ES_Air User Manual

1. Introduction

2. Formulae

  2.1 Mole Weight

  2.2 Enthalpy

  2.3 Sulfuric Acid Dew Point

  2.4 Water Dew Point

3. Screen



1.  Introduction

 ES_Air is the software to calculate wet air composition, mole weight, wet bulb temperature, absolute humidity, enthalpy and moisture dew point of flue gas, for user input of atmospheric pressure, dry bulb temperature, relative humidity and enthalpy reference temperature.

Wet air composition is calculated based on the US standard dry air composition.   The US standard dry air composition is as below.


: 20.947 %


: 78.086 %


: 0.934 %


: 0.033 %

Mole Weight

: 28.966

Flue gas enthalpy is calculated by either of "NASA Glenn Coefficients for Calculating Thermodynamic Properties of Individual Species"(NASA/TP-2002-211556, September 2002), ASME PTC 4.4, 1981 or Babcock & Wilcox "STEAM its generation and use, 40'th Edition".

Dry bulb temperature and enthalpy reference temperature to be input should be greater than 33.8 oF(1 oC) due to the limitation of the saturation steam table built in.


2. Formulae   (TOC)

2.1 Mole Weight

Refer to ES_FlueGas user manual.

2.2 Absolute Humidity

Water vapor in the air is one of the gas components of the air.   Therefore, the amount of water vapor to be contained in the air follows the partial pressure law of Dalton.

The water vapor partial pressure of the air is proportional to the mole quantity of water vapor in the air.  The maximum water vapor partial pressure is the saturation pressure at the dry bulb temperature, and the relative humidity is the ratio of the water vapor partial pressure to the maximum partial pressure.

Calculation of absolute humidity for an example of 14.7 psia atmospheric pressure, 60 oF dry bulb temperature and 60% relative humidity is as below.

The saturation pressure at 60 oF is 0.2562 psia.   That is, the maximum water vapor partial pressure(100 relative humidity) is 0.2562 psia.   Therefore, the water vapor partial pressure for 60% relative humidity is 0.2562 x 0.6 = 0.15372 psia.

Since the water vapor partial pressure is 0.15372 psia, the partial pressure of dry air is 14.7 - 0.15372 = 14.54628 psia.   Therefore, the mole quantity ratio of water vapor to dry air is 0.15372 / 14.54628 = 0.010568.

Since the mole weights of water vapor and dry air is 18.015 and 28.966 respectively, the weight ratio of water vapor to dry air, i.e. absolute humidity, is 18.015 / 28.966 x 0.010568 = 0.006572 weight H2O / weight dry air.

2.3 Wet Bulb Temperature

Wet bulb temperature is calculated using the equation of Carrier as below.

 Pv = Pvw - {(Pb - Pvw)(Td - Tw)} / (2830 - 1.44 * Tw)



: Partial pressure of water vapor = 0.01 * RH * Pvd, psia



: Water saturation pressure at dry bulb temperature, psia



: Water saturation pressure at wet bulb temperature, psia



: Atmospheric pressure, psia



: Dry bulb temperature, oF



: Wet bulb temperature, oF

 Since the wet bulb temperature(Td) and water saturation pressure at wet bulb temperature(Pvw) are in the equation, the wet bulb temperature is calculated by try-and-error method.

 2.4 Enthalpy (TOC)

Refer to ES_FlueGas for enthalpy calculation details.

In case of flue gas enthalpy calculation, the difference among three methods are negligible.   However, in case of air enthalpy there is considerable difference among three methods.

The reason is guessed as the enthalpies by ASME PTC 4.4 - 1981 and B&W STEAM are calculated by the enthalpy calculation of dry air as a whole, not by the enthalpy calculation of each components.   Whereas, the NASA Glenn Coefficients calculates the dry air enthalpy by the enthalpy calculation of each components.

It is recommended to use NASA Glenn Coefficients.

2.5 Water Dew Point

Water dew point is calculated using the following equations.

    WDP = Steam saturation temperature at Pv(partial pressure of water vapor)


3.0 Screen (TOC)

The green color boxes are for user input.   After inputting the atmospheric pressure, dry bulb temperature, relative humidity and enthalpy reference temperature in green color boxes, then press [Calculate] button for calculation.

Menu :

Main Menu

Sub Menu




exits the program.






[Set calculation units...]: sets the units in the current calculation.



[Set the current units as default] : sets the current units as default units.





[Air component unit]

selects the gas component unit in either of [Volume %] or [Weight %].





[Enthalpy calculation formula]

selects the enthalpy calculation method among the following formulae.



[NASA Glenn Coefficients], [ASME PTC 4.4 - 1981], [B&W STEAM its generation and use]





[End User License Agreement...]

shows the End User License Agreement for using the Free Execution Software of ENGSoft Inc.


Buttons :




performs calculation.


References :  (TOC)

1. NASA/TP-2002-211556, September 2002 - NASA Glenn Coefficients for Calculating Thermodynamic Properties of Individual Species

2. NASA/TP-2001-210959/REV 1, February 2002 - CAP: A Computer Code for Generating Tabular Thermodynamic Functions from NASA Lewis Coefficients

3. ASME PTC 4.4 - 1981, Performance Test Code for Gas Turbine Heat Recovery Steam Generator

4. B&W STEAM its generation and use, edited by S.C. Stultz and J.B. Kitto of Babcock & Wilcox, a McDermott company

Copyright (c) 2004 - 2005 ENGSoft Inc., Seoul, Korea, All right reserved.