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A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries, natural gas processing, and sewage treatment. The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air.
In the simplest of terms, the discipline of heat transfer is concerned with only two things: temperature, and the flow of heat. Temperature represents the amount of thermal energy available, whereas heat flow represents the movement of thermal energy from place to place.
On a microscopic scale, thermal energy is related to the kinetic energy of molecules. The greater a material's temperature, the greater the thermal agitation of its constituent molecules (manifested both in linear motion and vibrational modes). It is natural for regions containing greater molecular kinetic energy to pass this energy to regions with less kinetic energy.
Heat Transfer Mechanism
Regions with greater molecular kinetic energy will pass their thermal energy to regions with less molecular energy through direct molecular collisions, a process known as conduction. In metals, a significant portion of the transported thermal energy is also carried by conduction-band electrons.
When heat conducts into a static fluid it leads to a local volumetric expansion. As a result of gravity-induced pressure gradients, the expanded fluid parcel becomes buoyant and displaces, thereby transporting heat by fluid motion (i.e. convection) in addition to conduction. Such heat-induced fluid motion in initially static fluids is known as free convection.
All materials radiate thermal energy in amounts determined by their temperature, where the energy is carried by photons of light in the infrared and visible portions of the electromagnetic spectrum. When temperatures are uniform, the radiative flux between objects is in equilibrium and no net thermal energy is exchanged. The balance is upset when temperatures are not uniform, and thermal energy is transported from surfaces of higher to surfaces of lower temperature.
Simplified Heat Exchanger Principle
Heat exchangers work because heat naturally flows from higher temperature to lower temperatures. Therefore if a hot fluid and a cold fluid are separated by a heat conducting surface heat can be transferred from the hot fluid to the cold fluid.
Figure 1 Simplified Heat Exchanger
The rate of heat flow at any point (kW/m2 of transfer surface) depends on:
Heat transfer coefficient (U), itself a function of the properties of the fluids involved, fluid velocity, materials of construction, geometry and cleanliness of the exchanger
Temperature difference between hot and cold streams
Total heat transferred (Q) depends on:
Heat transfer surface area (A)
Heat transfer coefficient
Average temperature difference between the streams, strictly the log mean (DTLM)
Thus total heat transferred Q = UADTLM
But the larger the area the greater the cost of the exchanger
Therefore there is a trade-off between the amount of heat transferred and the exchanger cost
HEAT EXCHANGE CONCEPTS
Heat Exchanger Fundamentals
Heat exchange is a natural phenomenon occurring throughout our environment. It drives the weather cycles and energy exchange between ecosystems. Harnessing its utility through accurate control of heat exchange has been a focus of our industry for over a century.
Heat exchangers allow control over the dynamics of heat transfer between fluids. They are used in widespread applications, such as solar heating, pool heating, domestic water heating, radiant floor heating, food processing, marine applications, general industrial process control, and more.
Below are parametric thermodynamic equations that define the nature of heat exchange and performance of a heat exchanger for any given application. Once these thermal parameters are determined they can be used to calculate heat exchanger performance in order to select the most suitable product based on the specific application.
Theoretical Heat of a Fluid
The heat transfer principal in heat exchangers is based on a colder fluid gaining heat from a relatively hotter fluid separated by, and flowing over, a heat conductive material.
This is expressed by the following formula:
Heat exchanger equation1Â Â Â Â Â Â Â Â Â Â (eqn 1)
Q = Total heat loadÂ
m = Mass flow rate of fluid.Â
cp = Specific heat of fluid at constant pressure.
DT = Change in temperature of the fluid.
This formula provides the Theoretical Heat Yield to or from a given fluid undergoing a temperature change, DT at a mass flow rate, m with the fluid's specific heat property, cp.
Practical Heat Transfer Control
The theoretical heat yield of a fluid gives the amount of heat that needs to be transferred into or from a fluid. The practical heat transfer is a function of the physical geometry of the heat exchanger, its material composition, and the fluid condition.
The general form of the equation defining the maximum potential heat transfer through a heat exchanger is expressed by the formula:
Heat exchanger equation 2Â Â Â Â Â Â Â Â Â Â Â (eqn 2)
U = Overall heat transfer coefficient
A = Surface area
LMTD = Logarithmic mean temperature difference
The Practical Heat Transfer Control is determined by the molecular thermodynamic interactions between the fluids flowing through the heat exchanger and the geometry of the heat exchanger itself.
The overall U value is calculated by an equation specific to the geometric configuration of a Heat Exchanger. It is a function derived using dimensionless numbers such as Reynolds Number (Re), Prandlt Number (Pr), along with fluid flow parameters. The overall U value is calculated over the total surface area A of the heat exchanger, across which the fluids exchange heat.
The log mean difference of the inlet and outlet temperatures (LMTD) of the hot and cold fluids for a counter flow exchanger is expressed by the formula:
Heat exchanger equation 3Â Â Â Â Â Â Â Â Â Â (eqn 3)
Thi = Inlet temperature of hot fluid
Tco = Outlet temperature of cold fluid
Tho = Outlet temperature of hot fluid
Tci = Inlet temperature of cold fluid
Practical heat exchange value, Qp, can be compared to the theoretical, Qt, value to determine if the heat Exchanger has enough capacity to fulfill the application requirements.
Types of Exchangers
Heat exchangers come in a wide variety of types and sizes. Here are a few of the most common ones. Coil heat exchangers (Figure 1) have a long, small diameter tube placed concentrically within a larger tube, the combined tubes being wound or bent in a helix. One fluid passes through the inner tube, and the other fluid passes through the outer tube. This type of heat exchanger is robust-capable of handling high pressures and wide temperature differences. Although these exchangers tend to be inexpensive, they provide rather poor thermal performance because of a small heat-transfer area. Nevertheless, a coil heat exchanger may be the best choice for low-flow situations, because the single tube passage creates higher flow velocity and a higher Reynolds number. These exchangers are commonly used to establish a fixed temperature for a process-stream sample prior to taking measurements. These exchangers can also be used to condense high-temperature stream samples. Plate heat exchangers (Figure 3) consist of a stack of parallel thin plates that lie between heavy end plates. Each fluid stream passes alternately between adjoining plates in the stack, exchanging heat through the plates. The plates are corrugated for strength and to enhance heat transfer by directing the flow and increasing turbulence. These exchangers have high heat-transfer coefficients and area, the pressure drop is also typically low, and they often provide very high effectiveness. However, they have relatively low pressure capability. Shell-and-tube heat exchangers (Figures 2 & 5) consist of a bundle of parallel tubes that provide the heat-transfer surface separating the two fluid streams. The tube side fluid passes axially through the inside of the tubes; the shell-side fluid passes over the outside of the tubes. Baffles external and perpendicular to the tubes direct the flow across the tubes and provide tube support.
Tube sheets seal the ends of the tubes, ensuring separation of the two streams. The process fluid is usually placed inside the tubes for ease of cleaning or to take advantage of the higher pressure capability inside the tubes. The thermal performance of such an exchanger usually surpasses a coil type but is less than a plate type. Pressure capability of shell-and-tube exchangers is generally higher than a plate type but lower than a coil type.
Figure 1. Coil heat exchangers are capable of handling high pressures and wide temperature differences.C:\Users\admin\Desktop\figure 1.jpg
Figure 2. Thermal performance of shell and- tube exchangers is high.figure 2.png
Figure 3. Plate heat exchangers have high heat-transfer coefficients and area.figure 3.png
Figure 4. Stream temperatures through a heat exchanger in countercurrent flow.figure 4.png
Figure 5. Shell and- tube heat exchanger with counter-current flow.figure 5.png