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Air Diffusion

Air distribution for heating, ventilating and air conditioning systems ranges from directional jets for spot heating, cooling, or air make-up, to totally diffused air streams of uniform velocity and direction for scientific and other specialised uses. Each type of application demands that its own needs be satisfied and there is no single method of distributing supply air which will meet the variety of requirements encountered in practice. A design engineer must be familiar with the various means available to him and will make design decisions on major plant based upon the ability of the air stream to effectively offset the space load. The most common requirement is to meet the needs of human comfort, and in this regard, the thermal balance which matches the load to heated or cooled supply air, must be achieved within parameters beyond the simple issue of temperature equilibrium and must include air motion, relative humidity and noise level. The object of air diffusion in HVAC systems is to ensure an acceptable combination of these factors with the thermodynamic performance of the system. The key to the achievement of this is correct use of the principles of air diffusion.


Induction and Entrainment

The rate at which air from a jet can exchange heat with air from its environment is determined apart from ?t by induction and entrainment. For further discussion of this topic, refer to the section dealing with VAV air distribution. For the vast majority of HVAC applications, the faster this heat exchange is performed, the better, and this usually calls for modification of a free jet.


Ceiling Surface (Coanda) Effect

If a jet is directed parallel and in close proximity to any surface, and in particular a ceiling, its own motion creates a low pressure area between it and the surface, due to its inability to induce or entrain air from the environment bounded by that surface. This low pressure area causes a bodily shift of the jet so that it clings to that surface until its energy is either sufficiently dissipated, or some other influence such as high ?t or an obstruction separates the two. The resulting reduction in induced or entrained air flow allows the energy of the jet to carry it for a greater distance, and it is this feature which establishes the throw of ceiling or near ceiling outlets designed for comfort cooling. In the case of a sidewall outlet, this surface effect will be established if it is located such that its near boundary is within 300 mm of the surface in question - usually ceiling - and while it forms an angle of less than 40º with the surface.


Room Acoustics and N.C. Curves

The study of room acoustics is a science on its own, and it is normally beyond the scope of an HVAC engineer's brief to be precise in his specification of acoustic performance, this generally being a matter for specialists in acoustics for special purpose rooms. A general expectation however, can be identified by the use of NC Curves. In most cases, unintuitive understanding of the particular room acoustics is all that is possible and of course the room plays a most important part in achieving desired results.


For more information on these and many other engineering principles associated with Holyoake products  Holyoake users can download the Holyoake Engineering Data Catalogue here.


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