HEAT TRANSFER

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• HEAT TRANSFER

CHAPTER 11 Heat Exchangers
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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

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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

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Heat Exchanger Types
Example
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Heat Exchanger Types (Contd)
Shell and Tube (common in chemical process
industry)
Shell and Tube
Shell and Tube
Shell and Tube
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Heat Exchanger Types (Contd)

• Shell and Tube

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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

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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

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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

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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
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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

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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
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Analysis of parallel flow heat transfer (Contd)

• From the overall energy balance, total heat
  transfer
• thus
• Combining
• For parallel flow

Eq. 11.14
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Analysis of parallel flow heat transfer (Contd)

• Temperature profile for parallel flow

In
Out
Fig 11.7
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Analysis of counter flow heat transfer

• For counter flow
• Temperature profile for counter flow

Fig 11.8
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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)

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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
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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.
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Typical Example E11.1 ( textbook, pp619)
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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.
  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