Swift exchange

Heat exchangers sit at the core of every petrochemical facility, and represent a market segment projected to cross $12.7 billion by 2012. Petrochemicals Middle East investigates the systems available, and how to find the model best suited to your budget.

Heat exchangers are devices specifically designed for the efficient transfer of heat from one fluid to another over a solid surface. This transfer of heat can either take the form of absorption or dissipation of heat. Heat exchangers can be found in everyday equipment from boilers, furnaces, refrigerators to air conditioning systems.

The exchangers used transfer heat from one liquid to another, without allowing them to mix. The exchange can be to alter the temperature, or state of liquids, either condensing or boiling as required.

Regardless of the function the heat exchanger fulfills, in order to transfer heat the fluids involved must be at different temperatures and they must come into thermal contact, as heat can flow only from the hot to cold, thus the heat is transferred via a metal plate isolating the two fluids.

Heat exchangers are found in most chemical, electrical or mechanical systems and they serve as the system's means of gaining or rejecting heat. Some of the more common applications are found in heating, electronic equipment, ventilation and air conditioning (HVAC) systems, radiators or internal combustion engines, boilers, condensers and gas preheaters.

Classification

Heat exchangers are classified according to their flow arrangement. In parallel-flow heat exchangers, the two fluids enter the exchanger at the same end, and travel in parallel to one another to the other side. In counter-flow heat exchangers the fluids enter the exchanger from opposite ends. The counter current design is most efficient, in that it can transfer the most heat from the transfer medium. In a cross-flow heat exchanger, the fluids travel perpendicular to one another through the exchanger.

For efficiency, heat exchangers are designed to maximise the surface area of the wall between the two fluids, while minimising resistance to fluid flow through the exchanger. The exchanger's performance can also be affected by the addition of fins or corrugations in one or both directions, which increase surface area and may channel fluid flow or induce turbulence.

The petrochemical industry has specifications for manufacturing, which heat exchangers must meet. ASME (American Society of Mechanical Engineers), TEMA (Tubular Exchangers Manufacturers Association) and API (American Petroleum Institute) codes and specifications are the most commonly used. "Nowadays, the European standard PED (Pressure Equipment Directive) is also finding wider acceptability," says Perumal Sukumar, senior sales manager at Dolphin Radiators and Cooling Systems.

Shell & Tube

The most basic and the most common type of heat exchanger construction is the tube and shell, it consists of a set of tubes in a container called a shell. The fluid flowing inside the tubes is called the tube side fluid and the fluid flowing on the outside of the tubes is the shell side fluid. At the end of the tubes, the tube side fluid is separated from the shell side fluid by the tube sheet(s).

The tubes are rolled and press-fitted or welded into the tube sheet to provide a leak tight seal. On systems where the two fluids are at vastly different pressures, the higher pressure fluid is typically directed through the tubes and the lower pressure fluid is circulated on the shell side.

"Shell and tube as well as plate type heat exchangers find applications in the petrochemical industry. However, shell and tubes have more application due to their versatility and higher pressure capability. Plate heat exchangers are however more compact and useful below pressures of 25 bar", says Sukumar.

This is due to economy, because the heat exchanger tubes can be made to withstand higher pressures than the shell of the heat exchanger for a much lower cost. The support plates also act as baffles to direct the flow of fluid within the shell back and forth across the tubes.

Plate exchange

A plate heat exchanger uses metal plates to transfer heat between two fluids. This has a major advantage over a conventional heat exchanger in that the fluids are exposed to a much larger surface area, because the fluids spread out over the plates. This facilitates the transfer of heat, and greatly increases the speed of the temperature change.

It is not as common to see plate exchangers because they need well-sealed gaskets to prevent the fluids from escaping, although modern processes have made them feasible.

"Plate and frame heat exchangers are suitable for offshore platforms too, as it saves money and space" says Uttam Vishwasrao, service manager at Tranter Heat Exchanger.

A regenerative heat exchanger (regenerator), is a type of heat exchanger where the flow is cyclical and periodically changes direction. It is similar to a countercurrent heat exchangers. However, a regenerator mixes the two fluid flows while a countercurrent exchanger maintains them separated. The temperature profile remains at a nearly constant temperature, and this includes the fluid entering and exiting each end.

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