Calculations Of Fuel Savings

Assume that the feed water consists of 60% condensate and 40% make-up. By recovering additional condensate, the feed water quality is improved, resulting in a lower blow down rate. The blow down rate reduction and corresponding fuel savings can be calculated. Thus if the additional recovery results in a feed water of 67% condensate rather than 60%, the total alkalinity will be reduced from 70 ppm to 58 ppm and we can increase the feed water concentration from 10 to 12. The blow down rate can then be reduced from 10% to 8-1/3%. The actual blow down and feed water requirement in pounds can be calculated as follows:

Apply the following equation to determine reduction savings:

Assume a steam production of 1,000.000 lbs / day.

 F = S1-% BD

Where;

F = feed water requirement (lbs)
S = steam generated (lbs)
% B D = percent blow down, expressed as a decimal.

 At 10% blow down F = 1,000,0001-(0.10) = 1,000,0000.9) =1,111,110 lbs.

 At 8-13% blow down F = 1,000,0001-(0.0833) = 1,000,0000.9167) =1,090,870 lbs.

The difference represents the actual blow down reduction:
1,111,110 lbs. - 1,090,870 lbs. = 20,240 lbs. reduction

Apply the following equation to determine the fuel costs savings:

 Br * HV*%E *C = \$ Savings

Where:

Br = blow down reductions (lbs/day)
H = heat content of blow down (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 (adjust to market cost as desired), we can calculate the following daily savings:

 10,240 lbs. * 309 Btu/lb142,440 Btu/gal. * 0.80 *\$0.32 = 6254160113952 * 0.32 = \$17.56/day

In this example, by returning only an additional 7% of 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 blow down heat is not being recovered. A blow down heat recovery system would, 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 feed water 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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