A common comparison used to highlight the energy costs of these systems is to compare the energy impact of a single fume hood with that of a typical US household. On average a single lab fume hood uses as much energy as three typical US houses. And when you consider that a given facility may have many lab hoods in a single laboratory space, you can see how these energy impacts quickly add up.
In order to minimize the wasted energy associated with these laboratories, high-precision VAV lab controls have been developed to ensure operator safety, and to only provide the minimum amount of air necessary--And great savings have been realized by this sort of measure. But the energy efficiency of these systems can be improved even more.
Once the airflow has been taken down to a minimum, the energy associated with conditioning that air has been greatly reduced. But the energy associated with moving that air still can be reduced further. ASHRAE 90.1 states:
ASHRAE Standard 90.1 - 6.5.3.2.3:
“For systems with direct digital control of individual zone boxes reporting to the central control panel, static pressure setpoint shall be reset based on the zone requiring the most pressure; i.e., the setpoint is reset lower until one zone damper is nearly wide open.”
This calls for static pressure reset for VAV systems to minimize fan energy--ensuring that only the minimum amount of static pressure is provided to move the air. And this strategy is perfectly applicable to laboratory VAV systems as well as commercial air conditioning--as long as the system components are selected appropriately.
Tek-Air has published a white paper entitled Demand Based Static Pressure Reset Control for Laboratories That explores the energy benefits of this type of control scheme.
This paper analyzes system component selection, including control valves and sensors and illustrates the impact of these decisions on the overall energy use of the VAV system. In an analysis of a 50,000 cfm exhaust system, the reduced static from a pressure reset strategy can result in nearly $9,000 per year savings in fan energy (based on 0.75" savings, and $0.06/kwh electric costs).
These sorts of static pressure savings are easily attainable with a wise selection of air valve components. The commonly specified venturi-type valve has a minimum operating pressure that prevents these savings from being realized, and this added pressure drop often creates objectionable noise, which requires even more pressure drop for the system in the form of sound attenuators. This pressure reset strategy requires valves that can operate accurately and safely at low pressures.
The Tek-Air PRD valve provides unmatched pressure performance, and a quick examination of a cross section of the valve shows why:
Each blade of the damper is a smooth airfoil, greatly reducing turbulence and keeping the pressure and acoustic profile of the valve to a minimum.
If pneumatic air is not available, Tek-Air's new Accuvalve provides very similar performance with the convenience of electronic actuation. (And it won an innovation award at the 2008 AHR expo!)
A peek at the cross section of this valve shows how it attains these low pressure drops:
The airfoil shape of the valve assembly assures minimal pressure drop and sound generation for great efficiency in the fan system.
Energy savings cannot come at the cost of safety, and it is imperative that systems utilizing this method of pressure reset have sensors that can operate accurately and effectively in a wide range of pressure regimes. Tek-Air uses vortex shedding flow sensor technology to ensure the most accurate and linear control on the market.
Energy conservation is only going to become a bigger and bigger issue for designers of all building systems, and fume hood systems are a large opportunity for savings. It is important that designers and owners consider all the impacts of their design decisions and their system selections.
(Don't forget about checking the fan for stability: See this article for a review on this issue.)
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