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Structure of an industrial furnace

If the heat requirement of an industrial furnace or process will be supplied by the burning of a fuel, a combustion system must be designed. Apart from technical requirements, standards like EN 746-2 and ISO 13577-2 must be observed for safety. The combustion process plays a very important role in designing a system. For that reason, the process should be investigated under the following topics;

  • Process tempreature
    • Low temperature ( <600 °C). As in food or paint curing.
    • Medium temperature (600-1050 °C). As in stress relieving of metals or hardening.
    • High temperature (1050-1250 °C). As in steel forging.
    • Very high temperature (> 1250 °C). As in glass melting or technical ceramics.
  • Capactiy of the equipment and burners, burner types and quantitiy
  • Process operation type: Continuous or batch
  • Heating type: Indirect or direct
  • Furnace atmosphere: Oxidizing or reducing
  • Combustion air temperature: Ambient temperature or pre-heated air

A combustion system, planned according to the above listed aspects, consist of four parts;

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1. Gas Inlet Section

A gas train is needed for maintaining the pressure required for the equipment, for safety and measuring the consumption. A typical gas train built according to EN 746-2 consist of the following components;

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Pos. Example Component 746-2 Reference Tip
010 AKT Manual Valve 5.2.2.1
020 GFK Gas Filter 5.2.2.2 At maximum flow rate, filters should show a pressure loss of no more than 10 mbar
030 DH Pressure Gauge Valve 5.2.2.1 Gauge valve decreases the pressure exerted on the gauge after measurement and isolates the gauge from line pressure.
040 KFM Pressure Gauge
050 JSAV Safety Shut-off Valve 5.2.2.4 Use the same nominal diameter as for the gas pressure regulator so that a reduction in the pipe diameter is not necessary.
060 VGBF Pressure Regulator For Gas 5.2.2.4 Pressure regulators should be designed to be as small as possible. To achieve good control characteristics, the maximum pressure gradient (difference between the inlet pressure and the required outlet pressure of the regulator) should be fully utilized.
070 AKT Manual Valve 5.2.1.15 During operation, the valve must be secured open. Lever of the manual valve can be removed to prevent incorrect operation.
080 VSBV Safety Relief Valve 5.2.2.4 Safety relief valves are designed to relieve pressure peaks in pipe systems thus preventing the safety shut-off valve from being activated unintentionally.
090 DM, DE Flow Meter
100 DG Pressure Switch (Gas Min.) 5.2.2.5.2.1 Pressure switches should be installed preferably with horizontal or vertical diaphragm. Ensure that no dirt or moisture can penetrate the open ventilation ports. The min. pressure switch setpoints to be adjusted depend on the installation location and the process conditions.
110 DG Pressure Switch (Gas Max.) 5.2.2.5.2.2 Pressure switches should be installed preferably with horizontal or vertical diaphragm. Ensure that no dirt or moisture can penetrate the open ventilation ports. The max. pressure switch setpoints to be adjusted depend on the installation location and the process conditions.
120 VAS Solenoid Valve For Gas 5.2.2.3
130 DG Pressure Switch Adjust the pressure switch to half the inlet pressure to ensure correct functioning of the tightness control.
140 TC Tightness Control 5.2.2.3.4 The standard EN 746-2 stipulates a tightness control for capacities over 1200 kW and recommends for below.

Function

If the main shut-off valve (010) is open, the pressure and quantity required for the IThE are controlled by the gas pressure regulator (060). Any dirt in the gas or pipe system will be removed by the filter (020) first. If the gas pressure regulator (060) suffers a fault (e.g. a diaphragm fracture), the gas supply will be stopped safely by the high pressure shut-off device (050). The relief valve (080) prescribed here vents low leakage rates of the high pressure shut-off device and prevents unintentional tripping in the event of pressure surges. It therefore ensures the supply of the IThE is highly reliable.

The automatic shut-off valve (120) is part of the protective system, as are the low-pressure cut-off (100), the high gas pressure protection device (110) and the valve proving system (140). These compulsory safety devices are actuated separately by the process control system. In the case of insufficient or excessively high gas flow, the process control system will receive a signal and shut off the gas supply safely by closing the valve (120). Another valve, which must be installed directly upstream of the burner, will also be closed. Before each commissioning or after switching off the system, the valve proving system (140) checks the functioning of these valves. Here, the gas volume enclosed between the valves must be channelled off to a safe area by opening the venting valve (100).

The quantity of gas consumed can be read off the flow meter (090). The inlet and outlet pressure will be displayed on the pressure gauges (030/040).

The respective unit accuracy groups, lock up pressure classes and accuracy classes must be observed to ensure optimum adjustment of gas pressure regulator, high pressure shut-off device and relief valve. We recommend the generation of a pressure scaling diagram to exclude tolerance-related interferences of settings.

2. Air Supply

The majority of industrial burners operate with air pressures higher than atmospheric pressure. Air supply lines with a central fan are required for air supply and control. A common air supply line designed according to EN 746-2 is as follows;

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Pos Example Component 746-2 Reference Tip
010 Combustion air fan with intake filter An intake fan filter is recommended.
020 KFM Pressure Gauge
030 DH Pressure Gauge Valve
040 DG Pressure Switch 5.2.2.5.1 Hava Air deficiency cut-out
050 DG Pressure switch 5.2.2.5.1 Air Air flow detector
060 VMO Orifice Air orifice.

Function

The supply air pressure generated by the fan (010) is monitored by the pressure switch (040) acting as an air deficiency cut-out. When the fan is started by the process control system, contact changeover at the pressure switch is checked.

During the pre-purge, a differential pressure switch (050) monitors the air flow on the orifice (060). If there is no air pressure supplied or if there is no differential pressure on the orifice, the system will be blocked.

EN 746-2 recommends that the intake filter on the fan be monitored for soiling. This can occur directly through a differential pressure switch on the filter or indirectly through a second air pressure switch on the pressure side of the fan. The filter monitoring pressure switch triggers a warning, however it does not need to be linked into the safety interlocks.

In many cases it is recommended that the combustion air be taken in from outside the production hall so as to extend the cleaning intervals for the intake filter. However, drawing in air from the outside does not do away with the need for an intake filter.

3. Burner Systems

Before designing the burner control systems the following decisions must have been made;

  • Capacity control
    • Modulation
    • Staged control
    • Pulse firing
  • Fuel/Air Ratio Control
    • Mechanical control (Linkage / Cam Disk)
    • Pneumatic ratio control
    • Electronic ratio control
  • Flame control, safeguarding in the event of a burner fault
    • Ionisation control
    • UV control
    • Safeguarding over furnace temperature
  • Number of burners and zones
    • Single burner
    • Multiple burners, single zone
    • Multiple burners, multiple zones
  • Combustion air temperature
    • Ambient temperature (Cold air)
    • Prehated air, central recuperation
    • Prehated air, self recuperative burners
    • Preheated air, regeneration
  • Burner ignition
    • Direct ignition
    • Pilot ignition
    • Auto combustion in a temperature controlled atmosphere

Decisions regarding the aspects above are to be taken according to the requirements of the process. Apart from that, flameless combustion for NOx reduction, oxygen trimming for reduction of material oxidation, multi fuel usage, premix combustion are topics which should be considered depending on the process and regulation requirements.

As an example, a single, cold air burner with direct ignition, pneumatic linkage and ionisation controlled burner system is explained below;

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Pos. Example Component 746-2 Reference Tip
010 VAS Solenoid valve for gas 5.2.2.3.2
020 GIK Air/gas ratio control 5.2.3.3 Pressure regulators should be designed to be as small as possible. To achieve good control characteristics, the maximum pressure gradient (difference between the inlet pressure and the required outlet pressure of the regulator) should be fully utilized.
030 Valve combination VCG, as an alternative to the individual devices automatic shut-off valve (VAS) and air/gas ratio control (VAG)
040 GEHV, GEH Flow Adjusting Cocks
050 EKO Stainless steel bellows unit 5.2.1.5
060 BIO, ZIO, BIC, ZIC Burner 5.2.5.1 Burners can be combined with a burner quarl or ceramic tube. Selection is dependent on the process and system requirements such as capacity, temperature, flame shape and outlet velocity as well as furnace wall thickness and gas type.
070 BVA+IC Control valve with actuator Select the smallest control valves possible so that the opening characteristic can be fully utilized. Note the residual flow when the valve is closed.
080 BVA, DKR Butterfly valve for air Pursuant to EN 746-2, all manual air control valves must be blocked in the required positions and secured against unintentional maladjustment.
090 BCU 370 Burner control unit for intermittent or continuous operation for industrial forced draught gas burners. 5.2.6.1 The burner control unit with integrated ignition unit is mounted to the burner on site and ensures simple commissioning of the burner system.

Function

Simple, cost-effective burner system for processes which require high temperature accuracy and low circulation in the furnace, e.g. crucible and ladle heating, incineration installations, hot-air and steam generators, aluminium smelting furnaces…

The capacity can be adjusted continuously by activating the air control valve (070) (analogue or 3-point step signal). The pneumatic ratio control system (020/030) controls the gas pressure proportionally to the air pressure and thus maintains a constant air/gas ratio. At the same time, it acts as an air deficiency cut-out. Adjusting valves and/or butterfly valves (040, 080) are used for limiting the air and gas flow rates and for adjusting the air/gas ratio.

Furnace pressure fluctuations have the same effect on the gas and air flow so that the air/gas ratio will remain unchanged.

Ignition and monitoring are ensured by burner control unit BCU 370 (090). The BCU 370 activates the fan and sets the connected control valve (070) to pre-purge and ignition position. After pre-purge and burner start, the Enable signal is issued to the process control system which positions the control valve in accordance with the capacity demand.

For burner capacities between 120 and 360 kW, ignition with reduced capacity is required pursuant to EN 746-2:2010 (slow opening gas valves).

The pneumatic ratio control system in conjunction with a defined ignition position of the air control valve (070) is an equivalent alternative to the pilot gas bypass according to figure D.6 in Annex D of EN 746-2:2010. The ignition position of the air control valve (070) is to be monitored using limit switches or pressure switches in the air circuit. For burner capacities greater than 1200 kW, the valves must be equipped with an automatic valve proving system (TC). On the BCU 370, the DG min. input does not trigger a lock-out in the event of a fault, as required by EN 746-2:2010.

4. Process Control Systems

Process control system (PCC) for control and continuous temperature control of a single burner system, ionization-controlled with gas and cold air supply.

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Pos. Example Component 746-2 Reference
010 Power supply unit
210 Combustion air fan
300 Safety limits 5.2.2.3.1
311 DG Air min. air pressure monitor 5.2.2.5.1
321 DG Gas min. low pressure cut-off 5.2.2.5.2.1
322 DG Gas max. high gas pressure protection 5.2.2.4
331 Emergency stop button NTA 5.2.2.3.1
332 Over temperature 5.2.2.3.1
341 Pre-purge 5.2.3.2
350 TC 1-3 Tightness control 5.2.2.3.4
410 Valve control 5.2.2.5.2.2
450 BCU 460 Burner control unit 5.2.6.1
510 Temperature control

Function

The supply voltage is supplied to the control system via the power supply unit (100).Start-up of the combustion air fan (210) includes the “no flow” state check of the Air min. air pressure monitor (311). The control block for monitoring the safety limits (300) assumes the safety-related monitoring of the safety limits Air min. air pressure monitor (311), Gas min. low-pressure cut-off (321), Gas max. high gas pressure protection device (322), Emergency stop/Emergency off/Gas off (331) and ϑ max. overtemperature monitor (332).

Once the system has started and all safety limits (300) are present, pre-purging (341) of the thermoprocessing equipment begins and the tightness control (350) checks the automatic shut-off valves. Once pre-purge (341) has been completed and the OK signal has been issued by the tightness control (350), the safety interlocks (shown in yellow) are set and the burner is started in the ignition position. The burner is set to the ignition position by the valve control system (410). Once the presence of the flame has been signalled to the automatic burner control unit (450), the burner starts. The temperature controller (510) now takes over the temperature control of the heating equipment.

A system fault lock-out occurs if at least one of the safety limits does not lie within the system operating range, or the tightness control (350) has detected a leak on the main gas valves or burner valves, or the automatic burner control unit (450) has detected a burner fault. The system fault lock-out may only be reset manually. The safety limits listed refer to a typical item of thermoprocessing equipment. The required safety limits may differ, depending on the system type.

The automatic burner control unit (450) and the tightness control (350) control the same automatic shut-off valves on the equipment.

The automatic burner control unit (450) and the tightness control (350) control the same automatic shut-off valves on the equipment. In the diagram, a BCU 460 is shown for the automatic burner control unit (450). If another control unit is used, some signals must be provided by an additional control logic.

The information above is given as a general example and may not be suitable for your equipment or process. Please check docuthek for further information.