Boiler Feedwater
By James McDonald, PE, CWT
Originally Published: CSTN – January 2005
The boiler feedwater calculation is:
Feedwater =
Steam
1 - % BD
100
It is helpful to understand
understand how this equation is derived. The mass balance
around a boiler as illustrated in Figure 1 is:
Feedwater = Steam + Blowdown
Steam
Feedwater
Boiler
Blowdown
Figure 1: Boiler System Mass Balance
Balance
Calculations
414
Cycles of concentration (or cycles) is a comparison of dissolved solids level
of the blowdown with the feedwater. This can be expressed as:
BD concentration
Cycles =
FW concentration
where FW stands for feedwater and BD stands for blowdown.
Just like in a cooling tower system, cycles of concentration can also be
expressed as:
FW lb/hr
Cycles =
BD lb/hr
Now, if we solve for BD, we get:
BD =
FW
Cycles
Plugging this into the mass balance equation previously shown gives:
FW
FW = Steam +
Cycles
Solving for FW yields the following series of equations:
FW
FW -
FW
(
1-
FW =
1
Cycles
)
= Steam
Steam
(
1-
Calculations
= Steam
Cycles
1
Cycles
)
415
This last equation is a very good equation to have, because by simply
knowing the steaming rate and cycles, one can calculate feedwater.
Usually, this last equation is taken a step further to include percent
blowdown instead of just cycles. Percent blowdown is defined as:
100
% BD =
Cycles
Rearranging this equation reveals:
1
Cycles
=
% BD
100
Plugging this into the feedwater equation just derived gives us the familiar
feedwater equation:
FW =
Steam
1 - % BD
100
Now, with the two equations highlighted in blue, the feedwater flowrate can
be determined if the steam rate is known and either the cycles or percent
blowdown are known.
Calculations
416