Heat transfer coefficient is the most important factor to design the heat exchanger. Metals and metal alloys have the highest coefficient in the World.  So the material of the thermal system components such as tubes and plates, are chosen from that group. AISI 316, AISI 304, Titanium are the most popular materials in the exchanger industry.  In mostly heat exchangers, there are no external heat and mechanical interactions. Typical system works as a heating or cooling of a fluid that presented to the system and evaporation or condensation of single- or multi component fluid streams. In other usage of the exchangers are to recover heat, sterilize, concentrate or control presented fluid.

1. Classification of heat exchangers:

Heat exchangers transfer heat by using a thermal energy (enthalpy) from one fluid to another in bound of another fluid proper to the thermal systems laws. Typical applications of the heat exchangers involve heating or cooling applications such as concern and evaporation or condensation of single- or multicomponent fluid in streams.      

Commonly used heat exchangers contains heat transfer between fluids takes place through a separation, a wall or into and out of a wall surfaces temporary .Most of the heat exchangers separate the heat transfer liquids to have more control over the heat transition process between the hot & cold fluids.  The fluids are separated by a plate of a fin or a tube bundle exchangers further can be categorized in floating-head and U-tube exchanger.

      The most used of the heat transfer is the designing of a form of the heat exchanger to transfer/exchange heat from one fluid to another fluid by using universal Thermodynamic Laws. Such devices for efficient exchange of heat are generally called Heat Exchanger. Transferring the heat is the main objective of the design so the classification of the Heat exchanger is done by the process occurred inside them.  Classification of heat exchangers’ generalization can be seen in the Figure 1. The most commonly used heat exchangers are Shell and Tube exchangers.  The shell and tube exchanger’s models are:

 

1.1. Fixed tube-sheet exchanger: This model is welded to the fins to increase efficiency and lower the price. The tube bundles cannot be removed.

1.2. Removable tube bundle: To increase the life cycle of the product or eliminate the position factors of the heat transfer liquids, the tube bundles can be easily replaced or can be taken to be cleaned and replace it again for purposes.   Tube bundle may be removed for ease of cleaning and replacement. Removable tube sheet and the tube bundle can be removed for cleaning or inspection.

1.3. Floating-head exchanger: The exchanger that uses clams to fix the tube sheets to the frame of the heat exchanger. The other side of the bundle, the tubes may expand to the direction of the exchanger and freely float in tube sheet. A floating head fixed with bolts to the surface. So they do not mix or leak. Such exchangers are using the direct transfer of enthalpy, the type of the thermal energy storage and release from entire exchanger surface or matrix— are referred to as indirect transfer type such exchangers usually have fluid leakage from one fluid stream from one to another, due to pressure difference and matrix shape causes pressure drop in the exchanger. Examples of heat exchangers are shell-and tube exchangers, automobiles, boilers, radiators, condensers, evaporators, and cooling towers.

1.4. Plate Heat Exchanger: Plate heat exchangers consist of thin plates joined together, with a small amount of space between each plate, typically maintained by a small rubber gasket. The surface area is large, and the corners of each rectangular plate feature an opening through which fluid can flow between plates, extracting heat from the plates as it flows. The fluid channels themselves alternate hot and cold fluids, meaning that heat exchangers can effectively cool as well as heat fluid—they are often used in refrigeration applications. Because plate heat exchangers have such a large surface area, they are often more effective than shell and tube heat exchangers.

1.5. Regenerative Heat Exchanger: In a regenerative heat exchanger, the same fluid is passed along both sides of the exchanger, which can be either a plate heat exchanger or a shell and tube heat exchanger. Because the fluid can get very hot, the exiting fluid is used to warm the incoming fluid, maintaining a near constant temperature. A large amount of energy is saved in a regenerative heat exchanger because the process is cyclical, with almost all relative heat being transferred from the exiting fluid to the incoming fluid. To maintain a constant temperature, only a little extra energy is need to raise and lower the overall fluid temperature.

1.6. Adiabatic Wheel Heat Exchanger: In this type of heat exchanger, an intermediate fluid is used to store heat, which is then transferred to the opposite side of the exchanger unit. An adiabatic wheel consists of a large wheel with threads that rotate through the fluids—both hot and cold—to extract or transfer heat. 

 

Classification According to Functions Diagrams:

 

   

A heat exchanger tied to heat transfer factors such as a core or matrix containing the heat transfer surface or area, and fluid distribution elements such as headers, manifolds, tanks, inlet and outlet nozzles or pipes, or seals. The heat exchanger area is the most important between them; the surface that separates the fluids is the surface between the fluids so the interaction of the heat is limited by these surfaces. The heat conduction achieve with these surface areas. The surface design can increase or decrease the surface of the heat exchange surface more the area is more heat transfer. The fluid distribution elements must be isolated to reduce the heat loss of the system. Headers, nozzles etc. Must be tightened to proper tightness to prevent leakage or the fluids mix. For the safety of the thermal system there must be no estimated leakage or pressure drop in this case. The heat exchanger manuals contain the knowledge you to use, clean or repair the exchangers properly.  Generally there are no moving parts in a heat exchanger; except the rotary regenerative exchanger, they use mechanical driven rotary motor to rotate the main matrix of the heat exchangers in calculated speed to increase surface interaction.

      Even the small part of the surface of the thermal area contacts with the hot or cold fluid can be enough to exchange heat from one to other; this is called primary or direct surface thermal conductivity.  To increase the heat transfer area, some extra components can be placed in the exchanger to make bulges, the bulges are normal (900 angle) to the surface for higher efficiency. These extensions called fins.

 The heat transfer of the fins includes thermal heat conduction and convection from a surface and radiation to the fluids. As a result, the addition of fins to the primary surface reduces the thermal resistance on that side and there by increases the total heat transfer from the surface for the same temperature difference... These secondary surfaces or fins may also use.

 To decrease structural weak points and increase matrix strength allows the user use highly viscous liquids. The space between fins, fin pairing, has important effect to the temperature field inner side of the heat-exchanger. With higher spacing temperature rises inside heat-exchanger and theoretical cooling output is decreasing.

Fin figure: Presents the effectiveness of the angler fins.

      On the other side initial pressure decreases with higher spacing to so the surface resistance’s effect on fluid flow lowers. The heat exchanger design can be altered to fit the user’s needs to achieve the goal because of these small design changes allow us to use the heat exchanger in a large scale of sectors such as process, power, petroleum, transportation, air-conditioning, refrigeration, cryogenic, heat recovery, alternative fuel, and manufacturing industries, they also serve as key components of many industrial products available in the marketplace.

       These exchangers are classified in many different forms. We will classify them according to transfer processes, number of fluids, heat transfer mechanisms, and construction type and flow arrangements. Another arbitrary classification can be made, according to the heat transfer surface area/volume ratio, into compact and non-compact heat exchangers. This classification is made because the type of components, sectors of applications, and design parameter is changed. All these classifications are presented in the Diagrams above. Other ways to classify heat exchangers are by fluid type (gas–gas, gas–liquid, liquid–liquid, etc.), industry.

2. Transfer Process:

The transfer process’ principle is to classify the heat exchangers as direct contact type and indirect contact type. In direct contact type, heat is transferred between cold and hot fluids by the direct contact of the fluids such as cooling towers, spray and tray condensers.  In indirect heat exchanger, heat energy is transferred by the heat transfer surface

2.1. Gasketed heat Exchangers:

Gasketed plate heat exchangers contain the plates and frame. They are mainly for the food industries because of their stainless properties and cleaning factors. Their efficiency reached to maximum when active plate geometries used to increase thermal surface. The variety of the sector of applications has widened exponentially. They are capable of satisfying even the most extreme duties. Therefore they can be used as an alternative to shell and tube type heat exchangers for low and medium pressure liquid to liquid heat transfer applications.

Design of plate heat exchangers is mostly specialized in nature of the thermal conductions principles Considering the possibility of the designs; plates variety, quality of the plates (AISI 304,AISI 316,Titanium), number of the plates, plates thermal conduction area, etc. Unlike tubular heat exchangers for which have designs data and calculations are easy to access.

Figure of the gasketed heat exchanger

 

2.2. Advantages and disadvantages of the gasketed heat exchanger design:

Allow you to use of contra-flow design parameters, reduces heat transfer area.

The plates variable versions of surface geometry which allows flexibility in the thermal efficiency.

They can’t satisfy the required large volume flows related with low pressure vapors and gasses.

The liquid flows in thin streams between the plates. Pressed with matrix, plates produce  high turbulence stream, this combined with large heat transfer areas

Only the plate edges are exposed to the environment so heat loss is so little that no insulation is required.

Plate heat exchangers can’t deal high pressures due to the requirement for plate gaskets.

Thermal length depends on the design parameter of the plates’ surface area and the stream distance traveled by the flow. The low thermal length has smaller are of thermal length so the area of the stream’s flow is small.  The high thermal length has larger area to interact with the fluid so the stream causes more heat exchange.

Fıgure above presents the counter stream ways between the plates (left) and thermal surface area design matrix (right).

Figures above present the low thermal length of the matrix (top) and high thermal length of the matrix (bottom).

2.3. The pressure drop analysis:

Heat exchanger works under certain steady-state conditions.

Heat losses to the surroundings are negligible, because of the value is very small.

There is no need for a thermal energy sources.

Temperature of each fluid is uniform over every cross section of the counter and parallel flow.

Magnitude of the thermal resistance coefficient is unchanged in the entire heat exchanger surface.

There are no phase changes in the fluid streams when they are flowing the stream ways of the heat exchanger.

Heat conduction is stable if the system is secured and designed properly.

Individual and overall heat transfer coefficients are constant independent of time, temperature and position.

General assumption; fluid when it is pumped, is proportional to the pressure drop, which is indicates with fluid friction and other pressure drop contributions from the flow path of fluids.

Fluid pressure drop has direct relations with exchanger heat transfer factors such as operation method, size of the plates, mechanical characteristics of the frame.

Fluid pressure drop can be calculated with the heat exchanger, sum of the pressure drop across the matrix, and the distribution devices like pipes, headers, manifolds. Reduction in the connections causes the loss.

2.4.Pressure drop:

Flow is steady and isothermal and fluid properties are not relative to the time.

Fluid local temperature only affects the characteristic properties of fluid such as density.

The pressure points of the fluid are not relative to the direction.

Free-body forces are only the gravity (magnetic, electric etc.).

The Bernoulli equation shall be valid only for stream line flows.

Friction of the surface (roughness of the surface) is considered as constant along the length of flow.

 

Written by Çağatay KORZAY

3. References:

1. Kern, D.Q., Process Heat Transfer, McGraw-Hill, New York, 1950.

2. Tagore, J., Evolution of heat exchanger design techniques, Heat Transfer Eng. 1, No. 1, 15-29 (1979).

3. Perry, R.H., Green, D.W., Eds. Perry’s Chemical Engineers’ Handbook, 7th Edition, McGraw-Hill, New York, 1997.

4. Phadke, P.S., Determining tube counts for shell-and-tube exchangers, Chemical Engineering, September 3, 1984, pp. 65-68.