Pressurised environment

Air pressure control is a universally accepted principle in cleanroom and containment laboratory design as a means of maintaining air cleanliness and protecting personnel, both within the facility and outside, from airborne contaminants.

With the advent of building management systems (BMS), control engineers have been able to develop systems that can regulate room environmental conditions to constant levels using various sensors and controllers. Room air pressure is one such parameter that has come under the BMS system control. However the term 'constant' is not as absolute as one might hope. It is understandable that engineers can be lulled into a false sense of security by manufacturers' claims of 'constant' flow devices and high accuracy sensor signals. But how many times do we hear complaints about alarms that are regularly activated falsely, resulting in them being ignored or silenced because the cause cannot be identified? Two inter-related aspects of environmental air pressure control are airflow and air pressure. It is possible to engineer BMS systems to control these parameters and a common approach would be to use a volume control damper in conjunction with some form of pressure sensing to determine flowrate, an actuator and, of course, a microprocessor. This approach relies on the accurate interaction of all these elements, each of which has its own performance tolerance and, added together, greatly increase the variability of the 'constant' control. However, in any field of engineering, simplicity is, more often than not, by far the best policy. In contrast, an alternative method is available based on a combination of just two elements: • venturi valves, often used in cleanroom ventilation systems, which offer a direct means of accurate constant flow control; • air pressure stabilisers that provide accurate differential pressure control. Both the venturi valve and the air pressure stabiliser are able to provide these functions without the need for peripheral sensors, actuators or computers. By providing constant flow irrespective of upstream duct pressure conditions, venturi valves are highly suitable for distributed air supply systems. Air pressure stabilisers, working in conjunction with venturi valves, provide air pressure control to a constant level over a wide range of flowrates that compensate for variations resulting from alterations in room leakage or extract rates. Once commissioned, they will provide long-term stable and accurate control without the potential drift and variation associated with the more complex BMS approach.

Balancing system In terms of air pressure stabiliser specification, one solution is apreco's VARI-centric range, designed to maintain a sterile environment and allow accurate research and testing in the cleanrooms of pharmaceutical, healthcare and commercial companies. The patented balancing system is based on a built-in failsafe principle, whereby as soon as the pressure differential drops below the required level, the stabiliser closes. Differential pressure is controlled directly and airflow problems are immediately apparent when a failure in other parts of the system occurs. The range is simple to specify with a wide selection of sizes for different applications, including fire-rated models fully adjustable to operate within a 5-35 Pa pressure range. Systems are suitable for inclusion in partitions and walls from 50–1000m thick – without external horizontal flat surfaces, which is preferable in cleanrooms. Commissioning and installation costs are minimal and removable blades simplify adjustment and maintenance. Design engineers should consider very carefully the tolerance of performance of the various components they use in airflow control systems and the net effect of the combination of these tolerances. The outcome may be that they conclude that the constant flow/pressure systems currently being specified are not as constant as they would like them to be. If so, they should consider using air pressure stabilisers and venturi valves to provide a simpler and more robust solution.