Hoods canopy

Fixed systems are those where movement of the hood or other changes to the system, except perhaps opening and closing of lids and doors, is not possible. One example is the hood with a sliding door surrounding a drilling or a milling machine another is the laboratory fume hood and another is the canopy hood above or the enclosure around a paper machine. 

The exterior hoods described here are divided into basic openings, rim exhausts, low-volume high-velocity (LVHV) hoods, receptor hoods (canopy hoods), and downdraft ventilation tables. Many varieties of these types of hoods exist. Some of these have been described and investigated more thoroughly than others because they are used more often or they are of more general use and applicability than the more specialized hoods. 

Receptor hoods, also called canopy hoods, are designed to capture contaminants given off by heated processes. They take advantage of the thermal updraft caused by such processes by placing the hood in the path of the updraft, they receive the exhaust and capture the contaminants. 

FIGURE 10.31 Canopy hoods over a cold process (left) and a hot process (right). 

FIGURE 10.3 2 Canopy hood with an airflow rate less than the thermal updraft airflow from a hot process. 

FIGURE 10.33 Canopy hoods according to Hemeon left, low hood, right, high hood. 

FIGURE 10.34 Basic geometry for calculating necessary flow rate for high canopy hoods. 

FIGURE 10.35 Inappropriate use of a canopy hood, since worker exposure is not prevented by use of the hood. 

Capture efficiency measurements may be used to evaluate the function of a canopy hood (see Section 10.5). Capture velocity is not a feasible evaluation tool, since a canopy hood does not generate an air velocity close to the source. It is also possible to use exposure measurements for workers outside the plume area. Since most hot processes generate visible contaminants, visual inspection of the flow, especially around hood edges, might provide a qualitative evaluation. Many contaminants could however be invisible when diluted and smoke generators (Section 10.5) may be necessary to find leakages (temporary or permanent) around the hood edges. 

I he round jet is most common in general ventilation and is used in local ventilation for spot cooling, for cleaning surfaces, or to direct air and contaminants in specific direction, e.g., into a large canopy hood or away from a contaminant generation point. The radial jet is not used much in local ventilation. 

A rectangular or a canopy hood is placed close to a wall with the opening lacing down. This could be called an open exhaust hood. The hood may have... 

Two or more plane jets can be placed above and outside the rim (all sides) of a canopy hood and directed downward. Fhe exhaust flow into the hood makes the down-directed jets turn inward and upward when the jet velocity has slowed down enough to be influenced by the exhaust flow. In many cases, the aim is to diminish the general supply airflow rate into the room and sometimes to use the jets as separators. lliis method is quite often used on large kitchen hoods to increase their capture efficiency. If the jet is directed toward the front of the fireplace and just reaches the front before turning inward, a high capture efficiency can be achieved. 

FIGURE 10.105 Two principal ways of using air jets to increase efficiency of a canopy hood. 

Source capture Low-level hood Canopy hood... 

The use of canopy hoods or remote capture of fume is usually considered only after the rejection of source or local hood capture concepts. The common reasons for rejecting source or local hood capture are usually operating interference problems or layout constraints. In almost all cases, a canopy hood system represents an expensive fume collection approach from both capital and opetating cost considerations. Remote capture depends on buoyant ait curtents to carry the contaminated gas to a canopy hood. The rising fume on its way to the hood is often subjected to cross-drafts within the ptocess buildings or deflected away from the hood by objects such as cranes. For many of these canopy systems, the capture efficiency of fume may be as low as 30-50%. 

It is only recently that technology has developed so that canopy-hood efficiencies can be predicted and measuted in a quantitative way. One of the difficulties in this atea has been the use of diffetent definitions for canopy-hood efficiency. Goodfellow and Bender have proposed a standard... 

The preceding results can be extended to relate canopy-hood performance to opacity. This is a significant step because air pollution legislation in many countries has a reference to opacity levels (i.e., a fume concentration level which must be met at the point of discharge of fume from a process building), jations can be developed of this form ... 

Following a similar approach to that used for low-level hcxtds, small-scale modeling is often pursued for the design of canopy hoods for a new facility or for modifications to an existing installation. >-i-24 Bender describes rests carried out... 

Canopy hoods A capture hood located above a process, designed to provide a suitable capture velocity to ensure the safe removal of the contaminant produced by the process. 

Open surface tanks having canopy hoods used for ... 

Laboratory aisles must be no less than 5 feet wide and benches should have sufficient unobstructed width to accommodate modern analytical instrumentation. An overhead (filtered) exhaust system would permit small canopy hoods to be connected as necessary to scavenge fumes from areas near injection and exhaust ports of analyzers not located in hoods. Each room should have its own supply... 

Because of obvious (and not-so-obvious) dangers involved, it is the teacher who does the firing in the activities in Chapter 4. In schools, electric kilns are usually chosen for firing because fuel-fired kilns require wood, natural gas, oil, coke, coal, or charcoal as fuel, as well as chimney ventilation systems. Electric kilns also require ventilation systems, such as negative pressure systems and a canopy hood, but the placement is more flexible than room placement for a fuel-fired kiln with a chimney ventilation system. 

A six-foot canopy hood mounted over a bench is situated against one wall. This bench is used to store general solvents... 

Physical Chemistry/Instrumental Analysis The Physical Chemistry laboratory, containing 20 student stations, is used also for Instrumental Analysis and for certain courses in the Community and Technical College which administers two-year associate degree programs. The laboratory is equipped with three four-foot induced-air hoods and a five- and a six-foot walk-in hood, each with a built-in vacuum rack. Against one wall are located four peninsular work benches ventilated by a single canopy hood. 

Against another wall, a microscopy table and a refractory table are also ventilated by a common canopy hood. All canopy hoods are exhausted together. 

The abatement of VOCs is hindered by the fact that large voliunes of ambient air are entrained by the collection systems (e.g. canopy hoods) used. 

Because of the sensitivity of the unit to maintenance of the air balance. Class llA cabinets are not ducted directly to the outside but, if it is desired to exhaust the effluent from the cabinet to the outside, a small canopy hood is used, surrounding the exhaust from the cabinet but separated from it by an air gap of about an inch. The air flow through this canopy hood is sufficient to capture the cabinet exhaust but because it is not connected directly to the cabinet exhaust, wiU not affect the sensitive air balance within the cabinet. 

---