Unidirectional flow and be defined as an airflow moving in a single direction, in a robust and uniform manner, and at sufficient speed to sweep particles away from the critical processing or testing area with regularity. Hence the object of the unidirectional airflow is to push outward any contamination which might be deposited into the air-stream and to avoid the potential for contamination dropping out of the air, either though gravity or by striking a object, and falling onto a critical surface.
Part of the control of air rests with air direction and this is a factor of airflow velocity. Poor airflow uniformity leads to turbulent airflow and vortex formation. In terms of the velocity of the air, this is described in some regulatory documents: 0,. Whether achieving good airflow and thereby avoiding poor airflow needs to conform to the range specified in regulatory guidance documents has been a long-standing issue, particularly given the non-scientific origins of the regulatory guidance values.
This article considers regulatory guidance on airflow velocities and the way that these are verified, and whether satisfactory airflow can be achieved outside of these guidance values. The discussion extends to consideration of the verification of these parameters at working height, especially in light of if this the most appropriate location by which to measure air velocities.
HEPA filters function through a combination of three important aspects. With this, there are one or more outer filters that work like sieves to stop the larger particles of dirt, dust, and hair. Inside those filters, there is a concertina - a mat of very dense fibres - which traps smaller particles. The inner part of the HEPA filter catch particles as they pass through in the moving air stream.
It was noted that when air is introduced into the cleanroom at a high velocity which causes the air to travel along a unidirectional path over a required distance. In doing so, contamination is swept away from the critical area unlike the more random distribution and transition of contaminants in turbulent flow cleanrooms 2.
Atomic Energy Commission 3. This concept of laminar airflow led to the development of specialised airflow cabinets whereby greater levels of cleanliness could be achieved through air passing at a sufficient velocity. It was this work that showed that an airflow velocity of 90 feet per minute was adequate to achieve the necessary levels of particle cleanliness avoiding settling particles of a diameter of 5.
This velocity also fitted with the capabilities of the fans in use at the time in relation to noise reduction. While this was effective, a fuller range — such as 25 to feet per minute — was not explored 4. While the range of 90 feet per minute suited the Sandia Corporation conditions, this velocity part of the first U. At no stage was this velocity reconsidered to determine what the most applicable range was based on science, or even whether it was necessary to state a range at all. In the first U.
Regulatory standards for cleanroom unidirectional airflow velocities differ in terms of where measurements are to be taken from and in terms of how much weight should be placed upon specific velocities. In terms of position, the U. FDA guidance, the requirement is to measure airflow velocities below the filter face at a distance of 6 inches 6. Similarly, to meet ISO measurements of the airflow velocity should be at approximately mm to mm from the filter face 7. The velocity is assessed using an anemometer, a device for measuring wind speed.
There are two common designs — vane and hot wire. The typical testing frequency is six-monthly or following any maintenance work or filter changes 8. According ISO the number of measuring points should be sufficient to determine the supply airflow rate in cleanrooms and clean zones. This should be the square root of 10 times of area in square meters.
However, not less than 4 readings should be taken. At least one point per filter should be measured. In each case, the airflow velocity range is recommended to be in the range 0. Higher velocities may be appropriate in operations generating high levels of particulates. In theory, the risk from lower air velocities is from insufficient laminarity and an inability of the air velocity to effectively sweep away any particles in the air-stream.
The risk arising from faster airflow velocities is from turbulence, and a tendency for the air to potentially eddy. However, do the standards really seek to imply that, say, going to 0. If they do, then this is not based on sound science.
While the air velocities remain guidance, experience suggests that some regulators are more open to considering velocities outside of these ranges than others. Furthermore, it is often the case that lower airflows can provide the same level of particle control and unidirectional pattern; and sometimes faster airflows are required, either as a result of equipment balancing or due to remove particles from certain operations such as where powder is handled.
In such cases satisfactory air patterns can be demonstrated through airflow visualisation. Consideration of this is discussed next. As indicated above, an airflow velocity of 0. There has been regulatory drift towards seeing these airflow velocities are mandatory. This is a mistake, since lower velocities, requiring lower energy use, may achieve the same effect; and higher velocities may be appropriate in operations generating high levels of particulates.
What should be stipulated instead is where air velocity becomes related to performance expectations where the air in critical areas is supplied, via point of use as HEPA- filters, in a unidirectional manner and at a velocity sufficient to sweep particles away from the critical area during operations, irrespective of the velocity setting. This means setting air velocity parameters for each processing line or item of equipment and ensuring these are justified and appropriate to maintain air quality under dynamic conditions within a defined space.
Literature also supports this position. Work by Whyte, which looked at airflow velocities covering the range of 0. Increasing the airflow velocity up to 0. The assessment suggested that an airflow velocity of 0.
Where alternate airflow velocities are proposed, these can be assessed through the recording of particle counts and by airflow visualisation studies. The revised text to EU GMP Annex 1 not in force at the time of writing but profiled in a recent edition of the Journal of GxP Compliance 11 puts greater emphasis upon airflow movement than with air velocity:. Normally, such conditions are provided by a localised air flow protection, such as laminar air flow work stations or isolators.
Moreover, the inference that isolators require the same air velocities as to other Grade A devices is out of step with most studies such as Peters et al 12 and Midcalf et al The draft goes on to read:. During initial qualification and requalification air speeds may be measured either close to the terminal air filter face or at the working height, Where ever the measurement is taken it is important to note that the key objective is to ensure that air visualization studies should correlate with the airspeed measurement to demonstrate air movement that supports protection of the product and open components with unidirectional air at the working height, where high risk operations and product and components are exposed.
The maintenance of unidirectional airflow should be demonstrated and validated across the whole of the grade A area. Entry into the grade A area by operators should be minimized by facility, process and procedural design. However, it is suggested in this paper that airflow visualisation studies can provide the means to consider alternate airflow velocities. This approach recognises actual performance, in the operational state with equipment running and person el carrying out the necessary activities, ahead of velocity.
The purpose of flow visualization is to confirm the smoothness, flow patterns and other spatial and temporal characteristics of airflow in an installation. For this, the airflow is examined through airflow visualisation mapping whereby smoke is generated, and the behaviour of the smoke is studied and then captured by a video camera. Air-flow studies can demonstrate a significant amount of information. This is significantly different from filter face velocity. Velocities are expressed in units of length divided by units of time.
Examples would include meters per second and feet per minute. To understand airflow rates we first need to consider the type of airflow we are using. Clean room airflow is described as being either unidirectional laminar or non-unidirectional turbulent. We typically see full filter coverage in unidirectional cleanrooms and partial filter coverage in non-unidirectional cleanrooms.
Unidirectional cleanrooms have generally parallel streamlines and approximately uniform velocity throughout. The advantage of unidirectional flow is that airborne particles are carried out of the room in minimum time and the shortest path, thereby giving them less chance to start any trouble.
Non-unidirectional rooms have non-parallel flow streams and non-uniform velocities. This mechanism is similar to placing a garden hose into a bucket of muddy water and stirring the water while letting the excess drain out over the top. Eventually the clean water entering the bucket will dilute and displace the dirty water. Rather than trying to achieve laminarity we want to encourage turbulence in this type of room, much like stirring the muddy water.
Without this mixing, areas of increased contamination and temperature and humidity gradients will form. Describing airflow in terms of air changes per hour is common for non-unidirectional flow rooms ISO classes 6 through 9 and high-bay installations. Since the airflow in these rooms is non-uniform, attempting to directly measure the average air velocity is not feasible.
The average velocity may be calculated, however, using volumetric measurements from the terminal filters. Leaving it in terms of average velocity would be confusing and misleading and may result in someone attempting to use a velometer to directly measure this variable.
If we wish to compare one room to another we need to know the height of each. For the unidirectional flow rooms ISO classes 1 through 5 we typically specify airflow rate by velocity rather than the air change rate. Since the airflow is presumed to be uniform throughout, the concept of average room velocity conveys real information and can be determined by direct measurement with a velometer.
Expressing airflow in terms of velocity allows us to compare the airflow rates of one cleanroom to the next and to quantify any changes in a room airflow rate. The measured velocity is totally independent of the room height.
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