Plate and Frame Heat Exchanger
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Transcript of Plate and Frame Heat Exchanger
PLATE AND FRAME HEAT EXCHANGER
engineering-resource.com
PRESENTED BY
• HAFEERA SHABBIR 06-CHEM-19
• MUBASHRA LATIF 06-CHEM-23
• PAKEEZA TARIQ MEER 06-CHEM-65
• MAHPARA MUGHAL 06-CHEM-69
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OUTLINE
• Introduction• Construction• Principle of Operation• Applications• Advantages• Limitations of Operation• Comparison of with STH• Design steps with Solved example
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Introduction
• It is a type of compact heat exchanger
• A plate heat exchanger is a type of heat exchanger that uses metal plates to transfer heat between two fluids
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CONSTRUCTION
• Based on their construction plate and frame heat exchangers are classified into
• (a) Gasketed–plate
• (b) Welded-plate
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GASKETED-PLATE HEAT EXCHANGER(GPHE)
• Parallel corrugated plates clamped in a frame with each plate sealed by gaskets and with four corners ports, one pair for each of the two fluids.
• The fluids are at all times separated by 2 gaskets, each open to the atmosphere. Gasket failure cannot result in fluid intermixing but merely in leakage to atmosphere, hence a protective cover is there.
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Construction of GPHE
• Plates
• Gaskets
• Plate frame
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PLATES
• Plate thickness is 0.4 to 0.8 mm
• Channel lengths are 2-3 meters
• Plates are available in: Stainless Steel, Titanium, Titanium-Palladium, Nickel
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PLATES PATTERNS1)Induce turbulence for high HT coefficient2)Reinforcement and plate support points that
maintains inter-plate separation.
TYPES OF PATTERNS
• Mainly 2 types of patterns (corrugations) are used
1)Intermating or washboard corrugations2)Chevron or herringbone corrugations
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CHEVRON OR HERRINGBONE
• Most common type• Corrugations are pressed to same depth as
plate spacing• Operate at High pressure• Corrugation depth 3mm to 5mm• Velocity 0.1 to 1 m/s
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CHEVRON CORRUGATIONS
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INTERMATING TROUGH PATTERNS
• Pressed deeper than spacing• Fewer connection points• Operate at Lower pressure• Max channel gap 3mm to 5mm• Min channel gap 1.5 mm to 3 mm• Velocity range in turbulent region is 0.2 to 3 m/s
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DIMPLE CORRUGATIONS
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GASKETS
• They limit the maximum operating temperature for a plate heat exchanger. Material selection depends upon
1)Chemical resistance
2)Temperature resistance
3)Sealing properties
4)Shape over an acceptable period of time
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GASKET MATERIALS
• Typical gasket materials are
Natural rubber styrene
Resin cured butyl
Compressed asbestos fiber gaskets
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FRAMES
• Materials
1)Carbon steel with a synthetic resin finish
2)stainless steel
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WELDED PLATE HEAT EXCHAGERS(WPHE)
• Developed to overcome the limitations of the gasket in GPHE
• Inabilty of heat transfer area inspection and mechanical cleaning of that surface
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OPERATION
• Channels are formed between the plates and corner ports are arranged so that the two media flow through alternate channels.
• The heat is transferred through the thin plate between the channels, and complete counter current flow is created for highest possible efficiency. No intermixing of the media or leakage to the surroundings will take place as gaskets around the edges of the plates seal the unit.
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APPLICATIONS
3 major applications
• (1)liquid-liquid services
• (2)condensing and evaporative
• (3)Central cooling
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LIQUID-LIQUID SERVICES
• It is well-suited to liquid/liquid duties in turbulent flow, i.e. a fluid sufficiently viscous to produce laminar flow in a smooth surface heat exchanger may well be in turbulent flow in PHE.
• It has major applications in the food industry.
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CONDENSATION AND VAPORIZATION
• Condensation of vapor (including steam) at moderate pressure, say 6 to 60 Psi, is also economically handled by PHE’s, but duties involving large volumes of very low pressure gas or vapor are better suited to other forms of heat exchangers
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CENTRAL COOLING
• It is the cooling of a closed circuit of fresh non-corrosive and non-fouling water for use inside a plant, by means of brackish water. Central coolers are made of titanium, to withstand the brackish water
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ADVANTAGES
• Compactness• Flexibility• Very high heat transfer coefficients on
both sides of the exchanger• Close approach temperatures and fully
counter-current flow• Ease of maintenance. Heat transfer area
can be added or subtracted with out complete dismantling the equipment
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CONTD…..
• Ease of inspection on both sides• Ease of cleaning• Savings in required flow area• Low hold-up volume• Low cost • No Local over heating and possibility of
stagnant zones is also reduced• Fouling tendency is less
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LIMITATIONS
• Low Pressure
upto 300 psi
• Low temperature
upto 300 F
• Limited capacity
• Limited plate size
0.02 sq.m to 1.5 sq.m
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• Large difference b/w flow rates cant be handled
• High pressure drop
• Potential for leakage
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COMPARISON BETWEEN PHE AND STHE
FEATURES•Multiple duty•Hold up volume•Gaskets
•modifications
PHE
Possible
Low
On each plate
Easy by adding or removing plates
STHE
Impossible
High
On flanged joints
impossible
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FEATURES PHE STHE•Repair
•Detection of leakage•Access for inspection•Time reqd. for opening•Fouling
Easy to replace plates and gaskets
Easy to detect
On each side of plate
15 min
15 to 20 % of STHE
Requires tube plugging
Difficult to detect
Limited
60 to 90 min
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FEATURES PHE STHE
Sensitivity to vibrations
Not sensitive sensitive
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DESIGN STEPS WITH SOLVED EXAMPLE
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STATEMENT OF PROBLEM
• A plate heat exchanger was use to preheat 4 kg/s of dowtherm from 10 to 70◦C with a hot water condensate that was cooled from 95 to 60◦C.Determine the number of plates required for a single-pass counter flow plate and frame exchanger. Assume that each mild stainless-steel plate [kw=45j/s.m.K]has a length of 1.0m and a width of 0.25m with a spacing between the plates of 0.005m.Also,estimate the pressure drop of the hot water stream as it flows through the exchanger.
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DATA REQUIRED
• The performance characteristics for the chevron configuration selected for the plates are shown . For
• Re > 100,Nu and f can be represented by the following relationships:
• Nu = 0.4 Re0.64Pr0.4
• f = 2.78Re-0.18
• :
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ASSUMPTIONS
• The plate heat exchanger operates under steady state conditions.• No phase change occurs: both fluids are single phase
and are unmixed. • Heat losses are negligible; the exchanger shell is adiabatic.• The temperature in the fluid streams is uniform over the flow cross section.• There is no thermal energy source or sink in the heat exchanger.• The fluids have constant specific heats.• The fouling resistance is negligible.
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Properties of each fluid at the mean temperature in the exchanger are
property Dowtherm at 40◦C Water at 77◦C
Heat capacity CP 1.622*103 J/kg.K4.198*103J/kg.K
Thermal conductivity k 0.138 J/.m.K 0.668J/s.m.K
Viscosity µ 2.70*10-3Pa.s 3.72*10-4Pa.s
Density ρ 1.044*102kg/m3 9.74*102kg/m3
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SOLUTION
• APPROACH TO THE PROBLEM:• To avoid an iterative calculation because of the
interdependency between the heat transfer area and the total flow area, use the NTU approach to determine the NTUmin required, noting that NTUmin=UA/(mCp)min.the area of the plate and frame exchanger can be calculated once the overall heat transfer coefficient has been evaluated.
•
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CALCULATION OF HT AREA
• For a single pass configuration with Np plates and NP+1 flow passages ,solution of the problem can be simplified mathematically by assuming n flow passages and n-1 plates ,since flow velocities involve flow passages and not plates. with this modification, the heat transfer surface area of the exchanger in terms of n is
• A=(n-1)LW=(n-1)(1)(0.25)=0.25(n-1)m2
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CALCULATION OF FLOW AREA
• The flow area for each stream with n/2flow passages is given by:
S=n/2(W)(b)
=n/2(0.25)(0.005)
=(6.25*10-4)n.
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CALCULATION OF HEAT DUTYAND FLOW RATES
TOTAL RATE OF HEAT TRANSFER: FOR DOWTHERM q= (mCpΛT)c
=4(1.622*103)(70-10) =3.89*105W THE MASS FLOW RATE OF WATER : mh=q/(CPΛT)h
=3.89*105/(4.198*103)(95-60) =2.65 Kg/s VELOCITY OF WATER: Vh =mh /ρhS =2.65/(9.74*102)(6.25*10-4)n =(4.35/n)m/s
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• EQUIVALENT DIAMETER:
De=2b
=0.01m
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CALCULATION OF HOT SIDE HT COEFFICIENT
• REYNOLD NUMBER: Reh=DeVhρh /µh
=0.01(4.35/n)(9.74*102)/(3.72*10-4) =1.139*105/nThis indicates that Reynold number is greater than 100 and correlation
for Nu can be used. Pr NUMBER: Prh = Cpµ/k = (4.198*103)(3.72*10-4)/0.668 =2.34• hh = (0.4)(kh/De)Re0.64Pr0.4
=[0.668/0.01][1.139*105/n]0.64(2.34)0.4
=6.467*104/n0.64W/m2.K
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CALCULATION OF COLD SIDE HT COEFFICIENT
The same calculations are repeated for cold stream. V=mc/ρc S
=4.0/(1.044*103)(6.25*10-4)n =6.13/n Re=DeVcρc/µc
=0.01(6.13/n)(1.044*103)/(2.70*10-3) =2.37*104/n Prc=(1.622*103)(2.70*10-3)/(0.138) =31.73 This also indicates that Re>100 hc=(0.4)(kc/De)Re0.64Pr0.4
=(0.4)(0.138/0.01)(237*104/n)0.64 (31.73)0.4
=1.388*104 /n0.64 W/m2.K
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CALCULATION OF OVERALL HT COEFFICIENT
• The overall heat transfer coefficient can now be determined in terms of n. Since the surface areas on either side of the plate are the same, no correction for area is required.
• Assume a thickness of the plate xw of 0.0032m
1/U=1/hh+xw/kw+1/hc
=n0.64/(6.467*104)+(0.0032)/(45)+n0.64/(1.388*104)
=8.751*10-5n0.647+7.11*10-5 m. K/W
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USING THE NTU METHOD
• A NTUmin for cold stream with a minimum mcp is defined
NTUmin=UA/(Mcp)min
=Tc,out–Tc,in/ fΛT◦,log mean
LOG MEAN TEMPERATURE DIFF:
ΛT◦,log mean: =(Th,in-Tc,out)-(Th,out–Tc,in)/ln[(Th,in-Tc,out)/Th,out-Tc,in)]
=(95-70)-(60-10)/ln[(95-70)/(60-10)] =36.067 K.
• For a single pass counter flow plate and frame heat exchanger ,F=1.
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• NTU =70-10/36.067
=1.664
• To satisfy the other NTU definition of UA/(Mc) in terms of results in the relation
• 1
8.751*105n0.64+7.11*10-5 =1.664 ( )(
0.25(n-1)
4.0(1.622*103)
)
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ITERATIVE METHOD
• This equation can be solved with itreration to indicate that n=51.Thus 50 plates are required to meet to the heat transfer needs to preheat 4kg/s of dowtherm from 10 to 70◦C.
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HYDRAULIC DESIGN
• PRESSURE DROP IN WATER STREAM:
• Vh =4.35/51=0.0853m/s
• Reh=1.139*105/51=2233
• Since Re>100
• f =2.78Re-0.18
• =2.78(2233)-0.18
• =0.694
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CONTD..
• Neglecting friction due to entrance and exit losses as well as temperature effects on the viscosity between the wall and the bulk fluids.
• So pressure drop is calculated from the following equation:
• ΛP=4f(L/De)ρh Vh2 /2
• =4(0.694)(1/0.01) (9.74*102)(0.0853)2/2• =984N/m2
• =984Pa
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CONCLUSION
• Since the entrance and exit losses will be small, the pressure drop per plate
• is small, and a new configuration with modified dimensions should be considered.
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