Power absorbed by the fan

This will be obtained from the duty requirements of air/gas vol­ume flow, pressure to be developed, and known air/gas condi­tions at fan inlet. It is also necessary to consider how all these factors may vary during fan operation.

For example, it is usually difficult to assess accurately the fan pressure. The system designer often therefore adds a “safety margin” to his calculated pressure to ensure that he achieves the design flow. If he can subsequently add in additional resis­tance by orifice plates or similar to bring the flow back to specifi­cation then there will be no problem. Alternatively he may be able to partially close a damper in the system to dissipate the unwanted pressure. If this is not possible, and the speed can­not be changed, then the fan will handle more air and this may affect the power consumption.

With “non-overloading” fans fitted with backward inclined back­ward curved, or aerofoil, the volume flow against power curve is relatively flat over the working range, i. e. an increase in capac­ity with reduced pressure has only a small effect, if any, on the power absorbed. With impellers having blades radial at the out­let, i. e. shrouded radial, open paddle, backplate paddle, and ra­dial tipped, the power increases uniformly with capacity.

The forward curved impeller has a flow versus power curve, which increases ever more rapidly towards the “free air” or zero pressure condition. Forthis reason it is suggested that the mar­gins given in Table 13.1 be added to the fan absorbed power, simply to cater for the normal inevitable errors in system resistance calculations.

Where the system resistance is accurately known, or where a small loss of capacity is acceptable, then it may be possible to reduce these margins.

It is also important to know if the power absorbed can vary with time. In a ventilating system with a fan handling “outside” air the only variation will be that due to a variation of air density with changing barometric pressure or ambient temperature. Calcu­lations of both fan duty and system resistance are normally made under “standard” conditions i. e. with air having a density of 1.2 kg/m3. Typically this would correspond to dry air at a tem­perature of 20°C and a barometric pressure of 101.325 kPa. Al­ternatively air at 16°C temperature, 100 kPa barometric pres­sure, and 62% relative humidity also has the same density. Between summer and winter there will be variations in both temperature and barometric pressure, and these will affect the air density. Typically temperature could fall to -3°C (270° K) and barometric pressure could rise to 105kPa. The effect on air density would then be:

273 + 20

1.35 kg/m3

101.325 273-3

I. e. an increase of 12%.

If such variations in conditions do occur, then the necessary margin must be allowed. A possible alternative is for the motor to be “overloaded” for short periods of time. This is not necessarily a danger, as motor performance (usually determined by winding temperature) can improve at low temperatures.

A more important case of varying temperature would be for hot gas fans where the starting condition could be with ambient air, but the normal condition is at a reduced gas density. The motor may have to be rated to cover the higher horsepower, although where the working temperature is rapidly achieved, the margin can be minimal. Often in such cases a damper is incorporated in the system. This is closed either fully or partially on start-up and opened when the temperature is achieved. The fan motor need then only be rated to cover the hot gas conditions, pro­vided the power with damper closure is materially lower. An example will illustrate the problem.

Example of a hot gas fan starting “cold”

A fan has an absorbed power of 75 kW when handling gas at a temperature of 325°C. It is started on air at 20°C with the gas-tight damper in the system fully closed. Reference to the fan characteristic curve shows that the power at zero flow is 35% of that at the rated flow.

273 + 325 35

————————— x————- = 53.4 kW

273 + 20 100

If the fan had been started on air at 20°C with a fully open damper, the power would have been:

273 + 325 273 + 20

The power at zero flow is a function of the fan design. Generally the narrower the fan, the lower will be the percentage of maxi­mum. Backward bladed fans have a higher zero flow power than forward curved, with radial intermediate. If the percentage was 50% then the power at zero flow would be:

273 + 325 50

75 x——— x—- =76.5 kW

273 + 20 100

This is higher than the duty power. At intermediate flows, the power being a greater percentage of maximum, care will need to be taken to ensure that the temperature has risen sufficiently. If not, the power absorbed could rise significantly above the start-up and duty conditions. The motor will need to be rated for the highest power consumption.

It should be noted especially that many dampers are not com­pletely gas tight and allow a flow even when fully dosed. This may typically be of the order of 5% to 10% of the rated flow. The power underthese conditions can be significantly higher than at zero flow, dependent on the shape of the fan/power character­istic. Reference to the curves is therefore recommended. There is also an additional power loss in the transmission, be it a belt drive or coupling. This is discussed in Chapter 11.

Posted in Fans Ventilation A Practical Guide


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