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HEAT TRANSFER

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Heat Exchangers, LMTD Method KEY POINTS THIS LECTURE Various types of heat exchangers that are commonly used in industry and product designs. – PowerPoint PPT presentation

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Title: HEAT TRANSFER


1
  • HEAT TRANSFER

CHAPTER 11 Heat Exchangers
2
Heat Exchangers, LMTD Method
  • Where weve been
  • So far have focused on detailed heat transfer
    analysis of specific conditions, such as external
    heat transfer coefficient
  • Where were going
  • Investigate methods for larger system level
    analysis that combine all these modes of heat
    transfer in heat exchangers

3
Heat Exchangers, LMTD Method
  • KEY POINTS THIS LECTURE
  • Types of heat exchangers, advantages and
    disadvantages
  • Overall heat transfer coefficient, concept of
    fouling factor
  • Log mean temperature difference
  • Application of LMTD to heat exchanger analysis
  • Text book sections 11.1 11.3

4
Heat Exchanger Types
Example
5
Heat Exchanger Types (Contd)
Shell and Tube (common in chemical process
industry)
Shell and Tube
Shell and Tube
Shell and Tube
6
Heat Exchanger Types (Contd)
  • Shell and Tube

7
Heat Exchanger Types (Contd)
  • Plate and Frame
  • Series of plates with flow channels embossed in
    them.
  • The two fluids are guided through alternating
    rows of the plates
  • Advantages __________________________
  • Application pictured Electrocoat paint in
    automotive assembly plant

8
Heat Exchanger Types (Contd)
  • Plate and Fin
  • Dense array of plates that guide alternating
    channels of fluids (typically air)
  • Series of fins connect the plates and greatly
    increase the heat transfer area
  • Advantage very large heat transfer surface area
    per unit volume .
  • One common application Aircraft environmental
    control systems

9
Overall heat transfer coefficient for HX
  • Recall from earlier the overall thermal
    resistance concept
  • Types of resistances involved with heat
    exchangers (covered in previous sessions)
  • Cold side internal convection
  • Cold side fouling factor
  • Conduction through wall
  • Hot side external convection (smooth wall or may
    involve fins)
  • Hot side fouling factor
  • Review howthese werecalculated

10
Analysis of heat transfer
  • Total heat transfer rate is found through energy
    balance, regardless of the HX type or flow path
  • For hot fluid Energy balance
  • Define
  • For the cold fluid
  • (Note no minus sign - in this equation,
    since heat flow in)

q
11
Analysis of heat transfer (Contd)
  • Energy balance gives
  • For the entire flow length
  • A convenient way to compute the heat transfer is
    from the mean temperature difference between the
    hot and cold fluids
  • Next Evaluation of ?Tm different for parallel
    and counter flow

12
Analysis of parallel flow heat transfer
  • Parallel flow heat exchanger
  • At any location along the heat exchanger
  • Where
  • So
  • Integrating from the inlet to the outlet

dq
Eq. 11.13
13
Analysis of parallel flow heat transfer (Contd)
  • From the overall energy balance, total heat
    transfer
  • thus
  • Combining
  • For parallel flow

Eq. 11.14
14
Analysis of parallel flow heat transfer (Contd)
  • Temperature profile for parallel flow

In
Out
Fig 11.7
15
Analysis of counter flow heat transfer
  • For counter flow
  • Temperature profile for counter flow

Fig 11.8
16
Typical use of the LMTD method
  • Given
  • Need to cool a certain mass flow rate of fluid A
    from TA,i to TA,o using the fluid B at TB,i
  • Find
  • Design / size the heat exchanger
  • Solution Method
  • Use the overall energy balance to find
  • Select the heat exchanger type (based on the
    other project needs, available resources, size
    and weight considerations, etc., etc.)
  • Select tube diameters and types of heat transfer
    surfaces (fins, no fins, etc.)
  • Use to determine the
    needed heat exchanger heat transfer area (?
    length)

17
Special cases
  • For a condensing vapor
  • For an evaporating liquid
  • What if Ch Cc in a counter-flow HX?

T
x
In Out
T
x
In Out
T
x
In Out
18
Multipass and cross-flow heat exchangers
  • The equations are the same.

Counter-flow conditions
To find F, please refer to the figures 11.10-13.
19
Typical Example E11.1 ( textbook, pp619)
20
Heat Exchangers, LMTD Method
  • KEY POINTS THIS LECTURE
  • Various types of heat exchangers that are
    commonly used in industry and product designs.
    Understanding of when to consider using each
    type.
  • Defined the fluid heat capacity
  • Log mean temperature difference introduced again
  • Temperature distribution parallel vs. counterflow

Parallel
Counterflow
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