Downdrafting

Downdraft reactor wood, peach pits greenhouse... 

Sihca brick and large fireclay shapes are fired in circular downdraft kilns. These kilns vary in diameter and can accommodate up to 150,000 23-cm bricks or their equivalent in other si2es. The complete burning cycle for a typical periodic kiln ranges from 21 to 27 days as compared with four to seven days for a tunnel kiln. 

Care has to be taken to ensure that downdrafts do not cause the combustion products to be liberated in the occupied space. To achieve this, it is essential that the stack is of such a height that wind deflecting off adjacent structures does not influence the free passage of the products of combustion. 

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. 

Small versions of downdraft tables (less than approximately 0.5 m-) are used when small-sized chemical work is to be done on rabies instead of in laboratory-fume hoods (see Fig. 10.37). This includes work with low-momentum source (no initial velocity and near room temperature) such as laboratory animal experiments. 

FIGURE 10.37 Downdraft hood for small-scale laboratory work. 

Downdraft tables are also used for human dissection tables although rim exhausts (see Rim Exhausts) are usually used for this purpose. For manual shoe repair and during application of artificial fingernails, downdraft tables are the only type of exhaust possible. 

Very large downdraft tables have been used for sources such as electro-cutting and welding of large steel sheets. 

FIGURE 10.38 Downdraft table for ndtn etc. with side walls. This cable has two exhaust connections to distribute the flow over the enure surface. 

Downdraft tables are available for welding, but the efficiency could be quite low due to low suction velocities and large work pieces. 

The downdraft tables used tor small-scale chemical work are only recommended for small operations that generate contaminants near the working surface. The items that are used should not cover too large an area of the exhaust. Covering the surface increases the velocity and subsequently also the... 

Use of warm processes on a downdraft table should be avoided since the air velocity created by the exhaust is often lower than the velocity due to buoyancy effects. Effective use of a downdraft table for welding requires velocities high enough to counteract the buoyancy, which could result in disturbances of the welding process. 

Also, when using cold processes on a downdraft table, the worker should avoid leaning over the working place or sitting too close to it, since this could disturb the flow into the exhaust and increase the spread of contaminants into the workspace. The same effects as when using fume cupboards could easily be the result (Section 10.2.3.3). 

For small downdraft tables used for chemical work, a flow rate of 0.28 m s and m- table is used. This gives a mean velocity immediately above the surface of approximately 0.3 m s h This is a very low velocity, which can not capture moving contaminants. When these values are used, a maximum use height of 0.15 m is recommended, which should result in a small leakage from the source to the surrounding. This presumes there is at least 0.1 m of uncovered surface between the worker and the source and that the surface is covered to less than 30%. For these tables the pressure difference is between 50 and 100 Pa, depending on the density of holes. ... 

For the determination of downdraft risk in the winter case, three-dimensional and transient CFD computauons were performed using the TASC flow code. Boundary conditions were defined from the results of the thermal modeling. 

The three-diiuensional and dynamic CFD computations show a strong imer-mittenr behavior ol the cold-air downdrafts. Two-dimensional and steady-state models produce results which rarely reflect the real situation. 

Downdraft A natural or mechanical downward airstream, either that may, due to its temperature and/or velocity, cause thermal discomfort. In the case of a stack discharge, the term downwash may be used for the downward air current in the lee of the chimney that takes the smoke and other emissions below the emission discharge level causing ground-level pollution. 

Downdraft hood A hood positioned under a process that receives gases, vapors or dusts from the source above. 

Units preferably should be placed in the open and at least 75-100 ft from any large building or obstruction to normal wind flow. If closer, the recirculation from downdrafts may require raising the effective inlet air temperature 2-3°F or more above the ambient selected for unobstructed locations. If wind velocities are high around congested areas, the allowance for recirculation should be raised to greater than 3°F. 

The wind effect may assist or oppose the natural-draft flue, causing increased draft up the flue, or downdraft. For this reason, it is important to minimize their effects... 

Downdraft diverters Any flue should be designed as far as possible to produce a constant draft. If the draft is insufficient then spillage of combustion products will take place, and if too great, then excess air will be induced, leading to an unnecessary loss of efficiency. 

A typical downdraft diverter used with natural-draft flues is shown in Figure 19.5. This consists of a baffle plate mounted at the top of the primary flue. There are three modes of operation ... 

Adverse conditions with down draft - the down draft mixes with the products of combustion and diluted products enter the room for the brief period that the downdraft persists. The draft in the primary flue (and hence the performance of the appliance) is unaffected. 

The availability of very small areas of control can be advantageous where electronic apparatus with mild chimney effect would have its natural cooling upset by strong downdrafts, by confining air supply to gangways. 

Type of heating in use - Thirteen downdraft-type steam unit heaters, controlled by an optimizer and internal air thermostats. 

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