486

STEP 3. Optimum steam plant load-balancing conditions are satisfied when the total plant steam demand is met according to Table 5.8.

(Boiler No. 1 input) + (Boiler No. 2 input) + (Boiler No. 3 input) +... = minimum

By trial and error and with the use of Figure 5.12, optimum plant heat input is:

Boiler Steam Load Heat Input

1 173 226

2 172 250

3 (Banked standby) -

Plant totals 345

STEP 4. The annual fuel savings realized from optimum load balancing is the difference between the existing boiler input and the optimum boiler input.

Steam plant energy savings

= (existing input) - (optimum input) = 486 - 476 x 106 Btu/hr = 10 x 106 Btu/hr or annually:

Costs that were not considered in the preceding example are the additional energy savings due to more efficient fan operation and the cost of maintaining the third boiler in banked standby.

The cost savings were possible in this example because the plant had been maintaining a high ratio of total capacity in service to actual steam demand. This results in low-load inefficient operation of the boilers. Other operating modes which generally result in inefficient energy usage are:

1. Base-loading boilers at full capacity. This can result in operation of the base-loaded boilers and the swing boilers at less than optimum efficiency unnecessarily.

2. Operation of high-pressure boilers to supply low-pressure steam demands directly via letdown steam.

Table 5.8 Unit Efficiency and Input Tabulation
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