Flow regulation

This Chapter reviews a number of the factors affecting the efficient utilisation of energy in fans, their systems, their prime movers and especially their flowrate controls.

It is useful therefore to re-examine the fundamentals and it is hoped that the resulting conclusions may be of value to system designers, users and energy managers.

No one method of flow regulation is applicable to all applications. How the system resistance varies with flow, and whether there is a fixed element, very much determines the choice.

It is important to emphasise that no one method is applicable to all systems. Whilst speed control of induction motors by inverters is currently the most popular, there are situations where, because of a fixed element in the system resistance, other methods are more appropriate. This Chapter gives the necessary information.

Energy costs rose considerably during the 1970s following a succession of crises affecting the Middle East oil-producing na­tions. Despite a temporary respite in the 1980s following a rapid increase in North Sea oil production, and the discovery of other sources, this escalation continued in the 1990s. In the 21st cen­tury there is also now a “green” issue to be faced in the realiza­tion that continued burning fuels is leading to ever increasing levels of C02 in the upper atmosphere. Global warming is now largely accepted as a possible threat to mankind.

For all these, and many other reasons, the spotlight of effi­ciency has been directed to the reduction in energy consump­tion of all types of machinery, but none more so in fluid or turbo-machinery such as fans, pumps and compressors. Such concerns need not — indeed should not — be solely altruistic. The savings in running costs can usuallyjustify a small increase in first cost, even for the humble fan.

If “carrot" is not enough, however, we have in some areas to contend with a little “stick”. Recent changes to the UK’s building regulations, for example, encourage the installer to design air conditioning or mechanical ventilation systems to meet defined energy targets. We even have to contend with a new found en­thusiasm for “natural” ventilation. Preference will in any case be given to plants which incorporate efficient means of flowrate control such that supply and demand can be more closely matched all times.

This Chapter reviews a number of the factors affecting the effi­cient utilisation of energy in fans, their systems, their prime movers and especially their flowrate controls.

All or some of the following strategies should be considered.

A) Ensure that the plant is only in use when required.

B) Use some form of capacity control to match the flow to re­quirements.

C) Prime movers to be of high efficiency and matched to de­mand.

D) Keep plant and motors in largest possible units, consistent with a) and b) above.

E) Reduce system resistance to a minimum.

F) Make no unnecessary energy conversions.

None of these strategies should give rise to any surprise amongst practising fan engineers. We are, however, in an ad­vertising age where the advantages of high efficiency motors and of inverter controls have been trumpeted to the disadvan­tage of the others. It is useful therefore to re-examine the fun­damentals and it is hoped the resulting conclusions may be of value to system designers, users and energy managers.

Posted in Fans Ventilation A Practical Guide


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