# Turbo Expander

1. Feb 2, 2010

### Sc336

Can anyone please describe the method of calculation of power of a turbo expander?
Please write down the formula and the method of calculation for same.

2. Feb 2, 2010

### CFDFEAGURU

Welcome to the forum.

Could you possibly be anymore vague in what you are asking?

For starters what are you using this turbo expander for? What type of system is it in?

Have you done any calculations of your own yet?

Matt

3. Feb 2, 2010

### Sc336

Sorry: Rephrased question for clear understanding

True,
The question was vague, lemme try to rephrase it, being a rotating equipment engineer,
I am trying to find out the power of a turboexpander (API-617 chapter-4) type.It is a compressor and a gas turbine on the same shaft (Also called as expander compressor).
As per the data given the expanded gas is methane with temp of 1020 psi& -10 degf suc pressure and temp and 408 psi& -80 degf discharge pressure and temp .Mass flow rate is 992760 lb/hr(There is a computed value of 8480 hp in data)
which i am not able to figure out how? Kindly help me with volume flow rate and power calculation.
I have not tried any calculations as i am unaware of the laws governing the expander here.

4. Feb 2, 2010

### CFDFEAGURU

Ok now that we have some information maybe we can get something going here.

Re-specification of what you have.

Gas - 100% by Volume Methane (CH4)
Mass flow of 992760 lbs/hr of CH4
Inlet pressure to the compressor of 1020 psig
Inlet suction temperature of 10 F or is it -10 F

Outlet discharge of 408 psig
Outlet temperature of 80 F or is it -80 F

The volume flow rate is easily calculated from the specified mass flow rate.

V = mass flow rate / density of the methane.

The density of CH4 at -10 F and 14.7 psi is roughly 0.0489 lbs/ft^3 and at -80 it is roughly 0.0579 at 14.7 psi, that is a change of only 18%, so use the average of the two for a first pass. Then the volumetric flow is simply

V = 992760 / (0.0579+0.0489)/2 = 18747308.9 ft^3/hr

Again, please verify the above information and we can continue forward.

Thanks
Matt

5. Feb 2, 2010

### Sc336

hey dude,
You are on the right track, however the pressure in the above analysis of volume flowrate shall be the suction pressure to get the acfm, I think we can use the Molecular weight to find out the density at the suction pressure MW being 18.98 at -10 degf.

Last edited: Feb 2, 2010
6. Feb 3, 2010

### Sc336

hey, thanks for showing the path, i have calculated the volume flow rate based on the density.However can you further tell me the formula for calculating the power?

7. Feb 3, 2010

### CFDFEAGURU

Sc336, I am out of my office today. I wil be able to help you out tomorrow.

Thanks
Matt

8. Feb 4, 2010

### CFDFEAGURU

I am not sure of exactly how the manufacturer is calculating the power. It is probably a proprietary calculation since they have done testing to determine as accurately as possible the horse power of their unit.

However, from a basic 1st Law analysis, you should just be able to calculate the work of the turbine and then calculate the work of the compressor and then subtract the compressor work from the turbine work to achieve the actual work out of the turbine. However, this will over estimate the actual work out of the turbine since we do not know the compressor or turbine efficiencies.

Think you can take it from here now?

Thanks
Matt