1. Application
The gas fired bogie hearth furnace is mainly used for heat treatment of metal parts.
2. Structural Introduction
2.1 furnace body
The furnace body steel structure is made from 12-18# U steel and 4-14mm
steel plates. The side pillar and back pillars are made from sectional steel,
and are reinforced with sectional steel bracings. The external wall of the
furnace is painted with 2 layers of primers, and 2 layers of coating paint.
The key part of the furnace is painted with heat resistant paint. The furnace
door edge is made of 14mm heat-resistant steel casting material in several
pieces.
2.2 lining
The lining material is 1200 type 1000℃ high purity refractory fiber compacted
block. This refractory lining structure has the advantages of low heat conductivity,
strong anti-shock capacity, and anti-erosion. Aluminum silicate fiber compression
block with thickness of 320mm is designed as the high temperature refractory layer.
2.3 bogie
The bogie is comprised of heat preservation material, bogie frame and moving
mechanism. The refractory layer is divided into 3 sub-layers, i.e., 1st top
high-alumina brick layer, and 2nd and 3rd layer with both light clay bricks.
The bogie framework is made of 20# U-steel and 20# I-steel. The edges of the
bogie are made of 20mm I-steel. The bogie is equipped with 8 pieces of wheels.
Each wheel diameter is 300mm.
The heat preservation material is divided into 3 layers, the top layer being
high-alumina bricks, and 2nd and 3rd layer being both light clay bricks.
The bogie is driven by motor reducer gear driving mechanism. The moving speed
of the bogie is 6-8m/min.
2.4 Sealing System
Sealing between the furnace door and the furnace body is through the electric
push-rod. Sealing between the bogie and the real wall is through the spring
mechanism. When the bogie moves inside the hearth, the bogie contacts tightly
with the rear wall to prevent heat loss.
2.5 furnace door
The furnace door is comprised of all-fiber lining and steel structure shell.
The door is built with silicate aluminum compressed fiber blocks same as that
for the furnace body. The shell is a welding structure from sectional steel and
steel plates. The furnace door has the feature of simple, practical, reliable and
convenient maintenance. The furnace door is fixed on the bogie.
2.6 burning system
The burning system is comprised of AGS 125HB high speed burners, proportional
combustion control system, gas valve, solenoid valve, burner control box, etc.
The burner has the functions of automatic ignition, flame detection, and flame out alarm.
The burner controller receives the control signal of the temperature controller and
controls the large/small fire status of the burner based on the heat load requirement so
that adjustment of the temperature is realized.
A) The burner has adjustment function and the adjustment rate is 1:10 and the air
efficient is 1-5. The burner could meet the temperature uniformity of the heat treatment
process and effectively control the air-gas ratio so that the fuel consumption is reduced.
B) When the furnace is working, the burning system could automatically cut off the
electrical gas valve and general safety valve in the case of power failure detected by
the alarm system. When the power supply is resumed, the worker needs to open
the safety valve and restart the ignition program after purging function is confirmed.
The burner is set with a proportional control unit which enables alternative burning
of large and small fire, fire out alarm, and re-ignition. The burner control box has both
manual and self-run modes available. The worker could operate in front of the furnace
or in the control cabinet.
C) The pipe before the burner is equipped with solenoid valve, and manual gas
adjustment valve. The valve system could realize an ideal gas/air supply ratio so
that the air excess efficient is lower than 1.1.
2.6.1 air system
The air piping system is comprised of high pressure centrifugal fan, automatic
regulation valve, pressure gauge and pipes. The air volume is to match the gas
volume, and the air excess rate is lower than 1.20.
2.6.2 gas supply system
The general gas supply pipe is equipped with a pressure regulation device (equipped
with a filter), a low pressure switch and a pressure gauge. For the sake of safety there
is one fast cut-off valve on the general gas pipe.
2.6.3 discharge system
The furnace takes direct fume discharge method. The fume is directly vented out
through the flue pipe to the atmosphere. The fume conducts heat exchange with
the heat exchanger before emission to the atmosphere.
2.6.4 Furnace pressure control system
The pressure on the working table in the furnace is kept at (+15 Pa), which is very
beneficial to the uniformity of temperature and the thermal efficiency of the furnace.
When the pressure in the furnace is too high, the hot air in the furnace will escape
from the furnace mouth and other unsealed places, resulting in the heat loss caused
by the escape of the flue gas; because the high temperature flue gas in the furnace
escapes to the outside of the furnace, it will affect the door, the side seal and burner
of the furnace directly, which is related to the overall service life of the furnace; when
the pressure in the furnace is too low, a large number of cold air outside the furnace
will be absorbed into the furnace, as well. The heat loss of off-furnace flue gas is increased.
The low pressure of the furnace causes the diffusion of cold air outside the furnace into
the furnace, and secondary combustion is formed due to the entry of oxygen-containing
cold air, which has adverse effects on the uniformity of furnace temperature, oxidation
of workpiece and thermal efficiency of the furnace. Therefore, effective technical
measures must be taken to control the furnace pressure with full automatic control.
Our method is to control the furnace pressure effectively by using a system consisting
of pressure taking device, pressure transmitter, intelligent instrument and so on. The
furnace pressure is controlled in the optimum state (the surface of the trolley is in a
slightly positive pressure). At this time, the exhaust gas is in a dynamic equilibrium
state, which can not only ensure that the furnace gas does not overflow, but also
ensure that the cold air outside the furnace is not sucked into the furnace, so as to
save energy and maximize the efficiency of the furnace.
2.6.5 Heat exchanger and flue system
A flue is set at the back of the furnace, and it is carried out with the heat exchanger
(according to the national standard GB3486-83). The flue leads to the outside of the
factory building. The cold air is exchanged with the heat exchanger to preheat the
combustion-supporting air before entering the burner, so as to improve the heat efficiency
of the furnace.
Heat exchangers and flue gas exhaust pipes are insulated by internal insulation (using
fiber castables) to ensure the service life of heat exchangers and flues.
2.6.6 fault detection and alarm system
The furnace has a complete fault detection, alarm, diagnosis, and safety protection
system. On the control cabinet there is an alarm lamp.
2.7 control system
control system has the following parts: Ⅰ. Burning control system; Ⅱ. Electrical
power control system; Ⅲ.temperature control system .
Measures taken for burning control system:
K type thermal couple gets mV signal and sends it to temperature PID controller
(SHIMADEN brand). The temperature controller calculates the signal and outputs
4-20mADC control signal to the electrical actuator. The actuator further sends a signal
to the burner controller after calculation.
The burner controller is mainly responsible for ignition and flame detection. When the
terminal receives the ignition signal, the burner controller carries out flame simulation
and self testing stage. If during this simulation and self-test stage the burner couldn’t
detect the flame signal. The burner controller would open the gas solenoid valve and
convert the voltage 220VAC to 5KV for ignition (ignition time about 3S). If the burner
detects the flame signal in the safe time period (about 8S), the yellow indication light
of the burner would be on (ignition successful). If the burner couldn’t detect the flame
signal, the red light would be on (ignition failed). The burner controller is locked and
the gas solenoid valve is cut off.
B) electrical power control system: The electrical power control system mainly controls
ON/OFF of the fan, and IN/OUT of the bogie.
C) temperature control and record system
The system has the functions of temperature setup, control and digital display. And also
there is an audible and visual alarm at over-temperature situations.
Temperature control zone arrangement
A:temperature control system
The furnace is equipped with 5 thermal couples and an intelligent temperature controller.
The furnace is controlled by 5 zones.
B: instrument temperature control
The temperature controller allows setup of heating time, holding time, and cooling time.
It has self-tuning and self-adjustment function, and displays SV and PV.
