View source for Concentric tube heat exchanger

'''[[Concentric]] Tube (or Pipe) [[heat exchanger|Heat Exchangers]]''' are used in a variety of industries for purposes such as material processing, food preparation, and air-conditioning.<ref name="Naterer">{{cite book|author=Greg F. Naterer|title=Heat Transfer in Single and Multiphase Systems|publisher=CRC Press|year=2002|isbn=0-8493-1032-6}}</ref> They create a temperature driving force by passing [[fluid]] streams of different temperatures [[Parallel (geometry)|parallel]] to each other, separated by a physical [[Boundary (thermodynamic)|boundary]] in the form of a pipe. This induces [[forced convection]], [[Heat Transfer|transferring]] heat to/from the product.

==Theory ==

The [[thermodynamic]] behaviour of concentric tube heat exchangers can be described by both empirical and numerical analysis. The simplest of these involve the use of [[correlations]] to model heat transfer; however, the accuracy of these predictions varies depending on the design. For [[turbulent]], non-viscous fluids the [[Nusselt number#Dittus-Boelter equation|Dittus-Boelter Equation]] can be used to determine the [[heat transfer coefficient]] for both the inner and outer streams; given their diameters and velocities (or flow rates). For conditions where thermal properties vary significantly, such as for large temperature differences, the [[Nusselt number#Sieder-Tate correlation|Seider-Tate Correlation]] is used. This model takes into consideration the differences between bulk and wall viscosities. Both correlations utilize the [[Nusselt number]] and are only valid when the [[Reynolds number]] is greater than 10,000. While Dittus-Boelter requires the [[Prandtl number]] to be between 0.7 and 160, Seider-Tate applies to values between 0.7 and 16,700.

For calculations involving the outer stream, the equivalent diameter (or mean hydraulic radius) is used in place of the geometric diameter, as the cross-sectional area of the annulus is not circular. Equivalent diameters are also used for irregular shapes such as rectangular and triangular ducts. For concentric tubes, this relationship simplifies to the difference between the diameters of the shell and the outer surface of the inner tube.

[[File:Thermal Circuits.png|thumb|600px|center]]

<math>D_{\mathrm{eo}} =  \frac{4\cdot Area}{Wetted Perimeter}</math>

After the heat transfer coefficients (h_{i} and h_{o}) are determined, knowing the resistance due to fouling and [[thermal conductivity]] of the boundary material (k_{w}), the Overall Heat Transfer coefficient (U_{o}) can be calculated.

<math>{1\over U_{o}}  = {R_{fo}} + {R_{fi}}\cdot \frac{D_{o}}{D_{i}} + \frac{D_{o}}{2k_{w}}\cdot \ln {\frac{D_{o}}{D_{i}}} + {1\over h_{o}} + {1\over h_{i}}\cdot \frac{D_{o}}{D_{i}}</math>

The length of heat exchanger required can then be expressed as a function of the rate of heat transfer:

<math> A = \frac{Q}{U \Delta T} </math>

Where A is the surface area available for heat transfer and ∆T is the [[log mean temperature difference]].<ref>{{cite book|author=Barney L. Capehart|title=Encyclopedia of Energy Engineering and Technology|publisher=CRC Press|year=2007|isbn=978-0-8493-3653-9}}</ref> From these results, the [[NTU method]] can be performed to calculate the heat exchanger’s effectiveness.<ref name="Naterer"/>

<math>q_{max}\equiv C_{min} (T_{h,i}-T_{c,i})</math>

where

<math>E \equiv \frac{q}{q_{max}}</math>

==Concentric tube heat exchanger design==

The primary advantage of a concentric configuration, as opposed to a [[Plate heat exchanger|plate]] or [[shell and tube heat exchanger]], is the simplicity of their design. As such, the insides of both surfaces are easy to clean and maintain, making it ideal for fluids that cause [[fouling]]. Additionally, their robust build means that they can withstand high pressure operations.<ref name="Shah">{{cite book|author=Ramesh K. Shah|title=Heat Transfer Equipment Design|publisher=Taylor & Francis|year=1988|isbn=0-89116-729-3}}</ref> They also produce turbulent conditions at low flow rates, increasing the heat transfer coefficient, and hence the rate of heat transfer.<ref>{{cite book|author=J.M. Coulson and J. F. Richardson|title=Coulson & Richardson's Chemical Engineering: Fluid Flow, Heat Transfer and Mass Transfer|edition=Sixth|publisher=Butterworth Heinemann|year=1999|isbn=0-7506-4444-3}}</ref> There are significant disadvantages however, the two most noticeable being their high cost in proportion to heat transfer area; and the impractical lengths required for high heat duties. They also suffer from comparatively high heat losses via their large, outer shells.

The simplest form is composed of straight sections of tubing encased within the outer shell, however, alternatives such as corrugated or curved tubing conserve space while maximising heat transfer area per unit volume. They can be arranged in series or in parallel depending on the heating requirements.<ref name="Shah"/> Typically constructed from stainless steel, spacers are inserted to retain concentricity, while the tubes are sealed with O-rings, packing, or welded depending on the operating pressures.<ref name="Lewis & Heppell">{{cite book|author=Michael John Lewis and N. J. Heppell|title=Processing of Foods: Pasteurization and UHT Sterilization|publisher=Springer|year=2000|isbn=0-8342-1259-5}}</ref>

While both co and counter configurations are possible, the [[Countercurrent exchange|countercurrent]] method is more common. The preference is to pass the hot fluid through the inner tube to reduce heat losses, while the [[:wikt:annulus|annulus]] is reserved for the high [[viscosity]] stream to limit the pressure drop. Beyond double stream heat exchangers, designs involving triple (or more) streams are common; alternating between hot and cool streams, thus heating/cooling the product from both sides.<ref name="Lewis & Heppell"/>

==See also==
*[[Heat transfer]]
*[[Heat exchanger]]
*[[Shell and tube heat exchanger]]
*[[Plate heat exchanger|Plate and Frame Heat Exchanger]]
*[[NTU method]]

==References==
{{Reflist}}

==External links==
*[http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node131.html Thermodynamics of Heat Exchangers]

{{DEFAULTSORT:Concentric Tube Heat Exchanger}}
[[Category:Heat exchangers]]