About Psychrometric Calculator

Thermodynamic Properties of Humid (Moist) Air
 
The HumidAirWeb application calculates 35 thermodynamic properties of humid (moist) air. It allows 21 different combinations of input variables to be used for calculations. Input variables that can be used are: pressure, dry bulb temperature, wet bulb temperature, dew point temperature, relative humidity, humidity ratio, enthalpy, entropy, and volume.

Calculation of thermodynamic properties in the application is based on the most exact formulations of thermodynamic properties of moist air currently available. These formulations are approved by ASHRAE (American Society of Heating, Refrigeration and Air-conditioning Engineers) as documented in the 1997 ASHRAE Handbook "Fundamentals". Hyland and Wexler presented the equation of state in:
 

• Formulations for the thermodynamic properties of the saturated phases of H2O from 173.15 K to 473.15 K, W. Hyland and A.Wexler, ASHRAE Transactions, 89(2A) 500-519, 1983.
• Formulations for the thermodynamic properties of dry air from 173.15 K to 473.15 K, and of saturated moist air from 173.15 K to 372.15 K, at pressures to 5 MPa, R. W. Hyland and A. Wexler, ASHRAE Transactions, 89(2A) 520-535, 1983.

Thermodynamic properties

Range of validity
 
The entire set of equations used by the HumidAirWeb - Psychrometric Calculator application for calculation of thermodynamic properties, is defined by the following temperature and pressure range:

SI Units

• 173.15 K <= T < = 473.15 K
• 0.000001 MPa < p < = 5 MPa .

• -100 °C <= T < = 200 °C
• 0.00001 bar < p < = 50 bar .

English Units

• -148 °F <= T < = 392 °F
• 0.000145037 psi < p < = 725 psi .

Reference state

IMPORTANT

In the used formulations of thermodynamic properties of moist air, reference state (zero enthalpy) is set at 0 °C. On some psychrometric charts (especially in the US) reference state (zero enthalpy) is set at 0 °F.

As a consequence, enthalpy and entropy values calculated by the application will be shifted on these charts for the difference between reference points.

Since for any process calculations only enthalpy difference (in and out of a process) is important, reference point doesn't really matter.

If you want to reproduce enthalpy values from such charts, you should find the enthalpy difference for one point (doesn't matter which one), and then add that value for all calculated enthalpies. The same applies for entropy values.

Input variables

To calculate properties, Psychrometric Calculator allows up to 21 different combinations of input variables in SI or English units:

1. f ( dry bulb temperature, relative humidity, pressure )
2. f ( dry bulb temperature, wet bulb temperature, pressure )
3. f ( dry bulb temperature, dew point temperature, pressure )
4. f ( dry bulb temperature, humidity ratio, pressure )
5. f ( dry bulb temperature, enthalpy of moist air, pressure )
6. f ( dry bulb temperature, entropy of moist air, pressure )
7. f ( dry bulb temperature, volume of moist air, pressure )
8. f ( wet bulb temperature, relative humidity, pressure )
9. f ( wet bulb temperature, dew point temperature, pressure )
10. f ( wet bulb temperature, humidity ratio, pressure )
11. f ( wet bulb temperature, enthalpy of moist air, pressure )
12. f ( wet bulb temperature, entropy of moist air, pressure )
13. f ( wet bulb temperature, volume of moist air, pressure )
14. f ( dew point temperature, relative humidity, pressure )
15. f ( relative humidity, humidity ratio, pressure )
16. f ( relative humidity, enthalpy of moist air, pressure )
17. f ( relative humidity, entropy of moist air, pressure )
18. f ( relative humidity, volume of moist air, pressure )
19. f ( humidity ratio, enthalpy of moist air, pressure )
20. f ( humidity ratio, entropy of moist air, pressure )
21. f ( humidity ratio, volume of moist air, pressure )

Available properties

The following thermodynamic properties can be calculated with Psychrometric Calculator:

1. Dry bulb temperature [Tdb];
2. Wet bulb temperature [Twb];
3. Dew point temperature [Tdp];
4. Pressure [P];
5. Saturation water pressure [Ps];
6. Saturation pressure at wet bulb temperature [Pwb];
7. Saturation pressure at dew point temperature [Pdp];
8. Mole fraction of water [Xw];
9. Mole fraction of air [Xa];
10. Weight fraction of water [Ww];
11. Weight fraction of air [Wa];
12. Humidity ratio [W];
13. Saturation humidity ratio [Ws];
14. Degree of saturation [DS];
15. Relative humidity [RH];
16. Specific volume of dry air [Va];
17. Specific volume of saturated ice [Vi];
18. Specific volume of saturated water [Vw];
19. Specific volume of saturated vapor [Vv];
20. Specific volume of moist air [Vm];
21. Specific density of dry air [Da];
22. Specific density of saturated ice [Di];
23. Specific density of saturated water [Dw];
24. Specific density of saturated vapor [Dv];
25. Specific density of moist air [Dm];
26. Specific enthalpy of dry air [Ha];
27. Specific enthalpy of saturated ice [Hi];
28. Specific enthalpy of saturated water [Hw];
29. Specific enthalpy of saturated vapor [Hv];
30. Specific enthalpy of moist air [Hm];
31. Specific entropy of dry air [Sa];
32. Specific entropy of saturated ice [Si];
33. Specific entropy of saturated water [Sw];
34. Specific entropy of saturated vapor [Sv];
35. Specific entropy of moist air [Sm].

Calculate properties  

Calculation procedure
 
In order to calculate properties of the moist air use the following steps:

1. Choose units, SI or English;
2. Select one of the 21 different functions based on available input variables;
3. Enter data for the first parameter;
4. Enter data for the second parameter;
5. Enter data for the third parameter;
7. Press Calculate button to perform calculation.

Detailed result of a calculation is displayed in the main table, located in the lower part of the input form, and also 14 main properties are displayed in the Output Pane. Input arguments used for calculation are displayed in red.
 
In the Output Pane results are always inserted at the top, so you don't have to scroll to see the latest result.

Click here for a larger image.

Fig.1 Calculation of thermodynamic properties of moist air

Heating / cooling calculations

To calculate sensible heat flow, latent heat flow, total heat flow and sensible heat factor, use the following steps:

1. Select "Heating / Cooling" tab;
2. In the "Inlet Point" group box enter parameters of the first point;
3. In the "Outlet Point" group box enter parameters of the second point;
4. Press "Calculate" button.

Results of the calculation will be displayed in the first empty "results" column in the main pane. Depending on the input parameters heating or cooling flows will be calculated.

Before making the fourth calculation, press the "Clear results" button to clear results from the 3 available result columns in the main pane.

For each calculation, calculated inlet and outlet conditions will be displayed in the Output Pane as well. To distinguish heating / cooling results from other data, for heating process rows will be displayed in red and for cooling process in blue. 

In the Output Pane results are always inserted at the top, so you don't have to scroll to see the latest result.

Save data to Excel

To save calculated properties to your local computer, follow these steps:

1. Choose File, Save to Excel as CSV menu item. The File Download dialog appears;
2. Alternatively, choose File, Save to Excel as CSV Unicode menu item. The File Download dialog appears;
3. Choose Save to save the document to your computer.

The CSV Unicode format will save properly some special characters contained in the definition of units. Since CSV is just a text file you can open a saved file with any text editor, for example Notepad.