Organic
Water Treatment Chemicals
Steam Boilers, Cooling Towers,
Hot And Chilled Closed Systems
Fuel Oil Treatment
(Home of D.M Concentrate)
Reduced
Boiler Fuel Costs Through
Improved Feedwater Quality –
Part I
Introduction:
A
certain amount of water must be
removed, either continuously or
periodically, from an operating
boiler in the order to limit the
concentration of impurities in
the boiler water. Because this
removed water, or blowdown, has
been heated, it represents a
certain heat value imparted by
the fuel, but not available to
make steam. It follows,
therefore, that if the amount of
blowdown can be reduced, a
corresponding reduction in fuel
costs can be achieved, for the
same rate of steam generation.
At today’s fuel costs, even a
slight reduction in blowdown can
represent a significant cost
savings.
Determination
of Concentration Limits:
Blowdown
rate is expressed as a percent
of total feedwater required.
Thus a 5% blowdown rate means
that 5% of the boiler feedwater
is lost through blowdown and the
remaining 95% is converted to
steam. The blowdown rate
(expressed as a decimal) is
calculated as the reciprocal of
the number of times the
feedwater can be concentrated in
the boiler. For example, if the
feedwater can be concentrated
twenty times, the blowdown rate
required will be 1/20 or 5%.
Since the only reason for
blowdown is to limit the
concentration of undesirable
impurities in the boiler water,
and because these impurities
originate in the feedwater, it
follows that if feedwater
impurities can be reduced, the
feedwater can be concentrated
more times. Higher feedwater
concentrations, in turn, result
in a lower blowdown rate. Thus,
if feedwater quality can be
improved such that it can be
concentrated thirty times rather
than twenty, the blowdown rate
can be reduced from 5% to
31/3%.
Certain
feedwater impurities must be
limited due to their tendency to
form deposits in the boiler.
Calcium and magnesium (hardness)
fall into this category. Other
impurities, if allowed to
concentrate too high, can result
in boiler water carryover.
Alkalinity and total dissolved
solids are the primary offenders
in this case. Table I summarizes
the usually accepted limits for
various feedwater constituents.
This table can be used to
determine the maximum allowable
feedwater concentrations, based
on the present boiler feedwater
quality. To do this, have the
feedwater analyzed for all the
listed constituents. For
greatest accuracy, analyze
several samples or obtain a
twentyfour hour composite
sample.
Then
divide the maximum boiler water
value (from Table I) by the
feedwater value (in ppm) for
each constituent. Take the
lowest of these quotients as the
actual concentration limit. The
following example illustrates:
Feedwater
Analysis: 
PPM
in Feedwater 
Maximum
Allowed 
Concentration
Limited 
Alkalinity
as CaC0_{3} 
70 
700 
10 
Silica
as SiO_{2} 
4 
150 
37 
Total
Dissolved Solids 
50 
3000 
12 
The
limiting constituent in this
case is the total alkalinity,
which can be concentrated only
ten times. A blowdown rate of
1/10 or 10% would be required.
Condensate
Recovery:
Boiler
feedwater consists of returned
condensate plus whatever amount
of makeup water is required to
satisfy the demands of the
boilers. Condensate, unless
contaminated, is quite low in
dissolved solids. Thus, it adds
very few impurities to the
feedwater. Makeup water, on the
other hand, usually contributes
most of the feedwater
impurities. Hence, we can think
of condensate as diluting the
makeup water impurities; and
the greater amount of condensate
returned, the better the
feedwater quality.
A
note of caution at this point:
Condensate is corrosive and can
add dissolved and suspended iron
to the feedwater unless properly
treated.
Of
course, there will always be a
certain amount of condensate
that cannot be recovered for
various reasons. Also, some
condensate sources may be
unsuitable for reuse due to
unavoidable contamination.
Contaminated condensate should
be segregated from other plant
condensate as close to the
source as possible. All good
quality recoverable condensate
should then be returned to the
feedwater system.
Calculations
of Fuel Savings:
In
the example above let us assume
that the feedwater consists of
60% condensate and 40% makeup.
By recovering additional
condensate, the feedwater
quality is improved, resulting
in a lower blowdown rate. The
blowdown rate reduction and
corresponding fuel savings can
be calculated. Thus if the
additional recovery results in a
feedwater of 67% condensate
rather than 60%, the total
alkalinity will be reduced from
70 ppm to 58 ppm and we can
increase the feedwater
concentration from 10 to 12. The
blowdown rate can then be
reduced from 10% to 81/3%. The
actual blowdown and feedwater
requirement in pounds can be
calculated as follows:
Assume
a steam production of 1,000.000
lbs. Per day. The,
f=feedwater
requirement (lbs)
s=steam generated (lbs)
%=percent blowdown, expressed as
a decimal.
1.
At 10% blowdown f = 
1,000,000 
= 
1,000,000 
=
1,111,110 lbs. 
1(0.10) 
0.9 
1.
At 81/3% blowdown f = 
1,000,000 
= 
1,000,000 
=
1,090,870 lbs. 
1(0.0833) 
0.9167 
The
difference represents the actual
blowdown reduction:
1,111,110 lbs. – 1,090,870
lbs. = 20,240 lbs. reduction
We
can now apply the following
equation to determine the fuel
costs savings
Br
* H 
*
C = $Savings 
V
* %E 
Where:
br = blowdown reductions
(lbs/day)
H = heat content of blowdown
(from Table II)
C = cost of fuel ($/unit)
V = heating value of fuel
(Btu/unit)
%E = boiler efficiency
Using
our former example and burning
No. 6 fuel oil with a heating
value of 142,440 Btu/gallon at a
cost of $0.32 per gallon, we can
calculate the following daily
savings:
20,240
lbs. * 309 Btu/lb 
*
$0.32 = 
6254160 
*
0.32 = $17.56/day 
142,440
Btu/gal. * 0.80 
113952 
Thus,
in this example, by returning
only an additional 7% o0f
condensate, a significant
savings has been realized. Also,
the heating value of the
returned condensate would yield
additional savings.
These
calculations are based on the
assumption that blowdown heat is
not being recovered. A blowdown
heat recovery system would, of
course, reduce the potential
savings.
Drum
Pressure (psig) 
TDS
(ppm) 
Total
Alkalinity (ppm CoCO_{3}) 
Suspended
Solids (ppm) 
Silica
(ppm) 

Fire
Tube 
Water
Tube 
With
Turbines 
Without
Turbines 
0300 
3500 
700 
800 
(a) 
100 
150 
301450 
3000 
600 
400 
(a) 
100 
90 
451600 
2500 
500 
 
(a) 
40 
40 
601750 
2000 
400 
 
(a) 
25 
30 
(a)The feedwater for all
modern water tube boilers should
contain little or so iron,
copper or hardness for most
reliable operations.
Pressure
(psig) 
Heat
of Saturated Liquid
(BTU/lb) 
10 
208 
15 
219 
20 
228 
25 
236 
30 
243 
40 
256 
50 
267 
60 
277 
70 
287 
80 
294 
90 
302 
100 
309 
120 
322 
140 
333 
160 
344 
180 
353 
210 
366 
235 
376 
260 
385 
285 
394 
335 
410 
385 
424 
435 
437 
485 
450 
585 
472 
685 
493 
785 
512 
