Specific Heat in Absolute vs Relative Units

In summary: 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?
  • #1
ttrumble
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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?
 
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  • #2
ttrumble said:
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?
Yes and yes. 1 degree F = 1 degree R

Chet
 
  • #3
Chestermiller said:
Yes and yes. 1 degree F = 1 degree R

Chet

Thank you for your quick response. I appreciate the confirmation.
 
  • #4
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
dauto said:
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.

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:
  • #6
dauto said:
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.

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
 
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  • #7
Chestermiller said:
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

That's a terrible notation. Whoever had that idea needs to quit having ideas. Seriously...
 
  • #8
dauto said:
That's a terrible notation. Whoever had that idea needs to quit having ideas. Seriously...
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
 

Related to Specific Heat in Absolute vs Relative Units

1. What is specific heat and how is it measured?

Specific heat is the amount of heat energy required to raise the temperature of one gram of a substance by one degree Celsius. It is typically measured in units of joules per gram per degree Celsius (J/g·°C) or calories per gram per degree Celsius (cal/g·°C).

2. What are absolute units and how are they different from relative units?

Absolute units refer to a specific measurement system that is independent of external factors. In the case of specific heat, absolute units would be joules or calories. Relative units, on the other hand, are measurements that are relative to a reference point or standard. In the case of specific heat, relative units would be the specific heat capacity of water, which is often used as a reference point.

3. Why is it important to use absolute units when measuring specific heat?

Using absolute units allows for more accurate and consistent measurements, as they are not affected by external factors such as the substance's mass or the temperature scale used. This makes it easier to compare specific heat values between different substances and experiments.

4. Can specific heat be converted from absolute to relative units?

Yes, it is possible to convert specific heat from absolute units (joules or calories) to relative units (J/g·°C or cal/g·°C). This is done by dividing the specific heat value by the specific heat capacity of water, which is approximately 4.18 J/g·°C or 1 cal/g·°C.

5. How does specific heat in absolute units affect the study of thermodynamics?

Specific heat in absolute units is an important factor in the study of thermodynamics as it helps determine the amount of heat energy required to change the temperature of a substance. This, in turn, affects other thermodynamic properties such as enthalpy and entropy, which are crucial in understanding and predicting the behavior of substances in various processes.

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