Continuous Blowdown. The boiler water is continuously discharged
in an amount which maintains a constant boiler water concentration ratio of
solids. For a typical diagram, see Figure 25. Continuous Blowoff Piping
Boiler Blowdown Piping and Valve Standards. See the ASME Boiler
and Pressure Vessel Code, Section 1 Para. PG-59.3.3 and para. A-56 to A-60,
and ANSI/ASME B31.1, for minimum requirements.
Calculate percent blowdown as follows.
Percent Blowdown = 100 /CR
CR = concentration ratio
Calculate continuous blowdown rate as follows:
Z = B x C/(A - B)
Z = Blowdown rate in pounds per hour (pph).
A = Predetermined boiler water concentration as total
solids in parts/million (ppm) or mg/L.
B = Total solids in feedwater as fed to boiler in
parts per million (ppm) or mg/L.
C = Steam output in pounds per hour (pph).
For an example of design calculations for continuous blowdown equipment,
assume the following values:
Total solids in makeup
Total solids in condensate return
Total solids in boiler water (A)
(2 500 mg/L)
Steaming rate (C)
Losses in system
Blowdown (by trial and error)
Makeup = blowdown
plus losses =
Condensate = steam rate
minus losses =
B = (6,530 pph x 170 ppm) + (9,000 pph x 25 ppm)
(6,530 + 9,000) pph
= 86 ppm = 86 mg/L