HEAT EXCHANGER
Heat exchangers are devices designed to transfer heat between two or more fluids i.e., liquids, vapors, or gases of different temperatures. Depending on the type of heat exchanger employed, the heat transferring process can be gas-to-gas, liquid-to-gas, or liquid-to-liquid and occur through a solid separator, which prevents mixing of the fluids, or direct fluid contact. Other design characteristics, including construction materials and components, heat transfer mechanisms, and flow configurations, also help to classify and categorize the types of heat exchangers available. Finding application across a wide range of industries, a diverse selection of these heat exchanging devices are designed and manufactured for use in both heating and cooling processes.
Heat Exchanger Thermodynamics
The design of a heat exchanger is an exercise in thermodynamics, which is the science that deals with heat energy flow, temperature, and the relationships to other forms of energy. To understand heat exchanger thermodynamics, a good starting point is to learn about the three ways in which heat can be transferred – conduction, convection, and radiation. In the sections below, a review of each of these heat transfer modes is presented.
Conduction
Conduction is the passing of thermal energy between materials that are in contact with one another. Temperature is a measure of the average kinetic energy of molecules in a material – warmer objects (that are at a higher temperature) are exhibiting more molecular motion. When a warmer object is brought in contact with a cooler object (one that is at a lower temperature), there is a thermal energy transfer between the two materials, with the cooler object becoming more energized and the warmer object becoming less energized. This process will continue until thermal equilibrium has been achieved.
The rate at which heat energy is transferred in a material by thermal conduction is given by the following expression:
In this expression, Q represents the amount of heat transferred through the material in time t, ΔT is the temperature difference between one side of the material and the other (the thermal gradient), A is the cross-sectional area of the material, and d is the thickness of the material. The constant k is known as the thermal conductivity of the material and is a function of the material’s intrinsic properties and its structure. Air and other gases generally have low thermal conductivities, while non-metallic solids exhibit higher values and metallic solids generally showing the highest values.
Convection
Convection is the transfer of thermal energy from a surface by way of the motion of a fluid such as air or water that has been heated. Most fluids expand when heated and therefore will become less dense and rise relative to other parts of the fluid that are cooler. So, when the air in a room is heated, it rises to the ceiling because it is warmer and less dense, and transfers heat energy as it collides with the cooler air in the room, then becoming denser and falling again towards the floor. This process creates a natural or free convection current. Convection can also occur through what is termed forced or assisted convection, such as when heated water is pumped through a pipe such as in a hydronic heating system.
For free convection, the rate of transfer of heat is expressed by Newton’s law of cooling:
Where Q-dot is the rate of transfer of heat, hc is the convective heat transfer coefficient, A is the surface area over which the convection process is occurring, and ΔT is the temperature differential between the surface and the fluid. The convective heat transfer coefficient hc is a function of the properties of the fluid, similar to the thermal conductivity of the material mentioned earlier regarding conduction.
Radiation
Thermal radiation is a mechanism of heat energy transfer that involves the emission of electromagnetic waves from a heated surface or object. Unlike conduction and convection, thermal radiation does not require an intermediate medium to carry the wave energy. All objects whose temperature is above absolute zero (-273.15oC) emit thermal radiation in a typically broad spectral range.
The net rate of radiation heat loss can be expressed using the Stefan-Boltzmann Law as follows:
where Q is the heat transfer per unit time, Th is the temperature of the hot object (in absolute units, oK), Tc is the temperature of the colder surroundings (also in absolute units, oK), σ is the Stefan-Boltzmann constant (whose value is 5.6703 x 10-8 W/m2K4). The term represented by ε is the emissivity coefficient of the material and can have a value anywhere between 0 to 1, depending on the characteristics of the material and its ability to reflect, absorb, or transmit radiation. It is also a function of the temperature of the material.
CLASSIFICATION OF EXCHANGER
Regenerative heat exchangers
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"). Regenerative Heat Exchangers are sometimes known as Capacitive Heat Exchangers.
Regenerators are mainly used in gas/gas heat recovery applications in power stations and other energy intensive industries. 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. Examples of these are tubular exchangers and plate exchangers,
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 fluids can be single or two phase and can flow in a parallel or a cross/counter flow arrangement. The shell and tube exchanger consists of four major parts:
- Front end–this is where the fluid enters the tubeside of the exchanger.
- 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. to hold the bundle together.
- Shell this contains the tube bundle.
The popularity of shell and tube exchangers has resulted in a standard being developed for their designation and use. This is the Tubular Exchanger Manufactures Association (TEMA) Standard. In general shell and tube exchangers are made of metal but for specialist applications (e.g., involving strong acids of pharmaceuticals) other materials such as graphite, plastic and glass may be used. 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.A.D. Saunders [Saunders (1988)] provides a good overview of tubular exchangers.
Other types of tubular exchanger include:
- 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
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.
Heat Pipes, Agitated Vessels and Graphite Block Exchangers can be regarded as tubular or could be placed under Recuperative "Specials". 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. Each of the plates is separated by a gasket which seals the plates and arranges the flow of fluids between the plates, 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.
spiral heat exchanger
The spiral plate heat exchanger is made by rolling two long metal plates around a center core to form two concentric spiral flow passages, one for each fluid. The plate edges are welded shut so that each fluid stays within its own passage and there is no flow bypassing or intermixing. Channel plate width and spacing (gap between plates) are optimized for the specified duty, maximum heat transfer, and ease of access. The plate gap is maintained by welded spacer studs although some designs do not require them.
Due to its inherent circular design and large surface area to volume ratio, the spiral heat exchanger offers unique advantages over other types of heat exchangers like the shell and tube.
The spiral's single-flow passages induce high shear rates that scrub away deposits as they form. This self-cleaning effect reduces fouling and makes spiral heat exchangers ideal for handling tough fluids such as process slurries, sludge, and media with suspended solids or fibers.
Single and long curving flow passages with a uniform rectangular cross-section ensure superior flow distribution, intense turbulence, and high heat transfer coefficients (50-100% greater than shell & tubes).
WORKING OF SPIRAL TYPE EXCHANGER WITH VIDEO ANIMATION
MAIN CRITERIA FOR HEAT EXCHANGER SIZING AND SELECTION
- Function that the heat exchanger will perform (whether condensing, boiling, etc.)
- Pressure limits (high/low), which may vary throughout the process, and pressure drops across the exchanger
- Approach temperature and temperature ranges (which may vary throughout the process)
- Fluid flow capacity
- Materials requirements. Conditions like sudden temperature changes or corrosive media may require special materials. For a gasketed plate heat exchanger, the gaskets must be compatible with the fluids in the unit.
- Thermal fluid characteristics and product mix. If the heating or cooling fluid is susceptible to fouling, a corrosion resistant material may be needed.
- Location. Some exchangers may require cooling water, steam, or hot oil, and they may be relevant options only where these utilities are available.
- Footprint. Space limitations and layout may also affect which heat exchanger models are suitable. Keep in mind that lower approach temperatures generally correlate to larger units.
- Maintenance requirements. Depending on housekeeping procedures, it may be useful to choose a design lends itself to easy cleaning. Ease of repair or inspection may be a factor as well.
Generally, more than one heat exchanger model will work for a given application, so additional criteria may help in evaluating the best fit. Consider factors like future scalability, overall cost to purchase and operate, and efficiency/carbon footprint to narrow the options.
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