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Old 05-01-2016, 08:57 PM   #21
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Originally Posted by Elliot Naess View Post
The concept of air or water moving too fast to pick up heat has always puzzled me. I am NOT convinced this phenomenon exists -- UNLESS it has to do with the boundary layer I mentioned earlier. Heat transfer, I expect, primarily takes place in the boundary layer. Possibly, the thickness of the boundary layer matters. I wold love to learn more about this!
Ever walk through a doorway at a store that has a turbine fan blowing straight down to keep insects out while leaving the door open? Inside is cool from the ac, outside is normal weather temp. Just a thought.
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Old 05-04-2016, 06:28 PM   #22
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Like the experiment where they put golf ball dimples all over a car and found that it had less wind resistance.
Do you have a pointer to that experiment? A few minutes of Googling didn't turn up anything.
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Old 05-04-2016, 07:07 PM   #23
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Do you have a pointer to that experiment? A few minutes of Googling didn't turn up anything.
In Texas we call this the "hail stone effect".
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Old 05-04-2016, 07:51 PM   #24
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Originally Posted by dan-fox View Post
Do you have a pointer to that experiment? A few minutes of Googling didn't turn up anything.
Actually that was something they did on Mythbusters some years ago. That design doesn't seem to have gained any popularity, except by the hail damage crowd.

They took two identical CrownVics and applied a coating of clay to one so they could imprint the dimples, and the other was loaded with an equal weight of clay distributed inside the car. Not exactly the perfect surfaces for comparative purposes.
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Old 05-05-2016, 06:21 PM   #25
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Do you have a pointer to that experiment? A few minutes of Googling didn't turn up anything.
I should have said "A few minutes of Googling didn't turn up anything that indicated that it did any good." The aerodynamics of lift on a spinning sphere are pretty well understood, and the person who said that the dimples aid in keeping the boundary layer flow attached hit it on the head.

In another one of my previous lives, I worked as a tech at a wind tunnel. Waaay interesting job. The closest I have to a take-home from that experience applicable here is "It's gonna do what it's gonna do, and measuring airflow beats trying to predict airflow."

You can duct-tape tubing to various spots on the frame and undercarriage and compare it to static pressure with a U-tube manometer. No kidding; very cheap high-school-science-project level stuff that yields decent comparative results (as opposed to numerical results). Or you could tuft a bunch of surfaces (the ribbon suggestion although we used to use yarn) and video it to get an idea of surface flow.
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Old 05-05-2016, 07:31 PM   #26
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That would be an interesting job.

So originally the question was should Cadillac's AC fans in the skirt of his bus be able to draw air in through the skirt surface at highway speeds with the wind blowing past the side of the bus at 60 mph?

It seems like on a flat surface like that the air pressure would be nearly neutral and fans shouldn't have any problem drawing in air. I don't know if there would be a negative pressure under the skirt or not, but I assume not. I assume a lot of the times though and it gets me in trouble.
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Old 05-05-2016, 08:07 PM   #27
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Originally Posted by Elliot Naess View Post
The concept of air or water moving too fast to pick up heat has always puzzled me. I am NOT convinced this phenomenon exists -- UNLESS it has to do with the boundary layer I mentioned earlier. Heat transfer, I expect, primarily takes place in the boundary layer. Possibly, the thickness of the boundary layer matters. I wold love to learn more about this!
In my experience as an HVAC installer whether using steam,180deg.water for heat or 40deg. Water for A/C the manufacturer regardless of names always had the hottest/coldest on the leaving air side.
That was to heat or cool a building?
It does depend on coil position and is dependent on what you are trying to do?
Maybe exhaust fans drawing air in from the air skirt blowing into the coil or cowl induction for each coil?
I don't know the answer?
But to me if you made a cowl inducted hood and duct it between each box(depending on your arrangement ) the coolest air comes in and passes through box. If the first box is the hottest then the second box is warm and the third box tempered. As the air spends time passing through each box it loses a little bit/or alot of temp depending on the space it is trying to heat/cool? But sometimes you want outside air and sometimes you don't? And is all dependent on the outside air temp?
And our computer geeks on here might be able to suggest some 12v controllers/temperature regulator's that might/could control a temperature damper in a cowl induction hood on the side of a skoolie? To regulate space temps???
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Old 05-05-2016, 08:16 PM   #28
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The smaller ducting between each box is to help the air mix temperature's.
But it is all decided by the outside air that is being introduced to the space and the heat being produced by the space that decides the heat/cool of each space.
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Old 05-05-2016, 10:22 PM   #29
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Originally Posted by Jolly Roger View Post
In my experience as an HVAC installer whether using steam,180deg.water for heat or 40deg. Water for A/C the manufacturer regardless of names always had the hottest/coldest on the leaving air side.
That was to heat or cool a building?
It does depend on coil position and is dependent on what you are trying to do?
Maybe exhaust fans drawing air in from the air skirt blowing into the coil or cowl induction for each coil?
I don't know the answer?
But to me if you made a cowl inducted hood and duct it between each box(depending on your arrangement ) the coolest air comes in and passes through box. If the first box is the hottest then the second box is warm and the third box tempered. As the air spends time passing through each box it loses a little bit/or alot of temp depending on the space it is trying to heat/cool? But sometimes you want outside air and sometimes you don't? And is all dependent on the outside air temp?
And our computer geeks on here might be able to suggest some 12v controllers/temperature regulator's that might/could control a temperature damper in a cowl induction hood on the side of a skoolie? To regulate space temps???
for efficiency you want the Least-conditioned space air hitting the MOST extracted conditioning medium... for instance in heating.. I take the Coldest space return air and run it over the Heat exchanger just before the heat (in my case exhaust gasses fro ma furnace) is exhausted to the outside..

for cooling you get the Most humidity removal by the air being Cold.. if you temper the air first by running it over the coils closest to the suction line.. then you give your main heat exchanger a chance to cool that air way down.

with A/C, the actual air temperature over the coild plays a big part in how well the humidity is handled.. so if I run the CFM of air up (hugh speed, high volume) then that air is never going to be cooled-down to the point of removing the humidity(and it wont be as cold).. its not a matter of that air not having time to get cold.. its a matter the volume of air being moved is over-loading the heat exchanger...

those of you that have newer home A/C systems will notice in very humid weather your system fan will slow down.. the air coming from the vents is reduced bt is very cold.. the humidity is reduced at the expense of there becoming hot and cold spots in the rooms...

I employ a technique in my house for cool but very humid days where I run the coil temperature very cold, (right above freezing) and then I bypass a certain percentage of the air.. (it never hits the coils).. so I get a nice dry but temperate air from the vents... again its not the TIME that air needs to spend over those coild to condense the water.. its a matter of not expecting my heat exchanger to do more than what its designed to do..

as for the bus skirt A/C airflow.. I have no idea wich way the air flowed when those conditions were right... all i know is enough air flowed over the coils that the fans never turned on.. it couldve been negative pressure on the side of the bus due to traffic, wind, etc. . so the air may have flowed "under bus to skirt and out the left side".. or it couldve flowed in the same way that the fans intended which was in the skirt and out under the bus..

I NEVER hit a time when the air-flow was so backwards that the fans failed to move enough air.. I never tripped my High head pressure switch.. (and it was a manual reset type on purpose)

my new condenser is going to be fully under-bus so im guessing I'll always have to run a fan or 2 or 3 depending on the interior system load..

though I will note what it does once I get it installed and running...

-Christopher
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Old 05-06-2016, 04:53 PM   #30
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Originally Posted by cadillackid View Post
for efficiency you want the Least-conditioned space air hitting the MOST extracted conditioning medium... for instance in heating.. I take the Coldest space return air and run it over the Heat exchanger just before the heat (in my case exhaust gasses fro ma furnace) is exhausted to the outside..
That's intuitive.. after pondering it for a while. In heating as your example, if the coldest air were run over the hottest part of the exchanger first, it'd warm up some. But as the air continued across the exchanger it wouldn't gain any more heat. The exhaust would go out warmer, relative to the counter-flow situation you described, and so the system would be less efficient.

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Originally Posted by cadillackid View Post
with A/C, the actual air temperature over the coild plays a big part in how well the humidity is handled.. so if I run the CFM of air up (hugh speed, high volume) then that air is never going to be cooled-down to the point of removing the humidity(and it wont be as cold).. its not a matter of that air not having time to get cold.. its a matter the volume of air being moved is over-loading the heat exchanger...
This one I had a hard time with initially. At first blush I figured the same energy is being removed from the air (and from the water vapor suspended within the air) no matter the fan speed. Then it seems with the higher fan speed it'll just take longer to condense the water out of the air, whereas with the lower fan speed it'll take longer to cool the air. Finally I realized both approaches are equally effective only if they both run until the whole space is chilled below the dew point. Brrr! Normally the thermostat would quit calling for cooling when the temperature had dropped enough, but the higher speed fan system wouldn't have removed much humidity by that point. Thus it would "never" dehumidify, just as you said.

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Originally Posted by cadillackid View Post
I employ a technique in my house for cool but very humid days where I run the coil temperature very cold, (right above freezing) and then I bypass a certain percentage of the air.. (it never hits the coils).. so I get a nice dry but temperate air from the vents... again its not the TIME that air needs to spend over those coild to condense the water.. its a matter of not expecting my heat exchanger to do more than what its designed to do..
Ahh, so part of the air drawn in by the blower shunts around the evaporator while the other part goes through the evaporator. The two parts re-combine in the duct headed out to the building dried but not so cold. That's pretty clever. Is the bypass automated in some way, or is it manual?

Thanks for your comments above. The are good food for thought as I contemplate going fully custom for an A/C system.
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