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Thread: Positive Displacement Vs Centrifugal Pumps - How to Select

  1. #1
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    Positive Displacement Vs Centrifugal Pumps - How to Select

    The most obvious reason to use a PD pump is when you have a high viscosity application. It is common knowledge that a centrifugal becomes very inefficient at even modest viscosity. However, there are many other reasons to select a PD pump over a centrifugal other than high viscosity. In fact PD pumps are very commonly used on thin liquids like ammonia and solvents.

    A simple rule of thumb is you should consider using a PD pump whenever you might be operating a centrifugal at other than at the BEP (Best Efficiency Point). Of course the further away from the BEP you get the more likely a PD pump will be a better choice. This can typically happen at low flow conditions, modest to high head conditions, or any type of elevated viscosity. As you can see from the efficiency curve it takes more horsepower to operate a centrifugal outside of its BEP. This horsepower has a cost, the initial cost of the larger motor plus a higher life cycle cost in energy consumed. Many times the PD pump will have a lower initial cost as well as a lower operating cost.

    Another reason to use a PD pump would be if the application has variable pressure conditions. A centrifugal pump will “walk” up and down the curve which can cause process problems. A PD pump will give near constant flow that makes it possible to match the flow to the process requirements. The desire to have constant flow is the reason that a PD pump is the pump of choice for metering applications.

    Obviously, if there is changing viscosity in the application the PD pump is the best choice. As can be seen from the charts, viscosity has a major impact on the centrifugals performance. Even a small change in viscosity, like 200-400 SSU, has a large impact on the centrifugal.

    PD pumps generally can produce more pressure than centrifugals. This will depend on the design of each pump but pressures of 250 psi (580 feet) are not unusual for a PD pump with some models going to over 1000 psi (2,300 feet). This is a significant difference between the two principles. The capability for a PD pump to produce pressure is so great that some type of system overpressure protection is required.

    Generally speaking pumps tend to shear liquids more as speed is increased and the centrifugal is a high speed pump. This makes the PD pump better able to handle shear sensitive liquids. Shear rates in PD pumps vary by design but they are generally low shear devices, especially at low speeds. Internal gear pumps, for example, have been used to pump very shear sensitive liquids. It is important to contact the manufacturer for specific information on shear rates and application recommendations.

    By their nature, PD pumps create a vacuum on the suction side so they are capable of creating a suction lift. The standard ANSI centrifugal does not create a vacuum so it can not lift liquid into the suction port. There are self-priming centrifugal designs that can lift liquid an average of 15 feet. This corresponds to a vacuum of 13” hg. Wetted PD pumps (a pump that is not full of liquid but with some liquid in it) can often reach vacuums of 25 to 28” hg. So a PD pump is the logical choice when there is a suction lift required.

    As mentioned earlier, PD pumps tend to run at lower speeds than centrifugals. This will have an impact on seal life, so PD seals tend to last longer than seals in centrifugal pumps. In addition, to assure adequate seal life a centrifugal will typically require one of the seal flush plans. A PD pump, because of its lower shaft speed typically does not need an external flush plan. Also, generally speaking, low speed mechanical devices tend to operate longer than high speed mechanical devices.

    At certain combinations of flow and pressure centrifugals are inherently inefficient, due to the design of the impeller and the short radius turn the flow must make. These applications are generally under 100 GPM but particularly under 50 GPM. A PD pump, by contrast, is very well suited for low flow conditions. Centrifugals, by contrast, tend to do very well in high flow conditions.
    source

    PD pumps wiki

    Did anyone try PD pumps before?

  2. #2
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    A positive displacement pump may be a great fit for some watercooling applications, specifically loops with very high flow restrictions. That said you can’t just replace your centrifugal pump with a positive displacement pump without some serious design considerations. The article sourced above mentions plenty of benefits for positive displacement pumps but few pitfalls. It’s not my intention to scare people away from them but hope this will help some people considering a positive displacement pump.
    1. Positive displacement pumps produce pressure pulses as they operate. The frequency of these pulses depends of the type of pump. A piston type pump will produce a lower frequency than say a gear type. The nature of these pressure pulses can be significant and can exceed the material characteristics of the tubing in the system; pressure rating of the radiator could also be an issue. Even a small positive displacement could damage tygon tubing so make sure you understand a pump’s design characteristics. In the case of a lower frequency pulse tubing will want to move around with the surges.

    2. Never run a positive displacement pump without a discharge flow path. Some positive displacement pumps will have a built in overpressure relief valve and some don’t. If you have a pump that doesn’t have one built in, you need to install one in the system in the form of a relief valve that creates a flow path from pump discharge back to the suction if the discharge flow path is blocked.

    3. There are quite a few different types of positive displacement pumps. The best suited, in my opinion, to watercooling would be a gear type or some variation on the gear type. Even then some gear pumps are designed to pump a liquid like hydraulic oil or fuel oil. As such they depend on these liquids to lubricate. A peristaltic pump might also work well especially since they use the same type of hose as watercooling, but it might be difficult to find one with sufficient flow rate to be suitable. Also peristaltic pumps make a very irritating sound in my opinion.

    4. There a few design reasons why positive displacement pumps are used; such as pumping high viscosity liquids, compressible fluids, a need for a self priming pump, pumping slurries, applications where the suction source is significantly lower than the pump suction, and in general where high pressure is required.

    All these caveats aside, I think positive displacement pumps could be useful in some people’s design, especially in chilled water systems.
    Last edited by Akula; 08-12-2010 at 08:05 PM.

  3. #3
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    Thanks for the reply.

    There are many options for pumps of different types, just found an online engineering catalog sorted by

    Maximum Discharge Flow
    Maximum Discharge Pressure
    Inlet Size
    Discharge Size
    Horsepower
    Power type (AC/DC)

    You may need to register (free)
    http://www.globalspec.com/ProductFin..._Control/Pumps

    There are good brands other than Iwaki or Pan World, more than your typical aquarium stocks. Have a look.

    247 Centrifugal Pumps, DC Powered, <15psi
    http://www.globalspec.com/SpecSearch...8067&Comp=3943
    (Iwaki RD-30 is 14.31psi)

    Perhaps someone can find something suitable for WC.

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    Xtreme X.I.P. Martinm210's Avatar
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    Doubt you'll find anything in the sub 30 watt DC power consumption range and flow rate we're looking for. PD pumps are fairly complex and generally for very specialized applications...haven't seen any as small as we need.

    Also pretty hard to make a PD pump that would outlast something like a Laing DDC or D5 with 50,000 hour MTF. Only wearing part is that tiny ceramic bearing.

    Would be cool if you could find one to play with though.

  5. #5
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    Not only PD pumps, but centrifugal pumps too. The online catalog is just too BIG, but I'm sure there is something that beats what we are being offered now (DDC or Iwaki). Have a look at the number of models & manufacturers. I remember someone was looking for a pump to transport ethanol/alcohol, should be available from the catalog.

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  6. #6
    Xtreme X.I.P. Martinm210's Avatar
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    See if you can find something with the following specs:

    • DC Power
    • Consumes less than 30-40watts, preferrably less than 20
    • Max Head is > 10' H2O
    • Max Flow Rate > 3GPM
    • Noise Level is acceptably quiet


    We have several good pumps in WC in addition to the DDC/D5, they just are not as popular.

    The DD CPXpro is a good lower cost one that would give similar results to a stock DDC.


    It's not going to do as well as a DDC with a top though, but pretty darn close in stock form.

    The Alphacool AP1510 is a strong one too.
    http://www.xtremesystems.org/forums/...d.php?t=170509

    Just not sure they have the same durability of the DDC/D5s...havn't done any long term tests myself.

    And there are more too

  7. #7
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    How are these for a start? Unfortunately, there aren't any heat dump or noise level values given by the manufacturers.

    Centrifugal Pump selection criteria :

    for comparison, Iwaki RD-20 (max head 26ft/8m or 11.27psi, max flow 4gpm or 240gph or 900Lphr)

    DC powered (note : variable units used)

    http://www.clarksol.com/html/prodspecsNH_PXDPump.cfm head=0.91-4.80m, flow=3.0-30.0Lpm
    http://www.clarksol.com/html/PI-Z-DPump.cfm head=34.6-83.0psi, flow=1.60-5.40gpm (looks very similar to Pan World models)
    http://www.gripumps.com/productsV01-cp.asp (Hi-Speed Series) 12VDC head=up to 16psi flow=up to 1.3gpm

    AC powered (note : variable units used)

    http://www.clarksol.com/html/prodspecsNH_PXPump.cfm head=1.20-11.0m flow=7.0-80.0Lpm
    http://www.clarksol.com/html/chemical_pump.cfm head=22.0psi flow=55.00gpm
    http://www.gripumps.com/productsV01-cp.asp (149 series - midrange) head=13.9psi flow=31.0gpm, remaining series better/lower specs than Iwaki RD20/RD30

    Is it really very difficult to implement PD pumps in WC loops? I've seen some small ones used in medical IV lines, they use tygon tubes too.

    edit : Add Water Puppy



    Jabsco Water Puppy $95.00 12VDC head=6m when primed flow=6.3gpm specs in pdf manufacturer link
    Last edited by pokipoki; 08-13-2010 at 08:14 AM.

  8. #8
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    Quote Originally Posted by pokipoki View Post
    How are these for a start? Unfortunately, there aren't any heat dump or noise level values given by the manufacturers.

    Centrifugal Pump selection criteria :

    for comparison, Iwaki RD-20 (max head 26ft/8m or 11.27psi, max flow 4gpm or 240gph or 900Lphr)

    DC powered (note : variable units used)

    http://www.clarksol.com/html/prodspecsNH_PXDPump.cfm head=0.91-4.80m, flow=3.0-30.0Lpm
    http://www.clarksol.com/html/PI-Z-DPump.cfm head=34.6-83.0psi, flow=1.60-5.40gpm (looks very similar to Pan World models)
    http://www.gripumps.com/productsV01-cp.asp (Hi-Speed Series) 12VDC head=up to 16psi flow=up to 1.3gpm

    AC powered (note : variable units used)

    http://www.clarksol.com/html/prodspecsNH_PXPump.cfm head=1.20-11.0m flow=7.0-80.0Lpm
    http://www.clarksol.com/html/chemical_pump.cfm head=22.0psi flow=55.00gpm
    http://www.gripumps.com/productsV01-cp.asp (149 series - midrange) head=13.9psi flow=31.0gpm, remaining series better/lower specs than Iwaki RD20/RD30

    Is it really very difficult to implement PD pumps in WC loops? I've seen some small ones used in medical IV lines, they use tygon tubes too.

    edit : Add Water Puppy



    Jabsco Water Puppy $95.00 12VDC head=6m when primed flow=6.3gpm specs in pdf manufacturer link
    That first link looks like the old 50Z

    http://www.dtekcustoms.com/50z-dc12m...ne12vpump.aspx

    They weren't bad little pumps if you could tolerate the high pitch whine noise they were claimed to make.


    As for an actual PD pump, it's not great but here's one with almost acceptable numbers (and price).

    http://www.shurflo.com/rv-products/r...s/default.html

    http://www.jcwhitney.com/water-pump/...41&zmap=346615

    Many PD pumps are also kind of noisy with lower viscosity liquids like water, and most are rather expensive for this hobby. Another problem with them is most are also rated for intermittent usage.
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  9. #9
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    Imho one can choose a bit higher priced PD pumps to compare to similar centrifugal pumps, as it seems one can count out price of no more needed for loop filling reservoir (self-priming bit).

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