Conclusions Examples

Example 1 demonstrates how irreversibilites within the turbine can greatly reduce net power output and efficiency. In this case, ideal efficiency of 29.75% is reduced to 20.83% under actual conditions and net power output is reduced from 2,625 to 1,837 hp.

If the 70% efficient turbine in this example were replaced with a 50% efficient turbine, actual power output and overall system efficiency (including boiler losses) drop to 1,312 hp and 12.64%, respectively. On the other hand, if the inlet condition is increased to a higher pressure and temperature, or if the 250-psig steam is superheated, shaft power output and overall thermal efficiency would increase. Superheating the steam increases the Rankine cycle efficiency because the average temperature at which heat is transferred to the steam is increased. Note also that when steam is superheated, the quality of the steam leaving the turbine improves.

In Example 2, ideal back-pressure turbine efficiency of 14.09% is reduced to 7.04% under actual conditions and net power output is reduced from 1,143 to 571 hp.

However, because the back-pressure turbine exhausts all unused energy to process, the net efficiency remains at almost 100% (neglecting boiler losses and small parasitic losses), regardless of the actual cycle efficiency. If the backpressure steam load were large in relation to required power, a lower efficiency turbine would likely be most cost-effective. Conversely, if the back-pressure steam load is small, it may be more important to more fully utilize the power producing potential of each lbm (kg) of steam with a high-efficiency turbine.

As shown in Example 3, use of a back-pressure turbine in place of a PRV requires an added fuel energy input of 1,859 MBtu/h to produce 615 hp of power and deliver the equivalent amount of useful heat to process. This tradeoff would usually provide significant net energy cost savings at conventional fuel and electricity prices.

While the values will differ, similar observations and conclusions can be drawn from Examples 4 through 6 which follow.

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