Convection heating a hottub 2

[mccheynebeeman]

Dear Participants,

I would like to use an open wood fire to heat a hot tub in the outdoors. I plan on making an open system, letting the water flow from the bottom of the tub through a radiator network within the fire, to outlets on the top of the tub using convection. I would like to control the desired temperature in a way that would use of as little energy as possible and was thinking of a solenoid valve that would disconnect this simple pipe network when the optimal temperature was reached, and engage a network of passive radiators to dissipate the heat before that water entered the tub. I can easily build the electronic circuit, but was wondering if there are some concerns about the design? (ie. will the patrons be scorched by boiling water if they sat too close to the hot outlets?)

Frazer RM Ross

 

[IRstuff]

No, they won't be scorched.


They'll be scalded.

TTFN

 

[mccheynebeeman]

Which ever word you would like to use, as long as we understand my issue. If this is the case (which I am not sure do to the unexplained reply), should I incorporate a water pump?

 

[MintJulep]

There used to be plans available for wood-fired hot-tubs. If the Google link up above doesn't give you any leads, check the classifieds in the back of Popular Mechanics magazine.

I would use a pump.

Don't forget that water expands when heated, and if heated enough, turns to steam.

 

[moltenmetal]

Yes, you need a pump. A thermosiphon arrangement is very likely to cook your patrons unless very carefully designed, so some mixing to limit the temperature variation from place to place within the tub will be needed. The zone between comfort and cooking isn't all that large. You'll also find that large, non-pilot operated solenoid valves (i.e. not requiring pressure differential to open and close) are very expensive and consume a fair bit more power than you expect.

 

[pennpoint]

mccheynebeeman
Most people can only withstand 120F degrees for a very short time (threshold of pain). You need to control (limit) the heat input or mix cold water with heated water, so as to not exceed that threshold temperature. What will a pump do alone? You need to assure that the entering water temperature does not scald you or your guests.
You should not tackle this on your own without further invesigation, at this time you seem to lack the know how to complete a safe working system. Think it out and ask more questions.
Hey! what the heck! just buy a commercial hot tub. You may end up with less medical bills.

Good Luck



Best regards
pennpoint

 

[mccheynebeeman]

just buy a commercial hot tub. You may end up with less medical bills

I would hope NO medical bills if this was a commercially produced hot tub.

Thank you all for your suggestions and criticism.

 

[pennpoint]

mccheynebeeman
I conceed your point regarding the medical!

There are wood fired hottubs which you can built or purchase however one has to be very careful not load the fire box with too much tender.
An online search will rinder many hits.

I do wish you well

Best regards
pennpoint

 

[StoneCold]

How hard can it be? I have seen it done a ton of times in cartoons! Then again most of the stories had cannibals in them.

Happy cooking
StoneCold

 

[mccheynebeeman]

I have found two types (although, I am sure there are more) of convection fire heated hot tubs. One is made by placing a modified potbelly wood burning stove in the middle of the tub and the other uses a similar approach to the one I mentioned in my initial post, reducing the heat by limiting air flow to the fire. Both require a paddle to mix the water.

Since this has been proven to work over history, and there is no pump involved with these systems, the only thing I would like to change is the temperature control. Instead of changing the temperature of the heat source, I would like to remove the heat in the water before it enters into the tub. I will do this via a radiator with a fan. There is much concern about boiling water & steam, which can be buffered by an open tank beside the tub (20 liters or so). This will alleviate the problem of steam. If the water is +120 I can have a temperature sensor within the buffer tank to sound an alarm.

Temperature can be measured inside the tub to control the fan on the rad. The only need for a power source would be the electronics and the fan which could easily be powered by a car battery for a couple days (given 4 hours of use a day).

The heating coil will be a 1 ½ “ pipe curved in a U-shape, (one pass in, one pass back) with a higher exit point for the “hot” and lower entrance for the “cold”. I plan on using garden hose, or depending on recommendations, fire hose to connect the heating coil to the rest of the system. The smaller the tubing and more surface area, the quicker the heat will transfer to the water and hence – steam – right?

The hot water inlets to the tub will be around ½ down the gross water level to allow for fluctuation (and I do understand that there will be a boiling problem if the water drops below these inlets).

The pump I am considering, but would rather not use because of the large consumption of electricity.

 

[ivymike]

maybe this guy could help...

http://www.break.com/index/bb717.html

 

[mccheynebeeman]

He probably learned his trade from these guys:

 

[mccheynebeeman]

Just a note: I built the heat coil & a control box for the hot tub and it worked great out in the deep woods. I put an inline pump and a car rad (which would turn on via comparator circuit if too hot). There were no steam shots, and no other threats to safety. When the water was at the proper temp, I disconnected my second coil and only used the one in the pic to keep a constant temperature.. around 102-105f.