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General: MVHR units

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      CommentAuthorJSHarris
    • CommentTimeMay 19th 2012
     
    <blockquote><cite>Posted By: SteamyTea</cite>Are fans limited in the same way as turbines? S have a max of a nats under 60%, put a restriction in the way and it drops rapidly.</blockquote>

    Not really, the Betz limit doesn't apply. I used to play around with hovercraft years ago and we aimed to run lift fans at around 65%, more if we could get it. It's fairly easy to get a fan to operate at around 80% or more under fixed flow conditions, but efficiency falls off quite rapidly if the conditions change away from optimum.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 19th 2012
     
    Not sure I understand that fully, is is similar to putting a venturi cowl around a turbine and then claimant the limit is broken.
    Admit that I have not spent much time looking at fans, though I need to as I want to make a MkIII version of my home made heat exchanger (currently MkII is have a few days being constantly on to see what is happening with the temperatures).
    I have a gut feeling that bigger is better then it comes to fans, but then then changes the dynamics of the heat exchanger, though I think I can overcome this easily enough.
    •  
      CommentAuthorJSHarris
    • CommentTimeMay 20th 2012 edited
     
    Bigger is better, no question. A big fan turning slowly is more efficient than a small fan turning quickly. The same goes for propellers.

    Right now I'm finishing a small light boat for the Cordless Canoe Challenge, a race for boats powered by cordless tools (see here for last years event: http://www.youtube.com/watch?v=t82OL0YObw4). Much testing, development and experimentation has shown that I can get a measured prop efficiency of about 88%, by using a large diameter, narrow blade, prop turning quite slowly. In comparison, a typical outboard prop will be around 60% efficient.

    The same rules apply to props as to fans, but fans have the added advantage of running in a duct, so they have lower tip losses (much of the wasted power on a prop comes from vortices that spin off the tips).

    For good efficiency a fan needs to have a low pressure ratio for the required flow. The pressure ratio is just the dynamic pressure difference between the inlet and outlet. If you increase the area swept by the fan, then for a given volume flow rate the velocity drops. As dynamic pressure is proportional to the square of velocity, this results in a significant pressure ratio change.

    The other key factor is to get the flow resistance of the heat exchanger down to as low a figure as possible, as this then reduces the pressure ratio further.

    The Betz limit only applies to extraction of power from an airflow, as it is based on the fundamental principle that a 100% efficient device would result in no air flow from the outlet (because all the energy in the incoming air had been extracted). Clearly this cannot ever happen, so Betz calculated what he thought was the most you could get (and it's arguable whether he got it right).

    A fan can still operate with no inlet airflow, or no outlet airflow, although efficiency drops to zero for this case, so Betz doesn't apply. Just as well, really, as the fans that propel the majority of big aircraft run at around 80% or more efficiency, way over the Betz limit.
    • CommentAuthorJoiner
    • CommentTimeMay 20th 2012
     
    By heck, that 'King Canute' boat motored well!

    Brilliant. Must have been a LOT of fun. :bigsmile:
    •  
      CommentAuthorjoe90
    • CommentTimeMay 20th 2012
     
    I cannot wait to retire and spend my time doing pointless things that are fun. Beats working for a living Eh.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 20th 2012
     
    I have a kayak and a cordless drill, hope I loose my job soon as I just heard that we are in for some good weather.:cool:
    • CommentAuthorJoiner
    • CommentTimeMay 21st 2012
     
    Hedonist. :surfing:
  8.  
    Posted By: GaryBA good figure is 0.4-0.5 W/l/s and a bad one in my opinion (and EST's) is anything above 1.0 W/l/s.
    The DV71 is one of the stinkers at 2.31 to 3.06. The DV75 is the best performer of the Airflow Developments range at 0.64 to 1.0, depending on the fan speed/number of wet rooms.
    We measured the FiWi Breathing Window at 6W at 50 m^3/h over the weekend, which is 6 /50/1000 * 3600 = 0.432W/l/s, so we're happy. We're using a new fan soon that should improve that figure!
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 22nd 2012
     
    Posted By: Viking House0.432W/l/s,

    So 0.43 J.l^-1
    • CommentAuthorborpin
    • CommentTimeMay 22nd 2012 edited
     
    To bring this back on topic.....

    Posted By: GaryBYou should check out the specific fan power of any MVHR unit you wish to install - you will be paying the electricity 24/7 and I'm afraid to say that the SFP of the DV71 unit is especially poor.
    Had a chat with Airflow today and asked about the SPF of the unit.

    The response was that this was due to the inclusion of a pollen filter in the unit making it harder for the unit to draw the air due to greater resistance. I can see nothing on the SAPQ website that has any detail on what the filters in use were when tested.

    Am I being fed a line or is it a valid argument for higher SPF?
    • CommentAuthorEd Davies
    • CommentTimeMay 22nd 2012 edited
     
    First DuckDuckGo find for “air filter pressure drop” is:

    http://ateam.lbl.gov/Design-Guide/DGHtm/filterpressuredrop.htm

    which indicates not more than 0.2" water gauge for a good (i.e., low pressure drop) HEPA filter at what sounds like sensible operating conditions.

    32' water gauge is about 100 kPa so that's 100e3 / (32 x 12 x 5) = 52 Pa.

    Work done in pushing some fluid through something is force x distance = pressure x area x distance = pressure x volume. So the extra SPF required for a good filter would be 52 J/m³ or 0.052 J/l if you insist or 0.052 W/(l/s) if you really insist.

    If that calculation's right (beware: done while half way through a second can of beer) then, yes, you are being fed a line though that line might be a result of not-so-low-pressure-drop filtering either due to not-so-good filter material or restricted surface area - I guess.
    • CommentAuthornbwilding
    • CommentTimeMay 22nd 2012
     
    The G3 filters in my Sentinel Kinetic Plus (which fares well in SAP in terms of SFP) are stated to catch 70-85% of 10 micron particles. You can also fit F5 filters that catch 98% at 10 microns. Most pollen is over 10 microns I believe. I don't know which filters were used for the SAP test, though I doubt it would make a huge difference.

    I'd be interested to know whether there are clinical studies of whether MVHR helps much with pollen allergies. In my experience, the companies who use this as a selling point tend to only quote anecdotal evidence. It seems to me that to get any benefit you'd have to be prepared to keep your windows closed all the time in summer. Given the limited cooling capacity of MVHR with summer bypass, it's not clear to me that this is entirely feasible.

    Nigel
    • CommentAuthorborpin
    • CommentTimeMay 22nd 2012 edited
     
    The DS90 is quoted as having a G3 + F7 filter. No idea what that actually means!

    On further searching I found this diagram which goes some way to explaining it. (http://www.aimfiltration.com/products_syn_air.asp). It does seem to me to be a limitation of the testing. I have seen somewhere that the F5 filter for the Vent-Axia is 'optional' so presumable not fitted for testing?
      download.jpg
    • CommentAuthornbwilding
    • CommentTimeMay 23rd 2012 edited
     
    Probably means they are put one after the other on the supply side.

    As far as I can see the distribution of pollen grain sizes found in the UK tails off rapidly below 10 microns, so in terms of that graph, whether you catch 98% of a number close to zero, or only 80%, doesn't amount to much of a difference in real terms.

    Clearly one can put extremely efficient particle filters on an MVHR if one wants. My question would be whether there is any firm evidence that eg a F9 filter reduces allergy symptoms appreciably compared to eg a F5 or a G3, particularly when in high summer windows may need to be open for cooling purposes.

    When it comes to health benefits of MVHR vis a vis allergies, I would suspect that this mainly comes from the elimination of black mould (the spores of which can be allergenic) due to lowering of the humidity, rather than from the specific choice of filter, assuming you have a filter that catch most pollen as almost all seem to do.

    Regards

    Nigel
    • CommentAuthorborpin
    • CommentTimeMay 23rd 2012
     
    They are almost certainly on the supply side with G3 followed by F7. there is just a G3 on the exhaust.

    The point of the discussion though, is the effect different filters will have on the power required to drive/pull the air. If you have better filtration, the fan will have to work harder - the question is by how much. This then distorts the SPF on the SAPQ results as it apparently does not take this into account making the data very suspect - unless someone can tell me otherwise.

    On the flipside, the SPF is then a function of how good the filter is as that seems to vary widely.
    •  
      CommentAuthorJSHarris
    • CommentTimeMay 23rd 2012
     
    Any filter, whether on the inlet or outlet side of a fan, will increase flow resistance and so increase fan power consumption for a given air flow rate. I, too, suspect that the figures suppliers quote are the most optimistic, and exclude the losses induced by additional filters (and probably exclude duct losses, too).
    • CommentAuthorJoiner
    • CommentTimeMay 23rd 2012
     
    How else can they do it? :confused:
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 23rd 2012
     
    Posted By: JoinerHow else can they do it?

    Do you mean measure the power consumption to achieve a desired 'efficiency' or catch more particles?

    The first one is to measure different things the second would be to use a cyclone filter.
    • CommentAuthornbwilding
    • CommentTimeMay 23rd 2012
     
    I suppose my point is that a manufacturer might try to justify a higher fan power for their unit by claiming that this is due to better filtration, but if they can't produce clear evidence that that level of filtration has any tangible benefits with regard to pollen allergies then why take the financial hit?
    • CommentAuthorJoiner
    • CommentTimeMay 23rd 2012
     
    I was actually thinking about comparing all units on the same basis under 'standard' conditions, which in real world situations don't exist outside the lab.
    • CommentAuthorborpin
    • CommentTimeMay 23rd 2012
     
    Exactly my point. If for instance all units were tested with just a G3 filter then it would be a level playing field. At the very least there should be a statement of configuration in terms of filters and anything else that may affect the performance.

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