Sheet 4343-3 Page 5
TWINBED ® II COMBUSTION AIR BYPASS
PREVENTION
PURPOSE
To prevent the bed from overheating while in pilot-only mode, a controlled volume of air is by-passed around the combustion air cycle valve. This air flow is adjusted to satisfy the rate of exhaust valve leakage and achieve a neutral-to-slightly-positive regener- ator plenum pressure relative to the furnace chamber, eliminating the outward flow of hot gases. ADJUSTMENT While the burner pair is in pilot-only mode and at operating temperature, connect the positive side of a differential pressure gauge onto the pressure tap located on the rear of the TwinBed plenum. Connect the negative side of the pressure gauge onto the pressure tap located on the rear of the TwinBed burner head. Adjust the bypass valve until a slightly positive differential pres- sure is achieved to cause flow in the desired direction. This pres- sure is expected to be approximately + .02”w.c to + .04”w.c.
The purpose of the TwinBed II combustion air bypass system is to prevent overheating of the regenerator bed and support struc- ture while the burner pair is in pilot-only mode. While in pilot-only mode, both the combustion air and the exhaust air cycle valve are in the closed position. As the valves have metal-to-metal seats (not intended as tight shut-off) there is a potential for leakage depending on the differential pressure across the valve. Typically a TwinBed II system’s deadhead ex- haust fan suction is greater than the deadhead air fan pressure. Accordingly the exhaust valve natural leakage is greater than that of the combustion air valve, creating a negative pressure in the regenerator plenum which pulls hot gases from the furnace chamber into the regenerator bed. As a result while in pilot-only mode for extended periods of time, the depth of the hot zone in the regenerator media bed will increase and will potentially overheat the plenum.
WARNING: Situations dangerous to personnel and property may exist with the operation and maintenance of any combustion equipment. The presence of fuels, oxidants, hot and cold combustion products, hot surfaces, electrical power in control and ignition circuits, etc., are inherent with any combustion application. Components in combustion systems may exceed 160°F (71°C) surface temperatures and present hot surface contact hazard. Fives North American Combustion, Inc. suggests the use of combustion systems that are in compliance with all Safety Codes, Standards, Regulations and Directives; and care in operation.
CONTACT fna.sales@fivesgroup.com T +1 800 626 3477 - F +1 216 373 4237
Fives North American Combustion, Inc. 4455 East 71st Street - Cleveland, OH 44105 - USA www.fivesgroup.com
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