At this time, pressure drop across the steam
control valve is maximum. If boiler capacity is
large enough, the control valve inlet pressure will
stay at design, then flow rate will be limited by
control valve capacity.
Table 1 gives maximum condensate flow for
different inlet steam pressures. Interpolate for
intermediate pressures.
If steam demand on start-up can be held within
a controlled limit, the trap(s) may be sized
accordingly.
When traps are undersized, condensate wiU
back up in the generator with loss in machine
capacity and may cause dangerous water hammer.
Depending on boiler size, the boiler water makeup
system could operate and add water to the boiler.
Sooner or later, excess water would return to the
boiler room and either overflow the hot well to
drain, or if it flows directly into the boiler, it may
shut the boiler down on high boiler water level
control.
Condensate Cooler is used on some atmospheric
condensate systems to reduce or eliminate loss of
flash steam from the open receiver vent. The
condensate cooler must be sized for handling and
condensing flash steam as well as cooHng the
condensate. Condensate is normally cooled to
about 180 F. Pressure drop thru the condensate
cooler should be very low, as it must be added to
trap outlet pressure. If there is a liquid leg down to
the condensate receiver, the condensate cooler and
trap should be located at the bottom of this leg.
When a condensate cooler is used, it is desirable
to use either cold boiler feed water or other cold
water source which can benefit by heat rejected
from hot condensate. Cooling tower bleed water
can be used, but it may be heavy with dissolved
solids and may rapidly foul the cooler. Tower
makeup water can be used when large cooling
towers are part of the system. Extra load to the
tower would be insignificant.
Receiver and Condensate Pumps — When open
receivers are used, the vent should be directed
outside the equipment space to eliminate fogging.
Be careful in using small receivers and close-
connected condensate pumps. Some commercially
available systems may work well on standard
heating systems but can present problems in
handling condensate from absorption machines.
The basic difference in absorption machine oper
ation lies in higher condensate temperatures and
greater amounts of flash vapor. Commercial
heating systems normally deliver condensate to the
receiver thru long return runs. This lowers con
densate temperature to 200 F or lower with
relatively little flash steam.
Absorption machines commonly located close
to the condensate receiver have little or no
condensate cooling. During full load, condensate
may be delivered to the trap at close to 12 psig and
240 F. This creates large amounts of flash steam at
the trap outlet and in the condensate receiver.
Very hot condensate drawn into the condensate
pump may cause cavitation.
To minimize these effects, the following guides
are offered:
1. If equipped with a vented receiver, the inlet line
to the receiver should enter above the receiver
water level. Flash steam can go directly out the
vent without creating turbulence or frothing.
2. Locate the condensate pump as far below
receiver water level as possible to give maxi
mum Net Positive Suction Head (NPSH) to the
pump.
3. If pump suction pipe is located at bottom of
the receiver, use a vortex breaker at the receiver
outlet.
4. Locate pump suction at opposite end of re
ceiver from the condensate inlet. This will
minimize agitation and frothing at pump inlet.
5. A properly selected condensate cooler, as
previously described, will eliminate problems
with flashing.
VACUUM PUMP CONDENSATE RETURN
SYSTEMS are sometimes used to return con
densate from space heating installations. The
vacuum pump maintains the condensate return
system at a subatmospheric pressure and permits
the heating system to operate with subatmospheric
pressure when the heating load is small.
It is generally impractical to use an existing
vacuum pump condensate return system. Con
densate from the absorption machine is far higher
in temperature than condensate from the original
heating system for which the vacuum return pump
■; was selected. Hot condensate forms excessive
quantities of flash vapor when released into the
vacuum return system and will usually cause vapor
lock in either the return piping or the vacuum
return pump, or both. When the existing
condensate return system is a vacuum pump type,
the recommended method of returning condensate
from the machine is a separate wet-return system,
if possible.
As an alternate choice, condensate can be
discharged thru a steam trap to an atmospheric
vent receiver. The receiver discharges flash-cooled
condensate thru a second trap into the vacuum-
return system.
If a condensate cooler is used, condensate may
be cooled to an acceptable level and discharged
