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General Guidance for Polyfan Fan and Component Selection

For basic configuration of Polyfans units all parties concerned with the selection process need to consider suitability and compatibility of components that combine to provide the finished product. It is assumed that before embarking on this product design selection process the client or their appointed representative has satisfied themselves that a direct driven polypropylene centrifugal fan from the Polyfan range is suitable for the intended application.

In the example that follows an assumption is made that electrical motors will be used to drive the Polyfan unit, and where the purchaser obtains components only for self assembly that they have sufficient knowledge and skill to assemble the components.

All information given should be cross-referenced with the 'Polyfan catalogue' provided.

STAGE 1 - POLYFAN SIZE AND DUTY
STAGE 2 - MOTOR SELECTION
STAGE 3 - POLYFAN COMPONENT SELECTION


STAGE 1 POLYFAN SIZE AND DUTY.

Using the fan curves provided in the 'Polyfan catalogue' select a fan that can provide the performance required. The Polyfan performance curves provide information on
  • Volume flow Q (in m3/h)
  • Pressure HT (in mm)
    note: Pa =HT x 9.80665
  • Fan outlet velocity in Vu (in m/sec)
  • Fan outlet velocity pressure Hd (in mmH20)
    note: Pa = mmH20 x 9.80665
  • Absorbed Power HP
    note: kW = HP x 0.746
  • Rotational speed N (in revs per min)
  • Efficiency shown as %
It is preferable to select a fan that operates within its most efficient zone on the fan curves. For this exercise we have assumed that the requirement is for a direct driven fan to provide 400 m3/h against a total pressure of HT10mm (98 Pa) with an outlet velocity below 7m/s when operating at a 6 pole speed (less than 1000 RPM) with an absorbed power in the region of  0.15HP (0.11 kW) using a 240 volt single phase supply.

By reference to the Polyfan fan curves firstly consider the volume flow of 400 m3/h against a pressure of HT 10. There are several fans capable of providing this duty but as a direct driven machine the VSB 20 operating at 950 RPM (6 pole) is the one selected. See 'Polyfan Calculation Sheet VSB 20' below.

Follow the horizontal line for pressure HT10 (98 Pa) across to the right until it dissects the vertical line from 400 m3/h. You will note at this position the two lines dissect between the 54% and 56% efficiency line and are below  the dark arced line denoting the rotational speed of 950 rpm (6 pole).

Reference to the broken chain dotted arc line will confirm that the absorbed power is below the 0.15 HP (0.11 kW) specified. Projecting vertically down from the 400 m3/h point the outlet velocity is confirmed at 5.3 m/sec and extending this line indicates the outlet pressure as 1.8 mmH20 (18 Pa).

From the information obtained above, it is confirmed the VSB20 driven by a 6 pole motor should provide the performance required. The next stage will consider implications of the direct driven motor selection for the VSB20 in this example.  Selection should consider that motor manufacturers quote motor speeds as 2, 4, 6 & 8 pole. Dependant on supplier and type with a 230/240 volt 50 Hz 6 pole motor speeds can be in a band of 840 to 920 RPM + or - 10%.

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STAGE 2 MOTOR SELECTION

The Polyfan range can be connected to most electric motors subject to the frame size of the motor being suitable to accept the impeller, impeller fixing screw, back plate closing flange, sealing gland and mounting stool. In the design of the Polyfan range consideration was given to European metric framed motors and standard frame sizes available.

In response to request for standardisation of motor sizes within the range it was decided that 0.18kw would be the lowest power of motor used and kw powers below 0.18kw illustrated for the VSB14, VSB20, and VSA 20 have been uprated to 0.18kw.    A Polyfan VSB 20 operating at 1450 RPM (4 pole) would be fitted with a 0.18kw 4 pole motor in a 63 frame and 11 mm dia shaft.  For this example the 240 volt single phase Polyfan VSB 20 operating at 950 RPM (6 pole) will be fitted with a 0.18kw 6 pole motor in a 71 frame with a 14mm dia shaft.

All Polyfan units require the shaft to which the impeller is secured to have a tapped thread in the shaft and a keyway. The fundamental fixing arrangement relies upon the keyway providing rotational stability whilst the threaded shaft accepts the specially designed encapsulated fixing screw to secure the fan impeller  onto the shaft. A further requirement is for the shaft length to be sufficient to accept the impeller boss and fixing screw.

The voltage at which the fan will operate will also effect the motor fitted. It is often the case that the motor frames for an identical fan case to provide identical duties will vary in the motor frame and impeller bore as a consequence of one being 240 volt and the other being 38/440 volt.

For guidance we have prepared a schedule of the more commonly used impeller, back plate and fixing screw arrangements ' Polyfan impeller sizes/Rotations schedules'. It is important when specifying motors for Polyfans to ensure consideration is given to the following.

  • Motor power, voltage and pole speed.
  • Motor frame has a female thread to suit the Polyfan fixing screw for the fan size.
  • Motor frame has a standard metric keyway.
  • Motor full face flange is suitable for the Polyfan back plate closing flange.
  • 'Polyfan Impeller size and rotational schedule'.

For the VSB 20 shown in the example the motor selection is as follows

  • Motor Power 0.18KW  
    Sufficient for the 0.05KW absorbed power.
  • Motor Speed 6 pole (850 RPM) 
    Sufficient for RPM required.
  • Motor Shaft 14mm dia tapped M5 x 25 deep with 5mm keyway and B5 flange. 
    Suitable for VSB/20 impeller see 'Polyfan impeller sizes/Rotations schedule'.
  • Motor voltage 230/240 volt single phase 
    Suitable for clients specified voltage.
  • Motor mounting foot mounted B3 with B5 flange 
    Suitable for 71 Polyfan closing flange see 'Polyfan impeller sizes/Rotations schedule'.

For guidance the 'Polyfan catalogue' has a small table showing some typical performance duties with motor speed and power. In some instances to facilitate motor frame compatibility it may necessary to consider a motor with a kW capacity exceeding those shown.  Selection should consider that motor manufacturers quote motor speeds as 2, 4, 6 & 8 pole. Dependant on motor supplier and type with a 380/440 volt 50 Hz 6 pole motor speed can be in a band of 820 to 960 RPM + or - 10%.

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STAGE 3 POLYFAN COMPONENT SELECTION

The components that can be interchanged on a known Polyfan fan size (to suit the specified fan frame) are the back plate, closing flange and impeller. Reference must be made to the 'Polyfan impeller size/rotations schedule' to ensure the size required is available.

For a known rotational handing, for example with a Polyfan VSB 20, whilst the fan case, inlet, gasket and stool are a common components, there is a selection of three closing flanges and three impeller bores.

When ordering components state clearly the requirement. Italics are examples.

  • Polyfan Type and size - VSB 20
  • Handing of Casing - LGO
  • Closing flange type - 71 x 14 for 130mm PCD B5 standard mount
  • Impeller Rotation - LG clockwise at inlet
  • Impeller hub bore - 14 mm dia.
  • Impeller fixing screw - M5 x 25mm
All orders should confirm the numbers of each component required.

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