The ventilation system in your industrial building should properly control airflow, temperature, and humidity to ensure good indoor air quality. If it’s not doing its job, you may notice your workspace feeling overly hot, stuffy, or clammy during colder weather. This can be remedied by improved ventilation system design and retrofitting of components like your main industrial fan.
However, finding the right solution involves understanding some key thermodynamics and engineering principles. To solve your ventilation issues, we need to be able to measure and understand the physical characteristics of the air in your building. These measurements are displayed and examined using a psychrometric chart. A psychrometric chart visually represents the properties of air and how they relate to each other. Today we’ll break down this detailed tool into simple terms, and explain how we use it to design your ventilation system.
We have years of experience conducting technical assessments of the air qualities in many types of industrial buildings. For expert assistance in designing ventilation systems tailored to the specific needs of your building, contact Eldridge today.
Starting with the Basics: What is Air?
We often think of air as simply the invisible material around us that lets us breathe. And while it’s true that air contains the oxygen we need, it’s actually a mixture of different gases. Importantly for ventilation, air also holds water vapor. The amount of this water vapor in the air impacts your comfort level as well as the integrity of equipment and supplies. Too much moisture in your industrial building can lead to dampness and mold, while too little can dry out materials or irritate your respiratory system.
Just knowing the temperature of the air isn’t enough to understand these effects. We need a way to measure and understand all the different properties of air, particularly how much moisture it holds. We look at this moisture content from several different angles, as you’ll see below.
Different Ways of Looking at Moisture in the Air
We’re all familiar with the standard measurement of humidity as a percentage—we see it on the weather channel everyday. But in a comprehensive air quality assessment of your building, that’s only one of several ways we assess moisture. All of these measurements are part of a psychrometric chart graph.
Relative Humidity: This is the percentage of water vapor in the air compared to the maximum amount the air can hold at a specific dry bulb temperature.
Humidity Ratio and Vapor Pressure: These both describe the actual amount of water vapor present in the air. Humidity Ratio is the mass of water vapor per unit mass of dry air. Vapor Pressure is the partial pressure exerted by the water vapor in the air.
Saturation Temperature or Dew Point: This is the temperature at which the water vapor in the air will start to condense into liquid if the air is cooled down.
Wet Bulb Temperature: This is the temperature the air would reach if cooled by evaporating water into it until it is saturated.
Other Air Quality Factors We Assess
As well as all of the moisture level measurements, a psychrometric chart contains two other important relevant measurements of the air.
Dry Bulb Temperature: This is simply the “normal” air temperature you would read with a standard thermometer.
Enthalpy: This represents the total heat energy of the air, including both the sensible heat (used to change the dry bulb temperature) and the latent heat (used to change the wet bulb temperature).
Putting It Altogether in a psychrometric Chart
Now we take all these different ways to describe air – its temperature, how much moisture it holds, and its potential to condense – and put them onto one single graph, a psychrometric chart. Essentially, this chart gives us a map that tells us how to get to the sweet spot for your building’s optimal ventilation system design.
There are two main borders on the map. The horizontal border shows the dry bulb temperature. The vertical border shows how much actual moisture is in the air (either as humidity ratio or vapor pressure).
Now, instead of just points along these straight borders, a psychrometric chart also has curved lines and other angled lines drawn across it. Each of these lines represents another important property of the air. For example, there are curved lines that show the relative humidity – whether the air is 50% full of moisture, 80% full, or any other percentage. There are also angled lines that tell you the dew point temperature – the temperature at which condensation will start. Other lines show the enthalpy, the total heat energy in the air, and the wet bulb temperature, which relates to evaporative cooling.
So, if you know any two of these air properties, you can find a specific spot on the psychrometric chart where those two lines intersect. Once you find that spot, you can then read off the values for all the other air properties without having to do complex calculations. The chart visually connects all these different aspects of air together.
Using This Chart to Design the Best Ventilation System
A psychrometric chart helps visualize the entire air handling process. By plotting the current indoor air conditions and identifying the desired conditions, we can see the “path” the air needs to take. This path dictates the components we choose for your ventilation system design.
For example, if the chart shows hot, humid air inside your building (a point on the higher temperature and humidity side), and we want cooler, drier air (a point on the lower temperature and humidity side), this tells us we need to add a process that removes both heat and moisture, like cooling coils and dehumidification equipment.
The distance and direction of the path between the initial and desired air conditions on the chart also help determine the capacity of the components. A large change in temperature or humidity will require more powerful heating, cooling, or dehumidification units. The chart can even help estimate the energy needed for these processes.
The chart also helps in deciding how the air should be treated and moved. For instance, if we need to introduce fresh outdoor air, we can plot its condition on the chart and see how it needs to be modified (heated, cooled, humidified, or dehumidified) before being supplied to your building to achieve the desired indoor environment.
Expertise in Industrial Ventilation System Design – Eldridge
For truly effective industrial ventilation system design that stands the test of time in your building, count on the expertise of professionals who know these technical principles inside out. Contact our team at Eldridge to discuss how we can design a ventilation system that truly works for you.