NOTE: The scaling constant 1.08 is derived as follows:
Where:
Simplifying the equation:
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 sizing steam to hot water converter valves:
Where:
gpm = Gallons per minute of water flow through
converter.
TD
w
= Temperature difference of water entering
and leaving the converter.
0.49 = A scaling constant. This value is derived
as follows:
Simplifying the equation:
4. When sizing steam jet humidifier valves:
Where:
W1 = Humidity ratio entering humidifier, pounds
of moisture per pound of dry air.
W2 = Humidity ratio leaving humidifier, pounds
of moisture per pound of dry air.
= The specific volume of air at standard
conditions of temperature and
atmospheric pressure.
= Cubic feet per minute (cfm) of air from the
fan.
= A conversion factor.
Simplifying:
5. When Equivalent Direct Radiation (EDR) is known:
Where:
EDR (Total)=Radiators are sized according to
Equivalent Direct Radiation (EDR). If
controlling several pieces of radiation
equipment with one valve, add the EDR
values for all pieces to obtain the total
EDR for the formula.
0.24 = A scaling constant, lb steam/unit EDR.
See Table 4.
Table 4. Output of Radiators and Convectors.
STEAM VALVE PRESSURE DROP
Proportional Applications
When specified, use that pressure drop (h) across the valve.
When not specified:
1. Calculate the pressure drop (h) across the valve for good
modulating control:
h = 80% x (Pm-Pr)
NOTE: For a zone valve in a system using radiator ori-
fices use:
h = (50 - 75)% x (Pm-Pr)
Where
Pm = Pressure in supply main in psig or psia
(gage or absolute pressure).
Pr = Pressure in return in psig or psia. A
negative value if a vacuum return.
= the specific volume of air at standard
conditions of temperature and
atmospheric pressure.
