Forced air ventilation

[Zephod]

I'm playing with ventilation at the moment, trying to keep my bus cooler inside.

Year 1 - all grey paint, 140F inside at the height of summer.
Year 2 - the roof is painted white, solar powered extractor fans are placed at the back, near the roof 108F inside at the height of summer
Year 3 ?

I'm planning to suck air in from the side of the bus, pass it through a Honda lawnmower engine filter. The fan will be a CPU fan since they shift goodly quantities of air, run off 12v and are brushless. The air gets concentrated and piped in via 1.5 inch plumbing piping.

While one little fan won't shift a whole load of air, constant operation combined with the extraction fans should make an appreciable difference.

Out of curiosity I built a drop in window ventilation unit that had two Walmart battery powered fans. That made a huge difference almost instantly. The CPU fan shifts more air, more efficiently.

I'm welding together the basis for my ventilation unit at the moment.


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[windtrader]

Hey,
I use various computer fans for different hacks too. Do remember those fans are low power fans and spin fine with in free air. They do not do well under load. In a bus application it seems likely there may be significant air pressure differences. Unfortunately, the PC box fans are cheap compared to more powerful commercial fans.

[Zephod]

Hey,
I use various computer fans for different hacks too. Do remember those fans are low power fans and spin fine with in free air. They do not do well under load. In a bus application it seems likely there may be significant air pressure differences. Unfortunately, the PC box fans are cheap compared to more powerful commercial fans.

True, they are cheap. My extraction fans seem to work just fine. Not very high powered but they shift air out. If I put a Walmart bag over my exhaust vent, the fan fills it within a few seconds. I have two five inch CPU fans as my exhaust vents. In order to help them, I'm going to install my forced air intake st the other end of the bus.


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[Oonrahnjay]

    Zeph, there's a basic issue that's working against you here.  Air is heavy and pumping it around takes energy.  If you're putting milliamps into moving air, you're not going to get much air moved.
    A second part of this is that we need to look at the efficiency of fan.  A blade-type fan will move air to some extent in free air but once you get backpressure, they become very inefficient; a factor in this is that if you have a filter, a blade fan won't move much air through them.  Pretty much the only way to move air against pressure in a vehicle is a centrifugal (or "squirrel-cage") fan -- they WILL build up pressure that you can use. 
    Any fan that doesn't use much power isn't going to do much to circulate air.  It's that simple.  But there is one thing that we can do that is efficient and that's to move air in a way that it already wants to go.  That's a really good thing about what you're doing -- you're pushing cool air in at the bottom.  If you can build up a little pressure, you can circulate that incoming air against the interior surfaces in your bus; that will cool them down and as that air warms up, it will rise to the ceiling where your extractor fans will "encourage" that air to continue out of the bus and the cycle can repeat itself over and over.
    I think that you've seen the obvious -- if an outside body panel is in a place to get hot (usually in direct sunlight), then the heat is going to "soak" into the roof and ceiling, or vertical side walls, or the floor.  Once this has warmed up your interior panels, those interior panels will radiate the heat into the inside volume of the bus.

    There are three main ways to cut down on heat building up in the bus:
1)  Reflect it.  Doing something like painting a roof white, especially using a coating that has additional reflecting qualities, can make a big difference.  This applies in a similar way to side walls with only slightly reduced efficiency and can be used to reflect engine compartment and underbody heat away from the floor.
2)  Insulate it.  If you can get a good, well-sealed layer of insulation between the hot outside panel, it will make a substantial reduction in the amount of heat transferred to the inside of the bus.  A lot of people do this by putting wooden strips on their inside walls and putting insulation between these strips and then secure the insulation in place by their interior wall paneling.
3)  Remove it.  That's where a good air circulation system comes in.

    In my opinion, you have to do it all.  But air circulation can only help if you have a good supply of cool air.  If it's 94 degree in the shade outside,  PERFECT air extraction* is only going to give you a bus that's 94 degrees inside.  Our task is to put together a plan for design balanced against what we think we can live with.

(*  Of course, you're never going to get perfect air extraction, even using lots of machinery that's fed high amounts of power.)

[windtrader]

Bruce,

Same thinking as me but much better explained. One more point about backpressure or air pressure on the fan. You need to think through what dynamic pressure you are dealing with and which way the fans point. For example, if you have windows open while driving, maybe even some gaps will build positive pressure in the cabin, then you have another low power fan directing air flow into the cabin, thinking you are getting more air inside. That fan is likely to be seeing pressure on the exhaust side so it can not flow any air inside, defeating the idea of using that fan for additional flow. If the sme fan is exhausting air, the positive pressure in the cabin will work with the fan and exhaust air out.

Having worked with these cheap PC fans, the low amperage makes them really useless unless you are moving air in free space which they do nicely. I have one right on my desk that provide a very nice near silent breeze.

[muldoonman]

Have 2 fans in the top of my bus. (Fantastic 3 speed) Thermostat controlled if you want to set temp.  Never have,  just turn them on.They pull a ton of air though windows and can't see where these little fans could help much.

[Zephod]

I'm happy with the amount of air I'm extracting. I need to push cooler outside air in. I did an experiment where I built a window unit that used a pair of desktop battery powered fans and it rapidly cooled inside the bus. My CPU fans are more efficient.

The plan is to suck the air through the filter and blow it into the cabin via a tube. I did think of a simple unit sucking air from under the bus but it would have been a pain to change the filter. Having said that, it is the simplest design and would eliminate needing a louvered vent. I'm concerned about blowing air from the grey tank vents into the cabin.


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[Oonrahnjay]

I'm happy with the amount of air I'm extracting. I need to push cooler outside air in. I did an experiment where I built a window unit that used a pair of desktop battery powered fans and it rapidly cooled inside the bus. My CPU fans are more efficient.

The plan is to suck the air through the filter and blow it into the cabin via a tube. I did think of a simple unit sucking air from under the bus but it would have been a pain to change the filter. Having said that, it is the simplest design and would eliminate needing a louvered vent. I'm concerned about blowing air from the grey tank vents into the cabin. 

     There's an important thing here -- if you succeed in moving unfiltered air, and count the outside air that infiltrates into a partial vacuum space, then every degree worth of air, you're also drawing in a big gulp of dust and dirt.  We all get a lot of that rolling down the road but add that happening over and over while you're parked and you have a big build up of that stuff all over everything inside.  We really need to push air through a filter and keep a positive pressure inside or we're living in filth. Little fans and a few D-cells won't do it.

[Geoff]

One fact that I keep thinking about is that you cannot pump outside air into a bus without cooling it and expect to cool the bus anymore than the ambient temperature.  Is there anyway you can get enough water to improvise a swamp cooler?  That is a great way to get cool air into your bus.

--Geoff

[Zephod]

     There's an important thing here -- if you succeed in moving unfiltered air, and count the outside air that infiltrates into a partial vacuum space, then every degree worth of air, you're also drawing in a big gulp of dust and dirt.  We all get a lot of that rolling down the road but add that happening over and over while you're parked and you have a big build up of that stuff all over everything inside.  We really need to push air through a filter and keep a positive pressure inside or we're living in filth. Little fans and a few D-cells won't do it.

Oh, I'm way beyond d cells. I'm using a 12v gel cell with a solar panel and the air will pass through a filter before it reaches the fan.


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[Zephod]

     There's an important thing here -- if you succeed in moving unfiltered air, and count the outside air that infiltrates into a partial vacuum space, then every degree worth of air, you're also drawing in a big gulp of dust and dirt.  We all get a lot of that rolling down the road but add that happening over and over while you're parked and you have a big build up of that stuff all over everything inside.  We really need to push air through a filter and keep a positive pressure inside or we're living in filth. Little fans and a few D-cells won't do it.

Oh, I'm way beyond d cells. I'm using a 12v gel cell with a solar panel and the air will pass through a filter before it reaches the fan.


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[Zephod]

One fact that I keep thinking about is that you cannot pump outside air into a bus without cooling it and expect to cool the bus anymore than the ambient temperature.  Is there anyway you can get enough water to improvise a swamp cooler?  That is a great way to get cool air into your bus.

--Geoff

Parked up, power usage is minimal so keeping the bus at ambient temperature is the goal. Later, I might add some Peltier units to the underbus tubing with a fan to blow the hot air out through a side vent but that's for when I'm plugged in.


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[86102A3]

I would recommend a Saudi style roof, basically another layer of metal just above the roof. This will act as a heat sink and keep a lot of the heat from solar loading from migrating into the bus. I am planning on making a race deck on the roof of my bus, it will also act as a Saudi roof. Also some sort of awnings for your windows would be advisable. These will help bring those internal temps way down, so you will not need to work so hard to cool the interior.


Jeff LoGiudice
Tampa, Fl.
1986 TMC 102A3
1999 Reliabilt 6V92TA
Allison HT740

[Zephod]

I would recommend a Saudi style roof, basically another layer of metal just above the roof. This will act as a heat sink and keep a lot of the heat from solar loading from migrating into the bus. I am planning on making a race deck on the roof of my bus, it will also act as a Saudi roof. Also some sort of awnings for your windows would be advisable. These will help bring those internal temps way down, so you will not need to work so hard to cool the interior.


Jeff LoGiudice
Tampa, Fl.
1986 TMC 102A3
1999 Reliabilt 6V92TA
Allison HT740

Those are excellent suggestions. The safari roof is a great idea and I'm baffled given temperatures in the southern USA why nobody has them as standard on both homes and vehicles.

I'm trying to keep my bus looking as bus like as possible as a way of blending into the urban scene. I'm trying to find some good amorphous panels but they seem too low in power.

The awnings are another great idea though sadly I'm still stuck trying to be anonymous.

I'm pretty sure my air intake will reduce internal temperatures pretty well.


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[luvrbus]

I would start that puppy and head for the mountains in Idaho and forget the fans