HVAC Systems: Is there a Difference between Mixed Air and Recirculation Air Operation?

Designing HVAC systems for pharmaceutical production facilities often raises the question of the difference between recirculated air and mixed air operation. These terms are frequently used ambiguously in everyday technical practice, although they are clearly defined in technical standards and are relevant for system design. According to EN 16798-3, recirculated air refers to exhaust air from the room that is returned to the air handling unit (AHU), mixed with outside air to form mixed air, treated, and then supplied to the cleanroom as supply air. Recirculated air is therefore not an independent operating condition but rather an air stream that can be incorporated into the supply air in certain proportions. Mixed air is defined normatively as air consisting of two or more air streams, typically outside air and recirculated air.

For cleanrooms, this means that the supply air delivered to the room can be designed either as pure outside air (100% fresh air operation) or as mixed air consisting of outside air and recirculated air.

From a technical perspective, whenever recirculated air is used, the system operates with mixed air. This can be configured in different ways. In one variant, the mixed air consists of a constant proportion of recirculated air and a constant proportion of outside air. In practice, this mode of operation is often referred to as “recirculation operation,” even though, strictly speaking, the air supplied is still mixed air. In a second variant, the proportions of outside air and recirculated air are variable; the outside air fraction can be increased up to 100%, for example to utilize free cooling or to remove thermal loads. In both cases, it must be ensured that the specified minimum outside air proportion is maintained at all times.

The key difference between these concepts therefore lies not in the terms recirculated air or mixed air themselves, but in the control strategy and variability of the air fractions. Systems with constant proportions are designed to provide stable and reproducible conditions, whereas variable mixed air systems offer greater flexibility in terms of energy efficiency and climate control, but require more sophisticated planning, control systems, and higher investment costs. The selection of the appropriate concept is therefore based on a holistic assessment of GMP requirements, contamination risks, energy efficiency, operational strategy, as well as investment and life-cycle costs.