Honeywell AUTOMATIC CONTROL, Engineering Manual

Page: 455 / 518

ENGINEERING MANUAL OF AUTOMATIC CONTROL
VALVE SELECTION AND SIZING
445
Fig. 17. Equal Percentage Valve Hot Water Application.
Since the valve pressure drop (h) should be equal to or
greater than the drop through the heat exchanger and
fittings, 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 Cv formula shows that the valve should
have a Cv of 83.6 or 84.
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
indicates a pressure drop of 1.41 ft of water through the
exchanger coils.
Use the water valve Cv formula to determine capacity index
for Valve V1 as follows:
VALVE VI
C2339A
180 ° F
HOT WATER
SUPPLY
40 PSI
HOT WATER
RETURN
HEATING
COIL
LOCAL
PIPING
2.2 PSI
DROP
20 GPM AT DESIGN,
4.3 PSI DROP
30% PRESSURE DROP, Cv = 12
70% PRESSURE DROP, Cv = 5
CASE B: 62 PSI
CASE A: 50 PSI
100%
0% 100%
STEM TRAVEL
FLOW AT CONSTANT
PRESSURE DROP
C2340
70%
PRESSURE
DROP
30%
PRESSURE
DROP
IDEAL EQUAL
PERCENTAGE
VALVE
CHARACTERISTIC
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.
Select a linear valve providing close control with a capacity
index of 84 and meeting the required pressure and
temperature ratings.
STEAM VALVES
Calculate the required capacity index (Cv) 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 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.
Cv =
Q G
h
Cv =
(1 + 0.00075s)Q V
63.5 h
Cv =
7 0 1
0.70
= 83.6 or 84