The Pure Substance Property Calculator
Writing Functions to the Worksheet
The Generalized Property Calculator
Two
pop-up calculators are implemented in TPX, which may be used either as simple
calculators, or to write functions to the worksheet for later analysis.
The
Pure Substance Property Calculator is accessed by selecting Properties
/ Pure Substances. (Eventually TPX will support mixtures, hence the
qualifier pure)
To
use the calculator, first select the desired substance in the box in the
upper-left corner. The molecular weight, critical temperature, critical
pressure, and critical compressibility are displayed in the current units.
Then
select two properties to fix the thermodynamic state (e.g. T and P), and enter
their values in the boxes. When you press Calculate, the other
properties will be computed and displayed in the Current State
box.
The
property X (vapor mass fraction) may be used in conjunction with T or P
to specify saturation states. X may have any value from zero (saturated liquid)
to one (saturated vapor). If you try to specify X with T or P above the
critical value, an error message will result.
Some
additional features:
|
If you double-click in a box where a property value
is expected, the value from the Current State will be copied
into the box. This allows you to hold one property constant while changing
another. |
Example: What is the
minimum work required per kg to steadily pump liquid water initially at 300 K
and 1 atm to a pressure of 200 atm?
|
|
You can enter any valid Excel formula wherever
numerical input values are requested. Just remember to start the formula with
an "equals" sign (=). The formula may refer to cells on the
worksheet, defined names, and may call other TPX functions, Excel worksheet
functions, etc.
|
Although
simply displaying the computed property values is enough for simple problems,
to perform additional calculations or generate plots it would be desirable to
transfer these values (or better yet, functions which calculate them) to an
Excel worksheet. This is easily done - just check Write to Worksheet.
When you do this, a column of check boxes appears to the right of the
properties in the Current State box. Check the boxes for the properties you
want to write to the worksheet. Then when you press Calculate
the next time, these properties will be written, in the order listed, to the
active worksheet.
The
properties will be written either left-to-right across a row or down a column,
starting in the cell specified in the First Cell box. You can
choose which behavior you prefer under Options. When you first
open the Property Calculator, the First Cell box will contain
the address of the active cell. If you close and re-open it, the First
Cell box will retain its last value. If this is not the desired
location, enter the correct cell address, or double-click to enter the active
worksheet cell.
You
can choose whether to write numerical values or functions to the worksheet. For
most purposes, it is best to write functions, since then if any input to the
function changes, the property value automatically recalculates.
Wherever
possible, when writing out functions the calculator writes property inputs to
the functions as cell references, rather than using the numerical
property values. This can be done only if the properties used as inputs have
been, or are currently being, written out to worksheet cells. For this reason,
it is good practice to write out a table of all relevant properties for each
state, even those which may be fixed.
The
two specified input properties are color-coded in the output. A blue
cell means that a numerical value was entered, not a formula. A brown cell denotes a formula, which usually means that the input
property refers to some other state. This allows quick visual inspection of a
table of properties, showing which two properties specify the state, and which
cells contain numerical values that can be varied in parametric studies.
Here we see state 1 was
specified to be saturated liquid at T = 40 (C), and state 2 was specified by P
and S, with P = 12 (MPa), and s given by a formula (which is s2 = s1).
The
ability to write out property functions which refer to others for their inputs
is the feature that makes it very easy to set up cycle problems. This is
discussed more in the next section.
(to
be written)