Thermodynamics Enthelpy of water

AI Thread Summary
The discussion focuses on calculating the variation of internal energy and enthalpy for water transitioning from liquid at 212ºF and 1 atm to steam at 400ºF and 100 psi. The specific internal energy of liquid water is given as 180.02 Btu/lbm, while the specific enthalpy of steam is 1228.4 Btu/lb. Participants emphasize the importance of correctly converting units, particularly for pressure and specific volume, to ensure calculations are in BTU/lbm. The correct approach involves using the initial enthalpy formula and ensuring all unit conversions are accurately applied. The thread highlights common pitfalls in unit conversion and the need for clarity in thermodynamic calculations.
ruiwp13
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Homework Statement


Liquid water at 212ºF and 1atm has a specific internal energy of 180.02Btu/lbm. The water is transformed into steam at 400ºF and the pressure of 100psi where it's specific volume is 4.937ft^3/lb and it's specific enthalpy is 1228.4Btu/lb. Determine the variation of internal energy and the variation of enthalpy of the process.


Homework Equations


ΔH=ΔU+(ΔPV)
h=u+Pv


The Attempt at a Solution



So I know the specific internal energy in the liquid state (180.02Btu/lbm), and I can calculate the specific internal energy in steam (h=u+Pv ⇔ 1228.4=u+6.804*4.937) which gives me 1194.80. But now I'm stuck... I have the variation of the specific volume (vsteam-vliquid) , the variation of pressure (6.804atm-1atm) and the variation of specific internal energy. So, I can get the variation of the specific enthalpy(h). But how do I get the mass to get the variation of enthalpy(ΔH)? (if what I said is correct of course).

Thank you for your time and patience
 
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You had the right idea in the first part, but you need to express the pv part in BTU/lb. Right now, the units are handled incorrectly.

As far as the second part is concerned, you need to make use of the pressure and specific volume of liquid water at 212 F. The density is about 62.4 lbm/ft3 and the pressure is 14.7 psi.
 
Chestermiller said:
You had the right idea in the first part, but you need to express the pv part in BTU/lb. Right now, the units are handled incorrectly.

As far as the second part is concerned, you need to make use of the pressure and specific volume of liquid water at 212 F. The density is about 62.4 lbm/ft3 and the pressure is 14.7 psi.

I've corrected the units, I think. Converted 4.937ft^3/lb to Btu. 1ft^3=1.027Btu. It gave me 5.070299.
So, instead of using the 1atm I'll use the 14.7 psi and the specific volume I calculate it from the density you gave me to use in the (ΔPV) part? But I still don't get it how will I found the mass to convert it to ΔH and ΔU
 
Last edited:
ruiwp13 said:
I've corrected the units, I think. Converted 4.937ft^3/lb to Btu. 1ft^3=1.027Btu. It gave me 5.070299.

This conversion is incorrect and also irrelevent.

So, instead of using the 1atm I'll use the 14.7 psi and the specific volume I calculate it from the density you gave me to use in the (ΔPV) part? But I still don't get it how will I found the mass to convert it to ΔH and ΔU

The initial specific enthalpy is 180 + pv, where the pv is expressed in units of BTU/lbm. You know the final enthalpy already, so you can get the change in enthalpy.

The final internal energy is calculated the way you did it, but make sure that the units of the pv are, again, BTU/lbm.

chet
 
Chestermiller said:
This conversion is incorrect and also irrelevent.



The initial specific enthalpy is 180 + pv, where the pv is expressed in units of BTU/lbm. You know the final enthalpy already, so you can get the change in enthalpy.

The final internal energy is calculated the way you did it, but make sure that the units of the pv are, again, BTU/lbm.

chet

I'm not seeing how will I get the units in BTU/lbm.
 
ruiwp13 said:
I'm not seeing how will I get the units in BTU/lbm.
p=14.7 psi = 14.7x144 lbf/ft2
v=1/62.4 ft3/lbm

pv = units of ft-lbf/lbm

What is the conversion factor between ft-lbf and BTU?

Chet
 
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Chestermiller said:
p=14.7 psi = 14.7x144 lbf/ft2
v=1/62.4 ft3/lbm

pv = units of ft-lbf/lbm

What is the conversion factor between ft-lbf and BTU?

Chet

1.285x10-3 :p thank you for your patience and time.

Best Regards
 
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