desktop air user`s manual
Contents
1. A Naw 1y 2 2 2 2 Y 2t A Xa M 2x X M Naw E Xy M w 5 17 w 17 4M a M w 7 My hy M a M M M Y M 2 24 x 2 MM at er 1 EE 5 M 4MM Jm n M 7 viscosity of dry air at the specified pressure and temperature 7 viscosity of water vapor at the specified pressure and temperature 4 A2M M T M M na and C 266 93E 7 aw potential function parameter for mixture adjusted for one polar molecule Q6 transport property integral as a function of the reduced temperature Ai2 function of reduced temperature In a similar manner the equations for thermal conductivity of moist air is as follows DeskTop Air for Windows Version 4 0 User s Manual Chapter Theoretical Basis of Functions Page 17 X Y 1 2 1 4 where 4 thermal conductivity of the mixture 2 2 _ x 2x X X A 4 2 2 2 Y uo UO 2 1702 4 4 A a aw w Z xiu t 2x x UP xiu 9 2 Uo s A m 12 1 M 1 E 12 12 15 1215 M 2 2 pul d oe SEIS e iM 1 M M 12 12 15 124 5 M 2 MM 4 4 MEME x uu epe B EL 777 TE gt s TELAN A Thermal conductivity of dry air at the specified pressure and temperature A Thermal conductivity of water vapor at2. 0 0 Page 20 Chapter 4 Accuracy Page 21 COMPARISON OF RESULTS WITH NBS 1 DATA EXHIBIT 5 3 ENTROPY OF DRY AIR Temperature Range To Pressure rom g Max Error 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 E K 0 0 0 0 0 0 8 6 6 250 300 350 400 500 00 00 1450 1 1 1 1 1 COMPARISON OF RESULTS WITH NBS 1 DATA EXHIBIT 5 4 SPECIFIC HEAT OF DRY AIR Temperature Pressure Range K Avg Max Avg Max Avg Max Error Error Error Error Error Error 0 0 0 0 0 0 0 EJ 0 0 0 0 0 0 2 8 6 Notes Questionable reference data 0 0 0 0 0 0 0 0 0 Q amp 9 1 1 1 1 71 5 50 deg 0 250 0 300 0 350 0 400 0 500 00 00 0 1450 ad 250 200 M 360 40 500 eoo 1450 2000 DeskTop Air for Windows Version 4 0 User s Manual Chapter 4 Accuracy Page 22 COMPARISON OF RESULTS WITH ASHRAE 9 amp NBS 1 EXHIBIT 5 5 SPECIFIC VOLUME ft Ibm ft lbm of Dry Air ASHRAE ASHRAE 9 554 9 553 9 554 9 553 9 807 9 807 9 807 9 807 10 061 10 060 10 061 10 06 50 10314 10 313 10 314 10 314 40 10 567 10 567 10 568 10 568 30 10 820 10 820 10 823 10 822 20 11 073 11073 11 078 11 078 10 11 326 11 326 11 335 11 335 11579 11 579 11 594 11 594 10 11
3. where nA ideal gas molar entropy for air ideal gas molar entropy for water nA constant to adjust reference state for air nA constant to adjust reference state for water DeskTop Air for Windows Version 4 0 User s Manual Chapter Theoretical Basis of Functions Page 13 The molar specific heat at constant pressure of moist air can be described by the equation 2 ER 265 Bs 246 Cn y or 2 or T eB R 1 yh n 2 y y v eT or 2b AE I oss y y where c ideal gas molar specific heat of dry air at constant pressure c ideal gas molar specific heat of water vapor at constant pressure For combinations of pressure and temperature below the boiling point of water there is a maximum quantity of water that can exist at that temperature in the vapor state In this condition the air is said to be saturated For the case where moist air exists in a saturated state the mole fraction of water vapor Xws is given by and the mole fraction of air xas is given by P fp as P where the pressure of saturated vapor over pure phase water f a dimensionless quantity introduced by Goff 1949 called the enhancement factor or excess solubility which is equal to Xwsideal Xwsideal the ratio of the vapor pressure of the pure water to the
4. F and at pressures up to 5 0 MPa 725 psia The functions detect requests for calculations outside that range and return an error value 1 2 32 BIT AND 64 BIT VERSIONS The DeskTop Air package includes both 32 bit and 64 bit installation files If you are running a 64 bit operating system such as Windows 7 you should install the 64 bit version otherwise you should install the 32 bit version They are functionally identical DeskTop Air for Windows Version 4 0 User s Manual Chapter 2 Using DeskTop Air Page 2 2 USING DESKTOP AIR 2 1 OVERVIEW DeskTop Air is a Windows application which calculates the thermodynamic and transport properties of moist air It can be used as an interactive replacement for air property tables and psychrometric charts but does much more than that It automatically calculates all unknown properties when a state point is defined by known properties The program is flexible and designed to minimize keystrokes for common calculations DeskTop Air allows you to enter any number of state points label them and store the collection of points for later reference You can print a table of stored points or copy them to the clipboard and paste them into your favorite spreadsheet or word processor Major features of this program include e validity over a wide range of pressures and temperatures e flexible interactive design large choice of units for each property tools for heating cooling expansion and co
5. 2 Reference Conditions DEERE REED GEEN URE inr ERG GEGEE EN ate EER ENS 4 2 3 Datas u suasana 4 2 4 dies sede EE EE EER GR 5 2 5 Calculating State Points GERED EE oe ge EE puse vo eg geed 6 2 6 Formatting Property Values uses acted d oai ebd 6 2 7 Labeling and Storing State Points ER ie ak ie a i i ee 6 DES in ee OE OE EE EN 7 2 9 Saving And Opening Data 7 2 10 AdVanced Feallf8ss 8 2 10 1 0 8 2 10 2 Heating Cooling TOO s eiusm ux eta eet incar 9 2 10 3 Exchanging Data with Other 2 10 Getting 10 2 12 Eror MOesSdOBS Ceo N aaa 10 2 13 Exiting DESKTOP Alf is EE GE bac dele 10 3 THEORETICAL BASIS OF AIR FUNCTIONS a ee ee ee ee ee 11 91 Basis ot GalGulatlOns menaire D uu cM EE See De 11 3 2 Humidity Parameters ss ER ER ER ER DE 14 223 MANS POM Properties EE SE GE SE SSS SS ES 15 4
6. International Temperature Scale of 1990 ITS 90 described in reference 5 as the basis of temperature 2 3 ENTERING DATA Numeric data is entered in a specially designed edit box called an IO Box which behaves like a standard Windows edit box but includes some additional features To enter new data double click the IO Box and all the data will be highlighted As you enter new data the old data will be replaced To edit data hold the left mouse button down and drag the mouse over the characters that you wish to replace thereby highlighting them Release the mouse button and type new characters to replace the highlighted ones The IO Box has two modes of operation input and output When new data is entered the IO Box is placed in the input mode and red X appears to the left of the box After a new state point is computed all IO Boxes are placed in the output mode and the red X s are removed DeskTop Air for Windows Version 4 0 User s Manual Chapter 2 Using DeskTop Air Page 5 The IO Box accepts either numerical values or arithmetic expressions that can be evaluated to a numeric value A number can be entered in either decimal or scientific notation The expression can be any valid arithmetic expression using the following operators add subtract A multiply divide exponent parentheses Arithmetic calculations can be nested to any level using parentheses An example of a valid expression is 1004 2 99
7. Windows Version 4 0 User s Manual Page 26 Appendix Error Codes Page 27 5 REFERENCES Tables of Thermal Properties of Gases NBS Circular 564 1955 Vasserman A A Kazavchinskii Ya Z and Rabinovich V A Thermophysical Properties of Air and Air Components Nauka Press Moscow 1966 English translation by Israel Program for Scientific Translations Ltd available from NTIS Springfield Va 1971 as TT70 50095 Rabinovich V A Beketov V G Moist Gases Thermodynamic Properties Begell House Inc New York 1995 Hirschfelder J O Curtiss C F and Bird R B Molecular Theory of Liquids and Gases John Wiley and Sons 1954 Mangum B W and Furukawa T Guidelines for Realizing the International Temperature Scale of 1990 ITS 90 NIST Technical Note 1265 1990 Thermodynamic and Transport Properties of Steam ASME New York NY 1993 The IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance for General and Scientific Use International Association for the Properties of Water and Steam Paris France September 1995 R W Hyland A Wexler 1983 Formulations for the Thermodynamic Properties of Dry Air from 173 15 K to 473 15 K and of Saturated Moist Air from 173 25 K to 372 15 K at Pressures to 5 MPa ASHRAE Transactions 89 2A 520 35 American Society of Heating Refrigeration and Air Conditioning Engineers ASHRAE Fun
8. a dialog box that lists all of your DeskTop Air data files in the current folder You may select a different folder to view other files Select the file you want to open and press the Open button to load the file To start a new file select the New button from the Toolbar If you have stored any points and have not saved the file you will be prompted to do so 2 10 ADVANCED FEATURES DeskTop Air includes special tools that facilitate the calculation of some common air processes These tools include an expansion compression tool and a heating cooling tool 2 10 1 Expansion Compression Tool The Expansion Compression tool is used to evaluate performance of an air turbine or an air compressor If you are predicting the performance and know the efficiency of the turbine compressor the tool can be used to calculate the state point conditions at the expansion or compression end point If you are analyzing the performance of a turbine or compressor and know the conditions at the end point the tool can be used to calculate the efficiency The tool also computes the energy converted to shaft work generated by the turbine or used by the compressor To use the tool first select the active state point representing the start of the expansion or compression path Next select Expand Compress from the Tools menu or click the turbine icon on the toolbar which will open the Expansion Compression tool dialog box In the tool dialog box select
9. be used If you change units and then save a file those selected units will be restored whenever the file is opened If you wish to change the units back to your default unit set issue the command Restore Default Settings from the Format menu 2 5 CALCULATING STATE POINTS When you have finished entering values for the selected input variables click the on screen button labeled Compute to calculate all of the state point properties If the Compute button is grayed you have not checked enough properties to define the state point You may also compute the state point by pressing the Enter key on your keyboard Note that the Enter key serves two purposes in DeskTop Air Pressing the Enter key just after entering data in an IO Box tells DeskTop Air to evaluate the expression in the IO Box Pressing the Enter key a second time tells DeskTop Air to compute the state point If you move the focus to any other control after entering data in an IO Box the expression is evaluated automatically and you only have to press the Enter key only once to compute the state point The Enter key will not compute a state point if the Compute button is grayed 2 6 FORMATTING PROPERTY VALUES DeskTop Air normally displays all property values in fixed decimal notation You may increase or decrease the number of decimal places in a selected property IO Box by selecting Decimal Places or Decrease Decimal Places from the Format menu
10. entered The units for these results will be consistent with your input units You may change any of the data and repeat the calculation as required When you are satisfied with the result you may copy that state point back to the Active Point on DeskTop Air s main window by pressing the OK button Note that the values for moist air in the second column will be copied back to the active point If no condensation has occurred these values will be the same as in the first column labeled mixture If condensation has occurred then the values in the second column will be at saturated conditions If you do not wish to replace the Active Point on the main window with the heating cooling state point calculated just press the Cancel button DeskTop Air for Windows Version 4 0 User s Manual Chapter 2 Using DeskTop Air Page 10 2 10 3 Exchanqinq Data with Other Programs DeskTop Air includes two ways to share data with other applications running on your PC such as Excel Both methods use the Windows clipboard In the first data sharing method DeskTop Air s Edit command copies whatever text is highlighted in an IO Box e g a number or part of a number to the Windows clipboard From there that text can be pasted into other applications running on your PC This is typically done using those applications Paste command The second data sharing method also places text on the clipboard but uses an extended format tha
11. the conditions you will use the compute the end point from the combo box near the top of the screen Select Pressure amp Efficiency if you know the pressure and efficiency If you are checking an actual machine you will need to know the exiting air temperature or enthalpy Check either the Pressure Enthalpy or Pressure Temperature buttons as required The two variables you have chosen will have their value fields enabled all others will be grayed Enter the appropriate values and click the Compute button The Flow In column displays the state point properties entering the equipment which were copied from the active point The Total Flow Out box has three columns that display the state point properties at the end of the expansion The first column identifies the thermodynamic properties for the mixture of moist air and liquid water exiting the equipment The second column displays the thermodynamic properties for the moist air exiting and the third column shows the properties for the liquid water if any condensation occurs The units displayed are those currently in use on DeskTop Air s main window If you wish to use different units you should select those units on the main window before opening the expansion tool Below the state point properties you will find the shaft power based on the entering air flow rate that you entered The units for these results will be consistent with your input units You may change any of the da
12. 0 04594 70 0 03964 0 03968 0 03969 60 0 03357 0 03359 0 03360 50 0 02766 0 02766 0 02766 ASHRAE 40 0 02189 0 02187 30 0 01623 0 01621 20 0 01071 0 01069 0 00531 0 00528 0 0 00000 0 00000 10 0 00519 0 00517 20 0 01024 0 01023 30 0 01518 0 01519 40 0 02002 0 02004 50 0 02478 0 02480 60 0 02945 0 02947 70 0 03403 0 03406 80 0 03853 0 03855 90 0 04295 0 04297 100 0 04729 0 04730 110 0 05157 0 05156 120 0 05577 0 05575 130 0 05989 0 05986 140 0 06394 0 06391 150 0 06793 0 06790 160 0 07185 0 07181 170 0 07572 0 07567 180 0 07951 0 07946 0 07947 1 32184 1 32183 190 0 08325 0 08320 0 08321 2 12731 2 12733 200 0 08692 0 08689 0 08690 4 28462 4 28477 DeskTop Air for Windows Version 4 0 User s Manual Page 24 Chapter 4 Accuracy 1 0058 COMPARISON OF RESULTS WITH ASHRAE 9 EXHIBIT 5 8 ENHANCEMENT FACTORS 1 0056 1 0054 4 1 0052 1 005 1 0048 1 0046 1 0044 K 1 0042 1 004 1 0038 20 40 60 80 100 120 140 Temperature deg F EET Goff 3 0 DeskTop Air for Windows Version 4 0 User s Manual Page 25 Chapter 4 Accuracy COMPARISON OF RESULTS WITH ASHRAE 9 EXHIBIT 5 9 SATURATED HUMIDITY RATIO Saturated Moist Air at 1 atm 200 2 30454 2 30460 0 00006 0 00 DeskTop Air for
13. 7 2 2 5 which evaluates to 1000 506 Expressions are evaluated whenever you tab to or click on another control or press the Enter button Only the resulting value is shown in the box If you enter an incorrect expression your computer will beep and the edit cursor will highlight the offending character You must fix the error before DeskTop Air will allow you to continue If the expression you are entering is too long to fit in the box the box will temporarily increase in length so that you may view more of the expression When you are finished editing the box returns to its original length To recall the last expression used in an input box use the Recall Expression command on the Edit menu You l then have the opportunity to edit the expression and let the IO Box re evaluate it Note that the box retains the last number or expression entered even after a compute command is issued As a result the value in the box will not reflect the value of the recalled expression if a compute command has altered the value You may use the Cut Copy and Paste commands on the Edit menu or the toolbar to exchange numeric data between DeskTop Air s Boxes and any other application that supports the clipboard If you start entering data in an IO Box and wish to go back to the original data you may use the Undo command on the Edit menu However once the IO Box loses the focus the Undo command is no longer available 2 4 UNIT SELEC
14. 832 11 832 11 857 11 857 20 12 085 12 127 12 127 30 12 338 12 406 12 406 40 12 591 12 696 12 696 50 12 844 13 001 13 001 13 096 13 329 13 329 13 349 13 688 13 688 13 602 14 089 14 089 13 855 14 547 14 547 100 14 107 15 083 15 084 110 14 360 15 727 15 727 120 14612 14 613 16 519 16 519 130 14 865 14 865 17 520 17 52 140 15 118 15 118 18 826 18 825 150 15370 15 370 20 589 20 589 160 15 693 15 623 23 082 23 082 170 15875 15 875 26 835 26 834 180 16428 16 128 33 037 33 037 190 16 380 16 381 45 043 45 042 DeskTop Air for Windows Version 4 0 User s Manual Chapter 4 Accuracy COMPARISON OF RESULTS WITH ASHRAE 9 amp NBS 1 EXHIBIT 5 6 ENTHALPY Dry Air at 1 atm Saturated Moist Air at 1 atm Temp Btu lbm Btu Ibm of Dry Air i NBS GAi ASHRAE Air ASHRAE 80 19 217 19 218 19 221 19 213 19 215 70 16 815 16 814 16 806 16 803 16 817 60 14 412 14 410 14 414 14 388 14 392 50 12 009 12 007 12 011 11 964 11 968 40 9 606 9 605 9 609 9 522 9 526 30 7 203 7 203 7 206 7 050 7 053 20 4 802 4 802 4 804 4 525 4 527 10 2 401 2 401 2 402 1 914 1 915 0 0 000 0 000 0 000 0 836 0 835 10 2401 2 401 2 402 3 803 3 804 20 4 802 4 803 4 804 7 106 7 107 30 7203 7 205 7 206 10 916 10 917 40 9 606 9 606 9 609 15 233 15 233 50 12 00
15. 9 12 009 12 012 20 307 20 306 60 14 412 14 412 14 415 26 470 26 467 70 16 815 16 815 16 818 34 101 34 097 80 19 248 19 218 19 222 43 706 43 701 90 21 622 21 622 21 626 55 959 55 951 100 24 027 24 027 24 031 71 770 71 761 110 26 432 26 431 26 436 92 397 92 386 120 28 837 28 836 28 842 119 628 119 612 130 31 244 31 244 31 249 156 095 156 076 140 33 652 33 652 33 656 205 849 205 824 150 36 060 36 059 36 064 275 516 275 49 160 38 469 38 468 38 474 376 760 376 737 170 40 878 40 877 40 884 532 293 532 256 180 43 289 43 288 43 295 793 156 793 166 190 45 703 45 700 45 707 1303 287 1303 321 200 48 120 48 114 48 121 2688 073 2688 205 DeskTop Air for Windows Version 4 0 User s Manual Page 23 Chapter 4 Accuracy COMPARISON OF RESULTS WITH ASHRAE 9 amp NBS 1 EXHIBIT 5 7 ENTROPY Saturated Moist Airat 1 atm Btu lbm F of Dry Air Air ASHRAE 0 04592 0 04592 0 03965 0 03966 0 03354 0 03354 0 02754 0 02755 0 02165 0 02166 0 01583 0 01583 0 01001 0 01002 0 00414 0 00414 0 00192 0 00192 0 00832 0 00832 0 01528 0 01528 0 02315 0 02315 0 03187 0 03187 0 04192 0 04192 0 05389 0 05389 0 06845 0 06844 0 08643 0 08642 0 10896 0 10895 0 13754 0 13752 0 17421 0 17418 0 22183 0 22180 0 28460 0 28457 0 36894 0 36890 0 48529 0 48524 0 65193 0 65188 0 90433 0 90425 Dry Air at 1 atm Temp Btu Ibm F NBS Air 80 0 04589 0 04593
16. ACCURACY n DADE DEE IA EA 18 4 1 Accuracy of Calculations 2 EE ME terr Ee 18 5 REFERENCES aa ei 27 Chapter 1 Introduction Page 1 1 INTRODUCTION 1 1 OVERVIEW DeskTop Air is an interactive air property calculator based on data published by the National Bureau of Standards NBS 1955 1 NBS data is also the basis of the formulations used to develop the tables published in the American Society of Heating Refrigeration and Air Conditioning ASHRAE Fundamentals 1993 All of the properties calculated by DeskTop Air are based on real gas properties not perfect gas approximations As such they include the effects of pressure and interactions caused by the mixing of water vapor and dry air components Details on the theory and accuracy are provided in this user s manual The psychrometric properties facilitate conversion among various moist air parameters such as absolute humidity relative humidity wet bulb temperature and degree of saturation The thermodynamic properties provide forward and inverse relationships using specific volume specific enthalpy and specific entropy The transport properties provide specific heat dynamic viscosity and thermal conductivity from pressure temperature and humidity ratio The DeskTop Air functions are valid over a temperature range from 180 K 136 F to 2000 3140
17. DESKTOP AIR USER S MANUAL VERSION 4 0 COPYRIGHT NOTICE The Air software and manual are copyrighted and licensed for use by one user per copy purchased This manual and the software described in it are copyrighted with all rights reserved Under the copyright laws this manual or the software may not be copied in whole or part without written consent of Techware Engineering Applications Inc Techware Engineering Applications Inc grants permission to the purchaser to make a limited number of copies of the add in for backup purposes only provided that the copies are not in use at the same time as the original Additional reproduction of the add in is a violation of copyright law Violators will be prosecuted to the fullest extent of the law Copyright 1995 2012 Techware Engineering Applications Inc All rights reserved Table of Contents 1 INTEODBOUETIOR iiie di oue e ERE Qo ES Ee ee Re ER Cade cues N RE Re ee CENNE EREC EREA 1 let NEG UI II NEPOTES ee Ee ie ER a EE 1 1 2 32 Bit and 64 bit versions ooi rdi e desir ed uec N n 1 2 USING DESISTOPORIED RA tp PRU rettet tete 2 AMI jl 2 2 1 1 Valid Operating hande ss EE RR SE AG ae es 2 2 2 2 2 2 1 Wel DIY Basis DRR EDE DE DR DR ER N DU 3 2 2
18. TION DeskTop Air allows you to use any combination of units for input and output properties The units for each property can be set independently by using its associated combo box To change a unit select its combo box and scroll through the list of available units using either the keyboard cursor arrows or the mouse Whenever a new unit is selected the program converts the values displayed for that property to the new units Therefore when entering data you should first select the units and then key in the input values Otherwise the values will be converted to the new units and you will have to re enter them The first time the program is started the properties will be displayed SI units You can change all of the units to either standard English or SI units by issuing the English Units or SI DeskTop Air for Windows Version 4 0 User s Manual Chapter 2 Using DeskTop Air Page 6 Units command from the Format menu or pressing the Eng or SI toolbar button When you change to English units in this manner the reference temperature for dry air is changed to 0 F Similarly when changing to SI units the reference temperature is changed to 0 You may choose any combination of available units for your default unit set Simply select the units you desire for each property and issue the Current Settings Default command from the Format menu The next time you start DeskTop Air your default unit set will
19. better agreement with the original experimental data than the model used by ASHRAE Since the enhancement factor is the basis for calculating saturated humidity ratio we place a higher confidence on the saturated humidity ratio calculated by Air DeskTop Air for Windows Version 4 0 User s Manual Chapter 4 Accuracy COMPARISON OF RESULTS WITH NBS 1 DATA EXHIBIT 5 1 COMPRESSIBILITY OF DRY AIR Temperature Pressure Range deg K Avg Error Error Error Error 250 0 001 0 002 0 019 0 028 0 125 10 360 0 001 0 001 0 003 0 003 0 009 N 3 Co O 400 10 500 O A ojo 700 92 0 000 0 001 0 003 0 004 0 002 0 001 0 001 0 003 0 004 0 010 0 001 0 001 0 001 0 002 0 008 800 0 000 0 001 0 00 0 001 0 004 1450 0 000 0 001 0 001 0 002 0 002 500 2000 0 004 0 016 0 004 0 006 0 004 40 Error 0 279 0 014 0 013 0 004 0 013 0 013 0 010 0 006 0 004 0 010 COMPARISON OF RESULTS WITH NBS 1 DATA EXHIBIT 5 2 ENTHALPY OF DRY AIR Temperature Pressure Range eg K deg K Avg Max Avg Max Avg Error Error Error Error Error 13 0 0 04 0 0 0 0 i DeskTop Air for Windows Version 4 0 User s Manual Max Error 2690 07 0 01 0 01 0 01 0 01 0 01 0 01 0 10
20. damentals 1993 DeskTop Air for Windows Version 4 0 User s Manual
21. elect Exit from the File menu Press Alt F4 keys e Click the X button in the upper right corner of the program window DeskTop Air for Windows Version 4 0 User s Manual Chapter Theoretical Basis of Functions Page 11 3 THEORETICAL BASIS OF AIR FUNCTIONS 3 1 BASIS OF CALCULATIONS The formulations used in developing the air property functions treat moist air as a mixture of two real gases namely dry atmospheric air and water vapor Furthermore dry atmospheric air is assumed to be a homogeneous single component gas with an apparent molecular weight of 28 9645 g mol The P V T behavior of the mixture is described by a virial equation of state where P Total pressure T Absolute Thermodynamic Temperature v Total volume of gas R Universal Gas Constant B second order virial coefficient of the mixture third order virial coefficient of the mixture For the ranges of pressure and temperature used in these functions the higher order coefficients can be ignored It can be shown for the air water mixture that lt 2 2 w aw and C X RE Ct where x mole fraction of air in the mixture x mole fraction of water in the mixture virial coefficients of air aa aaa DeskTop Air for Windows Version 4 0 User s Manual Chapter Theoretical Basis of Functions Page 12 B C Virial coe
22. essibility of the air mixture or the water vapor as required V V p Saturation pressure of water vapor at temperature T y the volume of the condensed water wcond The solubility equation can be solved for Xws by iteration The value of is used to find the dew point temperature of a gas mixture to determine the amount of condensation that occurs in a cooling process and to verify that the value of x input by the user does not exceed Xws 3 2 HUMIDITY PARAMETERS In determining the thermodynamic properties of moist air it is required to know the relative quantities of water vapor and dry air in the mixture The formulations described above use the mole fractions of air and water for this purpose In practice it is common to use other parameters as well to define the proportions of air and water vapor in the mixture The most common of these parameters include humidity ratio relative humidity degree of saturation wet bulb temperature and dew point temperature and mass fraction of water vapor DeskTop Air allows you to input any of the various parameters for determining the level of moisture in the air and displays all of the others when the state point is calculated The definitions of each moisture parameter are as follows M Humidity Ratio M a where M mass of water vapor in sample M mass of dry air in sample DeskTop Air for Windows Version 4 0 User s Manual Chapter Theoretical Ba
23. fficients of water vapor Www www virial coefficients of the mixture aw aaw aww ASHRAE uses polynomial equations for estimating the virial coefficients of air and water vapor as presented by Hyland and Wexler 8 These equations are derived from the data presented by the NBS 1 While these equations produce reasonable results they are limited in temperature range from about 100 to 200 and are clearly inadequate for this application which supports temperatures up to 2000 K Instead of using polynomial equations Techware has developed a gas property database from the NBS 1 data The database includes virial coefficients as well as ideal gas properties for enthalpy entropy and specific heat that span the range of temperatures from 180 K to 2000 K Coefficients are extracted from the database at any desired temperature using non linear interpolation The molar enthalpy of moist air hm can be described by the equation h x hie hy x h RT i m c T 4 as dT v 2 dT v where h ideal gas molar enthalpy for air h ideal gas molar enthalpy for water h constant to adjust reference state for air a h constant to adjust reference state for water w The molar entropy of moist air sx can be described by the equation 5 ts x RIP x ge a Zi 8 JR rei Me x RT 2 dT w
24. g box In the tool dialog box select the conditions you will use to compute the end point from the combo box near the top of the screen Select Pressure Heat Flow if you know the pressure and heat added or removed If you are measuring performance an existing device you will need to know the exiting air temperature or enthalpy Check either the Pressure Enthalpy or Pressure Temperature buttons as required The two variables you have chosen will have their value fields enabled all others will be grayed Enter the appropriate values and click the Compute button The Air Flow In column displays the state point properties entering the equipment which were copied from the Active Point in the main screen window The Total Flow Out box has three columns that display the state point properties at the end of the expansion The first column identifies the thermodynamic properties for the mixture of moist air and liquid water exiting the equipment The second column displays the thermodynamic properties for the moist air exiting and the third column shows the properties for the liquid water if any condensation occurs The units displayed are those currently in use on DeskTop Airs main window If you wish to use different units you should select those units on the main window before opening the Heating Cooling tool Below the state point properties you will find the heat added based on the entering airflow rate that you
25. gh there does not appear to be any definitive standard of accepted values as there are for steam and water We found the most complete coverage by Vasserman Kazavchinskii and Rabinovich 2 who have surveyed the available literature and DeskTop Air for Windows Version 4 0 User s Manual Chapter Theoretical Basis of Functions Page 16 presented tables of values over a range of temperatures and pressures We have used their tabular values of viscosity and thermal conductivity as the basis of our database for dry air We have also used their recommended polynomial adjustment factors as a function of density to account for variations associated with pressure We have used the equations recommended by ASME 6 to compute the viscosity and thermal conductivity properties for water vapor These are the same equations used in our WinSteam product There is little information available regarding the viscosity and conductivity of moist air In most practical cases the relatively small quantity of water vapor in air has a small effect on the overall transport properties unlike the thermodynamic properties which are greatly affected by moisture Nonetheless we have selected a method to account for the effects of moisture based on kinetic theory as presented by Hirshfelder Curtis and Bird 4 The equation for viscosity of a binary mixture is as follows X Y 1 27 I Z where 7 viscosity of the mixture 2 X 2x x xt
26. kTop Air Page 3 point can be uniquely identified by specifying the moisture content and any two of the other five properties Psychrometric chart the state point is normally determined as the intersection of two lines which represent two variables Only two values are required on the Psychrometric chart because the chart is valid for only a single air pressure In most practical applications pressure is usually one of the known variables Most of the calculations require that the pressure be known In the rare case when pressure is not known it can be calculated from the equation of state if the dry bulb temperature specific volume and humidity ratio is known Each of the key properties has a check box associated with it When checked it signifies that this property is to be used in calculating the state point In general two thermodynamic properties and one other property which indicates the moisture content must be selected before DeskTop Air will allow a computation There are some combinations of only two properties that allow computation in the saturated state In these cases you will notice that the Compute Saturated button is enabled Once you select one property DeskTop Air will disable all the other check boxes whose properties are not allowed in combination with the first selected property You may change your selections by un checking one or all of the check boxes and selecting a new combination To compute an air proper
27. mpression processes instant response time extensive help screens If installed properly DeskTop Air is started by simply double clicking on its name icon in the DeskTop Air subgroup of the TechwareEng Group appearing in the Windows Start Menu 2 1 1 Valid Operating Range The DeskTop Air functions are valid over a temperature range from 180 K 136 F to 2000 K 3140 F and at pressures up to 5 0 MPa 725 psia The functions detect requests for calculations outside that range and return an error value 2 2 BASIC OPERATION The DeskTop Air display is arranged in a tabular format that remains constant although the program window can be re sized You may sometimes find it convenient to make the program window smaller Each of the rows is dedicated to one of the air properties pressure dry bulb temperature specific volume enthalpy entropy wet bulb temperature dew point relative humidity humidity ratio degree of saturation H2O mole fraction H2O mass fraction molecular mass specific heat dynamic viscosity and thermal conductivity There is a column that displays the property values for the active point and a column that displays data for one of the stored points The key properties that can be used to define the air state point are pressure temperature specific volume enthalpy entropy and one of the moisture indicators Theoretically a state DeskTop Air for Windows Version 4 0 User s Manual Chapter 2 Using Des
28. or by clicking either of 0 00 the toolbar buttons respectively If you wish to change the selected property to scientific notation select the Scientific Notation command from the Format menu or press the EE toolbar button The Format menu many be used to change the number format to Fixed Decimal or Percent as well Corresponding toolbar buttons are Fix and respectively Of course the percent format only makes sense for the non dimensional properties The formatting commands are only enabled when the focus is on an IO Box 2 7 LABELING AND STORING STATE POINTS DeskTop Air gives you the option of labeling and storing any number of state points for future reference After computing the state point DeskTop Air will automatically suggest a unique name for the new state point such as Point 1 or Point 2 but you will probably want to enter something more descriptive Just enter a new name in the Point Label box DeskTop Air for Windows Version 4 0 User s Manual Chapter 2 Using DeskTop Air Page 7 To store this point press the Store Point button The Store button will not be enabled if the displayed data is not a correctly calculated state point DeskTop Air requires all stored points to have a unique label If you try to store two points with the same label DeskTop Air will query whether you wish to replace the stored point which has the same label with the current point If not yo
29. s based on absolute zero temperature In SI units ASHRAE uses a reference condition of 0 and one atmosphere of pressure for dry air For water vapor ASHRAE used a reference condition of liquid water at the triple point temperature of 0 01 In English units however ASHRAE uses a reference condition of 0 F at one atmosphere of pressure for dry air properties while maintaining the convention of using the triple point as a reference temperature for water properties DeskTop Air allows you to set the reference temperature of 0 0 or 0 K Please note that in all cases the enthalpy and entropy of the water portion are set to zero for liquid water at the triple point temperature Some publications which are based on perfect gas assumptions assign the zero point for enthalpy and entropy to water in the vapor state at the triple point or some other specified temperature We believe that setting the zero point for enthalpy and entropy to water in the liquid state rather than in the vapor state has two major advantages First the values are numerically equivalent to standard international steam tables and Techware s WinSteam product This facilitates the handling of processes that include both moist air and liquid water streams without worrying about reference temperatures Second It greatly simplifies analysis of processes in which water is either condensed from or evaporated to the air stream All reference conditions use the
30. sis of Functions Page 15 As described in section on alternate moisture parameters mole fraction or mass fraction of water vapor can be substituted for humidity ratio through the unit set argument Xy Re lative Humidity x where x mole fraction of water vapor xy mole fraction of water vapor saturated at the same temperature and pressure W Degreeof Saturation m where W humidity ratio W humidity ratio of saturated air at the same temperature and pressure Dew Point Temperature D is defined as the solution to the equation W P D W 8 It represents the temperature to which a sample of moist air at a given unchanging pressure can be cooled before condensation begins Thermodynamic Wet Bulb Temperature B is the temperature at which water by evaporating into moist air at a given dry bulb temperature and humidity ratio can bring the air to saturation adiabatically at the same temperature B while the pressure is maintained constant It is defined as the solution to the equation h P T W W P B W P T h B h P B where h P T W enthalpy of the moist air sample at given pressure dry bulb temperature and humidity ratio hy enthalpy of water added at the wet bulb temperature h P B enthalpy of saturated air at the wet bulb temperature 3 3 TRANSPORT PROPERTIES Values for viscosity and thermal conductivity of dry air at atmospheric pressure can be found in many references althou
31. t is especially useful for calculations involving air properties This method is accessed using the Copy Point command on the Edit menu Regardless of whether any text is selected this command copies a table of text values to the clipboard using either the Active Point or all of the stored points The table includes one column for each point You can optionally direct DeskTop Airto also include property and unit labels in which case they appear as the first column in the table The table includes one row for each property in the same order as they are displayed on DeskTop Air s main window All values are copied to the clipboard using the same format displayed on the screen So if you need more accuracy in your Excel calculations set the DeskTop Air display accuracy accordingly before issuing the Point command 2 11 GETTING HELP The Help menu offers extensive on line help for DeskTop Air The About item in the Help menu presents the version and serial numbers of your copy of Air 2 12 ERROR MESSAGES When input values are out of range or any other errors occur a message box is displayed describing the error The calculated fields will not be updated until the error condition is resolved 2 13 EXITING DESKTOP AIR You may exit DeskTop Air by using any of the standard methods for closing Windows applications These include the following e Click the upper left corner of its program window and select close from the menu S
32. ta and repeat the calculation as required When you are satisfied with the result you may copy that state point back to the active point on DeskTop Air s main window by pressing the OK button Note that the values of moist air in the second column will be copied back to the active point If no condensation has occurred these values DeskTop Air for Windows Version 4 0 User s Manual Chapter 2 Using DeskTop Air Page 9 will be the same as in the first column labeled mixture If condensation has occurred then the values in the second column will be at saturated conditions If you do not wish to replace the current point on the main window with the expansion end point calculated just press the Cancel button 2 10 2 Heating Cooling Tool The Heating Cooling tool is used to analyze air stream in which heat is either added or removed If you know the quantity of heat that is being added to or removed from the air stream you can use the tool to calculate the resulting state point temperature and other conditions If you know the temperature or enthalpy of the air leaving the heating or cooling device you can use the tool to calculate the quantity of heat exchanged To use the tool first make sure that the active state point represents the conditions of the air entering the heating or cooling device Next select Heat Cool Tool from the Tools menu or click the flame icon on the toolbar which will open the Heating Cooling tool dialo
33. table consisting of all of the stored points by selecting the Print command from either the file menu or the toolbar The tables will include a column for each of the stored points with each of its properties listed in a row label for each row includes the property name and the current units selected The printing utility will attempt to fit as many points on a page as possible based on the paper size and orientation that you specify using the Print Setup command You can preview the output by selecting the Print Preview command from the File Menu 2 9 SAVING AND OPENING DATA FILES When you start DeskTop Air or open a new file it will be untitled You may save a collection of stored state points along with your selections of units and formats to a DeskTop Air file Select Save from the menu or click the disk icon on the toolbar which will open the Save As dialog box You should enter a name for the file the file extension will be added automatically You may choose a folder in which to save the file or accept the default folder If you have saved the file at least one time during the session you can save the work under a different filename by choosing the Save As command from the File menu To open a saved file select from menu or click the open file icon on the DeskTop Air for Windows Version 4 0 User s Manual Chapter 2 Using DeskTop Air Page 8 toolbar This will open
34. tables in reference 1 are in error One would expect the thermodynamic properties to approach the ideal gas properties as the pressure is reduced to zero While this is the case for temperatures up to 800 it is not the case above 800 K Curiously at 800 K the tables in reference 1 change from a temperature increment of 10 K to 50 at the point where the error begins Perhaps this change of temperature increment was not handled properly Comparison of data from Vasserman et al 2 in the temperature range from 850 K to 1300 K supports the conclusion that the tables from NBS 1 are in error Unfortunately the tables from Vasserman et al 2 does not contain data for temperatures above 1300 Moist air properties are compared against values published by ASHRAE 9 in Exhibits 5 5 5 7 which show excellent agreement It should be noted that there are small differences in the saturated humidity ratio between DeskTop Air and ASHRAE which are attributed to the different models used as explained in the section on basis of calculations Some of the table data was generated using the functions for convenience They use the exact same function library as used in DeskTop Air DeskTop Air for Windows Version 4 0 User s Manual Chapter 4 Accuracy Page 19 Exhibit 5 8 plots the enhancement factors from and ASHRAE with the original experimental data from Goff This clearly shows that the model used by Air provides
35. the specified pressure and temperature c STU 2 Lon aF r aw and 1989 1 7 function of reduced temperature DeskTop Air for Windows Version 4 0 User s Manual Chapter 4 Accuracy Page 18 4 ACCURACY 4 1 ACCURACY OF CALCULATIONS Thermodynamic properties for dry air calculated by Air have been checked against the values published by NBS 1 As shown on Exhibits 5 1 through 5 4 the calculated values show excellent agreement with the source data throughout most of the valid range Some small differences can be observed at low temperatures and high pressures This is due to the limitations of using a third order virial equation of state Even so the maximum error is less than 0 3 for thermodynamic properties and 3 17 for specific heat Another smaller difference is shown in the compressibility factor between 1500 and 2000 K This is due to the dissociation of Nitrogen and Oxygen at these temperatures This error is less than 0 02 The tables show some differences in enthalpy entropy and specific heat occurring at 850 K and growing with temperature This initially was thought to be attributed to dissociation but this was dismissed for two reasons First dissociation effects were not included below 1500 K in reference 1 Second the error is much greater than what can be attributed to dissociation above 1500 Further examination of the data tables suggests that the
36. total pressure of the mixture ASHRAE uses equations for estimating the enhancement factor as well as the virial coefficients of dry air and air water mixture developed by Hyland and Wexler 8 This version of DeskTop Air uses another method for calculating the enhancement factor presented by Rabinovich 3 We believe that this method produces better correlation with the original experimental data developed by Goff Even so the differences between the two methods are generally small DeskTop Air for Windows Version 4 0 User s Manual Chapter Theoretical Basis of Functions Page 14 Assuming that the condensed phase is pure water the value of can be found by equating the chemical potential of pure condensed water with the chemical potential of its vapor in the gas solution Since the equations describing chemical potentials are non linear and difficult to analyze phase equilibrium is often described in terms of a thermodynamic function called fugacity The equality of chemical potentials can be replaced by equating the logarithms of the fugacity of pure condensed water with that of its vapor in the gas mixture solution The equation of solubility based on this concept is derived by Rabinovich 3 and can be expressed as follows In P In z p T x 1 d udi dp 2 36 o P ws B z p T RT v M 2 epo IS Z Cun GENE es Ca y y ws qaw g ww ws where B C ek z l the compr
37. ty state point begin by selecting the input properties and set the corresponding check boxes as described above Enter values for those properties in the boxes to the right of the property names The next section describes various methods for entering data When you enter a value for any of the key properties a red X appears next to the property value to indicate that a new value has been entered This is a warning that the value being displayed is not consistent with the current state point After a compute command is completed successfully all property values are recalculated and the red X s are cleared Be sure to enter the values in units consistent with the unit displayed to the right of the value box If you wish to change units select the desired units before entering the value If you change units after the value is entered the value entered will be converted to the new units Next click the Compute button to find all the unknown properties If the Compute button is grayed you have not checked enough properties to define the state point Fields without check boxes are output only These include molecular mass specific heat viscosity and thermal conductivity After recalculation all fields contain property values for moist air at a particular state point The box labeled Reference identifies the reference conditions for the property values It indicates whether the values are on a wet or dry basis the reference press
38. u should rename the point and try to store it again After storing the point the state point values for the active point will be copied into a storage array and given the name you selected The point will be displayed in the stored points column in same units as the active point After a state point is stored its name is added to the point name combo box in the upper right hand corner of the main window You can view a stored point by pressing the arrow on the point name combo box and selecting a point from the drop down list If you wish to use a stored point with any of the special tools or as the basis for calculating a new point you must recall the stored point to the active point Begin by selecting the point from the point combo box s dropdown list Next press the Recall Point button to the stored point to the active point You may now use the active point for new calculations If you change any input data and press Compute the active point will be changed but the stored point from which you copied the values will remain intact You can delete a stored point by first selecting it from the Point Name combo box and then select Delete Point from the Edit menu This collection of state points can be saved as a DeskTop Air file In addition you can copy state points to the clipboard where they can be transferred to another application such as a spreadsheet program or word processor 2 8 PRINTING You can print a
39. ure for entropy and the reference temperature for dry air enthalpy and entropy 2 2 1 Wet or Dry Basis All mass dependent properties specific volume enthalpy entropy specific heat viscosity and conductivity can be based on either a dry or wet basis The default for DeskTop Air expresses the properties per mass of dry air This is consistent with ASHRAE conventions and is quite useful when dealing with processes that involve evaporation or condensation of water vapor In some cases you may wish to express these properties on a wet basis that is per mass of DeskTop Air for Windows Version 4 0 User s Manual Chapter 2 Using DeskTop Air Page 4 wet air dry air plus water vapor You can accomplish this by selecting Ref from the Toolbar or Reference Conditions from the Tools menu 2 2 2 Reference Conditions Enthalpy and entropy values are always expressed relative to particular reference conditions Many people forget that the values of enthalpy and entropy that are found in published tables are not absolute values but instead are relative to particular reference conditions Engineering calculations always deal with enthalpy or entropy differences typically between in flowing and out flowing streams For this reason it does not matter what you select as the reference conditions as long as you use them consistently DeskTop Air allows you to select reference conditions consistent with ASHRAE or an alternate set of condition
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