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
- 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

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:

- 173.15 K <= T < = 473.15 K
- 0.000001 MPa < p < = 5 MPa

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

## Moist Air Properties Limits

The upper limit of the absolute pressure is set at 5 MPa, and the temperature is restricted to the range -100 to 200°C. However, for any given absolute pressure, the highest temperature cannot exceed that temperature at which the saturation vapor pressure equals the absolute pressure.

In other words, moist air properties are valid for any dry-bulb temperatures below or equal to the saturation temperature of water at the same given absolute pressure, above that limit saturated moist air cannot exist.

## IMPORTANT:

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

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

Since for any process calculation only enthalpy difference (in and out of a process) is important, the 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.

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 )

For input pressure, always enter absolute pressure and not gauge pressure.

The following 35 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]**; - 10Weight fraction of water
**[Ww]**; - 11Weight fraction of air
**[Wa]**; - 12Humidity ratio
**[W]**; - 13Saturation humidity ratio
**[Ws]**; - 14Degree of saturation
**[DS]**; - 15Relative humidity
**[RH]**; - 16Specific volume of dry air
**[Va]**; - 17Specific volume of saturated ice
**[Vi]**; - 18Specific volume of saturated water
**[Vw]**; - 19Specific volume of saturated vapor
**[Vv]**; - 20Specific volume of moist air
**[Vm]**; - 21Specific density of dry air
**[Da]**; - 22Specific density of saturated ice
**[Di]**; - 23Specific density of saturated water
**[Dw]**; - 24Specific density of saturated vapor
**[Dv]**; - 25Specific density of moist air
**[Dm]**; - 26Specific enthalpy of dry air
**[Ha]**; - 27Specific enthalpy of saturated ice
**[Hi]**; - 28Specific enthalpy of saturated water
**[Hw]**; - 29Specific enthalpy of saturated vapor
**[Hv]**; - 30Specific enthalpy of moist air
**[Hm]**; - 31Specific entropy of dry air
**[Sa]**; - 32Specific entropy of saturated ice
**[Si]**; - 33Specific entropy of saturated water
**[Sw]**; - 34Specific entropy of saturated vapor
**[Sv]**; - 35Specific entropy of moist air
**[Sm]**.

To open a new file, do the following:

- Choose
**File**,**Open**menu item

Alternatively, press the "**New**" toolbar button.

This will clear all currently displayed data on the forms.

To save calculated properties to your local computer, use the following steps:

- Choose
**File**,**Save to Excel as CSV**menu item. The File Download dialog appears - Alternatively, choose
**File**,**Save to Excel as CSV Unicode**menu item. The File Download dialog appears - 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.

On the "**Setup Pane**" in the "**Units**" group box, set units as required. SI and English units are available.

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

- Choose units, SI or English;
- Select one of the 21 different functions based on available input variables;
- Enter data for the first parameter;
- Enter data for the second parameter;
- Enter data for the third parameter;
- Press the "
**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.

**Fig.1** Calculation of thermodynamic properties of moist air

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

- Select "
**Heating / Cooling**" tab - In the "
**Inlet Point**" group box enter parameters of the first point - In the "
**Outlet Point**" group box enter parameters of the second point - Press the "
**Calculate**" button

The solver can handle all of the following heating and cooling processes:

- Humidifying only
- Heating and humidifying
- Sensible heating only
- Chemical dehumidifying
- Dehumidifying only
- Cooling and dehumidifying
- Sensible cooling only
- Evaporative cooling

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, rows for heating process will be displayed in red and for cooling process in blue.

The last two columns in the Output Pane (**Process** and **Point**) provide a description of a calculated point.

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

To calculate properties of a mixing process, use the following steps:

- Select "
**Mixing**" tab - In the "
**Inlet Point 1**" group box enter parameters of the first point - In the "
**Inlet Point 2**" group box enter parameters of the second point - Press the "
**Calculate**" button

The solver can handle mixing points in the following areas:

- Super heat (
**non fog**) area, this is the standard case; - Super saturation (
**fog**) are, it also calculates created moisture as fog or droplets.

Results of the calculation will be displayed in the first empty "results" column in the main pane.

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 mixing results from other data, rows with calculated values will be displayed green.

The last two columns in the Output Pane (**Process** and **Point**) provide a description of a calculated point.

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