Usage-Based Controls: A Small Feature with Big Savings

Most critical airflow spaces such as hospitals, science laboratories, and research facilities require an immense operations and maintenance budget in order to stay afloat.

As a result, the amount of energy most conventional constant volume systems expense is eye-opening. In a generation where these constant volume systems are rapidly being replaced by more efficient variable air volume systems, it is important to understand and be able to address the small features and strategies that we can implement in order to introduce significant energy savings to the table. This is where usage-based controls comes in for fume hood applications.

Usage-based controls directly refers to controls that act on the occupancy state of a space where the ratio of occupied versus unoccupied hours becomes a key factor.

For example, in a university chemistry laboratory where there a certain number of bench fume hoods, for example, there will always be x number of individuals that forget to close their fume hood sashes and thus leave them open at the end of the day.

Doing this exerts a great deal of pressure on the exhaust systems because they need to maintain a constant 100 feet per minute face velocity requirement at the sash as stated in the ASHRAE Z9.5 standards, which ensures that anything that’s being worked on within the fume hood does not escape into the breathing space of the operator or any other adjacent spaces like the corridor.

In many applications, toxic chemicals and fumes result from the work being performed in the fume hood and can have serious health effects, if not fatal, on the operator if in direct contact.

Fortunately, Phoenix Controls has implemented a strategy that can help turn down the sash face velocity during unoccupied or hibernated states, such as holidays or vacation periods. For fume hood applications, there exists a range of sensors that work as a feedback to the system and help reduce the energy consumption of the exhaust systems when even, say, an operator is away from his working area at the hood.

By using a motion sensor that has a defined detection zone in front of the fume hood, known as the zone presence sensor, we can vary the fume hood exhaust airflow down to 60 feet per minute when the operator is away from his workspace, which is still compliant with ASHRAE guidelines during unoccupied hours and still be able to properly contain fumes, gases or particles in the fume hood because of a 1 second high-speed response time.

By doing this, we can achieve 40% less airflow when the fume hood is open and 80% when they are closed. The zone presence sensor also has the ability to recognize inanimate versus animate objects so that it only recognizes the operator and doesn’t confuse it with other nearby objects.

After years of market research, Phoenix Controls came up with the usage-based controls concept because they realized that the time the operator is actually present in front of the fume hood is significantly less than the unoccupied time for the large amount of airflow being exhausted.

Since laboratories have to be 100% outside air systems per ASHRAE standards, then the energy expensed is significantly more since there is no possibility of using return air in the building.

Another type of sensor that is part of Phoenix Control’s usage-based controls platform is the sash sensor, which is another device that can assist with turning down the fume hood exhaust airflow if needed. A sash sensor is placed along the inner vertical frame of the fume hood opening or sash. A magnet on the sash trips a series of electronic switches on the sensor, which sends a signal to the fume hood monitor to lower the exhaust airflow on the fume hood exhaust valve, thus reducing the face velocity at the sash opening.

Therefore, by raising the sash, we increase the exhaust airflow due to the increased open area in contact with the operator and vice versa. It is evident that by using a combination of both the zone presence sensor, the fume hood monitor, and the sash sensor, we can confidently present an energy savings strategy from Phoenix Controls for fume hood applications no matter the application.

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