What is Fresh Air Fraction in an HVAC system?

The fresh air fraction is a way to quantify the action of an economizer in an HVAC system. It is defined as the mass flow rate of fresh air divided by the total airflow rate. In other words, it is a measure of how much fresh air is being brought into the system compared to the total amount of air flowing through it.

While the fresh air fraction is a useful metric, it is important to remember that it only tells part of the story. The overall efficiency of an economizer also depends on factors such as:

  • the temperature,
  • the humidity of the outside air,
  • as well as the set point of the system.

Therefore, it is important to consider all of these factors when evaluating an economizer’s performance.

What is fresh air in HVAC?

Fresh air is air that has not been circulated through an HVAC system. It typically contains a higher concentration of oxygen and a lower concentration of pollutants than air that has been circulated.

Why is fresh air important in an HVAC system?

Fresh air is important in an HVAC system because it helps to keep the indoor air quality high. By bringing in the fresh air, the system can dilute contaminants that may be present in the indoor air. Additionally, fresh air can help to regulate the temperature and humidity levels in a space, which can improve the overall comfort level.

Minimum fresh air requirements for offices

The minimum fresh air requirements for office spaces vary depending on the country. In the United States, the minimum requirement is 20 cubic feet per minute (cfm) of fresh air for each person in the space. In Canada, the minimum requirement is 10 cfm per person.

What are some factors that affect Fresh Air Fraction?

There are a few different factors that can affect the Fresh Air Fraction in an HVAC system:

  1. The first is the set point of the system. The setpoint is the temperature at which the system is trying to maintain. If the setpoint is lower, then the system will need to bring in more fresh air in order to achieve it.
  2. Additionally, the outside air temperature and humidity can also affect the Fresh Air Fraction. If the outside air is cooler and less humid, then more fresh air will be needed in order to maintain the desired indoor conditions.
  3. Finally, the type of economizer being used can also affect the Fresh Air Fraction. Different economizers have different capabilities, so it is important to choose one that is appropriate for the application.

What are some benefits of a high Fresh Air Fraction?

A high Fresh Air Fraction can offer a few different benefits:

  1. First, it can help to improve the indoor air quality by bringing in more fresh air.
  2. Second, it can reduce the overall energy consumption of the HVAC system. This is because less energy is required to condition the outside air when a higher proportion of it is being brought into the system.
  3. Finally, a high Fresh Air Fraction can also help to prolong the life of the HVAC system. This is because less wear and tear occurs on the system when it is not working as hard to circulate air.

Related Links

Fresh Air Fraction Control

Building Re-Tuning Training Guide: AHU MinimumOutdoor-Air Operation

Heating fresh air by hot exhaust air of HVAC systems

Air Handling Units Explained

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