Preventing Air Tube Burn-Off

What causes air tube burn-off? Can it be prevented? These questions occasionally arise. First, let us see if we can understand exactly why this happens. The air tube is fabricated from cold-rolled steel, which has a maximum operating temperature range of 1000°F. This has proven adequate over the years, primarily due to the fact that air from the burner fan pressurizes the air tube, scrubbing its surface and keeping its peak temperature well below the limit. On burner shutdown, the air tube temperature rapidly decreases due to the loss of flame.

Adverse Conditions Detrimental to a Burner Application

• If the flue passages become restricted due to scale or soot accumulation, back-pressure can cause hot gases to be forced around the air tube enclosure or entry tunnel. These gases may be around 2000°F and can easily elevate the air tube temperatures to a marginal level or even exceed its maximum rating. If this condition persists, the end of the air tube begins to oxidize, distort and finally burn-off. The flame is of very poor quality and this condition usually leads to a safety lock-out or a service call.
  The back-pressure that led to this failure could also have been produced by down-draft, poor draft conditions, or firing an input above the appliance maximum input rating.
• Higher CO2 levels produce elevated flame temperatures. Less excess air means a hotter flame and reduced cooling air down the tube. Generally, you will not encounter problems with air tube burn-off if the CO2 level does not exceed 12.5%. Operating above this in some dry-base boilers and most older furnace designs has occasionally resulted in evidence of chamber erosion and air tube overheating. Remember that everything has its limit. The combustion refractories used in modern equipment are usually rated to 2300°F. A flame temperature at 14.0% CO2 can possibly exceed maximum refractory rating and deterioration occurs.
• Another potential problem is the chamber entry opening around the air tube and combustion head. Ideally, the annular space around these components should be as tight as possible and sealed with adequate insulation. Sometimes, the old entry erodes or is extremely large in diameter. This allows hot gases to penetrate into the entry tunnel, and raise the air tube temperature. This can precipitate a burn-off condition.
  
• Closely linked to this is the air tube insertion depth. If the chamber entry is tight, then the combustion head must be 1/4″ back from flush with the inside chamber wall. (See Figure 1.) Should the entry be quite large in diameter, then the head could be pulled back further. Be sure that no664804-1.gif (11k) impingement of oil spray occurs upon the chamber entry tunnel. Again, we must reiterate that combustion gases can be 2000°F or more and any overexposure of the air tube or head to these elements can lead to air tube burn-off. The combustion head normally serves as a protective cover for the tube and is constructed of 18SR stainless steel, which can survive in the hot environments. That is why you may find it laying in the chamber perfectly intact when the end of the air tube loses its structural integrity.

How can air tube burn-off be prevented?

We have identified the major factors involved, and you can see the need to:

• Make sure that the combustion head does not protrude into the combustion chamber. It must be 1/4″ back from the inside chamber wall.
• Keep the flue passages clear of soot and scale accumulations. An annual cleaning is a good policy and helps prevent unwanted “back pressure.”
• Set the over-fire draft for negative .02″ W.C., or that specified by the manufacturer of the heating unit. Never operate the burner with a positive over-fire pressure unless the appliance is designed specifically for this.
• Use instruments to adjust the burner for a trace of smoke, measure the CO2 and reduce the CO2 by one full percent to a zero smoke level, i.e., (13.5% at a trace to 12.0% at zero.) Note: For oxygen, increase by one full percent. This will give you good efficiency and allow a margin of reserve air before smoke generation occurs. The combustion temperatures should be within the refractory tolerance and there will be less chance of exceeding the air tube temperature limits. Do not be “faked out” by unusually low CO2 readings. Infiltration around viewports, gaskets, boiler sections, etc. can dilute your readings. Seal all such leaks with furnace cement. Sometimes a comparison of over-fire measurements with the stack readings will reveal evidence of leaks.
• In an application where there is danger of overheating, it is usually a good practice to protect the air tube by wrapping a sleeve of refractory material around its circumference. This can be a prefabricated sleeve or a wrap of cerafelt, fiberfrax or Kaowool (all various names for alumina-silica fiber insulation). If a wrapper is used, take care to secure it with wire that can endure the temperatures. The Wet-Pak cerafelt material will conform to the tube shape and harden in place without as much need for securing as the other “dry” materials. Even a thin wrapper is better than nothing and if a sleeve cannot be used, then fill the air gap with small pieces that can provide protection. This insulation will refract heat and shield the air tube.
  NOTE: Where space permits, a prefabricated fiber heat shield is recommended. (Beckett part number 31338U.)

Never fire the appliance above the manufacturer’s maximum rated capacity.

While the incidence of air tube burn-off may be quite rare, you should be aware of the cause and the prevention. Should you find one burned off and merely replace it with a new one, you have not solved the problem. You must determine which of the above-mentioned factors apply and skillfully take the appropriate action outlined to remedy the problem.