Checking and managing air flows in the clean room

In the clean room, the concentration of airborne particles must be as low as possible so that the semi-conductors produced there are not damaged or the pharmaceutical products contaminated. This is done through a high rate of air change. In clean rooms of the category A in which the product lies open and comes into direct contact with air, the aim is therefore to ensure a continuous low-turbulence displacement flow. For the monitoring of these low-turbulence displacement flows, flow sensors are used that with their speed precisely read the low flow speeds that occur in the clean room and in addition also comply with the requirements there with regard to their material and their design.

The term 'Good Manufacturing Practice' (GMP) defines quality guidelines that ensure the quality of the production processes and environment in the manufacturing of pharmaceuticals. One company that has committed itself to GMP-compliant methods is the company Neuberger from Rothenburg ob der Tauber. In addition to hardware and software for the control of air conditioning in buildings, Neuberger supplies monitoring systems that have been specifically designed for the pharmaceutical industry. Their task is to monitor, visualize and archive all the necessary core values of the clean room such as room temperature, room humidity, room pressure, particle concentrations and air flow.

One special feature here is the monitoring of the fan filter units (FFUs) installed in the ceiling of the clean room which supply carefully filtered air on a permanent basis and under constant control. It is thus ensured that polluting particles cannot reach the product or that contaminated air from outside cannot penetrate into the clean room. This low-turbulence displacement flow is captured by flow sensors that Neuberger uses in addition to monitor the filters or to regulate the speed of the FFUs.

Monitoring filters or regulating FFUs

In its function for filter monitoring, the sensor checks whether the flow speed is correct at the fixed ventilator speed or whether it has already slowed down due to increasing contamination of the filter. In the second application variant, the flow measured by the sensor is used directly to regulate the speed of the FFUs.

The GMPs stipulate a closely specified, permissible work area for the flow speed of; 0.45 m/s with a permissible deviation of ±20 %, corresponding to 0.09 m/s. 'The flow sensors used of the type SS 20.415 from SCHMIDT Technology have a measurement precision of just 0.05 m/s. SCHMIDT Technology guarantees this not only with a factory certificate but also issues a calibration certificate pursuant in the company's own precision wind channel at the customer's request.

GMP-compliant and nevertheless unique

The SS 20.415 has been specifically designed for use in clean rooms pursuant to GMP. This means firstly that the sensor only has smooth surfaces without undercuts and can thus be easily cleaned or disinfected. Secondly, only suitable materials such as stainless steel were used for the installation parts required in the clean room and these materials are resistant to common cleaning agents and disinfectants such as alcohol, peracetic acid and hydrogen peroxide.

A special feature is the recognition of contamination. If, contrary to expectations, the sensor element should become contaminated or be moistened by some of the cleaning liquid, the sensor can then no longer measure correctly and issues an error signal. The error is signalized by the SS 20.415 by issuing a value of 2 mA. Otherwise, it communicates the measurement values as a fault-insensitive 4 ... 20 mA signal with high linearity accuracy.

Another advantage of the SS 20.415 is its compact design. It is facilitated by the integration of the electronics in the sensor tube which, with its diameter of just 9 mm, does not create any turbulences in the flow. The two constructional variants are also an advantage. A version curved in a 90° angle can be installed directly under the ceiling of the clean room; the straight version is suitable for installation on a wall.