Page 84 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

4.3 Design considerations 81

shell and tube exchanger is more complex as compared to double-pipe exchanger and optimum design
involves the considerations given below.

Process

1. Process fluid assignment to shell and tube side.
2. Specifying stream temperatures.
3. Setting shell-side and tube-side pressure drop design limits.
4. Setting shell-side and tube-side velocity limits.
5. Selection of heat transfer models and fouling coefficients for shell side and tube side fluids.

Mechanical

1. Selection of heat exchanger layout as per the decided code (TEMA/BIS 4503, etc.) and the
number of passes.
2. Specification of tube parameters e size, layout, pitch and material.
3. Setting upper and lower design limits on tube length.
4. Specification of shell-side parameters e material, baffle cut, baffle spacing and clearance.
5. Setting upper and lower design limits on shell diameter, baffle cut and baffle spacing.

4.3.1 Input data for design
a. Input data pertaining to the shell- and tube-side fluids are listed in Table 4.3. In addition to this,
information on the nature of the fluids, e.g., flammability, corrosive nature, fouling tendency, solid
concentration, etc., as applicable are noted. These are used in deciding the type of heat exchanger
suitable for the service.
b. Constructional details like installation (horizontal/vertical/inclined); design pressure and
temperature; preferred options, if any, for the tube geometries (length, OD, thickness), tube pitch
and layout; materials of construction; nozzle size, rating and facing are also noted before
embarking on design.
c. Allowable pressure drop.

4.3.2 Design output
Process design
Output from process design are
- Heat transfer area required to deliver the specified heat duty based on assumptions of exchanger
type and configuration details. This involves detailing of exchanger configuration commonly
summarised by specifying the code classification, e.g., TEMA AES exchanger.
- Overall heat transfer coefficient, film coefficient of both shell and tube side, temperature
difference MTD.
- Pressure drop for shell- and tube-side fluids.
- Design references: process calculation references (procedure for calculation of shell-side and
tube-side heat transfer coefficients; design standard (TEMA, BIS, etc.)).
- Design considerations such as short-term conditions, if required.