# Specific Heat in Absolute vs Relative Units

1. Jun 3, 2014

### ttrumble

I'm running a simulation on a gas mixture that spits out its Specific Heat in a number of different units.

The values for Specific Heat in Btu/lbmolF vs Btu/lbmolR are the same, which confuses me at first glance.

I sort of remember in Thermo not having to worry about Btu/lbmolF vs Btu/lbmolR, even though looking at it from a unit perspective I would have to multiply by 460 (from 1F = 460.67 R) to convert between them. I have googled this and can't find much information.

Does this come from relative temperature being a temperature difference vs an absolute temp?
Can I trust the output from this simulation for my compressor efficiency calculations?

2. Jun 3, 2014

### Staff: Mentor

Yes and yes. 1 degree F = 1 degree R

Chet

3. Jun 3, 2014

### ttrumble

Thank you for your quick response. I appreciate the confirmation.

4. Jun 3, 2014

### dauto

The conversion from Fahrenheit to Rankine is TR = TF + 459.67. That's not the same as TR = TF * 460.67, though both lead to 1F = 460.67 R which is what you pulled from Google without fully understanding.

The units don't look right though. It should read BTU/lb.R instead of Btu/lbmolR which doesn't seem to make any sense.

5. Jun 3, 2014

### ttrumble

I didn't actually pull it from google, but I was stuck thinking from a unit conversion (multiplying by 1) perspective without understanding how absolute temperature is used. When I said that I was going to multiply by 460, I was referring to the heat capacity, not just to the value of temperature. I picked 1F arbitrarily.

The units seem fine to me, at least from an industry engineering standpoint where we use different definitions of the mole, which I believe the standard mole would be considered a gram mole (gmol). It's not actually mass multiplied by mols, but an identifier next to the mol units depending on the unit system we're using. The title mentioned that I'm dealing with specific heat, but heat capacity where it has not been divided by molar mass to give information on a per unit mass basis is what this program's output is.

Maybe another poster could clarify what I have stated.

Last edited: Jun 3, 2014
6. Jun 3, 2014

### Staff: Mentor

The molar heat capacity is given in units of BTU/ lbmol.R, where one lbmol is 1 MW of the substance in lbs. Go figure us crazy Americans, huh!!

Chet

7. Jun 3, 2014

### dauto

That's a terrible notation. Whoever had that idea needs to quit having ideas. Seriously...

8. Jun 3, 2014

### Staff: Mentor

Please try not to be so judgmental. We chemical engineers find this an extremely convenient unit to work with, especially in dealing with industrial heat effects from chemical reactions. Do you also feel that molar heat capacity expressed as cal/g-mole.C represents seriously terrible notation? The lb molar heat capacity in BTU/lbm-F is numerically equal to the gram molar heat capacity in cal/g-mole.C.

Chet