Honeywell MS4103 manual Substituting, 256

Appendix A: Valve Selection and Sizing

c.A manual balancing valve may be installed in the boiler bypass to equalize the boiler drop and the bypass drop.

WATER VALVE SIZING EXAMPLES

EXAMPLE 1:

A two-way linear valve is needed to control flow of 45F chilled water to a cooling coil. The coil manufacturer has specified an eight-row coil having a water flow pressure drop of 3.16 psi. Further, specifications say that the coil will pro- duce 55F leaving air with a water flow of 14.6 gpm. Supply main is maintained at 40 psig, return is at 30 psig. Select required capacity index (Cv) of the valve.

Use the water valve Cv formula to determine capacity index for Valve V1 as follows:

		Cv = Q----------G---
		h
Where:
Q	=	Flow of fluid in gallons per minute required
		is 14.6 gpm.
G	=	Specific gravity of water is 1.

h= Pressure drop across the valve. The difference between the supply and return

is 10 psi. 50% to 70% x 10 psi = 5 to 7 psi. Use 6 psi for the correct valve pressure drop. Note that 6 psi is also greater than the coil pressure drop of 3.16 psi.

Substituting:

14.6 1

Cv = -----------------= 6

6

Select a linear valve providing close control with a capacity index of 6 and meeting the required pressure and temperature ratings.

EXAMPLE 2:

A bypass valve is required to prevent flow through the chiller from dropping below 90 percent of design flow. When sizing valves for pump or chiller bypass applications (Fig. 16), sys- tem conditions that cause the valve to open or close com- pletely must be considered before a pressure drop can be selected.

Assume the following:

System flow at design, 1000 gpm Pump head at design, 48 ft Pump head at 90 percent flow, 50 ft

Pressure across mains at AHU 1 at design flow, 28 ft Chiller pressure drop, 12 ft

Chiller piping loop design pressure drop, 8 ft

With full system flow, Valve V5 is closed. Pressure drop across V5 equals the pump head minus the friction drops to V5. Pressure drop across Valve V5 is then 48 ft – 12 ft (chiller drop) – 4 ft (supply drop) – 4 ft (return drop) or 28 ft.

With system flow at 90 percent, the pump head rises to

50 ft, while the friction drops fall to the lower values shown in Figure 16. For additional information on chiller bypass opera- tion see Chiller, Boiler, and Distribution System Applications section. Pressure drop across V5 equals the pump head minus the friction drops to V5. Pressure drop across Valve V5 is then 50 ft – 9.6 ft (chiller drop) – 3.2 ft (supply drop) –

3.2ft (return drop) or 34 ft. Converting ft to psi, 34 ft x 0.4335 psi/ft = 14.7 psi.

Substituting the flow of water, specific gravity of water, and pressure drop in the Cv formula shows that the Valve V5 should have a Cv of 235.

900 1

Cv = --------------- = 235

14.7

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