So I have been meaning to ask this for a long time. We all know NCEES loves feedwater (open and closed) heaters as well as mixing problems.
I am confused as to NCEES's rational with calculating enthalpies. They seem to have a strong preference to using Cp (specific heat) rather than using steam tables (or combining the two). Hopefully in problems it does not matter, but I have ran across some where it does. Obviously stream tables are far more accurate.
In the problem referenced in the subject line (NCEES TF 512), at State 3 they give the following properties:
P3 = 60 psia
T3 = 102F
My approach to getting the enthalpy of at this point would be to go to the saturated steam tables and use the temperature (as the pressure is low). Maybe interpolate if I feel I must. In this case I will interpolate. The tables give the following:
at 100F hf = 68.037
at 105F hf = 73.028
If you interpolate to 102F you get h3=70.026
To me this seems the correct (and most accurate way) to get enthalpy.
Here is what NCEES did:
h3 = hsat - Cp(DeltaT)
from the saturated tables at P3=60psia Tsat = 262.69F & hsat=262.24
h3 = 262.24 - (1 Btu/(lbm-F)(292.69-102)
h3 = 71.6
I know 70.0 versus 71.6 may not seem like a lot, but I have run across NCEES sample problems were the mass flow rates are very high. In such cases differences in enthalpy values do make a difference.
So why do they calculate enthalpies in such a way? I could understand if they were trying to avoid the steam tables altogether, but for their equation you need hsat (steam tables).
I have only seen this in NCEES sample problems, nowhere else (MERMs, Kaplan and so on). And I have seen this on other NCEES problems as well. I think there is even one problem where if you do not do it this way (the less accurate way) you end up with the wrong answer.
Any thoughts?
I am confused as to NCEES's rational with calculating enthalpies. They seem to have a strong preference to using Cp (specific heat) rather than using steam tables (or combining the two). Hopefully in problems it does not matter, but I have ran across some where it does. Obviously stream tables are far more accurate.
In the problem referenced in the subject line (NCEES TF 512), at State 3 they give the following properties:
P3 = 60 psia
T3 = 102F
My approach to getting the enthalpy of at this point would be to go to the saturated steam tables and use the temperature (as the pressure is low). Maybe interpolate if I feel I must. In this case I will interpolate. The tables give the following:
at 100F hf = 68.037
at 105F hf = 73.028
If you interpolate to 102F you get h3=70.026
To me this seems the correct (and most accurate way) to get enthalpy.
Here is what NCEES did:
h3 = hsat - Cp(DeltaT)
from the saturated tables at P3=60psia Tsat = 262.69F & hsat=262.24
h3 = 262.24 - (1 Btu/(lbm-F)(292.69-102)
h3 = 71.6
I know 70.0 versus 71.6 may not seem like a lot, but I have run across NCEES sample problems were the mass flow rates are very high. In such cases differences in enthalpy values do make a difference.
So why do they calculate enthalpies in such a way? I could understand if they were trying to avoid the steam tables altogether, but for their equation you need hsat (steam tables).
I have only seen this in NCEES sample problems, nowhere else (MERMs, Kaplan and so on). And I have seen this on other NCEES problems as well. I think there is even one problem where if you do not do it this way (the less accurate way) you end up with the wrong answer.
Any thoughts?
