The Doctor has been making house calls and has found a disturbing trend – far too many heat treaters do not understand the function, design, adjustment or safe operating requirements for their flame curtains. It’s time to review a subject too often taken for granted. Let’s learn more.
FunctionFlame curtains across furnace doors (Fig. 1) have multiple functions:
- To produce a vertical stream of combustion products across the full width and height of the door opening, thereby minimizing both the infiltration of room air into the furnace chamber and disruption of the furnace atmosphere inside.
- To serve as a positive ignition source of the combustible atmosphere that escapes through the door when it is open.
DesignMost flame-curtain systems are engineered to utilize your plant natural gas, propane or aerated propane supply regulated down in pressure to a stable 4–6-inch water column (2.30–3.45 osig). Air is supplied by either your plant high-pressure air supply adjusted to 80–100 psig, a small combustion-air blower or by tapping into the existing combustion-air blower if your system is gas fired.
For high-pressure systems, the air line should include a venturi-type inspirator, which converts a relatively small flow of high-pressure air to a much larger flow of air at low pressure. A globe-type valve in the air line, ahead of the solenoid valve, is the “shutoff valve” used when shutting down the furnace and flame curtain. A needle-type valve in the air line, downstream of the solenoid valve, is the “flow-adjusting valve.” Having once been properly set at the time of initial start-up of the furnace, the setting of this valve normally will not be changed.
The flame-curtain gas- and air-solenoid valves are connected in parallel to the door position-sensing limit switch. Thus, as the door just begins to open, both solenoid valves open simultaneously and the curtain “lights-off” and continues to fire until the door has been closed. As the curtain is engineered with a very high firing rate, it generates considerable heat. To prevent possible damage from heat distortion to the doorframe and/or door plate, the door must never be held open any longer than required to turn on and initially adjust the flame curtain or to subsequently load or unload the furnace.
Whether you have a high- or low-pressure system, full coverage of the door opening must be routinely checked by operators, supervisors and maintenance personnel to determine if/when adjustments are needed. Variation in plant air pressure is the most likely reason flame curtains go out of adjustment.
Operating Safety RequirementsThe key operating safety requirements for most furnaces with respect to the flame curtain are:
1. Whenever routinely starting up a furnace and before opening the door to turn on and verify adjustment of the flame curtain, you must first turn on the flame-curtain pilot, ignite it and verify it to be burning with a good, strong blue flame. Be sure that drafts or other sources of air do not move the pilot flame into a position where it cannot ignite the flame-curtain air/gas mixture when the door opens.
2. Whenever the furnace is in operation, you must never open the door without first verifying the flame-curtain pilot to be burning with a strong, stable blue flame.
3. Whenever you wish to leave the furnace idling at temperature overnight or over a weekend with atmosphere flowing, you must verify – before you leave – that the flame-curtain pilot is burning and properly positioned.
4. Whenever you leave the furnace shut down and either still cooling or already cooled, you must make absolutely certain that the flame-curtain gas- and air-line shutoff valves have been fully closed. You must also do this if you leave the furnace idling at temperature with atmosphere not flowing.
Typical Initial Starting and Adjusting ProceduresThe following procedure is typically required to initially turn on and adjust a high-pressure flame curtain:
1. Remove the protective cap over the gas-air ratio-adjusting screw in the gas-air mixer on the end of the air-line inspirator. Then, turn the screw in all the way, clockwise, until you feel it “bottom.”
2. With the furnace door fully closed, open the shutoff cock in the curtain gas line and the small cock in the pilot gas line and, without delay, light the pilot. Adjust the pilot so that it burns with a good, strong, stable blue flame. A blue flame without orange or yellow tips indicates the pilot is burning at close to the required 10:1 air-gas ratio (perfect ratio) on natural gas, or 25:1 on propane or 14:1 on aerated propane. If the pilot is equipped with a supervising flame rod and flame-detection relay, the relay contacts are now closed.
3. Fully open the air-line shutoff valve ahead of the solenoid valve. Then, stand as far away from the furnace door and flame curtain as possible and open the door. As the door begins to open, the gas and air solenoid valves open but gas does not yet flow.
4. Set the air-line flow-adjusting valve downstream of the solenoid valve so that the air-pressure gauge reads 5–10 psig.
5. Verify that the pilot is still burning with a strong flame and start turning the gas-air ratio-adjusting screw in the gas-air mixer out, counterclockwise, to start gas flow to the curtain burner. Continue until the burner lights-off and the essentially raw-gas flame extends in height to approximately the top edge of the door opening (or the crown of the door vestibule arch). Note: If the flame curtain cannot cover the entire door opening, the system is undersized and must be upgraded before operating the furnace.
6. Without delay and only if required to make the flame transparent enough so that you can see through it, further open the curtain needle-type air-flow adjusting valve downstream of the solenoid valve to add more primary air to the mixture. As you add air, the height of the flame will lessen somewhat and the flame will become less orange or yellow in color.
Proper air-gas ratio adjustment will leave the flame curtain burning very rich (at approximately a 1.2:1 ratio). If not enough air is added, the flame curtain will produce soot (carbon deposits) around the door, which is undesirable and indicates an adjustment to the air-gas ratio is needed.
7. Without delay, close the furnace door to turn off the flame curtain.
8. Verify that the flame-curtain pilot is still burning with a strong blue flame of adequate size. Make any adjustments required to obtain a proper flame.
9. Lock the gas-air ratio-adjusting screw in the mixer in position with the jam nut and replace the protective cap.
Routine Starting and Adjusting ProcedureThe required procedures to routinely turn on the flame curtain are:
1. Verify that you have already brought the furnace to above 1400°F, that you have started atmosphere flow into the furnace, that you have observed atmosphere to be burning into the furnace chamber off the end of the inlet pipe with a good, strong flame and that you have fully closed the furnace door.
2. Without delay, fully open the flame-curtain gas shutoff cock ahead of the solenoid valve and light both the flame-curtain pilot and (if supplied) the furnace-atmosphere burnoff-can pilot. Verify both pilots to be burning with strong, stable blue flames.
3. Fully open the curtain air-line shutoff valve ahead of the solenoid valve.
4. Stand as far away from the furnace door and flame curtain as possible and open the door. As the door begins to open, verify that the curtain lights-off with a flame of adequate height to cover the full width and height of the door opening and has proper transparency.
5. Without delay, close the furnace door to turn off the flame curtain. IH