The majority of heat exchangers used in sanitary processing indirectly move heat from warmer to colder fluids without causing the fluids to mix.
Varies between its applications, type of heat exchangers employed differ and the heat transferring process can occur in such a way that gas-to-gas, liquid-to-gas, or liquid-to-liquid via a solid separator which prevents the direct fluid contacting each other.
The various kinds of heat exchangers that are accessible are further classified and categorized by other design features, such as flow configurations, heat transfer methods, and construction materials and components.
Numerous sectors find use for these heat-exchanging devices, which come in a variety of designs and capacities for heating and cooling operations.
In this article, we will get to understand in more details on heat exchangers, exploring the various designs and types available, as well as their respective working principles.
Heat Exchangers Thermodynamics
As a derivation in Thermodynamics, there are three methods in which heat can be transferred – Conduction, Convection and Radiation.
In Conduction, it is referred to as the transfer of thermal energy between materials which are in contact with one another.
When a warmer (higher temperature) object is in contact with a cooler object (lower temperature), it is said that there is a thermal energy transfer between two materials, until thermal equilibrium is achieved.
The heat energy transfer rate in thermal conduction can be performed by equation below:-
Where,
Q = Amount of heat transferred through the material in time, t
Delta T = Thermal gradient between two materials
A = Cross-sectional area of material
d = Thickness of material
k = Thermal conductivity of material
In Convection, heat energy is transferred from a surface through the movement of a heated fluid, such water or air. When heated, most fluids expand, becoming less dense and rising in relation to other, cooler portions of the fluid.
Therefore, a free convection current process occurs and in some occasions, forced or assisted convection when heated water is pumped through a pipe such as in a hydronic heating system.
By Newton’s law of cooling, rate of heat transfer in free convection is derived as:-
Where,
Q = Rate of heat transfer
hc = Convective heat transfer coefficient
A = Convection surface area
Delta T = Temperature difference between surface and fluid
In Radiation, it refers to the heat energy transfer which involves the electromagnetic waves emission from a heated surface or object. Radiation process is carried out without any mediums as all objects whose temperature is above absolute zero (-273.15°C) emit thermal radiation.
Types of Heat Exchanger
Several variants of heat exchangers are available in the market nowadays and the selection of types of heat exchangers is purely based on the application of use. Three of the most common types of heat exchangers are:-
Shell & Tube Heat Exchangers
Shell and Tube heat exchangers are made up of a single tube or series of parallel tubes enclosed within a sealed, cylindrical pressure vessel.
The working principle of this heat exchanger allows one fluid or gas to flow through the smaller tube(s), whereas the other fluid flows within its shell side.
The countercurrent flow, cocurrent flow, or crossflow configurations in design characteristics will further determine the efficiency of heat exchange.
Several common industries and applications of this type of shell & tube heat exchangers are:-
Oil refining
Preheating
Oil cooling
Steam generation
Boiler blowdown heat recovery
Vapor recovery systems
Industrial paint systems
These designs are typically employed in high pressure applications, but they can also be used in situations where a vacuum may call for a structure that can withstand high stresses.
Because of its containment properties, a shell and tube bank may be a better option for handling hot gases than a finned tube bank, especially in situations where the gases are toxic, potentially harmful to human health, or require special handling to prevent leakage into the atmosphere.
Another form of shell & tube heat exchanger, double pipe heat exchanger in HVAC air-conditioning system employs two or more concentric, cylindrical pipes or tube (one larger and one smaller tubes to allow one fluid flowing through the small tubes, and the other fluid flows in between the small tubes within the larger tube.
Double pipe heat exchangers can be constructed with co-current or counter-current flow arrangements, and they can be employed in series, parallel, or series-parallel combinations within a system.
This gives them some design flexibility as well to achieve required capacity and efficiency.
Finned Tube Heat Exchanger or Air Cooled Heat Exchanger
Fluids like air, water, oil, or gas can be heated or cooled using finned tube heat exchangers, which can also be used to recover or capture waste heat.
Numerous industries, such as oil and gas, power generation, maritime, and HVAC&R, can use these heat exchangers. Several applications of the finned tube heat exchanger includes:-
Diesel charge air coolers
Oil Coolers
Hydrogen coolers
Waste Heat Recovery
Driers
Air-conditioning
Air heaters
Steam Condensers
Generator Coolers
When air is the desired medium for cooling or heating, such as when there is little or low-quality water, finned tube heat exchangers are frequently utilized.
Heat is transferred between a fluid that transfers heat efficiently—such as a liquid with some viscosity—and a fluid that does not—such as air or gas with low density—in a finned tube heat exchanger.
In order to expand the tube's surface area and optimize its thermal performance, fins or other components like looped wires are added to the "air side" of the tube.
Fins can vary in height from high to low, and they can be molded into the surface of the tube or pressure-connected to its outside.
Finned tubes can be produced in a variety of designs and with a combination of different materials for the tubes and the fins, depending on the intended use and operating environment.
Plate Heat Exchangers
A plate type heat exchanger consists of multiple thin, corrugated plates bundled together and each pair of plates creates a channel through which one fluid can flow, while the pairs are binned via brazing or fastened in such a way that a second passage is created between pairs, allowing other fluid can flow through.
This process is referred to as heat exchange in plate heat exchangers.
Wider spaces between plates can aid in maintaining flow requirements when fluids are somewhat viscous (thick) or include trace amounts of particles. Particulates can move across plates with wide gaps without blocking flow.
Chevron patterns are commonly found on standard plates in order to optimize plate strength under high pressure. Different chevron angles on plates can be used to maximize heat transmission for particular pressure drops.
Due to having fewer points of contact, wider-stream plates aid in preventing clogs. When heated with liquid or steam, they work particularly well for applications using raw juice.
There are two characteristics of wide-gap plates that can facilitate the flow of fluids or particles when they are employed with fibrous, viscous, or particulate fluids:
1. Greater distances between plates than in typical designs
2. Pattern of plates
In the unlikely event that a break appears in one of the plates, the media cannot mix between the two sheets that make up a double-walled plate.
Gaskets can be clipped or glued to the plates, playing an important role in this type of heat exchanger to prevent misalignment which causes leaking.
Applications for plate-and-frame heat exchangers are usually on low to medium viscosity products with little to no particulate such as milk, cream, ice-cream mixes, beverages and beer wort.
In a Nutshell
Heat exchangers can exist in various forms and they are highly dependent on their applications.
TCW Group, as a one stop HVAC manufacturer based in Malaysia able to provide professional advice and offer solutions via manufacturing of the appropriate heat exchangers used in different applications.
Feel free to drop us an inquiry at support.hvac@tcw-my.com.