pondering on air conditioning...

[pbbelly]

I've spent that last couple of hours perusing the posts concerning air conditioning a TD/TTT. Most of the time we are nowhere near an electric hookup when we use our TTT, and the thought of listening to a generator all night doesn't appeal to me much either.

So, while pondering a solution my thoughts have turned to our first year of marriage; just the two of us, struggling through our first semester of junior college, no kids, living in a 28' Shasta travel trailer...ah, memories! (is it any wonder we love the cosiness of our TTT?)

I digress. During the whole 15 or so months that we lived in that thing the only time our electric bill exceeded the $12 minimum (1981/82) was when we used a small window unit and an oscillating fan simultaneously. (With just the ac it was still less than $12!) I attribute the low bill mainly to the fact that everything electrical in there was 12v, running off of a small inverter. But, the refrigerator was a 3-way, LP,12VDC, 110VAC unit. it worked beautifully in any mode. Instead of using a freon/compressor system it used ammonia that was heated by either a propane flame or an electric heating element. We used the 110 mode almost exclusively.

If that system works so well cooling 12 cubic feet to -40˚ (or colder in the freezer comp.) why couldn't it, or something similar, cool the 96 cubic feet of our TTT to 72˚?

Here's an explanation of how the system works: http://www.lpappliances.com/G-Absorp.html

Some discussion on an rv website http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/15434119/srt/pa/pging/1/page/1.cfm
kinda poo poos the idea, but I don't think it is so out of the question for our sized stuff. And as for the "nasty chemicals" concerns, they have ammonia in their fridges, what's the difference?

It appears that since it relies on gravity for circulation that you'd have to leave everything in a vertical position. Which is probably a bonus-it could be a thin unit built into a wall. An enterprising individual could start manufacturing ready-made wall units for TD/TTT's!

Another factor that I am uncertain of is would it dehumidify the air?

Anyone have any insights?

Thanks!

pbbelly

[Ira]

Without even knowing what you're talking about, I know it won't work:

Greater minds have covered this in depth and ad nauseaum, and you just can't reinvent the wheel.

[Wright]

Well said Ira, I was planning on just quoting the Caveman on the Gieco commercials, "uhhh... What?"

[jp03]

Believe you are asking, why cannot an ammonia system be used for air conditioning? Well, they are commercially, real issue is more like expense and safety. The ammonia gravity refrigerator does generate internal pressures up to 500 # easy. Without a pump to operate, just like a freon system, the effeciency is low. The ammonia u clean house with, is like 1/10 the strength in a system. If it leaked, ur lungs would be chemical burned before u got up out of bed!

[madjack]

PB, if you read the whole RV.net thread, you have seen why it is not practical for this application...another example is using the modules from a thermo-electric fridge...we have had a couple of members, try that out with very little or no success...the volume is just to great for them to work efficiently...and in the case of absobtion units...safely.........
madjack

[Alphacarina]

Yes, Servel air conditioning has been around since the 1930's - A neighbor across the street had an Arkla model for several years. Two things about his system come to mind - His gas bill was always higher than my electric bill in the summertime and he had 'techniicans' out there working on it several times per year. He eventually scrapped the entire thing and bought an electric unit

Using ammonia absorbtion cooling powered by electricity is WAY expensive - It's only big plus was that back in the '40's and '50's when natual gas was dirt cheap, it could be operated in areas of the country where electricity was expensive a little cheaper than an equivalent electric model . . . . . when you could keep it running

Don

[pbbelly]

Believe you are asking, why cannot an ammonia system be used for air conditioning? Well, they are commercially, real issue is more like expense and safety. The ammonia gravity refrigerator does generate internal pressures up to 500 # easy. Without a pump to operate, just like a freon system, the efficiency is low. The ammonia u clean house with, is like 1/10 the strength in a system. If it leaked, ur lungs would be chemical burned before u got up out of bed!

I have had three camping rigs with refrigerators in them with enough ammonia to kill me-I never worried one bit about whether it was going to, and I have never heard of a case where someone died in their RV from ammonia fumes. So, what's it matter if I have a three or four times the amount ammonia? We have a saying in mining "after the first ton it doesn't matter" (usually quoted when someone is trying to guess the weight of a rock that could potentially fall and kill someone). So the presence of more than enough to potentially kill me wouldn't be a deterrent to me.

Propane consumption. How much could it burn in 8 hours? It's not like I am trying to cool a house, just a tiny little travel travel-not much bigger than a refrigerator!(funny!-a guy I work with told me that our TTT looked like a refrigerator on its side!) If I had that system in my TTT this year it would've been used two nights so far... Again, not much of a deterrent.

The initial cost is what nixes it for me. I haven't found a new gas refrigerator (to cannibalize for components)for under $1,000 and used ones are non-existent in my neck-of-the-woods.

So, that takes me back to a generator/petcool or generator/shaker system if I want a/c. I don't have a practical spot to install a shaker, so it would have to be converted for external use. Not a big deal. The problem that arises is the fact that a generator an a cooling unit will take up virtually all of my cargo area in my tow rig (when I need the back seat for my daughter) which is a major downside.

Once again I started thinking. and this one is workable, and I have pretty much everything to do it on hand:

I have a portable generator with a good engine & bad generator/housing (fell off of a truck and damaged it) I'm thinking of removing the generator and using that engine to power an r134 automotive a/c system-the engine can pull the compressor on the condenser fan. use a 12v evaporator fan. and an underdash evaporator unit. I can duct the cold and return air to the TTT, and wire in start/stop and blower controls. so I would have some control of it from inside the camper. (the thermostat would probably need to be on the unit, but maybe not?)

This would have to work-the TTT has only a little more interior volume than a pickup cab. and it would take up less cargo space than either a generator/pet cooler or generator/shaker system. The main potential problem that I see is the close proximity of the exhaust to the circulating air and the possibility of (deadly) carbon monoxide problems.

I could also permanently mount the engine/compressor/condenser on the tongue and mount the evaporator inside-that would all but eliminate any a/c air contamination problems (I could still get carbon monoxide leaking in from outside though but there again, just about every modern large RV has a genset mounted on it that could potentially do the same thing)

The big question is, do I want to go to all of this trouble (not much expense for me to go this route) for something that I will probably only use a couple of nights a year? And if I mount it permanently do I want to haul it around even when it isn't needed? (So far, since mid-June, we have spent 12 nights in our TTT and only wished for a/c on 2 nights; one in New Mexico the other in Oklahoma-next year we plan a long fall excursion to New England) Or, I could go with a petcool and just seek out sites with hookups on hot nights? Or, I could just buck up & tough it out!

Thanks for all of the input. If I do something worthwhile I'll post some pics & details.

pbbelly

[cleonard]

I have also been thinking about using an automotive derived system. There are a few issues. First it likely runs afoul of the EPA regulations. Because of this you would have to be your own A/C tech. No shop could legally touch your system. One item that you must have is a good vacuum pump. For my own personal use I would not worry about it too much and I do all my own A/C work anyway. No matter what do not sell the trailer with this kind of A/C system installed. That could get you into trouble.

Automotive A/C systems have a big heat moving capacity. Most are 20,000 BTUs or so at highway speeds. This means that they also suck up 2 or 3 horsepower. To reduce the power demand a simple solution is to just turn the compressor slower. The problem with this is the expansion valve/orifice tube. These are sized with the 20,000 BTU in mind. I'm not sure if the expansion valve type system could be modified. The orifice tube type systems would be easier as the orifice tube is just a hole. The good thing here is almost all modern systems are orifice tube except for high end cars. All you need is a smaller hole in the tube. Since all automotive systems are large it's going to take a custom orifice tube.

If you target a lower BTU output like 2500, then the hp requirements will scale too. The EER will not match a good window shaker, but I will use an EER of 8. So if it's 2500 BTU then the compressor will need 312 watts. This is small enough that it might be possible to just run the thing off of a few 12 volt batteries. Use an efficient electric motor to spin up some kind of flywheel to speed and then hit the clutch on the compressor. The inertia of the flywheel will take care of the starting torque for the compressor and the electric motor only has to supply the running power.

Doing this will not be simple. There will be a lot of trial and error before it is working right. Then there is the expense. If you buy new, automotive systems are expensive ($1000). There will also be a lot of time invested as well. However, the idea does have potential for someone willing to expend the effort.

After doing this analysis I pretty much decided that this kind of system is just not worth it for me. Since I live in SoCal an evaporative system should meet most of my A/C needs.

[SteveH]

When I built my teardrop, I knew I would be camping where there was no power most of the time, so I did not build in A/C. I have since added a window shaker working thru a dryer hose...see my gallery. However, back to the point, I installed a roof vent with a fan, then bought one of the cheep Walmart tent fans, and with the roof vent going, and the tent fan setting in front of the upwind window blowing on me, I am cool at night in most any kind of weather.

From what I know about electricity and power, I don't believe a reasonable alternative to having AC power, or having a generator will be found. Well, not in our lifetimes, anyway.

[Alphacarina]

Having a small generator laying around (like a Honda EU200) can be handy for many different things - Mine will see lots of use, besides just on the tear

They are certainly quiet enough to sleep around when they are running . . . . they kick back to an idle when the compressor in the window unit kicks off and they don't run much above a fast idle when the compressor is running . . . . and you easily get 10 or 12 hours of use to a gallon of gasoline - Pretty economical to run

Having a generator enables you to run a small microwave oven, which is a blessing all by itself. If you're using 12 volt refrigeration (like the Engel I'm going to use) then you also have the added benefit that whenever the genset is running, it's charging your battery at a 10 amp rate

Small window units are dirt cheap (I paid $80 for mine at Lowes last week) and they are also small, lightweight and easy to build in to something as compact as a tear

My A/C will be built into the back (comletely built in with no visible sign of it from outside the tear) and my Honda Genset will be installed in a small (secure) 'cage' sitting on the tongue, just between the front of the tear and the spare tire

Other than the initial cost of the genset (roughtly $800 for the Honda EU2000's) there really is no 'downside' to having available air conditioning wherever you go - I'm saving enough building my own tear that I'm going to splurge about $800 each on the generator and the fridge and consider it money very well spent - If I was in love with tent camping, I might feel differently . . . . I guarantee you my wife would - She wouldn't come at all if she had to sleep in the heat and here in the south, when it's nice enough to go, it's usually too hot to sleep

You can't rent too many $50 a night motel rooms before you've spent the initial cost of an air conditioned tear

Don

[pbbelly]

cleonard:

I have a vacuum pump and lots of automotive a/c experience so that is no issue. I don't think I'll mess with trying to use 12v to turn the compressor though. The thought of using an inverter to run a petcooler(or small window unit) has occurred to me. I am already using the highest reserve capacity group 31 deep cycle that I could find. I will have to crunch the numbers and see what kind of run time I could get going that route. A window unit would have been the way to go if I had built my TTT with that in mind. Any location option that I can think of now has drawbacks. My galley space is crammed full of cooler and water tank...The solution may be to cannibalize a window unit and make a built-in and carry a small generator?

Steve H.:

I have a fan in my roof vent and a neat little 12v oscillating fan that were perfect when we were camping in the "low humidity" states even at nightly temps in the 80's. (we rarely ran the fans more than 15 minutes) It was the night in New Mexico with 80+% humidity that was killer. And the state park that we stayed at had all of the hookup sites taken and they did not allow generators after 10 pm so even if we had ac and a generator we still would've cooked! The next night in OK did have electric hookups. Less temp but just as much humidity. The fans ran pretty much all night and we were still sweat soaked.

Alphacarina:

Same problem with limited nighttime generator use (which I realize would kill my engine driven automotive a/c idea as well)



There's a solution out there! (I hope) I will look into the inverter viability and see what I can find.

pbbelly

[pbbelly]

Ok,

The inverter thing may be viable. The petcool unit pulls 380 watts (in cooling mode, heating mode is much more, but that isn't an issue-we slept with the windows open all night at 27°F)

Now, I know that there will be a certain amount of inefficiency in the inverter, but assuming none: 380 watts divided by 12 volts yields ~32 amps. My deep cycle is rated at 205 amp/hours which works out to almost 6 1/2 hours of run time. I know that there will be a certain amount of power loss in the inverter, but I also know that the compressor will not be running all of the time so the unit won't require 380 watts continuously.

A 5,200 btu window a/c (at least the one I have) requires 595 watts which translates to ~50 amps or 4 hours (again not accounting for inefficiencies or compressor cycling)

As an aside: While trying to dig up power consumption figures I ran across this nifty little dehumidifier . If I duct the warm air outside, it might make a huge difference in comfort(in conjunction with our fans), and not consume too much juice in the process (65w). For $39 (+ the cost of an inverter which I am probably going to invest in one any way I go)it would be worth a try...

[Alphacarina]

You need to study up on Peukerts Law which describes battery capacity as a function of current draw

In a nutshell, it says that the higher load you put on a bettery, the fewer amphours you get - In other words, your 205 AH battery only has that amount available at a 20 amp rate and it will be a good deal less than 205 at a 32 amp rate

The 205 AH capacity is also only then the battery is new and will decrease as the battery ages - Could be half of the 205 if the battery is a couple years old and has been 'well exercised' . . . . like running an A/C frequently

Also, lead acid batteries don't last long if discharged much deeper than about 50% or so - You can really only 'use' about half of that 205 if you don't want to have to replace the batteries every few months - They will age fast if you discharge them much deeper than 50% with any frequency

Boaters have tried for years to make battery powered air conditioning 'work' and they have many things going for them that you don't in a tear - They can pretty easily carry 500 or 1000 AH of batteries and they can use 150 or 200 AH alternators to recharge the huge battery banks, but you still almost never see any boat with battery/inverter powered A/C

Don

[ARKPAT]

You can think about a Water Cooled A/C system. No A/C licence required. Just an Evaporator/Condensor/Pump/holding tank-expansion tank/fan. Most large Hospitals and Shopping Malls use this system to heat and cool ( variation of ) to this day. K.I.S.S. That is the way is started after the ICE Blocks melted.

Pat

[madjack]

WH, I wish ya all the luck in the world and hope you come up with something viable....however as Don (alphacarina) stated...folks have been trying for years to accomplish your goals...not just on boats but here as well and no alternative has been found yet...physics is physics and we haven't found a way to bend 'em yet...by the way...the info Don gave on battery usage/life/longevity is right on the money...your usable power is only half of the rated 205 and you will only get about 3hours at 32A draw...double that because the unit will not be running all the time and you get 6hours of usable time....also consider, that as the voltage decreases in the battery with usage, the amperage will go up to maintain the wattage and at the end of that 6 hours, you may be drawin' 60Amps or more...truly, I am not trying to rain on your parade, just trying to point out that physics is physics thing...here's ta hopin'' you have better luck at this than anybody I know has......
madjack