Introduction:- Conventional clean rooms operate on the principle that the air supplied to the room is of sufficient quantity to dilute or remove the contamination generated within the room. Measurement of the air supply volume and determination of the air change rate (ACR) is a measure of the frequency of air turnover in the clean room. The ACR can be determined by measuring the mean air velocity at the supply HEPAs or grilles and calculating the air change rate based on the mean air supply volume or by using a flow measuring hood (balometer), which collects all of the air from the supply and gives an air supply volume directly. Whichever method is used to determine the ACR it should be consistent, as the two techniques will give different readings. Always compare like with like. Using a flow measuring hood will give a more accurate air change rate reading, whilst an anemometer will allow a gross measurement that can be used for trend analysis.
Equipment:- The equipment required for measuring the air velocity is a thermal anemometer, vane anemometer or equivalent. A flow measuring hood with appropriate measuring device can be used.
 Sample locations:- Where an anemometer is used to determine the mean air velocity sufficient measurements should be made across the HEPA filter or grille face. It is suggested that at least four positions are tested across the filter or grille face to obtain the mean supply air velocity. Where a flow measuring hood is used the flow hood opening should completely cover the filter or diffuser grille and three readings taken to obtain an average of the air supply volume from the filter or grille. It should be noted that when using a flow measuring hood, cross-reference to air velocity readings in the duct is required to calculate the correction factor for the equipment.
 Frequency of sampling:- The test should be carried out at least three monthly using the same test method each time in order to allow comparison of test results.
 Method of sampling
Using an anemometer:-

  1. Support the anemometer such that the airflow is unobstructed.
  2. Position the sample head of the anemometer such that it is against the supply air grill and approximately 10cm from the HEPA filter, where installed. If the filter is set back behind a grille it is acceptable to take the readings on or at the surface provided the vanes or eyelashes of the grille are not moved. The effect of the non-uniform velocity across the supply area can be minimised by taking more readings per unit area.
  3. Repeat for each supply filter or grill.
  4. Calculate the mean air velocity (m/s) at each HEPA filter/grille and then calculate the air supply volume (m³/s) for each HEPA/grille by multiplying the mean air velocity by the HEPA filter/grille area (m²).

Using a flow measuring hood: –

  1. Place the flow measuring hood opening completely over the filter or grille, seating the face of the hood against a flat surface to prevent air bypass and inaccurate readings.
  2. Measure and record the mean air supply volume (m³/s) by taking three readings at each supply HEPA or grill.

Results and interpretation of results: – The ACR (per hour) can be calculated using the following formula:
ACR = air supply volume (m³/s) x 3600/ Room volume (m³)
Where there is more than one supply HEPA in a room the air supply volume for each filter should be determined and the volumes summed (to give a total air supply volume) before multiplying by 3600 and dividing by the room volume. The ACR in some facilities may be determined using secondary spillover air from door grilles in addition to primary air supplied from HEPA filters. The air supply from grilles should be treated in the same manner as an additional HEPA filter and the air volume summed to give a total air supply before calculating the ACR.
To achieve the level of cleanliness in an aseptic room and a clean support room the ACR should be greater than 20 air changes per hour.
Action:- Where a facility has been designed to achieve a specific ACR this should be determined. The supply volume will drop as HEPA filters block and as a result the ACR will fall. Where the ACR does not meet design requirements an investigation into the problem should be carried out. Any corrective action taken as a result of the investigation should be recorded. The corrective action may require filters to be changed and the air supply system to be re-balanced to achieve the desired ACR. Major changes to the air supply system must only be undertaken by authorized personnel.
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