Heat transfer fundamentals (2 of 5)

  Fluids

The fluids for both product and Service systems with which the heat exchanger designer has to work are as varied as the processes which use heat exchangers. They can however be classified into two very broad categories:

• Newtonian - Where the inherent property defined as viscosity is independent of the rate of shear within the fluid.
• Non-Newtonian - Where the inherent property defined as viscosity is dependant on the rate of shear within the fluid.

In simple terms, the effective viscosity of a Newtonian fluid does not depend on the velocity with which it flows through a pipe or tube but for a Non-Newtonian fluid it does.

As well as the viscosity of the working fluids four other properties are of major importance when modelling heat transfer performance.

• Density - the mass of the fluid per unit volume which directly affects the velocity with which the fluids flow through a system.
• Specific heat - the amount of heat which a given mass of a fluid requires for the temperature to be changed by 1°.
• Thermal conductivity - the rate at which heat can flow through a fluid.
• Latent heat - the amount of heat which a given mass of a substance requires to change state - that is to melt if it is a solid, freeze if it is liquid, evaporate if it is a liquid or condense if it is a gas.

Equally important from the operational point of view are the corrosion characteristics of the fluid which influence the final choice of materials of construction that the designer must use.

It is particularly important to identify fluids which are known to be high in Chlorides as these can lead to stress corrosion cracking in some grades of stainless steel but any high acidity or alkalinity fluids should be checked with an expert metallurgist to confirm material suitability. In applications such as exhaust gas cooling it is important to check for condensation on the tube wall and the composition of the gas (or fuel) to check if any acids will form as the gas is cooled. If condensation is confirmed and the gas or fuel contains any Sulphur compounds then the assistance of an expert metallurgist should again be sought for advice on suitable materials.

Shell & Tube heat exchangers

When marketing surveys of heat exchanger usage are carried out they usually report that the most common type of heat exchanger in use in most industries is the shell and tube type. Other types are used when operational or process circumstances dictate, for example air cooled units when there is no source of cooling water readily available or plate types where weight and space are of paramount importance and adequate servicing facilities are available, but the highest proportion of units are still shell and tube types.

Shell and tube heat exchangers can be classified into two broad categories:

• Fixed tube units, where the tubes are fixed rigidly into the pressure retaining envelope of the heat exchanger.
• Removable tube units where the mechanical design allows the tube bundle to be removed from the pressure envelope for inspection and cleaning purposes.

In terms of initial cost the fixed tube type of heat exchangers is usually the least expensive but as the outer tube surfaces are not readily accessible for cleaning and inspection purposes they should only be used in circumstances where the shell side fluid is non fouling or where fouling deposits can be removed by chemical methods.

Removable tube units are made in a variety of different styles to suit the application but all have gasketted or "O" ring sealed joints in contact with the fluids which have to be sealed against the working pressures and temperatures and still be compatible with the application in terms of chemical resistance, approval for food industry use etc. The cost per square metre of heat transfer surface for these units is higher than the equivalent fixed tube design, the level of cost depending on mechanical design considerations.

As a very general rule, a shell and tube unit with a removable tube bundle will increase the cost of a heat exchanger designed for a specific thermal performance by approximately 30%.

An important factor to be considered is that the design and manufacture of shell and tube heat exchangers falls into two very distinct phases.

• The first phase is the completion of the thermal sizing calculations, where the skilful designer can use the enhanced performance characteristics of the corrugated tube to his advantage in reduction of size, weight, hold up volumes etc.
• The second phase is the identification of the appropriate design conditions, temperature, pressure etc. and the mechanical design of the unit where the designer has to follow a rigid set of rules for the pressure envelope of the heat exchanger. This may involve compliance with one or more of the international pressure vessel codes to satisfy the requirements of the purchaser or more frequently the purchasers Insurance Company and National legislation. Equally important is the selection or confirmation of the nozzle sizes, connection types and bellows design which are appropriate for the application and correct for the design conditions.

The designer has to work within an environment which may control his actions and reduce the opportunity of using the advantages of the product to full effect.