Boiling / Condensation Heat Transfer - جامعة نزوى€¦ · · 2015-04-082015-08-04 ·...
Transcript of Boiling / Condensation Heat Transfer - جامعة نزوى€¦ · · 2015-04-082015-08-04 ·...
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• A boiling fluid consists of a two-phase mixture of vapor and liquid. When
such a fluid flows through a tube, a number of distinct flow regimes can
occur depending on the flow rate and the relative amounts of vapor and
liquid present. Two-phase flow is thus more complex than single-phase
flow, and special methods are needed to calculate the pressure drop in
equipment handling boiling fluids.
• Condensation is the reverse of boiling, and the condensing curve is the
same as the boiling curve. Thus, many of the computational difficulties
encountered in the analysis of reboilers are present in condensers as well.
For wide-boiling mixtures, in particular, the nonlinearity of the condensing
curve and the variation of liquid and vapor properties over the condensing
range mean that a zone or incremental analysis is required for rigorous
calculations.
• In reboilers and vaporizers, boiling usually takes place either on the
exterior surface of submerged tubes or on the interior surface as the fluid
flows through the tubes. In the former case the tubes are oriented
horizontally, while in the latter they may be either horizontal or vertical
Boiling / Condensation Heat Transfer
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Condensation on Vertical Tubes
• The average heat-transfer coefficient, h, for the condensate
film can now be obtained by integrating the local coefficient
over the length, L, of the wall:
• For tube diameters of practical interest, the effect of wall
curvature on the condensate film thickness is negligible.
Hence, above Equation can be used for condensation on either
the internal or external surfaces of vertical heat-exchanger
tubes. The alternate form, below Equation is also applicable
with the appropriate modification in equivalent diameter.
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Determine laminar/turbulant conditions
For a bank of nt tubes, the wetted perimeter is ntπD, where D is either the inner
or outer tube diameter, depending on whether condensation occurs inside or
outside the tubes.
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For vertical tubes, A / P = L , whereas, for horizontal tubes A/P = 3.14 D
Condensation on Horizontal Tubes
• The Nusselt analysis for condensation on the external surface
of a horizontal tube is similar to that for a vertical surface. The
result corresponding to:
The alternate form is:
Where:
Above equations apply to a single tube or a single row of tubes.
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Turbulent/Laminar !!
In trying to calculate Re we found it is dependent on the mass flow
of the condensate but this depends on the heat transfer coefficient,
which is depending on Re. To solve the problem we assume either
laminar or turbulent flow. Then calculate the heat transfer
coefficient and check the Re number to confirm whether the
assumption was correct
Let us assume laminar film condensation.
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Example: Condensation on tube bank
One hundred tubes of 0.5 in (1.27 cm) diameter are
arranged in a square array and exposed to atm steam.
Calculate the mass of steam condensed per unit length of
tubes for a tube wall temperature of 98C
Solution
The condensate properties are obtained from the previous
example, and replacing d by nd,
Where n =10
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Heat-Transfer Coefficients for Pure Component Nucleate
Boiling in tube bundles
• Heat-transfer coefficients for boiling on tube bundles are generally higher
than for boiling on single tubes under the same conditions. Palen [4]
presented an approximate method for calculating convective effects. The
average boiling heat transfer coefficient, hb, is expressed as follows:
where hnc is a heat-transfer coefficient for liquid-phase natural convection and Fb can be
calcualted using:
The coefficient, hnc, can be estimated using equation:
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Bubbly flow: At low vapor fractions, vapor bubbles are
dispersed in a continuous liquid phase.
Slug flow: At moderate vapor fractions and relatively low
flow rates, large bullet-shaped vapor bubbles flow through
the tube separated by slugs of liquid in which smaller
bubbles may be dispersed. A percolating coffee pot
exemplifies this type of flow.
Churn flow: At higher flow rates, the large vapor bubbles
present in slug flow become unstable and break apart,
resulting in an oscillatory, or churning, motion of the liquid
upward and downward in the tube.
Annular flow: At high vapor fractions and high flow rates,
the liquid flows as a film along the tube wall while the vapor
flows at a higher velocity in the central region of the tube.
This condition, which is characteristic of flows with a high
mass flux, is referred to as wispy annular
flow.
Mist Flow: At very high vapor fractions, the liquid phase
exists entirely as droplets entrained in a continuous vapor
phase.
Two phase flow regimes
where ΔTx is the temperature difference between the surface and
saturated liquid in Celsius and p is the pressure in Mega pascals. The
heat-transfer coefficient has the unit of watts per square meter per
Celsius. This equation is valid for a pressure range of 5-170 atm
Convective Boiling heat transfer Coefficient
(Vertical Tubes)
Forced-convection inside vertical tubes can be found using the
following two methods
First Method
ℎ = 2.54(∆𝑇𝑥)3𝑒𝑝/1.551
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Convective Boiling heat transfer Coefficient
(Vertical Tubes)
• Chen Correlation
and
Note that the Reynolds number in this equation is calculated using the
flow rate of the liquid phase alone.
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A reboiler is a heat exchanger that is used to generate the vapor
supplied to the bottom tray of a distillation column. The liquid from
the bottom of the column is partially vaporized in the exchanger,
which is usually of the shell-and-tube type. The heating medium is
most often condensing steam, but commercial heat-transfer fluids and
other process streams are also used. Boiling takes place either in the
tubes or in the shell, depending on the type of reboiler. Exchangers that
supply vapor for other unit operations are referred to
as vaporizers, but are similar in most respects to reboilers.
Thermal and hydraulic analyses of reboilers are generally more complex than
for single-phase exchangers. Some of the complicating factors are the following:
• Distillation bottom liquids are often mixtures having substantial boiling
ranges. Hence, the physical properties of the liquid and vapor fractions can
exhibit large variations throughout the reboiler. Thermodynamic
calculations are required to determine the phase compositions and other
properties within the reboiler.
• A zone or incremental analysis is generally required for rigorous
calculations.
• Two-phase flow occurs in the boiling section of the reboiler and, in the case
of thermosyphon units, in the return line to the distillation column.
• For recirculating thermosyphon reboilers, the circulation rate is determined
by the hydraulics in both the reboiler and the piping connecting the
distillation column and reboiler. Hence, the reboiler and connecting piping
must be considered as a unit. The hydraulic circuit adds another iterative
loop to the design procedure.