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A heat exchanger is a device used to transfer heat between two or more fluids. The fluids can be single or two phase and, depending on the exchanger type, may be separated or in direct contact. Devices involving energy sources such as nuclear fuel pins or fired heaters are not normally regarded as heat exchangers although many of the principles involved in their design are the same.
In order to discuss heat exchangers it is necessary to provide some form of categorization. There are two approaches that are normally taken. The first considers the flow configuration within the heat exchanger, while the second is based on the classification of equipment type primarily by construction. Both are considered here.
This type of flow arrangement allows the largest change in temperature of both fluids and is therefore most efficient where efficiency is the amount of actual heat transferred compared with the theoretical maximum amount of heat that can be transferred.
Crossflow heat exchangers are intermediate in efficiency between countercurrent flow and parallel flow exchangers. In industrial heat exchangers, hybrids of the above flow types are often found. The first level of classification is to divide heat exchanger types into recuperative or regenerative.
A Recuperative Heat Exchanger has separate flow paths for each fluid and fluids flow simultaneously through the exchanger exchanging heat across the wall separating the flow paths. A Regenerative Heat Exchanger has a single flow path, which the hot and cold fluids alternately pass through. In a regenerative heat exchanger, the flow path normally consists of a matrix, which is heated when the hot fluid passes through it this is known as the "hot blow". This heat is then released to the cold fluid when this flows through the matrix the "cold blow".
A good overview of regenerators is provided by Walker The two main types of regenerator are Static and Dynamic. Both types of regenerator are transient in operation and unless great care is taken in their design there is normally cross contamination of the hot and cold streams. However, the use of regenerators is likely to increase in the future as attempts are made to improve energy efficiency and recover more low grade heat.
However, because regenerative heat exchangers tend to be used for specialist applications recuperative heat exchangers are more common. There are many types of recuperative exchangers, which can broadly be grouped into indirect contact, direct contact and specials. Indirect contact heat exchangers keep the fluids exchanging heat separate by the use of tubes or plates etc..
Direct contact exchangers do not separate the fluids exchanging heat and in fact rely on the fluids being in close contact. In this type, the steams are separated by a wall, usually metal. Tubular heat exchangers are very popular due to the flexibility the designer has to allow for a wide range of pressures and temperatures. Tubular heat exchangers can be subdivided into a number of categories, of which the shell and tube exchanger is the most common.
A Shell and Tube Exchanger consists of a number of tubes mounted inside a cylindrical shell. Two fluids can exchange heat, one fluid flows over the outside of the tubes while the second fluid flows through the tubes. The shell and tube exchanger consists of four major parts:. Rear end—this is where the tubeside fluid leaves the exchanger or where it is returned to the front header in exchangers with multiple tubeside passes.
Tube bundle—this comprises of the tubes, tube sheets, baffles and tie rods etc. The popularity of shell and tube exchangers has resulted in a standard being developed for their designation and use. In general shell and tube exchangers are made of metal but for specialist applications e. It is also normal for the tubes to be straight but in some cryogenic applications helical or Hampson coils are used.
A simple form of the shell and tube exchanger is the Double Pipe Exchanger. This exchanger consists of a one or more tubes contained within a larger pipe. In its most complex form there is little difference between a multi tube double pipe and a shell and tube exchanger. However, double pipe exchangers tend to be modular in construction and so several units can be bolted together to achieve the required duty.
The book by E. Saunders [Saunders ] provides a good overview of tubular exchangers. Furnaces —the process fluid passes through the furnace in straight or helically wound tubes and the heating is either by burners or electric heaters. Tubes in plate—these are mainly found in heat recovery and air conditioning applications.
The tubes are normally mounted in some form of duct and the plates act as supports and provide extra surface area in the form of fins. Electrically heated—in this case the fluid normally flows over the outside of electrically heated tubes, see Joule Heating. Air Cooled Heat Exchangers consist of bundle of tubes, a fan system and supporting structure. The tubes can have various type of fins in order to provide additional surface area on the air side.
Air is either sucked up through the tubes by a fan mounted above the bundle induced draught or blown through the tubes by a fan mounted under the bundle forced draught.
They tend to be used in locations where there are problems in obtaining an adequate supply of cooling water. A heat pipe consists of a pipe, a wick material and a working fluid. The working fluid absorbs heat, evaporates and passes to the other end of the heat pipe were it condenses and releases heat.
The fluid then returns by capillary action to the hot end of the heat pipe to re-evaporate. Agitated vessels are mainly used to heat viscous fluids. They consist of a vessel with tubes on the inside and an agitator such as a propeller or a helical ribbon impeller. The tubes carry the hot fluid and the agitator is introduced to ensure uniform heating of the cold fluid. Carbon block exchangers are normally used when corrosive fluids need to be heated or cooled.
They consist of solid blocks of carbon which have holes drilled in them for the fluids to pass through. The blocks are then bolted together with headers to form the heat exchanger.
Plate heat exchangers separate the fluids exchanging heat by the means of plates. These normally have enhanced surfaces such as fins or embossing and are either bolted together, brazed or welded. Plate heat exchangers are mainly found in the cryogenic and food processing industries.
However, because of their high surface area to volume ratio, low inventory of fluids and their ability to handle more than two steams, they are also starting to be used in the chemical industry. Plate and Frame Heat Exchangers consist of two rectangular end members which hold together a number of embossed rectangular plates with holes on the corner for the fluids to pass through.
This type of exchanger is widely used in the food industry because it can easily be taken apart to clean. If leakage to the environment is a concern it is possible to weld two plate together to ensure that the fluid flowing between the welded plates can not leak.
However, as there are still some gaskets present it is still possible for leakage to occur. Brazed plate heat exchangers avoid the possibility of leakage by brazing all the plates together and then welding on the inlet and outlet ports.
Plate Fin Exchangers consist of fins or spacers sandwiched between parallel plates. The fins can be arranged so as to allow any combination of crossflow or parallel flow between adjacent plates. It is also possible to pass up to 12 fluid streams through a single exchanger by careful arrangement of headers. They are normally made of aluminum or stainless steel and brazed together.
Their main use is in gas liquefaction due to their ability to operate with close temperature approaches. Lamella heat exchangers are similar in some respects to a shell and tube. Rectangular tubes with rounded corners are stacked close together to form a bundle, which is placed inside a shell. One fluid passes through the tubes while the fluid flows in parallel through the gaps between the tubes. They tend to be used in the pulp and paper industry where larger flow passages are required.
Spiral plate exchangers are formed by winding two flat parallel plates together to form a coil. The ends are then sealed with gaskets or are welded. They are mainly used with viscous, heavily fouling fluids or fluids containing particles or fibres.
This category of heat exchanger does not use a heat transfer surface, because of this, it is often cheaper than indirect heat exchangers.
However, to use a direct contact heat exchanger with two fluids they must be immiscible or if a single fluid is to be used it must undergo a phase change.
See Direct Contact Heat Transfer. The most easily recognizable form of direct contact heat exchanger is the natural draught Cooling Tower found at many power stations.
These units comprise of a large approximately cylindrical shell usually over m in height and packing at the bottom to increase surface area. The water to be cooled is sprayed onto the packing from above while air flows in through the bottom of the packing and up through the tower by natural buoyancy. The main problem with this and other types of direct contact cooling tower is the continuous need to make up the cooling water supply due to evaporation.
Direct contact condensers are sometimes used instead of tubular condensers because of their low capital and maintenance costs. There are many variations of direct contact condenser. In its simplest form a coolant is sprayed from the top of a vessel over vapor entering at the side of the vessel.
The condensate and coolant are then collected at the bottom. The high surface area achieved by the spray ensures they are quite efficient heat exchangers. Steam injection is used for heating fluids in tanks or in pipelines. The steam promotes heat transfer by the turbulence created by injection and transfers heat by condensing. Normally no attempt is made to collect the condensate.
Direct heating is mainly used in dryers where a wet solid is dried by passing it through a hot air stream. Another form of direct heating is Submerged Combustion. This was developed mainly for the concentration and crystallization of corrosive solutions.
The fluid is evaporated by the flame and exhaust gases being aimed down into the fluid which is held in some form of tank. The wet surface air cooler is similar in some respects to an air cooled heat exchanger. However, in this type of unit water is sprayed over the tubes and a fan sucks air and the water down over the tube bundle.
We think you have liked this presentation. If you wish to download it, please recommend it to your friends in any social system. Share buttons are a little bit lower. Thank you! Published by Trevor Willet Modified over 5 years ago. The set of tubes is called a tube bundle, and may be composed by several types of tubes: plain, longitudinally finned, etc.
Download Edward A. Find manufacturers of Heat Exchangers, Heat Exchangers, Construction Material in the process heating equipment industry that includes everything from drying and curing ovens to cooling and heat dissipation equipment. The thermal design. Heat exchanger: selection, design and construction by e. Design, construction, and performance of plastic.
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Shell and Tube Heat Exchangers are one of the most popular types of exchanger due to the flexibility the designer has to allow for a wide range of pressures and temperatures. There are two main categories of Shell and Tube exchanger :. Regardless of the type of industry the exchanger is to be used in there are a number of common features see Condensers.
A heat exchanger is a device used to transfer heat between two or more fluids. The fluids can be single or two phase and, depending on the exchanger type, may be separated or in direct contact. Devices involving energy sources such as nuclear fuel pins or fired heaters are not normally regarded as heat exchangers although many of the principles involved in their design are the same. In order to discuss heat exchangers it is necessary to provide some form of categorization. There are two approaches that are normally taken.
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