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offgrid ![]() Senior Member ![]() Joined: 23 Jul 2018 Online Status: Offline Posts: 5290 |
![]() Posted: 19 Feb 2019 at 11:58am |
It's not a precise calc at all. I just estimated the surface area A of the tank in ft^2 (about 31), assumed an air film R value (about 0.6 for no wind) and used a delta t of 60-15 = 45 degrees. That gives a heat flow of
Q= A/R*delta T = 2300 BTU/hr. Propane has a heat content of 91,600 BTU/gal and most gas heaters are about 80% efficient so that give 2300*24/91600/.8=0.75 gallons per day. Then as a total swag I assumed another 25% loss in the plumbing and water heater, so about 1 gpd of propane. Could easily be off by a factor of two or more. That's why I asked if you knew how much propane you started with, it would help to calibrate the calculation assumptions. The rigid foam should work fine but could be a hassle to box in. If you face the foil out that should be pretty good at protecting the board from road crud. Most everyone seems to like the Victron unit but its sure not cheap. I don't see much that could go wrong with the measurements of a cheap one but I've been surprised before. If you switch to a shunt meter be sure to put the shunt in the negative (grounded) leg and get one that is rated higher than the max current you ever need to provide. I don't think you'll blow the fuse in your ammeter but even if you do, that's what the fuse is there for. I've blown my share over the years ....
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1994 Chinook Concourse
1995 RV6A Experimental Aircraft 2015 Rpod 179 - sold |
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texman ![]() Senior Member ![]() ![]() Joined: 24 Jul 2014 Location: TeXas Online Status: Offline Posts: 446 |
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i ordered the paste you referenced. i'll take your word that it is what i need because i certainly dont know. ( i would have used silicone probably) so thanks again. i am thinking that i may ditch the Hall meter already. It reads negative (as in charging) when the pod is in an all off state. It should read about 250ma according to the klein using its' hall meter. i have tried every method i can think of to reset and always clears to -.1 amp no matter what. i guess it is reading the voltage from the earth somehow. ![]() as for the tank insulation, Off grid mentioned closed cell spray, which would be the cadillac for sure. I am considering using a closed cell polyisocyanurate foam faced available at lowes for $17 for 4x8 sheet and 3/4" thick. it is R5 and i would basically box the tank and reinforce the insulation for protection from high speed road objects somehow. still in design phase for sure on that. pipe insulation for the supply line. I have no idea how much propane i had or used. i know i was not full. how are you measuring propane usage so precisely? |
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offgrid ![]() Senior Member ![]() Joined: 23 Jul 2018 Online Status: Offline Posts: 5290 |
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For thermal paste I was thinking of this or similar:
Its used a lot in electronics to connect IC's and transistors to heat sinks. I've used it for temp sensors before too and its worked well as long as the temp changes are slow so everything has time to equalize. I think the overflow line would be the wrong place for the sensor because that will be warmest place. The pump suction line will only have the pressure from the water above it in the tank, so not much but it could still leak. So for sure try just attaching the sensor to the suction line and insulating first. Insulating the whole tank wouldn't be required to keep it from freezing but it would reduce propane use considerably over time. Depends on how long you intend to camp in freezing conditions. How much propane was in your cylinder the night you ran the test? With a water tank temp of 60 and an air temp of 15 I get about 1 gallon per day propane use with an uninsulated tank. If its windy that might double or triple. If you can tune things to get the average tank temp down to around 34 degrees the usage goes to about .4 gpd propane at 15F ambient, maybe 1 gpd if its windy. If you add an inch of foam that would drop to about 1/10 gpd propane, and wind wouldn't be much of a factor. Of course, my calcs could be wrong ![]() |
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1994 Chinook Concourse
1995 RV6A Experimental Aircraft 2015 Rpod 179 - sold |
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texman ![]() Senior Member ![]() ![]() Joined: 24 Jul 2014 Location: TeXas Online Status: Offline Posts: 446 |
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thank you Stephen, Lost again and Offgrid for the helpful ideas. My set point for the recirc is 33.5F. In my trial runs the sensor was basically reading air temps i think. It was place in a small hole that was left by FR mfg. where the water lines exit the interior and meet one of the low point drains. it was next to the water line but just setting there. Also, on the trial run i had not added the second Tstat to control the hot water heater, that is complete now. in full disclosure, i got up at 2am to check on it and temps were just below freezing and then went to bed and temps dropped to the teens so it caused the constant recirc. my fault but the bed felt too good. i think the best test will be on a cold day so i can babysit everything instead of staying up all night. i should be able to get the tstat sensor to fed through the floor and by affixed to the water supply line below the floor and insulate that pipe. i think that will be the best place since it is the most vulnerable to freezing. I'm don't think this is necessary, but the supply could be rerouted to go straight up from the tank through the floor and probably eliminate nearly 2.5 ft of exposed line leaving only maybe 8-9" of exposed line instead of 3 ft now. i will have to look for thermal paste, another new thing for me, and there have been many on this undertaking. so much fun for me. ![]() so it sounds like insulating the entire tank may not be required? pic of DPDT wiring with reverse polarity and led indicators and the other low point drain with exposed pex. ![]() ![]() |
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offgrid ![]() Senior Member ![]() Joined: 23 Jul 2018 Online Status: Offline Posts: 5290 |
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Even with zero insulation starting with 30 gallons of 32 degree water it would take something like 2-3 days of 0F weather to freeze the fresh water tank. So that is not the problem area. The exposed tubing is the issue, those can freeze pretty quickly, particularly the exposed winterizing drains which won't get heated well. Warm water doesn't like to move downward. If they can't be removed from above then it would probably be best to cap them off as high as possible from underneath and then insulate with spray foam. One other thought if using thermal paste and clamping the sensor on doesn't work well enough is to use an immersion type temp probe inserted in a fitting added into the supply tube where it exits the fresh tank.
The little tstats use a 10Kohm NTC (negative temp coefficient) thermistor, which is probably the most common temp sensor in use. Here is a link to one set up as an immersion probe: It looks like it uses an M8 male fitting so you'd need to work up a reducing tee fitting to go in the 1/2 inch pump supply tube that would reduce down to a female M8. I'd suggest trying the thermal paste, clamp, and insulate approach first. The other thing that will help is to slow the recirc flow rate way down and set the thermostat cut in point just above freezing. That will allow for the temps in the tank, supply tube, and sensor time to stabilize so the water in the fresh tank doesn't need to be heated much. All our houses here on the OBX are elevated so about 10-12 feet of the water supply line is exposed. Mine is 3/4 inch pex with foam insulation and if I leave one faucet barely dripping when its below freezing out I have no problems with that pipe freezing, at least down to 20 degrees ambient or so which is as cold as we ever get here. IIRC my incoming water temp in the winter is down in the low 50's so it really doesn't take much heating to do the job. |
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1994 Chinook Concourse
1995 RV6A Experimental Aircraft 2015 Rpod 179 - sold |
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lostagain ![]() Senior Member ![]() ![]() Joined: 06 Sep 2016 Location: Quaker Hill, CT Online Status: Offline Posts: 2595 |
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That's what I put on our water line from where it exits the tank to where it enters the trailer floor. I did it to protect the water from freezing on cold spring/fall Sierra Nevada mornings when it dips into the low 20's for a few hours while we're camping. I figured there is enough residual heat in the tank to keep it from freezing but the supply line hose is vulnerable. It was easy to put on and comes with adhesive already applied on the split edges that is pretty sturdy.
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Never leave footprints behind.
Fred & Maria Kearney Sonoma 167RB Our Pod 172 2019 Ford F-150 4x4 2.7 EcoBoost |
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StephenH ![]() podders Helping podders - pHp ![]() ![]() Joined: 29 Nov 2015 Location: Wake Forest, NC Online Status: Offline Posts: 6417 |
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What about using something like those foam covers for outside faucets on the low point drains? If you could rig a way to attach one, it might provide enough insulation to protect them.
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StephenH
Happy is the man that findeth wisdom,... ouR escaPOD mods Former RPod 179 Current Cherokee Grey Wolf 24 JS |
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offgrid ![]() Senior Member ![]() Joined: 23 Jul 2018 Online Status: Offline Posts: 5290 |
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texman, sounds like you're most of the way there, great! Just some tuning of sensor locations and settings. I think you're on the right track with your next steps. You'll for sure need to insulate any exposed tubing you can't remove. The drain lines are your freeze risk point as there won't be warm water flowing through those.
What temp setting are you using on the recirculation tstat? The challenge sounds like it might be to get the tstat temp sensor to read closer to the actual water temp in the pump feed line. You might try some thermal paste on the sensor to get it in better contact with the feed line, then wrap it tightly and insulate over that.
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1994 Chinook Concourse
1995 RV6A Experimental Aircraft 2015 Rpod 179 - sold |
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texman ![]() Senior Member ![]() ![]() Joined: 24 Jul 2014 Location: TeXas Online Status: Offline Posts: 446 |
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One of the low point drains. Other is almost identical
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texman ![]() Senior Member ![]() ![]() Joined: 24 Jul 2014 Location: TeXas Online Status: Offline Posts: 446 |
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Rear mixing valve
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