Heat transfer

Heat Transfer
Objectives, applications & Heat transfer mechanisms.
Fourier’s law.
Heat transfer by conduction, convection & radiation.
Heat interchangers &
Heat exchangers.

1. Heat Transfer Yogeshwary Bhongade Assistant Professor Gondia college of Pharmacy, Gondia

2. UNIT-II 1- Heat Transfer Objectives , applications & Heat transfer mechanisms. Fourier’s law . Heat transfer by conduction, convection & radiation. Heat interchangers & H eat exchangers.

3. Introduction Heat is a form of engery and transfer means movement Heat transfer is the movement of heat energy from one substance to another. A B

4. Objectives & Application 1. Model basic heat transfer processes and identify modes 2. Calculate thermal resistances 3. Perform an energy balance to determine temperature and heat flux 4. Predict heat exchanger performance 5. Determine radiation heat transfer

5. Mechanism

6. CONDUCTION “When heat flow in a body is achieved by the transfer of the momentum of individual atoms or molecules without mixing, such process is known as conduction.” It takes place in solids and fluids whose movement is restricted. e.g. - iron rod such as metal wall of an evaporator.There is no actual movement of molecules. On receiving energy from surface of heat, molecules vibrates and pass on energy to adjacent molecule. When both end have same temperature, heat transfer is stopped. Driving force: Difference of temperature between two end

7. CONVECTION “When heat flow is achieved by actual mixing of warmer portions and cooler portions of the same material, the process is known as convection.” There is actual physical movement of molecules. It takes place in some fluids (i.e. liquids and gases). e.g. If hot and cold liquids are mixed, the heat can transfer from hot liquid to cold liquid by physical movement of molecules. This method of heat transfer is called as convection.

8. Convection is the transfer of thermal energy by the movement of molecule fom one part of the material to another. As the fluid motion increase, so dose the convective heat transfer. The presence of bulk motion of the fluid enhances the heat transfer between the solid surface and the fluid. Convection heat transfer is mode of heat transfer between a surface and fluid moving over it. It is restricted to liquids and gases as movement of group of molecule is not possible with solid.

9. It involves two type of heat transfer. 1) Nature Convection When movement of group molecule occurs due to change in density resulting from temperature gradient it is known as natural convection. 2) Forced Convection When a fluid is forced past a solid body and heate transferred between the fluid and body, it is called forced convection heat transfer, heat convection.'' Forced convection is involved during drying in ovens, fluidised bed drier. etc In Forced convection, fluid surrounding the surface is constantly replaced; therefore rate of heat transfer is higher than for natural convection

10. Application Dimenational analysis offers a rational approach to the estimation of complex

11. RADIATION “When heat flows through space by means of electromagnetic waves, such energy transfer is known as radiation.” E.g. Black surface absorbs most of the radiation received by it & simultaneously the absorbed energy is quantitatively transferred into heat. In short The hot body emits radiant energy in all direction. If this radiation strikes a receiver then part of it may be absorbed and part of it may be refracted. This method of heat transfer is called as radiation.

12. Fourier’s law Rate = Driving Force/ Resistance i.e. dQ/dt = -KA.dt where, dl dQ/dt =Rate of heat transfer K= constant A= surface area dt= Temperature difference dl= length

13. Heat transfer through A Series A cylinder

14. H eat exchangers Heat exchangers ae devices that are used to transfer heat from hot gas to liquid through metal wall. Heat exchange can be done by different flows,.

15. Heat exchangers Different types of heat exchangers are a) Shell and tube heater b) Multipass Heater c)Two pass floating head heater

16. Shell and tube heater It is single pass tubular heater Construction: It consist of number of parallel tube. Both the end of tube are fitted into tube sheets, The tubes are enclose into casing which is cylindrical in shape Two distribution chembers are enclosed by cover. Liquid inlet is provided to the distribution chember There are also provision for steam inlet, condensate outlet and vent for non-condensed gas.

17. Working Steam is introduced through inlet into space around tubes. Therefore tube get heated. Condense vapours and condensed gases are escap through outlet. Liquid to be heated is pump to the first distribution chember. The fluid in tubes get heated due to heat transfer by conduction through metal wal followed by Stagnant layer and finally by convection. The hot liquid enters into second distrbution chember and leaves through outlet.

18. Advantages Large heatig surface can be packed into small volume Simple Method Less time required Cross sectional area of tube are large but velocity of fluid in the tube are low, hence heat transfer coefficient is also low. Slow Process. Limited amount of liquid can use Disdvantages

19. Multipass Heater The main advantage of multipass heater is to improve velocity of fluid. Construction It consist of number of parallel tubes. The end of the tube are fitted into tube sheet. The tubes are enclosed in the casing. Baffles are placed into distribution head. There are also provision for entrance and exit of the fluid.

20. Working The feed is introduced into the compartment A of one of the head. Then it is pass through tube into compartment B of the head. Then fluid back through other set of tube to compartment C of the first head and finally leave through compartment 1. The fluid is divered by using baffles. Since heater is multipass, so same liquid has to flow through several tubes back and forth

21. Advantages The main advantage of multipass heater is to improve velocity of fluid. Constuction is complicated. cost of pumping increases due to multiple number of entries and exist. Friction loss Disdvantages

22. Two pass floating heater

23. Construction It consist of bundle of parallel tubes which are enclosed in casing. There are two distribution chembers and one of the distribution chember is partitioned. The fluid inlet and outlet are also attached to the same chember Other distribution chember is like floating head. the end of tube is embeded into floating head. There are also provision for steam inlet and outlet for noncondensed vapours and condensate.

24. Working The steam is pass through the inlet as a result tube get heated. The non- condensed gases and condensed vapours pass throug outlet. The fluid to be heated is introduced into one side of partition of distribution chember. From there fluid reaches to floating head. The direction of fluid is changed and then fluid entered into second side of partitif distribution chember. Due to head tube, The hot liquid leaves through oulet provided.

25. Advantages The floating head arrangement is advantageous because the tube sheet is structurally independent of the shell.

26. Heat interchangers These divises are used to transfer heat fron one liquid to other liquid or gas to other gas via metal wall

27. Double pipe heat interchanger The double-pipe heat exchanger is one of the simplest types of heat exchangers. It is called a double-pipe exchanger because one fluid flows inside a pipe and the other fluid flows between that pipe and another pipe that surrounds the first. This is a concentric tube construction.

28. Construction This is usually consists of concentric pipes. One fluid flow in the inner pipe and the other fluid flow in the annulus between pipes. The two fluid may flow concurrent (parallel) or in counter current flow configuration; hence the heat exchanger are classified as: counter current double pipe heat exchanger cocurrent double pipe heat exchanger

29. Advantages Its excellent capacity for thermal expansion It is easy to install and clean Its modular design makes it easy to add new sections Replacement parts are inexpensive and always in supply Disadvantages It is not as cost effective as most shell and tube exchangers it requires special gaskets

30. THANK YOU