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Why are centrifugal pumps generally used to move water?

  1. May 15, 2015 #1
    Well the question says it all.
    On several websites I have found main 2 categories of pumps
    for air - diaphragm pumps
    for water - centrifugal pumps

    Why are centrifugal pumps not used generally for air applications?
     
  2. jcsd
  3. May 15, 2015 #2
    As I'm sure you have read, a centrifugal pump works by forcing a liquid to the outside of a doughnut-shaped vessel where it then exits the pump at high speed. The liquid experiences a 'centrifugal force' which causes it to press against the edges of the vessel. The amount of centrifugal force experienced by a rotating object depends on its mass. Since the mass of a given volume of air is tiny compared to the same volume of water, air won't experience very much centrifugal force and the pump would be very inefficient.
     
  4. May 15, 2015 #3

    Hesch

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    Generally centrifugal air pumps/air blowers are used when low noise is required.

    The major difference between air and water is density, whereas the force at 1 cm3 water will be much higher than the force at 1 cm3 air. The power supplied to water is higher than to air. The frictions as for air/water in the centrifuge ( resulting in losses ) will be comparable. Thus the efficiency of a water centrifugal pump will be higher than of an air centrifugal pump.

    ( I cannot calculate it, so the above written is based on intuition ).
     
  5. May 15, 2015 #4

    russ_watters

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    While this statement is true, it isn't the main issue -- though it does imply a key point: there is no fundamental difference between a pump and a blower or fan. Terminology-wise, "blower" and "fan" refer to air, so they can't be used to described water flow, but "pump" can refer to air or water. Regardless, the thing separating them when you focus on air is pressure.

    Fundamentally, any pump/blower/fan is a device that moves a fluid by generating a pressure difference. The difference between an air pump, blower and fan is colloquial in nature only: "fan" is generally used to describe low pressure, "blower" medium pressure and "pump" high pressure. So let's examine the different types of air movement devices:

    1. Fan. Low pressure. Application: The thing sitting on your desk, blowing air at you. If you can see the actual fan, that means there is no ductwork attached to it and therefore it operates at low pressure. These are axial, bladed, like propellers. They are capable of only very low pressures; Less than 1" W.G.

    2. Blower. Medium pressure. Application: HVAC air movement. These need to be able to generate some pressure so that they can force air through ductwork, filters and cooling/heating coils without those things having to be too large. Note, however, that for HVAC, "fan" and "blower" tend to be used interchangeably, though "fan" is the more common. It is only for specialty higher pressure applications (dust collectors, for example) that the term "blower" becomes favored. These are almost all centrifugal. Applicable pressure: 1-10" W.G.

    3. Pump. Higher pressure (above 10" W.G. or measured in psi, or to generate a vacuum). Most centrifugal, but sometimes positive displacement.

    And another level:
    4. Compressor. Anywhere from a few psi to hundreds. Depending on the flow rate and size needed, these may be positive displacement or still centrifugal.

    So as you can see, there are no hard and fast rules and the terminology overlaps, but the key difference is in how much pressure vs flow you need:
    -Low pressure and high flow = Axial
    -Medium pressure and medium flow = centrifugal
    -High pressure and medium flow = centrifugal
    -High pressure and low flow = positive displacement (piston/diaphragm)

    Regarding efficiency: for HVAC air movement, centrifugal fans are indeed generally the highest efficiency, generally running in the 65-85% range. You do sometimes see high pressure axial fans/blowers, but I'm not clear on the limitations that make them uncommon.
     
    Last edited: May 15, 2015
  6. May 15, 2015 #5

    Hesch

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    Maybe not, I just gave my answer to this question:
    In other words: Why are centrifugal pumps more popular in connection with water than with air. Then I commented that centrifugal air pumps indeed were popular if low noise was required. Example: ventilation of high-rise blocks.

    Subsequently I continued, speaking of efficiency which is higher with water ( 75 - 93% ) than with air.
     
  7. May 15, 2015 #6

    russ_watters

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    Just to be clear here, the main problem is that the OP's question was based on a false premise, so it can't be answered as-is, it needs to be corrected. I was trying to pay you a compliment by saying you were at least mostly correct. Here's the OP's wrongly premised question:
    The premise is wrong because centrifugal fans generally are used for air applications.
    Which is false. The efficiencies are pretty close between water and air.
     
  8. May 15, 2015 #7

    Hesch

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    What is false?
     
  9. May 15, 2015 #8

    russ_watters

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    Your statement that centrifugal pump efficiency is higher than centrifugal fan efficiency.
     
  10. May 15, 2015 #9

    Hesch

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    No, I wrote: Efficiency is higher with water ( 75 - 93% ) than with air.

    Full text:
     
  11. May 15, 2015 #10

    russ_watters

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    Yes, that's compatible with my paraphrase of what you said (I didn't specify the fluids). And it is, in general, not correct.

    More specifically:
    For the same volumetric flow, the first sentence is correct and the second is not. The friction (and pressure drag) for water is much, much higher than for the same volume of air because of the density.
     
  12. May 15, 2015 #11

    Hesch

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    Well, I think that almost all of the pressure drag is resulting in acceleration of the water/air.

    1 cm3 of water will at some speed will have an energy that is about 1000 times higher than air at the same speed. This factor must be compared to the difference in friction: You say that water-friction is much, much higher than air-friction, but is it a factor 1000 higher?
     
  13. May 15, 2015 #12

    OCR

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  14. May 15, 2015 #13

    russ_watters

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    Yes, but you are going about this question the wrong way. You can just look at the efficiency numbers of various pumps and blowers and compare them. They are pretty similar.
     
  15. May 15, 2015 #14
    Now that's an eye-opener.
    1. What about the power though? Would the fan used for water draw more power for an identical fan that moves air?

    2. Centrifugal blowers seem to be widely used in industry to move air and generate moderate to low pressures. They are also widely used in consumer items like gpu etc cooling or vacuum cleaners. What makes them different from pumps then?
    I guess both of them use the same principle?
     
  16. May 15, 2015 #15
    The air pump market seems to be dominated by the diaphragm pump (flow in upto few 10's of liters/min and pressure upto few 10's of psi).
    Comparatively why can't the centrifugal design compete with this.
    Is it singularly because of the density factor as 'sk1105' and 'Hesch' have pointed out?
     
  17. May 16, 2015 #16
    You might want to explain the air pump market to which you refer?
    The pump market is not just one but several.
    Also, what definition of "pump" are you using? - see Russ's post 4.

    Certainly an attempt to use a diaphragm pump to blow air through the radiator of a car would be a much more cumbersome and ill suited design than the bladed fan type that one generally sees.

    A positive displacement pump such as the diaphragm satisfies different necessary system characteristics over velocity pumps such as the centrifugal.
    One such characteristic from positive displacement pumps is that the Q of the pump is generally constant regardless of pressure.
     
  18. May 16, 2015 #17
    Probably I made a wrong statement, and I should delete it but I don't think the forum would allow me to do that now.

    Agreed!
     
    Last edited: May 16, 2015
  19. May 16, 2015 #18
    Hello.
    What kind of pump to use depends on service requirements. E.g. pressure, discharge rate, medium, temp, location, environment etc.
    Being a Marine Engineer I work with a variety.
    *Centrifugal pumps give a high discharge at low pressures. They generally produce a suction head of approx. 12mm water head when pumping water. They can pump air as well but require fine clearances and a very high speed. E.g. marine 2stroke engine blowers coupled to constant pr. Turbines (9k-13k).

    The answer to your question " why centrifugal pumps are GENERALLY used for water" is that generally requirements are for high discharge at low pressures. 2-10bar
     
  20. May 16, 2015 #19
    Agreed
    What are these clearances that you talk about?
     
  21. May 16, 2015 #20
    All clearances. As you can well expect air can leak out of more places than water. Usually these compressors are large but the clearances amongst the move parts is not equivalent to a pump of the same size.

    Centrifugal pumps of equivalent size are steam turbine driven Crude Oil Pumps (COPs) on Oil Tankers. They can pump out 8k m³/hr and run at very high speeds as well. Not surprisingly they can loose suction i.e. inability to pump air.

    Sorry that I'm out of touch and cant provide the exact data which the answer requires.
     
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