Heat exchangers design considerations
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Heat Exchangers: Design Considerations. Parallel Flow. Counterflow. Heat Exchanger Types. Types. Heat exchangers are ubiquitous to energy conversion and utilization. They involve heat exchange between two fluids separated by a solid and encompass a wide range of flow configurations.

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Heat Exchangers: Design Considerations

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Heat exchangers design considerations

Heat Exchangers:Design Considerations


Types

Parallel Flow

Counterflow

Heat Exchanger Types

Types

Heat exchangers are ubiquitous to energy conversion and utilization. They involve heat exchange between two fluids separated by a solid and encompass a wide range of flow configurations.

  • Concentric-Tube Heat Exchangers

  • Simplest configuration.

  • Superior performance associated with counter flow.


Types cont

Finned-Both Fluids

Unmixed

Unfinned-One Fluid Mixed

the Other Unmixed

Types (cont.)

  • Cross-flow Heat Exchangers

  • For cross-flow over the tubes, fluid motion, and hence mixing, in the transverse

    direction (y) is prevented for the finned tubes, but occurs for the unfinned condition.

  • Heat exchanger performance is influenced by mixing.


Types cont1

One Shell Pass and One Tube Pass

Types (cont.)

  • Shell-and-Tube Heat Exchangers

  • Baffles are used to establish a cross-flow and to induce turbulent mixing of the

    shell-side fluid, both of which enhance convection.

  • The number of tube and shell passes may be varied, e.g.:

One Shell Pass,

Two Tube Passes

Two Shell Passes,

Four Tube Passes


Types cont2

Types (cont.)

  • Compact Heat Exchangers

  • Widely used to achieve large heat rates per unit volume, particularly when

    one or both fluids is a gas.

  • Characterized by large heat transfer surface areas per unit volume, small

    flow passages, and laminar flow.

(a) Fin-tube (flat tubes, continuous plate fins)

(b) Fin-tube (circular tubes, continuous plate fins)

(c) Fin-tube (circular tubes, circular fins)

(d) Plate-fin (single pass)

(e) Plate-fin (multipass)


Heat exchangers design considerations

Tubular Exchanger Manufacturers Association


Overall coefficient

Overall Heat Transfer Coefficient

Overall Coefficient

1 / U A = 1 / h1 A1 + dxw / k A + 1 / h2 A2

where

U = the overall heat transfer coefficient (W/m2K)

A = the contact area for each fluid side (m2)

k = the thermal conductivity of the material (W/mK)

h = the individual convection heat transfer coefficient for each fluid (W/m2K)

dxw = the wall thickness (m)


Overall coefficient1

Overall Coefficient

  • where

    • kw - Thermal Conductivity of Fluid

    • DH - Hydraulic Diameter

    • Nu - Nusselt Number

  • where:

  • ν : kinematic viscosity, ν = μ / ρ, (SI units : m2/s)

  • α : thermal diffusivity, α = k / (ρcp), (SI units : m2/s)

  • μ : dynamic viscosity, (SI units : Pa s)

  • k: thermal conductivity, (SI units : W/(m K) )

  • cp : specific heat, (SI units : J/(kg K) )

  • ρ : density, (SI units : kg/m3 ).

  • V : Fluid velocity (SI units m/s)

  • L : Pipe Internal Diameter (SI units m)

n = 0.4 for heating (wall hotter than the bulk fluid) and 0.33 for cooling (wall cooler than the bulk fluid) 


Thermal conductivity

Thermal Conductivity


Lmtd method

  • Counter-Flow Heat Exchanger:

Evaluation of depends on the heat exchanger type.

A Methodology for Heat Exchanger

Design Calculations

- The Log Mean Temperature Difference (LMTD) Method -

LMTD Method

  • A form of Newton’s Law of Cooling may be applied to heat exchangers by

  • using a log-mean value of the temperature difference between the two fluids:


Lmtd method cont

LMTD Method (cont.)

  • Parallel-Flow Heat Exchanger:

  • Note that Tc,ocan not exceed Th,ofor a PF HX, but can do so for a CF HX.

  • For equivalent values of UA and inlet temperatures,

  • Shell-and-Tube and Cross-Flow Heat Exchangers:


Energy balance

Overall Energy Balance

Energy Balance

  • Application to the hot (h) and cold (c) fluids:

  • Assume negligible heat transfer between the exchanger and its surroundings

  • and negligible potential and kinetic energy changes for each fluid.

  • Assuming no l/v phase change and constant specific heats,


Special conditions

  • Case (a): Ch>>Cc or h is a condensing vapor

  • Case (b): Cc>>Ch or c is an evaporating liquid

  • Negligible or no change in

  • Negligible or no change in

Special Operating Conditions

Special Conditions

  • Case (c): Ch=Cc.


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