Thermal Engineering

  • Thermal Engineering  

Thermal Engineering is a specialized sub-discipline of mechanical engineering and chemical engineering that deals with the movement of heat energy and transfer. The energy can be transformed between two mediums or transferred into other forms of energy. A thermal engineer will have knowledge of thermodynamics and the process to convert generated energy from thermal sources into chemical, mechanical, or electrical energy. Many process plants use a wide variety of machines that utilize components that use heat transfer in some way. Many plants use heat exchangers in their operations. A thermal engineer must allow the proper amount of energy to be transferred for correct use. Too much and the components could fail, too little and the system will not function at all. Thermal engineers must have an understanding of economics and the components that they will be servicing or interacting with. Some components that a thermal engineer could work with include heat exchangers, heat sinks, bi-metals strips, radiators and many more. Some systems that require a thermal engineer include; Boilers, heat pumps, water pumps, engines, and more.

Part of being a thermal engineer is to improve a current system and make it more efficient than the current system. Many industries employ thermal engineers, some main ones are the automotive manufacturing industry, commercial construction, and Heating Ventilation and Cooling industry. Job opportunities for a thermal engineer are very broad and promising.

Thermal engineering may be practiced by mechanical engineers and chemical engineers. One or more of the following disciplines may be involved in solving a particular thermal engineering problem: Thermodynamics, Fluid mechanics, Heat transfer, or Mass transfer. One branch of knowledge used frequently in thermal engineering is that of thermofluids.


A gas/oil central heating boiler (heat generator) is like the engine of a car, this provides the heat that the facility needs to warm itself up. The size of the boiler is matched to the size of the facility.

    If the boiler is oversized, the fuel bills will be excessive.
    If the boiler is undersized, it may not generate enough heat in winter.

The ideal size for a boiler is one that just copes adequately on the coldest day of the year. Most boilers are oversized by at least 30%. This is due to the way systems used to be calculated with a card calculator. These were always over-calculated "to be on the safe side." Today, the emphasis is on energy conservation, and the fact that heat loss calculations can be done very accurately, means there is no need to oversize. This allows smaller radiators and less water in the system, which in turn, means a smaller boiler and reduced costs for both installation and fuel bills.

The boiler does not directly govern the amount of radiators fitted to the system. It is the power of the pump and circulation of the water through adequately sized pipes that determines the number of radiators you can have. But the total output of all the radiators, pipes, and cylinders determines the size of the boiler.

The boiler is not the heating system; it is only one of the parts in the global heating system. ', a heating system consists of four main parts:

    Boiler/burner combination (the part producing the heat)
    Piping with pumps and valves (the part distributing the heat)
    Radiators and convectors (the part emitting the heat to the room)
    Control equipment such as room thermostat and outside temperature control (the part controlling room and water temperature)

Fire-tube Boilers:-
In fire-tube boilers, combustion gases pass through the inside of the tubes with water surrounding the outside of the tubes. The advantages of a fire-tube boiler are its simple construction and less rigid water treatment requirements.

The disadvantages are the excessive weight-per-pound of steam generated, excessive time required to raise steam pressure because of the relatively large volume of water, and inability to respond quickly to load changes, again, due to the large water volume.

The most common fire-tube boilers used in facility heating applications are often referred to as ''scotch'' or ''scotch marine'' boilers, as this boiler type was commonly used for marine service because of its compact size (fire-box integral with boiler section).

The name "fire-tube" is very descriptive. The fire, or hot flue gases from the burner, is channeled through tubes that are surrounded by the fluid to be heated. The body of the boiler is the pressure vessel and contains the fluid. In most cases, this fluid is water that will be circulated for heating purposes or converted to steam for process use.

Every set of tubes that the flue gas travels through, before it makes a turn, is considered a "pass." So, a three-pass boiler will have three sets of tubes with the stack outlet located on the rear of the boiler. A four-pass boiler will have four sets and the stack outlet at the front.

Fire-tube boilers are:

    Relatively inexpensive
    Easy to clean
    Compact in size
    Available in sizes from 600,000 btu/hr to 50,000,000 btu/hr
    Easy to replace tubes
    Well suited for space heating and industrial process applications

Disadvantages of fire-tube boilers include:

    Not suitable for high pressure applications 250 psig and above
    Limitation for high capacity steam generation

Water-tube Boilers:-

In a water-tube boiler , the water is inside the tubes and combustion gases pass around the outside of the tubes. The advantages of a water-tube boiler are a lower unit weight-per-pound of steam generated, less time required to raise steam pressure, a greater flexibility for responding to load changes, and a greater ability to operate at high rates of steam generation.

A water-tube design is the exact opposite of a fire-tube. Here, the water flows through the tubes and is encased in a furnace in which the burner fires. These tubes are connected to a steam drum and a mud drum. The water is heated and steam is produced in the upper drum.

Large steam users are better suited for the water-tube design. The industrial water-tube boiler typically produces steam or hot water primarily for industrial process applications, and is used less frequently for heating applications. The best gauge of which design to consider can be found in the duty in which the boiler is to perform.

Water-tube boilers:

    Are available in sizes far greater than a fire-tube design , up to several million pounds-per-hour of steam
    Are able to handle higher pressures up to 5,000 psig
    Recover faster than their fire-tube cousin
    Have the ability to reach very high temperatures

Disadvantages of the water-tube design include:

    High initial capital cost
    Cleaning is more difficult due to the design
    No commonality between tubes
    Physical size may be an issue


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