Figure 13-3. Flash steam recoverable from continuous blowdown systems.

This chart is used to calculate the percent of boiler water discharged by a continuous blowdown system that can be flashed into steam at a reduced pressure and is recoverable as low pressure steam for heating or process.

Example: A boiler operated at a pressure of 450 psig. Continuous blowdown amounts to 10,000 lb/hr. What percentage of blowdown water can be recovered as flashed steam at 10 psig?

Solution: Locate 450 psig on the left axis. Follow horizontally toward the right to the intersection with 10 psig "flash" curve (point A). Drop vertically downward to the bottom axis and read 24.5%. (24.5% of 10,000 lb/hr blowdown=2450 lb/hr of flash steam at 10 psig pressure.)

These curves have been prepared from the formula:

% flashed steam = Hb -Hf X 100
Vf

where

Hb = heat of liquid at boiler pressure, Btu/lb

Hf = heat of liquid at flash pressure, Btu/lb

Vf = latent heat of vaporization at flash pressure, Btu/lb

 

Note: For boiler pressures between 100 and 800 psig, use "flash" pressure curves slanting from lower left to upper right corner and the bottom axis. For boiler pressures above 800 psig, use "flash" pressure curves slanting from lower right to upper left corner and top axis.