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The Property Calculators

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

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.

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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?

Assuming the pump inlet state has just been calculated (see above), we first need to find the outlet state.  We know that the minimum work will result for an ideal pump which produces no entropy. If the pump is also adiabatic (a reasonable approximation), then the process is isentropic: s2 = s1. Since we are given P2, state 2 may be found very simply. We first change the specified property pair from "T P" to "P S", enter "200" in the P box, and double-click in the S box, which causes the value 0.3932 to appear.

cs.gif (3241 bytes)Pressing Calculate produces the desired state 2, shown at the right. The pump work is h2 - h1, which is 132.84 - 112.69 = 20.69 kJ/kg.

 

 

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.

Example: to set the pressure to twice the critical pressure of nitrogen (no matter what the selected substance is), enter in the P box: "= 2 * Pcrit("N2")"

 

Writing to the Worksheet

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.

color.gif (3868 bytes)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.

The Generalized Property Calculator

(to be written)

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