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Single Pass or Recirculating Airflow Systems
Advantage and Disadvantages of Cleanroom Airflow Designs

florida cleanroom systems 072   Correctly designed and engineered cleanroom airflow and air change rates are the heart of the cleanroom envelope assuring compliance with ISO, FDA, cGMP, and USP - 797  regulatory standards. They also greatly impact operational and maintenance cost and requirements upon completion of the ISO Class cleanroom environment.

   Besides particulate levels, airflow and air changes rates will ultimately determine the temperature, relative humidity, heat load removal, static, and gradients of all depending on the percentage of treated conditioned air introduced, number of times it is recirculated, and total heat load of the cleanroom environment including personnel, process equipment, lighting levels , and type of HEPA filtration modules utilized.

Single Pass Cleanroom Airflow Systems:

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   Cleanroom single pass airflow, typically utilized by low cost cleanroom systems under 400 square feet is usually accomplished with HEPA fan filter units pulling in ambient or conditioned air and exhausting it out the sides of the cleanroom though exhaust grills back into the ambient environment. It can also exhaust air through the door thresholds. When recirculating ambient air supplies it is important to note that the ambient air must be  treated and conditioned to below the desired operating temperature and relative humidity desired in the cleanroom enclosure. It must also be conditioned low enough to offset the heat loads of the HEPA fan filter motors, light fixtures, process equipment, air change rates and number of  personnel in the cleanroom enclosure.

   Cleanroom installed in typical commercial laboratory spaces seldom maintain a comfortable operating temperature and relative humidity within the cleanroom environment if dependent on the ambient areas cooling equipment only. Additional cooling equipment, dedicated to the cleanroom environment and process is typically required to control temperature and relative humidity to a comfortable level for occupants with low enough temperature and relative humidity to mitigate the growth of fungus or bioburden.

   This makes single pass treated air one of the least expensive options to cleanroom construction in the beginning but can lead to one of the most expensive options to employ in the end if adequate temperature and relative humidity is not attained by use of the existing cooling equipment. 

   As an initial design it is usually only utilized as a design when odors, fumes, or other contaminants need to be 100% exhausted from the cleanroom environment and further treated by a scrubber system before being exhausted into the outside ambient environment.  

Recirculating Cleanroom Airflow Systems:

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   Recirculating cleanroom airflow filters recirculate air through a dedicated air handling system specifically designed and engineered to maintain the cleanroom at the required operating environmental tolerances.

  Recirculating airflow systems offer temperature, relative humidity, and pressurization controls through a controls system on the air handling unit itself and are typically paired with static HEPA filtration modules versus fan powered modules.

   Unidirectional airflow can be adjusted and directed by placement of the HEPA filtration modules and low level returns to direct airflow and heat load removal in areas of the cleanroom environment that require higher degrees of airflow.  

   Lower cleanliness class cleanrooms can also be filtered by a HEPA bank filtration module versus ceiling mount modules to reduce cost and maintenance requirements in the future. 

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Florida Cleanroom Systems HVAC Engineering Services:

  • Cleanroom Zone ISO Classifications
  • Air Flow Velocity / Changes per Hour
  • Laminar or Unidirectional Airflow
  • Temperature & Relative Humidity
  • Noise & Vibration Dampening
  • ESD - Electrostatic Discharge Levels
  • Cleanroom Zone Pressurization & Air Balance
  • CW - Chilled Water or DX - Direct Expansion Designs
  • Sustainability

   Designing and engineering the cleanroom air handling systems correctly the first time through is always less costly than attempting to repair an  incorrectly designed system or retrofitting a system after the fact. Air temperatures and relative humidity along with pressurization of the ambient air should be measured and documented over a twenty four hour timeframe. 

   When attempting to rely on a single pass ambient air controlled cleanroom enclosure for environmental tolerances, depending on process equipment, function, and number of personnel, it is not improbable that the cleanroom environmental conditions can run 10° F higher than the ambient with relative humidity up to 85%. These environmental conditions will ultimately lead to contamination with fungus and microbial growth with possible rust particulates generated by any metal surfaces.    

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