What is Heat Exchanger?
Heat exchangers are the equipment’s used for transfer of thermal energy (enthalpy) between two or more fluids. These fluids can be liquids, vapors or gases. It can be between a solid surface and fluid, between solid particulates and fluid, liquid to liquid, gas to gas, and liquid to gas at various different temperature, where T1 (fluid 1 temp) ≠ T2 ( fluid 2 temp) and in thermal contact. These fluids can be single or two phase, may be separated or in direct contact depending on the exchanger type. To increase the efficiency of the heat exchanger, there should be increment in the surface area of the wall between the two fluids and minimize the resistance of fluid flow through the exchanger.
Typically the process involves the heating and cooling of a fluid or evaporation and condensation depending upon the application and usage. In some other applications these are used to reject or recover heat, pasteurize, sterilize, crystallize, fractionate, distill or control a process fluid.
Selection of a particular type of heat exchanger depends on various factors such as –
- Nature of the two fluids
- their pressure
- their temperature
- flow rate
- Required heat transfer rate
Therefore, it is important to understand the basic principle of a heat exchanger and which heat exchanger can be used according to the required condition.
Basic Working Principle of Heat Exchanger
Irrespective of their design all heat exchangers work under the same roof. These are the 3 laws of thermodynamics, namely the zeroth, First, and Second law of thermodynamics. These laws facilitate the transfer or exchange of heat from one fluid to another.
A heat exchanger is a device that includes 2 or more fluids that transfer the heat from one medium to another. In general, one of the fluid is significantly at a high temperature compared to another.
So now we have a cold and hot fluid. As when the two try to move towards the stability the heat exchanger enables the heat to pass from hot fluid or medium to cold fluid either directly or indirectly.
As a result in the end the hot fluid becomes a little colder and cold fluid become a little hotter.
How Heat is Exchanged- Mechanism of Heat Transfer
Heat can be exchanged or transfer through mainly 3 process
It is a process by which heat is transferred from high vibrating molecular temperature to low vibrating molecular temperature which is in physical contact with each other. The former is at higher and later is at lower kinetic energy. This most preferably takes place in solid due to the collision of molecules. These are also known as heat conduction or thermal conduction. E.g. – Hot cup of tea is placed on the table. Heat transfer from cup to table.
2. Convection –
This heat transfer occurs within the fluids because of the density difference. It happens when fluid moves and carries the thermal energy away. E.g. – Boiling of water. The hot water molecules at the lower surface heat up and become lighter and move upward while at the same time, the dense water molecules move from top to bottom.
3. Radiation –
It is a process by which heat is transferred through electromagnetic waves. It doesn’t require any medium for heat transfer and can be transferred in a vacuum or any transparent medium. The rapid movement of charged protons and electrons generate these waves.E.g.- Microwave radiation emitted in the oven.
It can be calculated by Stefan Boltzmann’s law.
Fluids used in the Heat Exchanger
A heat transfer fluid can be liquid or gas depending upon the application or the type of heat exchanger and material being used. It can be air, water, oil, water glycol, chlorinated salt water, acids, etc.
Before fluid selection there are mainly three different parameters which need to be looked out;
Which involves their physical state, chemical properties, and thermal properties. If we elaborate this more , a fluid needs to be selected based on their temperature, alkaline or acidic, flow rate, pressure, Phase change, etc.
Water as fluid- It is the most common type of fluid used in heat exchanger because it is cheaper, have high heat capacity (the amount of heat a fluid can hold without changing its temperature), and are easy to transport.
Oil, synthetic hydrocarbons or silicon based fluids are used for high temperature ranges.
Gases such as water vapor, Nitrogen, Argon, Helium, Hydrogen are used where liquids fluids are not suitable. so, as to get the efficient working conditions of heat exchanger using gases as a fluid, pressure is elevated, to increase the flow rate of gases.
In general, the Boiling point and heat capacity should be high. Because high boiling points prevent the fluid from vaporizing at larger temperatures and high heat capacity enables a small amount of fluid to transfer large amount of heat very efficiently.
Some basic characteristic of fluids
- Low viscosity- helps in the easy flow of fluid and reduces the pumping cost.
- Non- Corrosive- the flowing fluid should avoid the corrosion on the walls of the tube which reduce the maintenance cost.
- High thermal diffusivity and conductivity
- High boiling point and low freezing point- helps to remain in the same phase while exchanging heat.
Material Selection for Heat Exchanger
Material selection is based on various factors such as on application, thermal efficiency, corrosion resistance, durability, cleanability, and cost & availability of the material.
There are various materials such as metals, ceramics, plastics, composites, etc. and each has advantages over others.
- Graphite heat exchanger– High thermal conductivity and corrosion-resistant.
- Ceramic heat exchanger – They can withstand at high temperature of over 1000 degree Celsius. At this range metals like iron, copper, steel easily melt.
- Composite heat exchanger- composites are the amalgamation of two or more materials. You can mix a high thermal conductivity material with a low weight material with better corrosion resistance of plastic.
What is Temperature Cross Over
Let’s suppose a scenario in which the inlet temperature of hot fluid (Fluid 1) is 70°C and inlet temperature of cold fluid (fluid 2) is 30°C and outlet temperature of fluid 1 is 45°C and outlet temperature of fluid 2 is 47°C. Now this is the condition of the temperature cross over, when the outlet temperature of cold fluid exceeds the outlet temperature of hot fluid. This is an important factor that needs to be considered while designing the heat exchanger. It reduces the efficiency of the exchanger.
To avoid this one can increase the flow rate of cold fluid (fluid 2) or in some cases, it can’t be avoided so plate heat exchanger is the best option for that.
Classification or Types of heat Exchangers
Heat Exchangers can be classified into 5 categories-
Classification on the basis of flow arrangements
1. Parallel Flow-
These heat exchangers are also known as the co-current heat exchangers. These are the arrangements in which both the fluids enter at the same end, moves in the same direction to the other end. In these designs, the temperature difference at the inlet is high, but the fluid temperature reaches the similar value at the outlet. Although these arrangements have lower efficiencies compare to counter flow arrangements but have thermal uniformity across the wall of the exchanger.
2. Counter flow –
Also known as the Counter-current heat exchangers are the flow in which the fluids enter from the opposite ends and travel anti-parallel to each other. It allows the highest flow of heat transfer and are the most efficient in all flow arrangements.
3. Cross flow –
In cross flow arrangements fluids flow in perpendicular direction to each other. These have efficiencies in between of parallel and counter flow arrangements.
In industries, Hybrid type of flow arrangements are used such as counter-cross flow and multi pass flow heat exchangers.
Why Counter flow heat exchangers have better efficiency than Parallel flow?
There are mainly 3 points to explain why the counter flow has better efficiency than parallel.
- Temperature Gradient – As you can see in graph average temperature difference or temperature gradient is vary largely in parallel flow which cause thermal stress on the wall. On the contrary, counter flow has a uniform distribution of thermal stress throughout the process.
- The outlet temperature of the cold fluid reaches the highest temperature of the hot fluid in counter flow.
- As there is a more uniform temperature difference throughout the process, the rate of heat transform is also uniform.
What is LMTD (Logarithmic Mean Temperature Difference)?
LMTD is used to calculate an idea of performance and effectiveness of the heat exchangers. LMTD stands for Logarithmic mean temperature difference. It is the logarithmic average of the temperature difference between the hot and cold fluid. LMTD is directly proportional to the amount of heat exchanged. So, as the value of LMTD increases, the value of heat transfer between the fluids increases or the other way around.
As we know the amount of heat exchange can be expressed as –
LMTD Correction factor
Now before understanding LMTD correction factor lets understand what is heat exchanger pass.
A heat exchanger pass is refer to the flow of fluid from one end to other. If it is one shell pass then the fluid enter from one end and exit at another.
If it is 2 shell pass or double shell pass then the fluid enter and exit at the same end. And so on
The greater no. of pass, the greater rate of heat transfer, but at the same time can also lead to high-pressure loss and high velocity.
Let’s understand this from the diagram.
So coming back on the topic LMTD is valid for only one shell pass and one tube pass. For multiple no of shell and tube pass the flow of fluids is neither parallel nor counter flow. Hence for this geometric irregularity LMTD correction factor(F) is calculated to obtain corrected mean temperature difference ( Corrected MTD) or the effective driving force.
Corrected LMTD = F* LMTD
What is Effectiveness of Heat Transfer – The NTU method?
When the heat transfer area of the surface is known, but more than one of the outlet and inlet temperature are unknown, LMTD might be applied with hit and trail approach, but the effectiveness-NTU method is always preferable and applied to precession results.
The effectiveness is the measure of the thermal performance of a heat exchanger. It is the Ratio of actual rate of heat transfer from hot to cold fluid to the maximum rate of heat transfer thermodynamically permitted.
Since the actual heat transfer can never be larger than maximum heat transfer the value of ∈<1 always. For any specified fluid pair, the counter-current flow have the maximum effectiveness.
The increase of effectiveness is non-linear with NTU.
Classification on the basis of Phase
1. Single-phase and two-phase heat transfer
In single phase heat exchanger, the fluids do not experience phase change throughout the heat transfer process. The inlet and outlet state or phase of the fluid remain same. For e.g. in liquid-liquid heat transfer applications, the warmer liquid loses heat and transferred to the cooler liquid and neither of the fluid changes to gas or solid state.
In two- phase heat transfer, the fluids do experience the phase change during the heat transfer process. The phase change can occur in both or one of the fluids resulting in a change of state from liquid to gas or gas to liquid. Generally the designs of two-phase heat exchangers are more intricate and complex as compare to the single phase exchangers.
Condenser, boilers, evaporators are some applications where 2 phase mechanism are being used.
Types of two-phase heat exchangers
A condenser is a heat exchanger device which is used to condense or cool down the fluid from a gaseous state to liquid state. It is a 2 phase heat exchanger which changes the phase of gaseous to liquid. During this process, the latent heat is released by the substance and transferred into the surrounding. These are used for efficiently heat rejections in many industrial process. It comes in many sizes ranging from small to very large.
It is designed to transfer heat from a hot working fluid to another fluid or surrounding air. The heat transfer between fluids takes place during phase change, in this case during the condensation of a vapor into a liquid.
The vapor typically enters into the condenser at a relatively high temperature compare to another fluid. As the vapor cools, it goes down to saturation temperature, condenses into liquid and releases large amount of latent heat.
As shown in the figure below the fluid is cooled from a gas(g) to a liquid(f)
Heat transfer rate in condenser is denoted as below
An evaporator is a heat exchanger device which is used to convert a chemical substance from liquid into a gaseous or vapor form. In other words evaporator is a cooling system that extracts the heat from the environment. During this process, the heat is injected with the substance from the surroundings or from hot fluid.
– Working principle
The solution is fed into the evaporator and passes through the heat source. The applied or injected heat convert the liquid or water into vapor.
Vaporization takes place from m to g.
A boiler is in the shape of a closed vessel in which fluid (generally water) is heated. It’s not mandatory the fluid should boil in boilers. The heated fluid or vaporized fluid exit the boiler and pass on to another chamber or use for various process or heating applications. It’s include water heating, cooking, sanitation, central heating.
The basic working principle of boiler is very easy to understand. The fuel, generally coal, is burnt in a furnace and hot gases are produced. These hot gases come in contact with the water bodies, which are in a closed vessel. This leads to the heat transfer process from hot gases to water and consequently the steam is produced in the boiler.
After this steam is passed to the turbine of the thermal or steam power plant.
Recuperative Heat Exchangers VS Regenerative heat exchangers
Recuperators are the one in which two fluids are separated all the time either direct or in indirect contact (through a solid barrier) to each other. A recuperator is a countercurrent flow heat recovery heat exchanger positioned within the supply in order to recover the waste heat.
In Recuperative heat exchangers, each fluid all together flow through its own channel within the exchangers. On the other hand, regenerative heat exchanger, also known as capacitive heat exchangers allows the fluid to flow alternatively through the channel. In these above 2 types explained, recuperative heat exchangers are widely used in industries.
Working Principle of Recuperative Heat exchangers
Recuperators are the heat exchanger in which energy from hot combustion gases, known as flue gases, is transferred to the cold air supplied for combustion. For e.g. in Gas turbine engines, air is compressed then mixed with fuel, which is then burned and drives the turbine. The recuperator, as shown in above figure, exchange these cold gases with hot air which is later on used to preheat the air before entering to the burning of fuel stage and thus save the fuel and make it more efficient.
These Recuperators are further categorized into Direct, Indirect and Special types.
Working Principle of Regenerative heat exchangers
In regenerative heat exchangers, the flow path normally consists of matrix, as you can see in figure, which is heated or energy gets stored when the hot fluid passes through it ( also known as hot blow). This stored energy is released when the cold fluid passes through the matrix (known as cold blow).
These are used in heat recovery applications in power stations and other energy-intensive industries.
These are further classified into Static and Dynamic.
Static vs Dynamic
These regenerators also known as fixed bed, stationary, valved, and periodic flow. For continuous operation the exchanger must have at least two identical matrices in parallel, but typically have three, or four matrices which reduces the temperature variation effectively in the outlet-heated cold gas. Unlike in rotary regenerators single matrix is sufficient for continuous operation.
In fixed or static regenerators, the heat exchanger material and components remains stationary or fixed as fluid moves through it. While in dynamic, the material components move throughout the process. Both are at risk for developing cross- contamination between fluids stream.
In one of the static type regenerator, the warmer fluid runs into one channel and colder fluid runs into another for a fixed period. After, sometime using quick operating valve the flow of both fluids reversed with each other which leads to the heat transfer process.
In dynamic, it typically consists of a rotating, thermally conductive element ( a drum or disc) through which warmer and colder fluid continuously flow, in separate sealed sections or channels. As the drum or disc rotates the given section first passes through the warmer, and then colder fluid or vice versa. Which allows to absorb the heat from the warmer fluid and pass it to the colder fluid.
Classification on the basis of compactness
Compact vs Non compact Heat Exchangers
Compactness and non-compactness of any heat exchanger depend upon the quantity known as area density, denoted by β (Beta). Area density is the ratio of the surface area of the heat exchanger to the volume of that exchanger.
Typically when β is greater than 700 then it is considered as Compact Heat Exchanger otherwise Non- Compact Heat Exchanger.
Some examples and their area density value-
- Car Radiator ,β = 1000
- Glass ceramic gas turbine Heat exchanger, β = 6000,
- The Regenerator of Sterling Engine , β = 15,000
- Human lungs, the most effective and most compact exchanger, β = 20,000.
Classification on the Basis of Transfer Process
1. Direct contact-
Recuperative heat exchangers either subject to direct or indirect contact heat transfer process to exchange heat between the two fluids. In direct contact heat exchangers, there is no wall or separator between the two fluids. So the heat is directly transferred from warmer fluid to colder fluid. The devices which employ this process include cooling towers, Steam injectors etc.
2. Indirect contact-
In indirect contact heat transfer process, there is always a wall or barrier such as tube or plate (a thermally conductive material) between the fluids. The heat first transfer from the warmer fluid to the wall and then to another fluid.
Classification on the Basis of Construction
These exchangers are generally build in a tubular shape, although also in rectangular, oval, elliptical or round/flat twisted tubes have also been used in some of the applications. There is a lot of flexibility in the design because the core geometry can be easily altered or varied by changing the length, diameter or arrangements of the tube. Tubular exchangers can be simply designed for high pressure relative to the environment and for high-pressure fluids.
These are majorly used for liquid to liquid or for liquid to phase change ( condensing or evaporating) heat transfer application.
These are also used for liquid- to -gas and gas -to –gas heat transfer application when the operating temperature and pressure is very high and prone to fouling problem.
These are further classified as Double pipe, Shell and tube, Spiral tube.
Double Pipe Heat Exchanger
Design and construction
It is the simplest type of heat exchanger in design and configuration which consists of two concentric cylindrical pipes or tubes, which means that both the pipes have common center point. One of the tubes have a larger diameter as compared to another.
These are applied where flow rates of the fluids and heat duty are small ( less than 500kw).
These exchangers are generally used where heat transfer surface area requirement is 50 m2 or less.
These are simple to design but require large space to get the desired heat transfer rate. Stacks of double concentric pipes are used in some process applications with fins.
- It has two section, Inner and outer. It is one of the most convenient and efficient double type of heat exchanger.
- Several hairpins are connected in series in order to obtain a larger heat transfer rate.
- Return bend of Inner pipe do not contribute in the heat transfer rate.
During the flow of fluids deposition of any undesirable material on the heat transfer material surfaces is known as Fouling.
Due to this situation, an undesirable resistance is created against the flow which is known as fouling factor or dirt factor and is denoted by Rd and is generally 0 for newly build heat exchangers.
These fouling increases the overall thermal resistance and also lower the heat transfer coefficient of heat exchangers.
– Types of fouling
- Chemical fouling
- Corrosion fouling
- Crystallization fouling
- Biological fouling
Shell and Tube Type Heat Exchangers
Design and Construction
- These are the indirect contact types of exchangers. It consists of many small tubes which are located inside a shell. These tubes are positioned inside the shell either in the form of bundle or stack, which can be fixed (permanent fix) to the body, floating head, or U-Tube. Fixed tube provides maximum are for heat transfer. The floating type accommodate the tube bundle to expand or contract according to heat flow rate. The floating type also allows the tube to be easily removed for servicing and maintenance. U-Tube provides differential thermal expansion between shell and tube and to the individual tubes.
- The tube bundle comprises tubes, tube sheets, tie rods, and baffles which hold the bundle together. Baffles are used to prevent the tube bundle from vibrating.
- These are the most versatile and most commonly found in all the plants. They also provide high rate of heat transfer rate in a confined space. They can easily operate at high pressure. The shell is an enclosure and has a circular cross section
- The selection of the heat transfer material depends upon the corrosive nature of the fluid and the working pressure and temperature. In general tube and shell are made up of metal but for some applications, other materials such as graphite, glass, plastics may be used. SiC- based heat exchangers are generally used in chemical industry for processing of Sulfuric acid.
- Tubes having a diameter in the range of 19 mm to 20 mm are most commonly used. These tubes have a triangular or square pitch.
In general all heat exchangers have the same principle which is to transfer heat from one medium or fluid to another. A shell and tube, indirect contact heat exchanger, has an outer cylindrical shape as shell and bundles of tubes enclosed inside a shell. One fluid pass in tube and another outside of a tube or over the tubes. Each one have different temperature at the entrance of the heat exchanger.
Some Applications of shell and tube heat exchangers
- Generator oil cooler
- Turbine oil cooler
- Furnace oil heaters
- Compressor air cooling
- High pressure boiler feed water heater
Spiral tube Heat exchanger
Spiral tube or coil are high efficiency heat exchangers. These consist of a multi-tube spiral assembly inserted inside a shell, where two paths are created for counter-flow heat transfer between fluids. It is more efficient than the shell and tube type heat exchanger, since the spiral geometry is compact and require less space compare to shell and tube type heat exchanger
It is applied for high pressure applications..
Fluid flow in the spiral tube and other on the outer surface of tube or in shell.
- Compact, light-weight, and easy to install
- Gives high resistance to hydraulic and thermal shock
- Optimized design for corrosive fluids.
- Pump seal coolers
- Instant hot water heaters
2. Plate type Heat Exchanger
Plate type heat exchangers worked in the similar manner as shell and tube type heat exchanger, using series of stacked corrugated plates rather than tubes. It allows the formation of series of channels for fluids to flow between them.
The space between two adjacent plates forms the channel which allows the flow of fluids. It has a major advantage over other conventional heat exchangers because of the fluids are exposed more in this over the surface area. This easily facilitates the transfer of heat and also increases the temperature change. Advancement in gasket and brazing technique increased the practical use of plate type heat exchangers. These exchangers have low volume and cost as compared to shell and tube. In HVAC (Heat, ventilation, and air conditioning) applications, plate and frame heat exchangers are used.
There are many types of permanently bonded plates heat exchangers such as Dip- brazed, Vacuum-brazed, and welded plates.
Plates heat exchangers can also be categorized on the basis of type of plates used, such as spiral plate, lamella, Gasketed plates.
Why corrugated plates?
- Provide better heat transfer
- Produce turbulent flow
- Provide strength
Lamella Heat Exchangers
Design and construction
A lamella heat exchanger consist of a bundle of tube, known as lamella which are enclosed by a outer tubular shell. These tubes or elements referred to as Lamella are thin plates, welded edges and are high aspect ratio rectangular channels. There is an opening at the end of the lamella through which fluid passes and ranges from 3 to 10 mm and have thickness of 1.5 to 2 mm. Lamella are stacked or bundle with each other which forms the channel and allows the flow of another fluid. In large heat exchangers, two or more tubes are stacked together to sustain high pressure. There are no baffles in this. However, one end of the tube is fixed and another is floated, which allows thermal expansion.
One fluid flows inside the lamella tubes and other flows longitudinally in the spaces between them. In general the exchanger has single pass with a counter current flow of fluids.
Also have high heat transfer coefficient because of small hydraulic diameter and no leakage.
Spiral Plate Heat Exchangers
Typically It consists of a two strips of sheet metal, a split mandrel or a shaft, welded studs, covers. These strips of sheet metal wrap around the split mandrel in helical shape to form a pair of spiral channels for two fluids. These plates have spacing by using welded studs.to complete the arrangements, covers are fitted at each end. Generally, carbon steel and stainless steel are used for this.
A spiral plate exchanger has comparatively large diameter because of those spiral turns.The core spiral element or plates are either welded at each side of the channel or gasketed.
3 arrangements can be obtained for fluids.
- Both the fluids are in spiral counter-current flow or counter flow.
- One fluid in spiral flow, and other in cross flow to the spiral.
- One fluid in spiral flow, and other in combination of cross and spiral flow.
The heat transfer coefficient is not as high as in plate type heat exchangers unless if plates are corrugated. However, these spiral plates have high heat transfer coefficient than shell and tube exchangers because of high surface area.
The benefits or advantage of these exchangers are –
- It can easily handle viscous and fouling fluids.
- If fouling started, velocity in the passage increases which leads to low fouling rate as compare to in Shell and Tube heat exchangers.( as you know fouling rate decreases with increasing velocity).
- Easy to service and maintain because of single passage.
The gasketed plate or Plate-and-frame are the heat exchangers which consist of bundle of thin rectangular metal plates sealed around the edges through gaskets and are held in a frame. The plates pack is assembled between the pressure plate and frame plate. Typically materials like stainless steel, titanium, and non-metallic are used. The design of gasketed plate allows easy cleaning and maintenance.
In this, the plates are sealed with gaskets which seals the channel and allows the fluids to flow alternate channels. The figure shows that one of the fluid or hot fluid enters through the upper channel or connections then goes down and move out in counter-current direction. And in a similar fashion another fluid or cold fluid enters through the lower connections then move upwards and move out in counter direction. As the fluids passes through the channel, heat from hot to cold fluid passes efficiently.
3. Extended Surface Heat Exchangers
The tubular and plate heat exchangers are considered as surface heat exchangers, except shell and tube with low finned tubing. These exchangers effectiveness is nearly equal to 60% or below, and the heat transfer surface area density is typically less than 700 m2/m3. In some applications, the effectiveness requirement is more than 98% with volume and mass parameters are limited. So, because of these constraints more compact surface area is required. But on the contrary, in a heat exchanger with gases or some liquids, the heat transfer coefficient is low which results in large heat transfer surface area requirement. So, to increase both the surface area and compactness, extended surface area ( Fins) with high density is added on either one or both the side of the fluids according to the design requirement. Additionally, the fins can increase the surface area by 5 to 10 times of the primary surface area in a general.
Tube-Fin and Plate-fin are the two most common geometries types of Extended surface heat exchangers.
1. Plate-Fin Heat Exchangers
These types of exchangers have corrugated fins, typically triangular and rectangular cross section, or have spacers sandwich between 2 parallel plates. Sometimes the fins are fitted in flat tubes with rounded corners which eliminates the need of the sidebar.
The flat tubes or plat, which separate the two fluids and the fins formed the individual passage flow.
Fins are die or roll-formed which are joint to the plates through soldering, brazing, welding, adhesives, mechanical fit or extrusion.
In gas-to-gas heat exchanger fins may be applied on both sides, but in gas-to-liquid, fins are fitted on gas side, if applied on liquid side its purpose would be different generally for structural strength and for mixing different flows.
Corrugated fins geometries for plate fin exchangers
- Plain rectangular fins
- Plain triangular fins
- Wavy fins
- Offset strip fins
- Perforated fins
- Multilouver fins
- In automobile industries
- In electric power plants( Gas turbine, Fuel cells)
- Heat pump, refrigeration, AC
- Gas Liquefaction
2. Tube-Fin Heat Exchangers
Tube-Fin heat exchanger is classified into conventional and specialized tube-Fin heat exchanger. In conventional tube-fins the heat transfer between fluids take place by conduction through the walls. However, in specialized tube-fin or Heat pipe exchanger, the ends of the tube are closed which acts as a separating wall for the fluids and also responsible for heat transfer medium by conduction, evaporation and condensation of the heat pipe fluid.
Let first understand conventional tube-fin heat exchanger
In gas-to-gas fluid heat exchanger, the heat transfer coefficient is a one order of magnitude is higher on the liquid side as compared to the gas side. So, to make balanced thermal conductance, fins are incorporated on the gas side which increases the surface area for heat transfer.
In tube fin heat exchanger, round, rectangular, or sometimes helical tubes are used. Typically, fins are used on the outside of the tube, but also sometimes inside according to the applications. Tube fins exchangers can withstand ultra-high magnitude of pressure on the tube side. These are generally less compact relative to the plate-fin heat exchangers
- These are generally employed where one fluid is at high pressure or has a high heat transfer coefficient than the other fluid. Hence, as a result of this, these are extensively used as condensers and evaporators in Air conditioning.
- As condenser in power plants.
- As oil coolers in propulsive power plants.
Now let understand Heat-Pipe-Exchangers
These are similar to tube-fin-exchangers with having individual fin tubes. The tube is the heat pipe, and continuous movement of hot and cold fluids takes place in separate parts of the exchanger.
Let’s understand the operation in 2 steps-
- Heat is transferred from the hot gas to the evaporation segment of the heat pipe by convection.
- Then thermal energy is carried away by the vapors in the condensation segment where it transfer the heat to the cold fluid by convection.
As shown in the figure, the heat pipe is the closed vessel or tube which is partially filled with the heat transfer fluid( as you can see liquid in wick) and closed permanently at both the ends.