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- Heat Exchangers
1.Types of Heat Exchangers
What are they?
Devices for effecting heat transfer from one fluid to another
Mechanical Configurations
›Tube within a tube
›Multiple tubes with a shell (vessel)
›Finned tubes
Flow Arrangements
›Counter flow
›Parallel flow
›Cross flow
Main Design Considerations
›Heat transfer efficiency
›Structural strength
›Pressure drop
›Size and Weight
›Operation and maintenance requirements
›Cost
Design Codes
›ASME Codes for Unfired Pressure Vessel
2.Log - Mean T Method (LMTD Method)
Overall Heat Transfer Coefficient
Consider a parallel flow, tube-within-tube heat exchanger
where: = effective temperature difference
R = thermal resistance
U = overall heat transfer coefficient
A = effective surface area
Thermal conduction analysis showed:
plane wall:
cylinder wall:
Note:
›For cylindrical systems,
›To consider the effects of surface fouling and fins, overall heat transfer coefficient becomes:
Parallel Flow
Define:
Noting , we can write:
Newton’s law of cooling gives:
After simplification, it becomes;
where:
Counter Flow
Note:1) (when are they equal?)
2)Only for counter flow, could be larger than
Complex Heat Exchangers
See Figures 11.10 to 11.13 for F values
Note:1) must be evaluated under counter flow condition
2)F = 1 if or
3.NTU - Effectiveness Method
›Alternative to determine the exchanger performance
›Used most conveniently if two or more of the inlet or exit temperatures of both streams are unknown
Definition
›Heat exchanger effectiveness
Assuming , the energy balance gives
Realizing the maximum possible heat transfer occurs when the fluid of smaller C undergoes the maximum temperature difference available, we can write
where:Cmin = min(Ch, Cc)
or
›NTU (Number of Transfer Units)
Parallel Flow Heat Exchanger
Assume Cmin =Ch
where:Cr = heat capacity ratio
Recall:
In terms of and Cr, it becomes:
After the further transformation, it can be written as:
Note:1)Valid also for Cmin =Cc
2)
Other Configurations
›Mathematical relations:Tables 11.3 and 11.4
›Graphical relations:Figures 11.14 to 11.19
Note:1) Cr = 0 for condensers and boilers
4.Method Applications
›Design problem: all the inlet and outlet temperatures known, find the type and size of heat exchangers LMTD method
›Performance evaluation problem: the type and size of heat exchanger known, find the heat transfer rate and fluid outlet temperatures for the selected fluid flow rate and inlet temperatures NTU method