Mono or Multi tube water heater

Our hot tub easts through electric at a scary rate - nothing wrong with tub or heater its just how it is. I got to thinking that I could easily make a stand alone heating unit for it running off propane (We use a lot of propane as it is). Then I tried to determine if it should be a mono (as in flash steam) type heater or a multi cross tube type heater...and I cant get any decent answers. Obviously it will be designed and built along boiler type materials so it can be well insulated, it only has to heat the water to a max 45deg C, 800 litres over about 8 hours. Whilst I have an abundence of copper tube and small bore pipe I would like to keep it as simple as possible. I will make the burners as standard PSB boiler burners as they put out good heat and dont use much gas (they will have to be tweaked to run on Propane though). Any suggestiond?

anyone?

Looking at this link it takes 73 mins for a 6kw heater through a 15l/m coil to heat 800L, the coil size is a mystery.

Why not go solar?

Interesting problem, raise a body of water to temperature, and simultaneously stabilise it against heat losses. A a general rule, gas is one third the cost of electricity generated from gas, due to conversion losses.

Dave's comment is interesting,regarding solar. Assuming direct solar heating, we've all felt the heat of water from a hose pipe on a hot day. But how doable? Today, in Cardiff, it's cool and overcast, so it depends on where Fizzy is located.

What's the power of the electrical heater?

Well, assuming no losses, heating 800 litres of water from 12°C to 45°C in 28800 seconds (8 hours), would require a 3.5kW heater and in that time burn about 31kWh of electricity. ( Heat value of 31kWh approx 107000btu )

Engineering Toolbox gives the Net heating value of Propane as 19834 btu/lb, so about 2.5kg of gas would be burned heating 800 litres (5.4lbs of gas).

Electrical heating is more efficient in that the element can be buried inside the water and the whole lot insulated. Not difficult to waste less than 10% of the heat input.

Gas is cheaper than electricity but it's much harder keep the heat in. The firebox part of the tank can't be lagged, and whatever is done in the way of lagging a fair about of heat disappears up the chimney. A full-size steam locomotive boiler won't be more than 80% efficient, and 60% or less is more likely. Big boilers are more efficient than small ones because they have a better ratio between volume and surface area. Low-temperature heaters are less wasteful than high because heat loss is proportional to temperature difference. Water tube boilers are more efficient than fire tube types, and most Marine boilers leave steam locos in the dust! (Not least because ships can accommodate heat recovery systems and thick lagging.)

There's a relationship between the size, number and length of fire tubes needed. Too few and too many both waste heat. I can't find the book covering this aspect of boiler design, it's to do with balancing the surface area of the tubes with hot gas flow so that the maximum heat is transferred: a single big tube is fast and wasteful, many small tubes are too slow and choke the fire. Perhaps someone else knows - it may not matter much in this application?

I don't know how efficient a home-made gas heated 800 litre boiler at 45°C could be. I guess a big fire tube boiler would be good rather than bad, so maybe assume 70% efficiency and expect to burn about 3.2kg of Propane over 8 hours.

Assumes all 800 litres of hot water are used and the tank is completely refilled with cold water every 8 hours. Consuming less than 800 litres would save energy, as would using waste chimney heat to pre-warm a feed water tank. (If there's space!)

I don't know anything about PSB burners either, but to give an idea of size small cooker gas-rings are about 1kW and large rings 3 to 4kW. Nothing terrifying needed! All seems do-able to me, just make sure there's plenty of air and the tubes flow well enough to stop an over-lean flame producing Carbon Monoxide. (Definitely buy a CO detector!)

Dave

Edited By SillyOldDuffer on 19/06/2020 14:28:48

Great care would have to be taken with any design with thermostat control and water level and safety valves.

Solar water heating kind of depends on how much cloud cover you experience in the country you live in, I have a vacuum tube water heater installed and supplies very hot (boiling) water all year round apart overcast days (which are rare in Malta where I live)

It's heating capacity is not affected by the outside temp, but by the amount of direct sunlight the tube can receive. So it could be a sunny day in cold weather and you'll get hot water. I doubt you could use solar power efficiently in the UK tho, I believe it's mainly overcast up there, right?

A thought. As central heating boilers became more efficient they had to have a fan to ensure a draft. If you cool your exhaust too much you too will need a fan.

Adrian

Why not use an old central heating boiler,the pump would circulate the tub water through the tub.Thermostat and controls built in.Need to change to propane. Most of the work is done .This is just an idea ,would probably work.The heat output is also known ,making any calculations easier.

Diesel fuel/bio-fuel powered waterheaters as used in trucks to keep the engine warm and any thing else that moves, have to admit not seen one used for a hottub yet! Try google " Hydronic M8" it's the type of water heater that I'm on about. Failing that theres always the hot water wash as used to clean trucks etc. Simple unit just a length of pipe coiled around a burner plus a couple of safety devices...............Stay safe.

Thanks..in burnley solar is a complete non starter, indeed rain power would produce more output. I have thought about converting an old gas boiler, no health risks as outdoors, stock leccy heater is 2kw and raises temp roughly 2 deg C per hour....and seems to cost a bomb.

Ok Burnley not known for its sunshine but if you used a solar unit as background heating it would then reduce the amount of electric used, even on a cloudy day solar units do heat the water albeit not as efficient as if you lived in Malta etc. I think there would be a useful heat gain from the solar and it would work all the time, except at night of course, therefore keeping the mass of water at a steady temperature thereby making a significant reduction in electric costs.
Dave W