Physical performance model for turbo-expander

[rotw]

For centrifugal compressor, if I know (or can approximate or regress) a functional relationship (characteristics) as such

(work input factor) = F(Inlet flow coefficient, Mach number, impeller diameter) -----(1)
(polytropic efficiency) = G(Inlet flow coefficient, Mach number, impeller diameter) -----(2)

I can derive useful performance outputs in physical units for a given inlet condition parameters (pressure, temperature, gas mixture):

(physical output X) = H(Mass or volume flow) ------(3)

X : Polytropic head, Polytropic efficiency, outlet pressure, outlet temperature, or absorbed power

Starting from centrifugal compressor performance model above, how would you go about adapting it to the turboexpander case in similar fashion/structure?

- How would express characteristics (1) & (2) that would be pertinent to turboexpander ?
- Would be able to derive outputs such as in (3) ? Which are those outputs (most commonly considered, found in spec, curves, proposals)? Are they same or different ?

 

[georgeverghese]

If it helps in any way, there are graphs (no numerical expressions) for predicting expander isentropic eff based on speed and impeller dia for radial flow expanders in Perry Chem Eng Handbook, 6th and 7th edn in the chapter on process machinery. Engineers use isentropic eff and not polytropic eff values for expanders, unlike that for compressors where polytropic eff values are preferred. Same applies to steam turbines also.

 

[rotw]

I had a look at the chapter in question. A good point to start cracking on..
Thank you