# Which one of the following mode of heat transfer would take place predominantly from boiler furnace to a water wall?

Which one of the following mode of heat transfer would take place predominantly from boiler furnace to a water wall?

#### SOLUTION

A furnace is the primary part of the boiler where the chemical energy available is converted into thermal energy by the combustion process.

HEAT TRANSFER IN BOILERS

An understanding of heat transfer is a fundamental requirement for a boiler operator. There are three ways that heat is transferred, conduction, radiation, and convection and all three mean occur in a boiler. The fundamentals of these heat transfer processes are individually simple, but the applications are complex because of the simultaneous occurrence of several modes of heat transfer, irregular shapes involved, dynamic conditions, and so on. That is why heat transfer is a not-so-exact science, as the variables are many and the situations dynamic Also, a very large body of data on the fundamental properties of materials is required.

Heat Transfer by Conduction

Heat energy can be transferred from one substance to another when they are in direct contact. This type of heat transfer takes place in solid objects. Heat transfer in this method is linearly related to the temperature difference. This method of heat transfer is called conduction.

The conduction method of heat transfer is mainly seen in solid objects. It happens when any ho material comes in contact with a cold material. When heat is transferred through conduction the substance itself does not move; rather, heat is transferred internally by vibrations of atoms and molecules. Electrons can also carry heat. Metals have many free electrons which mov^t around randomly. They can transfer heat from one part of the metal to another. So, metals art is generally a good conductor of heat.

Some solid materials are a better conductor of heat than others. For example, metals are good conductors of heat while material like wood is not. When a metal is heated at one end, the other end also becomes hot while that does not happen with a piece of wood. Good conductors of electricity are often good conductors of heat.

Since the atoms are closer together, solids conduct heat better than liquids or gases This means that two solid materials in contact would transfer heat from one to the other better than a solid in contact with a  gas or a gas with a liquid.

Heat flow from flue gases to water/steam across the metal wall in a boiler, loss of furnace heat to the surrounding atmosphere through insulation, and heat loss from hot steam/water pipes through insulation are all examples of conduction.

HEAT TRANSFER BY CONVECTION

Molecules of a liquid and gas are not confined to a certain point. They change their position, In this case, heat energy is transported from one point to another by the movement of these molecules. This phenomenon of heat transfer is called convection.

Heat transfer in fluids is generally done through convection. The hotter part of the fluid is not as dense as the cooler part, so there is an upward buoyant force on the hotter fluid, making rise while the cooler and denser fluid sinks. The convection current is set up in the fluid.

Convection may take place naturally through the creation of convection current or by applying some external force. Convection in a boiler is due to the movement of hot gases over the stock surface.