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Cleanroom Air Balancing
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Cleanroom Air Balancing

1. Introduction


In this article, we will discuss cleanroom air balancing, a critical aspect of commissioning a pharmaceutical HVAC system. We aim to describe the process and sequence of activities, as well as the prerequisites. The procedure outlined here applies to both statically and dynamically balanced systems. Statically balanced systems requires manual adjustment of return dampers through trial and error. Dynamic systems, featuring VAV (Variable Air Volume) actuators, usually require the Building Management System (BMS) to adjust control parameters. For further information on dampers, refer to our post titledĀ Dampers in Pharmaceutical HVAC.

2. Description


When addressing cleanroom air balancing during commissioning, we typically identify two types of adjustments: rough balance and final balance.


2.1 Rough Balance


Initially, adjust the supply and return or exhaust airflows of the air handling system to match design values. Additionally, set the supply airflows of each HEPA terminal filter to their designated capacities. Typically, leave return dampers fully open during this stage.


2.2 Final Balance:


Fine-tune room pressures in the final step. This requires adjusting return dampers to achieve the necessary room differential pressure.

3. Pre-Requisites


Before proceeding with the actual adjustment, you must meet several prerequisites. Failure to meet any of these requirements can compromise, delay, or hinder the balancing process. Generally, we can list the following minimum requirements:


3.1. Mechanical Completion (MC)


This marks a milestone in the construction process where the HVAC system or systems are completed and ready for handover to the commissioning team. When referring to a complete HVAC system, this encompasses not only the HVAC system itself (AHU, fans, ductwork, dampers, etc.) but also the electrical and control system associated with it (BMS & QBMS). It also includes connected utility systems (chilled water, hot water, steam, heat recovery auxiliary systems, etc.). At this stage, walkdowns have been completed, and all punch items have been closed. It’s worth noting that HEPA filters are not yet installed at this phase. Diffusers and returns are protected by film.


3.2 Installation Qualification:


This step should be straightforward and pose no difficulty or delay if all previous phases have been carried out with the involvement of the C&Q personnel.


  • Preliminary verification: Commissioning reports, fan analysis (amps, rpm, vibration analysis), FAT (Factory Acceptance Test), SAT (Site Acceptance Test), etc.
  • P&IDs (Piping and Instrumentation Diagrams) are verified.
  • Ductwork routing plans.
  • Correct damper positions.
  • Labels installed.
  • Instrumentation installed, identified, and calibrated.
  • Spare parts list available.


3.3 Safety Inspection:


It is essential to conduct a safety inspection to ensure that the entire system is correctly installed and, before commissioning, to prevent potential safety hazards. For example, safe access to walk-in ceilings, grounding, blocking access doors to fans, etc.


3.4 Cleaning:


Once the previous steps are completed, all systems must be cleaned. Remove all diffusers and return protections. Clean the HEPA filter housings. In technical areas, ducts and the exterior of AHUs should be cleaned of dust accumulated during construction (this should be minimal if the clean-as-you-go philosophy is applied correctly). The interior of AHUs should be carefully cleaned and, if applicable, disinfected. Normally, units include two types of filtration; at this initial stage, it is common to install filters for this single use, not the final ones, as they will tend to accumulate more particles than in normal operation. HEPA filters are not yet installed!


Under the concept of clean construction, the construction phase may still be ongoing, but contamination control measures on-site have been implemented, such as the use of specific clothing and limiting activities that generate particles.


3.5 Utility Connection:


Chilled (or glycol) water, hot water, steam, electrical power, and control systems can now be connected. While it is not essential to have fluids available at this stage for comfort, it is desirable, especially in cold climates to prevent coil freezing!


3.6 System Blowing:


For a successful cleanroom air balancing, it’s very important the initial blowing of the system, as it aids in removing metallic particles and other impurities from the ductwork that could compromise the HEPA filters. Ideally, the duct cutting process (installation of dampers, fitting placement, etc.) should always be followed by local vacuuming to collect these particles, but this is not always feasible. The blowing process can take from a few hours to even days, depending on overall observations. Once blowing is completed, all filters can be installed. The decision to install HEPA filters will be based solely on whether all construction activities have finished, and a thorough clean of cleanrooms has been carried out.

4. Rough Balance


As mentioned in the introduction, rough cleanroom air balancing aims to closely approximate both the design airflow rates and the differential pressure values. The degree of approximation logically depends on the system’s complexity. An AHU serving one or two rooms will almost coincide with the fine balance.

However, systems can be quite complex. Having Airflow & Instrumentation Drawings (AF&ID) is essential for our purpose. A good AF&ID should contain, at a minimum, the following information to achieve a successful and efficient outcome: total supply airflow of the AHU (even if oversized, if applicable), airflow rates for each duct branch, the position of CAVs and VAVs, leakage flows (incoming and outgoing), arrangement of air terminal diffusers with individual airflow for each, individual return airflows, if there is a variable extraction airflow (which affects differential pressure, such as the presence of BSCs, depyrogenation tunnels, etc.).

These values in the AF&ID will be design values, and they may not exactly match the final values after balancing, but they will undoubtedly be an excellent starting point. How we calculate these values is explained in our post, Cleanroom Air Balance. In any case, whether it’s simpler or more complex, we should almost always follow the following order:

4.1. Adjust the supply and exhaust fan airflow rate.


This can be done either by manually inputting the Hz into the variable frequency drive (for which we will need the assistance of a Pitot tube or a similar element in the main supply duct to determine airflow) or by reading the static pressure using a transmitter (knowing the manufacturer’s K value for the fan).

Similarly, adjust the airflow rate of the exhaust or return fan, if applicable in our installation.

4.2. Adjust the airflow rate of constant air volume (CAV) dampers.


Once again, based on design schematics, we will adjust the values for each diffuser. Ideally, we will have a CAV for each diffuser, which would make our work much easier. In other cases, we may have one CAV per duct branch supplying air to a set of diffusers. In this case, we may have some airflow variation.

4.3. Return dampers


If manual, can be left fully open. In this initial phase, we are not aiming to adjust room differential pressures.

5. Final Balance


For the final clean room air balancing, completing construction activities is crucial. Personnel responsible for balancing should control access to doors and rooms. Another preliminary step involves checking for leaks through openings of elements pending installation (e.g., pipes passing through walls, communication with technical areas of autoclaves, part-washers, etc.), or defects in door seals. Generally, we follow these steps:

a) Calibrate room differential pressure transmitters.

b) Clearly label room access (e.g., “no entry” labels on doors).

c) Measure airflow rates at each diffuser using a balometer of appropriate dimensions for the filter. Adjusting the CAVs provides the most accurate values to match design airflow rates.

d) Adjust the outdoor air intake airflow rate to the design value, preferably with a CAV in this location.

e) Ensure process equipment affecting differential pressures remains static, without fluctuations in airflow being drawn or introduced into rooms.

f) Establish a pressure balancing strategy, starting with rooms with the highest differential pressure and proceeding in descending pressure order. Begin adjustments with rooms having balanced infiltration and exfiltration rates.

g) Adjust room pressure based on design type. For statically balanced systems with manual return dampers, closing the damper increases room pressure, while opening it decreases pressure. For dynamically balanced systems with motorized VAV dampers, input the desired pressure setpoint into the BMS.

h) If differential pressure values are too low, consider increasing the outdoor air intake airflow rate, bearing in mind the potential effects on overall room pressure. Before adjusting, check for leaks.

Repeat the procedure for each room as necessary, as it often requires iteration. Modifying the pressure of one room will likely affect adjacent rooms’ pressure.


A good PID should contains the design airflow values to assist the commissioning engineer

5. Conclusion


In conclusion, proper cleanroom air balancing in pharmaceutical HVAC systems is essential for ensuring optimal performance and maintaining stringent cleanliness standards. The rough balancing phase aims to approximate design airflow rates and differential pressures, laying the foundation for subsequent fine adjustments. Utilizing detailed airflow and instrumentation drawings as a guide, personnel systematically adjust supply and return airflow rates, damper positions, and room pressures. The final balancing stage demands meticulous attention to detail, with completed construction activities and a focus on calibrating pressure transmitters and implementing a comprehensive pressure balancing strategy. Adherence to these systematic procedures and prerequisites enables pharmaceutical facilities to establish HVAC systems that meet regulatory requirements, promote product integrity, and ensure the safety of personnel and environments.

You can find more information about cleanroom air balancing and commissioning in rhe ASHRAE Design Guide for Cleanrooms.

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