Development in Double Pipe HEAT EXCHANGER for Concurrence & Better Economy! More New Geometric Ideas : Just for Economy !!! P M V Subbarao Professor Mechanical Engineering Department I I T Delhi

Double Pipe Heat Exchanger A double pipe heat exchanger is one of the simplest form of Heat Exchangers. The wall of the inner pipe is the heat transfer surface. The major use of these HX is sensible cooling or heating applications. But Very long, even for moderate capacities. Unviable to accommodate in an industrial space. To make a Unit Isotropically Compact, the arrangement is made in Multiple Times and Continuous Serial and Parallel flow.

Hairpin Heat Exchanger The inner tube is connected by U – shaped return bend enclosed in a return bend housing

Hairpin Heat Exchangers in series

Well preferred for heat transfer areas upto 50 m 2

 NTU Curves: Counter flow NTU 

Ideas for Thermodynamic Betterment U-tube Annular in series & tubular in parallel Annular in parallel & tubular in series

More Innovative Configurations of DTHXs

Annular flow in series & Tubular Flow in Parallel Tubular stream mass flow is equally split between the two units. Counter Flow HX-

Annular flow in series & Tubular Flow in Parallel

Flow Parameters Annular flow: Hot Fluid & Tubular flow : Cold Fluid. ParameterHX 1HX 2 Hot fluid inlet temperatureT h,in1 T h,in2 Hot fluid outlet temperatureT h,ex1 T h,ex2 Cold fluid inlet temperatureT c,in1 T c,in2 Cold fluid outlet temperatureT c,ex1 T c,ex2 Hot fluid flow rate Cold fluid flow rate Surface areaA/2

Analysis of HX1

Analysis of HX2

Analysis of Global ASTP HX

New Dimensionless Parameters For same value of U in both the HXs Cold (Tubular) stream in parallel: Hot (annular) stream in series:

For simple counter flow heat exchanger: For HX1 of ASTP: For HX2 of ASTP:

Mean Temperature of Global ASTP

For two level ASTP For n level ASTP

Two level Annular flow in series & Tubular Flow in Parallel

n level Annular flow in series & Tubular Flow in Parallel

For simple counter flow heat exchanger: For n level ASTP

F P avg n

Comparison

 NTU Curves: Counter Vs parallel flow

Need for Compact HXs Double Pipe Hxs are long, even for moderate capacities. Unviable to accommodate in an industrial space. The size of heat exchanger is very large in those applications where gas is a medium of heat exchange. Continuous research is focused on development of Compact Heat Exchangers --- High rates of heat transfer per unit volume. The rate of heat exchange is proportional to –The value of Overall heat transfer coefficient. –The surface area of heat transfer available. –The mean temperature difference.

Large surface area Heat Exchangers The use of extended surfaces will reduce the gas side thermal resistance. To reduce size and weight of heat exchangers, many compact heat exchangers with various fin patterns were developed to reduce the air side thermal resistance. Fins on the outside the tube may be categorized as –1) flat or continuous (plain, wavy or interrupted) external fins on arrays of tubes, – 2) Normal fins on individual tubes, –3) Longitudinal fins on individual tubes.

Innovative Designs for Extended Surfaces

Geometrical Classification Longitudinal or strip RadialPins

Anatomy of A STRIP FIN thickness x xx Flow Direction

profile PROFILE AREA cross-section CROSS-SECTION AREA Basic Geometric Features of Longitudinal Extended Surfaces

Complex Geometry in Nature An optimum body size is essential for the ability to regulate body temperature by blood-borne heat exchange. For animals in air, this optimum size is a little over 5 kg. For animals living in water, the optimum size is much larger, on the order of 100 kg or so. This may explain why large reptiles today are largely aquatic and terrestrial reptiles are smaller.

Straight fin of triangular profile rectangular C.S. Straight fin of parabolic profile rectangular C.S. L x=0 b x=b x qbqb L b qbqb x=0 x Longitudinal Extended Surfaces with Variable C.S.A

For a constant cross section area:

Most Practicable Boundary Condition Corrected adiabatic tip: thickness x xx b b

Rate of Heat Transfer through a constant Area Fin Fin Efficiency:

How to decide the height of fin for a Double Pipe HX ?

LONGITUDINAL FIN OF CONCAVE PARABOLIC PROFILE The differential equation for temperature excess is an Euler equation: L b qbqb x=bx=a=0 x

The particular solution for temperature excess is: And the heat dissipation (L=1) is: Efficiency:

Gardner’s curves for the fin efficiency of several types of longitudinal fins. Longitudinal Fins

n th order Longitudinal Fins

Helical Double-tube HX

Secondary Flow in Helical Coils The form of the secondary flow would depend on the ratio of the tube diameters and other factors. A representative secondary flow pattern is shown below: Thirdly, this configuration should lead to a more standard approach for characterizing the heat transfer in the exchanger. The ratio of the two tube diameters may be one of the ways to characterize the heat transfer.

Heat Transfer in Helical Tubes Acharya et al. (1992, 2001) developed the following two correlations of the Nusselt number, for Prandtl numbers less than and greater than one, respectively.

Heat Transfer in Helical Annulus  Nusselt numbers for the annulus have been calculated and correlated to a modified Dean number.  The modified dean number for the annulus is calculated as it would be for a normal Dean number, except that the curvature ratio used is based on the ratio of the radius of the outer tube to the radius of curvature of the outer tube, and the Reynolds number based on the hydraulic radius of the annulus.  Thus the modified Dean number is:

Helical Coils: Laminar flow De is Dean Number. De=Re (a/R) 1/2. Srinivasan et al. (7 < R/a < 104): Manlapaz and Churchill: Correction for vp:

Helical coils: turbulent flow

Expenditure in A Heat Exchanger The capital investment on Heat exchanger material is proportional to double the Heat transfer Area. Investment on both cold side and hot side of a heat exchanger for a given surface area of heat exchanger. Another expenditure is running cost or operational cost. Main operation cost is pumping power cost. This again increases the size of the pump and capital cost. This arises a question of inner and outer flow pressure drop calculations and a suitable innovation for the same.

Idea 1: Multi-tube Heat Exchanger An exclusive continuous multi tube exchanger is used in laundry, textile, or paper mill applications. Using "stacked" design the unit can be expanded as required by the addition of more sections. Design is based on pure counter flow of fluids for most efficient heat transfer. Temperature approaches as close as 3°C can be economically achieved for certain applications.

Idea 2: Multi Pass Heat Exchanger

Heat lost by hot fluid:

Temperature Variations in Multi Pass HX.

1-2 Shell with Better Flow COnfiguration

TEMA – E : One parallel & Two counter Tube Flows

TEMA – E : Two parallel & Two counter Tube Flows

TEMA – E : One parallel & One counter Tube Flows : Devided Shell Flow

TEMA 1-2 G Shell & Tube

TEMA 1-2 H Shell & Tube

TEMA 1-2 J Shell & Tube

TEMA 1-4 J Shell & Tube

Multiple Shell-Side Passes In an attempt to offset the disadvantage of values of F less than 1.0 resulting from the multiple tube side passes, some manufacturers regularly design shell and tube exchangers with longitudinal shell-side baffles. The two streams are always countercurrent to one another, therefore superficially giving F = 1.0.

Multiple Shells in Series

Double Pipe HEAT EXCHANGERS with Low Thermal Resistance P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Ideas for Better Heat Transfer!!!

Enhanced Heat Transfer…..

Double Pipe HX with finned inner Tube Equivalent diameter of annulus heat transfer, D e :

Helical Double-tube HX

Secondary Flow in Helical Coils The form of the secondary flow would depend on the ratio of the tube diameters and other factors. A representative secondary flow pattern is shown below: Thirdly, this configuration should lead to a more standard approach for characterizing the heat transfer in the exchanger. The ratio of the two tube diameters may be one of the ways to characterize the heat transfer.

Heat Transfer in Helical Tubes Acharya et al. (1992, 2001) developed the following two correlations of the Nusselt number, for Prandtl numbers less than and greater than one, respectively.

Heat Transfer in Helical Annulus  Nusselt numbers for the annulus have been calculated and correlated to a modified Dean number.  The modified dean number for the annulus is calculated as it would be for a normal Dean number, except that the curvature ratio used is based on the ratio of the radius of the outer tube to the radius of curvature of the outer tube, and the Reynolds number based on the hydraulic radius of the annulus.  Thus the modified Dean number is:

Helical Coils: Laminar flow De is Dean Number. De=Re (a/R) 1/2. Srinivasan et al. (7 < R/a < 104): Manlapaz and Churchill: Correction for vp:

Helical coils: turbulent flow