# Special purpose low-power air conditioning - metal involved!

• posted

I'm trying to cool the boat for a few days at a time...

A normal 5000 BUT marine air conditioner can pull up to 30 amps of 12 volt power. If my house battery were new and fully charged, that's 4 or

5 hours. That won't get it for even for a weekend.

If we had a diesel engine (and ran it all day) we could use an automotive approach. But we don't - and can't.

So, quoting Kelly Johnston (one of my favorite heroes), "Simplicate, and add lightness".

Statement of Problem: I want to air condition the boat for up to 3 or maybe 4 days at a time. Independent of dock power. With as low of a battery load as possible.

Proposed Solution:

A cooler type container with a load of dry ice and a way to move large amounts of air across the cooler.

A sketch at:

I'm already thinking glycol for the fluid.

But what to make the rest of it from? Cheap, off the shelf stuff preferred! A pump that can handle antifreeze? High torque low power 12 volt DC motors? What to use for the heat exchangers? Coils of tubing? Or auto parts?

Other than a drip pan (and a handful of brain cells), what am I missing?

Thanks all,

Richard

• posted

have you calculated the heat capacity of dry ice? there is a reason we employ equations first to determine feasability

• posted

Lookup swamp coolers if you want to go the ice route. Biggest problem with any non a/c solution is the amount of humidity produced by other cooling methods - which usually ends up with hot damp(er) air rather than cold dry air.

• posted

What is the water temperature? It might be possible to simply use a heat exchanger to dump heat into the water. In thory, it would even be possible to get the coolant to circulate without a pump.

• posted

Ok, show and tell!

• posted

• posted

If he's on the water and needs a/c, probably. As a matter of fact, he'll need to calculate the load from condensation, about 1000 btu's per lb. of condensate. That's a major load on a/c's on the Texas coast. You could get a lb. of condensate out of every 1000 cf or so of outside air, so it's important to recirculate the air so you only have to dry it once.

Humidity isn't a problem here in central Texas right now. However, no outside grilling, smoking, welding, or any other open flame or sparks are allowed, immediate arrest if caught. I don't know about shooting, I guess muzzleloaders would be a bad idea. Come on, rain!

Pete Keillor

• posted

What is the humidity? I wonder whether a simple "swamp cooler" might work. (or sleep on deck :-)

• posted

Richard, it will take one *hell* of a lot of ice. Dry ice would be better than water ice but I know the water numbers off hand

9 btu to melt one pound of ice, another 31 btu goin up to 60, so use 40btu per pound.

That 5000 BTU ac running 5 hours is 25000 total btu

25000/40 is 600 lbs. I won't look it up but dry ice might be 6 times better or 100 lbs. for five hours. Do the math to get better that an approximation but you can see you'd literally need a ton of dry ice to do the weekend.

Karl

• posted

I just looked it up. Dry ice packs more punch than i thought 250 BTU per pound. or twenty pounds an hour to equal that 5000 BTU ac. You'd only need 1000 lbs. to do the weekend.

• posted

Not the numbers I remember from high school. But it would take a lot of ice. If I remember correctly a ton of airconditioning ( one ton of ice melting in 24 hours) is 12,000 btu's per hour.

Dan

• posted

Is that water temperature at the surface? What is the water temperature ten feet down?

Another approach might be to use a water bed. That 87 degree water is about ten degrees cooler that your body and in a water bed without foam insulation between you and the waterbed mattress will feel down right chilly. Doing something like that could keep you comfortable while you sleep and reduce the power needed in a 24 hour period.

Dan

• posted

Maybe at the surface, or in really shallow water. You've got a huge heatsink to work with, and as long as the water from down deep is cooler than the dewpoint, it should also remove humidity. Not sure how you could do it under way, but lowering a hose over the side long enough to get to cooler water would work when you are tied up.

Doug White

• posted

The question that comes to my mind is "how big is the boat". A water bed is a large almost-free surface and it could to really bad things to stability.

• posted

" snipped-for-privacy@krl.org" fired this volley in news:38bf5938- snipped-for-privacy@d25g2000yqh.googlegroups.com:

Yes, Dan, but you missed a nit.

Water absorbs 80 calories per gram going from 0C ice to 0C water, but then it also absorbs 1 calorie per gram to increase in temperature one degree C.

So a gram of water would absorb 80+(say)20 calories going from 0C ice to

20C water.

If you were a creature that was comfortable at 110C, you'd gain an extra boon, because water absorbs another 540 calories changing from 100C water to 100C steam.

LLoyd

LLoyd

• posted

He hasn't "missed a bit". The "ton" rating is a well accepted industry standard in the US.

• posted

Gunner Asch on Sat, 10 Sep 2011 03:56:22 -0700 typed in rec.crafts.metalworking the following:

On a boat?

• posted

"Stormin Mormon" on Sat, 10 Sep

2011 09:17:29 -0400 typed in rec.crafts.metalworking the following:

So, pump the CO2 gas into another tank for reuse. Refreeze it, or use it to charge "soda water".

• posted

"J. Clarke" fired this volley in news: snipped-for-privacy@hamster.jcbsbsdomain.local:

I wasn't talking about the ton rating... he surmised the figures were wrong because he didn't account for the additional heat absorption from

32F to 60F.

I know what a cooling "ton" is... I'll even bet Stormin' Morman knows. , but it only expresses a number of BTUs or calories absorbed per hour... it does not calculate (or even intend to calculate) how many BTUs are necessary to cool from one temperature to another. It's only purpose is to give a unit name to how much heat it takes to melt one ton of ice

-- that means from 0C ice to 0C water; none other.

LLoyd

• posted

I should have said " something like a water bed." You would not that much mass. With a lot less mass you would probably want some way to circulate water through the " marine water bed ". And a PIC to monitor the temperature and turn the pump on and off as necessary.

Dan

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.