Feature several types and configurations of filters too ) will be different , so it is recommended to check maximum performance / airflow of the intended filters to be used in system design . If your calculation requires 1 000m 3 per hour and your filter capacity is 500m 3 per hour , two filters would be required in the filter bank in this simple example .
The filtration , however , is not the only main consideration in a cleanroom . Air volume and psychometric calculations are paramount . An application may require an extremely low relative humidity ( RH ) figure of 2.5 % at -6 ° C . This adds significantly to the complication level of the system . The fresh air quantities and cooling requirements can also become tricky to balance . -6 ° C continuous operation must take into account allowing periodic defrosting .
It is not uncommon to design cleanroom HVAC systems to be able to supply a slightly higher number of ACHs and a slightly higher airflow considering that the filters will accumulate ‘ particulate ’ over time that will result in a higher need to maintain the air flow rate and in turn consume more energy .
Depending on what will occur within the cleanroom , another major up-front design consideration will be the introduction and exhausting of air . Some chemicals and fumes are lighter or heavier than air and so this factor will determine either introducing clean air at a high level or a low level , and whether the extract point would be at a low level ( most common ) or a high level .
Each facility will further have its own configuration around the return air – some of which will include additional filtration , while others will include only single pass and exhaust ( possibly with filtration in between depending on the process / use of the room ).
Cleanrooms are further held in either positive pressure or in negative pressure . Positive pressure will force the air flow out of the room instead of into it . This means that the air in the cleanroom will continually be pushed out of the space , thus preventing any unfiltered air or contaminants from entering . As mentioned , the barrier between the uncontrolled space and the classified area is known as an airlock . This barrier may also include a number of airlocks or double door systems . Here the pressure cascade will force any particulate or contaminants to a lower pressure zone .
With hazardous processes or particulate , the opposite is applicable : the pressure is negative in order to prevent particulates from escaping the room .
Depending on the facility , ultraviolet lights may also form part of the HVAC system needs – particularly in medical and laboratory environments where these devices are used to kill ( or neutralise ) micro-organisms and pathogens . Some facilities may use high heat treatments for certain similar functions .
Cleanroom applications , although most commonly associated with high tech or precise environments could also include more simple processes where for example coatings are applied to products or spray booths need to maintain a clean air to avoid spoiling or possible blistering . For such applications it is common to have a “ water trap ” that will catch the overspray or an air scrubber – a wet or dry device typically functioning by a centrifugal force . Where moisture ( water or a chemical ) is introduced , this will adhere to any particulate . The whirlpool motion will then naturally separate the heavier particles that fall down into a hopper mechanism and is thus extracted from that air . The downside to this method is that an effluent will be the by-product of the process that must be disposed of in the correct way .
Typically , when toxic fumes are generated within a space this is normally handled by standalone fume cupboards or fume hoods . These fume cupboards or hoods directly exhaust to an allocated outside point in a safe area via the HVAC system . The HVAC system feeding the room must also control pressure cascading , so that there is no backdraft . Some facilities that deal with harsh odours or dangerous chemicals would require the inclusion of suitable carbon filters to neutralise such factors . This would be another important aspect to know up front as different carbon compounds would be required for mercury droplets versus chlorine versus plant oils .
www . hvacronline . co . za RACA Journal I March 2022 29