Honeywell MS4103 manual Scaling constant, Pressure drop in psi, Superheat in degrees F

Appendix A: Valve Selection and Sizing

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

Q G

Cv = -------------

h

Where:

Q= Flow of fluid in gallons per minute required to pass through the valve is 70 gpm.

G = Specific gravity of water is 1.

h= Pressure drop across the valve. Plans of the heating system indicate three-inch supply and return mains. From an elbow equivalent table and pipe friction chart found in the ASHRAE Handbook or other reference manuals, the calculated pressure drop through a three-inch tee and the piping from the valve and the tee to the exchanger is

0.09psi. Heat exchanger pressure drop is

1.41ft of water or 1.41 ft x 0.433 psi/ft =

0.61psi. Total pressure drop from bypass connection through the heat exchanger and to the hot-water input of the three-way valve is 0.61 + 0.09 or 0.70 psi.

Since the valve pressure drop (h) should be equal to or greater than the drop through the heat exchanger and fit- tings, 0.70 psi is used as the valve pressure drop.

For optimum control, a manual balancing valve is installed in the bypass line to equalize the pressure drops in the exchanger and bypass circuits.

70 1

Cv = -------------- = 83.6 or 84

0.70

Substituting the flow of water, specific gravity of water, and pressure drop in the Cv formula shows that the valve should have a Cv of 83.6 or 84.

Select a linear valve providing close control with a capacity index of 84 and meeting the required pressure and tempera- ture ratings.

Steam Valves

Calculate the required capacity index (Cv) for a valve used in a steam application, using the formula:

(1 + 0.00075s)Q V Cv = -------------------------------------------------

63.5h

Where:

Q = Quantity of steam in pounds per hour required to pass through the valve.

V= Specific volume of steam, in cubic feet per pound, at the average pressure in the valve. For convenience Table 5 at the end of the STEAM VALVES section lists the square root of the specific volume of

		steam for various steam pressures.
		Therefore, use the value in this column of
		the table as is; do not take its square root.
63.5	=	A scaling constant.
h	=	Pressure drop in psi.
s	=	Superheat in degrees F.

Determining the Cv for a steam valve requires knowing, the quantity of steam (Q) through the valve, the pressure drop (h) across the valve, and the degrees of superheat. See QUANTITY OF STEAM and STEAM VALVE PRESSURE DROP. Then select the appropriate valve based on Cv, temperature range, action, body ratings, etc., per VALVE SELECTION guidelines.

NOTE: When the superheat is 0F, then (1 + 0.00075s) equals 1 and may be ignored.

QUANTITY OF STEAM

To find the quantity of steam (Q) in pounds per hour use one of the following formulas:

1.When Btu/hr (heat output) is known:

	Btu ⁄ hr
	Q = 1000------------------------------------------------Btu⁄ lbsteam
Where:
Btu/hr =	Heat output.
1000 Btu/lb=	A scaling constant representing the
	approximate heat of vaporization of
	steam.

2.For sizing steam coil valves:

		CFM ⋅ TDa ⋅ 1.08
		Q = 1000------------------------------------------------Btu⁄ lbsteam
Where:
cfm	=	Cubic feet per minute (ft3/min) of air from
		the fan.
TDa	=	Temperature difference of air entering and
		leaving the coil.

1.08= A scaling constant. See NOTE.

1000 Btu/lb= A scaling constant representing the approximate heat of vaporization of steam.

258