Introduction
Compact heat exchanger can be characterized by its high ‘area density’ this means that it has a high ratio of heat transfer surface to heat exchanger volume. So Compact heat exchange is characterized by high heat transfer surface-area to volume ratios and high heat transfer coefficients compared to other exchanger types. A heat exchanger having a surface area density greater than about 700 m2/m3 for a gas is quite arbitrarily referred to as a compact heat exchanger.
The
heat transfer surface area is increased by fins to increase the surface area per unit
volume and there are many variations available
Compact heat exchangers are not a new technology; there is a continuous need to
produce innovative designs to suit market requirements. Such designs are more
efficient in terms of heat transfer although fouling and pressure are important
design considerations that make compact heat exchangers not suitable for all
applications.
Working
The essential principle of a heat exchanger is that it transfers the heat without transferring the fluid that carries the heat. Heat is exchanged by the fluids: the hot fluid cools down and the cold fluid warms up, without them actually coming into contact and mixing. The threshold area/volume densities of a range of generic heat exchanger types that could all be classed as ‘compact’ are listed below:
Compact Heat Exchanger Types
There are many types available from suppliers, below is a short list:
● Plate and Frame Heat Exchangers
● Brazed Plate Heat Exchangers
● Welded Plate Heat Exchanger
● Plate-Fin Heat Exchangers
● Brazed Plate-Fin Heat Exchangers
● Diffusion-Bonded Plate-Fin Heat Exchangers
● Spiral Heat Exchangers
● Printed Circuit Heat Exchangers
● Plate and Shell Heat Exchangers
● Polymer Heat Exchangers
I. Plate and frame heat exchanger :
● Plates pressed from stainless steel or higher grade material
– titanium
– incoloy
– hastelloy
● Gaskets are the weak point.Gaskets are made of
– nitrile rubber
– hypalon – viton
– neoprene
Advantages of plate and frame HE
High heat transfer - turbulence on both sides
• High thermal effectiveness - 0.9 - 0.95 possible
• Low ΔT - down to 1K • Compact - compared with a S&T
• Cost - low because plates are thin
• Accessibility - can easily be opened up for inspection and cleaning
• Flexibility - Extra plates can be added
• Short retention time with low liquid inventory hence good for heat sensitive or
expensive liquids
• Less fouling - low r values often possible
Disadvantages of plate & frame HE
• Pressure - maximum value limited by the sealing of the gaskets and the
construction of the frame.
• Temperature - limited by the gasket material.
• Capacity - limited by the size of the ports
• Block easily when solids in suspension unless special wide gap plates are used
• Corrosion - Plates good but the gaskets may not be suitable for organic solvents
• Leakage - Gaskets always increase the risk
• Fire resistance - Cannot withstand prolonged fire (usually not considered for
refinery duties)
Scope of plate & frame HE :
Principal Applications :
● Gasketed plate and frame heat exchangers have a large range of applications
typically classified in terms of the nature of the streams to be heated/cooled
as follows:
❖ Liquid-liquid.
❖ Condensing duties.
❖ Evaporating duties.
● Gasketed units may be used in
❖ refrigeration
❖ heat pump plants and
❖ extensively used in the processing of food and drinks.
II. Spiral heat exchangers :
• The classic design of a spiral heat exchanger is simple
• The basic spiral element is constructed of two metal strips rolled around a central
core forming two concentric spiral channels.
• Normally these channels are alternately welded, ensuring that the hot and cold
fluids cannot intermix.
Operating Limits :
● Maximum design temperature is 400°C set by the limits of the gasket
material.
● Special designs without gaskets can operate with temperatures up to
850°C.
● Maximum design pressure is usually 15 bar, with pressures up to 30
bar attainable with special designs.
Applications :
• It is ideal for use in the food industry as well as in brewing and wine making.
• Spiral heat exchangers have many applications in the chemical industry
including TiCl4 cooling, PVC slurry duties, oleum processing and heat
recovery from many industrial effluents.
• Spiral heat exchangers also provide temperature control of sewage sludge.
III. Extended Surface Heat Exchangers :
Extended surfaces have fins attached to the primary surface on one side of a two-fluid or a multifluid heat exchanger. Pins are primarily used to increase the surface area (when the heat transfer coefficient on that fluid side is relatively low) and consequently to increase the total rate of heat transfer.
IV. Brazed Plate Heat Exchangers
Brazed plate heat exchangers have no headers, tie bars or sealing gaskets because the plates are furnace brazed at temperatures of 1100°C.During the assembly phase, a sheet of brazing material (generally copper but also nickel) is placed between the plates, the pack is pressed and subsequently baked for some hours. The BPHE exchanger is more compact, lighter and less bulky than one with gaskets. The image shows the path made by the hot and cold
fluid. The brazing material carries out the function both of the gaskets and the frame.
These exchangers are generally used with chevron corrugated plates, which are assembled alternating the corrugation directions in order to create a lattice contact. The crossing points between the corrugations of two coupled plates form a dense network of contact points that confer pressure tightness and induce swirling streams that improve heat exchange.
In this way, the turbulence of the fluids is high even at low nominal input speeds and the flow passes from laminar to turbulent for low flow rates. The figure shows a cross section of an exchanger with 8 plates in total (6 of which are useful for heat exchange) in which the 3 channels used for the passage of the refrigerant fluid (in light blue) and the 4 for water (in red) are seen.
Advantages of brazed plate heat exchangers
● Widespread use in industries
● Compact
● Low Cost
● Resistance to changes in pressure and temperature
● Low heat loss
CONCLUSION
This blog tells everything about compact heat exchangers, their types and working. The compact heat exchangers have been proven as a boon in many applications may it be any small application or any big industrial application. Compact heat exchangers are widely used for heat integrating process and provide advantages over shell-and-tube heat exchangers, such as compactness, large surface area per volume ratio, the low temperature difference etc.
References
1.GULF COAST ENGINEERED SOLUTIONS, COMPACT HEATEXCHANGERS
https://www.gces-inc.com/products/compact-heat-exchangers/
2.WEBBUSTERZ ENGINEERING - What is a compact heat exchanger?https://www.webbusterz.org/compact-heat-exchanger/
3. How does a plate heat exchanger work
https://www.alfalaval.my/products/heat-transfer/plate-heat-exchangers/gasketed-plate-and-frame-heat-exchangers/heat-exchanger/how-plate-heat-exchanger-work/
4. ONDA - PLATE HEAT EXCHANGER WORKING PRINCIPLE
https://www.onda-it.com/eng/news/how-a-plate-heat-exchanger-works/plate-heat-exchanger-working-principle
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