About Air Changes

What are Air Changes?

Air changes per hour (ACH) are defined as the volume per unit time in hours, of air entering a closed space, divided by the total volume of that space. Mathematically is expressed by means of this simple equation:

Where:

ACH = number of air changes per hour (h^-1)

Q = air flow (m³/h)

V = space volume (m³)

Why is Air Changes important?

This is the most important design parameter (but not the only) when designing pharmaceutical airflow (Q) requirement. This value is often part of the User Requirement Specifications (URS).

The underlying idea that gives us ACH is how often the air volume of a room pass through a HEPA filter, and consequently, how much air is cleaned up. According to this, is easy to deduce that as high ACH we are using, higher cleanliness is achieved. Thus, grade B will require more ACH than grade C, and grade C more than grade D. Higher ACH also reduces the recovery time of a space. The recovery time test is often used as a validation acceptance criteria and this will be covered in another post.

According to 2019 ASHRAE HVAC Applicatons (9.4), there is a correlation between  air changes and particle removal:

The Design

There is no regulatory GMP either ISO 14644 rule that requires a determined minimum air change number for pharmaceutical applications. This has been traditionally a rule of the thumb for many designers through the years, based on accumulated experience. ISPE recommends the following values:

Do not confuse with values from the ANSI/ASHRAE Standard 62.2-2013 table. This table is to calculate air changes to obtain the fresh air volume needed for ventilation, based on the number of occupants, dilution of contaminants, etc. The values shown in the table above includes fresh air (whatever is recirculated or not).

These ranges in the table above are almost universally accepted and applied in the Pharmaceutical Industry. Airflow obtained from these values are normally higher than those obtained from cooling load calculations (but not always!).

It is important to remark, although it is obvious, that increasing ACH rate, leads to increase the operation costs (fan size, number of filters, etc). Some companies decide, according to their production needs, to reduce the working ACH during non-production and non-occupied hours. This requires validation.

We are not discussing in this post about Grade A (ISO 5), unidirectional flow so that air changes are not the most critical design value, but pattern and laminarity are important.

As mentioned, the source of design ACH is from empirical results and experience. Nevertheless, we could use complex calculation methods to optimise, considering as minimum, the following variables:

1. 1. 1. The cleanliness to achieve (grade B, C, D). The ratio of particles entering/exiting the space.
      2. Activity in the cleanroom. That means, the number of particles generated in the room.
      3. The layout of the room. Existence of nooks, areas difficult to arrive air barrier.
      4. The type of HEPA filters, or their capacity to retain a determined percentage of particles.
      5. Location of air returns.
      6. Air pattern through the room
      7. Differential pressure
      8. Existing of exhaust
      9. Percentage of recirculating air

Other considerations

We must consider the following situations, that can cause the number of ACH could increase:

Heat Gain

In conventional HVAC design, the required airflow (and consequently the resulting ACH) is calculated to overcome the heat generated in the room, according to the following equation:

Where:

Q = air flow (m³/s)

Hs = sensible heat generated in the room (kW)

ρ = air density (Kg/m³)

DT = Temperature difference between supply and room temperature (°C)

Normally, the resulting airflow calculated from this formula is lower than the calculated from the ACH. But with high room sensible heat gain (Hs) the heat gain criteria can exceed the ACH, and so, we must use this highest value.

Leakages

Achieving a good air room cleanliness involves a large amount of filtered air. But this is not only the sole condition. Imagine the very common situation of a bubble airlock, which is over-pressurized over surrounding spaces:

After a leakage calculation (explained here) we can obtain that the leakages through the doors can be very important, and close to the air volume supply. In this case, the air return is very low that produces a balloon effect, avoiding that particles are removed. Consequently, we need to increase the ACH so that air return volume is significant.

Fresh Air Requirements

This is the less common scenario. It is when the required airflow to accomplish with ANSI/ASHRAE Standard 62.2-2013, or local requirements is higher than the ACH, heat gain, or leakages.

Conclusions

• Use URS as Air Changes design criteria (most of the cases)
• Compare calculated airflow from ACH, heat gain, leakages and use the highest value
• Increasing ACH criteria make your particles number decrease, but has an impact on energy consumption!