Design Analysis of Plate Heat Exchangers

P M V SubbaraoProfessor

Mechanical Engineering DepartmentI I T Delhi

Understadning of Highly Specialized Design Features……

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.

LMTDTT outin

Central Idea

PlatesPlates

Distribution area

Inlet / outlet Media 1

Heat transfer area

Distribution area

Inlet / outlet Media 1Inlet / outlet Media 2

Inlet / outlet Media 2

Fully supported gasket groove

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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.

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.

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:

where

Then

Heat Transfer Coefficient

• With plate heat exchangers, heat transfer is enhanced. • The heat transfer enhancement will strongly depend on the

Chevron inclination angle , relative to flow direction, • Both the heat transfer and the friction factor increase with

. • On the other hand, the performance of a Chevron plate will

also depend upon the surface enlargement factor , corrugation profile, gap b.

• In spite of extensive research on plate heat exchangers, generalized correlations for heat transfer and friction factor are not available.

•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

Flow Reynolds Numbers

where Nt is the total number of plates and Np is the number of passes.

EFFECTIVE TEMPERATURE DIFFERENCE

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.

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.

Engineering Creations : THERMAL MIXING

• A pack of plates may be composed of all high-theta plates ( = 30º for example), or all low-theta plates ( = 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.

Plate Mixing

Asymmetrical plates

Only Driving force in heat Exchanger is MTD/LMTD so far !!!

Can we get the help any other Driving Force?

Windcatcher (Bagdir)

Direct Contact Heat Exchangers

P M V SubbaraoProfessor

Mechanical Engineering DepartmentI I T Delhi

Mediation through loss of Matter ……

Gas –Liquid Contactors

Spray Column Packed Column

Steam Power Plant

Historical Development of Cooling Towers

Natural Draft Cooling Tower : Counter Flow

Structure of Cooling Tower

Makeup water

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.

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.

• 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.

Specifications of A General TowerSl. 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

Natural Draft Cooling Tower : Cross Flow

Mechanical Draught Cooling Towers