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Navigating Psychrometric Charts: A Beginner's Guide

Psychrometrics is the study of the thermodynamic properties of air-vapor mixtures and a measure of the amount of energy per pound of dry air. It is the fundamentals to understanding how energy works in an HVAC system.

The greatest tool to use in analyzing air patterns is called a psychrometric chart. These charts are like maps that show the properties of air that help us determine how a system is operating based on factors such as outdoor and indoor air conditions. They help us understand how these properties interact and change with each other and how they affect the overall system performance.  

Psychrometrics also involves the comfort level of the space relative to how a person feels. There is a balance between the dry temperature and humidity that we feel. This is known as the sensible load (dry temperature) and latent load (humid temperature). For example, a space may read that it is 70°F but if the humidity levels are up in the 65% to 70% range, it will cause people to feel sweaty and uncomfortable.  

Terminology of a Psychrometric Chart: 

  1. Temperature: There are two types shown on the chart: dry bulb (normal air temperature) and wet bulb (temperature when air is cooled by evaporation). 
  2. Relative Humidity: This tells us how much water vapor is in the air. It can be relative humidity (compared to the maximum air can hold) or absolute humidity (mass of water vapor per unit volume of air). This value is measured as a percentage up to 100%.  
  3. Humidity Ratio: the amount of moisture contained per unit of dry air. Basically how much moisture the air can hold. This is measured in grains of water per pound of dry air.  
  4. Dew Point: The temperature at which water vapor in the air condenses. Dew point occurs at the saturation line where relative humidity equals 100%. 
  5. Density/ Specific Volume: Density represents the mass of air per volume. Air is going to always be more dense at colder temperatures. Specific volume is the reciprocal of density. So as density increases, the specific volume will decrease.  
  6. Enthalpy: A measure of the total energy in a thermodynamic system.  
  7. Sensible Heat Ratio: this value is used to describe the ratio of sensible heat load compared to the total load of the space.  
  8. Mixed Air: this is the air temperature when mixing the outside fresh air with the indoor return air. This is typically done with an energy recovery wheel or a mixing box with dampers. This is the air that will be entering the cooling coil.
  9. Comfort Zone: This is where people feel comfortable with the air. It varies depending on factors such as air properties, time of the year, and how many people are in the space.  

Using a Psychrometric Chart: 

  1. Reading Data Points: Each point on the chart shows the conditions of air at a specific time and place. By looking at these points, we can understand how air properties change. When it comes to finding data values, it starts by only needing two data points of information. Many times, this would be dry bulb and wet bulb or dry bulb and Relative Humidity. Some formats you might see for the first pair would read as 95°F/ 75°F or for relative humidity, 80°F/ 70%. Once two points are known, all other pieces on the chart can be found.  A diagram of temperature and temperature

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  2. Understanding Relationships: The chart helps us see how temperature, humidity, and energy are related. For example, warm air can hold more moisture, and cooling moist air can make water droplets form (condensation). Using the chart can help to predict the leaving supply air temperature of a system as well as find the changing cfm at part load conditions. The way in which a process moves on the chart can be shown in the graph below. For example, if the only temperature changing is the drybulb temperature, the system is only sensibly heating or cooling. If only the wetbulb temperature changes, the system is humidifying or dehumidifying.  
    Diagram of a diagram of heat and air conditioning

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  3. Analyzing Processes: Psychrometric charts help us calculate and understand air conditioning processes such as mixing the outside air with the return air. We can plot the values to see what the system cooling curve would look like. This plot shows not just the cooling process but also the dehumidification taking place. These trends are important to see when it comes to things like a VAV (Variable Air Volume) system versus a CAV (Constant Air Volume) system. A VAV system is typically going to pull more moisture from the air, resulting in a steeper slope for the curve.  
  4. Creating Solutions: The properties of air in an HVAC system are important for maximizing the best unit performance as well as creating better solutions. Air patterns are ever changing and the psychrometric chart is an important tool to use for HVAC design, especially when designing a unit for various climates.  

In Conclusion 

Mastering the art of navigating psychrometric charts is crucial for anyone working in the field of HVAC engineering and air conditioning. By understanding how to interpret these charts, engineers and technicians can make informed decisions, design efficient air conditioning systems, and solve complex problems related to air treatment and climate control. With practice and experience, navigating psychrometric charts becomes second nature, empowering professionals to excel in their roles and deliver optimal HVAC solutions.