Frequently Asked Questions

Table of Contents

1. I am upgrading my operating system.  Do I need a product upgrade ?
2. I am upgrading my version of Office.  Do I need a product upgrade ?
3. I just installed WinSteam.  Why don't the functions work in Excel ?
4. Excel's add-in Manager does not list "WinSteam for Excel".  What should I do ?
5. I have just upgraded to WinSteam 3.0 and an old Excel spreadsheet is returning errors.  What is wrong ?
6. I am using XLInterp with Office 97 and it caused Excel to crash. What can I do?
7. Why are the values of enthalpy that I get from @Gas so different from the the tables I am used to?
8. @Air does not have a function that returns specific heat at constant volume.  How can I estimate this value?

I am upgrading my operating system.  Do I need a product upgrade ?

The following matrix shows the compatibility of Techware products with various Windows operating systems.

Back to Top

I am upgrading my version of Excel.  Do I need a product upgrade ?

The following matrix shows the compatibility of Techware products with various Office versions.

Back to Top

I just installed WinSteam.  Why don't the functions work in Excel ?

Before you can use the functions in Excel, you have to tell Excel's Add-in Manager to make the functions available.  To do this, select the Add-ins command on Excel's Tool menu.  This brings up a dialog box listing various add-ins.  Towards the bottom of the list, you should find and entry, "WinSteam for Excel".  Check the box next to it and press OK.  The WinSteam functions will now be available in Excel every time you start Excel.

The same procedure must be done for @Air, @Gas and XLInterp as well.

Back to Top

Excel's add-in Manager does not list "WinSteam for Excel".  What should I do ?

Excel's Add-in Manager will usually look in the Office/Library or Excel/Library folder for add-ins.  The installation program usually copies the Add-in file to the proper folder but sometime it cannot.  You should search you disk drive for the file "xlstm32.xll" and move it to the appropriate Library folder.  If you do not wish to move the add-in file, you may open the add-in Manager (see above) and press the "Browse" button.   Then locate the add-in file and press OK.  Then check the "WinSteam for Excel" check box and press OK.

Back to Top

I have just upgraded to WinSteam 3.0 and an old Excel spreadsheet is returning errors.  What is wrong ?

First make sure that the Excel Add-in Manager has loaded "WinSteam for Excel".  See the two topics above for more details.

If this does not solve your problem, check the unit set argument.  In older versions of Excel. the unit set argument was optional.  If you omitted the unit set argument, WinSteam assumed you wanted "English" units.  In this version of WinSteam, the unit set argument includes more options and we recommend that you always include the unit set argument with each function call.  When you installed WinSteam you were asked whether to make the unit set argument optional or mandatory.  If you accepted the default, it made the unit set argument mandatory.  If your old spreadsheet omitted the unit set argument, this would cause an error.

To remedy this, re-install WinSteam and make sure that you make the unit set argument optional.  We still recommend that you explicitly enter the unit set argument in all new spreadsheet. 

Please note, if you are running Windows 2000, you  must install WinSteam as an Administrator or else the selection of optional unit set argument will not take effect.

Back to Top

I am using XLInterp with Office 97 and it caused Excel to crash. What can I do?

This stems from a known problem with Excel for Microsoft Office 97. It will happen when one of the cells in the table that you sent to an XLInterp function contains labels instead of numbers. Microsoft has fixed this problem with a patch, Microsoft Office 97 Service Release 1, (SR-1) designed to upgrade your Office 97 installation. You can get this patch from Microsoft for free at http://office.microsoft.com/assistance/9798/sr1howtoget.aspx

Back to Top

Why are the values of enthalpy that I get from @Gas so different from the the tables I am used to?

There are a few reasons.  First, with the default units sets, all specific properties are presented on a dry basis just like the ASHRAE air property tables.  You can change this to a wet basis by adding a "W" to the unit set argument (for example, "ENGW".) 

Second, some tables reference the enthalpy of the gases to absolute zero while others choose different temperatures.  @Gas, by default, uses the same reference conditions as the ASHRAE air tables.  This is zero deg F when using English unit sets and zero deg C when using SI or metric unit sets.  You can choose to use a reference temperature of absolute zero by adding an "A" to the unit set argument (for example, "ENGA".)  Recognize that enthalpies are meaningful only when taken as a difference between two conditions.  You will see that the enthalpy differences will be the same regardless of the choice of reference temperature.

Third, regardless of what reference temperature is selected for the gas components, @Gas always uses the reference condition for the water component as liquid water at the triple point.  This is done to be consistent with the ASME steam tables and is also consistent with the way ASHRAE handles moist air properties.  Many references treat the water vapor component as a perfect gas and reference the enthalpy to absolute zero or some other temperature.  With our method, water either added or removed from the process can be treated using the standard ASME steam tables or Techware's WinSteam product.  This makes it easy to handle situations where evaporation or condensation occur.  Since the water component is referenced to liquid water, you should use higher heating values when adding heat from the combustion of fuels in the gas stream.

Finally, the enthalpies and other properties calculated by the @Gas functions take into account the compressibility effects of the gas and therefore are not perfect gas approximations.

Back to Top

@Air does not have a function that returns specific heat at constant volume.  How can I estimate this value?

We know from thermodynamics the following equation:

Cp - Cv = T (dV/dT)_p (dP/dT)_v

where ,

T is the absolute temperature

(dV/dT)_p is the partial derivative at constant pressure

(dP/dT)_v is the partial derivative at constant volume

We can estimate the partial derivative (dV/dT)_p as follows:

1. Calculate the volume V using the function AirPTWV
2. Take a small increment of temperature, T' = T + 1 (deg K)
3. Calculate V' at P and T' using AirPTWV
4. (dV/dT)_p =(V' - V)/(T' - T)

Similarly, we can 

estimate the partial derivative (dP/dT)_v as follows:

1. Calculate the volume V using the function AirPTWV
2. Take a small increment of temperature, T' = T + 1 (deg K)
3. Calculate P' at T' and V using AirTVWP
4. (dP/dT)_v =(P' - P)/(T' - T)

Finally, calculate Cv = Cp - T (dV/dT)_p (dP/dT)_v

You may download a text file, AirPTWCv.txt, that will create two functions for Excel in a visual basic module.  One function AirPTWCvEng(P, T, W) calculates the specific heat at constant volume on a wet basis in English units.  The other function AirPTWCvSIF(P, T. W) calculates the specific heat at constant volume on a wet basis in SIF units.

If you are not familiar with VBA for Excel, note the following:

1. Open the Excel spreadsheet in which you wish to use the functions
2. Go to the Tools/Macro menu in  Excel and open the Visual Basic Editor
3. In the Project window, click on the VBAProject that has the name of your spreadsheet
4. From the VBA menu choose Insert Module
5. Copy the text from the file AirPTWCv to the VBA module you created
6. The two functions should now be available in Excel

Copyright © 2001  Techware Engineering Applications . All rights reserved.
Revised: September 25, 2003 .