I am convinced that an understanding of how heat is transferred from the fire to the water in a packaged fire tube boiler will enable operating staff to take a new look at how they manage the fire on the stoker grate.
For all practical purposes a steam boiler is nothing more than a furnace connected to or integrated with a heat exchanger. Apart from combustion, heat transfer in a boiler is one of the more critical processes for the production of steam. This is why a steam boiler is designed to absorb and transfer the maximum amount of heat released from the combustion process to the water drum where the heat of combustion is converted into steam energy.
Please keep in mind that I am still dealing with boilers firing pea coal on a moving chain grate stoker. With a horizontal steam boiler the stoker typically divides the furnace space into two halves down its length – a top half where combustion takes place, and a lower or bottom half which is actually wasted space, except that it collects some spilt ash that does not drop straight into the ash port. The significance of this arrangement is that heat transferred from the fire through the furnace wall only passes through approximately 50% of the available furnace heat transfer surface. In this respect gas and oil fired boilers offer significantly more heat transfer surface per similarly sized furnace.
Without proper heat transfer from the fire to the water:
- the steaming capacity of the boiler is greatly impaired; and
- useful heat is carried along in the flue gas and discharged to waste.
With this in mind, let us look at the factors influencing heat transfer in a boiler.
- Heat transfer within a steam boiler takes place in three different ways – radiation, convection and conduction.
- The rate of heat transfer is driven by the temperature difference between the fire/flame/flue gas and the water. Since the water temperature is normally fixed by the steam pressure, the only way of maximizing the temperature difference is to maximize the temperature of the fire/flame. In Boiler Bits 15 I have indicated how the flame temperature of fuel can be determined, but what it boils down to is that increasing excess air above the optimum level decreases the temperature of the flame. Thus the rate of heat transfer is strongly affected by how well excess air is being controlled. It is a known fact that increasing excess air also increases the flue gas exit temperature.
- The modern fire tube boiler with its water cooled walls absorbs approximately 60% of the heat from the burning of the fuel by means of radiation heat. That heat travels in the form of light waves from the glowing hot fire directly to the furnace walls.
- The magnitude of radiation heat transfer is driven by a factor (Tf⁴ – Tw⁴), where Tf is the absolute temperature of the fire and Tw the absolute temperature of the furnace wall.
- All wall surfaces (furnace flue, boiler tubes and tube plates) in contact with the water absorb heat by convection from the hot combustion gases. This mode of heat transfer is primarily driven by gas velocity and turbulence. The higher the turbulence, the higher the rate of heat transfer from hot combustion gas to colder metal surfaces.
- Heat absorbed by the wall surfaces pass through the steel by conduction. The rate of conduction is driven by the temperature difference between the heated steel surface and the colder wet surface of the water side.
- Lastly then heat is transferred from the water side steel surfaces to the boiler water by means of convection, although the process is more complex because of the simultaneous presence of steam bubbles and water at the heat exchange interface. This is a phenomenon known as “nucleate boiling”.
What then is the significance of all of this for steam plant operations?
- Since the largest portion of heat transfer in a boiler is by means of radiation, it calls for huge emphasis to be placed on the management and performance of the furnace.
- A long fire provides for a greater furnace surface area to be exposed to the temperature of the fire, increasing the amount of radiation heat transferred and reducing the amount of heat carried along in the flue gas. This definitely enhances boiler steaming capacity and leaves less energy in the flue gas which may be lost through the stack. Short fires are generally bad news, both in terms of steaming capacity and stack heat loss.
- Regardless of heat transfer processes and magnitudes, the efficiency of a steam boiler still reflects in the temperature of the exit flue gas. The closer the temperature gets to the saturated steam temperature, the better the heat transfer and efficiency of the boiler are. It is therefore essential that the exit flue gas temperature be continuously monitored as a means of verifying the integrity of heat transfer within the steam boiler.
- Keep surfaces exposed to radiation clean, specifically the walls of the furnace flue.
- Convective heat transfer takes place through a stagnant boundary layer of flue gas and air which exists between the furnace or tube wall and the flue gas. This layer acts to insulate the metal from the flue gases. The slower the velocity of the gases, the thicker is the boundary layer and the slower the rate of heat transfer becomes. Unfortunately limited means are available to increase velocity and/or turbulence in coal fired boiler tubes to scrub the boundary layer down to minimum thickness. Best is to make sure the boiler is correctly matched with the steam demand and that minimum fan speeds at minimum load are carefully selected.
- Over firing of the boiler may cause departure from nucleate boiling (DNB). When this occurs, steam forms so rapidly on tube walls that heat transfer to the water is impeded, causing steaming capacity to drop and flue gas temperature to rise.
- Scale deposits on heat exchange surfaces increases the resistance of heat flow through the steel walls and adds to stack heat loss. By way of example, a 2 mm layer of fire scale can cause 5% energy loss, a 1 mm layer of water side scale between 2% and 6%, depending on the density of the scale. It is therefore imperative that every effort be made to ensure that:
- Water treatment programmes are in place, and are monitored and managed.
- Coal procurement is a well managed process, including regular assessment of coal for fouling tendencies.
- Fire side tube fouling is monitored continuously. A modern boiler control system should be able to raise alarm when tube fouling reaches an unacceptable level.
This post was compiled by René le Roux for Le Roux Combustion, all rights reserved. Do you want to know more about efficiency of combustion or combustion optimization? Please contact us for your professional boiler automation, steam system efficiency and coal characterization needs.
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