- Summary
- Request for clarification as to the use of enthalpy vs internal energy data for a phase change from saturated steam to saturated water.

Is there anybody who can advise wether the heat lost / gained during vaporisation, is best calculated from enthalpy, or from internal energy, in the steam tables?

I am trying to establish the increase in the volume of milk when the temperature is raised by adding saturated steam at 1 bar (1 atm, for a coffee). I have been over the problem a couple of times. Generally applying equation of the form Q=dU, where Q is the heat input by the steam and dU change in internal energy of the milk.

In determining Q there is both a condensation component and a cooling component.

For the condensation component, the change in energy comes straight from the steam tables. My book (Moran and Shapiro) gives the enthalpy of vaporisation, however the internal energy of vaporisation must be calculated from the internal energy at saturated liquid and saturated vapour states. Despite the assumptions for a supercooled fluid at constant pressure suggesting that using either enthalpy or internal energy should give the same result, they dont.

As such I am hoping someone might be able to shed some light on why they dont match (i get a 10% difference in the outcome from this component alone), which is the correct one to use. I am presuming that internal energy is the way to go because it is more conservative (condensation) and the enthalpy introduces the pressure component, but on the other hand, enthalpy of vaporisation is the one provided in the book.

I am trying to establish the increase in the volume of milk when the temperature is raised by adding saturated steam at 1 bar (1 atm, for a coffee). I have been over the problem a couple of times. Generally applying equation of the form Q=dU, where Q is the heat input by the steam and dU change in internal energy of the milk.

In determining Q there is both a condensation component and a cooling component.

For the condensation component, the change in energy comes straight from the steam tables. My book (Moran and Shapiro) gives the enthalpy of vaporisation, however the internal energy of vaporisation must be calculated from the internal energy at saturated liquid and saturated vapour states. Despite the assumptions for a supercooled fluid at constant pressure suggesting that using either enthalpy or internal energy should give the same result, they dont.

As such I am hoping someone might be able to shed some light on why they dont match (i get a 10% difference in the outcome from this component alone), which is the correct one to use. I am presuming that internal energy is the way to go because it is more conservative (condensation) and the enthalpy introduces the pressure component, but on the other hand, enthalpy of vaporisation is the one provided in the book.