Blog
Definition of Heat Exchanger
A heat exchanger is a device used to transfer thermal energy between two or more fluids, which can be liquids, gases, or a combination of both. The main objective of a heat exchanger is to efficiently transfer heat from one fluid to another, to either heat or cool the fluid, while preventing direct mixing between the fluids. This technology plays a crucial role in many industrial applications, including energy production, chemical reactions, and air conditioning systems.
Thermodynamics of Heat Exchangers
According to thermodynamics, heat transfer occurs in three ways: conduction, convection, and radiation.
Conduction refers to the process of heat transfer between materials that are in direct contact. When a hotter object comes into contact with a cooler object, heat is transferred between the two materials until thermal equilibrium is reached. The rate of heat transfer through conduction can be calculated using the following formula:
Where:
- (Q) = heat transferred by the material over time (t)
- (\Delta T) = temperature difference between the two materials
- (A) = cross-sectional area of the material
- (d) = thickness of the material
- (k) = thermal conductivity of the material
In convection, heat is transferred through the movement of the heated fluid (such as water or air) over a surface. Most fluids expand when heated, reducing their density, and rise relative to the cooler portions of the fluid.
For example, when hot water is pumped through a pipe (as in a hydronic heating system), natural convection occurs, and in some cases, forced or assisted convection may also take place. According to Newton’s law of cooling, the heat transfer rate in natural convection can be expressed as:
Where:
- (Q) = heat transfer rate
- (h_c) = convective heat transfer coefficient
- (A) = convective surface area
- (\Delta T) = temperature difference between the surface and the fluid
Radiation refers to the transfer of heat through electromagnetic waves emitted by a heated surface or object. Radiation does not require any medium, as all objects with temperatures above absolute zero (-273.15°C) emit thermal energy.
Classification of Heat Exchangers
Based on Heat Transfer Principle
Surface Heat Exchangers
Surface heat exchangers involve two fluids with different temperatures flowing in separated spaces divided by a wall. Heat is transferred through the wall via conduction and convection at the wall-fluid interface. Surface heat exchangers include shell-and-tube, double-pipe, and other types.Regenerative Heat Exchangers
Regenerative heat exchangers transfer heat from a high-temperature fluid to a low-temperature fluid through a solid heat storage medium. The hot medium first heats the solid material to a certain temperature, and then the cold medium passes through the solid to absorb heat, achieving heat transfer. Examples include rotary and valve-switching regenerative exchangers.Indirect Fluid-Connected Heat Exchangers
Indirect fluid-connected heat exchangers connect two surface heat exchangers via a circulating heat transfer medium. The medium absorbs heat from the high-temperature fluid exchanger and releases it to the low-temperature fluid exchanger, transferring heat indirectly between the two fluids.Direct Contact Heat Exchangers
Direct contact heat exchangers allow two fluids to come into direct contact for heat transfer. Examples include cooling towers and gas condensers.
Based on Function
Heaters
Heaters raise the temperature of a fluid to the desired level without changing its phase.Preheaters
Preheaters preheat fluids to provide standard process parameters for subsequent operations.Superheaters
Superheaters raise the temperature of fluids (process gas or steam) to a superheated state.Evaporators
Evaporators heat fluids to their boiling point or higher, causing phase change and vaporization.
Based on Structure
Heat exchangers can be classified into: floating head type, fixed tube sheet type, U-tube type, plate type, and others.
Common Types of Heat Exchangers
There are various types of heat exchangers available in the market, and the selection depends entirely on the application scenario. The following are three of the most common types:
Shell-and-Tube Heat Exchanger
The shell-and-tube heat exchanger (also called a tubular or column-type exchanger) is widely used in industry as a wall-separated heat exchange device. Its core principle is that two fluids at different temperatures flow through a closed metal shell, with one fluid inside the tubes (tube side) and the other in the space around the tubes within the shell (shell side).
It mainly consists of a shell, tube bundle, tube sheets, end caps, and baffles. The fluid in the tube side usually handles high-temperature, high-pressure, or fouling-prone fluids, while the shell-side fluid flows in the space between the tubes and the shell wall.
The working principle is based on thermodynamic laws. When hot and cold fluids flow through the tube and shell sides respectively, heat is transferred through the metal tube wall from the hot side to the cold side. Baffles are usually installed in the shell to force the fluid to flow multiple times across the tube bundle, increasing velocity, breaking the boundary layer, and enhancing heat transfer efficiency.
Shell-and-tube exchangers are highly adaptable, capable of withstanding extreme temperatures and pressures, making them suitable for demanding industrial conditions. Materials such as carbon steel, stainless steel, titanium, or copper can be chosen based on fluid corrosiveness. Some structures (e.g., floating head or pull-out tube bundle) facilitate cleaning and maintenance.
Finned Tube Heat Exchanger
Finned tube heat exchangers enhance heat transfer efficiency by adding fins to the surface of the tubes. The core idea is to increase the heat transfer area, improving thermal transfer between fluids, often used between gas and liquid.
Fins (usually aluminum or copper) are attached to the outer tube surface and can be spiral, straight, or embedded. The liquid typically flows inside the tubes, while gas or another fluid flows outside around the fins. Heat transfers through the tube wall and fins, and the fins significantly increase the heat transfer area, boosting the thermal performance per unit tube length.
Key features include:
High heat transfer efficiency: Fins greatly increase the exchange area, especially useful for low-efficiency gas heat transfer.
Compact structure: Takes up less space than a shell-and-tube exchanger of equivalent capacity.
Wide applicability: Suitable for heat exchange between air, steam, flue gas, and liquids.
Flexible design: Fin type, spacing, and tube diameter can be selected based on fluid properties and working conditions to optimize heat transfer and minimize pressure drop.
Common applications include air conditioning condensers, industrial air coolers, flue gas heat recovery, refrigeration evaporators, and cooling tower heat exchangers.
Additional Notes
Common heat exchanger types can be further subdivided, but details are not elaborated here. If you are interested in learning more, feel free to leave a comment or contact JECICOOL directly for professional support on heat exchangers, chillers, and other refrigeration equipment.
Share This Article：
