Axial and Centrifugal Fans Frequently Asked Questions

1. What Is Standard Air?

You will find many different figures for standard air but at Fans & Blowers we use a figure of 1.2kg/m3 at sea level at a temperature of 20 deg.C and RH of 65%.


2. How Do I Correct Standard Catalogue Fan Data To My Site Conditions?

You must correct for density differences. Eg if you require a pressure of 50mbar at 100 deg C you must correct this back to standard air by ratio of absolute temperatures.


3. What Is Absolute Temperature?

It is 273 deg.C or 460 deg.F ( deg F may be specified in enquires from the USA). for example above you would correct 50mbar by multiplying by 273+50 / 273+20 this would give you an equivalent pressure of 55.1 mbar as your selection figure from standard catalogue data.


4. How Does Electrical Regularity Change My Industrial Fan Choice?

This is a classic catch that various folks fall into. Standard list information would regularly be at 50Hz recurrence. This implies say 2 post engines keep running at ostensible 2900 rpm or 4 shaft at 1450 rpm. On a 60 Hz supply it would keep running at ostensible 3500rpm and 1750 rpm individually. (real full load speeds change by a team %) If you buy an industrial radial fan intended to give a predefined obligation at a 50Hz speed yet utilize it on a 60Hz supply it will run quicker in this manner giving more stream, weight and fundamentally, more power, likely over-burdening the engine. It is basic that the electrical supply recurrence is determined to the fan provider.Fan terms SUCH AS FAN, BLOWER,


5. Exhauster, Booster Etc Confuse Me, What It’s All Mean?

The easy respond is that it is just jargon that means diverse things to diverse people. However in general terms a blower would usually be a relatively low flow high pressure industrial centrifugal fan. Positive displacement machines, eg Rootes type are referred to as blowers and they work on a different principle to a centrifugal fan. An exhauster operates on a suction duty rather than blowing. A booster would normally be found in a closed system boosting from a pressure in excess of or less than atmosphere at its inlet to a higher pressure at its outlet. If you have any doubt contact Fans & Blowers Ltd Technical Sales dept.


6. What If My Industrial Fans OR Blowers Requirements Have All Sort Of Difference, Eg Altitude, Temperature, Humidity, Gas other Than Air Etc?

You must give the industrial fan manufacturer all the relevant data and tell them how your design figures are calculated. At Fans & Blowers Ltd our technical sales department will make all the necessary corrections to give the correct industrial fan selection.


7. Can I Just Speed My Industrial Fan Up To Make It More Powerful?

Most industrial fan motors these days are suitable for speed increase by invertor control. (But beware motors for hazardous areas may not be certified for use with speed control.) However you should be aware of the industrial fan laws because you will get an increase in flow, pressure and power and could overload your motor. You will also need to contact Fans & Blowers Ltd Technical Sales department to ensure the centrifugal fan is mechanically suitable.


8. So What Are These Industrial Fan Laws?

  • Speed Change.

flow varies directly proportional, pressure as speed squared, power as speed cubed.

For example double your speed and you will get twice the flow and four times the pressure but take  eight times the power!


  • Density Change.

Flow (volume) will remain constant (but not mass which will vary directly proportional)

Pressure and power will vary directly proportional to density.


There are other industrial fan laws based on size change or combination of all, but speed and density are the ones that the fan purchaser will be mainly involved in.


9. If My Centrifugal Fan Is To Operate On Suction Can I Use Standard Catalogue Data?

Yes but you must first make a correction for density difference just as you do for temperature. Standard centrifugal industrial fan performance data is usually as inlet volume and discharge pressure. If your industrial fan operates under suction conditions the density at the inlet will be less than at test conditions. The site data will need correcting by the ratio of absolute densities, consult Fans & Blowers technical sales department.


10. What Is The Differentiation Among An AXIAL Flow And A Centrifugal Fan?

An axial flow fan move its air beside its affiliation. They are generally easy fan, impeller mount on a motor shaft and contained in a circular casing for mounting directly into a run of ducting. Ideal for a low pressure, low temperature, low cost applications although special variations are available. A centrifugal fan takes the air in at its centre and turns through 90 degrees to discharge radially. The centrifugal fan is the ‘all singing all dancing’ fan as it can develop high pressures and in its different forms handle high temperatures, high dust burdens, be gas tight construction, fabricated in a wide range of materials etc.


11. What Information Must I Tell The Fan Manufacturer When I Am Looking For A Quotation?

Minimum basic data needed is flow and pressure. Additional data required is:-

  • Type of gas (eg air, methane, nitrogen etc) and its density if possible.
  • Any erosive and or corrosive elements in gas stream.
  • Temperature
  • Altitude
  • Is flow actual or mass (eg m3/hr or kg/hr etc)
  • Is pressure at discharge or suction or combination of both.
  • Is performance calculated at standard density conditions or actual site density.
  • Preference for direct or indirect drive.
  • Electrical supply.
  • Site conditions ( eg indoors/outdoors, dusty, hazardous etc)
  • Any specific requirements for materials, construction standards, motor standards etc.
  • Noise level requirements.
  • General description of application to assist the fan manufacturer to use their expertise in selecting the best fan for the job. ( eg Ventilation, Combustion air, pneumatic conveying, material handling etc)
  • Consult Fans & Blowers Ltd Technical sales dept for their assistance.
  • We cannot be experts in all applications but we have a combined wealth of experience in many.

To Select A Centrifugal Fan Or An Axial Fan You Can See

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