Commonly Used Furnace Door Classifications for Boiler Grate Accessories

Standard furnace doors for biomass chain and reciprocating grates fall into manhole doors, observation doors, ash cleaning doors, fuel charging doors and explosion doors, with observation, ash cleaning and manhole doors bought most often. Standard materials are HT200 cast iron and heat-resistant nodular iron, with RTCr heat-resistant iron for high-temperature duty.

Common furnace door types

1. Observation door: used to watch combustion conditions, bed thickness, flame and grate operation, with a heat-resistant quartz glass sight glass set in the door. Round doors are common on small biomass boilers, while square doors are standard on medium and large chain grates and reciprocating grates above 6 tonnes; the door is compact, easy to install and seals well, but the opening is small, no one can enter and large clinker cannot be cleared. It is fitted on both side walls, the front and rear arches and the upper side walls of the grate.
2. Ash cleaning and clinker door: used to clear leakage slag, clinker and ash that fall below the grate, most often on both sides of a reciprocating grate; the door plate is larger with a thicker sealing flange and some versions include a small ash shovel. Cleaning is convenient without a full shutdown and a large manhole, but it only suits loose ash and small clinker, and it is fitted at the lower left and right sides of the grate and on the air box walls.
3. Manhole door: lets personnel enter during shutdown to replace grate bars, service the chain and clear large areas of clinker; it is a standard large square opening with hinged clamping bolts and a heat-resistant asbestos or ceramic fibre gasket. Space is ample for servicing the grate itself, but the door is bulky, takes furnace space and must never be opened during operation; it suits the front and rear ends and side-wall service points of boilers above 4T/h.
4. Fuel charging door: for manual fuelling, mostly on small hand-fed biomass boilers, and generally no longer fitted to fully automatic units. It allows emergency manual charging, but opening the door tends to spit flame and leak flue gas, so medium and large chain and reciprocating grates rarely order it separately.
5. Explosion door: biomass fuel has high volatile content, and the door relieves pressure if combustible gas in the furnace deflates, protecting the furnace body; gravity and diaphragm types are available. It must be noted that an explosion door cannot replace a normal service door and serves only as safety protection, fitted at the furnace roof and flue inlet.
6. Air duct inspection door and air control door: accessories specific to the grate air box, used for air box service, leakage checks and auxiliary primary air adjustment, widely fitted to chain and reciprocating grate wind boxes; they are small, simple cast iron sealed doors in strong demand from overseas customers.

Selection notes

Furnace doors are chosen to suit the grate type and boiler capacity; heat-resistant cast iron is preferred for hot areas, manhole and observation doors are laid out according to service and viewing needs, and explosion doors should be placed clear of walkways.