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Use the water valve C
v
formula to determine capacity index
for Valve V1 as follows:
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
0.09 psi. Heat exchanger pressure drop is
1.41 ft of water or 1.41 ft x 0.433 psi/ft =
0.61 psi. 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.
Substituting the flow of water, specific gravity of water, and
pressure drop in the C
v
formula shows that the valve should
have a C
v
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 (C
v
) for a valve used in a
steam application, using the formula:
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 C
v
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 C
v
,
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:
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:
Where:
cfm = Cubic feet per minute (ft
3
/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.
C
v
QG
h
-------------=
C
v
70 1
0.70
-------------- 83.6 or 84==
C
v
1 0.00075s+()QV
63.5 h
--------------------------------------------------=
Q
Btu hr⁄
1000Btu lbsteam⁄
-------------------------------------------------=
Q
CFM TD
a
× 1.08×
1000Btu lbsteam⁄
-------------------------------------------------=
Appendix A: Valve Selection and Sizing