# Superheated steam/isentropic expansion.

1. Aug 3, 2012

### maistral

I have a quiz tomorrow and I hope somebody sees this and answers in time.

Alright, so I have superheated steam at 8600kPa and 500 degrees C. It expands isentropically to 2900 kPa.

Since it's isentropic, S2 must be equal to S3, assuming it's ideal. S2 = S3' = 6.6858.

Getting the saturation entropies at 2900 kPa, Sl = 2.6283 and Sv = 6.1969. x is 1.13.

Now this is the problem; it's still superheated. How on earth do I get enthalpy (H) 3? I've been looking at the freaking steam table for three hours with no luck. apparently the answer is 3071 and I have no idea how did they even get that.

Thanks.

2. Aug 3, 2012

### andrien

Re: Superheated steam/isentropic expansion. URGENT

you will have to extrapolate the enthalpy from saturated vapour enthalpy point to the point you want.

3. Aug 3, 2012

### andrien

Re: Superheated steam/isentropic expansion. URGENT

you can also use h-s chart.

4. Aug 3, 2012

### maistral

Re: Superheated steam/isentropic expansion. URGENT

Can I get numerical values? All this thinking made my brain go broke.

I just need to know the entire process of getting the value, then I'll probably rest a bit. Thanks.

5. Aug 3, 2012

### andrien

Re: Superheated steam/isentropic expansion. URGENT

take a look at mollier diagram.a big chart,just follow the constant pressure line corresponding to the pressure and see what you get for that entropy.

6. Aug 3, 2012

### maistral

Re: Superheated steam/isentropic expansion. URGENT

we're required to use steam tables though.

7. Aug 3, 2012

### maistral

Re: Superheated steam/isentropic expansion. URGENT

OMG i finally got it. I take 2900kPa. Look for the appropriate interpolation wrt to 6.6858, then interpolate it to take the enthalpy which has the entropy of 6.6858. thanks!