why thermal effect should be considered in designing the engineering system?

Answer 1

Because your engineered system needs to operate in the real world, and not just on your desktop.

- Simple weather conditions such as temperature changes and humidity can cause condensation that may have adverse affects on your system.

- Different materials in the system will expand and contract at different rates. Gaskets and seals are often used to account for the changes. Will your system hold together over time?

- Does your system generate heat? Will it be able to dissipate enough heat to maintain reliable operation?

- Users of your system will be looking for guidance on where it can be operated? What are the acceptable operating temperatures for your system? For example: Antarctica, Iraq, and the Amazon jungle have far difference climate conditions.

Answer 2

*Effects of temperature changes:
When subatnces are subjected to changes in temperature, in one or more of the following phenomena may occur:

changes in colour
changes in size
changes in the state
chemical changes

*An increase in temperature generally causes bodies to expand, while a reduction in temperature causes bodies to contract.

Solids: The linear expansivity of a solid material is the increase in length of a unit length per degree rise:

Linear expansivity
=increase in length/[original length*temperature rise]
*Liquids: Liquids generally expand with increasing temperature.
Water behaves in an anomalous manner. It contracts as the temperature increases from 0 ºC to 4 ºC, reaching its highest density at that temperature. This has enormous significance for aquatic life in regions with severe winters.

Gases: The volume of a sample of gas kept at a constant pressure will expand or contract with temperature changes according to Charles' law (to be studied in Grade 11). If the volume of the gas is kept constant, for example if the gas is kept in a closed container, the pressure of the gas will increase with increasing temperature.
*Changes in state:
The physical state of bodies can be brought about by increasing temperarture, from solid to liquid and from liquid to gas or the other way round. When a substance passes from the solid phase to the liquid phase, it is said to melt. The temperature at which this occurs is called the MELTING POINT of that substance. The reverse process, i.e. the passing of a substance from a liquid to a solid, is called freezing or solidification. For most solids, pressure increases the melting point of pure substances. Water is a notable exception.

Impurities tend to lower the melting point of pure substances.

*When a liquid passes from the liquid phase to the gas phase, the liquid is said to evaporate. The reverse process, i.e., the passing from the gas phase to the liquid phase is called condensation. The temperature at which a liquid boils is called the boiling point of that liquid. The boiling point of a liquid is dependent on the pressure - the lower the pressure, the lower the boiling point. For this reason, boiling points of liquids should always state the pressure at which they were measured. If the pressure is not mentioned, it is assumed that it is one atmosphere.

Water normally boils at 100 ºC at sea-level, where the pressure is about 1 atmosphere.. On top of Mount Everest, where the atmospheric pressure is much lower, water boils at about 70º. In a pressure cooker, water boils at about 120 ºC.

Note that evaporation takes place at any temperature, and only at the surface of the liquid. Boiling, on the other hand, only takes place at a definite temperature, and occurs within the liquid, as shown by the appearance of bubbles.

Some substances, under certain conditions of temperature and pressure, pass directly from the solid to the vapour phase. This is know as SUBLIMATION. Solid carbon dioxide ("dry ice") and iodine crystals are example of substances which sublimate at normal atmospheric pressure.

CLICK:
http://www.physchem.co.za/Heat/Effects.htm