Boiler Plants - Energy Conversion and Management - page 85

Table 6.7 Effect of treatment techniques on makeup water contaminants under ideal operating
conditions

Suspended
Solid

Dissolved
silica

Dissolved
solids Treatment techniques Alkalinity Hardness

Filtration Excellent No

change

No
change

No change No change

Cold-process softening Good Fair Fair Good Fair

Hot-process softening Good Good Good Good Fair

Sodium-cycle cation
exchange

Excellent No

change

Excellent No change No change

Demineralization Excellent Excellent Excellent Excellent Excellent

Reverse osmosis Excellent Excellent Excellent Excellent Excellent

The main systems used to recycle the condensate into the boiler are: the
atmospheric system, pressurized systems, and deaerators.
The atmospheric system consists of an open tank where condensate is received
downstream of steam traps or drains. At atmospheric pressure the condensate
temperature is reduced to 373 K (100 C; 212 F): a portion of the condensate,
about 10-15 %, will flash to steam and will be lost in the atmosphere; the remaining
hot water will feed the boiler together with as much treated makeup water added as
the boiler requires.
The semi-pressurized and fully pressurized systems work with a receiver at
intermediate pressure below the lowest process steam pressure or at process
steam pressure. The flashed steam is piped to a low-pressure steam main where it
can be utilized for process purposes; the lost flashed steam is conveyed to the outlet
through relief valves or automatically operated vents which are installed to prevent
abnormal pressure surges. This loss, too, can be reduced if some additional

provisions are made for steam recovery.
The condensate is generally cooled by the makeup water to at least 283 K (10 C;
18 F) below the saturation temperature to prevent vapor generation in the boiler
feedwater pump system.
Notice that although the use of flash steam in low-pressure mains seems to offer
a significant heat recovery, its practical application involves a number of problems,
essentially economic, that must be carefully considered. In addition to inherent
factors which must be taken into account when planning a condensate recovery
system, the quantity of flash steam must match potential needs at any time since
steam cannot be stored economically for later use.
Spray deaerators, also called barometric condensers, consist of a tank where
steam rises through a cold water spray which condenses it (see Fig. 6.13 ). This
system, which achieves complete flash steam recovery to heat boiler feedwater, has
the additional advantage of producing a deaerating effect: if the temperature of the
tank is kept above about 363 K (90 C; 194 F) by a proper metering of the spray,
dissolved gases in the condensate and feedwater, such as oxygen and CO 2 , will
come out of the solution and will be released through the atmospheric vent.