4 ,580


Fig. 31-39 Flow Rate During Electric Billing Periods.

Fig. 31-38 Full- and Part-Load Performance Curves for Engine Pump Driver. Source: Waukesha Engine Division requirement remains constant at 2,546 Btu/bhp, even though the turbine mechanical efficiency would decrease significantly under lower load at lower speed. This is due to the fact that all unused steam energy from the topping cycle is passed on to process. The simple payback for the steam turbine driver versus the constant-speed motor base case is 2.0 years, as shown in Figure 31-46.

Reciprocating Engine Option

The first-cost premium of the reciprocating engine versus the constant-speed motor is assumed to be $70,000, with an annual incremental O&M cost of $0.01/bhp-h. It is assumed that no heat is recovered from the engine. Figure 31-47 calculates reciprocating engine energy requirement and operating cost. In this case, the minimum flow and TDH is limited by the reciprocating engine minimum operating speed, which is assumed to be 50% of the design rating of 1,800 rpm. The simple payback versus the base case is shown in Figure 31-48 to be 3.6 years.

Driver Option Comparison

All three driver options show excellent economic performance in these simplified analyses. The VFD option produces a very short simple payback period of 1.6 years, making

Fig. 31-39 Flow Rate During Electric Billing Periods.

assumed to be $60,000, with an incremental O&M cost of $2,500. Steam boiler efficiency is 83% and it is assumed that all back-pressure steam is passed on to process. Energy requirement and operating cost are calculated in Figure 31-45. Notice the net steam energy


TDH, (ft)

Pumpage hp

Pump Eff.

Pump hp

Motor Eff.

Input Power (kW)



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