Types and Principles of Heat Exchangers

https://en.wikipedia.org/wiki/Heat_exchanger

Heat exchanger
Tubular heat exchanger

Partial view into inlet plenum of shell and tube heat exchanger of a refrigerant based chiller for providing air-conditioning to a building
A heat exchanger is a system used to transfer heat between two or more fluids. Heat exchangers are used in both cooling and heating processes.[1] The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact.[2] They are widely used in space heating, refrigeration, air conditioning, power stations, chemical plants, petrochemical plants, petroleum refineries, natural-gas processing, and sewage treatment. The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air. Another example is the heat sink, which is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant.[3]

Contents
1Flow arrangement
2Types
2.1Shell and tube heat exchanger
2.2Plate heat exchangers
2.3Plate and shell heat exchanger
2.4Adiabatic wheel heat exchanger
2.5Plate fin heat exchanger
2.6Finned tube heat exchanger
2.7Pillow plate heat exchanger
2.8Waste heat recovery units
2.9Dynamic scraped surface heat exchanger
2.10Phase-change heat exchangers
2.11Direct contact heat exchangers
2.12Microchannel heat exchangers
3HVAC and refrigeration air coils
4Helical-coil heat exchangers
5Spiral heat exchangers
5.1Construction
5.2Self cleaning
5.3Flow arrangements
5.4Applications
6Selection
7Monitoring and maintenance
7.1Fouling
7.2Maintenance
8In nature
8.1Humans
8.2Birds, fish, marine mammals
8.3Carotid rete
9In industry
10In aircraft
11Current market and forecast
12A model of a simple heat exchanger
13See also
14References
15External links
Flow arrangement

Countercurrent (A) and parallel (B) flows
Fig. 1: Shell and tube heat exchanger, single pass (1–1 parallel flow)
Fig. 2: Shell and tube heat exchanger, 2-pass tube side (1–2 crossflow)
Fig. 3: Shell and tube heat exchanger, 2-pass shell side, 2-pass tube side (2-2 countercurrent)

There are three primary classifications of heat exchangers 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 the most efficient, in that it can transfer the most heat from the heat (transfer) medium per unit mass due to the fact that the average temperature difference along any unit length is higher. See countercurrent exchange. In a cross-flow heat exchanger, the fluids travel roughly perpendicular to one another through the exchanger.

For efficiency, heat exchangers are designed to maximize the surface area of the wall between the two fluids, while minimizing 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 driving temperature across the heat transfer surface varies with position, but an appropriate mean temperature can be defined. In most simple systems this is the "log mean temperature difference" (LMTD). Sometimes direct knowledge of the LMTD is not available and the NTU method is used.

Types
Double pipe heat exchangers are the simplest exchangers used in industries. On one hand, these heat exchangers are cheap for both design and maintenance, making them a good choice for small industries. On the other hand, their low efficiency coupled with the high space occupied in large scales, has led modern industries to use more efficient heat exchangers like shell and tube or plate. However, since double pipe heat exchangers are simple, they are used to teach heat exchanger design basics to students as the fundamental rules for all heat exchangers are the same.

1. Double-pipe heat exchanger (a) When the other fluid flows into the annular gap between two tubes, one fluid flows through the smaller pipe. The flow may be a current flow or parallel flow in a double pipe heat exchanger. (b) Parallel flow, where at the same point, the hot and cold liquids join, flow in the same direction and exit at the same end.

(c) Counter flow, where at opposite ends, hot and cold fluids join, flow in the opposite direction and exit at opposite ends.

The figure above illustrates the parallel and counter-flow flow directions of the fluid exchanger. If this is done under comparable conditions, more heat is transferred to the counter-flow device than to the parallel flow heat exchanger. Owing to the large temperature