14 Nov Dampers in Pharmaceutical HVAC
Today, we delve into a topic of significant importance in HVAC system design: dampers we use in pharmaceutical HVAC. As discussed in previous instances, air handling systems in the pharmaceutical industry exhibit two main characteristics:
- The supply air volume remains constant, ensuring hourly air exchanges within the space as per design. These refer to CAV (Constant Air Volume) dampers.
- The return air volume needs to be adjustable to ensure room pressure differentials, thus requiring variability within a certain range. These are VAV (Variable Air Volume) dampers.
We can achieve to control both constant and variable air volumes through manual dampers. However, this necessitates increased supervision from maintenance personnel and poses challenges in controlling installations within statically balanced systems.
Today, our focus is on CAV (1) and VAV (2) dampers. In an airflow and instrumentation drawing, we represent like this, although symbology may vary:
2. Constant Air Volume
These dampers prove particularly valuable in the pharmaceutical industry as they ensure a constant flow to terminal filters, regardless of:
- Filter clogging
- Pressure losses
This greatly facilitates the balancing of the HVAC system. During commissioning, one simply needs to set the flow at the terminal unit, typically an absolute filter.
They can be individually installed at each terminal filter. To reduce costs, they can be grouped in branches if pressure differences are minimal in the terminal filters of the branch. The actuator can be mechanical, using a spring, which has the advantage of not requiring electrical installation. However, the drawback is the inability to determine the position of the damper.
While a constant air volume damper is typically suitable for installation in the air supply, there are occasions when motorization may be desirable. Some scenarios where motorization may be beneficial include:
- Multiple Setpoints for Air Supply: when there are two or more setpoints for air supply. This can occur, for instance, in an energy-saving system where the airflow is reduced during non-production hours.
- Complete Closure Requirement: situations where it is necessary to completely close the damper. A typical example of this is during room decontamination cycles. Motorizing the damper in such cases provides the flexibility needed to adapt to varying operational requirements and optimize energy efficiency.
3. Variable Air Volume
The VAV concept should be regarded as the standard system for dynamic control of room pressure differentials. This damper allows for varying airflows depending on the pressure set in the room, accounting for leaks and/or infiltrations (refer to the cleanroom balance post). A necessary component is an actuator that, based on the pressure, adjusts the damper to open more or less, thus regulating the airflow. In this post, we won’t delve into the details of extraction fans in this scenario, which must be carefully defined to accommodate these airflow variations.
Similar to the case of CAV dampers, motorization for VAV dampers should control, if necessary, the two possibilities mentioned earlier:
Automatic Pressure Adjustment for Setpoint Changes: the pressure adjustment will be carried out automatically in the event of a change in the air supply setpoint. Although the room’s pressure differential depends solely on the outside air volume, special attention must be given to maintaining pressure during the switchover. This should be carefully adjusted during the commissioning phase.
Motorized VAV dampers play a crucial role in ensuring dynamic control and optimization of air pressure differentials, especially when adapting to changes in setpoints. Careful commissioning is essential for their effective operation.
When there is more than one air return per room, it is crucial to carefully study the placement of the VAV damper. If necessary, multiple dampers may be installed in larger rooms, positioned in opposite locations to handle variable airflows effectively. This strategic placement ensures balanced and efficient control of variable air volumes within the space.
It is good practice to follow the manufacturer’s recommendations meticulously. While these may vary, the general guidelines to be adhered to include:
- Install in the Direction of Airflow: typically, an arrow will indicate the correct installation direction. Ensure that the damper is installed in line with the airflow.
- Install on a straight section of the duct: bends, junctions or a narrowing or widening of the duct causes turbulence that may affect volume flow rate measurement. A minimum distance of 1.5 times the diameter is common, but some manufacturers may require up to 5 diameter
5. Design Considerations. Noise
The noise generated by a damper falls into two categories: radiated (towards the exterior) and discharged (into the ductwork). For the former, it is essential to assess the need for a damper with acoustic insulation. As for the discharged noise, the evaluation should focus on the necessity of installing a duct silencer.
Balancing these considerations is crucial for creating an effective and efficient HVAC system while minimizing undesirable noise levels.
To decide whether to install a silencer, consulting the manufacturer’s noise data based on airflow and pressure is crucial. The airflow is typically predetermined by design. Pressure, on the other hand, can vary depending on the damper’s closure angle. For sizing purposes, we can consider a worst-case scenario with a pressure drop between 200 and 250 Pa. This corresponds to a damper being partially open (or partially closed) with an angle of 45º. It’s important to note that we must appropriately select the damper so that its size aligns with the nominal airflow.
With the known airflow and pressure values, we can determine the noise generated for each octave band frequency. Here’s an example: