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Author Topic: Positive and negative pressure zones  (Read 700 times)
Tikvah
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« on: August 04, 2014, 08:35:52 AM »

I know this has been talked about before, but....
Thinking of a gray tank vent through the bay floor so was thinking where my pressure zones are.  Below rear-most bay, ahead of drive wheels, negative pressure?   What about the bay doors in the same area?  Positive?
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I couldn't repair my brakes, so I made my horn louder.
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chessie4905
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« Reply #1 on: August 04, 2014, 09:00:10 AM »

   Where do the fumes go when sitting still and tank percolating? Do you want the odor  coming out from underneath when camping? Through the roof is better.
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Tikvah
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« Reply #2 on: August 04, 2014, 09:10:41 AM »

I'm through the floor now, but not permanent, I want do do a permanent vent.  Never had odor outside the bus, but struggle with some odor while driving.  Black is vented through the roof
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I couldn't repair my brakes, so I made my horn louder.
1989 MCI-102 A3
DD 6V92 Turbo, Alison
Tons of stuff to learn!
Started in Cheboygan, Michigan (near the Mackinaw Bridge).  Now home is anywhere we park
bevans6
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« Reply #3 on: August 04, 2014, 09:12:28 AM »

While driving there is a high pressure zone at the front face of the bus, quite low pressure zones for about 4 to 6 feet back on the sides and roof of the bus, flow re-attaches and is neutral to a little bit high pressure along the sides of the bus to about two-thirds back, pressure grows higher as the sides and roof reach the rear of the bus, pressure is quite low directly behind the rear face of the bus.  Pressure under the bus is always low.  While not driving pressure depends on wind speed, but you can expect that pressure under the bus will the higher than pressure at the roof, particularly if there is a wind of almost any speed.  The curved top of the bus acts like a wing, reduces pressure as the wind curves past the surface.  The curved top of the Pressure inside the bus will depend on windows and wind speed/direction.  The recommendation for a roof-top vent has a lot more to do with dangerous sewer gases being lighter than air and so always trying to rise than it does with pressure, but using pressure differentials always helps.

Brian
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Oonrahnjay
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« Reply #4 on: August 04, 2014, 11:23:10 AM »

I'm through the floor now, but not permanent, I want do do a permanent vent.  Never had odor outside the bus, but struggle with some odor while driving.  Black is vented through the roof

     I'd attach to that existing stack.  Won't hurt anything, and if you get a bad colony of germs in that grey tank, you'll be glad that the smells are going out the roof.
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Bruce H; Wallace (near Wilmington) NC
1976 Daimler (British) Double-Decker Bus; 34' long
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Tikvah
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« Reply #5 on: August 04, 2014, 01:37:15 PM »

I've been contemplating connecting the two vents into one.  Anyone else do this successfully?
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I couldn't repair my brakes, so I made my horn louder.
1989 MCI-102 A3
DD 6V92 Turbo, Alison
Tons of stuff to learn!
Started in Cheboygan, Michigan (near the Mackinaw Bridge).  Now home is anywhere we park
Oonrahnjay
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« Reply #6 on: August 04, 2014, 03:18:20 PM »

  I've been contemplating connecting the two vents into one.  Anyone else do this successfully? 

    I have a side vent that I bought from Camping World that both my tank vents (and the vents for the bathroom and kitchen sink) are connected to.  It's right below the roofline on the side wall (about 13 feet off the ground).  Never had *any* trouble with it -- driving or parked. 
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Bruce H; Wallace (near Wilmington) NC
1976 Daimler (British) Double-Decker Bus; 34' long
6-cyl, 4-stroke, Leyland O-680 engine

(New Email -- brucebearnc@ (theGoogle gmail place) .com)
bevans6
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« Reply #7 on: August 04, 2014, 03:55:20 PM »

Connecting the vents together is perfectly fine.

Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
Spicer 8844 4 speed Zen meditation device
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1978 Lola T440 Formula Ford
1972 NTM MK-4 B/SR
akroyaleagle
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« Reply #8 on: August 04, 2014, 04:21:53 PM »

Vent it out the roof.

Try driving with the side driver toll window shut.

If it is open, and someone flushes the toilet, The Toll window will siphon the odor from the tank.

Installing a venturi on the top of the vent will help.
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Joe Laird
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Jeremy
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« Reply #9 on: August 04, 2014, 04:26:37 PM »

I read an article somewhere that described how to find high and low pressure areas on a car body by means of a length of flexible rubber tube passed-through one of the car windows, and the open end taped to different parts of the car body. Then you drove along with the other end of the tube in a jam jar of water, or some such arrangement - I forget exactly how it worked but basically it was using the water level to determine if the outside air pressure was higher or lower than the stable air pressure inside the vehicle.

On a shape a simple as a bus body you can more than likely predict what the air is doing as it passes over it - but it can be done a little more empirically if you wish!

Jeremy
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georgemci102a2
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« Reply #10 on: August 04, 2014, 05:05:32 PM »

That darn toll window. Grin..George
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Seangie
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« Reply #11 on: August 09, 2014, 05:46:06 AM »

Dave,

We have our black and gray tied together.  They vent out the rear wheelwell.  The only issues we run into are...

1.  The airvent is one inch which is not wide enough to let enough air (through the connected hose) when we dump water so the water in the traps typically gets sucked out when we dump which will allow smells into the bus if we don't immediately run water in the sinks and shower after dumping.

2. When parked we never smell anything unless....the black and gray tanks are left open and there is nothing to block the smell of the campground sewage that comes up through the hoses.  Our fix for this is to leave a little loop (basically a trap) when we setup our sewage hose to block the smell.

3.  When driving up steep climbs sometimes our traps will empty...again letting smells into the bus.

4.  Because of where the vent is located...if we flush while going hiway speeds the pressure blows air back through the toilet and can get you wet.  So we can only pee while on the highway.  Slower than 45 isn't a problem when flushing.  This was a suprise to me as I thought there would be negative pressure in the wheel wells and the air would be sucked out while moving.

None of these are dealbreakers.  Just nusiances aand things to think about.

-Sean
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HB of CJ
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« Reply #12 on: August 09, 2014, 10:16:05 PM »

Years ago Crown Supercoach schoolies exploited the hi-low pressure thing.  On the older Crowns, the radiator was mounted on the drivers side just behind the front right tire.  A sheet metal duct thing directed the air from the radiator into and through a big slow turning forward facing fan on the left side of the pancake engine that pulled air through the BIG radiator.  144 qt. capacity on mine.

There also was a big wide deep mud flap do hickie thing that ran cross wise just behind the radiator just below the forward engine compartment firewall.  Once I busted both huge drive belts on my daily ride 1963 Crown 10 wheeler schoolie.  The shop mechanic told me to raise all the side windows up and come home.  The water temp never got above 190F; the oil temp above 210F.  Warm spring day.

No radiator fan.  Seemed one was not absolutely necessary?  High pressure, low pressure.  The exhaust pipe was at the very end of the bus through the rear bumper.  I have seen both gray and black tanks sharing a common vertical vent through the roof.  I do not know how well it worked.  Best guess is that the holding tanks vents would have to exploit hi-low pressures and be located such.

HB of CJ (old coot)
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