SHELLBELL.ppt Shell-side Calculations Using Bell

About This Presentation

Title:

SHELLBELL.ppt Shell-side Calculations Using Bell

Description:

Thermal performance analysis (NTUs) for co- & counter-current exchangers. ... Pitch = 1.25 x tube diameter. 30o triangular best for clean fluids ... – PowerPoint PPT presentation

Number of Views:151

Avg rating:3.0/5.0

Slides: 33

Provided by: chemEng4

Category:

more less

Transcript and Presenter's Notes

Title: SHELLBELL.ppt Shell-side Calculations Using Bell

1
SHELLBELL.pptShell-side Calculations Using
Bells Method
2
Components of Course What Stage are We Up To?

Types of exchangers, revision of OHTCs, fouling
factors.
Heat exchanger selection.
Thermal performance analysis (NTUs) for co-
counter-current exchangers.
Multi-pass exchangers (ST).
Condensation boiling.
Radiation.

3
Reference

This is an advanced topic more details can be
found in the book by Hewitt et al
Hewitt et al, ch. 6.3.2, pp. 275-285
(Bells method for shell tube exchangers)
This lecture will focus on the concepts general
design considerations.

4
Basis of Bells Method Motivation for Use of
Method

Consider flow patterns (first) effects on heat
transfer pressure drop (not done by older
methods).
Still simple enough to be done by hand.
Can be set up on spreadsheet.

5
Flow Patterns, Basic Geometry

Why shell--tube exchangers are built this way.
What effects do these manufacturing details have
on performance (heat transfer pressure drop)?

6
Manufacturing Details
7
Manufacturing Details (cont.)
8
So What?

These details mean bypass leakage virtually
inevitable

9
Leakage Bypass Diagram
10
Leakage Bypass
11
(No Transcript)
12
Flow Patterns

Want flow to go across tubes (stream B).
However, some flow goes through
gap between tubes baffles (stream A)
tube bundle shell (stream C)
baffle shell (stream E)
between tube passes (stream F)
Bypass leakage streams degrade heat-transfer
performance some also reduce pressure drop.

13
Leakage (cont.)
14
Bypassing
15
Why Tube-Lane Partition?
16
Reduce Bypassing?
17
Common Calculations

Some calculations common to both heat transfer
pressure drop estimation
Shell-side Reynolds number
free cross-stream flow area
maximum inter-tube velocity, Vmax
Re(shell)
Bypass leakage Good design Combined effect
decreases actual h to no less than 40-50 of
ideal value.

18
Design Considerations HT Performance Bought with
Pressure Drop

Reducing pressure drop
Double, multi-segmental, disk doughnut baffles
Shell type to split-flow
Decreasing L
Increasing tube pitch
Removing sealing strips
Removing baffles

19
Pressure-Drop Limited Design

Tend to increase size to reduce pressure drop
Heat-transfer requirements easily satisfied
Common example low-viscosity fluids (water)

20
Heat-Transfer Limited Design

Opposite case to pressure-drop limited design
Common example high-viscosity fluids
Worth accepting higher running costs

21
Design Considerations Velocities

Too low excessive fouling
Too high tube vibration, erosion
Liquids
tube-side 1-2 ms-1, up to 4 ms-1 if fouling
shell-side 0.3-1 ms-1
Gases
atm pressure 10-30 ms-1

22
Design Considerations Temperatures

Very good heat recovery justified if value of
value of heat is high
series 1-2 or 2-4 shell--tube
plate
compact
1-2 shell--tube guidelines
coolers max dTgt20oC, min dTgt5oC
heat recovery min dTgt20oC

23
Design Considerations Fouling

Put fouling fluids inside tubes
Inside of tubes easier to clean than outside
Velocities easier to control
No dead zones

24
Design Considerations Fluid Property Effects

High pressure into tubes (containment)
High temperatures, corrosive fluids, into tubes
special alloys may be cheaper in tubes
reduces heat losses
High viscosity, low flowrate
easier to get turbulence on shell-side (Re 100)
if turbulence not possible on shell-side, better
to have laminar flow in tubes (prediction better)

25
Design Considerations Tube Diameter

Small tubes give improved heat transfer
BUT
Tube cleaning practices require d gt 20mm (approx.)

26
Design Considerations Tube Length

Longer tubes mean
fewer tubes (fewer pieces to handle)
smaller shell diameter
both cheaper
Usually (tube length)/(shell diameter) from 5-101

27
Design Considerations Tube Layout, Pitch

Pitch 1.25 x tube diameter
30o triangular best for clean fluids
Square with large spacing for fouling fluids

28
Design Considerations Baffle Type, Spacing, Cut

25 segmental is standard
Low pressure drop segmental, disk doughnut
Baffle spacing window size is important should
be about 1 with 25 cut, this gives spacing
diameter/4

29
Design Considerations Baffle Type, Spacing, Cut
30
Design Considerations Baffle Type, Spacing, Cut
31
Pressure Drop Heat Transfer

Pressure drop proportional to (velocity)1.8
Heat transfer proportional to (velocity)0.6
Pressure drop harder to predict than heat
transfer because it is very sensitive to flow
maldistribution flow-pattern details