Basic principles of heat transfer

kadelv (37)in #steemstem • 6 years ago

Thermal Engineering deals with the processes of heat transfer and the methodology to calculate the speed with which they are produced and thus be able to design the components and systems in which they are applicable. The transfer of heat covers a wide range of physical phenomena that must be understood before proceeding to develop the methodology that leads to the thermal design of the corresponding systems. Whenever there is a difference in temperature, the energy is transferred from the higher temperature region to the lower temperature region; According to thermodynamic concepts, the energy that is transferred as a result of a temperature difference is heat. However, although the laws of thermodynamics deal with the transfer of energy, they only apply to systems that are in equilibrium; they can be used to predict the amount of energy required to modify a system from one equilibrium state to another, but they do not serve to predict the speed (time) with which these changes can occur; the phenomenology that studies the transmission of heat complements thermodynamic principles, providing analytical methods that allow predicting this rate of thermal transfer. To carry out a complete analysis of the heat transfer it is necessary to consider three different mechanisms, namely: conduction, convection, and radiation.
These three forms of transfer are the simplest that can be considered in isolation, although
in practice, it is normal for at least two of them to occur simultaneously, making the phenomena more complex to study. The transfer of heat covers a wide range of physical phenomena that must be understood before proceeding to develop the methodology that leads to the thermal design of the corresponding systems. Some design examples can be:

Those that require reducing the amounts of heat transferred through a thermal insulator, or
amplify them by fins or other systems.
Those that involve processes of heat transfer from one fluid to another through exchangers
of heat
Those that thermally control a process, maintaining the operating temperatures of
the elements sensitive to heat within predetermined ranges, etc.

1.- Conduction, convection, and radiation

Source

Conduction

The transmission of heat by conduction can be carried out in any of the three states of matter: liquid solid and gaseous.
The conduction is basically a mechanism of transfer of energy between contiguous particles. The energy of the molecules increases when the temperature rises. This energy can pass from one molecule to another contiguous and from this to the next and so on either by a collision between particles, in fluids or by reticular vibrations in solids.
The conduction in the solids, therefore, enjoys a material support, which are the molecules of the body itself, which vibrate in fixed positions without moving, therefore the transfer of energy by conduction, macroscopically does not involve the transport of matter. The reasoning is valid both for the transfer of energy within a solid, and for solids in contact.
In the fluids, the conduction is explained thanks to the exchange of kinetic energy of its molecules, which occurs as a result of collisions between them. The transmission of heat by conduction in the fluids occurs mainly in what we will define as the boundary layer and has little importance in the rest of the mass.

The formula for calculating the amount of heat per conduction according to the Fourier law is:

Where:

|it is the heat transmitted per unit of time.
|It is the thermal conductivity.

A is the area of the contact surface.

|is the temperature difference between the hot and cold focus

x is the thickness of the material.

Convection:

Convection is one of the three forms of heat transfer and is characterized because it is produced by means of a fluid (liquid or gas) that transports heat between zones with different temperatures. The convection is produced only by means of fluid materials. What is called convection itself, is the transport of heat through the movement of fluid, for example: by transferring the fluid by means of pumps or by heating water in a saucepan, which is in contact with the bottom of the pan moves upwards, while the water on the surface descends, occupying the place left by the hot one.
Heat transfer involves the transport of heat in a volume and the mixing of macroscopic elements of hot and cold portions of a gas or a liquid. It also includes the exchange of energy between a solid surface and a fluid or by means of a pump, a fan or another mechanical device (mechanical convection, forced or assisted). In the free or natural heat transfer a fluid is hotter or colder and in contact with a solid surface, causes a circulation due to differences in densities that result from the gradient of temperatures in the fluid.

Convective heat transfer is expressed by Newton's Law of Cooling:

Radiation:

Thermal radiation is the energy emitted by matter that is at a finite temperature. We will focus our attention on the radiation of solid surfaces, although this radiation can also come from liquids or gases. In thermal radiation, heat is transmitted by electromagnetic waves, like light, but of different wavelengths. The radiant energy depends on the characteristics of the surface and the temperature of the emitting body. When influencing a receiver, part of the energy passes to this other body, depending on the characteristics of the receiver and its absorption power.

2.- Thermal conductivity.

Thermal conductivity is an intrinsic property of materials that values the ability to conduct heat through them. The conductivity value varies depending on the temperature at which the substance is located, so measurements are usually made at 300 K in order to compare some elements with others. It is high in metals and in general in continuous bodies, and it is low in gases (although in them the transfer can be done through free electrons) and in ionic and covalent materials, being very low in some special materials such as fiber of
glass, which are called by those thermal insulators. For thermal conduction there is a need for a substance, hence it is null in the ideal vacuum, and very low in environments where a high vacuum has been practiced.
In some industrial processes, work is being done to increase the heat conduction, either using high conductivity materials or configurations with a high contact area. In others, the desired effect is just the opposite, and it is desired to minimize the effect of conduction, for which materials of low thermal conductivity, intermediate voids are used, and are arranged in configurations with little contact area. The coefficient of thermal conductivity (λ) expresses the
quantity or flow of heat that passes through the surface unit of a sample of the material, of infinite extension, parallel flat faces and unit thickness, when between its
faces a temperature difference equal to unity is established, in stationary conditions. The thermal conductivity is expressed in units of W / m · K (J / s · m · ° C).

Thermal conductivity can also be expressed in units of British thermal units per hour per foot per degree Fahrenheit (Btu / h · ft · ° F)

3.- Thermal Balances of an oven

It consists of determining the quantities of heat entering and leaving the kiln per unit of time or per unit of weight of the finished product or piece produced.

The thermal balance serves:

To determine in which the heat generated by the fuel is used.
To determine the heating performance.
As a basis to determine how to save fuel.

In the thermal balance of the rotary kiln, in the same way, as in any other type of kiln, an equal heat must be established in the process. That is to say, that the heat that is delivered to the furnace, when burning the fuel, must be equal to that which is
consumes in the process and the losses in the furnace or that take the gases that leave by the chimney.

General consideration of the thermal balance of a furnace.

The following considerations should be taken:

The conditions of the process between the beginning and the end of the data collection must not present uncontrolled alterations.

As a rule:

Discontinuous processes: data collection will be done during a full load.
Continuous processes: the data collection begins once the operating conditions stabilized and continues for a period of time such that the small variations do not
influence the results.
Cyclic processes: the data collection begins and ends at corresponding points of the
cycle.

The flowchart should clearly delineate the different areas of the process to facilitate an exhaustive collection of data that allows the corresponding balances to be made
partial heat and, once the calculations have been made, quickly locate the points where the losses are above the admissible limits.
It must present a special attention to the measurement systems because the errors that originate from them directly affect the results.

4.- Energy efficiency.

Renewable energies are mistakenly understood as energy savings, however the savings are more in the habits and mentality of the consumer.

Energy efficiency is a practice used during the consumption of energy that aims to reduce energy consumption. Individuals and organizations that are direct consumers of energy can reduce energy consumption to reduce costs and promote economic, political and environmental sustainability. Industrial and commercial users may wish to increase efficiency and thus maximize their benefit. Current concerns include energy saving and the environmental effect of generating electricity. It is also called energy saving.

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