Where We Go Wrong In Heat Exchangers

Dick Hawrelak

Presented to ES317Y in 1999 at UWO

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.

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).

Poor Mass Balances

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

Fouling

Rd = (Uc - Ud) / (Uc)(Ud) Corrected LMTD for Uc. Rd build-up versus time not well

known. Cleaning versus spares.

Pressure Drop

Many variables affect pressure drop.

Nozzle sizes, baffle spaces, tube dia., tube length, no. tube passes.

Selection

Poor reboiler selection. Many configurations - which one to

select?

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?

Temperature Profiles

Partial condensers temperature profiles with inert gases are difficult to model.

Good VLE data hard to obtain.

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?

Maldistribution

Shell side maldistribution with small window cuts.

Tube side maldistribution with low tube side pressure drops.

How do you design a Chinese hat?

Acoustics

Shell side geometry can cause acoustic vibrations.

May require tuning baffles.

Entrainment

Kettle exchanger design depends on entrainment calculations.

Entrainment levels often ignored on mass balances.

See Shell Size v1.2

Expansion Joints.

Avoid expansion joints at all costs. No. flexes per hour usually

unknown. Paper clip example.

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.

Thermosyphon Layout

Previous Exam Problem

The students were presented a paper on Union Carbide’s 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.

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.

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.