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  Reduce boiler fuel cost Part I   


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 3-1/3%.

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 twenty-four 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 CaC03 70 700 10
Silica as SiO2 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 make-up 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. Make-up water, on the other hand, usually contributes most of the feedwater impurities. Hence, we can think of condensate as diluting the make-up 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% make-up. 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 8-1/3%. 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 =    s    , where
1-%bd

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 8-1/3% 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 CoCO3) Suspended Solids (ppm) Silica (ppm)
  Fire Tube Water Tube With Turbines Without Turbines
0-300 3500 700 800 (a) 100 150
301-450 3000 600 400 (a) 100 90
451-600 2500 500 - (a) 40 40
601-750 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
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