Location via proxy:   [ UP ]  
[Report a bug]   [Manage cookies]                
SlideShare a Scribd company logo

Heat Exchangers.pptx

•Download as PPTX, PDF•
•
Mustafa Dahham
Mustafa Dahham

heat exchanger is a device that transfers heat between two or more fluids. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact. Heat exchangers are widely used in a variety of applications, including: Heating and cooling systems Power plants Chemical processing Food processing Refrigeration Air conditioning

1 of 20

1

Heat Exchangers Created by Mustafa Jabbar Dahham Dep: Power Mechanics

2

1.1 Introduction Heat exchangers are devices that provide the transfer of thermal energy between two or more fluids at different temperatures. Heat exchangers are used in a wide variety of applications, such as power production, process, chemical and food industries, electronics, environmental engineering, waste heat recovery, manufacturing industry, air-conditioning, refrigeration, space applications, etc.

3

1.2CONSTRUCTION OF HEAT EXCHANGERS A heat exchanger consists of heat-exchanging elements such as a core or matrix containing the heat transfer surface, and fluid distribution elements such as headers or tanks, inlet and outlet nozzles or pipes, etc. Usually, there are no moving parts in the heat exchanger; however, there are exceptions, such as a rotary regenerator in which the matrix is driven to rotate at some design speed and a scraped surface heat exchanger in which a rotary element with scraper blades continuously rotates inside the heat transfer tube. The heat transfer surface is in direct contact with fluids through which heat is transferred by conduction. The portion of the surface that separates the fluids is referred to as the primary or direct contact surface. To increase heat transfer area, secondary surfaces known as fins may be attached to the primary surface

4

1.3 Classification of Heat Exchangers 1.3.1 Types of Heat Exchangers Double-pipe exchanger Shell and tube exchangers Plate and frame exchangers Plate-fin exchangers. Spiral heat exchangers. Air cooled heat exchangers Agitated vessels. Fired heaters.

5

1.3.2 Based on transfer process Indirect Contact – Shell & Tube Heat Exchangers Direct Contact – Cooling Towers 1.3.3 Based on phase of fluids Gas-Liquid exchangers Liquid-Liquid exchangers Gas-Gas heat exchangers 1.3 Classification of Heat Exchangers

6

1.3.4 Based on construction Tubular a. Double pipe heat exchanger b. Shell and tube heat exchangers c. Spiral heat exchangers Plate-type a. Plate and frame heat exchangers b. Spiral plate heat exchangers Extended Surface a. Plate-fin exchanger b. Tube-fin exchanger 1.3 Classification of Heat Exchangers

7

1.3 Classification of Heat Exchangers 1.3.5 Based on flow arrangements Parallel flow / Co-current flow Counter flow Cross flow

8

The most uncomplicated heat transfer device is Double-pipe heat exchanger. Which has the characteristics of two pipes overlapping as shown in the picture. Divided into Parallel Flow and Counter Flow

9

2.1 THE OVERALL HEAT TRANSFER COEFFICIENT A heat exchanger typically involves two flowing fluids separated by a solid wall. Heat is first transferred from the hot fluid to the wall by convection , through the wall by conduction, and from the wall to the cold fluid again by convection. Any radiation effects are usually included in the convection heat transfer coefficients. The thermal resistance network associated with this heat transfer process involves two convection and one conduction resistances, as shown in Figure. Here the subscripts i and o represent the inner and outer surfaces of the inner tube. For a double-pipe heat exchanger, we have 𝑨𝒊 = 𝝅𝑫𝒊𝑳 and 𝑨𝒐 = 𝝅𝑫𝒐𝑳, and the thermal resistance of the tube wall in this case is 𝑅𝑤𝑎𝑙𝑙 = ln( 𝐷𝑜 𝐷𝑖 ) 2𝜋𝑘𝐿

10

where k is the thermal conductivity of the wall material and L is the length of the tube. Then the total thermal resistance becomes: In the analysis of heat exchangers, it is convenient to combine all the thermal resistances in the path of heat flow from the hot fluid to the cold one into a single resistance R, and to express the rate of heat transfer between the two fluids as: Where :  U = Overall heat transfer coefficient  h = convection heat transfer coefficient  R = thermal resistance Total heat transfer coefficient (Overall heat transfer coefficient):

11

For the case where the pipe wall is very thick and has a very high k value, the conduction resistance is approximately zero (Rwall ≈ 0 ) Therefore, the Overall heat transfer coefficient (U) value can be written as follows: 𝟏 𝑼 ≈ 𝟏 𝒉𝒊 ≈ 𝟏 𝒉𝒐 Representative values of the overall heat transfer coefficients in heat exchangers

12

2.2 Analysis of Heat Exchangers In considering the analysis of heat transfer of heat exchanger equipment Must assume the following assumptions: The heat exchanger must be a Steady flow device. Fluid properties And the velocity of the fluid is constant. No changes in Kinetic and potential energy. Has a specific heat capacity.  No heat conduction in the direction of the length of the pipe. The outer walls of heat exchanger are insulated. No heat loss to the environment. Heat transfer occurs between only 2 types of fluids.

13

The rate of heat transfer from the hot side fluid = The rate of heat transfer into the cold side fluid. Where: the subscripts c and h stand for cold and hot fluids, respectively, and 𝑚𝑐, 𝑚ℎ = mass flow rates Cpc, Cph = specific heats Tc, out, Th, out = outlet temperatures Tc, in, Th, in = inlet temperatures

14

2.3 THE LOG MEAN TEMPERATURE DIFFERENCE METHOD Consider Newton’s Law of Cooling 𝑄 = 𝑈𝐴𝑠∆𝑇𝑚 where ∆𝑇𝑚 is the difference between average temperatures Both types of fluids To find the value ∆𝑇𝑚 is determined by the flow of Fluid that flows in the same direction in double pipe heat exchanger. will get the equation for ∆𝑇𝑚 as: ∆𝑇𝑙𝑚 = ∆𝑇1 − ∆𝑇2 ln( ∆𝑇1 ∆𝑇2 )

15

To calculate ∆𝑇𝑙𝑚, dividing the flow of fluid into 2 types which are: Both types of fluids flow in the same direction (Parallel flow) Two fluids flowing in opposite directions (Counter flow) For (Parallel flow) (Counter flow) ∆𝑇𝑙𝑚 = ∆𝑇1 − ∆𝑇2 ln( ∆𝑇1 ∆𝑇2 )

16

2.4 Multipass and Cross-Flow Heat Exchangers: Use of a Correction Factor The Calculation for ∆𝑇𝑚 using the formula mentioned above Will be used for double pipe heat exchangers that are parallel flow or counter flow only. For Cross flow or Multipass shell and tube heat exchanger, Correction factor: F is required to calculate ∆𝑇𝑚. where F will depend on The shape of the heat exchanger The temperature of the fluid at the entrance And the fluid temperature at the exit ∆𝑇𝑙𝑚 = 𝐹∆𝑇𝑙𝑚,𝐶𝐹

17

 Bellow the correction factor F charts for common shell-and-tube and cross-flow heat exchangers (from Bowman, Mueller, and Nagle, Ref. 2):

18

2.5 THE EFFECTIVENESS–NTU METHOD Heat exchanger method analysis method 2 is the NTU Effectiveness method. Which is commonly used to find the rate of heat transfer And the fluid temperature at the device exit. By knowing the various values such as the type and size of the device. Fluid flow rate And the temperature of the fluid at the entrance. NTU method is calculated using the relationship of Number of Transfer Unit (NTU) and Heat Transfer Effectiveness (𝜀) 𝜀 = 𝑄 𝑄𝑚𝑎𝑥 = 𝐴𝑐𝑡𝑢𝑎𝑙 ℎ𝑒𝑎𝑡 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 𝑟𝑎𝑡𝑒 𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝑝𝑜𝑠𝑠𝑖𝑏𝑙𝑒 ℎ𝑒𝑎𝑡 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 𝑟𝑎𝑡𝑒 Where: 𝑄 = 𝐶𝑐 𝑇𝑐,𝑜𝑢𝑡 − 𝑇𝑐,𝑖𝑛 = 𝐶ℎ 𝑇ℎ,𝑜𝑢𝑡 − 𝑇ℎ,𝑖𝑛 𝑄𝑚𝑎𝑥 = 𝐶𝑚𝑖𝑛∆𝑇𝑚𝑎𝑥= 𝐶𝑚𝑖𝑛 𝑇ℎ,𝑖𝑛 − 𝑇𝑐,𝑖𝑛 Number of Transfer Unit (NTU) 𝑁𝑇𝑈 = 𝑈𝐴𝑠 𝐶𝑚𝑖𝑛 = 𝑈𝐴𝑠 (𝑚𝑐𝑝)𝑚𝑖𝑛 Capacity Ratio (c) 𝑐 = 𝐶𝑚𝑖𝑛 𝐶𝑚𝑎𝑥

19

2.6 SELECTION OF HEAT EXCHANGERS Heat Transfer Rate Cost Pumping Power Size and Weight Type Materials Other Considerations

20

References: Heat exchangers, Selection, Rating, and Thermal Design, Third Edition, Sadik Kakaç, Hongtan Liu, Anchasa Pramuanjaroenkij. Heat Transfer, A Practical Approach, Second Edition, Yunus A. Cengel. Fundamentals of heat exchanger design, Ramesh K. Shah, Dusˇan P.Sekulic. Heat Exchanger Design Handbook, Second edition, Kuppan Thulukkanam

More Related Content

Heat Exchangers.pptx

  • 1. Heat Exchangers Created by Mustafa Jabbar Dahham Dep: Power Mechanics
  • 2. 1.1 Introduction Heat exchangers are devices that provide the transfer of thermal energy between two or more fluids at different temperatures. Heat exchangers are used in a wide variety of applications, such as power production, process, chemical and food industries, electronics, environmental engineering, waste heat recovery, manufacturing industry, air-conditioning, refrigeration, space applications, etc.
  • 3. 1.2CONSTRUCTION OF HEAT EXCHANGERS A heat exchanger consists of heat-exchanging elements such as a core or matrix containing the heat transfer surface, and fluid distribution elements such as headers or tanks, inlet and outlet nozzles or pipes, etc. Usually, there are no moving parts in the heat exchanger; however, there are exceptions, such as a rotary regenerator in which the matrix is driven to rotate at some design speed and a scraped surface heat exchanger in which a rotary element with scraper blades continuously rotates inside the heat transfer tube. The heat transfer surface is in direct contact with fluids through which heat is transferred by conduction. The portion of the surface that separates the fluids is referred to as the primary or direct contact surface. To increase heat transfer area, secondary surfaces known as fins may be attached to the primary surface
  • 4. 1.3 Classification of Heat Exchangers 1.3.1 Types of Heat Exchangers Double-pipe exchanger Shell and tube exchangers Plate and frame exchangers Plate-fin exchangers. Spiral heat exchangers. Air cooled heat exchangers Agitated vessels. Fired heaters.
  • 5. 1.3.2 Based on transfer process Indirect Contact – Shell & Tube Heat Exchangers Direct Contact – Cooling Towers 1.3.3 Based on phase of fluids Gas-Liquid exchangers Liquid-Liquid exchangers Gas-Gas heat exchangers 1.3 Classification of Heat Exchangers
  • 6. 1.3.4 Based on construction Tubular a. Double pipe heat exchanger b. Shell and tube heat exchangers c. Spiral heat exchangers Plate-type a. Plate and frame heat exchangers b. Spiral plate heat exchangers Extended Surface a. Plate-fin exchanger b. Tube-fin exchanger 1.3 Classification of Heat Exchangers
  • 7. 1.3 Classification of Heat Exchangers 1.3.5 Based on flow arrangements Parallel flow / Co-current flow Counter flow Cross flow
  • 8. The most uncomplicated heat transfer device is Double-pipe heat exchanger. Which has the characteristics of two pipes overlapping as shown in the picture. Divided into Parallel Flow and Counter Flow
  • 9. 2.1 THE OVERALL HEAT TRANSFER COEFFICIENT A heat exchanger typically involves two flowing fluids separated by a solid wall. Heat is first transferred from the hot fluid to the wall by convection , through the wall by conduction, and from the wall to the cold fluid again by convection. Any radiation effects are usually included in the convection heat transfer coefficients. The thermal resistance network associated with this heat transfer process involves two convection and one conduction resistances, as shown in Figure. Here the subscripts i and o represent the inner and outer surfaces of the inner tube. For a double-pipe heat exchanger, we have 𝑨𝒊 = 𝝅𝑫𝒊𝑳 and 𝑨𝒐 = 𝝅𝑫𝒐𝑳, and the thermal resistance of the tube wall in this case is 𝑅𝑤𝑎𝑙𝑙 = ln( 𝐷𝑜 𝐷𝑖 ) 2𝜋𝑘𝐿
  • 10. where k is the thermal conductivity of the wall material and L is the length of the tube. Then the total thermal resistance becomes: In the analysis of heat exchangers, it is convenient to combine all the thermal resistances in the path of heat flow from the hot fluid to the cold one into a single resistance R, and to express the rate of heat transfer between the two fluids as: Where :  U = Overall heat transfer coefficient  h = convection heat transfer coefficient  R = thermal resistance Total heat transfer coefficient (Overall heat transfer coefficient):
  • 11. For the case where the pipe wall is very thick and has a very high k value, the conduction resistance is approximately zero (Rwall ≈ 0 ) Therefore, the Overall heat transfer coefficient (U) value can be written as follows: 𝟏 𝑼 ≈ 𝟏 𝒉𝒊 ≈ 𝟏 𝒉𝒐 Representative values of the overall heat transfer coefficients in heat exchangers
  • 12. 2.2 Analysis of Heat Exchangers In considering the analysis of heat transfer of heat exchanger equipment Must assume the following assumptions: The heat exchanger must be a Steady flow device. Fluid properties And the velocity of the fluid is constant. No changes in Kinetic and potential energy. Has a specific heat capacity.  No heat conduction in the direction of the length of the pipe. The outer walls of heat exchanger are insulated. No heat loss to the environment. Heat transfer occurs between only 2 types of fluids.
  • 13. The rate of heat transfer from the hot side fluid = The rate of heat transfer into the cold side fluid. Where: the subscripts c and h stand for cold and hot fluids, respectively, and 𝑚𝑐, 𝑚ℎ = mass flow rates Cpc, Cph = specific heats Tc, out, Th, out = outlet temperatures Tc, in, Th, in = inlet temperatures
  • 14. 2.3 THE LOG MEAN TEMPERATURE DIFFERENCE METHOD Consider Newton’s Law of Cooling 𝑄 = 𝑈𝐴𝑠∆𝑇𝑚 where ∆𝑇𝑚 is the difference between average temperatures Both types of fluids To find the value ∆𝑇𝑚 is determined by the flow of Fluid that flows in the same direction in double pipe heat exchanger. will get the equation for ∆𝑇𝑚 as: ∆𝑇𝑙𝑚 = ∆𝑇1 − ∆𝑇2 ln( ∆𝑇1 ∆𝑇2 )
  • 15. To calculate ∆𝑇𝑙𝑚, dividing the flow of fluid into 2 types which are: Both types of fluids flow in the same direction (Parallel flow) Two fluids flowing in opposite directions (Counter flow) For (Parallel flow) (Counter flow) ∆𝑇𝑙𝑚 = ∆𝑇1 − ∆𝑇2 ln( ∆𝑇1 ∆𝑇2 )
  • 16. 2.4 Multipass and Cross-Flow Heat Exchangers: Use of a Correction Factor The Calculation for ∆𝑇𝑚 using the formula mentioned above Will be used for double pipe heat exchangers that are parallel flow or counter flow only. For Cross flow or Multipass shell and tube heat exchanger, Correction factor: F is required to calculate ∆𝑇𝑚. where F will depend on The shape of the heat exchanger The temperature of the fluid at the entrance And the fluid temperature at the exit ∆𝑇𝑙𝑚 = 𝐹∆𝑇𝑙𝑚,𝐶𝐹
  • 17.  Bellow the correction factor F charts for common shell-and-tube and cross-flow heat exchangers (from Bowman, Mueller, and Nagle, Ref. 2):
  • 18. 2.5 THE EFFECTIVENESS–NTU METHOD Heat exchanger method analysis method 2 is the NTU Effectiveness method. Which is commonly used to find the rate of heat transfer And the fluid temperature at the device exit. By knowing the various values such as the type and size of the device. Fluid flow rate And the temperature of the fluid at the entrance. NTU method is calculated using the relationship of Number of Transfer Unit (NTU) and Heat Transfer Effectiveness (𝜀) 𝜀 = 𝑄 𝑄𝑚𝑎𝑥 = 𝐴𝑐𝑡𝑢𝑎𝑙 ℎ𝑒𝑎𝑡 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 𝑟𝑎𝑡𝑒 𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝑝𝑜𝑠𝑠𝑖𝑏𝑙𝑒 ℎ𝑒𝑎𝑡 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 𝑟𝑎𝑡𝑒 Where: 𝑄 = 𝐶𝑐 𝑇𝑐,𝑜𝑢𝑡 − 𝑇𝑐,𝑖𝑛 = 𝐶ℎ 𝑇ℎ,𝑜𝑢𝑡 − 𝑇ℎ,𝑖𝑛 𝑄𝑚𝑎𝑥 = 𝐶𝑚𝑖𝑛∆𝑇𝑚𝑎𝑥= 𝐶𝑚𝑖𝑛 𝑇ℎ,𝑖𝑛 − 𝑇𝑐,𝑖𝑛 Number of Transfer Unit (NTU) 𝑁𝑇𝑈 = 𝑈𝐴𝑠 𝐶𝑚𝑖𝑛 = 𝑈𝐴𝑠 (𝑚𝑐𝑝)𝑚𝑖𝑛 Capacity Ratio (c) 𝑐 = 𝐶𝑚𝑖𝑛 𝐶𝑚𝑎𝑥
  • 19. 2.6 SELECTION OF HEAT EXCHANGERS Heat Transfer Rate Cost Pumping Power Size and Weight Type Materials Other Considerations
  • 20. References: Heat exchangers, Selection, Rating, and Thermal Design, Third Edition, Sadik Kakaç, Hongtan Liu, Anchasa Pramuanjaroenkij. Heat Transfer, A Practical Approach, Second Edition, Yunus A. Cengel. Fundamentals of heat exchanger design, Ramesh K. Shah, Dusˇan P.Sekulic. Heat Exchanger Design Handbook, Second edition, Kuppan Thulukkanam