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HEAT EXCHANGERS

Introduction

Heat exchangers are equipment that transfer heat from one medium to another. The properdesign, operation and maintenance of heat exchangers will make the process energyefficientand minimize energy losses. Heat exchanger performance can deteriorate with time,off design operations and other interferences such as fouling, scaling etc. It is necessary toassess periodically the heat exchanger performance in order to maintain them at a highefficiency level. This section comprises certain proven techniques of monitoring theperformance of heat exchangers, coolers and condensers from observed operating data of theequipment.

Classification of Heat Exchangers

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Components/Nomenclature of Heat Exchanger

1. Stationary Head-Channel 21. Floating Head Cover-External

2. Stationary Head-Bonnet 22. Floating Tubesheet Skirt

3. Stationary Head Flange-Channel or Bonnet 23. Packing Box

4. Channel Cover 24. Packing

5. Stationary Head Nozzle 25. Packing Gland

6. Stationary Tubesheet 26. Lantern Ring

7. Tubes 27. Tierods and Spacers

8. Shell 28. Transverse Baffles or SupportPlates

9. Shell Cover 29. Impingement Plate

10. Shell Flange-Stationary Head End 30. Longitudinal Baffle

11. Shell Flange-Rear Head End 31. Pass Partition

12. Shell Nozzle 32. Vent Connection

13. Shell Cover Flange 33. Drain Connection

14. Expansion Joint 34. Instrument Connection

15. Floating Tubesheet 35. Support Saddle

16. Floating Head Cover 36. Lifting Lug

17. Floating Head Cover Flange 37. Support Bracket

18. Floating Head Backing Device 38. Weir

19. Split Shear Ring 39. Liquid Level Connection

20. Slip-on Backing Flange 40. Floating Head Support

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Complete Energy Balance of Heat Exchanger

For hot fluid side of a heat exchanger let,

mH : mass flow rate of the hot fluid in kg/hrCpH : mass heat capacity of the hot fluid in Joules/kg0CTiH and ToH : Respectively inlet and outlet temperatures on exchanger hot side in 0CmC : mass flow rate of the cold fluid in kg/hrCpC : mass heat capacity of the cold fluid in Joules/kg0CTiC and ToC : Respectively inlet and outlet temperatures on exchanger cold side in 0CHeat lost by the hot fluid = -Q = mH × CpH × (ToH – TiH) … (1)Heat gained by the cold side = Q = mC × CpC × (ToC – TiC) … (2)Comparing equations (1) and (2),

mH × CpH × (TiH – ToH) = mC × CpC × (ToC – TiC) … (3)This energy balance equation can be solved for one variable for any given case. Out oftotal six variables in the equation (3), five should be fixed to determine the unknownvariable.

It should also be noted that the mass balance equation is already applied in this case todevelop equation (3).

The fact that mHin = mHout = mH and mCin = mCout = mC is already considered whilewriting equations (1) and (2).

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Procedure to Carried Out Energy Audit

What is the purpose of performance test?

To determine the overall heat transfer coefficient for assessing the performance of the heatexchanger. Any deviation from the design heat transfer coefficient will indicate occurrence offouling.

Performance Terms and Definitions:-

Overall heat transfer coefficient, UHeat exchanger performance is normally evaluated by the overall heat transfer coefficient Uthat is defined by the equation

Q=U*A*LMTDwhereQ=Heat TransferA=Heat transfer surface areaLMTD=Log Mean Temperature DifferenceU=Overall heat transfer coefficient

When the hot and cold stream flows and inlet temperatures are constant, the heat transfercoefficient may be evaluated using the above formula. It may be observed that the heat pickup by the cold fluid starts reducing with time.

Heat duty of the exchanger can be calculated either on the hot side fluid or cold side fluidas given below.Heat Duty for Hot fluid, Qh = Wx Cph x (Ti–To) ………..Eqn–1,

Heat Duty for Cold fluid, Qc = wx Cpc x ( to–ti) ………...Eqn–2If the operating heat duty is less than design heat duty, it may be due to heat losses, fouling intubes, reduced flow rate (hot or cold) etc. Hence, for simple performance monitoring ofexchanger, efficiency may be considered as factor of performance irrespective of other para-meter. However, in industrial practice, fouling factor method is more predominantly used.

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Equipment required to carry energy audit

The test and evaluation of the performance of the heat exchanger equipment is carried out bymeasurement of operating parameters upstream and downstream of the exchanger. Due careneeds to be taken to ensure the accuracy and correctness of the measured parameter. The instru-ments used for measurements require calibration and verification prior to measurement.

Parameters Units Instruments used

Fluid flow kg/h Flow can be measured with instruments likeOrifice flow meter, Vortex flow meter, Venturimeters, Coriollis flow meters, Magnetic flowmeter as applicable to the fluid service andflow ranges

Temperature °C Thermo gauge for low ranges, RTD, etc.

Pressure Bar g Liquid manometers, Draft gauge, Pressuregauges Bourdon and diaphragm type, Absolutepressure transmitters, etc.

Density kg/m3 Measured in the Laboratory as per ASTMstandards, hydrometer, etc

Viscosity MpaS Measured in the Laboratory as per ASTMstandards, viscometer, etc.

Specific heat capacity J/(kg.K) Measured in the Laboratory as per ASTMstandards

Thermal conductivity W/(m.K) Measured in the Laboratory as per ASTMstandards

Composition+ %wt (or) % Vol Measured in the Laboratory as per ASTMstandards using Chemical analysis, HPLC, GC,Spectrophotometer, etc.

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Terminology used in Heat Exchangers

Terminology Definationnnn

n

Unit

Capacity ratio Ratio of the products of mass flow rate andspecific heat capacity of the cold fluid to that ofthe hot fluid. Also computed by the ratio oftemperature range of the hot fluid to that of thecold fluid.Higher the ratio greater will be size of theexchanger

Co current flowexchanger

An exchanger wherein the fluid flow directionof the cold and hot fluids are same

Counter flowexchanger

Exchangers wherein the fluid flow direction of thecold and hot fluids are opposite. Normally preferred

Cross flow An exchanger wherein the fluid flow directionof the cold and hot fluids are in cross

Density It is the mass per unit volume of a material kg/m3

Effectiveness Ratio of the cold fluid temperature range to thatof the inlet temperature difference of the hotand cold fluid. Higher the ratio lesser will berequirement of heat transfer surface.

Fouling The phenomenon of formation anddevelopment of scales and deposits over theheat transfer surface diminishing the heat flux.The process of fouling will get indicated by theincrease in pressure drop.

Fouling Factor The reciprocal of heat transfer coefficient ofthe dirt formed in the heat exchange process.Higher the factor lesser will be the overall heattransfer coefficient.

(m2.K)/W

Heat Duty The capacity of the heat exchanger equipmentexpressed in terms of heat transfer rate, viz.magnitude of energy or heat transferred pertime.

It means the exchanger is capable ofperforming at this capacity in the given system

W

Heat exchanger Refers to the nomenclature of equipmentdesigned and constructed to transmit heatcontent

(enthalpy or energy) of a comparatively hightemperature hot fluid to a lower temperaturecold fluid wherein the temperature of the hotfluid decreases (or remain constant in case oflosing latent heat of condensation) and thetemperature of the cold fluid increases (orremain constant in case of gaining latent heatof vaporisation). A heat exchanger willnormally provide indirect contact heating. E.g.A cooling tower cannot be called a heatexchanger where water is cooled by directcontact with air

Heat Flux The rate of heat transfer per unit surface of aheat exchanger

W/ m2Heat transfer The process of transport of heat energy from ahot source to the comparatively coldsurrounding

Heat transfersurface or heatTransfer area

Refers to the surface area of the heat exchangerthat provides the indirect contact between thehot and cold fluid in effecting the heat transfer.

m2

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Individual HeattransferCoefficient

The heat flux per unit temperature differenceacross boundary layer of the hot / cold fluidfilm formed at the heat transfer surface. Themagnitude of heat transfer coefficient indicatesthe ability of heat conductivity of the givenfluid. It increases with increase in density,velocity, specific heat, geometry of the filmforming surface

W/( m2.K)

LMTDCorrection factor

Calculated considering the Capacity andeffectiveness of a heat exchanging process.When multiplied with LMTD gives thecorrected LMTD thus accountingfor thetemperature driving force for the cross flowpattern as applicable inside the exchanger

LogarithmicMeanTemperaturedifference,LMTD

The logarithmic average of the terminaltemperature approaches across a heatexchanger

°C

Overall HeattransferCoefficient

The ratio of heat flux per unit difference inapproach across a heat exchange equipmentconsidering the individual coefficient and heatexchanger metal surface conductivity. Themagnitude indicates the ability of heat transferfor a given surface. Higher the coefficientlesser will be the heat transfer surfacerequirement

W/(m2.K)

Pressure drop The difference in pressure between the inletand outlet of a heat exchanger

Bar

Specific heatcapacity

The heat content per unit weight of anymaterial per degree raise/fall in temperature

J/(kg.K)

TemperatureApproach

The difference in the temperature between thehot and cold fluids at the inlet / outlet of theheat exchanger. The greater the differencegreater will be heat

transfer flux

°C

TemperatureRange

The difference in the temperature between theinlet and outlet of a hot/cold fluid in a heatexchanger

°C

Terminaltemperature

The temperatures at the inlet / outlet of the hot /cold fluid steams across a heat exchanger

°C

ThermalConductivity

The rate of heat transfer by conduction thoughany substance across a distance per unittemperature difference

W/(m2.K)Viscosity The force on unit volume of any material thatwill cause per velocity

Pa

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Complete Procedure of Heat Exchanger Performance AssessmentProcedure for determination of Overall heat transfer Coefficient, U at field

This is a fairly rigorous method of monitoring the heat exchanger performance bycalculating the overall heat transfer coefficient periodically. Technical records are tobe maintained for all the exchangers, so that problems associated with reducedefficiency and heat transfer can be identified easily. The record should basicallycontain historical heat transfer coefficient data versus time / date of observation. Aplot of heat transfer coefficient versus time permits ratio- nal planning of anexchanger-cleaning program.

The heat transfer coefficient is calculated by the equationU = Q / (A x LMTD)

Where Q is the heat duty, A is the heat transfer area of the exchanger and LMTD is temperature driving force.

The step by step procedure for determination of Overall heat transfer Coefficient aredescribed below

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Density and viscosity can be determined by analysis of the samples taken from the flowstream at the recorded temperature in the plant laboratory. Thermal conductivity and specificheat capacity if not determined from the samples can be collected from handbooks.

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Conclusion:-

Proper maintenance required to overcome the difficulty of fouling so thatheat exchanger work efficiently.