Heat exchanger and its type,ntu method
- 1. 1. Introduction of heat exchanger 2. Type of heat exchanger 3. NTU (Number of Transfer Unit) Method 4. Heat exchanger design Content Prepared By GROUP 3 SECTION D 1
- 2. 1. IntroduCtIon of heat exChanger are device built for efficient heat transfer from one fluid to another. may be define as an equipment which transfer the energy from a hot fluid to a cold fluid with maximum rate and minimum investment and running cost. Example q intercoolers and preheaters q condenser and boiler in steam plant q evaporator and condenser refrigeration unit 2
- 3. 2. tYPe of heat exChanger Ø Classification of heat exchanger according nature of heat exchanger, relative direction of fluid motion, design and construction features, physical state of fluid. i. Direct contact heat exchanger ii. Indirect contact heat exchanger (A) Regenerator (B) Recuperator (i) Direct contact heat exchanger In a direct contact or open or open heat exchanger the exchanger of heat takes place by direct mixing of hot and cold fluids and transfer of heat and mass takes place simultaneously. 3
- 4. (ii) Indirect contact heat exchanger In this type of heat exchanger the heat transfer between two fluids could be carried out by transmission through wall which separates the two fluids. This type include (a) Regenerators (b) Recuperators or surface exchanger (a) Regenerator: In a regenerator type of heat exchanger the hot and cold fluid pass alternately through a space containing solid particle (matrix) these particle providing alternately a sink and a source for heat flow. Example:- I.C. engine and gas turbines, Open hearth and glass melting furnaces (b) Recuperator: is the most important type of heat exchanger in which the flowing fluid exchanging heat are on either side of dividing wall (in the form of pipes or tube generally). These heat exchangers are used when two fluids cannot be allowed to mix. Example:- Automobile radiators, Oil coolers, intercoolers, air preheaters i. Parallel flow or unidirectional flow ii. Counter flow iii. Cross flow 4
- 5. (i) Parallel flow heat exchanger In parallel-flow heat exchanger the two fluids enter the exchanger at the same end and travel in parallel to one another to the other side. Example:- Oil coolers, oil heaters, water heaters etc. (ii) Counter -flow heat exchanger In counter-flow heat exchangers the fluids enter the exchanger from opposite end. The counter current design is the most efficient. 5
- 6. (iii) Cross-flow heat exchanger In a cross-flow heat exchanger the fluids travel roughly perpendicular to one Another through the exchanger. Example:- the cooling unit of refrigeration system, automobile radiator etc. i. Concentric tube Shell and tube Multiple shell and tube passes Compact heat exchangers 6
- 7. (i) Concentric tube Two concentric tube are used each carrying one of the fluid. The direction of flow May be parallel. 7
- 8. (ii) Shell and tube In this type of heat exchanger one of the fluids flows through a bundle of tubes enclosed by a shell. The other fluid is forced through the shell and it flows over the outside surface of the tube. (iii) Multiple shell and tube passes Multiple shell and tube passes are used for enhancing the overall heat transfer. (iv) Compact heat exchanger There are special purpose heat exchanger and have a very large transfer surface area per unit volume of the exchanger. i. Condensers ii. Evaporators 8
- 9. (i) Condenser In a condenser the condensing fluid remain at constant temperature throughout the exchanger while the temperature of the colder fluid gradually increase from inlet to outlet. (ii) Evaporators In this case the boiling fluid (cold fluid) remain at constant temperature while the temperature of hot fluid gradually decrease from inlet to outlet. 9
- 10. 3. ntu (number of transfer unIt) method The heat exchanger effectiveness (ℇ) is defined as the ratio of actual heat transfer to the maximum possible heat transfer. Thus Ø By compiling non-dimensional grouping, ℇ can be expressed as function of three Ø non-dimensional parameter, this is known as NTU method. 10
- 11. 11 The heat exchanger dQ through an area dA of the heat exchanger is given by From expression we have
- 12. 12 We have expression for effectiveness
- 13. 13 v We find the effectiveness of parallel flow and counter flow heat exchanger is Given by the following expression
- 14. 4. heat exChanger desIgn 14 Apart from predicting heat transfer a host of considering arise in the design of heat exchanger two primary ones are minimizing pumping power and fixed cost. o The pumping power mathematically expressed as where:- is the mass flow rate of stream the pressure drop of the stream as it passes through the exchanger the fluid density Determining the pressure drop can be relatively straight forward in a single-pass pipe in tube heat exchanger. The pressure drop in a long straight run of pipe is given by where:- is the length of is the hydraulic diameter is the mean velocity of the flow in the pipe
- 15. 15 f is the Darcy-weisbach friction factor o Optimizing the design of an exchanger it can reduce the fixed cost of an exchanger by increasing U and reducing the required area. o Early in the process the designer should assess the cost of the calculation in comparison with (a) The converging accuracy of computation (b) The investment in the exchanger (c) The cost of miscalculation o Specification of inlet and outlet temperature