.3) | 1.15 | 1.5 | |
| 2.70 (.17) | 8.9 | (26.5) | 1.38 | 1.5 | |
06 | 3.31 (.21) | 4.2 | (12.6) | 2.46 | 2.5 | |
| 3.59 (.23) | 5.7 | (17.0) | 2.29 | 2.5 | |
| 3.39 (.21) | 4.0 | (11.9) | 2.58 | 2.5 | |
08 | 4.11 (.26) | 6.9 | (20.7) | 2.38 | 2.5 | |
| 4.45 (.28) | 9.4 | (28.1) | 2.21 | 2.5 | |
| 4.32 (.27) | 6.8 | (20.4) | 2.52 | 2.5 | |
10 | 5.55 (.35) | 13.4 (40.2) | 2.30 | 2.5 | ||
| 6.00 (.38) | 18.3 (54.8) | 2.13 | 2.5 | ||
| 4.99 (.32) | 9.6 | (28.6) | 2.45 | 2.5 | |
12 | 6.10 (.38) | 10.5 (31.4) | 2.86 | 2.5 | ||
| 6.48 (.42) | 7.3 (21.8) | 3.65 | 4.0 | ||
17 |
