Online Psychrometric Calculator for Chemical Engineers

Q: Dry-Bulb Temperature (TDB)

DBT is the temperature of air measured by a regular thermometer (in °C or °F ) i.e natural condition unaffected by the moisture content of the air. It represents the air’s thermal condition and is fundamental for psychrometric calculations. Dry Bulb Temperature, TDB is present on the horizontal axis (x-axis) at the bottom of the chart.

Q: Wet-Bulb Temperature (TWB) & Wet bulb Depression

WBT is the temperature indicated by a thermometer (in °C or °F ) covered with a water-saturated wick and exposed to air flow. It shows the cooling effect due to water evaporation and is useful for calculating humidity. TWB is along the saturation curve (100% RH line) . A diagonal line extending from the dry-bulb temperature to the saturation line. Wet Bulb Depression is the difference between dry bulb temperature and wet bulb temperature at any point. The wet bulb depression indicates relative humidity of the air.

Q: Dew Point Temperature (TDP) Meaning

Dew Point is the temperature at which air becomes saturated (RH = 100%) when cooled at constant pressure. For a substance it is the temperature at which saturation of that substance occurs without change in vapour pressure. It is the temperature infinitesimally above that at which the first dew forms. It determines the temperature at which condensation will occur.

Q: Specific Enthalpy (h) & Specific Volume (v)

Specific Enthalpy ( h ) is defined as the total heat content of moist air, considering both sensible and latent heat , per unit mass of dry air (kJ/kg dry air). Enthalpy is used to calculate the energy required for heating, cooling, humidification, or dehumidification. \[h = c_p \cdot T_{DB} + \omega \cdot h_{fg}\] where: cp is specific heat of dry air (≈1.005 kJ/kg·K), TDB is the dry-bulb temperature (°C), hfg is the latent heat of vaporization (≈2501kJ/kg at 0°C). Specific Volume represents the volume occupied by 1 kg of dry air , including the space taken up by water vapor (m3/kg). Specific volume is the inverse of density. It is helpful in duct sizing and airflow calculations. \[v = \frac{R_{air} \cdot T_{DB}}{P_{atm}}\] where: Rair=0.287 kJ/kg·K which is specific gas constant for air, TDB is the dry-bulb temperature (K), Patm is the atmospheric pressure (Pa).

Q: Saturation Line Meaning

Saturation line is the curve on the psychrometric chart where air is fully saturated (RH = 100%). It Indicates maximum moisture content the air can hold at a given temperature. At saturation, \[\omega = 0.622 \cdot \frac{P_{vs}}{P_{atm} - P_{vs}}\]

In chemical engineering, analyzing air-water vapor systems is necessary for various processes like drying and cooling. Traditional psychrometric charts are used to know the thermodynamics properties of moist air at a given pressure. But visually seeing this can be time-consuming.

Here we are providing the Online Psychrometric Calculator which is faster, more precise and provides instant calculation of properties like humidity and enthalpy.

Online Psychrometric Calculator

Psychrometric Calculator is useful for computing various atmospheric properties related to air moisture content. User enters the values for dry bulb temperature (DBT), wet bulb temperature (WBT), relative humidity (RH), and altitude

Based on the input values (either DBT & WBT or DBT & RH), the calculator finds the enthalpy, humidity ratio, air density, partial vapor pressure, dew point temperature, specific volume, and saturated vapor pressure.

Note: This calculation is done based on ASHRAE Fundamentals (SI) 2017 chapter 1.

Learn More about the Psychrometric Calculator Code and Concept

Also Read: Van der Waals Equation Calculator and PV Isotherm for Real Gases

What is Psychrometric Chart?

The Psychrometric Chart is a graphical representation of the thermodynamic properties of moist air at a given pressure. This chart gives a detailed visualization of various parameters like dry-bulb temperature, wet-bulb temperature, specific humidity, relative humidity, dew point temperature, enthalpy, and specific volume.

This tool is useful for chemical engineers in designing and analyzing systems involving air-water vapor mixtures, such as HVAC (heating, ventilation, and air conditioning) systems, drying processes, and cooling towers.

This Psychrometric chart is plotted using python – visit here to see full code

Example Problem on Psychrometric

The humidity ratio of atmospheric air at 28°C dry bulb temperature and 760 mm of mercury is 0.016 kg / kg of dry air. Determine: 1. partial pressure of Water vapour; 2.relative humidity; 3. dew point temperature; 4. specific enthalpy; and 5. vapour density

Given:

Dry Bulb Temperature, T_db = 28°C
P_b = 760 mm of Hg ;
Humidity Ratio, W = 0.016 kg/ kg of dry air

1. Partial pressure of water vapour

The humidity ratio is given by the equation:

\(W = 0.622 \cdot \frac{P_v}{P_a}\)

where P_v is the partial pressure of water vapour and P_a is the partial pressure of dry air. From the relationship:

\(W = 0.622 \cdot \frac{P_v}{P_b – P_v}\)

Rearranging the equation:

\(W \cdot (P_b – P_v) = 0.622 \cdot P_v\)

on solving, we get:

P_v = 12.16/0.638 = 19.06 mm of Hg

P_v = 19.06 x 133.3 = 2540.6 N/m²

2. Relative humidity

From steam tables, we find that the saturation pressure of vapour corresponding to dry bulb temperature of 28`C is P_s = 0.03778 bar i.e 3778 N/m2

Relative humidity is given by:

\(\text{Relative Humidity} = \frac{P_v}{P_s} \times 100\)

Substituting the values:

\(\text{RH} = \frac{2540.6}{3778} \times 100 \)

\(\text{Relative Humidity} = 67.3\%\)

Therefore, the relative humidity is 67.3 %

3. Dew Point Temperature

The dew point temperature is the temperature corresponding to the partial pressure of water vapour P_v. From steam tables, for Pv = 2540.6 N/m², the corresponding dew point temperature is T_dp = 21.1°C

4. Specific Enthalpy

From steam tables, latent heat of vaporization of water corresponding to a dew point temperature of 21.1 °C, h_fgdp = 2451.76 kJ/kg

We know that specific enthalpy, specific heat of water vapor is taken as 2.3 kJ/(kg·K),

h = 1.022 t_d + W (h_fgdp + 2.3 t_dp)

= 1.022 x 28 + 0.016 (2451.76 + 2.3 x 21.1)

h = 28.62 + 40 = 68.62 kJ/kg of dry air

FAQs on Psychrometric Terms

Psychrometry is the study of the thermodynamic properties of moist air and their application in analyzing and understanding air conditions and processes involving humidity.

Also Read: Antoine Equation Calculator for Vapour Pressure versus Temperature Calculations

Air conditioning processes are commonly analyzed using psychrometric charts and Mollier diagrams. These tools provide a graphical representation of various properties of air, Here we will discuss the terms used in the Psychrometric chart as follow:

Dry-Bulb Temperature (T_DB)

DBT is the temperature of air measured by a regular thermometer (in °C or °F) i.e natural condition unaffected by the moisture content of the air. It represents the air’s thermal condition and is fundamental for psychrometric calculations.

Dry Bulb Temperature, T_DB is present on the horizontal axis (x-axis) at the bottom of the chart.

Wet-Bulb Temperature (T_WB) & Wet bulb Depression

WBT is the temperature indicated by a thermometer (in °C or °F) covered with a water-saturated wick and exposed to air flow. It shows the cooling effect due to water evaporation and is useful for calculating humidity.

T_WB is along the saturation curve (100% RH line). A diagonal line extending from the dry-bulb temperature to the saturation line.

Wet Bulb Depression is the difference between dry bulb temperature and wet bulb temperature at any point. The wet bulb depression indicates relative humidity of the air.

Also Read: Relation Between Van der Waals Constants and Critical Constants

Dew Point Temperature (T_DP) Meaning

Dew Point is the temperature at which air becomes saturated (RH = 100%) when cooled at constant pressure. For a substance it is the temperature at which saturation of that substance occurs without change in vapour pressure.

It is the temperature infinitesimally above that at which the first dew forms. It determines the temperature at which condensation will occur.

Relative Humidity (RH) & Humidity Ratio (ω)

Relative Humidity is defined as the ratio of the actual water vapor pressure (P_v) in the air to the saturation vapor pressure (P_vs) at the same temperature. Mathematically RH is written as:

\[RH = \frac{P_v}{P_{vs}} \times 100 \%\]

It shows how close the air is to saturation. High RH can cause discomfort, mold growth, and material degradation.

Humidity Ratio (ω) or Specific Humidity is defined as the mass of water vapour present in 1 kg of dry air. Mathematically, in terms of pressure it can be written as:

\[\omega = 0.622 \cdot \frac{P_v}{P_{atm} – P_v}\]

where:
P_v is the partial pressure of water vapor (Pa or mmHg),
P_atm is the atmospheric pressure (Pa or mmHg).

Psychrometric Chart

Tool: Comprehensive Psychrometric Chart

An essential tool for HVAC engineers and professionals, enabling accurate analysis of air properties and system performance. Perfect for understanding moisture-air interactions.

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Specific Enthalpy (h) & Specific Volume (v)

Specific Enthalpy (h) is defined as the total heat content of moist air, considering both sensible and latent heat, per unit mass of dry air (kJ/kg dry air). Enthalpy is used to calculate the energy required for heating, cooling, humidification, or dehumidification.

\[h = c_p \cdot T_{DB} + \omega \cdot h_{fg}\]

where:
c_p is specific heat of dry air (≈1.005 kJ/kg·K),
T_DB is the dry-bulb temperature (°C),
h_fg is the latent heat of vaporization (≈2501kJ/kg at 0°C).

Specific Volume represents the volume occupied by 1 kg of dry air, including the space taken up by water vapor (m3/kg). Specific volume is the inverse of density. It is helpful in duct sizing and airflow calculations.

\[v = \frac{R_{air} \cdot T_{DB}}{P_{atm}}\]

where:
R_air=0.287 kJ/kg·K which is specific gas constant for air,
T_DB is the dry-bulb temperature (K),
P_atm is the atmospheric pressure (Pa).

Saturation Line Meaning

Saturation line is the curve on the psychrometric chart where air is fully saturated (RH = 100%). It Indicates maximum moisture content the air can hold at a given temperature. At saturation,

\[\omega = 0.622 \cdot \frac{P_{vs}}{P_{atm} – P_{vs}}\]

Engineering Thermodynamics

Edition: 6th Edition, By: P.K. Nag

A comprehensive textbook on thermodynamics for engineering students, covering fundamental concepts and advanced applications with clear explanations and practical examples.

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Resources

ASHRAE Fundamentals (SI) 2017 chapter 1 by ashrae.org
“Thermodynamics: An Engineering Approach” by Yunus A. Çengel and Michael A. Boles
“Introduction to Chemical Engineering Thermodynamics” by J.M. Smith, H.C. Van Ness, and M.M. Abbott
NPTEL Lectures on Thermodynamics
Engineering Thermodynamics: A Textbook – by P.K Nag

Disclaimer: The content provided here is for educational purposes. While efforts ensure accuracy, results may not always reflect real-world scenarios. Verify results with other sources and consult professionals for critical applications. Contact us for any suggestions or corrections.

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Online Psychrometric Calculator for Chemical Engineers

Table of Contents

Online Psychrometric Calculator

What is Psychrometric Chart?

Example Problem on Psychrometric