UNIT OPERATIONS III: HEAT TRANSFER

Heat transfer (or heat) is energy in transit due to a temperature difference. Whenever a temperature difference exists in a medium or between media, heat transfer will occur. This is the case in the unitary operations of this classification in the industries as a whole. Shall we delve into them?

The study of unitary operations and transport phenomena has very important applications in the engineering field, since it allows to “know” different subjects, such as the transport of fluids through pipes or the quantification of heat dissipation in motors. They are indispensable for design, operation and process and equipment optimization in all fields of engineering.

The transfer phenomena basically deal with the movement of a physical quantity from one point to another in space, and they are: movement of quantity transport, mass transport and thermal energy transport. The latter is fundamental for all branches of engineering, especially for and chemical engineering, and important for most industrial and environmental problems.

Heat transfer or heat transfer processes also affect the performance of propulsion systems (combustion engines and rockets). Other fields that require heat transfer analysis are heating systems, incinerators, cryogenic storage, electronic equipment refrigeration, refrigeration and air conditioning systems, and many others.

Heat transfer can be defined as the transfer of energy from one region to another as a result of a temperature difference between them. It is necessary to understand the physical mechanisms that allow heat transfer in order to quantify the amount of energy transferred in the unit of time (rate).

The mechanisms are:

  • Conduction: A process whereby energy is transferred from one region of high temperature to another of lower temperature within a medium (solid, liquid or gas) or between different media in direct contact;
  • Convection: The process by which energy is transferred from the hot portions to the cold portions of a fluid through the combined action of: heat conduction, energy storage and mixing motion;
  • Radiation: The process by which heat is transferred from a high temperature surface to a lower temperature surface when such surfaces are separated in space, even though there is a vacuum between them. The energy thus transferred is called thermal radiation and is made in the form of electromagnetic waves;

Based on these concepts arises the operations that involve thermal exchanges and which are based on the principle of transfer. The heating and cooling technologies and the heat transport that results from these operations are key steps in all processes of the chemical industry.

The process of heat exchange between two fluids that are at different temperatures and separated by a solid wall occurs in many engineering applications. The equipment used to implement this exchange is called heat exchangers, and specific applications can be found in heating and environmental conditioning, heat recovery, chemical processes etc. The most common applications of this type of equipment are: heaters, chillers, condensers, evaporators, cooling towers, boilers, etc.

We can classify the exchangers in several ways. One of them is:

  1. The mode of heat exchange;
  2. the number of fluids;
  3. Type of construction, etc.

Figure 9: Heat Exchanger Classification Diagram
Source: author

The best known heat exchangers in the industrial field are:

Tubular Exchanger – Shell and Tube

This changer is constructed with tubes and a housing. One of the fluids passes inside the tubes, and the other through the space between the housing and the tubes. They are most commonly used for any capacities and operating conditions such as high pressures and temperatures, highly corrosive atmospheres, very viscous fluids, etc.

Double Tube Changer

They are formed by two concentric tubes, as illustrated in the figure below. Inside the tube of the first (innermost) one fluid passes, and in the space between the outer surfaces of the first and inner of the second, the other fluid passes. The heat exchange area is the area of ​​the first tube. This is perhaps the simplest of all types of heat exchanger for the easy maintenance involved. It is generally used in small capacity applications.

Streamer Changer

They are formed by a coiled spiral-shaped tube forming the coil which is placed in a shell or container. The heat exchange area is the coil area. Thermal expansions are no problem, but cleanliness is very problematic. It allows larger heat exchange area than the previous one, has great application flexibility and is used especially when you want to heat or cool a bath.

In addition to heat exchangers, there are cooling towers and boilers, equipment used regularly in the industries in general. The first equipment mentioned is a structure that is designed to eliminate heat waste from a particular process step or process fluids, such as industrial water, which in many cases is the main component to be cooled.

Heat is pulled out from a procedure where it is generated and routed to it so that it can be dispersed into the environment, cooling this process step. These structures are used in a number of different industries, and many people are familiar with these buildings, even if they are not completely sure what they are, because they have become visually associated with nuclear power plants. The distinct towers seen for the sake of such facilities are actually cooling towers, not the plant itself, they can come in many different forms.

Figure 10: Industrial Cooling Tower Model
Sourcehttp://www.jmatos.com/pt/torres-arrefecimento.html

Boiler or steam generator is equipment that is designed to generate steam through a thermal exchange between fuel and water. This is done by this equipment built with plates and tubes whose purpose is to make water heat and pass from the liquid to the gaseous state, taking advantage of the heat released by the fuel that causes the metal parts of it to heat and transfer heat to the water producing the steam.

The purpose of generating steam came from the industrial revolution and the means of the era was of little use, but steam at first served the purpose of moving machinery and turbines for power generation and locomotives, with the advent of industrial necessity. The need for cooking and sanitation and the manufacture of food was necessary for the evolution of the boilers. With this, steam is used in dairy products, food factories (tomato extract, sweets), jellies, tanneries, refrigerators, vulcanization industries, sugar and alcohol mills, weaving, pulp and paper factories, among others.

Heat transfer unit operations also have another type of transfer that is directly related to matter or material, usually fluid and / or solid, contained in a given equipment. Thus, the study of this second transference is fundamental to the understanding of the process as a whole. All its losses, efficiency, yield and applications of a given equipment are important throughout the processes generated by companies, regardless of their area of ​​expertise.

It is necessary to deepen the knowledge about the actions of the subject and its functionalities in the operating systems. Therefore, let us know the unit operations that involve mass transfer. See you in the next episode!

REFERENCES

– Caldeiras

– QUITES, Eduardo Emery Cunha; LIA, Luiz Renato Bastos. Introdução à transferência de calor. Acesso em 12 de Julho de 2019.

Torres de Resfriamento

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