Phoenix Controls

6 Factors Affecting Airflow Measurement In Critical Environments

Indoor environments such as hospitals, laboratories and life science facilities require stable, efficient and safe airflow management. They are often referred to as critical environments or critical applications because it is vital to maintain specific airflow thresholds to protect the health and safety of the buildings’ occupants. Many flow measurement device companies/manufacturers claim to have viable solutions to maintain accurate flow control, however, their process of using airflow measurements are vulnerable to 6 real-world challenges described later in this post.

Conversely, Phoenix Controls’ theories of airflow control use sound physical principles. Their equipment is designed specifically for the demand of critical flow environments and for avoiding the inherent challenges of providing precise, repeatable control. They meter flow by pre-characterizing their valves and monitoring the flow with a feedback signal and differential static pressure sensor.

Ultimately it is not about measuring vs metering. It is about ensuring the safest environment that can operate reliably for the life span of the primary exhaust device. The following (6) factors should be taken into consideration when selecting ventilation controls.

1. Speed of Response

Maintaining sufficient equipment reaction time is imperative to maintain the containment of the space under control. Measuring flow changes and making control changes based on the proxy parameters have significant time delays. Phoenix Controls’ equipment uses closed-loop control to set the valve relative to a factory-characterized position versus flow relationship. The time to accomplish this is minimal. Static pressure changes in the system are compensated virtually instantaneously through the use of a mechanical function independent of flow control. Contrarily, flow-measuring systems imply a flow measurement derived from air velocity. When static pressure changes the velocity changes and the system will constantly be measuring and readjusting (always trying to catch up).

2. Accuracy

Maintaining containment or pressurization within critical environments requires accuracy across a broad range of flows. While flow measurement systems are typically very accurate at high flows, the error can increase to greater than 40% as the flow decreases (see table 1 below). Additionally, measurements obtained with velocity readings below 400 fpm are not practical because the accuracy of the measurements is compromised further. This limits the effective turndown to less than 5:1 with respect to flow.

In this example assume a 0.25-inch water column (WC) 1% full-scale accuracy pressure transducer for flow measurement in a 10-inch diameter duct.

Phoenix Controls’ approach to precise flow metering is plus/minus 5% accurate of command, regardless of desired flow. Specifically, a flow command of 1000 CFM will be provided within 50 CFM and a flow command of 50 CFM will be provided within 2.5 CFM. This accuracy is achieved by commanding the valve to precise position and allowing an independent, highly accurate static pressure compensation mechanism to maintain this flow regardless of significant static pressure changes.

3. Stability

Two factors should be considered regarding stability.

  1. To obtain an accurate flow reading when measuring flow, several samples must be taken over a period of time to obtain an average airflow reading. Another option is the transducer signal must be dampened in some way to produce a stable command signal to the flow control mechanism. Either of these attempts to stabilize an inherently fluctuating flow signal leads to inaccuracies or time delays. The airflow controller’s response time must be slowed or dampened to make it stable.
  2. The process of flow measurement must compensate for changes in system static pressure, as well as changes to flow command based on changes at the fume hood or room control. While these changes are underway, the building system is typically trying to maintain system static pressure at a plenum or common manifold. The interaction between the flow measurement and control devices on the same branch of the manifold or with the system static pressure controller can lead to troublesome oscillations. These oscillations are also known as breathing buildings.

The Phoenix approach does not need to be slowed to maintain stability and no external control adjustments are made to compensate for static pressure changes.

Field Tech Derek working hard.

4. Maintenance

All flow measurement systems require regular maintenance to clean the pressure sensing points and recalibration to compensate for drift in the transducers. Most manufacturers recommend annual maintenance on these systems.

Cleaning is required for two reasons:

  1. Pressure sensing ports are typically tiny orifices that are subject to blockage, thereby disrupting the system readings.
  2. Large volumes and broad varieties of contamination may pass the sensing element. These contaminants may be dangerous to personnel and equipment resulting in extensive safety precautions and potentially disruptive decontamination efforts.

Recalibration is necessary because the pressure transducers drift over time based on changes in their internal physical characteristics and sensitive electronic components. Many employ drift compensation methods during the interim period but verification to a known reference pressure is still typically required at least yearly.

The Phoenix control system does not require regular maintenance because simplified methods and components are used to control the flow. The position is set relative to a potentiometer over 20 years of life. Pressure independence is maintained through a simple spring designed well below the material’s limits and tested for more that 20 years of performance with no variance.

5. Installation Issues

Flow measurement devices require straight duct runs upstream and downstream of the sensing element to provide a smooth, stable velocity profile to the sensing element. In most experimental situations, good engineering practices recommend 10 diameters upstream and five diameters downstream of a straight duct. While the standards vary on the number of diameters, the ASHRAE Handbook: 2005 HVAC Applications advocates, “If possible, measuring points should be located at least 7.5 hydraulic diameters upstream from a disturbance.”

Although typical flow measurement suppliers for commercial and industrial applications say shorter straight runs are acceptable, recommendations on only four diameters upstream can be difficult and costly to design into a facility. Installations with less than the recommended straight lengths before a sensing element seriously compromise the accuracy of the measurement. While many systems have been designed with proper duct configurations, it is not uncommon that actual installations were not provided according to the design.

Phoenix Controls’ equipment is not measuring flow. Their products are inlet and exit condition insensitive because of the inherit nature of the venturi design and pressure compensating mechanism. These products can be installed in the duct in any configuration upstream or downstream of the valve without any impact on the accuracy, repeatability or stability of the flow control.

6. Balancing

Providing 10-14 diameters of straight duct around a measurement device is highly impractical in the field, and measurement device manufacturers have reduced this recommendation to significantly lower levels (4 to 6 diameters). Therefore, devices that measure flow must typically be field calibrated for the installation condition to which these are subjected. Coefficients specific to installation are obtained in the field and used to provide better accuracy to these measurement devices. However, the coefficients are specific to the airflow rate at which they were determined and contribute to further inaccuracies at lower or higher airflow rates. Balancing the system at building start-up becomes a significant effort. In addition, any significant changes to the system will require rebalancing to some, if not all, components in that branch of ductwork.

Phoenix’s approach is to factory characterize each venturi valve. As mentioned above, the components are insensitive to field conditions. Therefore, no further fieldwork is required other than the standard verification of appropriate flow. If changes are made to the system the Phoenix valve compensates automatically. It’s essentially a self-balancing valve. The testimony of numerous balancing agents is confirmation that the initial start-up of a Phoenix system is far simpler, shorter and less costly than any flow measurement system.

In summary, it is imperative not to rely on calculated flow measurement valves as a basis for airflow control in critical environments and applications. These types of devices are clearly subject to significant challenges. Phoenix Controls’ approach to critical airflow controls relies on a product specifically designed for this purpose. The products are provided as a systematic solution to a challenging problem. Each system starts with job application engineering, continues with 100% characterization of every valve that leaves their factory and ends with years of trouble-free performance.

We’re the proud Phoenix Controls representative for Connecticut and Western MA. A substantial portion of this post’s content was take from Phoenix Control’s White Paper, Factors Affecting the Performance of Airflow Measurement Devices in Critical Applications. We encourage you to download the complete whitepaper to further your knowledge on this subject.

Please contact us with questions, visit our website to learn about the other equipment and systems we can provide to improve the comfort, safety and efficiency of your facility. Follow us on social media to stay up-to-date on the our latest news and events: Facebook, LinkedIn and YouTube.

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