Honeywell MS4103 For hot water coil valves, Where, = the specific volume of air at standard, 255

Appendix A: Valve Selection and Sizing

2.For hot water coil valves:

		Q =	cfm ⋅ 1.08 ⋅ TDa
			------------K-----⋅-----TD--------w--------------
Where:
cfm	=	Airflow through the coil.
1.08	=	A scaling constant. See Note.
TDa	=	Temperature difference of air entering and
		leaving the coil.

K= Value from Table 3; based on temperature of water entering the coil (pounds per gallon x 60 minutes per hour).

TDw = Temperature difference of water entering and leaving the coil.

NOTE: The scaling constant 1.08 is derived as follows:

1.08 =	0.24-----------BTU---------------⋅ 60---min--------- ⋅		---1---lbair--------------
	lbairF	1hr	13.35ft3
Where:
---1---lbair--------------	= the specific volume of air at standard
13.35ft3	conditions of temperature and

atmospheric pressure.

Simplifying the equation:

14, 40Btumin 1.08 = -----------------------------------

Fhr13.35ft3

To find the scaling constant for air conditions other than standard, divide 14.40 Btu by specific volume of air at those conditions.

3.For fan system chilled water coil valves:

		cfm ⋅ Btu ⁄ lb
		Q = -----------------------------------
Where:		113 ⋅ TDw
cfm	=	Airflow through the coil.
Btu/lb=		Heat per pound of dry air removed.
		Includes both sensible and latent heat.
113	=	A scaling constant.
TDw	=	Temperature difference of water entering
		and leaving the coil.

WATER VALVE PRESSURE DROP

To determine valve pressure drop:

1.For two-way valves consider the following guidelines for valve pressure drop:

a.Include the pressure drop in the design of the water circulating system.

—In systems with two-way valves only, it is often necessary to provide a pump relief bypass or some other means of differential pressure control to limit valve pressure drops to the valve capabilities. For control stability at light loads, pressure drop across the fully closed valve should not exceed triple the pressure drop used for sizing the valve.

—To avoid high pressure drops near the pump, reverse returns are recommended in large systems.

b.The pressure drop across an open valve should be about half of the pressure difference between system supply and return, enough so that the valve, not the friction through the coil or radiator, controls the vol- ume of water flow or the valve pressure drop should be equal to or greater than the pressure drop through the coil or radiator, plus the pipe and fittings connect- ing them to the supply and return mains.

c.Verify allowable full open and full closed pressure drops for all proportional and two-position water valves with appropriate manufacturer literature.

d.Make an analysis of the system at maximum and min- imum rates of flow to determine whether or not the pressure difference between the supply and return mains stays within the limits that are acceptable from the stand point of control stability and close-off rating.

2.For two- and three-way valves consider the following guidelines for valve pressure drop:

a.In load bypass applications (Fig. 13) such as radia- tors, coils, and air conditioning units, the pressure drop should be 50 to 70 percent of the minimum dif- ference between the supply and return main pressure at design operating conditions.

b.A manual balancing valve may be installed in the bypass to equalize the load drop and the bypass drop.

3.When selecting pressure drops for three-way mixing valves in boiler bypass applications (Fig. 13), consider the following:

a.Determine the design pressure drop through the boiler including all of the piping, valves, and fittings from the bypass connection through the boiler and up to the three-way valve input.

b.The valve pressure drop should be equal to or greater than the drop through the boiler and the fittings. If the valve drop is much smaller than the boiler pressure drop at design, effective control is obtained only when the disc is near one of the two seats. The mid-portion of the valve lift will be relatively ineffective.

APPENDIX

255