Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual, Page: 380 / 512

ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
370
TYPES OF LOW PRESSURE STEAM
DISTRIBUTION SYSTEMS
One-Pipe Steam Systems
One-pipe systems were often used in small buildings and
apartments because of their simplicity and low cost. The same
pipe carried both steam and condensate (Fig. 110). They used
gravity returns and, therefore, required no traps.
Control valves on individual radiators are a potential problem
in one-pipe systems. As valves close, water is trapped in the
radiators resulting in low boiler water and water hammer noise at
the radiators when the valves reopen. Oversized valves controlled
two-position are required to allow condensate to flow out of
radiators while steam is entering. Thermostatic air vents allow
steam to completely fill the radiator. Many one-pipe systems have
no valves and temperature is controlled by cycling the boiler.
TYPES OF CONDENSATE RETURN SYSTEMS
In addition to gravity return used for small systems, either
open or vacuum return systems can be used.
In open return systems (Fig. 112) condensate flows by gravity
to one or more receivers where it is pumped back to the boiler.
Pumps are started and stopped by float controls in the receiver.
Fig. 110. One-Pipe System.
Two-Pipe Steam Systems
Two-pipe systems (Fig. 111) are used for all systems using
automatic control of terminal units. A trap at each unit removes
condensate.
C2925
MANUAL OR
AUTOMATIC
VALVES
THERMOSTATIC
AIR VENT
THERMOSTATIC
AIR VENT
STEAM
SUPPLY
RETURN
F & T TRAP AT
END OF MAIN
AND BOTTOM
OF EACH
RISER
C2924
MANUAL
OR
AUTOMATIC
VALVES
STEAM
SUPPLY
RETURN
TRAP AT
EACH
TERMINAL
UNIT
Fig. 112. Open Return System.
A vacuum return system requires a vacuum pump assembly
to discharge noncondensable gases to the atmosphere and to
force the condensate into the boiler (Fig. 113). A vacuum pump
assembly consists of a vacuum pump, a separator tank, and an
air vent.
C2926
PUMP
RECEIVER
CONDENSATE
RETURN
TO
BOILER
Fig. 113. Typical Two-Pipe Vacuum Pump System.
The vacuum in the return line draws a condensate/air mixture
into the receiver. This mixture is forced into a separating tank
where the air is vented to the atmosphere and the condensate
returned to the boiler. The air is discharged through a check
valve or a nonreturn vent to maintain a vacuum in the return
line under operating conditions. The vent on the vacuum pump
is usually the only vent in the system.
Typically, a selector switch allows the vacuum pump to be
operated automatically or manually. In the automatic position
a vacuum switch or float switch cycles the vacuum pump.
A steam system may be either open or closed. In a closed
system all condensate is returned to the boiler. In an open system
the condensate is expelled or used for other purposes, such as
humidifiers, dishwashers, and autoclaves and must be replaced
by makeup water.
Fig. 111. Two-Pipe System.
AIR
VENT
STEAM
LINE
C2928
VACUUM
PUMP
SEPARATOR
TANK
BOILER
PUMP
CONTROL
DRIP
TRAP
VALVE
TRAP
VALVE
TRAP
EQUALIZER
LINE
RETURN
LINE
BOILER
WATER
LINE