Phoenix Controls is a recognized leader in the design and manufacture of precision airflow control systems, most notably variable air volume (VAV) controls for laboratory ventilation. Our systems ensure the environmental integrity of critical spaces, including research laboratories, vivariums, pharmaceutical process areas, and biocontainment facilities by controlling pressurization, temperature, and humidity.

The design of HVAC systems for laboratories presents special challenges to the design team. These systems require large quantities of makeup and exhaust air for the fume hoods and must be maintained at precise pressures relative to the adjacent spaces. These requirements are essential to maintain the safety of the building occupants. This mandate for safety coupled with the desire to save energy presents a great challenge to the design team. Traditionally, labs were designed as constant volume systems, often with a separate exhaust fan and makeup air source for each hood. These systems were not only expensive to install, but were also extremely expensive to operate due to high energy usage. About 15 years ago, designers and equipment manufacturers began to experiment with the concept of modulating the airflow in the system based on actual hood usage. This was a good idea in theory, but due to the limitations of available controls and equipment, it often created more problems than were solved. Phoenix controls introduced a VAV system that will maintain the highest level of safety in the lab, while giving the building owner the most energy efficient system available. The Phoenix system is superior to other lab airflow control systems for the following reasons:

1. Fast Speed of response
2. True Pressure Independence
3. Very High turndowns and accuracy
4. Usage Based Controls

1. Speed of Response

Many laboratory air control systems attempt to measure the face velocity at the hood using thermistors, or equivalent devices, and flow measuring devices in the ductwork to adjust the exhaust air damper accordingly. The control loop takes the input from these flow measuring devices and continues to make adjustments to the air damper until the proper airflow is achieved. This process is too slow and is also very unstable. The addition of a supply air valve, general exhaust valve, and other labs on the same duct system further complicates this process. As the airflow from the hood is adjusted it will cause a change in the room pressurization, requiring these additional valves to make adjustments. Unless response time is very fast and system operation is very stable, wide fluctuations in room pressure can result, leading to unsafe operating conditions, not to mention a lab that does not meet OSHA, ANSI and ASHRAE requirements.

The Phoenix system is faster and more stable than others because of their unique approach to face velocity control and air flow control. The Phoenix system utilizes a potentiometer to sense the position of the fume hood sash and then uses the following simple formula to calculate the required airflow:

CFM = Required Face Velocity x Free Area

The flow command is known immediately using simple analog controls within the fume hood monitor and is used to position the hood exhaust valve for proper airflow control.

The airflow control approach used by Phoenix is different from their competition because they utilize a venturi air valve along with high speed analog circuits and pneumatic actuators. The venturi valve provides extremely accurate flow control, particularly at the low flow set points, and offers large turndown ratios allowing maximum energy savings. Each valve uses a closed-loop position controller that is factory calibrated using extremely accurate airflow measuring devices. Once the valve has been calibrated, the controller knows exactly how much air is moving through the valve for each position of shaft movement. The flow command from the monitor controls the venturi valves position, and thus the flow. Because of the nature of the high speed analog controls and the pneumatic actuators, the valve stabilizes at the new flow in less than one second from the time a fume hood sash is raised or lowered.

2. Pressure Independence:

Traditional VAV systems have accomplished pressure independence by measuring the airflow in the inlet of the VAV box and adjusting the damper as necessary to provide the amount of air required. This system works very well in HVAC systems, because speed and accuracy are not that critical. In laboratory environments, however, speed and accuracy are very important. Many laboratory airflow control systems attempt to maintain pressure independence in the same manner, by measuring airflow in the duct. The problem with these flow measuring devices is that although they may be very accurate at the flow they are rated for, they lose accuracy as the flow decreases. In other words, a device rated for +/- .5% accuracy at full scale, will not maintain accuracy at low flows. If this air valve is designed for a range of 100 to 1000 CFM, it could have as much as a 50% error at it=s minimum position. Also, their accuracy is dependant on ideal duct conditions, i.e. long straight runs of duct, which is very rarely a realistic expectation. Because large manifolded duct systems are constantly in a state of change, these controls tend to constantly hunt. Each time a hood sash is opened or closed, every airflow measuring device in the system will read a change in airflow and try to make an adjustment.

The Phoenix air valve maintains pressure independence in a much different manner. The valve has a cone in the inlet and this cone rides on an engineered spring connected to the shaft of the damper. As the pressure in the ductwork increases or decreases, the pressure on this spring is either increased or decreased, causing the cone to move. This floating action is able to accurately maintain pressure independence over a range of .6" to 3" of pressure drop across the valve, and because this is a mechanical process (i.e. a spring) it happens instantly without having to make any electric control adjustments. Click on the Phoenix air valve hyperlink above to visit the animated valve demonstration on the Phoenix controls home page.

3. Very high turndowns and accuracy:

Many laboratory airflow control systems use traditional VAV air flow valves to modulate the airflow in the duct. Unfortunately, these air control valves are only realistically accurate for turndowns of about 3:1. Some control manufacturers claim to be able to do a 4:1 or a 5:1 turndown accurately, and claim this to be adequate; however, turndowns of 9:1 and greater are usually necessary. For example, a typical fume hood would be sized for 1000 CFM with a minimum airflow of 200 CFM, which is a 5:1 turndown. A typical lab would require a negative pressure of -100 CFM. This would require that the supply valve be modulated from 900 CFM to 100 CFM to maintain this offset. This is not possible with a 5:1 turndown. A Phoenix 10" air valve is rated for a range of 50 CFM to 1000 CFM with a +/- 5% accuracy over the entire range, and a 12" valve is rated for 90 CFM to 1500 CFM with the same accuracy rating.

4. Usage based controls (Zone Presence Sensors)

The latest addition to the Phoenix Controls line of products is called the Zone Presence Sensor (ZPS). This is truly one of the most innovative products Phoenix offers. The ZPS simply maps the area in front of the hood for any activity. If it senses a period of inactivity (i.e. the zone is not occupied) it can reduce the face velocity at the hood to a predetermined standby setting. Many studies of hood occupancy have found that very often, hoods are left open when they are not in use. Many owners believe that they get very good sash management from their employees, which is probably true. However, although the lab workers will close their hoods when they go to lunch or go home, often, they will leave the hoods open when they are performing an experiment even if they are at their desk writing notes. This causes engineers to be reluctant to design much diversity in the system. The result is higher first cost and increased energy consumption. Use of the ZPS assures better management of hood exhaust making it possible to achieve significant cost savings.

It is possible that all the hoods in a lab building could be open at the same time; however, it is very unlikely. In fact, an average lab building with 100 fume hoods will never have more than 18 people in front of fume hoods at one time, to an accuracy of 99.9%. OSHA recommends that the face velocity of an occupied fume hood should be between 60 feet per minute and 100 feet per minute. This would mean that for this example, the exhaust and makeup air design air flows at 100 feet per minute would be 100,000 CFM. If it is assumed that there will be times when all the fume hoods will be left open, the system would have to be sized for 100,000 CFM. However, if zone presence sensors are used, based on a math model that can be established, we can say that only 18% of the hoods will actually be occupied at one time. Therefore, we would have 18,000 CFM consumed at the 18 occupied hoods, and only 49,200 CFM (600 CFM x 82 hoods) consumed at the unoccupied hoods. This method still maintains a face velocity of 60 feet per minute even when the hood is unoccupied. Using this simple and very conservative method for determining the diversity, we would be able to reduce the total CFM of the system from 100,000 CFM to 67,200 CFM saving thousands of dollars in installation costs. Further analysis of diversity can be performed and the designer should contact Gorham/Schaffler Inc. or their local Phoenix Controls representative to evaluate these projects on a job by job basis.