Canopy units

FIGURE 7.3.4.3 Canopy Unit Exhaust for Class II Type AI BSC. The canopy unit (sometimes called the thimble) is designed to fit over the exhaust of the BSC but not tightly. There is a gap of about 1 inch between the canopy and the BSC exhaust so that air is also exhausted from the room. Canopy units are often used in small lab rooms where some exhaust ventilation is needed to remove chemical contaminants that might be in the air. (From Figure 4 in CDC/NIH, Primary Containment for Biohazards Selection, Installation, and Use of Biological Safety Cabinets, 3rd edition, CDC/NIH, Washington, DC, 2007 available at http //www.cdc.gov/od/ohs/biosfty/primary containment for biohazards.pdf.)... 

Some BSCs are exhausted through canopy units (also called thimble units). Figure 7.3.4.3 shows a canopy unit. This unit is not directly connected to the BSC but rather is placed in close proximity to the exhaust. There is about an inch of space between the BSC exhaust pipe and the inside lower lip of the canopy unit. The canopy unit serves two purposes it removes any volatile chemical vapors that might be exhausted from the BSC or in the laboratory and it provides a source of ventilation for the laboratory room especially when the BSC is off, thus preserving the directional airflow into the laboratory. 

Some BSCs are exhausted through canopy units (also called thimble units). Figure 7.3.4.3 shows a canopy unit. This unit is not directly connected to the BSC but rather is placed in close proximity to the... 

A = Damper B = Flexible Connector to Exhaust C = Exhaust HEPA Filter Housing D Canopy Unit E = BSC... 

Figure 4. Diagramatic representation of the environmental monitoring units installed in gaps and adjacent canopied sites of the montane coniferous ecosystem of Camels Hump.

The isopentane for the catalyst preparation unit is stored as a liquid in a 60-ton horizontal (bullet) storage tank. The aluminum alkyls and other required chemicals for this process are received in small truck trailers and kept beneath a metal canopy. 

The catalyst preparation area is positioned between the two polyethylene production units with 60 feet separating each one. The aluminum alkyls storage canopy and isopentane horizontal storage tank are located at a remote area at an approximate distance of 250 feet away from the production and utility areas. The isopentane is transported to the catalyst preparation area through a 3-inch pipeline. A remote actuated isolation valve on this supply line that fails closed is located at the isopentane storage tank. This control valve and an associated isopentane feed pump are managed by the operator in the control room. 

Dispensers for propane can be configured like dispensers for gasoline with readouts for gallons dispensed and accumulated total purchase price. Dispensers should be mounted on a concrete pad for stability and protection from corrosion, unless the dispenser is part of a complete storage and dispensing unit where the dispenser is mounted securely within the system. Dispensers cannot be mounted inside buildings, but can be mounted under canopies and can be protected from the elements as long as the area is ventilated and the perimeter is not more than 50% enclosed. The heart of a propane dispenser is the meter that measures the... 

Toxaphene emissions for the entire United States were estimated by Li et al. [26], using application rates in the current year and residues carried over in the soil from past years, the latter estimated by assuming a 10-yr dissipation half life in soils. After the final year of toxaphene use (1986), only soil residues contributed to the emissions. Emission factors were calculated on a 1/4° latitude xl/6° longitude grid using the soil-air exchange and canopy models of Sholtz et al. [35,36]. About 80% of the emissions were from the southeastern, delta, and Appalachian states. The total quantity of toxaphene emitted to the atmosphere in 2000 was estimated as 364 tonnes. 

This expression yields an empirical parameter with units of velocity (cm s-1) which are independent of atmospheric concentration. Vg may be calculated on the basis of exposed leaf area, in the case of spaced plants, or in terms of the horizontal surface area for continuous swards such as cereal or grass crops. Both soil and plant are potentially available for contamination and the total velocity of deposition to a canopy can then be defined as... 

During the daytime, a transpiring and photosynthesizing plant community as a whole can have a net vertical flux density of CO2 (/coz) downward toward it and a net vertical flux density of water vapor (71W) upward away from it into the turbulent air above the canopy. These flux densities are expressed per unit area of the ground or, equivalently, per unit area of the (horizontal) plant canopy. Each of the flux densities depends on the appropriate gradient. The vertical flux density of water vapor, for example, depends on the rate of change of water vapor concentration in the turbulent air, c, with respect to distance, z, above the vegetation ... 

Figure 9-2. Schematic illustration of small packets or eddies of air swirling about in the turbulent region above vegetation. The eddies, which tend to increase in size with distance above the canopy, carry all of the molecules that they contain more or less as a unit. They are constantly changing— breaking up or coalescing with other eddies —making their actual size rather hypothetical.

A. If the light compensation point for CO2 fixation is at a PPF of 8 pmol m-2 s-1, what are the cumulative leaf areas per unit ground area for light compensation when 2000, 200, 20, and 0 pmol m-2 s-1 are incident on the canopy ... 

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