1. Introduction

Psychrometric processes are the building blocks of HVAC systems, orchestrating the intricate dance of air properties like temperature and humidity. For HVAC engineers and technicians, delving into these processes is not just an option but a necessity, serving as the key to achieving mastery in the realm of indoor environmental control.

Understanding psychrometric processes is vital on multiple fronts. It’s the cornerstone of efficient system design, allowing engineers to strike the right balance between comfort and energy efficiency. Technicians depend on this knowledge for troubleshooting and system maintenance. Moreover, it contributes to energy conservation and creates indoor spaces that are not just comfortable but also health-enhancing. In this guide, we’ll dive into the world of psychrometric processes, equipping you with the knowledge to master HVAC systems and create indoor environments that are efficient, comfortable, and conducive to well-being.

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2. Basic Processes

To understand the basic process, we will delve into the four fundamental psychrometric processes that form the cornerstone of HVAC systems. These processes are:

  1. Sensible Heating: Learn about the process of sensible heating, where the temperature of the air stream increases without altering its moisture content.
  2. Sensible Cooling: Explore how heat is removed from the air without changing its moisture content in the process of sensible cooling.
  3. Humidification: Understand the latent energy addition involved in humidification, where moisture is added to the air without changing its dry-bulb temperature.
  4. Dehumidification: Delve into the latent energy removal in the dehumidification process, where moisture is removed from the air without altering the dry-bulb temperature.

By understanding these basic processes, you’ll gain insights into how HVAC systems create and maintain comfortable indoor environments. These processes involve phase changes in water content, and we’ll explore their significance in the subsequent sections of this comprehensive guide.

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3. Sensible Heating

Sensible heating process.
Sensible heating process with air moving over a heating coil. Source: IIT Delhi

Dive into the fascinating process of sensible heating, a fundamental aspect of HVAC systems. Sensible heating involves increasing the temperature of the air stream without altering its moisture content. This process plays a crucial role in creating comfortable indoor environments.  To read sensible heating on a psychrometric chart, start by locating the initial air condition point. Identify the final condition point along the diagonal line representing sensible heating, with the vertical position indicating the dry-bulb temperature and the horizontal position showing the change in humidity ratio. Calculate the temperature change by subtracting the initial dry-bulb temperature from the final temperature, as sensible heating keeps humidity constant and only raises air temperature.

As we explore sensible heating, you’ll also discover the various heating devices that engineers and technicians use to achieve this vital HVAC function. These devices include:

  • Steam Coil: Learn how steam coils efficiently transfer heat to the air stream.
  • Hot Water Coil: Explore the use of hot water coils as a heating method in HVAC systems.
  • Heat Pipe: Understand the principles behind heat pipes and their role in sensible heating.
  • Air-to-Air Heat Exchanger: Discover how air-to-air heat exchangers contribute to the sensible heating process.
  • Sensible-Only Rotary Heat Wheel: Gain insights into the specialized rotary heat wheels used for sensible heating.
  • Electrical Heating Coil: Learn about electrical heating coils and their role in HVAC systems.
  • Furnace: Explore the traditional use of furnaces for sensible heating.
Sensible heating processes can be read off of the psychrometric chart be moving from point1 to point 2.
Sensible heating processes can be read off of the psychrometric chart be moving from point1 to point 2. Source: SlidePlayer

4. Latent Heating 

A latent heating process occurs when water (w) is evaporated without changing the dry bulb temperature (t).This process is represented as a vertical line on the psychrometric chart. When water is evaporated, it adds latent heat (L) to the air, increasing its enthalpy (h). The change in enthalpy during this process can be calculated using the following formula:

Δh = ma * L


  • Δh = Change in enthalpy (kJ/kg)
  • ma = Mass flow rate of dry air (kg/s)
  • L = Latent heat (kJ/kg)

During this latent heating process, the dry bulb temperature remains constant while the air’s moisture content increases.

5. Sensible Cooling

Sensible cooling
Sensible cooling process on a psychrometric chart can be read by moving from point A to point B. Source: CIBSE

Sensible cooling is a vital process in HVAC systems that involves removing heat from the air without altering its moisture content. This is achieved using various devices designed to carry out this specific task.

  1. Chilled Water Systems: Chilled water is a common medium for cooling. It circulates through coils or pipes, and air is passed over these cooled surfaces, resulting in heat transfer. Sensible cooling occurs as the air temperature decreases without any change in humidity.
  2. Refrigerant Cooling Coils: Refrigerant coils function in a manner similar to chilled water systems but use refrigerants instead. The air passes over these coils, and as the refrigerant inside evaporates and condenses, heat is absorbed and released, causing sensible cooling.
  3. Indirect Evaporative Coolers: These systems employ an adiabatic process that involves sensible cooling and humidification. Incoming hot and dry air passes through a heat exchanger where it indirectly cools down by transferring heat to a wet surface. This process maintains a constant wet-bulb temperature.
  4. Heat Pipes: Heat pipes are used to transfer heat efficiently. They operate by vaporizing and condensing a working fluid within a closed system. When air is passed over the condenser side of the heat pipe, sensible cooling is achieved.
  5. Air-to-Air Heat Exchangers: These devices transfer heat between two separate air streams, leading to sensible cooling in one stream while heating the other. They are valuable for energy recovery and maintaining indoor air quality.
  6. Sensible-Only Rotary Heat Wheels: Rotary heat wheels use rotating segments to transfer heat between incoming and exhaust air streams. This process allows for sensible cooling without altering humidity levels.
  7. Air Washers: Air washers use water spray systems to cool and clean the incoming air. Sensible cooling occurs as air is passed through the wetted surfaces.

On a psychrometric chart, the sensible cooling process is depicted as a horizontal movement to the left along a line of constant humidity ratio, towards the saturation line. During this process, there is no change in dew-point temperature, water vapor pressure, or humidity ratio, ensuring efficient cooling without introducing excess moisture into the air.


6. Heating and Humidification

Heating and humidification.
Humidification can be determined by moving from point A to B on the psychrometric chart. Source: CIBSE

Heating and humidification are essential processes in HVAC systems that work in sequence to prepare the air for comfortable and controlled indoor environments. In this combined process, air undergoes a two-step transformation to achieve the desired conditions.

First, the air enters a heating coil, where sensible heating takes place. This initial step elevates the air temperature without changing its moisture content. The energy added during this phase can be calculated using the following equation:

Q = m_a \left({h_2 – h_1}\right)


  • \(Q\) = Rate of energy added, KJ/hr
  • \(m_a\) = mass flow rate of dry air through the process
  • \(h_2\) = Specific enthalpy of moist air downstream of heating coil
  • \(h_1\) = Specific enthalpy of moist air upstream of heating coil

During the humidification process, the energy equation is:

m_a \left({h_3 – h_2}\right) = m_w \cdot h_w


  • \(h_3\) = The specific enthalpy of the moist air downstream of the humidifier
  • \(h_2\) = Specific enthalpy of moist air upstream of the humidifier
  • \(h_w\) = Specific enthalpy of the steam
  • \(m_w\) = Mass flow rate of the steam

The rate of moisture addition to the air, \(m_w\), is determined by a water vapor mass balance:

m_w = m_a \left({w_3 – w_2}\right)


  • \(w_2\) = Humidity ratio of the moist air upstream of the humidifier
  • \(w_3\) = Humidity ratio of the moist air downstream of the humidifier

Combining the equations:

\frac{{h_3 – h_2}}{{w_3 – w_2}} = h_w

This equation represents the slope of the humidification process on a psychrometric chart, allowing us to determine the direction of the process based on the enthalpy of the steam added to the air stream and the enthalpy-moisture protractor on a psychrometric chart.

It’s important to use dry and saturated steam during the injection process to prevent condensation. Steam can’t be sprayed below 100°C (at atmospheric pressure) due to nozzle requirements for higher pressure, and the lowest possible enthalpy carried with steam is the total heat of steam at 100°C when the steam is fully dry and saturated.

The amount of steam sprayed per kilogram of air is given by:

m_w = m_a \left({w_3 – w_2}\right)

7. Cooling and Dehumidification

Dehumidification is the process of removing moisture from the air, achievable by cooling the air below its dew point temperature. Effective dehumidification requires the cooling coil’s surface to stay below the dew point temperature of the air.

Dehumidification can be represented by moving from point A to point B on the psychrometric chart. Source: CIBSE

For example, consider cooling air from 35ºC dry bulb (DB) and 24ºC wet bulb (WB) to 20ºC DB and 17.6ºC WB. Plotting these values on a psychrometric chart and drawing a line from one point to another reveals a 0.74 intersection on the sensible heat factor line, indicating 26% latent heat removal and 74% sensible heat removal.

The cooling and dehumidification process is illustrated in the chart below, starting at point 1 and ending at point 2.

To calculate the required refrigeration capacity (\(Q_R\)), an energy balance is employed:

Energy Balance:

m_a \cdot h_1 = Q_R + m_a \cdot h_2 + m_w \cdot h_w

The mass flow rate of water in the air is determined by:

m_a \cdot w_1 = m_w + m_a \cdot w_2

Combining the above two equations yields the refrigeration capacity (\(Q_R\)):

Q_R = m_a(h_1 – h_2) – m_a(W_1 – W_2)h_w

Where \(h_w\) represents the enthalpy of saturated liquid at temperature \(t_2\). The enthalpy related to liquid condensate is small in comparison to \((h_1 – h_2)\), which represents the enthalpy difference for cooling the air and condensing the water. The process is often approximated by dividing it into sensible (S) and latent (L) components:

Q_{RS} = m_a(h_2 – h_a)


Q_{RL} = m_a(h_a – h_1)


Q_R = Q_{RS} + Q_{RL}

The sensible heat ratio for the process is then given by:

SHR = \frac{Q_{RS}}{Q_{RS} + Q_{RL}}

The rate at which moisture is removed from the air is calculated as:

m_w = m_a(W_1 – W_2)

8. Adiabatic Cooling

In the adiabatic cooling process, air is directed over a spray chamber equipped with nozzles that disperse water. The temperature of the sprayed water exceeds the wet-bulb temperature (WBT) of the incoming air but remains lower than the air temperature. As the air flows over this chamber, a portion of the water evaporates and is carried away by the air, thus increasing the specific humidity of the air. This phenomenon can be visualized in the figure below.

Crucially, the air supplies the necessary heat for water evaporation during this process, causing the air temperature to drop while maintaining the total enthalpy constant.

Complete air humidification is typically never achieved. Thus, we can define the effectiveness of the spray chamber using the following formula:

E = \frac{{T_1 – T_3}}{{T_1 – T_2}}


  • \(T_1 – T_3\) represents the actual drop in dry-bulb temperature (DBT)
  • \(T_1 – T_2\) represents the ideal drop in DBT

The humidification efficiency(%) can be calculated as:

\text{Efficiency } = 100 \times E

9. Chemical Dehumidification

In the chemical dehumidification process, when highly humid air is directed over a solid absorbent bed or subjected to a liquid absorbent spray, a portion of the water vapor is absorbed. This absorption reduces the water content in the air. The latent heat released during this process is absorbed by the air, leading to an increase in its dry-bulb temperature (DBT), while the total enthalpy of the air remains constant. Consequently, the chemical dehumidification process follows a path along a constant enthalpy line.

The effectiveness of the dehumidifier can be expressed as:

E = \frac{{T3 – T1}}{{T2 – T1}}


  • \(T3 – T1\) represents the difference in dry-bulb temperature (DBT)
  • \(T2 – T1\) represents the ideal change in DBT

10. Evaporative Cooling Systems

Evaporative cooling encompasses various types, including:

10.1 Direct Evaporative System

Direct evaporative cooling is a straightforward and efficient process where conditioned air directly interacts with a wetted surface for cooling and humidification. The steps involved are as follows:

  1. The hot and dry outdoor air undergoes filtration and then comes into contact with a wetted surface or water droplets within an air washer.
  2. The air experiences simultaneous cooling and dehumidification due to the exchange of sensible and latent heat (process o-s).
  3. The cooled and humidified air is delivered to the conditioned space, extracting sensible and latent heat (process s-i).
  4. Finally, the air is exhausted at state i.

In an ideal scenario, if the air washer were perfectly insulated with an infinite contact area, the process would follow a constant wet bulb temperature line, resulting in an adiabatic saturation process. In practice, the exit temperature may be slightly higher due to heat leaks and finite contact area.

The saturation efficiency (\(ε\)) of the direct evaporative cooling system can be defined as:

ε = \frac{{T1 – T3}}{{T2 – T1}}

10.2 Indirect Evaporative System

The indirect evaporative cooling process involves two separate air streams:

  • Primary air stream, which gets cooled and humidified through direct contact with a wetted surface (o-o’).
  • Secondary air stream, used as supply air to the conditioned space, only exchanging sensible heat with the cooled and humidified primary air stream (o-s).

Notably, in an indirect evaporative cooling system, the moisture content of the supply air remains constant, while its temperature drops. This can provide greater comfort in regions with higher humidity levels compared to direct systems. Commercially available indirect evaporative coolers can have saturation efficiency as high as 80%.

10.3 Multi-Stage Evaporative Cooling Systems

Multi-stage evaporative cooling systems offer advanced cooling solutions, with a typical two-stage configuration:

  1. In the first stage, the primary air is cooled and humidified (o -o’) by direct contact with a wet surface. Simultaneously, it cools the secondary air sensibly (o -1) in a heat exchanger.
  2. In the second stage, the secondary air stream is further cooled via direct evaporation (1-2).

In an ideal case, the final exit temperature of the supply air (\(t2\)) is several degrees lower than the wet bulb temperature of the inlet air to the system (\(t0\)). To enhance efficiency, it’s possible to sensibly cool the outdoor air before sending it to the evaporative cooler by exchanging heat with the exhaust air from the conditioned space.

11. Conclusion

In conclusion, this comprehensive guide has provided a deep understanding of psychrometric processes and their critical roles in HVAC systems. These processes are the building blocks for engineers and technicians, allowing them to create indoor environments that balance comfort and energy efficiency. From the fundamental principles of sensible and latent heating and cooling to the intricate details of heating, humidification, dehumidification, and advanced concepts like adiabatic cooling and chemical dehumidification, we’ve explored the complete spectrum of psychrometric processes. These processes are pivotal in optimizing indoor environments for human comfort and well-being.

Furthermore, we’ve unveiled the world of evaporative cooling systems, including direct, indirect, and multi-stage approaches. Each method presents its unique advantages and challenges, offering HVAC professionals a diverse toolkit to address varying environmental conditions and client requirements. By mastering these psychrometric processes, HVAC engineers and technicians can not only design efficient systems but also excel in troubleshooting, maintenance, and creating indoor spaces that promote both comfort and health. As the HVAC industry continues to evolve, this knowledge remains indispensable for achieving the perfect balance between human needs and energy conservation. Remember, HVAC professionals always know how to keep their cool, even during heated discussions, because they understand the science behind comfort and climate control. Whether you’re an experienced HVAC expert or just starting your journey in the field, the mastery of psychrometric processes will guide you towards a brighter and cooler future in indoor environmental control.


Table of Contents

  1. Understanding Psychrometric Charts
  2. Dry Bulb Temperature (Tdb)
  3. Wet Bulb Temperature (Twb)
  4. Dew Point Temperature (Tdp)
  5. Humidity Ratio or Moisture Content
  6. Specific Air Volume
  7. Sensible Heat Ratio (SHF)
  8. Relative Humidity (RH)
  9. Enthalpy
  10. Combination of Properties

Welcome, HVAC enthusiasts! Understanding psychrometric charts might seem daunting, but fear not. We’re here to make it as easy as A.B.C., well in this case H.V.A.C! In our guide on psychrometric charts, we will reveal the mystery of the powerful tools that drive the world of heating, ventilation, and air conditioning (HVAC). While these charts may sound complex, we’re here to break them down into simple, relatable terms, so everyone, from the average person on the street to seasoned HVAC professionals, can grasp their importance.

Think of psychrometric charts as the key to indoor comfort. They’re like the wizards behind the curtain, ensuring the air in your home, office, or any indoor space is just right. Whether it’s keeping you cool on a scorching summer day or toasty warm during the winter chill, psychrometric charts are the unsung heroes of HVAC.

In this guide, we’ll explore the ins and outs of psychrometric charts, making them accessible and understandable. So, let’s embark on this journey to demystify HVAC’s secret sauce and discover the top 10 things that every HVAC engineer and technician must know. By the end of it, you’ll have a newfound appreciation for the role these charts play in our daily comfort.

Psychrometric Chart: Visualizing Air Properties
Psychrometric Chart: Visualizing Air Properties. Source: Researchgate

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1. Understanding Psychrometric Charts

To understand what a psychromteric chart is and how it used I would recommend that you first read  Introduction to Psychrometry: Understanding the Properties of Moist Air.  In addition to that, a practical understanding of where you will find examples of the use of psychrometric charts in your everyday life can be found by reading Practical Applications of Psychrometry in Various Industries and Environments.  Now that that you understand that the psychrometric is graphical tool used by HVAC professionals to analyze and control the air’s temperature, humidity, and other properties for efficient heating, ventilation, and air conditioning systems, we can get into the thick of things!  Let’s learn about all the properties of air.

To understand what a psychrometric chart is and how it’s used, we recommend diving into the fascinating world of psychrometry. For students and engineers eager to master the art of psychrometrics, there’s no better guide than “A Guide in Practical Psychrometrics for Students and Engineers.

This comprehensive resource takes you on a journey through the intricacies of psychrometric charts, making the complex seem simple. Whether you’re a student embarking on your HVAC studies or an experienced engineer fine-tuning air conditioning systems, this guide is a must-have. It’s your key to unlocking the mysteries of psychrometrics.

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This guide equips you with the knowledge to analyze and control air temperature, humidity, and other properties efficiently. It’s not just a book; it’s your companion in the realm of psychrometrics, ensuring you’re well-prepared to tackle the challenges of heating, ventilation, and air conditioning systems. Take your understanding of psychrometric charts to the next level with this invaluable resource.


2. Dry Bulb Temperature (Tdb)

Dry Bulb Temperature, often referred to as “ambient air temperature,” is essentially the temperature of the air in your immediate surroundings. To measure it, you simply place a thermometer in the air where you are. This reading accurately reflects the Dry Bulb Temperature (Tdb), provided that the thermometer is shielded from direct sunlight, or any other radiation, to prevent interference from other heat sources. Additionally, it’s crucial that the measurement is taken in a dry environment, as excessive moisture in the air can distort the readings. Tdb is the metric we rely on when we simply want to know how hot or cold it is outside.

Tdb is measured with a standard thermometer that isn’t exposed to moisture or radiation. Tdb is usually expressed in degrees Celsius (°C) or Fahrenheit (°F). Kelvin (K) is also used, where zero Kelvin means absolutely zero heat and is equivalent to -273.15 °C. To determine the dry bulb temperature from a psychrometric chart, you take the temperature at a vertical line on the X-axis. It’s the reference point, representing the heat content, and the constant dry bulb temperatures appear as vertical lines in the psychrometric chart. So, whether you’re a newbie exploring the HVAC realm or a seasoned professional fine-tuning an air system, remember Tdb. It’s the unsung hero of temperature measurement, straightforward and reliable.

These are the top 3 psychrometers that we suggest for taking dry bulb temperature readings:

Dry Bulb Temperature on Psychrometric Chart
Dry bulb Ttemperature on psychrometric chart represented as vertical lines. Source: NC State University

3. Wet Bulb Temperature (Twb)

Wet Bulb Temperature, often abbreviated as Twb, is a crucial parameter in psychrometry. It represents the temperature of air undergoing adiabatic saturation, a process where air becomes saturated with moisture without any heat exchange.

Measuring Twb involves using a thermometer with its bulb covered by a wet muslin cloth. As the moisture on the cloth evaporates into the air, it has a cooling effect on the thermometer bulb, causing the temperature reading to drop. This temperature, the wet bulb temperature, is typically lower than the dry bulb temperature (Tdb) of the surrounding air.

The rate of evaporation from the wet cloth and the temperature difference between the dry bulb and wet bulb are influenced by the humidity of the air. In more humid conditions, evaporation is slower.

It’s important to note that Twb will always be lower than the dry bulb temperature (Tdb), except under one specific circumstance – when the air is completely saturated with moisture, reaching 100% relative humidity (RH). In that unique case, the wet bulb and dry bulb temperatures are identical.

By plotting the dry bulb and wet bulb temperatures on a psychrometric chart or Mollier chart, you can gain insights into the state of the humid air. Look for lines of constant wet bulb temperatures on the chart, which run diagonally from the upper left to the lower right. These lines are invaluable tools for understanding and working with psychrometric data.

Wet bulb temperature represented on a psychrometric chart as constant diagonal lines.
Wet bulb temperature represented on a psychrometric chart as constant diagonal lines. Source: NC State University.

4. Dew Point Temperature (Tdp)

Ever wondered about the magic temperature at which air starts to create dew, and everything feels a bit… well, saturated? It’s called the Dew Point, and it’s like the air’s own personal saturation point. Think of it as the moment when air just can’t hold its moisture any longer, and voila, you get dew!

So, what’s the deal with the Dew Point Temperature (Tdp)? When it’s close to the ambient air or dry bulb temperature (Tdb), you know you’re in a high humidity situation, and things might feel a tad muggy. But when the Dew Point is way lower than the air temperature, you’ve got low humidity on your hands, which can feel pretty crisp.

For a fun experiment, think about that cold soda bottle in your fridge. When it’s super chilly, you’ll see moisture droplets forming on the outside. Well, that’s the Dew Point of the air being higher than the temperature inside the fridge.  Now, to measure the Dew Point Temperature, all you need is a metal can, some ice cubes, and a trusty thermometer. Mix the ice and water in the can, give it a good stir, and watch what happens. When the air’s vapor decides to turn into droplets on the can’s surface, you’re pretty close to the Dew Point of the actual air.

On a psychrometric chart, the lines that represent the Dew Point Temperature are the horizontal lines running from the left to the  right. These lines display how the Dew Point Temperature changes with variations in the Dry Bulb Temperature and can be a handy reference for HVAC engineers and technicians. When working with the chart, follow these diagonal lines to identify the Dew Point Temperature, a crucial factor in assessing moisture levels and potential condensation in air.

So, when you’re consulting those psychrometric charts or geeking out on all things HVAC, remember the Dew Point – it’s the air’s way of saying, “I’m feeling a bit saturated today!”

Dew point temperature represented on a psychrometric chart as constant vertical lines.
Wet bulb temperature represented on a psychrometric chart as constant horizontal lines.  Source: NC State University

5. Humidity Ratio or Moisture Content

Specific Humidity is like the “water content” of the air. It’s measured in grams of water vapor per kilogram of dry air. Think of it as the amount of moisture that the air is carrying. Now, air can be a bit picky – it can only support a certain amount of moisture at a given temperature. This limit is what we call saturation humidity.

On the psychrometric chart, we find humidity ratio represented by lines that run horizontally. You’ll spot these lines on the right-hand side (Y-axis) of the chart. They start at the bottom and rise to the top, indicating how the humidity ratio changes with varying conditions. So, if you ever wondered how much moisture your air can handle, this is where you’ll find your answers!

Humidity Ration represented on a psychrometric chart as constant vertical lines read off the right hand side.  Source: NC State University

6. Specific Air Volume

Specific Volume is like the personal space of air – it’s all about how much room a certain amount of air takes up under specific conditions. In simple terms, it’s the opposite of air density. When the air gets warmer, it’s like it’s had a few extra cups of coffee; the molecules get excited and start jiggling around, making them spread out more. This makes warm air less dense than cool air – and that’s why it rises, kind of like a helium balloon at a birthday party. So, remember, warm air has more specific volume and is a bit of a lightweight.

Now, things get interesting when we throw humidity and atmospheric pressure into the mix. The more moisture vapor the air holds, the more spacious it becomes. And when the overall atmospheric pressure is cranked up, the air gets a bit shy and huddles closer, reducing its specific volume. You’ll find specific volume marked on the Psychrometric Chart as lines that slant from the lower right-hand corner to the upper left-hand corner. They’re like the rebels of the chart, always going against the flow.

Specific volume represented on a psychrometric chart as constant diagonal lines from the top left to the bottom right.
Specific volume represented on a psychrometric chart as constant diagonal lines from the top left to the bottom right.  Source: NC State University

7. Sensible Heat Ratio (SHF)

The Sensible Heat Ratio is a critical parameter used to determine the proportion of sensible heat and latent heat contributing to the overall cooling load. On the ASHRAE psychrometric chart, a protractor is employed to precisely measure and interpret this ratio by plotting the slope of the corresponding line. This information is valuable for optimizing cooling systems and ensuring efficient operation.

The ASHRAE psychrometric chart provides us with a handy tool, a protractor in the top left corner, to plot the slope of the line representing the Sensible Heat Ratio.  To get the SHR, take a ruler and put it along the slope slope ot the line showing the psychrometric process being studied(long red line on Figure[…].  Then move the ruler towards the protractor in the top let, while still keeping the ruler parallel to the line of the of psychrometric process.  Pleace the ruler at the center point of the protractor and the point where the arc of the protractor meets the the psychrometric process line will give the SHF(short red line on the psychrometric chart protractor.)

The Sensible Heat Ratio (SHF) is calculated using the following formula:

SHF = Sensible Heat (Qs) / (Sensible Heat (Qs) + Latent Heat (QL))


  • SHF = Sensible Heat Ratio
  • Qs = Sensible Heat
  • QL = Latent Heat

This equation is instrumental in determining the distribution of sensible and latent heat in cooling processes, providing valuable insights for efficient HVAC system design and operation.


Sensible heat ratio shown on a psychrometric chart protractor.
Sensible heat ratio shown on a psychrometric chart protractor. Source: Facility Dynamics Engineering.

Section 8. Relative Humidity (RH)

Relative Humidity (RH) serves as a yardstick for quantifying the moisture content air can retain at a specific temperature. It’s no secret that the warmth of the air plays a pivotal role; as temperatures rise, so does the air’s moisture-carrying capacity. Within the realm of psychrometric charts, the lines that denote constant relative humidity take shape as curves, originating at the lower left and gracefully sweeping their way up to the upper right of the chart. Notably, the boundary of absolute saturation, which marks 100 percent relative humidity, in the chart’s upper-left corner.

Understanding Relative Humidity provides HVAC professionals with critical insights into air moisture levels, an invaluable asset when optimizing heating, ventilation, and air conditioning systems.

Relative humidty curves on a psychrometric chart.
Relative humidty curves on a psychrometric chart.  Source: NC State University

9. Enthalpy

Enthalpy, a fundamental metric in the world of thermodynamics, quantifies the heat energy contained within the air. It consists of two distinct sources: sensible heat, originating from the air’s temperature, and latent heat, rooted in the air’s moisture content.

The unison of these two forms of energy gives birth to what we term ‘air enthalpy.’ This essential value is usually expressed in Btu per pound (Btu/lb.) of dry air or kilojoules per kilogram (kJ/kg).

Enthalpy plays a pivotal role in the realm of air heating and cooling. It’s the magic that determines whether you’re dealing with dry, scorching hot air, loaded with sensible heat, or the pleasantly cool, moist variety carrying an abundance of latent heat. On the psychrometric chart, the enthalpy scale can be found to the top left of the chart’s saturation boundary, marked with lines of constant enthalpy, flowing diagonally from left to right. These lines follow a path quite similar to the constant wet bulb temperature line, providing valuable insights into the air’s energy content.

When calculating the enthalpy of moist air, you’ll want to use the formula:

h = (1.007 * t – 0.026) + g * (2501 + 1.84 * t)

Where ‘g’ symbolizes the water content in kg/kg of dry air and ‘t’ represents the dry bulb temperature in degrees Celsius.

Understanding enthalpy is a must for HVAC engineers and technicians who aim to master the art of optimizing air conditions.

Enthalpy lines represented on a psychrometric chart.
Enthalpy lines represented on a psychrometric chart.  Source: NC State University

Section 10: Combination of Properties

You now know the different areas of a psychrometric chart and how to find the various properties of air under specific conditions, but that is just the begining! The most important part of understanding psychrometric is putting it all together, better referred to as combination of properties.  Combining properties will allow you to intepret the chart and determine exactly which process is taking place based on the air conditions given.  The chart below is the complete chart combining most of the lines and other parameters so far

Complete psychrometric chart combining most of the lines and other parameters
Complete psychrometric chart combining most of the lines and other parameters. Source: Testbook.


In wrapping up our exploration into the world of psychrometric charts, it’s essential to highlight how they benefit HVAC engineers and technicians in real-world scenarios. These charts may seem complex, but in plain terms, they are like maps guiding professionals in making indoor spaces comfortable and efficient.  Think of it this way: just as a GPS helps you find your way on the road, psychrometric charts help HVAC experts navigate the world of air conditioning and heating. They provide insights on how to control temperature and humidity, ensuring your home or office stays cozy and healthy.

Whether you’re a seasoned HVAC pro or someone starting out, these charts are your secret weapon for creating the perfect indoor environment. They bridge the gap between theory and practice, offering practical solutions for better indoor air quality and energy-efficient systems. So, next time you step into a comfortably cooled or heated space, you’ll know there’s a bit of science behind it, making your surroundings just right.