The fume hood is one of the most common pieces of laboratory equipment and also one whose performance is most subject to question. The primary purpose of its design was, and is, safety; viz. the protection of the worker within the laboratory, medical or industrial environment by effective prevention of "spillage."
The ideal operating characteristics of a fume hood include:
1. Minimum face velocity to ensure capture.
2. Constant face velocity in all sash positions.
3. Hood operation, including sash positions, will not impact the room and corridor space static pressure relationship.
4. Be energy conservative by exhausting only the amount of conditioned air necessary to maintain the required face velocity.
5. Minimum impact of capture capability by room environmental conditions.
6. Absence of obstruction, including dampers in hood exhaust system that act as collectors of contaminants.
7. Maintenance of exhaust duct velocities to prevent loss of conveyance capabilities.
8. Maintain exit air velocities so as to minimize any re-entry of exhaust.
The present invention provides these desired operating characteristics.
A number of operational problems are experienced with fume hoods constructed in accordance with the prior art. These include:
1. Widely varying face velocities from too low to ensure "capture" to higher than necessary. Tests prove that higher face velocities as 150 f.p.m. are not necessarily safer, and that the 80 to 100 range can provide good protection, and in some cases as low as 65 f.p.m., provided the hood's surrounding environment does not adversely impact its operation. Conventional hoods and most bypass hoods have unnecessarily high face velocities in low percent sash open position.
2. High energy costs. Fume hoods notoriously exhaust large amounts of conditioned air, and with the increased fuel costs the older air systems with constant volume supply and reheat coils, coupled with the hood exhaust always on and set for maximum sash openings, is very inefficient from an energy standpoint.
3. Disruption of room/corridor space static pressure relationship from changing of sash positions, or from cycling of hoods on and off.
4. Complaints from "users" with auxiliary air hoods when minimally conditioned air is used. It has also been reported that auxiliary air below room temperature may impair hood performance.
5. Condensation of vapors, collection of dusts, potential explosive conditions, etc. in hoods and exhaust ducts when exhaust volumes are reduced below design. These conditions may occur as a result of attempts to reduce energy by reducing flows based on sash position and by shutting down non-used hoods on a system.
6. Exit velocity from hoods exhaust system so low that re-entry becomes a problem. This condition is a companion result to paragraph 5 above.
Attempts to reduce fume hood energy include auxiliary air, heat recovery systems ("heat wheels" and "run around"), and reduction of exhaust flows to match lower sash positions and shutting down, or reducing hood flow, when units are not in use. Several surveys have shown that fume hoods are only needed or used at capacity (full flow exhaust) about 15% to 20% of the time. These kinds of energy reduction attempts have been mixed in their effectiveness. Auxiliary air hoods are expensive to install, savings are limited, and operator discomfort has been a common complaint. Problems with energy recovery systems include contamination of supply air, limited effectiveness, and high cost. The systems developed to reduce hood exhaust as functions of face velocity have generally been expensive and not able to maintain face velocity much better than the bypass units. Control has usually been with dampers, variable speed drive, or two speed fans. While energy use has been reduced, problems have arisen from chemical precipitation, below minimum conveyance velocities, explosive conditions, and exhaust air reentry because of below designed exit velocities.
In accordance with the present invention, a better system has been developed for fume hood operation that ensures the "capture" capability with the lowest practical face velocity.
Assuming the hood can be properly located and installed, the single most effective method of energy conservation is the control of face velocity or, in other words, exhaust only the amount of room air necessary to maintain a pre-determined or set face velocity.