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 four-row coil. Select the best modulating valve size for this unit.

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 GD-3. Modulating Valve Selections for Horizontal Concealed Units, High Static Motor, 70 F (21 C) EAT, 180 F (82 C) EWT, 30 F (-1 C) ∆T

Unit

 

 

Coil

Coil

Valve

Size

Coil

GPM (L/s)

WPD (kPa)

Cv

Cv

 

2-Row

1.19 (0.08)

6.0

(17.8)

0.74

0.7

02

3-Row

1.52 (.10)

13.8

(41.2)

0.62

0.7

 

4-Row

1.59 (.10)

3.8

(11.5)

1.24

1.5

 

2-Row

1.53 (.10)

10.3

(30.8)

0.72

0.7

03

3-Row

1.82 (.11)

4.3

(12.8)

1.33

1.5

 

4-Row

1.98 (.12)

6.2

(18.6)

1.21

1.5

 

2-Row

1.73 (.11)

3.3 (9.8)

1.45

1.5

04

3-Row

2.57 (.16)

9.1

(27.0)

1.29

1.5

 

4-Row

2.81 (.18)

13.4

(39.9)

1.17

1.5

 

2-Row

2.87 (.18)

9.9

(29.5)

1.39

1.5

06

3-Row

3.96 (.25)

5.9

(17.7)

2.48

2.5

 

4-Row

4.37 (.28)

8.2

(24.6)

2.32

2.5

 

2-Row

3.71 (.23)

4.7

(14.2)

2.60

2.5

08

3-Row

4.74 (.30)

9.1

(27.1)

2.39

2.5

 

4-Row

5.22 (.33)

12.7

(37.8)

2.23

2.5

 

2-Row

4.71 (.30)

8.1

(24.1)

2.52

2.5

10

3-Row

6.50 (.41)

18.1

(54.0)

2.32

2.5

 

4-Row

7.13 (.45)

25.3

(75.6)

2.15

2.5

 

2-Row

5.48 (.35)

11.4

(34.0)

2.47

2.5

12

3-Row

7.19 (.45)

14.5

(43.2)

2.87

2.5

 

4-Row

7.83 (.51)

10.5

(31.4)

3.67

4.0

Table GD-4. Modulating Valve Selections for Vertical Cabinet Units, Free Discharge Motor, 70 F (21 C) EAT, 180 F (82 C) EWT, 30 F (-1 C) ∆T

Unit

 

 

Coil

Coil

Valve

Size

Coil

GPM (L/s)

WPD (kPa)

Cv

Cv

 

2-Row

1.06 (.07)

4.8

(14.4)

0.74

0.7

02

3-Row

1.31 (.08)

10.5 (31.4)

0.61

0.7

 

4-Row

1.34 (.08)

2.8 (8.4)

1.22

1.5

 

2-Row

1.40 (.09)

8.8

(26.3)

0.72

0.7

03

3-Row

1.70 (.11)

3.8

(11.3)

1.33

1.5

 

4-Row

1.81 (.11)

5.3

(15.7)

1.20

1.5

 

2-Row

1.71 (.11)

3.2 (9.5)

1.45

1.5

04

3-Row

2.12 (.13)

6.4

(19.0)

1.27

1.5

 

4-Row

2.28 (.14)

9.1

(27.3)

1.15

1.5

 

2-Row

2.70 (.17)

8.9

(26.5)

1.38

1.5

06

3-Row

3.31 (.21)

4.2

(12.6)

2.46

2.5

 

4-Row

3.59 (.23)

5.7

(17.0)

2.29

2.5

 

2-Row

3.39 (.21)

4.0

(11.9)

2.58

2.5

08

3-Row

4.11 (.26)

6.9

(20.7)

2.38

2.5

 

4-Row

4.45 (.28)

9.4

(28.1)

2.21

2.5

 

2-Row

4.32 (.27)

6.8

(20.4)

2.52

2.5

10

3-Row

5.55 (.35)

13.4 (40.2)

2.30

2.5

 

4-Row

6.00 (.38)

18.3 (54.8)

2.13

2.5

 

2-Row

4.99 (.32)

9.6

(28.6)

2.45

2.5

12

3-Row

6.10 (.38)

10.5 (31.4)

2.86

2.5

 

4-Row

6.48 (.42)

7.3 (21.8)

3.65

4.0

CAB-PRC001-EN

17

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Trane CAB-PRC001-EN manual Selecting the Correct Modulating Valve Size
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CAB-PRC001-EN specifications

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