Published Date 9/6/18 7:00 AM
The constant increase in the technological level of production processes (in particular in the electronics industry) and the increasingly stringent regulations on hygiene and quality (chemical and pharmaceutical industries) have led to the growing use of controlled-contamination environments. These are areas with specific air requirements in terms of “contaminant presence”, so as to ensure optimal conditions for the processes carried out inside. Controlled contaminants generally refer to airborne particulate (number and size of particles suspended in the volume in question in a given time interval), but can also be aeriform (gases or vapours), present in the environment as part of the process itself or generated during it. Normally, a room in which the level of particulate is controlled is called a “clean room”, however it would be more correct to consider this as a “controlled-contamination system” that also includes filtration, distribution, recovery and handling of air inside the room using one or more air handling units installed specifically for this purpose. The latter also include devices to control the ambient temperature-humidity conditions , both for requirements related to the processes and the comfort of operators.
Contamination control is ensured by suitable design of the various parts of the system, based on the assessment of the level of acceptable risk for the specific application; however, especially in the AHU, it is important to be able to manage the dynamic behaviour of the components and the variation in surrounding conditions: in this sense, good control is essential to guarantee the system’s effectiveness and efficiency over time.
Here are some examples of “control loops” that are useful for managing a clean room AHU.
AHUs must primarily control the fans in order to guarantee a suitable recirculation flow-rate, as specified by ISO 14644, together with the type of filters to obtain the required level of particulate separation: nowadays, inverter-controlled fans are used quite frequently in order to modulate speed and obtain the desired flow-rate. Flow control means the air outlet speed from the absolute filters can be adjusted; the latter are often placed in the false-ceiling to create a laminar downwards flow, which avoids movements of the particles in an uncontrolled manner. A modulating fan is essential to maintain a constant air flow-rate (and therefore speed), which otherwise would decrease over time due to the progressive accumulation of particles in the filters. To effectively solve this problem, it is common practice to adjust fan speed according to the pressure difference read by a calibrated nozzle which, for radial fans, is often incorporated into the fan itself (see the figure).
This law can be applied: where ρ20 is the air density at 20°C and ρT is the density at the measured air temperature; k20 is a coefficient that depends on the geometry of the fan’s air outlet opening and is provided by the manufacturer. Blocking of the filters, at the same speed, causes a reduction in flow-rate and therefore a pressure drop at the outlet; consequently, fan speed is increased to bring the flow-rate to the desired value.
Control of fan speed based on differential pressure sensors is also very useful for maintaining pressure gradients between adjacent rooms. To prevent infiltration of outdoor air with high levels of contaminants, pressure inside the rooms must be higher than in adjacent, less controlled rooms. At times there are multiple levels with different classes, between which there must be a positive pressure gradient towards the less clean areas; generally this is at least 10-15 Pa or at least 5 Pa between pre-sterile areas and the outside environment. In this case, control involves maintaining a differential pressure between two rooms through introducing more suitably-filtered outdoor air than the air that is extracted (or naturally leaks out). The following image shows the qualitative pressure trend inside a typical air handling unit. The fan ensures an increase in pressure so as to keep the room in overpressure conditions.
Pressure gradient control is also important in some systems in which contaminants (mostly aeriform, such as solvents or disinfectants) are generated during the process; in this case, the air is taken in from the outside, treated and then delivered into the room. A separate device then extracts the air from the room and releases it into the atmosphere, after being treated; the pressure gradient in this case can be negative, to prevent contaminants from escaping.
Maintaining the temperature-humidity conditions inside clean rooms is important for both the processes and for the comfort of operators. Furthermore, a stable temperature is important because abrupt changes can create gradients and generate convective motion, affecting the controlled laminar air flow and lifting dust from surfaces. Consequently, controls is implemented based on the supply air probe reading; the number of recirculation cycles is such that in a short time, the entire clean room reaches the same temperature as the supply air. Alternatively, a more sophisticated method is to control the supply temperature using a floating set point, according to the temperature inside the room. In this way, air flow is stable and changes slowly if warm or cool air is needed to maintain the desired temperature in the clean room.
In summary, the type and positioning of the components inside the air handling unit, and above all the know-how needed to manage its operation, ensure the required specifications can be met the highest efficiency.
Humidification plays a primary role in this, and is worth looking at more in detail. This aspect will thus be examined in the next article.
Relative humidity: our first ally in the invisible war against bacteria
We will fog you!