Defined overpressure ensures product and process safety in cleanrooms. If the required pressure measurement is additionally backed up by a flow sensor, the controlled air volume adjustment can help to improve energy efficiency significantly. The bi-directional SS 20.400 flow sensor described below is perfect for this complementary use in cleanrooms.
The author: Rolf Bürssner Sales and marketing director, Schmidt Technology
To obtain excess pressure in a cleanroom, very high volume flows are usually essential in the air conditioning system. Not least for safety reasons and due to the reliable pressure measurements facilitated by differential pres-sure sensors, the EN ISO 14644 standard specifies excess pressure values in the range from 5 to 20 Pa. In pharmaceutical cleanrooms, for example, these values often lie between 15 and 30 Pa or even higher. The air volume needed to control the room pressure accounts for the significant share of the power consumption of an air conditioning system that is used to supply cleanrooms. Practical experience proves that the electricity consumption of the fans alone makes up approximately 57 % of the total costs for energy. The biggest power-saving potential derived from this fact involves adjusting the air volume to the requirements of the process. In order to leverage this potential, the excess pressure in the room must be reduced to a value as close as possible to the minimum requirements of the standard and then regulated or stabilised with the minimum possible supply air, i.e. with the smallest pos-sible fan output from the HVAC system.
Reserve and additional safety
In addition to the pressure measurement, a flow sensor installed in a suitable wall opening of approximately 50 mm diameter can be used to measure the so-called excess flow, i.e. the air flowing out of the cleanroom due to the overpressure prevailing there. The measuring ranges of modern flow sensors, such as Schmidt Technology’s SS 20.400, start at a flow velocity (WN) of 0.05 m/s. This value is much less than the differential pressure of 0.01 Pa and thus cannot be determined by a differential pressure sensor. A flow sensor therefore detects excess flow in a special opening in the wall of the cleanroom, even at very low differential pressure. Furthermore, SS 20.400 flow sensors are capable of measuring the flow directions bi-directionally, in other words including any reverse flows. They are based on a thermopile sensor which functions according to the principle of thermal anemometry. The flow direction can be determined by connecting the two semiconductors in parallel and measuring which one is hotter.
As even the smallest air flows are detected in both directions, SS 20.400 flow sensors are perfect for use in cleanrooms. In combination with the existing differential pressure sensors, they improve the functional safety and energy efficiency of cleanrooms significantly. An empirical simulation performed by AL-KO Therm, one of the largest manufacturers of components for ventilation and air conditioning systems, has proved that it is possible to reduce the costs for energy by 50 %.
Hall 11.1 Booth G16