heat transfer & heat exchangers chbe 446 – group5 stephan donfack benjamin harbor nguyen huynh...

HEAT TRANSFER&

HEAT EXCHANGERS

CHBE 446 – Group5Stephan Donfack

Benjamin Harbor

Nguyen Huynh

Cyndi Mbaguim

AGENDA Concept and Mechanism Heat Transfer Equations Design Material Selection Conclusion

CONCEPT Definition

• Discipline of thermal engineering that involves the generation, use, conversion, and exchange of thermal energy and heat between physical systems.

• The driving force of heat transfer is as result of temperature gradient between two regions.

• During heat transfer, thermal energy always moves in the same direction:

• HOT COLD

Mechanism for Heat Transfer

Three types of energy transfer:

- Conduction: Transfer of heat within a substance by molecular interaction.

- Convection: During macroscopic flow, energy associated with fluid is carried to another region of space.

- Radiation: Heat transferred through wave energy (electromagnetic waves)

Q hot Q cold

Th Ti,wall

To,wall

Tc

Region I : Hot Liquid-Solid Convection

NEWTON’S LAW OF CCOLING

dqx hh . Th Tiw .dA Region II : Conduction Across Copper Wall

FOURIER’S LAW

dqx k.dT

dr

Region III: Solid – Cold Liquid Convection

NEWTON’S LAW OF CCOLING

dqx hc . Tow Tc .dA

THERMAL

BOUNDARY LAYER

Energy moves from hot fluid to a surface by convection, through the wall by conduction, and then by convection from the surface to the cold fluid.

PROJECT FLOWSHEET

HEAT EXCHANGERS in INDUSTRY

• Commonly used throughout the chemical process industries as a means of heating and cooling process in product streams.

• Common industry utilization:• Space heating• Refrigeration• Air conditioning• Power plants• Petrochemical plants• Petroleum refineries• Natural gas processing • Sewage treatment

TYPES of HEAT EXCHANGERS

•Double-pipe•Shell and tube •Plate and frame•Spiral•Pipe coil

CONFIGURATIONS IN HEAT EXCHANGERS

Co-current flow Counter-current flow

Double tube – Single Pass Heat Exchanger

TEMPERATURE PROFILE

HEAT TRANSFER EQUATION IN HEAT EXCHANGERS

•

Log Mean Temperature Difference (LMTD)

CO-CURRENT CONFIGURATION COUNTER CURRENT CONFIGURATION

 Used to determine the temperature driving force for heat transfer in flow systems, most notably heat exchangers.

Heat Duty (Q)

• Amount of heat needed to transfer from a hot side to the cold side over a unit time.

• Derived from energy balance.

Where:

= flow rate

Hfluid = Fluid enthalpy (temperature dependent)

ASSUMPTIONS

- Steady State- No phase changes- Negligible heat loss- Constant overall heat transfer

generatedsin out

outin ewQhmhmdt

dE

ˆ.ˆ.

Overall Heat Transfer Coef (U)

• The overall HT coefficient is used to analyze heat exchangers.

• It contains the effect of hot and cold side convection, conduction as well as fouling and fins.

U

Xw: wall thickness

Km: thermal conductivity of wall

hi, ho: individual convective heat transfer coef

coefficients in & out of tubeDi, Do: Inner & outer diameter 

)/,/Pr,(Re, oiDLfNu

DIMENSIONLESS ANALYSIS TO CHARACTERIZE H.E

..Dv

k

C p .

Nu a.Reb .Prc

𝑵𝒖=𝑪𝒐𝒏𝒗𝒆𝒄𝒕𝒊𝒗𝒆 𝑯 .𝑻𝑪𝒐𝒏𝒅𝒖𝒄𝒕𝒊𝒗𝒆𝑯 .𝑻

𝒉 .𝐷𝐾

h = convective H.T coefK = conductive H.T coefµ = dynamic viscosityρ = densityCp = heat capacity

ν = mean velocityD & L = Length scale parameters

ESTIMATED U

Overall Heat Transfer Coefficient can

be estimated for different fluids as well

as the type of heat exchanger system

involved (Shell & Tube).

Frequently used sources:

o Perry’s Handbook

o ChemE Design Textbook

o Aspen Tech Software…

Area (Sizing)

Sizing a Heat Exchanger Equipment (by area calculation):

Costing (Base Cost Installation Cost) Approximating number of pipes needed in the heat

exchanger• Shell diameter and tubes pitch

Performance

HEAT EXCHANGERS IN GAS SWEETENING

Simplified schematic of gas sweetening process

HEAT EXCHANGER DESIGN

• The main heat exchanger called rich/lean amine interchanger.

It requires:Good heat recovery the thermal length of heat exchanger is a

key feature.

To minimize the fouling tendencies: high pressure drop (above 70 kPa) to keep shear stress high (50Pa)

GASKET MATERIAL SELECTION

• Normal ethylene propylene diene monomer (EPDM): used in amine systems due to its inherent resistance to H2S and CO2.

• Disadvantage: suffers degradation from hydrocarbons or other fluids on an increasing severity based on the degree of the non-polar nature of the fluid

Plate with EPDM gasket

CONT’d

• EPDM XH is a combination of EPDM and other rubber

resins creating an extra hard EPDM rubber, developed for

applications with hydrocarbon exposure.

• Other rubber materials: Aflas gaskets can be used for amine

duties, but not longer lifetime and increase capital investment

and replacement cost.

SHELL & PLATE HEAT EXCHANGER

• Using a shell and plate heat exchanger as a reboiler allows a small

temperature difference between the hot and cold sides-> prevent amine

from overheated and degradation

• A shell and plate heat exchanger followed by a separator vessel is

recommended for condenser.

A typical shell and plate heat exchanger

CONCLUSION

• Select the fit for purpose heat exchanger will improve the performance

of the amine plant, reduce investment costs and overall costs of

ownership.

• Selecting the right gasket plate will increase the efficiency while

maintenance costs and intervals can be reduced.

• Shell and plate heat exchangers are more commonly used than shell

and tube heat exchangers.

REFERENCE

• Middleman, Stanley. An Introduction to Mass and Heat Transfer, Principles of Analysis and Design.Wiley, Dec 1997.

• McCabe, Smith, and Harriott. Unit Operations of Chemical Engineering

• http://www.tranter.com/literature/markets/hydrocarbon-processing/Hydrocarbon-Eng-A-Sweet-Treat.pdf

• www.authorstream.com/Presentation/baher-174192-heat-exchangers-ent..