شرح Optimize design for protection of flow Heat exchanger plates

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15 يوليو 2015
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Optimize design for protection of flow Heat exchanger plates

Ammar I. Naji


Mechanical Department/ Alkut technology Institute .

Abstract:
In the present study an exergy-economic criteria that combine the exergy destruction caused by system irreversibilities- due to operation of the heat exchanger on temperature and pressure difference- and the total cost accompanied with this destruction. The prediction of the outlet conditions using EES software for different shape parameter also established, furthermore the optimization has been done to find the optimum heat exchanger size by calculating different aspect ratios defined as Dratio.

Keywords Heat exchanger, Exergy-economic analysis, Thermodynamic design

Nomenclature

W
width of the heat exchanger
Cm
L
length of heat exchanger in flow direction
Cm
Dratio
thickness ratio

m
mass flow rate
kg s-1
P
Pressure
kPa
R
Heat capacity ratio

thm
Km
thickness of plate
metal conductivity at average temperature
mm
W. m-1 K-1
thc
thH Nch hav U Tav Q
NTU C
Pr Re St
I I0
channel width on cold-side
channel width on hot-side number of channel pairs
average convective heat transfer coefficient overall heat transfer coefficient
average temperature
heat capacity of the heat exchanger
No of transfer unit Total annual cost Prandtl Number Reynolds Number Stanton Number Annual capital cost
Fixed maintenance cost
mm
mm

W. m-2 K-1
W. m-2 K-1
K
kW

$ Year-1




$ Year-1
$ Year-1




IF NN t
f
Capital cost conserved with the heat transfer area
Payback period
annual operating time relative lost of exergy
$ Year s
J
Loan rate of interest

ic Ce nt
SgenT
SgenP
tax rate
Unit price of exergy
Exergy-economic criteria
Entropy generation due to temperature difference
Entropy generation due to pressure difference

$ J-1
$ J-1
W k-1
W k-1
Greek symbols
Ρ
Density
kg m -3
Effectiveness of the Heat exchanger
Subscripts
C
Cold

H
Hot

i, in
Inlet

o, out
Outlet


clip_image002.gif




Introduction:
Parallel-plates Counter flow Heat exchanger is thermal equipment that widely used in many engineering applications to deliver energy between systems, the classical example of this kind that has been used in the cryogenics or refrigeration at very low temperatures, where the power requirement is highly related to the entropy generated in the cold space. The performance improvement of this type of heat exchangers is very important in order to save energy and improving the efficiency.
Engineering thermodynamics methodology is one way to analysis such thermal systems. However the exergy analysis technique and entropy generation minimization are the most used one for nowadays engineering thermodynamic applications.
Although the applicability of a system or process is usually based on various factors such as technical performance, efficiency and the environmental impact, the economics aspects can also play a critical role in the designing of such systems. Hence the Analysis, design and optimization that combine technical disciplines like thermodynamics with the economical aspects often provides acceptable compromises and also emphasizes the applicability of the system. In the design of Parallel-plates counter flow heat exchangers the goal of the optimization procedures from the economical point of view is to minimize the initial and operating cost while maintaining better heat exchanger effectiveness.
Therefore the employment of exergy-economic criteria to evaluate heat exchanger performance may open the floor for a new approach that can give practical range of operation for better trends when designing and optimizing these thermal applications.
Different design problems of thermal systems based on the thermodynamic optimization techniques [1–4] were used to find the least exergy destroys while taking into account the tradeoff between two or more competing irreversibilities.
An entropy generation minimization has been done for the counter flow heat exchanger that used Cry cooler systems. The detailed analysis presented the optimal CFHX configuration which produces lesser entropy generation and heat leakage, using different CFHX length, width and high parameters [5].
The parallel-plate heat exchanger in a counter flow configuration has been optimized based on thermodynamic methodology subject to volume constant by Juan et al., [6] the optimization carried out based on the spacing between the two channels and the total heat transfer area between the two sides. Furthermore the entropy generation has been calculated to show the system irreversibility.


Energy efficiencies not always the criteria to assess how the performance of a system approaches near ideality and also not properly describe factors that cause performance to deviate from ideality, several studies based on thermo- economic and exergy-economic criteria has been developed on Heat Exchangers to enhance the overall system performance [7-10], These criteria provide the practical link and can give the full picture to the designers of such systems.
Sahin et al., [11] investigated the single pass counter-flow heat exchanger by using the actual heat transfer rate per unit total cost considering lost exergy and investment costs as an objective function to evaluate the performance. This model assumed that the irreversibilities only due to heat transfer between the hot and cold streams and no irreversibilities such due to pressure drops and flow imbalance.
Yourong et al., [12] developed a model based on a exergy-economic criteria for describing the performance of different heat exchanger configurations. This criteria which are defined as the total costs per unit heat transfer rate has been examined using an illustrative example and finally concluded that the total costs per unit heat transfer rate decreases with the increment of
clip_image004.gif
for the same flow arrangement.




Conclusions
The exergy-economic criteria presents an efficient optimization technique for including the capital costs and the amount of heat transfer rate while calculating the irreversibility loss cost in the Heat exchanger. The design parameter
clip_image006.gif
play a key role to predict the outlet conditions then the total performance of the heat exchanger. Also it provides a good link to the exergy-economic criteria that leads to the Global thermo economic performance. The detailed analysis presented here may offer a real design analysis and performance investigation for this kind of Liquid-to-Liquid Heat exchanger that operate normally in the district cooling systems to provide Buildings with a child water.
 

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