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Design Analysis of Plate Heat Exchangers

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Design Analysis of Plate Heat Exchangers P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understadning of Highly Specialized Design Features – PowerPoint PPT presentation

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Title: Design Analysis of Plate Heat Exchangers


1
Design Analysis of Plate Heat Exchangers
  • P M V Subbarao
  • Professor
  • Mechanical Engineering Department
  • I I T Delhi

Understadning of Highly Specialized Design
Features
2
A Plate HX is said to be Optimally Sized, if the
thermal length required by the duty can match the
characteristic of the channel, by utilizing all
the available pressure drop with no
over-dimensioning, For any chosen channel
geometry.
Central Idea
3
Plates
Inlet / outlet Media 1
Inlet / outlet Media 2
Distribution area
Fully supported gasket groove
Heat transfer area
Distribution area
Inlet / outlet Media 2
Inlet / outlet Media 1
4
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5
Mean Channel Flow Gap
Flow channel is the conduit formed by two
adjacent plates between the gaskets. Despite the
complex flow area created by Chevron plates, the
mean flow channel gap b, can be identified as
where p is the plate pitch or the outside depth
of the corrugated plate and t is the plate
thickness, b is also the thickness of a fully
compressed gasket, as the plate corrugations are
in metallic contact. Plate pitch should not be
confused with the corrugation pitch.
6
Mean flow channel gap b is required for
calculation of the mass velocity and Reynolds
number and is therefore a very important value
that is usually not specified by the
manufacturer. If not known or for existing
units, the plate pitch can be determined from the
compressed plate pack (between the head plates)
,Lc which is usually specified on drawings. Then
p is determined as
where Nt is the total number of plates.
7
Channel Flow Area
One channel flow area is given by Ax
where Lw is the effective plate width.
The channel equivalent diameter De is given by
8
where
Then
9
Heat Transfer Coefficient
  • With plate heat exchangers, heat transfer is
    enhanced.
  • The heat transfer enhancement will strongly
    depend on the Chevron inclination angle b,
    relative to flow direction,
  • Both the heat transfer and the friction factor
    increase with b.
  • On the other hand, the performance of a Chevron
    plate will also depend upon the surface
    enlargement factor f, corrugation profile, gap b.
  • In spite of extensive research on plate heat
    exchangers, generalized correlations for heat
    transfer and friction factor are not available.

10
Flow Reynolds Numbers
  • The transition to turbulence occurs at low
    Reynolds numbers and, as a result, the
    gasketed-plate heat exchangers give high heat
    transfer coefficients.
  • The Reynolds number, Re, based on channel mass
    velocity and the equivalent diameter,De , of the
    channel is defined as

where Ncp is the number of channel per pass and
is obtained from
where Nt is the total number of plates and Np is
the number of passes.
11
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12
EFFECTIVE TEMPERATURE DIFFERENCE
13
Discussion on Plate Hx
  • Depending on b and Reynolds number, Chevron
    plates produce up to five times higher Nusselt
    numbers than those in flat-plate channels.
  • The corresponding pressure drop penalty, however,
    is considerably higher Depending on the Reynolds
    number, from 1.3 to 44 times higher friction
    factors than those in an equivalent flat-plate
    channel.

14
Guide Lines for Thermal performance/Design
Calculations
  • Design of PHE requires considerable skill and
    experience to produce the optimum.
  • A plate having a high Chevron angle provides high
    heat transfer combined with high pressure drop.
  • These plates are long duty or hard plates.
  • Long and narrow plates belong to this category.
  • On the other hand, a plate having a low Chevron
    angle provides the opposite features, i.e. low
    heat transfer combined with low pressure drop.
  • These plates are short duty or soft plates.
  • Short and wide plates are of this type.
  • A low Chevron angle is around 25º - 30º, while a
    high Chevron angle is around 60º - 65º.
  • Theta is used by manufacturers to denote the
    number of heat transfer units.

15
Engineering Creations THERMAL MIXING
  • A pack of plates may be composed of all
    high-theta plates (b 30º for example), or all
    low-theta plates (b 60º for example).
  • Alternately high- theta and low-theta plates
    (MIXED) may be arranged in the pack to provide an
    intermediate level of performance.
  • Thus two plate configurations provide three
    levels of performance.
  • A further variation is available to the thermal
    design engineer.
  • Parallel groups of two channel types, either
    (high mixed) theta plates or (low mixed)
    theta plates, are assembled together in the same
    pack in the proportions required to achieve the
    optimum design.
  • Thermal mixing provides the thermal design
    engineer with a better opportunity to utilize the
    available pressure drop, without excessive over
    surface, and with fewer standard plate patterns.

16
Plate Mixing
17
Asymmetrical plates
18
Only Driving force in heat Exchanger is MTD/LMTD
so far !!!
Can we get the help any other Driving Force?
19
Windcatcher (Bagdir)
20
Direct Contact Heat Exchangers
  • P M V Subbarao
  • Professor
  • Mechanical Engineering Department
  • I I T Delhi

Mediation through loss of Matter
21
Gas Liquid Contactors
22
Steam Power Plant
23
Historical Development of Cooling Towers
24
Natural Draft Cooling Tower Counter Flow
25
Structure of Cooling Tower
Makeup water
26
Anatomy of the cooling tower
  • The cooling tower is divided into three major
    zones namely,
  • The rain zone
  • The fill packing zone
  • The hot water distribution and pipes constituting
    the spray zone.
  • All these zones abet in meeting the demand of the
    cooling tower and can be termed as the supply
    parameters of the cooling tower.

27
Theory of Cooling Towers
  • A cooling tower cools the incoming water by a
    combination of heat and mass transfer.
  • Warm water supplied to the tower is sprayed or
    splashed over fill, which breaks up the water and
    exposes a very large surface area of the water to
    the air.
  • In a typical power plant cooling tower, the air
    flows upward through the tower counter to the
    water flow direction, either due to convection
    (natural draft tower) or to cooling tower fans
    (mechanical draft tower).
  • A portion of the water is evaporated into the
    air, with the necessary latent heat being
    transferred from the remaining water, thus
    lowering its temperature.

28
  • There is also some sensible heat transfer from
    the water to the air.
  • The driving force for this heat and mass transfer
    process is the difference between the entering
    wet bulb temperature of the air and the
    temperature of the water.

29
Specifications of A General Tower
Sl. No    
1 Water flow 6944 kg/sec
2 Atmospheric pressure 96726.6 Pa
3 Height of the tower 117 m
4 Height of fill zone 7.5 m
5 Height of measuring plane 9 m
6 Diameter of the rain zone 78.9 m
7 Diameter of the fill zone 77.744 m
8 Height of the throat of the tower 89.12 m
9 Diameter of the throat 46.7 m
10 Diameter of the top of the tower 49.7 m
11 Hot water temperature 43 degree C
12 Dry bulb temperature 36 degree C
13 Wet bulb temperature 30 degree C
14 Loss coefficient in spray referred to fill, Kspfill 0.794
15 Loss coefficient at tower supports referred to fill, Ktsfill 4.104
16 Loss coefficient at cooling tower inlet referred to fill, Kctfill 9.961
17 Loss coefficient at fill supports referred to fill, Kfsfill 7.37
18 Loss coefficient in water distribution system referred to fill, Kwdfill 0.514
19 Viscous resistance of fill 22370 m-2
20 Inertial resistance of fill 8 m-1
30
Natural Draft Cooling Tower Cross Flow
31
Mechanical Draught Cooling Towers
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