## Filtration

Particle sizes Atmospheric contaminants fall into four classes: solid, liquid, gaseous and organic. Distinction between some members of these classes is not clear cut and not particularly important, but recognition of their existence is relevant. For this reason the following broad statements are made; they should not be regarded as a set of definitions. In […]

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## An application of the decay equation to changes of enthalpy

Throughout this book it has been usual to consider the steady-state case. Air has been supplied to the room being conditioned at a nominally constant temperature and since this temperature is lower than the design temperature in the room, the sensible heat gains to the room are offset. Under these conditions the cooling capacity of […]

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## The decay equation

Although from the point of view of a purist it might be better to derive the decay equation in general terms, it is certainly easier to understand a derivation phrased with reference to a particular application. Accordingly, the equation is derived in terms of the rate at which a contaminant decays in a ventilated room […]

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## Ventilation And a Decay Equation

The need for ventilation The minimum amount of fresh air required for breathing purposes is really quite small; about 0.2 litres s“1 per person (see section 4.10). For comfort conditioning, however, it is insufficient to supply this small amount of fresh air; other factors enter into consideration and enough fresh air must be delivered to […]

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## Fans in parallel and series

A pair of identical fans connected in series handles the same volume as a single fan but develops twice the fan total pressure. Thus, the characteristic data for a single fan running at 8 rev s”1 in example 15.14 would become, for two in series: 0 0.5 1.0 2.0 2.5 3.0 3.5 720 770 820 […]

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## The effect of opening and closing branch dampers

In any system it is usual to ensure that the correct quantity of airflows along the branches off the index run by partially closing regulating dampers in the various branches. (The alternative to this would be to size down the branch ducts so that they absorbed the correct amounts of energy by virtue of their […]

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## Methods of varying fan capacity in a duct system

There are four basic ways in which the capacity of a fan-duct system can be altered: (1) Changing the fan speed (2) Partly closing a damper in the duct system (3) Partly closing variable position guide vanes at the inlet eye of a centrifugal fan or changing the blade pitch angle of an axial flow […]

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## The performance of air handling units

The majority of fans used in building services today are fitted in air handling units. Under such circumstances the performance of a fan is quite different from that established by BS 848. Air handling units should be tested according to BS 6583 (1985). If a centrifugal fan is installed in a cabinet, as is the […]

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## Fan testing

In the UK performance testing is covered by BS 848: Part 1: 1980. The standard is comprehensive and internationally well accepted. It covers three types of installation: A: Free inlet and free outlet. B: Free inlet and ducted outlet. C: Ducted inlet and free outlet. D: Ducted inlet and ducted outlet. The standard prescribes the […]

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## Power-volume and efficiency-volume characteristics

Two further characteristic curves, power-volume and efficiency-volume, are also used to express the performance of a fan at a given speed and these are shown in Figure 15.20(a), (b) and (c), for three different types of fan. In Figure 15.20(a), for a centrifugal fan with backward-curved blades on its impeller, we see that peak efficiency […]

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