0 Bhp 269 Bhp 539 Bhp 808 Bhp 1077 Bhp

-Exhaust Heat to 350F (Btu/hr)

Exhaust + Jacket Water Heat (Btu/hr)

Fig. 29-26 Recoverable Heat from Reciprocating Engine Operating under Variable Speed.

Figures 29-25 and 29-26 are representative examples of reciprocating engine performance under variable speed operation. Figure 29-25 compares simple-cycle heat rates, in Btu/bhp (HHV basis), over operating loads ranging down to 25% of full load for both constant (C) speed and optimal (O) variable speed operation. Figure 29-26 shows recoverable heat available for the same engine when operating at variable speed.

In steam turbine-driven pump applications, for example, the efficiency of the pump increases as speed is reduced, but the efficiency of the steam turbine is reduced. While the steam turbine provides excellent control over a very wide speed range, the benefits of reduced speed operation of driver and driven machine are not complimentary as with reciprocating engines.

A typical control sequence for prime movers applied to mechanical drive services is to first reduce speed, then use standard part-load control mechanisms to modulate the driven machine. Minimum speed will be set either by limitations of the prime mover or the driven equipment.

Figures 29-27 and 29-28 show SPRs for two mechanical drive applications using speed control for part-load operation. In Systems 3 and 4, speed control allows the driven machine to maintain constant performance (10 hp-h per unit of product) at two-thirds of full load. At one-third of full load, it is assumed that driver speed is at minimum and throttling increases the power requirement to 12 hp-h per unit of product output.

In System 3, driver thermal efficiency improves at two-thirds load to 9 energy units input per hp-h, producing an SPR of 90:1. Driver efficiency degrades to 11 energy units

Fig. 29-27 System 3 SPR.
Fig. 29-28 System 4 SPR.

input per hp-h at one-third of full load, producing an SPR of 132:1. This type of performance could be expected with a reciprocating engine operating under variable speed. Compared with System 2, which is assumed to have a similar driver and driven machine, the SPRs for part-load operation using speed control are superior by a significant margin.

In System 4, driver thermal efficiency gradually decreases as speed is reduced, typical for a steam turbine. However, compared with constant speed operation in System 1, this performance loss is more than offset by the performance improvement in the driven machine. Hence, the resultant SPRs for both part-load conditions are superior to that achieved at constant speed. Operation at two-thirds and one-third of full load produces SPRs of 110:1 and 144:1, respectively, compared with 120:1 and 160:1 in System 1

Figure 29-29 illustrates the affinity laws for centrifugal loads, showing percent of full-load power when speed is reduced proportionately with flow. Input power requirements have a cubed relationship with flow rate, while flow rate varies directly with rotational speed. Application of variable speed control can offer dramatic energy savings in centrifugal fan, pump, or compressor applications where the above relationships apply. For example, at 65% speed, a centrifugal pump may require as little as 27% of full-load input power.

Figures 29-30 and 29-31 show SPRs for two different systems in which the driven machines follow the affinity laws somewhat. As with Systems 3 and 4, it is assumed that at one-third of full-load system output, minimum

300 energy units ^


10 energy units/hp-h

30 hp-h

Driven Machine

10 hp-h/unit output

3 units output ^

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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