Inside every flight manual are the operating limitations for the balloon. Among those items is the minimum propane pressure. That psi figure is probably around 60-70 psi- a pressure below which the aerostat's burner cannot generate sufficient heat to keep the balloon in the air.
Propane pressure is a function of temperature, as the temperature becomes colder you will have less pressure in the tanks. What is a safe pressure depends on the conditions, load, pilot experience, and other considerations. What we will focus on in this article are two methods to raise the pressure in the tank.
First, however, let's examine some methods that are not acceptable. In the fall of 1983 a balloonist and restaurant owner elected to run his balloon tanks through a commercial dishwasher where final rinse temperatures exceed 180 F. The pressure in the tanks caused the relief valve to open and the propane came in contact with a ignition source causing a number of severe injuries and $125,000 in damage to the restaurant. Not a good idea.
Balloonists have been known to carry their tanks inside a motel room during the night at an out of town rally. Sometimes bathing the tanks in hot water. Not only is that against the fire code but bathing the tanks in warm-hot water does not allow you to properly control the temperature environment.
In days of old (at least we trust days of old) pilots were known to place the tanks in front of heating units and even resort to heating the tank with a burner flame. No kidding, there is actual video footage of a balloon pilot using his burner to warm aluminum tanks. These are definitely not acceptable.
Today the two most common, not to mention safe and effective, methods for raising the pressure in propane tanks are heat tapes wrapped around the tank and nitrogen injected into the tank. Both are effective. Both have their advantages and disadvantages. Below is a discussion of each with their relative pros and cons.
Heat tapes were designed to prevent freezing of water pipes. Generally these tapes have a thermostatic device to prevent them from over heating at around 40 F. A temperature that is too low to be effective for propane cylinders used in a balloon. You can remove the thermostatic control, however you remove a fail-safe feature, which can allow the tank to over heat if the pressure is not monitored. The tapes can either be used directly on the tank or in a casing such as a padded tank jacket.
Even if a heat tape does not have a thermostatic control the amount of heat output can be regulated, by the number of times it is wrapped around the tank and the wattage of the heat tape. One manufacturer of heat tapes uses an 85 watt tape in a padded tank cover. The tank can be plugged in all night and the pressure will not exceed 140 psi.
Disadvantages exist with heat tapes that are used directly contacting the tank. Because they were designed for use with water pipes and thermostatically controlled they were not designed for the intense heat use associated with heating a propane cylinder to 70 F. The protective shielding of the wire can become weakened, causing it to fray. This creates the possibility of an arc-through of the heat tape insulation to the fuel tank, causing a weak point in the cylinder. One balloon manufacturer conducting an annual inspection discovered such a situation. When the nick created by the arc was measured, they found that it had penetrated two-thirds of the depth of the tank wall. If it had completely penetrated the tank wall a fire and resulting explosion would have made a mess. With heat tapes there is also the ever present danger of an electrical spark near a possibly leaking tank valve or other source.
The heat tapes cannot have a bend or kink in them. If such a condition exists the heat tape will short at the bend and eventually cause a fire. A repair station last year discovered that there had been an internal fire inside of a tank jacket. The tank jacket contained a heat tape which had been bent in a zig-zag pattern.
Another drawback to heat tapes is that once the source of heat is no longer applied (when the tank is unplugged to allow the drive to the launch site) the tank begins to cool to the ambient temperature with the resulting loss of fuel pressure.
With the use of the heat tape in an insulated/padded tank cover the dangers of arc- through are lessened considerably. The potential for the tape's insulation to weaken and a short to develop are still present and will eventually shorten the life of the unit. Also the insulated/padded tank cover will help hold the heat and maintain the pressure for a longer period of time. However, if a short or other problem develops, the padded jacket can hamper efforts to get to the tape to repair it.
Heat tapes also have the disadvantage of taking a long time to raise the temperature/pressure. Depending on the tapes and temperature it can take many hours to reach a desired level.
With heat tapes you must monitor the tank pressure to insure that they are working correctly. And once they are working nothing can guarantee they will continue to work.
Still, the heated tank covers offer a superior performance over just the tape itself. Several reputable companies such as Stumpf Balloons in Providence, Rhode Island, offer them for sale.
If you are going to buy heat tapes to use there is one brand that Aerostar recommends-Frostec II. The heat tape was designed for use with plastic pipes and is self regulating to 70 degrees Fahrenheit.
Several balloonists are using a different method to heat their tanks. They have installed between the tank and a padded tank cover a waterbed heater. The heater has a thermostat that allows them to regulate the temperature to what ever level they want.
For all these problems, padded heated tank covers do have their advantages. A set will probably cost you less than a nitrogen system. They are portable and serve the double duty of protecting you from the tank.
But, more importantly they not only raise the pressure of the fuel, they warm the fuel. So what you ask? Remember in the article on preparing for winter there was the discussion of "O" ring leaks? Those pesky little leaks are caused by various component parts contracting at different rates. Most propane systems are designed to handle temperatures down to 40 F. (about 70 psi). If you are running cold fuel through your fitting there is the potential for a leak to develop. If you have warmed the fuel it will eliminate this problem.
Dick Wirth in his book Ballooning... talked about the 1979 World Championship where the temperature reached -20 F. Bruce Comstock was the only competitor to use electric blankets to warm his tanks. He was the only competitor to not experience severe icing problems.
Nitrogen pressurization has been used in Europe since the late 1970s, and very successfully. In the late 1980s a number of balloonists in this country started using it, particularly in Colorado. Nitrogen is an inert gas and acts as an artificial pressurant in propane tanks.
By using nitrogen you eliminate the problems addressed above with heat tapes. A fuel cylinder, pressurized with nitrogen, then used to a small percentage of its total volume, will continue to have a final total pressure much greater than that of the propane's vapor pressure alone.
Using nitrogen is convenient, it can be applied a few moments before inflation in the morning and you can repressurize for an afternoon flight. The process takes only a few minutes.
This might sound like an ideal system but there are disadvantages. It takes special equipment. In fact all parties contacted by Balloon Life stressed that it is extremely important that the proper equipment, procedures and storage techniques be used when handling nitrogen and nitrogen pressurized fuel tanks. To properly outfit a system with the necessary gauges and systems will cost around $200. Add to that a monthly rental fee for the nitrogen bottle that is holding your supply ($5-10/month). Before investing in and using a nitrogen system contact your balloon manufacturer to get their recommendations. Not all approve the use of nitrogen in their system.
Nitrogen is not recommended for tanks with vapor withdrawal for the pilot light. The nitrogen rests in the top of the tank where your propane vapor is. The nitrogen reduces the reliably and operating efficiency of the pilot light. As the temperature decreases and the proportional amount of nitrogen increases in the vapor, the volume of propane vapor reaching the combustion area (pilot light) decreases. Should you pressurize the vapor withdrawal tank, the pilot light might work for a while and then go out. You can agitate the tank (shake it thus bringing the two gases back to equilibrium) and relight the pilot light. Since you never know exactly when you need fire power or when the pilot light might fail, pressurization of the tank with vapor withdrawal is not recommended.
As mentioned above running cold fuel through the fittings can cause potential fuel leak problems. Also combustion efficiency is reduced slightly by the presence of nitrogen. However, this reduction is about 2% and is negligible.
Nitrogen that was added to the system when the ambient temperature was cold can cause a potential problem should the temperature rise. Particularly in the Spring, as temperatures warm, the system should be purged. What is happening? If the ambient temperature goes up the propane liquid temperature goes up. Not only does the propane pressure incease but, some of the nitrogen comes out of solution as well, causing a geometric rise in pressure rather than linear.
While tank pressure increased by nitrogen will remain relatively high there is a downside. Once the amount of fuel drops below 25 percent the pressure in the tank will drop dramatically. The pressure can be raised by shaking the tank. Its the same principle that takes place when you shake a can of soda. When you pressurize a tank with nitrogen, 60-80 percent goes into solution with the propane, and the remainder stays with the liquid in vapor state, giving you the pressure increase. When you shake it, the pressure will go up because some comes out of solution.
Where are the manufacturers today on the question of nitrogen usage? Aerostar International has certified nitrogen pressurization of certain propane cylinders in Aerostar or Raven hot air balloons. The Balloon Works still does not recommend it, says chief engineer Sidney Conn. Conn told Balloon Life that TBW has not had enough experience with nitrogen to approve it. He did anticipate that one day compressed natural gas would be an approved method for pressurization. However, he did not have a time table for when that might be approved. Cameron offers a liquid pilot light option on its Mark IV Super and Mark IV Ultra double burner units, but liquid pilots and thus nitrogen pressurization are not available for Cameron single burner units.
Tarp Head, of Head Balloons, says the use of nitrogen makes sense but he has never had any demand from customers to certify a nitrogen system. Personally, Head says the weather he flies in just isn't cold enough to demand using nitrogen. Likewise, Aerostar told Balloon Life that nitrogen is not for everyone. They said those who would benefit most from using nitrogen were those who flew on a regular basis in cold weather.
Whether you choose to supplement your fuel pressure with heat or nitrogen keep in mind the discussion above. It is by no means exhaustive. For some pilots a well made, padded, heated tank cover is the most effective means of fuel pressurization. For others nitrogen may be the best alternative.
For a more technical explanation of nitrogen and its use as an alternative fuel pressure supplementation contact Aerostar and ask for the September/October 1988 Technically Speaking, Alternative Fuel Pressure Supplementation. You may write them at P.O. Box 5057, Sioux Falls, SD 57117-5057 or call (605) 331-3500. Tell them you read about it in Balloon Life for a free copy.
Have a safe winter and keep your fuel pressure up.