Why is airflow important for environmental test chambers?

When considering a new test chamber, you may be concerned about airflow speed? How fast should it be? Most test specifications don't say much on this subject, which makes it even harder to decide. In some cases low speed is acceptable and/or desirable, and others high speed is better.

Test chambers need to circulate the air so that it can pass over heaters and coolers, ensuring a uniform air temperature. This is called "forced convection". You may think that "natural convection" is better to simulate real-world conditions, but it doesn't make a difference in most cases.

Low speed: If your test application doesn't specify temperature change rate, or if it is relatively slow (less than 5 degrees per minute), then lower air speed is okay. Instead, concentrate on how the air is distributed to ensure it uniform.

High speed: If your primary concern is changing the temperature of your test samples quickly, then you should be more interested in airflow. The faster the airflow, the quicker heat can be transferred from the air to your samples. There are diminishing returns from faster flow rates. Generally, it is accepted that 1,000 feet per minute (FPM) is the highest reasonable rate for test chamber applications. But in high performance applications, even 500 FPM is very effective.

What is the cost of high airflow? Temperature change rate is reduced due to increased heat transfer. In addition, as it moves the air, it puts extra energy (i.e. heat) into the air, making cooling more difficult. ESPEC's line of Environmental Stress Screening chambers have high airflow and reasonable change rates. But with a 10°C/min. air change, your sample change rate will be about 8°C/min. A standard chamber with the same air change rate, but lower airflow, may only get a sample change rate of 3-5°C/min. Of course, the smaller your sample is, the faster it will change temperature.

We sometimes run into confusion between the terms FPM (linear feet per minute) and CFM (cubic feet per minute). FPM is calculated by taking the CFM and dividing it by the cross-sectional area that it will be blowing through. CFM numbers can run very high, but when reduced to FPM, the actual air flowing past your product may not be as extreme as you thought it might be. For this reason, limiting the chamber size and cross-sectional area are important criteria for true ESS applications.