Kerns Description of Shell Side Flow in SHELL-AND-TUBE HEAT EXCHANGER P M V SubbaraoProfessorMechanical Engineering DepartmentI I T DelhiAnother Peculiar Averaging Method..

Thermal Analysis for Shell-Side Conventional Methods are based on Non-dimensional Analysis Convection Heat Transfer demands Definition of Nusselt Number Output

Reynolds Number Input

Prandtl Number -- Input

Shell Side Fluid Flow

Shell-Side Reynolds Number Reynolds number for the shell-side is defined based on the equivalent diameter and the velocity based on a reference flow:

Simplified Classification of Shell Side Flow

Fluid dynamic Similarity of Counter & Cross Flow Heat Transfer ?!?!?!

Thermodynamic Similarity of Counter & Cross Flow Heat Transfer

Tube Layout & Flow StructureA Real Use of Wetted Perimeter !

Tube LayoutTube layout is characterized by the included angle between tubes.Two standard types of tube layouts are the square and the equilateral triangle.Triangular pitch (30o layout) is better for heat transfer and surface area per unit length (greatest tube density.)Square pitch (45 & 90 layouts) is needed for mechanical cleaning.Note that the 30,45 and 60 are staggered, and 90 is in line.For the identical tube pitch and flow rates, the tube layouts in decreasing order of shell-side heat transfer coefficient and pressure drop are: 30,45,60, 90.The 90 layout will have the lowest heat transfer coefficient and the lowest pressure drop.

The square pitch (90 or 45) is used when jet or mechanical cleaning is necessary on the shell side. In that case, a minimum cleaning lane of in. (6.35 mm) is provided.The square pitch is generally not used in the fixed header sheet design because cleaning is not feasible.The triangular pitch provides a more compact arrangement, usually resulting in smaller shell, and the strongest header sheet for a specified shell-side flow area.It is preferred when the operating pressure difference between the two fluids is large.

Tube Pitch The selection of tube pitch is a compromise between a Close pitch (small values of PT/do) for increased shell-side heat transfer and surface compactness, and anOpen pitch (large values of PT/ do) for decreased shell-side plugging and ease in shell-side cleaning.Tube pitch Pt is chosen so that the pitch ratio is 1.25 < PT/do < 1.5.When the tubes are to close to each other (PT/do less than 1.25), the header plate (tube sheet) becomes to weak for proper rolling of the tubes and cause leaky joints.Tube layout and tube locations are standardized for industrial heat exchangers.However, these are general rules of thumb and can be violated for custom heat exchanger designs.

Identification of (Pseudo) Velocity Scale

Shell Side Pseudo Flow AreaThe number of tubes at the centerline of the shell is calculated by where is Asthe bundle cross flow area, Dsis the inner diameter of the shell, C is the clearance between adjacent tubes, and B is the baffle spacing

Pseudo Shell side Mass VelocityThe shell-side mass velocity is found with

Selection of Shell Diameter

Shell Diameter The number of tubes is calculated by taking the shell circle and dividing it by the projected area of the tube layout. That is where Apro-tube is the projected area of the tube layout expressed as area corresponding to one tube, Ds is the shell inside diameter, and CTP is the tube count calculation constant that accounts for the incomplete coverage of the shell diameter by the tubes, due to necessary clearances between the shell and the outer tube circle and tube omissions due to tube pass lanes for multitude pass design.

Projected area of Tube LayoutWhere PT is the tube pitch and CL is the tube layout constant.

Coverage of Shell Area

The CTP values for different tube passes are given below:

Pseudo Shell side Mass VelocityThe shell-side mass velocity is found with

Shell side Equivalent (Hydraulic) DiameterEquivalent diameter employed by Kern for correlating shell side heat transfer/flow is not a true equivalent diameter.The direction of shell side flow is partly along the tube length and partly at right angles to tube length or heat exchanger axis.The flow area at right angles is harmonically varying.This cannot be distinguished based on tube layout.Kerns experimental study showed that flow area along the axis showed excellent correlation wrtTube layout, tube pitch etc.

Equivalent Counter Flow : Hydraulic or Equivalent DiameterThe equivalent diameter is calculated along (instead of across) the long axes of the shell and therefore is taken as four times the net flow area as layout on the tube sheet (for any pitch layout) divided by the wetted perimeter.

Free Flow Area for Square Layout:Free Flow Area for Triangular Layout:

Shell-Side Reynolds Number Reynolds number for the shell-side is based on the equivalent diameter and the velocity based on a reference flow:

Identification of (Pseudo) Velocity Scale

Pseudo Shell side Mass Velocity: Perpendicular FlowThe shell-side mass velocity is found with

Hydraulic or Equivalent Diameter : Axial FlowA Hydraulic radius based on cross flow cannot recognize the importance of tube layout.The equivalent diameter is calculated along (instead of across) the long axes of the shell and therefore is taken as four times the net flow area as layout on the tube sheet (for any pitch layout) divided by the wetted perimeter.

Free Flow Area for Square Layout:Free Flow Area for Triangular Layout:

Shell-Side Reynolds Number Reynolds number for the shell-side is based on the equivalent diameter (based on axial flow) and the velocity on the cross flow area at the diameter of the shell:

Justification for this peculiar Definition of ReIt is true that, this method of evaluating the hydraulic radius (based on axial flow) and pseudo velocity (based on perpendicular flow) does not account for relative percentage of cross flow and parallel flow.The proportions of normal flow and axial flow will be influenced by baffle spacing. Out of many possible definitions, this particular definition could generate an accurate correlation for h & f.

Correlation for Shell side Heat Transfer Coefficient

Overall Heat Transfer Coefficient for the Heat Exchanger The overall heat transfer coefficient for clean surface (Uc) is given by Considering the total fouling resistance, the heat transfer coefficient for fouled surface (Uf) can be calculated from the following expression:

Outlet Temperature Calculation and Length of the Heat Exchanger The outlet temperature for the fluid flowing through the tube is The surface area of the heat exchanger for the fouled condition is :

and for the clean condition where the LMTD is always for the counter flow. The over surface design (OS) can be calculated from :

The length of the heat exchanger is calculated by

Hydraulic Analysis for Tube-Side The pressure drop encountered by the fluid making Np passes through the heat exchanger is a multiple of the kinetic energy of the flow.

Therefore, the tube-side pressure drop is calculated by

The second term in above equation is the additional pressure drop introduced by the change of direction in the passes. The tube fluid experiences sudden expansions and contractions during a return that is accounted for allowing four velocity heads per pass.

Hydraulic Analysis for Shell-Side The shell-side fluid experiences a pressure drop as it passes through the exchanger, over the tubes, and around the baffles. If the shell fluid nozzles (inlet and outlet ports) are on the same side of the heat exchanger, then the shell-side fluid makes an even number of the tube bundle crossings, but if they are on opposite sides, then it makes an odd number of the bundle crossings. The number of bundle crossings therefore influences the pressure drop.

Where,Based on experiments, the pressure drop experienced by the shell-side fluid is calculated by

b is the viscosity of the shell-side fluid at bulk temperature, and w is the viscosity of the tube-side fluid at wall temperature.

The wall temperature can be calculated as follows:

Evaluation & Fine tuning of DesignInsufficient Thermal RatingInsufficient Pressure Drop Rating

Insufficient Thermal RatingIf the output of the rating analysis is not acceptable, a geometrical modification should be madeIf the required amount of heat cannot be transferred to satisfy specific outlet temperature, one should find a way to increase the heat transfer coefficient or increase exchanger surface areaOne can increase the tube side heat transfer coefficient by increasing the fluid velocity - Increase number of tube passesOne can increase the shell side heat transfer coefficient by decreasing baffle spacing and/or baffle cutOne can increase the surface area by Increasing the heat exchanger length Increasing the shell diameter Multiple shells in series

Insufficient Pressure Drop RatingIf the pressure drop on the tube side is greater than the allowable pressure drop, thenthe number of tube passes can be decreased orthe tube diameter can be increased which may result to decrease the tube length (Same surface area)increase the shell diameter and the number of tubesIf the shell side pressure drop is greater than the allowable pressure drop then baffle spacing, tube pitch, and baffle cut can be increased or one can change the baffle type.

THERE IS ALWAYS A TRADE-OFF BETWEEN THERMAL & PRESSURE DROP RATINGS!

The Trade-OffBetween Thermal Balance & Flow LossHeat transfer and fluid friction losses tend to compete with one another.The total energy loss can be minimized by adjusting the size of one irreversibility against the other .These adjustments can be made by properly selecting physical dimensions of the solid parts (fins, ducts, heat exchanger surface).It must be understood, however, that the result is at best a thermodynamic optimum.Constraints such as cost, size, and reliability enter into the determination of truly optimal designs.

Roadmap To Increase Heat TransferIncrease heat transfer coefficentTube SideIncrease number of tubesDecrease tube outside diameterShell SideDecrease the baffle spacingDecrease baffle cutIncrease surface areaIncrease tube lengthIncrease shell diameter increased number of tubesEmploy multiple shells in series or parallelIncrease LMTD correction factor and heat exchanger effectivenessUse counterflow configurationUse multiple shell configuration

Roadmap To Reduce Pressure DropTube sideDecrease number of tube passesIncrease tube diameterDecrease tube length and increase shell diameter and number of tubesShell sideIncrease the baffle cutIncrease the baffle spacingIncrease tube pitchUse double or triple segmental baffles