Thermo-Hydraulic Analysis of SHELL-AND-TUBE HEAT EXCHANGERS

1. Thermo-Hydraulic Analysis of SHELL-AND-TUBE HEAT EXCHANGERS P M V Subbarao Professor Mechanical Engineering Department I I T Delhi

2. Thermal Analysis of Heat Exchanger Known as heat exchanger specification problems and their solutions. These are ‘rating’, ‘design’, and ‘selection’.

3. Rating Analysis The rating problem is evaluating the thermo-hydraulic performance of a fully specified exchanger. The rating program determines: the heat transfer rate and the fluid outlet temperatures for prescribed fluid flow rates, inlet temperatures, and the pressure drop for an existing heat exchanger; therefore the heat transfer surface area and the flow passage dimensions are available.

4. The Rating Analysis

5. The Design (Sizing) Analysis ‘Design’ is the process of determining all essential constructional dimensions of an exchanger that must perform a given heat duty and respect limitations on shell-side and tube-side pressure drop. In the Design (sizing) Analysis, An appropriate heat exchanger type is selected. The size to meet the specified hot and cold fluid inlet and outlet temperatures, flow rates, and pressure drop requirements, is determined. Constraints: Minimum or maximum flow velocities, Size and/or weight limitations, Ease of cleaning and maintenance, erosion, tube vibration, and thermal expansion. Each design problem has a number of potential solutions, but only one will have the best combination of characteristics and cost.

6. Basic Design Procedure

7. Basic Design Procedure Heat exchanger must satisfy the Heat transfer requirements (design or process needs) Allowable pressure drop (pumping capacity and cost) Steps in designing a heat exchanger can be listed as: Identify the problem Select an heat exchanger type Calculate/Select initial design parameters Rate the initial design Calculate thermal performance and pressure drops for shell and tube side. Evaluate the design. Is performance and cost acceptable?

8. The Selection Analysis ‘Selection’ means choosing a heat exchanger from among a number of units already existing. Typically, these are standard units listed in catalogs of various manufacturers. Sufficient manufacturer’s data usually exist to allow one to select comfortably oversized exchanger with respect to both area and pressure drop.

9. Macro Steps in Analysis Initial Decisions. Tube side Thermal Analysis. Thermal analysis for Shell side. Overall Heat Transfer coefficient. Hydraulic Analysis of Tube side. Hydraulic Analysis of Shell side.

10. Fluid Allocation Tube side is preferred under these circumstances: Fluids which are prone to foul The higher velocities will reduce buildup Mechanical cleaning is also much more practical for tubes than for shells. Corrosive fluids are usually best in tubes Tubes are cheaper to fabricate from exotic materials This is also true for very high temperature fluids requiring alloy construction Toxic fluids to increase containment Streams with low flow rates to obtain increased velocities and turbulence High pressure streams since tubes are less expensive to build strong. Streams with a low allowable pressure drop Viscous fluids go on the shell side, since this will usually improve the rate of heat transfer. On the other hand, placing them on the tube side will usually lead to lower pressure drops. Judgment is needed

11. Options for Shell-Side Thermal Analysis Kern's integral method  Bell-Delaware method  Stream analysis method Recent Methods

12. Kern Method of SHELL-AND-TUBE HEAT EXCHANGER Analysis

13. Kern’s Integral Method The initial attempts to provide methods for calculating shell-side pressure drop and heat transfer coefficient were those in which correlations were developed based on experimental data for typical heat exchangers. One of these methods is the well-known Kern method, which was an attempt to correlate data for standard exchangers by a simple equation analogous to equations for flow in tubes. This method is restricted to a fixed baffle cut (25%) and cannot adequately account for baffle-to-shell and tube-to-baffle leakages. Although the Kern equation is not particularly accurate, it does allow a very simple and rapid calculation of shell-side coefficients and pressure drop to be carried out

14. Thermal Analysis for Tube-Side

15. Number of Tubes The flow rate inside the tube is a function of the density of the fluid, the velocity of the fluid, cross-sectional flow area of the tube, and the number of tubes.

16. Tubes in Shell and Tube Hx The number and size of tubes in an exchanger depends on the Fluid flow rates Available pressure drop. The number and size of tubes is selected such that the Tube side velocity for water and similar liquids ranges from 0.9 to 2.4 m/s. Shell-side velocity from 0.6 to 1.5 m/s. The lower velocity limit corresponds to limiting the fouling, and the upper velocity limit corresponds to limiting the rate of erosion. When sand and silt are present, the velocity is kept high enough to prevent settling.

17. Tube Length Tube length affects the cost and operation of heat exchangers. Longer the tube length (for any given surface area), Fewer tubes are needed, requiring less complicated header plate with fewer holes drilled. Shell diameter decreases resulting in lower cost. Typically tubes are employed in 8, 12, 15, and 20 foot lengths. Mechanical cleaning is limited to tubes 20 ft and shorter, although standard exchangers can be built with tubes up to 40 ft. There are, like with anything limits of how long the tubes can be. Shell-diameter-to-tube-length ratio should be within limits of 1/5 to 1/15 Maximum tube length is dictated by Architectural layouts Transportation (to about 30m.) The diameter of the two booster rockets is dictated by the smallest highway tunnel size between the location of manufacturer and Florida. Scientific hah!

18. Tube Length : Tube & Header Plate Deformation