PC fan directions

Discussion in 'DIY' started by brettarider, Nov 5, 2012.

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  1. I'm looking to carry out a mod to help the cooling of my fridge in the campervan I'm planning on having a 12vt supply feeding a pair of PC fans on each of the air vents in the side wall were the fridge goes.

    The fridge works by drawing cool air via the bottom vent and up the cooling vents at the rear before making it's way out the top vent.

    Now by adding a pair of fans at each vent I'm hoping it'll help cool the fridge down a bit better and maybe perform better they will be run by a toggle switch so they can run on either 12/240v/gas

    So which direction should the fans move on the bottom vent and the top vent to circulate the air?

    I did try this a few years ago in my old van but with only one fan when I was pissing about and I could buy a kit to do it properly but it costs a few hunderd quid the las time I priced it up!
  2. Grumblegrunt

    Grumblegrunt LE Book Reviewer

    you just need a fan draw at the top close to the radiator I reckon seeing a heat rises you may as well encourage it. blowing air in wont do anything but making a chute over the radiator should improve the draw as it narrows.

    if you grew up with an open fire and used to do the newspaper on the fireguard trick to get it going then you'll understand what I mean
  3. Sympathetic_Reaction

    Sympathetic_Reaction LE Book Reviewer

    I'm not going to get too techy about it - depressingly this is my job (or at least part of it)

    You want to put both fans sucking out at the top of the system, if you put them one in and one out you may have problems as fans tend to have a bad habit of effecting each other when placed in series. Also having just one forced end of the system means that additional air can be drawn in from any other 'holes' and hence increasing airflow further.

    As Grumblegrunt says you could try and speed the airflow up a bit by forming a cowl over the top of the condensor (that's the bit that gets hot at the back) so that all the airflow goes up and through the fans, this wouldn't be necesary if both fans next to each other covered most of the output vent.

    To aid in the airflow, try to reduce any constriction points - if the output vent has a grill on it, replace this with a fixing for the fans, if you need a grill to cover the fans (i.e. to stop fingers or bits falling in) then make it with the largest slots you can get away with.

    If you want to reduce the power draw of your system you could put a thermocouple inline with the fan circuit so they only come on when the condenser temperature (or air around it) exceeds a certain level.


    (I'll go back to my CFD model of an airconditioning ducting system now and be geeky)
  4. Got me interested now.

    What bad effects could you have if you put the one on top blowing out and the one on the bottom sucking in?

    This is what I would have instinctivly set it up as but it is always good to ask an expert.
  5. Sympathetic_Reaction

    Sympathetic_Reaction LE Book Reviewer

    The issue comes when the 2 fans aren't balanced, so you may have one 'pushing' while the other is 'pulling' at different rates, this can cause weird pressure effects which can effect the efficiency of the fans, recirculating areas within the linked area and sometimes pressure waves moving between the two fans - very difficult to state which if any would occur without looking at the specific system setup.

    If you can balance the system then 2 fans inline are an option and can be beneficial in some cases (I'm doing a system at the moment where one leg needs a boost, so I've added a fan inline) but they tend to be annoying to setup and match across varying conditions.

    These are probably unlikely to be extreme issues for a simple low pressure system as you are suggesting but it is not the simplest way to set up the system, ideally the forced part of the system should be as close to the 'hot' section that needs cooling, the input to that hot section should be as large as possible (i.e. bring in air from as large a volume as possible) and the output should be as small as possible but covering the majority (ideally all) of the hot section.

    So in your case I'd think the fans are probably 4 inch diameter? if your outlet grill is 8 inches long and 2 or 3 wide then stick the fans next to each other covering the whole grill....if it is wider then block off any outlet that doesn't go through the fans (stops recirculation occuring - where you bring back in the air you have just pulled out). ideally you should make up a cowl system which at one end exaclty matches the output vent size and at the other exactly matches the fans, then smoothly transitions between them - possibly a bit extreme for what you are after. But do note that for a forced air system to be effective the system should really be sealed with the inlet and outlet well defined and all the flow going through the fans.

    I've probably gone a bit geeky there - sorry.


    • Like Like x 1
  6. Sympathetic_Reaction

    Sympathetic_Reaction LE Book Reviewer

    Just a quick caveat

    All systems are different, they all have quirks and other issues apart from airflow to balance (space, weights, ambient conditions etc)

    Pros and cons of all designs.

  7. I bow to Sympathetic_Reactions superior knowledge on this subject but I would add that I did a DIY hood for housing tubes to light a marine aquarium full of corals. The tubes gave off a lot of heat so I built two fans each end into the hood, two pushing, two pulling.

    I went through fans quite quickly, the bearings tending to go. It was solved by making all the fans suck air from outside in. Apparently it was due to pressure waves (as stated in the previous post) that made the fans run uneven.

  8. You know what, that actually made sense.

    Thanks for taking the time to explain and have a bukshee like :)
  9. Sympathetic_Reaction

    Sympathetic_Reaction LE Book Reviewer

    That's worrying I've been trying to explain this sort of thing to managers (using diagrams, CFD analysis and physical demonstrations) for about 2 years now and they still don't get it.


  10. Excellent a £5 bodge solution results in some great explanations! There was a cowl fitted to the rear of the top vent when I stripped the caravan I'm assuming it was to clear the air vent.

    A bit of google-fu found this thread which is pretty good explaining it
    Fridge cooling fans - Workshop - Touring and Tenting

    If I can I'll take a pic of the cowl in place on Sat and I'm not bothered about fitting a thermo-stat as the PC fans use so little juice I can leave them running all day
  11. Grumblegrunt

    Grumblegrunt LE Book Reviewer

    if you are sleeping in there then you might notice, also a good vent should develop enough airflow to not need the fans.

    doing kitchens the rules for fridges used to be mental as some companies insisted you stuck a whacking great vent in the plinth and the worktop which looks bad and was due to cheap kit not being designed properly.

    give a fridge room and airflow and its usually allright.
  12. I can always switch it off via a toggle switch which is what I'm planning to do I think the last time I looked the fans used something like about 0.02 amps per hour so they can just run all day
  13. Grumblegrunt

    Grumblegrunt LE Book Reviewer

    try a solar powered relay or a thermocouple
  14. When I first saw this thread I thought it was for cooling computer cases.

    If you have a three way fridge then the B. things are pretty useless when the ambient temperature tops 28C or more. So if you're going to the south of France or similar in the summer they are a problem. Dometic are well aware of this problem but still choose to do nothing about it. Dometic do make a purpose built fan for three way units but, knowing Dometic, they'll be expensive.
    12vDC compressor fridges are better in this respect.
    I have a 3 way fridge freezer in the present van and it's useless compared to the 12vDC units I've used in the past
    On all my self builds I have always used PC case fans as extractors and have not had any problems.
    As for the noise factor I've never found the fan to be too intrusive.
    It does pay to buy an expensive unit though. I've found the cheap fans to be noisier than the better quality kit. RS Components are a good source for fans. RS do show the Db levels for their fans