Air Jets

10.3.2.1 General

Air jets are used for many purposes. Some of these are described in other parts of this chapter, but it is not possible to describe all the possible types and uses. The fundamentals regarding velocity, flow rate, and spreading of round, radial, and plane jets are described in Sections 7.4 and 7.7. When jets are used inside rooms, they do not need to have any corresponding exhaust air. Ex­hausted air is needed for supply jets in general ventilation, but if the jet’s air is taken from the room and blown into the room again, no exhaust is needed. If the air is taken from outside the room, it is necessary to have the same flow rate exhausted from the room.

10.3.2.2 Principle

The main principle is to blow with a velocity higher than that of the sur­roundings And In this way create a transport of the air in one or more defined directions. The air could be blown to form plane, round, or radial jets. The function of the jet could be to shield a surface, to cover an opening, to coo! a person, to transport the air a long distance, or just to change the air velocity inside a closed volume. The air blowing creates a suction effect along the jet and the jet mixes surrounding air into itself and increases the moving flow rate, at the same time decreasing the jet velocity.

Jets used in local ventilation have the same forms and performance as jets in general ventilation, described in Sections 7.4 and 7.7. These sections de­scribe usable equations for flow, velocity, temperature, and concentration dis­tributions. The buoyancy plumes that can result at the end of a jet or from a warm source are described in Section 7.5.

10.3.2.3 Applicability of Sources

The plane jet, mostly blowing vertically, is used to increase the isolation between two volumes, one of which could be the inside of a cabinet, e. g., a laboratory fume cupboard. The plane jet could also transport air along a wall or be used to dry or cool the material in a pulp or paper line.

The 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.

10.3.2.4 Different Forms and Boundaries Relative to Other Types

Sometimes it is necessary to include the exhaust air when designing an air jet system (see Sections 10.4.2 and 10.4.3). Normally the descriptions assume that the jet is not disturbed by exhaust air, nearby surfaces, or other air move­ment in the room (see Chapter 7).

10.3.2.5 Specific Issues

Each jet must be designed for a specific function. A jet could easily trans­port or increase the capturing of contaminants, but could also very easily de­stroy the intended function of a local ventilation system.

10.3.2.6 Design Equations and/or Parameters

Many different equations describe jets. Some of these are used in Chapter

7. All jets must be placed in such a way that the mixing of surrounding air is not restricted. If it is restricted the jet can behave in an unpredictable manner. For example, a wall close to a plane jet nearly always makes the jet attach to the wall and changes the jet from a free jet to a wall jet.

10.3.2.7 Disturbances

It is quite easy to disturb the flow in a jet either by inserting a small surface at right angles close to the outlet or by using a small air velocity far from the outlet. Usually the latter needs a large flow rate, but a small flow rate with high velocity may also change the direction of the jet, especially as jet velocity decreases.

To change the direction of the jet using another air jet requires that the second jet have a very high velocity near the outlet. To change the direction of a jet far from the outlet using a surface requires a very large surface.

One of the most essential parameters for a jet is the direction. A small de­viation in the outlet angle could easily change not only the direction of the jet but also its intended function, be it as a shield or for air transport.

10.3.2.8 Changing Flow Rates

Isothermal jets are not influenced much by small changes in flow rate; the size of the influence can be seen from the different equations. Nonisothermal jets could be changed substantially by small differences in both outlet velocity (flow rate) and/or temperature.

10.3.2.9 Evaluation Procedures

A jet intended for mixing or for a long blowing range could be evaluated by measuring the velocity at different points along and across the jet. It could be necessary to measure the turbulence intensity of the jet, since this changes the transport properties in the jet. This is more important for plane jets intended to function as shields. For evaluating the shielding effect, tracer gas or particles is necessary. In these cases, the measures described in Section 10.5 can be used.

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