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The Theory Behind Heat Transfer

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The Theory Behind Heat Transfer The theory behind heat transfer Plate heat exchangers Inside view Heat transfer theory 2 Heat transfer theory The natural laws of physics always Heat transfer theory Heat exchangers allow the driving energy in a system to Heat can be transferred by three Heat transfer theory flow until equilibrium is reached. Heat methods. Heat exchanger types leaves the warmer body or the hottest fluid, as long as there is a temperature • Radiation – Energy is transferred by 4 Calculation method difference, and will be transferred to the electromag netic radiation. One example Temperature program cold medium. is the heating of the earth by the sun. Heat load Logarithmic mean temperature A heat exchanger follows this principle • Conduction – Energy is transferred difference in its endeavour to reach equalisation. between solids or stationary fluids by Thermal length With a plate type heat exchanger, the the movement of atoms or molecules. Density heat penetrates the surface, which Cooling separates the hot medium from the • Convection – Energy is trans- Flow rate cold one very easily. It is therefore ferred by mixing part of a medium with Pressure drop possible to heat or cool fluids or gases another part. Specific heat which have minimal energy levels. Viscosity The theory of heat transfer from one a) Natural convection, where the move- Overall heat transfer coefficient media to another, or from one fluid to ment of the media depends entirely Calculation method another, is determined by several basic upon density difference, and tempera- Construction materials rules. ture differences are evened out. Pressure and temperature limitations • Heat will always be transferred from b) Forced convection, where the move- Fouling and fouling factors a hot medium to a cold medium. ment of the media depends entirely or partly upon the results of an outside 8 Product range • There must always be a temperature influence. One example of this is a difference between the media. pump causing movement in a fluid. 9 Applications Heat exchanger selection • The heat lost by the hot medium is water/water equal to the amount of heat gained by Heat exchanger selection the cold medium, except for losses to water/oil the surroundings. Heat exchanger selection water/glycol Heat exchangers A heat exchanger is a piece of equip- 10 Plate heat exchanger ment that continually transfers heat construction from one medium to another. Plate heat exchanger components Brazed plate heat exchangers There are two main types of heat Fusion-bonded plate heat exchangers. exchangers • Direct heat exchanger, where both 11 Assembly media are in direct contact with each other. It is taken for granted that the 11 Installation media are not mixed together. An example of this type of heat exchanger is a cooling tower, where water is cooled through direct contact with air. • Indirect heat exchanger, where both media are separated by a wall through which heat is transferred. Radiation 2 Alfa Laval heat exchangers Heat exchanger types • Thin material for the heat transfer • Flexibility − the plate heat exchanger In this brochure only indirect heat surface − this gives optimum heat consists of a framework containing exchangers are discussed, i.e. those transfer, since the heat only has to several heat transfer plates. It can easily where the media are not mixed, but penetrate thin material. be extended to increase capacity. where the heat is transferred through Furthermore, it is easy to open for the heat transfer surfaces. • High turbulence in the medium − purpose of cleaning. (This only applies this gives a higher convection, which to gasketed heat exchangers, and not Temperature losses through radiation results in efficient heat transfer between to brazed or fusion bonded units.) can be disregarded when considering the media. The consequence of this heat exchangers in this brochure. higher heat transfer coefficient per unit • Variable thermal length − most of the Indirect heat exchangers are available area is not only a smaller surface area plate heat exchangers manufactured by in several main types (plate, shell-and- requirement but also a more efficient Alfa Laval are available with two different tube, spiral etc.) In most cases the operation. pressing patterns. When the plate has plate type is the most efficient heat a narrow pattern, the pressure drop is exchanger. Generally it offers the best The high turbulence also gives a higher and the heat exchanger is more solution to thermal problems, giving the self-cleaning effect. Therefore, when effective. This type of heat exchanger widest pressure and temperature limits compared to the traditional shell-and- has a long thermal channel. within the constraint of current equip- tube heat exchanger, the fouling of the ment. The most notable advantages of heat transfer surfaces is considerably When the plate has a wide pattern, a plate heat exchanger are: reduced. This means that the plate heat the pressure drop is smaller and the exchanger can remain in service far heat transfer coefficient is accordingly • Takes up much less space than longer between cleaning intervals. somewhat smaller. This type of heat a traditional shell-and-tube heat exchanger has a short thermal channel. exchanger. When two plates of different pressing patterns are placed next to each other, the result is a compromise between long and short channels as well as between pressure drop and effective- ness. Convection Conduction Alfa Laval heat exchangers 3 Calculation method To solve a thermal problem, we must Temperature program Heat load know several parameters. Further data This means the inlet and outlet Disregarding heat losses to the can then be determined. The six most temperatures of both media in the heat atmosphere, which are negligible, the important parameters are as follows: exchanger. heat lost (heat load) by one side of a plate heat exchanger is equal to the • The amount of heat to be transferred T1 = Inlet temperature – hot side heat gained by the other. The heat load (heat load). T2 = Outlet temperature – hot side (P) is expressed in kW or kcal/h. T3 = Inlet temperature – cold side • The inlet and outlet temperatures on T4 = Outlet temperature – cold side Logarithmic mean the primary and secondary sides. temperature difference Logarithmic mean temperature difference • The maximum allowable pressure drop The temperature program is shown in (LMTD) is the effective driving force in the on the primary and secondary sides. the diagram below. heat exchanger. See diagram to the left. • The maximum operating temperature. Temperature Temperature Thermal length • The maximum operating pressure. Thermal length (Θ) is the relationship between temperature difference δt on • The flowrate on the primary and T1 one side and LMTD. secondary sides. ∆T1 δt Θ = LMTD If the flow rate, specific heat and T4 T2 temperature difference on one side are Thermal length describes how difficult a known, the heat load can be calculated. ∆T2 duty is from a thermal perspective. See also page 6. T1 - T2 LMTD = ∆ ∆ T3 In ∆T1 Density ∆T2 Density (ρ) is the mass per unit volume and is expressed in kg/m3 or kg/dm3. 4 Alfa Laval heat exchangers Cooling Flow rate Specific heat For some duties, cooling applications This can be expressed in two different Specific heat (cp) is the amount of for example, the temperature program terms, either by weight or by volume. energy required to raise 1 kg of a sub- is very tight with close approaches on The units of flow by weight are in kg/s stance by one degree centigrade. The the different temperatures. This gives or kg/h, the units of flow by volume specific heat of water at 20°C is 4.182 what we refer to as high theta duties in m3/h or l/min. To convert units of kJ/kg °C or 1.0 kcal/kg °C. and requires high theta units. High theta volume into units of weight, it is neces- duties are duties that have Θ > 1 and sary to multiply the volume flow by the Viscosity are characterized by: density. Viscosity is a measure of the ease of flow of a liquid. The lower the viscosity, • Long plate, longer time for the fluid The maximum flow rate usually deter- the more easily it flows. to be cooled mines which type of heat exchanger is the appropriate one for a specific pur- Viscosity is expressed in centiPoise (cP) • Low pressing depth that gives less pose. Alfa Laval plate heat exchangers or centiStoke (cSt). fluid per plate to be cooled can be used for flow rates from 0.05 kg/s to 1,400 kg/s. In terms of volume, Overall heat transfer coefficient Plate heat exchangers are superior this equates to 0.18 m3/h to 5,000 Overall heat transfer coefficient (k) is a compared to shell-and-tube heat m3/h in a water application. If the flow measure of the resistance to heat flow, exchangers when it comes to theta rate is in excess of this, please consult made up of the resistances caused values. Shell-and-tube heat exchangers your local Alfa Laval representative. by the plate material, amount of foul- can go up to a maximum value of theta ing, nature of the fluids and type of ~1 while plate heat exchangers goes up Pressure drop exchanger used. to theta values of 10 and more. For a Pressure drop (∆p) is in direct rela- shell-and-tube to climb over theta value tionship to the size of the plate heat Overall heat transfer coefficient is of 1 or more, several shell-and-tube exchanger. If it is possible to increase expressed as W/m2 °C or kcal/h, m2 °C. needs to be put in series. the allowable pressure drop, and inci- dentally accept higher pumping costs, Temperature Temperature then the heat exchanger will be smaller and less expensive.
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