Confused about AC

[Catigale]

1.09 * CFM * deltaT = BTU/hr

Did the math in my head. Did I get it right?

You left off the transfer efficiency factor, which on this unit, kills it.



Short version .,,,the rate at which heat is transferred is proportional to the temp delta.

[mastreb]

Interesting concept providing some cooling at only 1amp / 12V according to the manufacturer:

http://www.cabincooler.eu

Supposed to be $415 including freight according to their add in "cruising world". I did a quick calculation on the example numbers they used and came up with about 4,000 btu/hr when the air temp in the cabin is 30C and the water temp at 16' depth is 14C. Won't work as well for those shallow bays and anchorages.

So that unit isn't actually an A/C, it's just a heat exchanger which means that it won't condense moisture out of the air except to drip a bit off the radiator.

You could easily make one out of a motorcycle radiator, electric cooling fan, pump, some hose, and some coiled copper tubing to make the heat exchanger. The first three parts can probably be found as a unit at a wreck yard.

[Jimmyt]

Several issues have to be checked to evaluate the concept. First, if you need 4,000 btu/hr, you need to pump enough cold water through the heat exchanger to transfer the heat. This can be evaluated by 500 x gpm x delta Tw (500 x water flow rate x water temp rise in deg F) to get btu/hr cooling (or heating if you're using hot water). For cooling, tHe water needs to be cooler than the dew point of the cabin air you want to achieve (typically 50F water or below coming into the cooling coil). The capacity formula presented in the earlier post is for the "sensible" cooling only (no condensation). That will give you the correct value of the heat required to change the air temperature (heating, or cooling without condensing). The total capacity with condensation is given by 4.5 x cfm x delta H where delta H is the change in enthalpy of the air in btu/lb between the entering air and leaving air conditions (coming into the cooling coil and leaving the cooling coil). If you live in a humid area, you're going to need pretty cold water to create comfort. If you live in a dry area where you don't need to remove moisture from the air, the little heat exchanger/pump/fan, might provide some value - if you can get cool water. If you're lucky enough to be in an arid climate, evaporative (swamp cooler) cooling might be better than the pump and coil arrangement. Typical commercial cooling water is supplied at 45 degrees or lower to produce indoor environments in the 75 deg F, 50% RH range. You need to remember that unless your coil is infinitely large, the air temp leaving cannot be equal to the water temp entering. Typical commercial temp spreads would be 45 deg F water entering for a 55 deg F air temp leaving. Just a few generalizations to get those creative juices flowing.
I'm considering some type of a/c myself, since it is hot and humid down here in Mobile AL. If I could just figure out how to condition the launch area where I'm setting up and taking down the rig...

[Starscream]

Yup, just a heat exchanger. A pump, two coils and a fan.

Capacity calculations were based on the example on their website. 30C entering air and 14C discharge air. I agree this situation is totally unbelievable since you can't produce 14 degree air with 14 degree water.

The website is really bad. http://www.cabincooler.eu. It's an interesting concept that could certainly work in certain environments, and the price is right if it could actually deliver useful cooling. Like Mastreb says you could make your own with some junk parts, but I like the idea of having a pre-engineered system with some level of corrosion resistance.

Sounds like the perfect opportunity for someone knowledgeable with an established business to become a distributor. Someone who could present at least a half-decent website.

In Canada, a system like this could be workable. Even in a shallow anchorage the water temps are rarely over 65 to 70F, and the air temp is almost never of 90F. A system like this could be just the ticket to bring the cabin temp down just a few degrees and make it more liveable. For those 4 or 5 days in our summer when you need it.

[mastreb]

Yup, just a heat exchanger. A pump, two coils and a fan.

Capacity calculations were based on the example on their website. 30C entering air and 14C discharge air. I agree this situation is totally unbelievable since you can't produce 14 degree air with 14 degree water.

The website is really bad. http://www.cabincooler.eu. It's an interesting concept that could certainly work in certain environments, and the price is right if it could actually deliver useful cooling. Like Mastreb says you could make your own with some junk parts, but I like the idea of having a pre-engineered system with some level of corrosion resistance.

Sounds like the perfect opportunity for someone knowledgeable with an established business to become a distributor. Someone who could present at least a half-decent website.

In Canada, a system like this could be workable. Even in a shallow anchorage the water temps are rarely over 65 to 70F, and the air temp is almost never of 90F. A system like this could be just the ticket to bring the cabin temp down just a few degrees and make it more liveable. For those 4 or 5 days in our summer when you need it.

Well, it's a closed-loop system, so just putting coolant in the radiator would work. The only corrosion issue would be with the hose and copper tubing that goes into the water, and that stuff is both cheap and easily replaced. If you're serious about it, I really think doing your own with an existing radiator of some sort is a pretty simple way to prove out the concept. It would likely work well here in SoCal as well, since the water is cool and the air is not humid.

[Jimmyt]

Starscream - Sounds like you have some nice summer weather! Down here, the heat and humidity is so bad, we have 4 or 5 days when we don't need gills to breathe... Of course, our winters are pretty sweet.

I'm with you on the pre-engineered system idea - with certified performance data. Unless there is published performance data over a wide range of conditions, so you can verify that it will work in your specific application, I'd be wary. Of course, I've got a couple of kids in college and one getting married, so $415 is still real money to me. Some may be able to get one just for grins to check it out.

[Russ]

Yup, just a heat exchanger. A pump, two coils and a fan.

Capacity calculations were based on the example on their website. 30C entering air and 14C discharge air. I agree this situation is totally unbelievable since you can't produce 14 degree air with 14 degree water.

The website is really bad. http://www.cabincooler.eu. It's an interesting concept that could certainly work in certain environments, and the price is right if it could actually deliver useful cooling. Like Mastreb says you could make your own with some junk parts, but I like the idea of having a pre-engineered system with some level of corrosion resistance.

Sounds like the perfect opportunity for someone knowledgeable with an established business to become a distributor. Someone who could present at least a half-decent website.

In Canada, a system like this could be workable. Even in a shallow anchorage the water temps are rarely over 65 to 70F, and the air temp is almost never of 90F. A system like this could be just the ticket to bring the cabin temp down just a few degrees and make it more liveable. For those 4 or 5 days in our summer when you need it.

Interesting idea. Not sure how well it would work. A typical A/C unit has an evaporator coil with temps around 40F (4C) with a high speed fan to exchange the heat. Assuming 100% efficiency exchanging water temp on the underwater bits you need some cold water below. Now if you could get water from under your boat that cold and have a fast blower, perhaps it would work. Water temp at the surface for us is 72 degrees. I have no idea how cold it is down deep.

Clever contraption. First test would be a thermometer on a string and see how cold it is below. Anything warmer than say 50 degrees seems like it won't work IMO.

1amp also seems way too low for power consumption. It requires a pump and a fan. In order to get to the 1A max, those would have to be weak IMO.

Down here, the heat and humidity is so bad, we have 4 or 5 days when we don't need gills to breathe.

I literally laughed out loud at this. Humidity is the killer isn't it? Especially sleeping. Fortunately we're currently at 25% humidity so heat isn't so bad.

[Jimmyt]

Spent most of my career (so far) battling heat and humidity in buildings, and trying to prevent the growth of our city flower (black mold).

You're right on the mark - Nothing worse than trying to sleep while marinating in a puddle of your own sweat.

I grew up in this stuff, so 25% RH turns me into a cracking, bleeding, pile of skin flakes. It sure sounds nice, though. When we hit 50-60%, and below 80 deg F, it feels like heaven to me.

[Russ]

Spent most of my career (so far) battling heat and humidity in buildings, and trying to prevent the growth of our city flower (black mold).

You're right on the mark - Nothing worse than trying to sleep while marinating in a puddle of your own sweat.

I grew up in this stuff, so 25% RH turns me into a cracking, bleeding, pile of skin flakes. It sure sounds nice, though. When we hit 50-60%, and below 80 deg F, it feels like heaven to me.

I can only imagine the heat there now. My business partner is in Mobile. I forgot how he put it the other day, something like the time it takes to walk from his car to the office door his back is covered in sweat.
I grew up on the East coast (NJ) and spent many a night on the boat with a fan blowing trying to dry my skin. Your description "marinating in a puddle of your own sweat" strikes a memory.

Big difference for me here. We were on the boat Wednesday night looking at the meteor shower and were COLD. Down right cold with blankets on. Gets cold at night so sleep is nice. Right now, I'm looking at 89 degrees and 25% RH. Hot, but your clothes don't stick. And by 10pm it will be cool and dry outside, windows come open.

If I was going to do A/C on my boat, I'd probably do a built in unit with water cooling. Use a small genset like the Honda 1000. Proper cooling just takes a lot of energy. More than can be stored in a battery. That water heat exchange gadget does look intriguing.

Or, as I used to do, use those small O2 fans. Work great. We've never spent a night at the marina (plugged in).

[vertex2100]

Check out the Yamaha 2000 generators, too, to run your AC, absolutely amazing machines. I bought three many years ago, one for boat, one for house and spare in case destroyed by salt water but spare is still in the box 7 years later. I have had great success with them. I put synthetic oil in and run it pretty much the whole 2 weeks we usually go out. The advantage of cheap window AC units are cheap, easily replacable when they die ( although I still use the same 7 year old one on my boat) and you don't need the boat in the water to use it. I run my AC any time I'm working in the cabin in the heat, day and night while preparing the boat for a trip, and, when we trailer from home, we run the gen., AC, battery chargers and freezer the whole way there. The boat is nice and cool when eventually boarding. Ran the system all the way from Jacksonville Fl to the Keys to keep the cabin and all the different food cold (dry goods and fresh fruit, frozen and in cooler). Bob

[WASP18]

Air conditioners efficiently cool, clean and dehumidify air. This unit only cools air at the level of the water's temperature it circulates.

[kurz]

Could you use an AC to dehumidify air in the cabin during winter? Does it work in low temperatures?

[vertex2100]

Could you use an AC to dehumidify air in the cabin during winter? Does it work in low temperatures?

If you were trying to use it as a regular AC, with the condenser outside, you would just freeze up the evaporator. It should work as a small dehumidifier if you put the whole thing inside the cabin and had a drain for the water. Used like that, it would warm the cabin somewhat from energy losses. You could make the world's cheapest reverse cycle heater by sticking it in the doorway backwards in the winter.

[Seapup]

kurz wrote:
Could you use an AC to dehumidify air in the cabin during winter? Does it work in low temperatures?

If you were trying to use it as a regular AC, with the condenser outside, you would just freeze up the evaporator. It should work as a small dehumidifier if you put the whole thing inside the cabin and had a drain for the water. Used like that, it would warm the cabin somewhat from energy losses. You could make the world's cheapest reverse cycle heater by sticking it in the doorway backwards in the winter.

If you get a portable you can run it without the vent hoses, vents both the hot and cold air inside. They have a dehumidify mode, but on mine its just full cold.

[Catigale]

Could you use an AC to dehumidify air in the cabin during winter? Does it work in low temperatures?

You have to get the coil temperature below the dew point of the dry winter air.

That will be difficult. In upstate NY, dew points are below freezing from OCT-APRIL!