Downward ventilation

When gas pressure produced in upward ventilated roadways, its direction is the same as fan pressure, which is positive for the fan when gas pressure produced in downward ventilation roadways, its direction is opposite to fan pressure, which is negative for the fan. 

Current mixing-type air distribution methods typically consider ventilation of the occupied zone with jets intercepting its upper boundary (e.g.. Fig. 7.6l2, c). Also, the occupied zone can be ventilated by the reverse flow produced as the supply jet degrades above the occupied zone level (Fig. 7.6h]. Mixing-type air distribution methods include air supply with jets projected vertically downward, inclined jets, jets directed vertically upward, and horizontal jets along room surfaces. 

Air supplied in confined space by downward vertical jets creates a similar flow pattern as in the case of air supply by horizontal nonattached jets. With vertical air supply, the occupied zone is ventilated directly by air jets. Grimitlyn suggests that the area of occupied zone ventilated by one jet be sized based on the jet s cross-sectional area at the point it enters the occupied zone. The jet cross-sectional area and configuration depend upon the height of the air supply, the type of air jet, and diffuser characteristics ( K, and K, ). 

Low-momentum air supply systems designed for local ventilation purposes mainly use vertically downward airflow. Some systems with an inlet of low-momentum horizontal airflow for a whole workroom are on the borderline between general and local ventilation and are therefore briefly described here. A more complete description will be found in chapters 7 and 8 dealing with general ventilation. 

A dripproof machine is an open machine in which the ventilating openings are so constructed that successful operation is not interfered with when drops of liquid or solid particles strike or enter the enclosure at any angle from 0 to 15 degrees downward from the vertical. The machine is protected against solid objects greater than 2 inches. 

In general, dilution ventilation alone is inappropriate for highly toxic substances, carcinogens, dusts or fumes or for widely fluctuating levels of pollutants. Since hygiene standards are often revised (usually downwards), specifications of existing systems may prove inadequate. 

The filter unit was located entirely inside the respective buildings and was independent of the ventilation system. The unit consisted of a suction box (usually floor standing) to which the filter material was attached usually as a flat vertical surface of an area to give a face velocity of about 0.75 m/s. Dust collected on this surface and was removed with a vacuum cleaner. Cleaned air was directed into an overhead discharge duct with a permeable lower surface. The capacity of the fan in the filter unit was made the same order as that of the main ventilating fan. The overhead duct was located above and as near to the stock as possible so that the clean air flowed downwards towards the stock. This air did not cause a draught because it was discharged at a low speed (0.3 m/s) and as it was recirculated it was at the same temperature as the air in the room. 

Fig. 15. Relationship between the alfentanil plasma concentrations and the probability of needing naloxone to restore adequate spontaneous ventilation. The diagram at the upper part shows the alfentanil plasma concentrations of the patients who required naloxone (upward deflection) or did not require naloxone (downward deflection). The plasma concentration-effect curve for this clinical endpoint (lower part) was defined from the quantal data shown in the upper diagram using logistic regression. Bars indicate SE of C5o%. (From Ausems ME, Hug CC, Stanski DR, Burm AGE. Plasma concentrations of alfentanil required to supplement nitrous oxide anaesthesia for general surgery. Anaesthesiology 1986 65 362-73, reproduced by permission.)...

Adequate ventilation in a downward direction because phosphine is heavier than air. 

Our studies showed that the biogeochemical system of the redox layer is subjected to temporal variability on a seasonal scale (connected with the seasonality of OM production) and interannual changes. Surface ventilation of dissolved oxygen down to the depth of the CIL (ag = 14.5 kg nr3) occurs in the winter from a combination of the NW shelf and the centers of the gyres. The intensity of ventilation is determined by climate forcing which may be determined by large-scale climate patterns like the NAO. This ventilation sets the upper boundary conditions for the downward transport of O2. Therefore, the position of the hydrogen sulfide boundary in the density field is connected with the climate variability, related to the NAO index. 

Mean room temperature was 20°C., and no thermostatic measures were taken, but the rates were recorded relative to that of one of the standards. Water and xylene received closer study (described below) to illustrate particular points. For the most volatile group three ventilating conditions were used (1) no deliberate ventilation, (2) gentle fanning, (3) more violent fanning with the wind directed about 45° downward onto the grid of dishes. The third condition visibly disturbed the liquid surfaces. The wind velocity at dish level was about 10 m.p.h. (used only for the less volatile groups). 

Electronic control enclosures as well as other electrical equipment are best placed at a lower level, and away from gas generating units, process lines and storage containers. This is because hydrogen rises rapidly away from the ground and normally disperses quite quickly. Such placement is, however, not a substitute for appropriate enclosures since a slight breeze or draft can blow a hydrogen emission sideways and/or downwards, even when adequate ventilation has been provided for the gas to escape upwards and not collect. 

In the subtropics, where wind-stress is convergent, water tends to be forced downward from the surface ocean into the thermocline. This downwelling is an important process for ventilation of the thermocline, and for driving the shallow gyre circulation. Figure 8 shows the measured tritium-helium age as measured... 

It has also been proposed that pyrite was formed by downward diffusion of hydrogen sulfide due to limited amount of iron in the deposited sediment (Passier et al, 1996). During the protosapropel formation the bottom water was probably still ventilated to some extent and the formation of HS was absent firom the bottom water or restricted to a thin blanket of water at the sediment-water interface. 

Ventilation air velocity is probably the single most significant parameter in combination with hoisting speed. Simulations have been carried out for ventilation velocities of 0,7.5 and 15 m/s. In all cases, the ventilation velocity is assumed to be downwards (intake). The base case skip orientation is cissiuned to be East-West as this represents the worst Ccise in terms of Coriolis force, cis it acts in the same direction as the steady state aerodyncunic forces (in the case of West skip travelling upwards). In all simulations, eight cases were modelled namely. East cuid West skips travelling up and down and headrope torque imbalance applied clockwise or counter-clockwise. 

When siting a capture hood or slot, advantage should be taken of the natural movement of the pollutants as they are released. For example, hot substances and gases are lighter than air and tend to rise, thus overhead capture might be most suitable, whereas some solvent vapours when in concentrated form are heavier than air and tend to roll along horizontal surfaces and pour downwards, so capture points are best placed at the side or below. Care must be taken to ensure that all contaminants are drawn away from the breathing zone of the worker - this particularly applies to places where workers have to lean over or get close to their work. It is important to note that whenever extract ventilation is exhausted outside, a suitably heated supply of make-up air must be provided to replace that volume of air discarded. 

FIGURE 3J8 Weather-cap dispersion characteristics. Deflecting weather cap discharges downward. (From Industrial Ventilation A Manual of Recommended Practice, 23d ed.. Committee on Industrial Ventilation, American Conference of Governmental Industrial Hygienists, copyright 1998, pp. 5-67.)... 

In synchronous interactions, the ventilator is sensitive to the initiation, modulation, and termination of a patient s effort (37). Synchrony can be assessed by noting patient comfort and by examining the airway pressure graphic. With synchronous gas flow, the airway pressure graphic remains convex and smooth, whereas when flow is not synchronous the airway pressure graphic is literally sucked downward by patient effort (Fig. 2) (38). 

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