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FYI: Fuel Heaters - The Uncelebrated Hero Of Modern Jet Propulsion

Turbine engines rely heavily on these devices of extraordinary precision for their safe and reliable operation.

Text and photos by Wayne Thomas

It is just about impossible to keep moisture from finding its way into most any type of fuel supply during storage, transport, or even as it is being consumed. Jet fuel in particular seems to be most adept at holding H2O molecules in suspension. Unlike with a tank of Av-Gas, water in a tank of jet fuel is very reluctant to simply fall to the lowest part of the tank in a single, virtually drinkable sphere. Therefore, a certain amount of moisture, water or H2O - whichever you care to call it - is expected to be present, at any given point in time, in a turbine-powered aircraft’s on-board fuel supply.

The real danger caused by moisture in jet fuel occurs when the H2O molecules freeze and form ice crystals as the fuel is introduced to the engine. Needless to say, when clumps of ice meet up with a turbine engine's fuel nozzles - things stop happening in rapid fashion. The fuel heater's job, therefore, is to maintain the incoming fuel at a temperature sufficient to prevent ice crystals from forming. This way, the moisture content will pass with no harm caused.


YOU’LL LIKELY ENCOUNTER A SOUTH WIND There are other brands of fuel heaters, but of all the ones in use today, those made by Stewart Warner South Wind Corp. are the most numerous. For this reason, they will be the primary component of this discussion.

According to Stewart Warner, the ideal fuel delivery temperature for turbine engines is 70°F to 90°F (21C to 32C). Not surprisingly, this also happens to be the targeted fuel delivery temperature range that is designed into their fuel heaters.

With only a 20° F temperature range to stay within - and with hot engine oil as the only source of heat to work from - requires that the fuel heating device be able to both quickly and accurately gage fluctuating fuel and oil temperatures - and react accordingly to them.

Interestingly enough, fuel heaters have a relatively low internal parts-count; a liquid-to-liquid heat exchanger - and a thermal element mechanism which acts both as a temperature sensing device and as an oil control valve, make up perhaps 90% of the components. A welded-together aluminum housing with integrated oil and fuel passageways make up the fuel heater body and gives permanent residency to the heat exchanger assembly. A very sturdy design which is also very light in weight.

HOW IT GETS DONE

All Stewart Warner fuel heaters use a pressure fed supply of hot engine lubricating oil to heat the engine-bound fuel to the desired temperature range of between 70° F and 90° F, as measured at the fuel outlet port of the fuel heater. But, before the fuel exits the fuel heater, it must first come in contact with the thermal element located inside the fuel discharge passageway. The thermal element consists of an expansive (and expensive), thermally sensitive material which changes in size according to the temperature of the fuel that is contacting it. This change in size results in movement of an oil control valve which, in turn, very precisely regulates the volume of hot engine that is allowed to flow through the fuel heater (similar to the way a vernatherm works in an engine oil cooling system).


Under normal conditions, engine oil circulates through the fuel heater’s heat exchanger core to heat the fuel. Should the temperature of the fuel rise to the upper threshold of 90° F, the oil will then be diverted through a bypass route, leading directly to the fuel heater’s outlet port. The intermediate positions of oil flow control through the fuel heater are infinitely variable in order to maintain a constant fuel output temperature of between 70° F and 90° F.


NO FUELING AROUND Being attached right there, to the side of an operating turbine engine, places the fuel heater at a reasonably high risk of becoming victim of thermal persecution in the form of direct flame exposure (from an engine fire, for example). Being the trustee of large volumes of pressurized fuel in this situation also means that the fuel heater must be capable of not only surviving such an occurrence, but it must also continue to perform its original task of controlling the fuel temperature without - quite literally - adding fuel to the fire.

To help protect the fuel heater from fire induced meltdown (and to reduce the chance of subsequent catastrophic damage to the airship), Stewart Warner used to apply an intumescent paint coating to the exterior surfaces of some of their fuel heater models. This coating is characterized by having a thick, pancake batter-like appearance - and amazingly was capable of withstanding direct exposure to a 2,000° F flame, for fifteen minutes. Which seems even more extraordinary when you take into account the fact that the fuel heater’s aluminum housing has a melting point of just 1,100° F (which is reached after only a few brief seconds when exposed to a 2,000° flame).

TOO GOOD TO BE TRUE

Unfortunately, the intumescent paint’s beauty was quick to pass and is no longer used. The problem, discovered after many hundreds of fuel heaters were produced and made their way into service with the pancake-batter attire in place, was that it would become brittle after relatively few hours in service. Cracks would develop, allowing moisture to become trapped between the intumescent coating and the fuel heater housing, causing corrosion damage. Sometimes with such severity that holes would open up, creating just the type of fire hazard it was meant to protect from. Eventually, large pieces of the coating would fall away, leaving large areas of the fuel heater with no thermal protection at all.

 

All S/W fuel heaters currently being manufactured come finished in an aluminized epoxy paint. True, the epoxy paint cannot provide the level of fire protection that the intumescent paint once offered, but it does offer the fuel heater’s aluminum exterior a high level of protection from other hazards of its environment like corrosion.

THE ONES TO WATCH OUT FOR The now abandoned intumescent paint was used on perhaps only two Stewart Warner fuel heater part numbers; P/N 10585 and P/N 10718. These two part numbers, however, are used on a great number of Pratt & Whitney’s most popular engines. You’ll find that most variations of the PT6 type engine uses the P/N 10585 fuel heater. While fuel heater P/N 10718 was used on most of the PW118 thru PW127 Pratt & Whitney turbine engines which were original equipment in airframe makes ranging from Aerospatiale to Jetstream. The bottom line is this: Any fuel heater still in service with the intumescent paint on it should be dealt with right away, in the manner specified for that part number.

Stewart Warner Service Bulletin #10585-73-08R1 details the corrective steps for intumescent coated fuel heaters bearing P/N 10585. In a nutshell, this S.B. calls for removing the thermally resistant coating and then replacing it with aluminized epoxy paint. Be advised, however, that performing the actions laid out in this Service Bulletin are best left to well qualified repair facilities. Highly toxic, hazardous chemicals are required to properly remove the intumescent paint (what’s left of it). In addition, applying the replacement finish, an aluminized epoxy paint, is an experts-only affair because it employs a paint system consisting of a three-part primer, then a two part aluminum metallic epoxy top coat.

Dealing with any remaining P/N10718 fuel heaters you encounter, on the other hand, is a super simple, two-step process; 1.) Remove unit from service. 2.) Replace it with the new P/N 10839F fuel heater. There is no corrective plan of action for P/N 10718 units - but don’t throw them in the trash dumpster! You will find they’ve actually got considerable trade-in value which can be applied toward the cost of the new P/N 10839F replacement unit. Contact your favorite authorized Stewart Warner distributor for details.

Photo at right; Part number 10718 S/W fuel heaters, like this one, are supposed to be removed from service and condemned (and replaced with P/N 10839F unit, per S/W directive dating back to 12/29/00). Unlike the P/N 10585 fuel heater, there’s no approved method for replacing the intumescent coating with epoxy paint. The P/N 10839F fuel heaters come from the factory finished in aluminized epoxy paint. According to Stewart Warner, the P/N 10839F fuel heaters are constructed with internal heat shielding, not found in the P/N 10718 units, allowing them to qualify for the 2000F flame exposure for 15 minutes, rating (so long as there’s engine oil flowing through the fuel heater, that is).

 

CARE AND FEEDING
Any disassembly, repair, or overhaul of the fuel heater is something that’s best left to a qualified facility, or factory authorized repair outfit. The internal components, though few in number, are manufactured to extremely close tolerances. The fuel heater overhaul process requires much in the way of specialized equipment and trained technical expertise. In addition, some fuel heater models have undergone a vast number of evolutionary design changes which can require an expert to correctly determine what upgrades or modifications are appropriate for any given unit.

Basically, the fuel heater should be overhauled at a time which coincides with any major engine work being performed. Aside from that, it is always a good idea to make a close visual examination of the fuel heater any time the opportunity presents itself. Any signs of corrosion or un-painted (exposed) areas of the fuel heater housing indicate that immediate attention is - or will soon be - needed. Any evidence of external fuel or oil leakage from the fuel heater should be viewed as very serious and not left unresolved.




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