Conditions of the heat transfer processes in industrial apparatus extremely diverse. These apparatuses are used for working environments with different structure and state of aggregation (gas, vapour, droplet liquid, emulsion, etc.) in a wide range of temperatures, pressures and physico - chemical properties.

Because of the variety of established requirements for heat exchangers, related to their application conditions, different heat exchangers design with wide size range of heat transfer surface (from a few to thousands of square meters in one unit) can be applied. The dimensional series of heat exchangers vary in permitted pressures and temperatures of the working environment, as well as materials from which the device is made.

A strict classification of heat exchangers does not exist. With regard to the petroleum refining industry heat exchangers can be classified according to the following main characteristics (Figure 1.1).

Figure 1.1  – General classification of HEA

а) according to the method of heat transfer apparatus are divided into the following groups (Figure 1.2):

Figure 1.2 –  Classification of HEA according to heat transfer

This HEA group can also be classified according to the construction (Figure 1.3):

• Apparatus made of tubes (“shell and tube” HE (Figure 1.4), "pipe in pipe" type (Figure 1.5), air coolers, subversive (Figure 1.6), coiled, etc)

Figure 1.3 – Classification of HEA according to the construction

Figure 1.4 – Shell and tube HEA

Figure 1.5  –  “Tube in tube” type of HEA

Figure 1.6 – Submersible HEA

• Apparatus that heat exchange surface is made of sheet metal (plate (Figure 1.7), spiral, etc.)
• Heat exchange apparatus with a surface made of non-metallic materials (graphite, plastics, glass, etc.)

1 – fixed plate with connecting pipes, 2 – back pressure plate, 3 – heat exchanger plates with gaskets, 4 – top rail, 5 – lower guide, 6 – Back, Front, 7 – a set of threaded rods.

Figure 1.7 – Plate heat exchangers

- Direct contact apparatus where heat transfer takes places between directly commingled flows. For the construction of direct contact heat exchangers generally less metal required; moreover, in many cases, they provide more efficient heat exchange. However, despite these advantages, these HE often can not be used due to infeasibility of direct contacting flows. These also include apparatus such as cooling towers. Structurally, the plate heat exchanger (PHE) is a package of heat exchanging plates and gaskets, installed in a special frame and bonded tightly with threaded rods of a certain size.

1. b) Depending on the purpose HEA are divided into the following groups (Figure 1.8).

Figure 1.8 – Classification of HEA according to the purpose

- Heat Exchangers - heat recovery devices that were entrained by-effluent streams. The general processes that take place might be heating of cold or cooling of hot flows or both the processes equally.

This heating and cooling of the flows reduces the heat supplied externally (the fuel consumption, amount of heating steam and etc.) and cooling agent.

This group of devices purposed to heat the oil on the distillation unit that is carried out with the use of outgoing heat of distillates, residue, as well as pumparound. This also includes heat recovery steam generator (HRSG) where steam is produced through the use of heat of oil distillation products, flue gases or catalyst of the FCC unit.

This group also includes cold regenerators:

- Heaters - apparatus for heating distillates or reagents with the heat of the heat transfer agent.

The heating process is central in this apparatus. Generally water vapors are used as a heat transfer agent, characterized with a high coefficient of heat transfer at a large value and the condensation of latent heat of condensation. Heat transfer agents could also serve as high-boiling point oil, heated in the tubular furnace;

- Condensers – the devices for cooling and condensing the vapor where the heat transfered to the cooling agent;

- Refrigerators – devices for cooling liquid flows. For the safe transport and storage of distillates cooling to the required temperature, usually ends up in the refrigerators.

If cooling of the liquid streams forms crystals then this apparatus called crystallizer.

The condenser, refrigerators and crystallizers are purposed for the cooling of the hot medium.

- Evaporators, boilers, in which the heating or partial vaporization is carried out with the use of high boiling point distillates flow and specific heat transfer (steam, hydrocarbon vapors, special high-boiling liquids, etc.). In these devices the heating or evaporation of a medium is the main target process, while cooling the hot stream is a byproduct due to the necessity of heating of the initial cold stream.

Examples of devices of this group may serve as the raw material heaters utilizing the heat of steam boilers by means of which the bottom of the distillation column is fed.

1. c) depending on the direction of flow, co-current, counter-current, cross-current and others.
2. d) depending on the number of moves on the tube and shell side: one, two, four - and multipass.

Shell and tube heat exchangers occupy about 80 % of the whole petrochemical industry. These heat exchangers are fairly simple to manufacture and reliable in operation and at the same time sufficiently universal. It can be used for heat exchange between gases, vapors, liquids, coolants, in any combination and in a wide range of pressures and temperatures.