Paul M. Sinacore, Doyle & Roth Manufacturing Co.

Because the process industry has become more competitive, with product development, process construction, and product life times drastically shortened, designers and engineers have less time between initial design and final fabrication. Fastback construction has become more the rule than the exception. Design changes are typically made after the start-up date, and constant improvements continue until demand for the product has diminished. At this point, a company must assess its manpower and equipment to tailor them to new market demands.

The constant demand on companies to be market- and customer-driven requires designers and engineers to be more creative than ever. Not only must equipment be efficient, effective and inexpensive, it must be designed for flexibility and longevity, often in multiple processes.

To the Shell and Tube designer, this means that processes currently being considered may not reflect where the unit will be utilized three weeks or three years from now. The design parameters may cause the unit to be so case-dependent that no flexibility in operating conditions will be possible.

This paper addresses design constraints and discusses criteria that assist designers in considering more flexible designs.

Basic Shell and Tube Design Parameters Before proceeding, certain design conditions and parameters must be established. While the following case study focuses on heat exchangers utilizing liquid as both the process and service fluids, it is also possible to apply these protocols procedures to more complicated designs, i.e. thermal syphon reboilers and falling film evaporators.

Basic considerations necessary for exchanger designs are: flow rates, terminal temperatures, physical properties of fluids, tendency of fluid fouling (fouling resistance), allowable pressure drop across the equipment, construction material and the physical space and/or floor space available to install equipment.

Secondary factors that should be considered for a unit to be designed as an asset (a reusable investment) as opposed to liability (nonflexible single process design) include: variability of previous factors, maintenance schedule, expected life of the process, and mechanical and thermal stress imposed on the equipment.

Flexibility Engineers must redefine design constraints for heat exchanger selection. Worse case scenarios must be weighed against limitations created by a constraint that may be of minimal probability while limiting future use, or even current efficiency.

Several heat exchanger design parameters may, if improperly specified, have undesirable effects on the versatility of the equipment. Special care should be taken in considering the cost and design impacts of such factors as coolant flow, maximum temperature, pressure drop, and fouling factors.

When designing a shell and tube exchanger for multipurpose use, special care should be taken to monitor fluid velocities. It is often the case that inflexibility in the aforementioned parameters ultimately decreases fluid velocity and leads to a design that becomes flow-dependent and completely inflexible in other process applications.

As an example, consider a case in which fouling resistance is over-specified. In high fouling fluids, a high velocity may be as important as the extra surface supplied by specifying a high foul. For cases where a fluid's tendency to foul causes extensive downtime, an increase in fouling coefficient may be counterproductive. Before adding additional foul, consider designs that allow for a flow velocity that keep flow in turbulent condition. In turbulent flow, lower fouling factors will offer the same amount of extra surface while fulfilling the same requirement. As an added benefit, higher fluid velocity decreases the probability of fouling, which in turn reduces the need for downtime.

Table 1 depicts the effect of typical fouling factors on the surface area of shell and tube heat exchangers relative to service coefficients. The percent of extra surface area added to each case is given. The table shows that care must be taken when specifying a fouling factor in an effort to achieve the desired result.

Where a high velocity cannot be achieved due to limits in floor space area, multiple smaller units may be cost-effective. In these cases each unit is designed with high velocity and lower fouling, allowing one unit to act as an online spare. During standard scheduled maintenance, one unit is cleaned while the other remains online. The switchover can be accomplished with minimal piping changes.

Now consider a situation in which an arbitrarily specified pressure drop is arbitrarily specified. This limitation decreases velocity through the exchanger and hinders the heat transfer coefficient. Where possible, if less pressure drop is drawn throughout the system, additional allowable pressure drop should be added to the heat exchanger.

Another factor is operating temperatures, which are often fixed or offer little flexibility. In cases where cooling water is used, a realistic "worse case" temperature should be considered. The designer should keep in mind the probability of this situation occurring, in relationship to the design effects it will cause. For example, cooling tower water at 90°F causes a 30% increase in surface area in equipment design, over the more typically available 80°F water. If this temperature increase occurs rarely, then instead of increasing the unit size the designer should explore other options, such as what effects a decrease in product flow rate would have over a short time period, or whether an alternate coolant source is available for the short term.

Such questions are frequently addressed on the plant floor, but are rarely posed in the initial design stage.

Remember that the cost of equipment is not limited to the initial purchase price. Proper operation and maintenance are key to the success of any design. Become familiar with plant operations to address these concerns accurately. Shell end tube exchanger maintenance should coincide with planned shut-downs and operational variability. Two typical concerns are cleanability and stress placed on units during turn-around.

Cleanability How a unit is cleaned, and how often, can factor in the design of the exchanger. For a high fouling unit it is desirable to keep the fluid on the tubeside. This allows both chemical and mechanical cleaning of the heating surface without major labor expenditure. Exchangers can also be supplied with removable covers to allow for ease in cleaning while maintaining piping seals.

Thermal Stresses Whenever equipment is put through a thermal cycle it experiences stress. The severity and frequency of such stresses should be considered as what may be considered normal operating procedures, on a plant level, may reduce the usable life of an exchanger. This can be addressed by reviewing the need for various types of expansion devices. The need for these potentially expensive changes can only be assessed through a clear understanding of operational procedures.

The physical size of the equipment may play an unexpected role in design. Fluid velocities and coefficients are directly related to the physical size of the exchanger. The location of the unit dictates maintenance costs due to relative ease in accessibility of the equipment.

Conclusion When designing a shell and tube heat exchanger, it is essential to define the customer and understand their individual requirements. Different aspects of each design address a different customer's need. The customer process design, project management, capital procurement and maintenance departments each have specific requirements:The Process designer demands efficiency and effectiveness. The project manager insists upon quick response and flexibility. Procurement requires cost-effectiveness. Maintenance looks to minimize down-time.By addressing each of these concerns, equipment can be designed to meet today's needs and therefore be more capable of meeting future demands. A clear understanding of design parameters and choices will allow the designer to select the most suitable design for the specific project being undertaken. As a result the equipment will be an asset to the customer for years to come.

For more information: Paul Sinacore, Doyle & Rothman Manufacturing Co., Inc., 26 Broadway, New York, NY 10004. Tel: 212-269-7840.

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