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Heating Boiler Combustion Controls
Learning Outcome
When you complete this chapter you will be able to… Explain the design and operation of various combustion controls on heating boilers.
Learning Objectives
Here is what you will be able to do when you complete each objective.
- Describe the construction and operation of heating boiler flame failure detectors.
- Describe the testing of hot water heating boiler, flame failure safety devices.
Introduction
In gas- and oil-fired boilers, the fuel enters the furnace in a gaseous or vaporized state, mixes with the combustion air and is ignited to produce a flame. If, for any reason, the burner should go out during normal operation, or the fuel should fail to ignite during start-up, the fuel to the burner should be shut off immediately. If this is not done, the unburned fuel collecting in the furnace and mixing with the combustion air may ignite and cause a serious explosion. Although various flame detection devices are in use, their application depends on the type of heating boiler used, the type of fuel burned, and which flame it is monitoring. Ideally, the pilot flame and burner should be monitored simultaneously. If this is not possible then the detection device will be used to monitor the pilot flame. If the pilot is not continuously lit then the main burner will be monitored. The thermocouple, flame rod, and photoelectric cells are most commonly used and will be discussed in this chapter.
Objective One
When you complete this objective you will be able to… Describe the construction and operation of heating boiler flame failure detectors.
Learning Material
Thermocouple
A thermocouple consists of two dissimilar wires welded together at one end to form a measuring or hot junction, while the opposite ends are connected to an electric circuit at the cold junction. A basic thermocouple is shown in Fig. 1. When the hot junction is exposed to the heat of a flame its temperature is higher than that of the cold or reference junction. A small voltage is generated causing current to flow through the circuit. Figure 1. Basic Thermocouple Construction The thermocouple, as used with a pilot burner, is made of a copper outer sheath which acts as one conductor of the thermocouple. An insulated wire inside the sheath and welded to the top of it forms the second conductor. Fig. 2 is an exterior view of one type. By inserting the thermocouple in the pilot flame, the current generated is used to energize a small electromagnet or solenoid that keeps a switch or relay closed in the power supply to the main fuel control valve. It may also keep a safety shutoff valve (pilotstat) in the gas line in open position as indicated in Fig. 3. If the pilot flame fails, no power is generated by the thermocouple. This prevents the main fuel control valve from opening on boiler start-up, or during operation, the safety shutoff valve closes, stopping the gas flow to the burner. Quite often the pilotstat or pilot gas valve and main burner gas valve are combined in a single unit. Figure 2. Pilot Burner With Thermocouple
The voltage supply generated by the thermocouple is usually not more than 30 millivolts so its electrical power is not sufficient to operate the full control system. A transformer is used to provide sufficient power to operate the main fuel valve. By using a number of thermocouples connected in series, as shown in Fig. 4, sufficient voltage can be generated to operate the fuel control system independently from any source of power. This system is often called a thermopile. Figure 4. Basic Thermopile Construction
Flame Rod
Another type of flame detection system uses a flame rod or electrode as a flame sensing device. It uses the principle that a flame is capable of conducting electricity. The flame rod, inserted in the flame, forms an electrical circuit using the rod as one electrode and the burner as the other and the flame as the medium to conduct the electrons. Without the presence of a flame, the circuit is open. It can detect the presence of a gas pilot or main gas burner flame. The basic system, illustrated in Fig. 5, shows the flame rod extended into the burner flame. When the flame is present, the rod, burner, and flame form a closed electronic circuit to a relay. This relay amplifies the signal from the flame unit and provides sufficient power to keep the safety shutoff valve open. If the flame fails, the circuit is opened and the fuel valve closes. During boiler start-up, the fuel valve cannot be opened unless the pilot burner flame is sensed by the rod.
Figure 5. Basic Burner System With Flame Rod The flame rod offers a reliable protection against flame failure. However, when used on automatically fired boilers, the system becomes more complex than that shown in Fig. 5. The firing equipment also includes a pilot burner, pilot valve, and an ignition system. In this system, the relay not only acts as a safeguard device but it programs the start up and the shut down of the firing equipment as well. Flame rods are commonly used with gas burners only. When used with oil-fired burners, the rod can become coated with carbon deposits which will affect the operation of the flame sensing system.
Photoelectric Cell
A photoelectric cell is a device that reacts either to the infrared or ultraviolet rays emitted by the flame of the fire. The cell is mounted on the front of the boiler in such a way that it can scan the flame. For this reason a photocell is commonly called a "scanner". Fig. 6 shows one method of mounting a scanner on the furnace front and Fig. 7 shows a scanner itself. Figure 6. Scanner Mounting
The flame relay controls the power supply to the solenoid of the main fuel valve. When a flame failure occurs, the scanner cannot sense a flame so its resistance increases causing the flame relay to open. The power supply to the fuel valve is cut off so the valve closes to shut off the fuel supply to the burner. During boiler start-up, when the scanner senses the pilot flame, the relay or switch in the electrical circuit to the main solenoid fuel valve is closed, permitting fuel flow to the main burner. If the main burner does not light, or if the pilot flame goes out, the scanner will open the burner circuit which will cause the main fuel valve to close. Most heating boilers, equipped with a photoelectric safeguard system, use a single scanner to sight both the pilot flame and the main burner flame. A basic diagram of a lead sulphide scanner installation is shown in Fig. 9. The scanner is mounted on the end of a sighting tube in such a way that its temperature will not exceed 50°C. The tube will be aimed so the scanner can sight the pilot as well as the main burner. Fig. 9 also shows the size of a pilot flame required to keep the burner circuit energized. Figure 9. Application of Scanner
Ultraviolet Scanner
The principle of operation and mounting of the ultraviolet scanner is quite similar to the infrared except that this scanner responds only to the ultraviolet rays emitted by the flame. This scanner is mounted as close to the flame as physically possible without subjecting it to a temperature over 100°C. Care should be taken that the scanner cannot sight the spark of the igniter since it reacts to it as well.
Rectifying Photocell
The rectifying photocell, shown in Fig. 10, is also known as a "cad" cell. It consists of a glass vacuum tube containing a curved metal cathode made of cadmium oxide and an anode wire, all forming part of an electronic circuit. As long as the cathode is exposed to the visible light of the burner flame, it emits electrons that are picked up by the anode. This creates a signal that closes the contact points of a flame relay which, in turn, allows the solenoid in the fuel valve to be energized and kept in the open position. Failure of the flame will cause the fuel valve to close. Figure 10. Rectifying Photocell The cell is often mounted inside the combustion air supply tube of a gun-type oil burner as shown in Fig. 11. Here it is kept relatively cool and clean while it still has an unobstructed view of the flame. Figure 11. Photocell Mounting in Combustion Air Tube
Objective Two
When you complete this objective you will be able to… Describe the testing of hot water heating boiler, flame failure safety devices.
Learning Material
a. With the main burner in normal operation, close the main gas cock. Note the time it takes for the automatic gas valve to close. This should take no longer than four seconds. b. If the test is successful, open the main gas cock, relight the burner, and check the burner for correct operation.
5. Gas Burner With Electronic Flame Scanner (Photoelectric Cell) a. Method 1: same as (4) above. b. Method 2: i. Remove the scanner from its sighting tube and cover it. Note the time it takes for the automatic gas valve on the main burner supply to close. This should take no longer than four seconds. ii. If the test is successful, reset the controls, light the burner, and verify correct operation. 6. Oil Burner With Electronic Flame Scanner (Photoelectric Cell) a. With the burner operating normally, close the manual fuel valve in the oil supply line. Note the time it takes for the oil solenoid valve to close. This should take no longer than four seconds. b. If the test is successful, open the manual oil supply valve, reset the boiler controls, light the burner, and check for correct operation. 7. Oil or Gas Fuel With Electronic Scanner and Proven Pilot Detection This test is required to prove the operation of the flame safeguard device during the starting cycle.
Checking Pilot Flame Failure Protection
- With the burner off, close the manual shutoff cocks in the main burner and pilot fuel lines.
- Start the boiler programming control. After the prepurge period, the pilot assembly will be energized, but because no flame is detected, the automatic pilot valve will close in about ten seconds. The main automatic fuel valve will not be energized.
- The programming timer will continue its cycle and during this time, the lockout switch will trip causing a safety lockout and an alarm, if used, will be actuated.
Checking Main Flame Failure Protection
- Reset the lockout switch and open the cock in the pilot fuel line. The gas cock in the main fuel line remains closed for the next test.
- Start the programming control. After the prepurge period, the pilot will be lit and since the scanner will sense the flame, the main fuel valve will be energized. The main burner will not light since the fuel supply cock is closed. After the trial period for the main burner ignition, the pilot is shut off. The scanner does not detect the flame and the main solenoid fuel valve is deenergized as well. The timer will continue its cycle, but the lockout switch will trip and an alarm will be actuated.
Maintenance of Flame Safeguard Devices
The programming control or relay requires no maintenance other than occasional blowing out of any accumulated dust. Never file the contact points. If the contacts are dirty or slightly oxidized, clean them by drawing a piece of hard finished paper between the contacts while holding them closed. It is advisable to keep spare parts, such as electronic tubes etc., available. It is important that the scanner should never get so hot that it cannot be held comfortably in the hand, as high temperature affects its operation as well as its life span. The scanner lens should be cleaned at regular intervals. Be sure that the scanner sight tube is always clear of any obstructions. Flame rods should be checked at regular intervals for build up of deposits as this will affect the operation.
