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Q1.  What is ASTEMMCP?

Q2.  What is the difference between the 1967 IFC formulation and the “new international formulation” in the NIST Chemistry WebBook?

Q3.  Why is there a caution note on ST_tpdata?

Q4.  What are the Programmer Functions?

Q5.  When would you use the Programmer Functions?
 
 
 
 
 
 
 
 
 
 

A1.  Top

ASTEMMCP is A Steam Table Evaluation Module for Mathcad Professional (version).  It provides a simple interface to water and steam properties based on the 1967 IFC formulations for ordinary water.  By simple, ASTEMMCP uses the vector capability in Mathcad to describe the state point with between 7 to 12 elements.  The minimum set includes {P,T,v,u,h,s and x}.  The full set adds {cp,cv,k,B and a}.  It is not necessary to know whether the fluid is a liquid, vapor or two-phase.  Given two parameters to define the state (with pressure or temperature being one of the two parameters), ASTEMMCP does the rest. ST_props and ST_ptdata will handle any fluid state.  If two-phase properties are needed, a simple user set flag will provide the information, or ST_2phase can be used. (See Screen Shot 1)

A2.  Top

The new formulation uses a different definition for the critical point: 705.10 F, 3200.1 psia, and 20.102 lbm/ft3 density.  The new normal boiling point is 211.95 F.

A3.  Top

ST_tpdata attempts to find the pressure at the user specified temperature and second variable. For pressure above 2398 psia (or temperatures above 660 F), the variables are not very sensitive to changes in pressure and the convergence coding may fail. You should always experiment with ST_tpdata in the range of interest for you particular problem. At this time, ST_props, ST_ptdata or ST_2phase should be considered as your first choice. Or, you may want to use the Programmer Functions to develop an alternative solution techniques. (See Screen Shot 2, uses ST_tpdata at 650 F)

A4.  Top

The Programmer Functions are the basic building blocks to the IFC equations.  They are low level routines on which the Standard Functions were built.  They are coded in the IFC reduced format (normalized to the critical point).  It is likely that majority of ASTEMMCP users will not need them.

A5.  Top

ASTEMMCP uses a vector representation for the state point, consisting of between 7 to 12 elements.  The minimum set includes {P,T,v,u,h,s and x}.  There may be occasions when it is only necessary (or required because of PC memory constraints) to track fewer variables, for example generating a 3-D graph of {P,T,h}.  If you have 100 P-points and 100 T-points, then the vector storage would be 70,000 (100 * 100 * 7 ) (e.g. Si,j := ST_props(Pi,Tj,Uflag), and graph S(i,j)4.  With the Programmer Functions you can reduce the storage to 10,000 (100 * 100 * 1). The Programmer Functions also allow for the development of specialized solutions and ST_root may have practical use for some problems not readily covered by ST_props, ST_2phase or ST_ptdata (exercise caution when using ST_tpdata).  It is also likely that use of the Programmer Functions will reduce problem run time when a problem requires an iterative solution, by reducing the internal math to its minimum and not calculating extra variables. (See Screen Shot 3)

Copyright 1998 Edward D. Throm
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e-mail: mister-ed@cox.net