Fans and ducting systems

A theme of this book has been that the fan and its system interact. Performance is not solely the responsibility of the fan manufacturer or the system designer. Each has his own tasks in achieving that harmony, when the two are in balance.

Fans and their ducting systems have to be in balance i. e. the system resistance (or back pressure of a system) and the fan pressure are equal. This normally only occurs at one volumetric flowrate if the fan characteristic has a negative slope and the system characteristic is rising.

A system will have a number of components each of which will have a pressure loss which is a function of the velocity of air or other gas which is flowing through it.

It is essential to realise that the capacity of a fan is not fixed, but is determined to a great extent by the system which is attached. Hence this concept is continually repeated in many of the chapters.

This Chapter looks at the problems in more detail and perhaps emphasises the need for continual dialogue between fan and system engineers. Buying fans through a purchasing department committed to spending the fewest bucks is fraught with danger.

But ductwork designers appear to know little of system effect factors — an aim of this Chapter is therefore to rectify that deficiency. Hopefully, it will lead to the reader looking for the other references given.

Just as fans have laws which govern their behaviour, so too have their systems. Fan systems can be an assembly of ducts, filters, coolers, heaters, dampers, louvres, terminal devices, screens etc. Alternatively, it might be a boiler, economiser, pre-heater chimney stack and associated flues. Yet again, it could be a dryer, heater and ducting or a dust collector, hoods and ducting. The variety of systems is virtually endless, but some of the more popular are described in more detail in Chapter 21.

Most systems draw air, or some other gas such as flue gas, from one space and discharge it into another. The means of producing this air movement in a controlled fashion is by the use of a fan with its prime mover.

Posted in Fans Ventilation A Practical Guide


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