The Specification Process

Practical selection of heat exchanger designs is a complex matter usually performed by a progressive series of actions including:

• Initial feasibility/selection of generic heat exchanger types based on the duty required including pressures, temperatures, characteristics of hot and cold process streams, previous experience, physical footprint etc. This eliminates unsuitable technologies at the first stage and feeds forward potential technologies to a more detailed feasibility assessment. Includes process integration aspects.

• Calculation of heat transfer areas including fouling characteristics for different exchanger configurations and different exchanger technologies.

• Comparative capital costing of heat exchanger options including different construction materials.

• Final selection including all the previous aspects plus installation costs, operational costs, energy efficiency, safety implications, maintenance requirement and other process specific selection criteria.

3.6.2.1 Initial Selection

The initial feasibility selection is a crucial step as experience shows that once a favoured technology is selected, then changes are unlikely to be made later. Consequently, it is important to ensure that compact heat exchanger designs are included in the initial technology selection (when appropriate).

The initial selection process takes account of a number of factors, all of which are related to the specific application (therefore no general rules can be given). These factors include the following:

• Thermal and Hydraulic Requirements.

The amount of heat to be exchanged, the fluid inlet and outlet temperatures and the allowable pressure drop (or pumping power) are often specified as a result of overall process optimisation. Clearly, any selected heat exchanger must be capable of meeting this specification. Many heat exchangers are limited by a maximum operating temperature and pressure.

The objective of the initial selection and preliminary costing exercises is to identify options that can be submitted for detailed design evaluation and costing by suitable organisations.

• Compatibility with fluids and operating conditions.

The materials of construction of the selected exchanger must be able to contain the fluids on the respective sides without excessive corrosion. This is a particular concern where aluminium or gaskets are used. Wall thickness in some compact heat exchangers is less than in conventional shell and tube exchangers, so corrosion rates and allowances need careful assessment. Upstream corrosion products are more likely to block a compact heat exchanger than a conventional shell and tube exchanger. Small fluid passages in a compact heat exchanger can also corrode and become blocked. The exchanger must be capable of being designed and constructed to withstand the stresses due to fluid pressures and temperature differences (thermal stresses). Another consideration is the consequences of failure allowing the mixing or leakage.

• Maintenance.

The characteristics of the process streams should be carefully considered to assess the requirements for cleaning (mechanical or chemical) and periodic replacement of all or part of the unit. Ease of modification could also be an important factor if process conditions are likely to change.

• Availability.

Project time-scales will often dictate the use of standard designs, which can be delivered rapidly. There may also be limitations in the available design methods for some of the possible units. Sometimes multiple exchangers must be used in parallel to overcome size limitations.

• Economic factors.

Obviously, if several possible heat exchanger types meet the physical requirements listed above, the final choice must be based on economics. For fixed pressure drop (pumping power), the main economic factor will be capital cost. However, it should be noted that there maybe a trade-off between pumping power and capital cost.

• Space and weight.

In a number of applications the volume occupied by a heat exchanger is a vital consideration while in others, the weight is a crucial factor. In some applications, both volume and weight are important. Examples of such limitations are found in the aerospace industry where limitation of payload is economically and technically important and offshore structures where there is a large incentive to reduce the size and loading of platforms. Some compact exchangers can be mounted directly onto the top of distillation columns or similar. Exchangers should be suitably robust and capable of standing mechanical shock if necessary.

• Fouling.

The potential for fouling with the respective fluids must be carefully assessed and the exchanger must be able to operate for the required period under the predicted conditions.

The following points give general guidance:

• Closed loops are unlikely to present significant fouling problems. Working fluids in refrigeration or power cycles, for example, should not cause any fouling in a well-engineered and maintained system.

• Open loops are prone to fouling, and may require the installation of filters to remove particles, fibres etc., as well as chemical treatment to prevent biological growth, deposition of scale, or corrosion.

• Once-through streams need to be examined on a case-by-case basis and appropriate action taken if the stream warrants it.

• Spiral heat exchangers are particularly able to handle highly fouling liquids and slurries although other wide-gap designs are available.

Where fouling and/or corrosion are causes for concern, consider installing a closed cycle system as an intermediate loop between the heat source and the ultimate sink.

Where a closed cycle system is not an option, consult with the equipment supplier(s) and give detailed consideration to:

• Fouling margins.

• Optimal flow rates.

• Control of heat exchanger operation.

• Upstream fouling prevention.

• In-exchanger fouling control/removal.

For further information on dealing with the fouling issue, see Module 3.2.

• Temperature and Pressure Cycling.

Continuous temperature and pressure cycling can induce stress-related failures in some exchanger designs. Also, vibration stress typical of some two-phase exchanges may also be a design consideration.

Further initial selection information for common heat exchanger designs is given in Table 3.6.2, or for other information, see Table 3.6.1.

Heat

Double pipe Shell 300

Tube 1400 -100 to +600 Few 0.25 to 200 5 0.90

Table 3.6.2 - Performance Summary of Some Heat Exchangers (adapted from HTFS Website)

With close approach temperatures in counter-current flow, exit temperatures can crossover;

for example in spiral or plate-fin heat exchangers.

In principle, the initial selection process is usually independent of cost as this is determined in subsequent steps taking into account the heat transfer coefficient, materials of construction and other installation factors.

3.6.2.2 Preliminary Cost Estimation

The installed cost of a heat exchanger comprises the capital cost of the exchanger plus the installation cost (which may be substantial). Other factors may also be relevant such as operating cost.

Usually order of magnitude capital cost is used to compare different technology options modified as necessary by other impinging factors.

There are a variety of methods available to estimate cost for a given duty. ESDU publish

3.6.2.3 Detailed Design and Costing

This stage is usually performed by equipment suppliers' proprietary software according to criteria supplied by the client. As this stage tends to be costly, the number of potential technology variants should be minimised allowing the equipment supplier to optimise the exchanger design.

The following section is a plate heat exchanger design example illustrating a basic selection methodology.

3.6.2.4 Design Procedure for a Plate Heat Exchanger (adapted from an Alfa Laval Thermal Division example)

This example calculation illustrates the steps in the specification of a plate heat exchanger from fundamental equations. The derivation of the design includes empirical factors specific to this manufacturer’s equipment.

Design Procedure

For definitions and values, see Tables 3.6.3 and Table 3.6.4.