Title: HEAT TRANSFER
1CHAPTER 11 Heat Exchangers
2Heat 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
3Heat 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
4Heat Exchanger Types
Example
5Heat Exchanger Types (Contd)
Shell and Tube (common in chemical process
industry)
Shell and Tube
Shell and Tube
Shell and Tube
6Heat Exchanger Types (Contd)
7Heat 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
8Heat 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
9Overall 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
10Analysis 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
11Analysis 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
12Analysis 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
13Analysis of parallel flow heat transfer (Contd)
- From the overall energy balance, total heat
transfer - thus
- Combining
- For parallel flow
Eq. 11.14
14Analysis of parallel flow heat transfer (Contd)
- Temperature profile for parallel flow
In
Out
Fig 11.7
15Analysis of counter flow heat transfer
- For counter flow
- Temperature profile for counter flow
Fig 11.8
16Typical 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)
17Special 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
18Multipass and cross-flow heat exchangers
- The equations are the same.
Counter-flow conditions
To find F, please refer to the figures 11.10-13.
19Typical Example E11.1 ( textbook, pp619)
20Heat 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