2000 Hook-up Book - Spirax Sarco

SYSTEM DESIGN
24
How to Size Temperature and Pressure Control Valves
Calculating Condensate Loads
When the normal condensate load is not known, the load can be
approximately determined by calculations using the following formula.
General Usage Formulae
Heating water with steam (Exchangers)*
GPM x (1.1) x Temperature Rise °F
lb/h Condensate =
2
Heating fuel oil with steam
GPM x (1.1) x Temperature Rise °F
lb/h Condensate =
4
Heating air with steam coils
CFM x Temperature Rise °F
lb/h Condensate =
800
Steam Radiation
Sq. Ft. EDR
lb/h Condensate =
4
*Delete the (1.1) factor when steam is injected directly into water
Specialized Applications
Sterilizers, Autoclaves,
Retorts Heating Solid Material
lb/h Condensate =
W x Cp x ∆T
L x t
W = Weight of material—lbs.
Cp = Specific heat of the material
(∆)T = Temperature rise of the material °F
L = Latent heat of steam Btu/lb
t = Time in hours
Heating Liquids in Steam Jacketed
Kettles and Steam Heated Tanks
lb/h Condensate =
G x s.g. x Cp x (∆)T x 8.3
L x t
G = Gallons of liquid to be heated
s.g. = Specific gravity of the liquid
Cp = Specific heat of the liquid
(∆)T = Temperature rise of the liquid °F
L = Latent heat of the steam Btu/lb
t = Time in hours
Heating Air with Steam;
Pipe Coils and Radiation
A x U x (∆)T
lb/h Condensate =
L
A = Area of the heating surface in square feet
U = Heat transfer coefficient (2 for free convection)
(∆)T = Steam temperature minus the air temperature °F
L = Latent heat of the steam Btu/lb
Valve Sizing For Steam
Satisfactory control of steam flow to give
required pressures in steam lines or
steam spaces, or required temperatures
in heated fluids, depends greatly on
selecting the most appropriate size of
valve for the application.
An oversized valve tends to hunt, with
the controlled value (pressure or temperature),
oscillating on either side of the
desired control point. It will always seek to
operate with the valve disc nearer to the
seat than a smaller valve which has to be
further open to pass the required flow.
Operation with the disc near to the seat
increases the likelihood that any droplets
of water in the steam supply will give rise
to wiredrawing. An undersized valve will
simply unable to meet peak load requirements,
startup times will be extended and
the steam-using equipment will be unable
to provide the required output.
A valve size should not be determined
by the size of the piping into
which it is to be fitted. A pressure drop
through a steam valve seat of even a few
psi means that the steam moves through
the seat at high velocity. Valve discs and
seats are usually hardened materials to
withstand such conditions. The velocities
acceptable in the piping are much lower
if erosion of the pipes themselves is to
be avoided. Equally, the pressure drop of
a few psi through the valve would imply
a much greater pressure drop along a
length of pipe if the same velocity were
maintained, and usually insufficient
pressure would be left for the steamusing
equipment to be able to meet the
load.
Steam valves should be selected on
the basis of the required steam flow
capacity (lb/h) needed to pass, the inlet
pressure of the steam supply at the
valve, and the pressure drop which can
be allowed across the valve. In most
cases, proper sizing will lead to the use
of valves which are smaller than the
pipework on either side.
Steam Jacketed Dryers
1000 (Wi - Wf) + (Wi x ∆T)
lb/h Condensate =
L
Wi = Initial weight of the material—lb/h
Wf = Final weight of the material—lb/h
(∆)T = Temperature rise of the material °F
L = Latent heat of steam Btu/lb
Note: The condensate load to heat the equipment must be added
to the condensate load for heating the material. Use same formula.