Page 317 - Steam Turbines Design, Applications, and Rerating

Shortcut Graphical Methods of Turbine Selection 291

Similar calculation for AYR turbine results in corrected steam rate of 30.3
kg/(kW⋅h).
Preliminary selection of single-valve, multistage steam turbines is
facilitated by similar sets of curves that are often available from the
turbine manufacturer.
Using Figs. 14.22 through 14.33, we can ascertain, quickly and eas-
ily, approximate steam rates for selected single-valve, multistage
impulse turbine frames that cover the more common horsepower/speed
requirements. For guaranteed steam rates, prices, and other specific
information, the manufacturer will have to be consulted. There are six
steps in estimating performance of these turbines.

Step 1: Select the performance curve that is most applicable to the
horsepower and speed requirements of the driven machine.
Step 2: Using your Theoretical Steam Rate Tables, determine the
TSR in lb/kWh (or kg/kWh) for the given steam conditions.
Step 3: Locate this TSR on the horizontal axis of the appropriate
curve selected in Step 1.
Step 4: Moving upward on the proper TSR line, locate the required
speed. Interpolation may be necessary.
Step 5: At this point, read the overall efficiency (η) on the vertical
axis.
Step 6: The TSR divided by this overall efficiency is equal to the
approximate steam rate.
TSR
SR =
η

Note: The calculated SR will be in the same units as the TSR from
the steam tables. When TSR is in lb/kWh, and SR is desired in
lb/hp⋅h, TSR must be multiplied by 0.746:
TSR lb / kWh (0.746)
SR lb/hp⋅h =
η
Again, we can visualize the estimating procedure by two examples.
Example 14.3: English units What is the steam rate of a condensing turbine
driving a compressor requiring 6000 hp at 5000 r/min? Steam inlet conditions are
600 psig, 650°F; exhaust pressure 4-in. Hga.
Step 1: The horsepower and r/min requirements suggest that turbine model
2E7 (Fig. 14.24) should be selected.