Last winter's surge in fuel prices provoked concerns that natural gas supplies might have to be curtailed in some areas. Most companies dodged that bullet, but the close call left a lot of people wondering what to do in case of a repeat this coming winter.
Natural gas curtailments haven't been much of an issue since the 1970s. For many companies, they fall into the realm of ancient history, and no one remembers them, let alone what was done to cope with them.
Well, let an old-timer who lived through it tell you -- most of them installed a backup fuel facility. In the desperate days of the early '70s, nearly everything was tried -- propane, butane, light and heavy fuel oil, gas made from coal, even sawdust. Heating equipment manufacturers were busy developing burners that could handle at least two fuels, sometimes more. The term "dual-fuel" was a standard part of the process heating vocabulary.
So, if natural gas supplies shrivel up during the winter, we can just go back to what they did in the 1970s, right?
Not really. Once reliable natural gas supplies returned to most areas, development of multifuel technologies ended. Many of the technologies and companies that thrived on the fuel pinch of the '70s are gone. The industry moved on to other challenges -- lower NOX emissions and higher efficiency. Today, if you want an oven burner capable of burning both gas and fuel oil, you'll probably have to pick a design that is at least 20 years old, with emissions and efficiency performance from that time, too. Of all the energy sources people resorted to back then, the only one that remains really viable is LP gas, or more correctly, LP vapor.
LP stands for liquefied petroleum. It's a catch-all term for propane, butane and mixtures of the two. It also includes propylene and butylene, which are sometimes included in blends, mostly outside the United States and Canada. In North America, propane is the most likely to be available in industrial quantities. Nearly any natural gas burner will operate on propane with little or no sacrifice in performance, so it's the most logical first choice for anyone looking for a backup fuel for ovens, dryers or other low temperature processes.
So, how do you get into the LP business? First, determine if there's an adequate source of supply in your area. No point in buying a tank if there's nothing to put into it. Second, make sure your combustion equipment will burn propane or butane. Most will, but some burners may need orifice changes to handle straight LP. The burner manufacturers can set you straight on that. Third, decide what type of backup system best fits your needs. You can go one of two ways: straight propane or propane-air. Straight propane systems (figure 1) require a storage tank, vaporizer (unless your demand is low) and three-way valve system to shut off the natural gas and bring on the propane. It's the simplest and least expensive system, but at a cost -- whenever you switch fuels, you'll have to reset your control valves and regulators. Otherwise, you'll get more propane than you need. This means making adjustments every time you change over.
Separate ControlsOne way around this is to run a separate propane piping and control system to each oven and dryer, preset them to the correct flows and simply switch over right at the burners instead of back at the head of the line. If you're going to go to this kind of expense, you're probably better off using a propane-air system (figure 2). In addition to the storage tank and vaporizer, this system incorporates a blender or valve that mixes air with the propane. A mixture of about 40% propane and 60% air can be substituted directly for natural gas without making any flow control adjustments. This eliminates all the hassle of a fuel switchover. In fact, some plants have their propane-air systems set to automatically cut mixture into the line whenever the natural gas pressure drops below a certain level. It's the most convenient of all the systems, and for a plant operating multiple pieces of heating equipment, it makes the most sense.
One final point: LP vapors are heavier than air, so they pose some unique operating hazards. For one thing, leaks tend to accumulate in low places. There are various codes and standards covering the construction and installation of LP systems. Make sure you're thoroughly familiar with them and follow them.