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Heating Hot Air Balloon Propane Tanks

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I’m looking for ideas for heating the propane tanks in my hot air balloon.

My hot air balloon feeds liquid, not gas, to the vaporizing coils of the burner. Heat loss from the balloon is about 1 million btu’s per hour. Our burners are rated for at least 12 million btu’s per hour. Burner output depends on fuel pressure. Propane fuel pressure is a function of fuel temperature. As fuel is burned off the pressure tends to drop and burner output drops. My burner requires a pressure of 60 to 140 psi. To achieve this pressure the fuel tanks need to be between 40 to 100 degrees F. Fuel temperature during winter flying can be an issue.

Some pilots tape self regulating heating tape to the exterior of their tanks. I have used a heat lamp suspended inside the basket with an insulating cover over the basket. Overnight, this system gives me about 10 degrees F rise, as compared to outside temperature.

I’d like to place a heater in the basket. But I must consider the possibility of an inflammable environment. Most heating elements get red hot. That’s not good. My more serious concern is the thermostat. These often have contacts, which can spark, at a temperature to set off propane.

A flammable mixture of air/propane should not exist in the basket, but prudence suggests this is not a good assumption for design purposes. This is especially true as I heat my tanks outside a motel room.

My idea is to use one of the heated-oil electrical radiators found in hardware stores. The thermostat would be eliminated, replaced with a backed up electronic temperature sensor mechanism and electronics to power the heater through a solid state relay.

Reasonable temperatures can be achieved with no more than 500 watts. A 1500 watt heater in the basket is overkill.

I’d like other suggestions for this issue.
Expensive, but for it being a fully tested and certified device and considering where it's being used I'd pay for it =O
Can you use flexible ducts from your balloon to a shell around your tank, then use a small fan to provide draft?

That might be a bit physically awkward having duct work like that, but sounds like a good idea to me, the energy is already there. I would however be worried about the ductwork catching on fire =P
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Some balloons use 43 lb (10gallon) aluminum tanks. Othes use 15 gallon vertical stainless steel tanks. My wife flies with three 10 gallon aluminum tanks. My balloon has a horizontal stainless tank of 20 gallons.

Our tanks have a foam insulated cover that helps retain heat, or cold, and protects knees from bumps. I actually saw a tank, wrapped with a tank heater that had scorch marks from a short in the wiring. This was particularly bad as the tank was aluminum, a material that loses much of its tensile strength with a small rise in metal temperature.

I think the concept of a ducted heater, using forced air through a tube to a remote basket has merit. Maintaining positive pressure makes it difficult for a propane leak to flow up the duct to the fan/heater. If the fan is placed above the basket it would be unlikely to experience gas in a propane leak, as propane gas is heavier than air, especially at low temperatures.

I feel better if the source of heat is below the ignition temperature for propane, say below 700 degrees F. That probably means a heating unit contained in some type of liquid, like the oil bath heaters I mentioned above.
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A good heating blanket would do fairly well also.

It was a common trick an old friend of mine used years ago during winter construction when they ran heater off of propane cylinders.
They just wrapped up the tanks in heating blankets and left them on.

The blankets have built in thermostats that limit them from ever getting to hot. If you ever felt around on a heating blanket and found a number of hard lumps along the elements thats where the internal thermostats are located.
The tanks would stay warm to the touch but not too hot and could sustain pressure right down to the bottom in most weather.
Have you considered a completely different solution. Are you able to maintain pressure a different way? Say you used a small argon cylinder with a regulator that fed into the gas portion of the propane tank to maintain 100psi. Small argon cylinders are available for welding.

Nitrogen is used in some balloons for pressure augmentation. It has several limitations.

1. If nitrogen is added and the tank is not used, dramatic pressure rises can occur in the tank during heating of the day. This means the tank has to be vented to remove the nitrogen. The nitrogen doesn't just say on the surface of the propane. The gas goes into solution with the propane making its removal more difficult.

In high dessert areas temperatures can go from freezing to the nineties. This can result in the pressures high enough to blow the pressure relief valve (about 270 psi.)

2. Some balloons use pilot lights fed by the gas vapor in the top of the propane tank. Adding nitrogen can reduce the dependability of the pilot light.

3. Burner and fuel feed valves generally contain o-rings. These rings stiffen at cold temperatures making leaks more likely. Heated tanks are not likely to face this problem.
Once an o-ring starts to leak it tends to get worse. The vapor temperature of propane is -44 degrees F. So once a leak starts the area around the leak cools to this temperature. The best way to solve this problem is to turn off the propane, let the leak area warm. Once warm the fuel can often be turned back on without the leak.

I'd really like to find a heat source that doesn't get red hot.
I used electrically heated coveralls (ex airforce bomber crew) from a military surplus store. One for each cylinder powered by two 12V gel packs in series.
Cylinders were stored overnight in standard electric blankets and the coveralls were fitted just before lift off.
If the cylinders are kept warm overnight you really only need inflight warming for the master cylinder due to the chilling from the vapour takeoff for the pilot light.
Our flying conditions allow us to fly with warm tanks without the need for supplemental heating during the flight.

I'm interested in beginning flight with warm tanks. During that hour of a flight heat loss is not a problem.

My current method using a 75 watt heat lamp is effective for temperatures that drop a few degrees below freezing. These lamps heat by radiation and I prefer to use convection. I think it is safer in a very small enclosed space. My basket interior is about 30 cubic feet.
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