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  #1  
Old 06-09-2017, 11:51 AM
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rvanstory rvanstory is offline
 
Join Date: Dec 2016
Location: New Braunfels
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Default Chilled Water AC

I am interested in building in a chilled water "A/C system" into my RV10. I do not want a full blown freon system due to expense, weight, maintenance. But, I do prefer to have a more sophisticated option than a portable cooler in back so I can have chilled air come through overhead vents for taxi and takeoff.

Here's a link to a 6 year old thread on one builders solution. http://www.vansairforce.com/communit...ad.php?t=75235

Has anyone else done something similar?

Could you give me as many details as possible? Don't want to reinvent an old wheel if possible, plus want to learn from others experiences to the good and bad of a plan.
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  #2  
Old 06-09-2017, 03:09 PM
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Weasel Weasel is offline
 
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Location: Brooksville, MS
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Default

Quote:
Originally Posted by rvanstory View Post

Here's a link to a 6 year old thread on one builders solution. http://www.vansairforce.com/communit...ad.php?t=75235

.
I have quite a few hrs in that airplane.

It does blow nice cold air for a while but has some down sides:

Really it weighs almost as much as an air conditioning system if you load enough ice for 2 taxi - climb out to cool air events. Say you want to go to that noon fly-in on Saturday with your buds and then leave in the middle of the afternoon with enough capacity to get back up high here in the deep south when the ambient temp. is 95 deg and the wet bulb temp is 71.

Will take ~35 lb of ice and another 6-9 lb of water for it to circulate. The fan, coil, pump and associated parts put you up to 45-50lb range total.

It takes up a large part of the baggage area. I was able to travel with my family and luggage with it but we packed lighter than most couples could even if they tried .

Trying to get ice at the destination is usually problematic. Even if you beg the leftover from the drink cooler that they are going to throw away, you sometimes are the scrounge




The good is.

You can remove most of it.

Cost is a lot less.

Installation is much easier.


I ended up installing a refrigerant air conditioner in N45678 based on experience with this one.
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  #3  
Old 06-09-2017, 03:55 PM
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Weasel Weasel is offline
 
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Default A little on the science

don't mean to talk down the idea.....and hey go ahead if you want but would like you to consider the results prior to pushing the ambition...


You need more than 12,000 btu/h to keep an RV-10 cool in our area. The Flightline AC in our airplane is rated at 12,000 btu/h and the measured output is a little over that ~12,800 btuh. (I measure the output of air conditioner installations for a livelihood...). This is not enough in my mind. Oh yes you can get in it and it feels a whole lot better than without AC but it will not make you cold like a Toyota when the sun is directly shining in.

OK so the definition above tells us a little bit of what is needed to cool an RV-10 at least in my mind

Now consider 1 lb of ice will absorb 144 btu when converted into water.

The coil we used needs ~47 deg water or less to make cold enough air for some dehumidification.

So lets say you start with 0 deg ice and warm to 32 deg ice, this will absorb .5 btu per lb so you get a total of 16btu for warming the ice.

You will get 144 btu when the same lb of ice turns into water.

You will get another 15 btu for warming the same lb of water from 32 to 47.

So for each lb of 0 deg ice you can expect 16+144+15=175 btu.

Now if you want to run the cooling system for 1 hr. (which will give you only 15 min taxi and climb to altitude, 15 min decent and taxi in for 2 flights) you would need. 12,000 btu / 175 btu per lb = 68 lb of 0 deg. ice
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Last edited by Weasel : 06-09-2017 at 04:54 PM.
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  #4  
Old 06-09-2017, 08:15 PM
Bill Boyd Bill Boyd is offline
 
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Location: Landing field "12VA"
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Default liquid N2

...latent heat of evaporation 856 Btu/lb.

The problems immediately apparent are availability (welding supply company), cost ($2/gallon in a dewar, 50 cents/gal in bulk per the internet) and finding a working fluid that can transfer heat from the nitrogen dewar at -320F.

I'd probably opt for a solid heat exchange medium (styrofoam insulated dewar with a metal plate LN2/air heat exchanger in one side. Probably expensive and unwieldy, but one cannot help but ponder how much liquid nitrogen you can buy with the funds required for a closed loop refrigerant AC system. Plus you can shed the weight when not in use on a given flight, as pointed out.
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  #5  
Old 06-10-2017, 06:58 AM
leok leok is offline
 
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Location: Clarkston, MI
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Default LN2 thinking extended

I like your thinking...
All you would need to do with the liquid nitrogen (LN2) is dribble it out into the airstream from a fan, (Perhaps onto a furnace filter medium) and let it evaporate. The small amount of added nitrogen in the airstream would quickly dissipate with the air exchange rate inherent in the cabin.

The real difficulty is getting it and handling it. Any touch to the skin resulted in freeze/burn in a few seconds. I would want to be real sure it couldn't spill in flight. Also, I used LN2 in a lab I ran some years ago. Even in a lab grade Dewar it only lasted a week or two before it all boiled off. So you would have to replenish often.
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  #6  
Old 06-10-2017, 11:14 AM
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Paddy Paddy is offline
 
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Default A little more Science (and why I chose the icebox)

Weasel is right on with the math, except that he understates the btu requirement by a considerable margin (for simplicity's sake) by only considering sensible heat transfer in his example. In reality, the cooling system's output is consumed to a large extent by the latent heat transfer required to condense ambient humidity. Specifically, about 1065btu/lb is required with a condenser coil at 50 deg F. But that's OK because...
In a vehicular application, the humidity is lowered quite quickly because the limited air volume inside the vehicle is rapidly recirculated through the cooler, providing what feels like immediate relief for the occupants. Now, this drier air is subjected to more efficient cooling (requiring less btu per degree temp drop) as it continues to circulate through the cabin. Lastly, the air in a vehicle is dynamic, meaning it's moving around, blowing nice (relatively) dry air on the occupants and keeping them cool, so the air temp itself doesn't need to be ice cold. And this is why....
I chose to go with the portable icebox solution. I just couldn't bear the idea of lugging a permanent setup around with the extra weight so far back in the CG envelope, eating up my useful load 100% of the time when I only want it for less than 30 min of each flight, and even then only on some flights, not all. Add to that the extra drag of the condenser intake hanging out in the airstream 100% of the time and well, you get the picture...
Results: One 22lb bag of ice gets me a nice comfy cabin on a hot sticky mid-western summer day (benchmark - no sweaty back) for taxi, takeoff and climb, lasts about 2 hrs turned off in cruise and provides another 15-20 min of cooling to cover descent, landing and taxi. If I have another leg to fly, well I - have to find more ice to be cool, but I'm OK with that and so is the wife (usually)
Now don't get me wrong - I want my air cold in my Toyota, and I'm not too pushed if it adds another 75lb or eats up a few HP or adds some cooling drag to my car, but not in my airplane - no way, no how. But that's just me, everyone has their own needs hierarchy
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  #7  
Old 06-10-2017, 02:58 PM
Bill Boyd Bill Boyd is offline
 
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Default I guess my thinking is colored by

daily access to a dewar of LN2 we use in the office for cryosurgery on warts and skin tags, with delivery service from the local welding gas company - and the added temptation of a spare dewar sitting in a closet. The real upside is the weight and size savings with LN2 vs ice for a comparable BTU capacity. Refuelability on the road would be virtually nil.

My cryo dewar will keep a load of nitrogen for about a month, undisturbed, before it all boils away. Might be able to ferry some excess LN2 in an ice chest refrigerator setup and expect it to keep for a couple days to power the return trip. Not sure. It certainly won't keep long in a container that allows significant contact with room temperature.
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  #8  
Old 06-10-2017, 03:25 PM
civengpe civengpe is offline
 
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Default

I use the B-kool unit in my rv-10 and love it. It knocks the edge off of the high Texas temps while on the ground and flying under the Houston Bravo air space. When I hit 5k, I turn it off and have plenty of ice remaining for my descent. I can always leave it on the ground if I have a full load of people/bags. It weighs 32# fully loaded and takes the place of my water ballast in the back.
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  #9  
Old 06-10-2017, 04:55 PM
BobTurner BobTurner is offline
 
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Quote:
Originally Posted by leok View Post
I like your thinking...
All you would need to do with the liquid nitrogen (LN2) is dribble it out into the airstream from a fan, (Perhaps onto a furnace filter medium) and let it evaporate. n.
On a humid day you might reach 100% humidity in the cooled cabin - not comfortable. Or have condensation dripping everywhere. Your fan or filter will need a de-icing system.
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  #10  
Old 06-10-2017, 06:10 PM
artrose artrose is offline
 
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Location: San Antonio area
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Depending on the applications requirements, residential and commercial building heat load calculations for equipment selection can be very complicated, or relatively straightforward if the engineer is experienced, (and has a little common sense). "there's art, and there's science". If he needs additional information or assistance, there's plenty of it readily available for these types of applications.

But, with respect to small aircraft, does anyone know where one might find published load calcs for small piston aircraft cabins? I'm really curious to know how the engineering community has approached this problem. How do they handle the sensible and latent loads, solar gains, infiltration loads, and all the other variables that come into play? The solution certainly isn't ice cubes in coolers, and even the majority of the factory installed systems seem to have less than stellar performance. The military likes cool vests. Maybe that's the solution?

Last edited by artrose : 06-10-2017 at 06:14 PM.
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