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FAQ about dryPAK v.3
by Z. Pakowski
PSYCHROMETRY
| 1. | Q. | I have to produce psychrometric
charts for a specific liquid component and air. What data should I have to
place this liquid in dryPAK's library ? |
| A. | You will have to provide the
following: component name, formula (*) and CAS number (*), diffusional volume according to Fuller (w), molar mass, critical temperature, melting point temperature, Antoine equation for vapor pressure, Watson equation for latent heat of vaporization, polynomials for specific heat of vapor and liquid, thermal conductivity of vapor (w), dynamic viscosity of vapor, liquid density. Please note that data marked (*) are used for book keeping only and data marked (w) are only used for calculation of the wet bulb temperature. | |
| 2. | Q. | We deal with two-component
solvent and need a psychrometric chart for this solvent. Can we use
dryPAK ? |
| A. | Psychrometric charts for multicomponent
solvents do not exist. Because of that dryPAK deals only with single component
liquids, however, gases can be multicomponent just like air is. | |
| 3. | Q. | Our calcination process uses
combustion gases at 1800oC. Can we use dryPAK to obtain
properties of this gas ? |
| A. | No, dryPAK can deal with combustion gases but the upper temperature limit is 1000oC. | |
| 4. | Q. | I have an old FORTRAN software
that performs a dryer simulation. I have to plot the results manually on
a psychrometric chart. Can dryPAK help me in this chore ? |
| A. | If your software can dump the results
to a text file on disk you can use dryPAK to further process your results.
By little manual editing of the text file you can produce a text file
readable by dryPAK. It is necessary to place temperature and humidity in
two columns for each continuous line you want to plot on a psychrometric
chart. You can select line thickness and color, mark points by circles or
labels etc. Plotting of the lines will be performed on a chart specified in the file header. The graph can be printed on a HP laser printer, a plotter or exported to MS WORD, COREL DRAW and other programs. | |
| 5. | Q. | Can I use dryPAK for calculation
of air humidity below melting point of water ? |
| A. | Yes, for air-water system dryPAK offers
a separate set of high accuracy ASHRAE equations for calculation of
termodynamic and property data in the range -100 to 200oC.
To use these equations one selects water and air marked HD from the
component list. For other systems this range is limited to temperatures
above melting point and below 1000oC (including water and air
components of ordinary accuracy). | |
| 6. | Q. | What is the advantage of the
Salin chart over the Mollier and Grosvenor charts ? |
| A. | The Salin chart uses axis deformation
and therefore it is highly compressed at higher absolute humidities. For
processes which start at high temperatures of liquid moisture (eg. drying
of paper) the high humidity part (unimportant) is significantly compressed
and the low humidity part (important) has higher resolution. Of course
other charts may be more advantageous for other processes. | |
| 7. | Q. | I would like to use the Mollier
chart produced by dryPAK as an illustration in a publication. Can I do it without cutting and pasting ? |
| A. | All graphical and text output of
dryPAK can be directed to a file instead of a printer. Graphic files are
produced in HPGL - graphic language recognized by a popular text and graphics
editing software. If you have a HPGL filter installed with your MS WORD
or WORD PERFECT you can directly import dryPAK graphics to your publication.
Further on the graph may be touched up a bit by using more suitable fonts,
line thickness etc. | |
| 7a. | Q. | My Word from MS Office97
could not import dryPAK plot files. What can I do ? |
| A. |
Word 97 has dropped HPGL import from its list of supported filters.
The
HPGL filter available in Office95 (Word 7.0) still works after
simple installation procedure described. The idea is to add this filter from the old Office95 to the new one. In order to do that proceed as follows:
NB. When importing dryPAK files to WORD it doesn't matter if the file type is hgl or plt. PLT files are optimised for Corel or Word import while HGL are better for HP LaserJet black&white landscape printout. | |
| 8. | Q. | Am I limited to atmospheric
pressure in my psychrometric calculations ? |
| A. | Not at all, in dryPAK you can set the
lowest ambient pressure to 1 and the highest pressure to 250 kPa for all
components except for water and air HD components where these pressures
are 1 and 1500 kPa. | |
| 9. | Q. | How can I calculate the
diffusional volume of 1,4 - dioxane ? Its stoichiometric formula is
C4H8O2. |
| A. | From the lower table in the
Appendix A of the manual one picks the diffusional volumes of elements: 15.5 for C, 2.31 for H and 6.11 for O. Since dioxane is heterocyclic one needs to subtract -18.3 for the heterocyclic ring. Thus the diffusional volume is: The above results must be multiplied by 10-3 to obtain the result in m3/kmol, however, it is typed as is to the dryPAK library files. | |
| 10. | Q. | What does "Sat" mean in the psychrometric chart SCRIPT file ? It is not explained in the manual. |
| A. | Indeed, it is not explained in the manual, however, it is explained in the dryPAK help file under CHART/SCRIPT item. SCRIPT routine will plot user supplied points on a user defined psychrometric chart. The points and all pertinent info need being placed in an ASCII text file of a dryPAK readable format. Data points have two coordinates: temperature and humidity. Sometimes we only know one of them and also know that the point lies on a saturation line. In this case it is enough to write only one parameter: temperature or humidity and write Sat instead of the second one. The other coordinate will be computed accordingly without time consuming calculations on the user side.
|
LIQUID PROCESSING
| 1. | Q. | Can I design a natural draft
cooling tower with dryPAK ? |
| A. | Yes, provided that your tower will
have constant cross-section and you can accept constant heat and mass
transfer coefficients in the tower. The coefficients are entered by the
user and are not calculated by the program. Therefore it requires some
previous knowledge to find realistic values of these coefficients. Even
with this defficiency dryPAK is very handy if non-water solvent must be
contacted with gas in order to dehumidify or humidify it. | |
| 2. | Q. | In textbook descriptions of
water cooling towers and related gas-liquid operations it is assumed that
water flowrate remains constant throughout the tower despite of some
evaporation or condensation. Can it be also applied to highly volatile
organic liquids ? |
| A. | Of course not. DryPAK solves
differential balances of liquid and gas phase and the liquid flowrate is
known locally. Evaporation as high as 50% of the initial flowrate may take
place. One should, however, remember that the local variation of liquid
flowrate will not have any influence on the heat and mass transfer
coefficients which are considered constant in dryPAK. |
SOLID PROCESSING - DRYING
| 1. | Q. | We fit sorption isotherm
equations to agricultural products that we investigate. Can we use our
own equation with dryPAK ? |
| A. | No, but dryPAK uses over 28 most
popular sorption isotherm equations including BET, GAB, Harkins & Jura,
Oswin, Henderson, Day & Nelson etc. On request your equation may be added
to the list in your own copy of dryPAK. | |
| 2. | Q. | Can we design a complete dryer
using the BALANCE routine of dryPAK ? |
| A. | The BALANCE routine is a tool for
solving heat and material balances over continuously operating dryers with
or without gas recycle. It can solve any combination of missing variables
describing all four gas and solid streams and additional heat streams
(e.g. heat losses) involved. Of course there may be only two missing
variables (there are two equations) and they should not belong to one
equation alone. The BALANCE interface checks this fact before passing the
data to the solver. The dryer model used here is a black box. We do not know anything about the process kinetics inside therefore the size of a dryer cannot be designed. Furthermore if the data entered do not come from experiment or earlier experience some of them may be unrealistic and therefore the calculated results may also be not realistic although physically valid. Summing up, the BALANCE can be used in the design process if the process variables of the exit streams are known from experience except for two of them which will be calculated. Of course the value of this routine is out of question when making recycle ratio studies or balancing experimentally measured values of an operating dryer. | |
| 3. | Q. | Can I predict the kinetics of
a potato drying process with dryPAK ? |
| A. | Yes, you can approximately. DryPAK
has a routine for calculation of process kinetics based on solving Fick's
diffusion equation. It can be done in three basic geometries: plate,
cylinder and sphere. If water diffusivity in potato is known (e.g. from
literature) and one can assume that potato is a sphere the kinetics of
drying can be calculated (isothermally or adiabatically). In the present
version dryPAK will not, however, consider shrinkage of the potato during
drying and possible changes of water diffusivity with moisture content
and temperature. | |
| 4. | Q. | I experimentally measured
the drying kinetics of my product. Can I use this kinetics to calculate
continuous drying processes with dryPAK ? |
| A. | Yes, you can if you present your
results in the form of the so called Characteristic Drying Curve.
The CDC has the following form
At present dryPAK will not accept drying kinetics approximated by numerous empirical equations including the so called "thin layer equations". |
TECHNICAL PROBLEMS & PRODUCT SUPPORT
| 1. | Q. | How soon the new release of
dryPAK can be expected ? Do I need to pay the full price if I already
have the actual version ? |
| A. | New dryPAK will probably be
released in two years from the announcing of the current version. Registered users of the earlier version can get an upgrade at fraction of the normal price. Upgrades cannot be further upgraded. | |
| 2. | Q. | What if I have a specific
technical question ? |
| A. | Please e-mail your question to
pakowski@chemeng.p.lodz.pl
or send a fax or letter to OMNIKON Ltd. If your question is of general
interest it will be answered here otherwise you will receive a personal
message. | |
| 3. | Q. | We would like to install dryPAK
in a LAN for our students to use during a drying technology course. Can we
do it? |
| A. | The network installation of dryPAK
is possible. In fact dryPAK need write to only three types of files: the
gas library file, temporary result files and text or graphic output files.
Other files are read only. For that reason the whole dryPAK system can be
installed on the server with the sole exception of gas library file and
initialization file which need to be copied to as many local disks or user
directories as necessary. The INI file must contain the gas library
path/filename pointing to the user local directory. When dryPAK is started
it will first look for gas library file at the local directory and, if not
found, at the directory where the main program is installed. All other
output i.e. temporary files and result files will be directed to the local
directory. If not installed locally on the network, trying to modify gas library by writing a gas mixture to it (MIXTURE routine) can produce a network protection error. |
Working Examples
- Thermodynamic properties of humid air at high pressures
- Countercurrent dryer
- Calculation of the wet-bulb temperature
- Drying kinetics by Fickian diffusion
- Combustion process of an organic fuel
e-mail pakowski@wipos.p.lodz.pl
visit on this page since 28th December 96
R.J. Zylla 28.12.96