Honeywell MS4103 manual Effect of Pressure Drop in Hot Water Valve Sizing, 257

Appendix A: Valve Selection and Sizing

	4			32
36		PD	SUPPLY	37.2
	3.2
40.4

CHILLER

129.6 PD

		HEAT/	HEAT/	HEAT/	180 GPM	HEAT/
		COOL	COOL	COOL	PER	COOL
DP SETPOINT = 34'		COIL	COIL	COIL	AHU COIL	COIL
	1		2	3		4
	DP

V5	V1	V2	V3	V4
PUMP
48
50

				B1	B2	B3		B4
	4	PD	RETURN
0	3.2
	ZERO REFERENCE		SYSTEM STRAINER
				4	NUMBERS IN CIRCLES =		GAGE PRESSURES
				3.2	PUMP INLET	=	ZERO FOR SIMPLICITY
					TOP NUMBERS	=	FULL FLOW
					BOTTOM NUMBER	=	90% FLOW
					PD	=	PRESSURE DROP	M10605

Fig. 16. Chiller Bypass Application.

EXAMPLE 3:

Sizing water valves for heating coils is especially critical. In Figure 17, a valve with a Cv of 12 will have 30 percent of the available pressure drop when full open, while a valve with a Cv of 5 will have 70 percent of the available pressure drop.

As shown in Figure 18, the valve with 70 percent of the avail- able pressure drop essentially provides the equal percent- age water flow control, resulting in linear coil heat transfer and stable temperature control. The valve with only 30 per- cent of the available pressure drop has a more linear flow control which results in nonlinear coil heat transfer. See EQUAL PERCENTAGE VALVE section for further informa- tion.

	CASE A: 50 PSI VALVE VI		HEATING
	CASE B: 62 PSI		COIL		40 PSI
180F				LOCAL		HOT WATER
HOT WATER				PIPING
SUPPLY				2.2 PSI			RETURN
30% PRESSURE DROP, Cv = 12				DROP
70% PRESSURE DROP, Cv = 5		20 GPM AT DESIGN,
		4.3 PSI DROP					C2339A

100%		30%
CONSTANTATFLOW	DROPPRESSURE
		PRESSURE
		DROP
			70%
				PRESSURE
				DROP
			IDEAL EQUAL
			PERCENTAGE
			VALVE
			CHARACTERISTIC
	0%	STEM TRAVEL	100%
				C2340

Fig 18. Effect of Pressure Drop in Hot Water Valve Sizing.

EXAMPLE 4:

A three-way mixing valve is needed for a heat exchanger application with a bypass line. Water flow is specified at the rate of 70 gpm. Manufacturer data for the exchanger indi- cates a pressure drop of 1.41 ft of water through the exchanger coils.

Fig. 17. Equal Percentage Valve Hot Water Application.

APPENDIX

257