Conclusions

The advantages of maintaining a good fan efficiency across the range of operating points are clear — low running costs which can lead to the additional capital cost being recouped in a very short period of time — often less than two years. A high efficiency impeller may not necessarily be more expensive as, with a re­duction in internal losses, the fan may even be reduced in size for a specific duty.

In an age of aggressive marketing, care must be taken to read beyond the advertising “blurb”. No form of flowrate control is applicable to all types of system and the user must distinguish between the different types of system. Speed control bytheuse of inverters with induction motors is not a universal panacea.

Graphs of the type shown in Figures 6.12 and 6.21 are com­mon, but attention is again drawn to some of the assumptions made and to the fact that they are only applicable to fully turbu­lent constant orifice systems, where PiQ3« N3. It must be ap­preciated that they are approximate and that they refer to spe­cific items of equipment. The full cubic power saving is never achieved in practice. The general conclusions are, however, valid.

In the analysis, the backward bladed fan has an assumed static efficiency of 80%, whilst for the forward curved and variable pitch axial, this is 60% both at the design flowrate. The differ­ences would be smaller if both axials and centrifugals were se­lected on a total pressure basis as recommended in the fan test standards ISO 5801/2. Special attention is drawn to the use of wide backward bladed centrifugal fans with 2 speed (dual wound 4/6 pole shown) motors and disc throttle dampers. This is a relatively cheap installation rivalling more sophisticated methods in its control efficiency.

DC motors with thyristor control surpass all others, but AC mo­tors with inverter drives are almost as efficient and much more reliable. Both enable high efficiency centrifugal fans to match the power savings of variable pitch axial flow fans.

A • Backward bladed fan • ouM damper ■ B — Backward bladed fan • radial vana in tat control

— —————— B2 • Backward biadad fan • 2 apaad motor — radial vana control

——————————— C — Forward curved fan • outlet damper

——————————— D — Variable pitch axial Row fan

— —- E — Backward biadad fan • diac throttle

————— ————- Ej — Backward biadad fan — 2 speed motor — diac throttle

——————————— F • Backward biadad fan — DC motor — thy rt»tor control

Conclusions

Percent of design flow

Figure 6.21 Power savings for damper and speed control

Speed control, whilst the preferred method for constant orifice or fixed systems, and also usable in many constant flow sys­tems, is not applicable to constant pressure systems.

You would expect a fan manufacturer to say it, but more care should be devoted to selection of appropriate equipment. Where comparisons are to be made on the basis of absorbed power, certification schemes such as those provided by AMCA and Eurovent become necessary. Performance data needs to be independently validated.

Remember that:

Q

= flowrate (m3/s)

Pf

= fan (total) pressure (kPa)

Nf

= fan (total) efficiency (decimal)

Tim

= motor efficiency (decimal)

Lit

= transmission efficiency (decimal)

Tic

= control efficiency (decimal)

P

= input power (kW)

P(Power input) kW =————————

T]f x rim x X Tic

The need to avoid unnecessary energy conversions is obvious, and direct drive fans should be considered wherever possible.

ETSU, BRESCU and their more recent successors, and oth­ers can take justifiable pride in the manner in which they brought to public attention, the reduction in running costs by changing from normal to high efficiency motors, when a saving of perhaps 5% can be made. How much greater would be the savings if the many fans with impeller efficiencies of 50 to 60%

Were changed for units having efficiencies of greater than 75%, and if appropriate fan regulators were fitted which were matched to their systems.

There is, of course, one foolproof method of saving power. Don’t leave a fan idling! Switch it off when it is not doing any use­ful work.

A particular example of this technique may be found in some bulk storage grain drying plants. Here the fan is controlled by a hygrostat and can only be run when the ambient air has a mois­ture content below the equilibrium moisture content of the grain, thus permitting some useful drying to take place without the need for auxiliary heat.

Posted in Fans Ventilation A Practical Guide


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