Where We Go Wrong In Heat Exchangers

1
Where We Go Wrong In Heat Exchangers

• Dick Hawrelak
• Presented to ES317Y in 1999 at UWO

2
Introduction

• 3 of large property damage losses are caused by
  failures in heat exchangers.
• Average Trended Loss was 24MM, the second lowest
  of all unit operations.

3
Exchanger Problems

• The following list of problems have been drawn
  from my personal experience as a design engineer
  with Dow Chemical for 33 years (1960 - 1993).

4
Poor Mass Balances

• Normal mass balance.
• Start-up, shut down or upset.
• Recycle conditions.

5
Fouling

• Rd (Uc - Ud) / (Uc)(Ud)
• Corrected LMTD for Uc.
• Rd build-up versus time not well known.
• Cleaning versus spares.

6
Pressure Drop

• Many variables affect pressure drop.
• Nozzle sizes, baffle spaces, tube dia., tube
  length, no. tube passes.

7
Selection

• Poor reboiler selection.
• Many configurations - which one to select?

8
Heat Transfer

• Poor understanding of design parameters in HTRI
  or HTSF.
• Pressure drop versus heat transfer area.
• Which steam pressure or refrigerant should be
  used?

9
Temperature Profiles

• Partial condensers temperature profiles with
  inert gases are difficult to model.
• Good VLE data hard to obtain.

10
Mechanical Design

• High RHO-V-SQUARE on inlet shell nozzle can
  rupture tubes.
• Impingement plate design not well defined.
• Tube vibrations with long tube spans.
• How to join tubes to tubesheet?

11
Maldistribution

• Shell side maldistribution with small window
  cuts.
• Tube side maldistribution with low tube side
  pressure drops.
• How do you design a Chinese hat?

12
Acoustics

• Shell side geometry can cause acoustic
  vibrations.
• May require tuning baffles.

13
Entrainment

• Kettle exchanger design depends on entrainment
  calculations.
• Entrainment levels often ignored on mass
  balances.
• See Shell Size v1.2

14
Expansion Joints.

• Avoid expansion joints at all costs.
• No. flexes per hour usually unknown.
• Paper clip example.

15
Recirculation Problems

• Low Recirculation due to inert build-up in shell,
  high condensate level, tube resistance, low
  liquid level in column.
• Low recirculation promotes fouling and unwanted
  heavies production.

16
Thermosyphon Layout
17
Previous Exam Problem

• The students were presented a paper on Union
  Carbides Seadrift, Texas, EO tower explosion.
  They were asked to comment on the explosion with
  respect to what they had learned in this safety
  course. Heat exchanger design played an important
  role in the explosion.

18
Other Possible Exam Questions

• In an exchanger, what type of joint must be
  avoided in construction?
• What is the problem of this type of joint?
• List four mechanical problems in exchanger design.

19
Summary

• This short list is indicative of some of the
  problems caused by poor engineering discipline in
  heat exchanger design.
• Recommend you obtain a copy of the Chemical Plant
  Design programs and follow the procedures built
  into the exchanger spreadsheets.