HEAT TRANSFER CH-307
SHAHZAD SHAIKH
LECTURER
CHEMICAL ENGINEERING DEPARTMENT
NED UNIVERSITY OF ENGINEERING AND TECHNOLOGY
Complex Engineering Problem
Instructions for Submission:
It should be submitted in proper report format in hard and soft copy.
Maximum number of the students per group is four (04)
Hand written reports will be rejected and considered to be zero (0) marks.
Complex Engineering Problem maximum marks (10)
Your reports should reach latest 17th January, 24 via e –mail (shahzadshaikh@neduet.edu.pk) and hard copy in a file.
Group assessment will be conducted on 18th January, 24.
• Title Page
• Topic summary
• Table of contents
• List of figures/Tables
• Objective of the Topic
• Background theory
• Methodology
• Calculations
• Graph and Analysis
• Conclusion
• Reference
What should be included in t
he report?
Student Name
XYZ
Contribution
25%
Report Marks
5
Assessment Marks
5
Design of shell and tube type exchangers using multi-objective optimization
(CLO 3)
Problem Statement:
This is a complex engineering problem for your heat transfer. Assume that you are a part of the engineering design team
at a company. You are providing consulting support on the design of shell and tube type exchangers using multi-
objective optimization . The main objective is to heat water from 35 C to 70 C. Your analysis should be reasonable,
professional, and demonstrate a strong grasp of the concepts of heat transfer. Please justify and ANALYZE with
supporting calculations.
We are going to learn Boiling and
Condensation as a Process Engineer.
LESSON OBJECTIVES
• By the end of session, students will know the
• Boiling and condensation
• Boiling, evaporation and vaporization
• Boiling types
• Boiling curve
BOILING
EVAPORATION
VAPORIZATION
BOILING & CONDENSATION
Boiling: When the temperature of a liquid at a specified
Pressure is raised to the saturation temperature Tsat at that
pressure boiling occurs.
Boiling process involve a phase
change from liquid to vapor state.
BOILING & CONDENSATION
Condensation occurs when the temperature of vapor at a specified
pressure is reduced to below its saturation temperature. (vapor to liquid)
In Boiling and Condensation, heat flux during
phase can be calculated in a process industry.
BOILING & CONDENSATION
A t s pe c i f i e d pre s s ure  Te mpe r at ur e of l i qui d > Ts at ur at i o n  B O I L I N G  L I Q U I D
A t s pe c i f i e d pre s s ure  Te mpe r at ur e of vapor < Ts at ur at i o n  C O N D E N S AT I O N  VA P O R
Boiling occur at the solid liquid interface
When the atmospheric pressure is below the vapor pressure
of the liquid.. Evaporation occurs.
Evaporation takes
place at liquid- vapor
interface
Water at 20 °C
P vapor = 2.3 KPa
P air = 1.4 KPa
P atm < vapor pressure of the
liquid
Vaporization
Evaporation is nothing but a type of vaporization which
mostly occurs at temperatures below the boiling point.
Liquid changes form into gas
BOILING
In Boiling, surface temperature
is more as compare to the liquid
Types of Boiling
Pool Boiling
Absence of
Bulk motion
of fluid
Flow Boiling
Presence of
Bulk motion
of fluid
POOL BOILING
• Boiling is called pool boiling in the absence of bulk
fluid flow.
• Any motion of the fluid is due to natural convection
currents and the motion of the bubbles under the influence
of buoyancy.
• The boiling curve which is a log-log plot between heat
flux (q/A) or heat transfer coefficient (h) and excess
temperature (ΔT).
• Excess temperature (ΔT = Tw - Tsat) is the temperature
difference between heating surface (Tw) and saturated
temperature of the liquid (Tsat).
STAGES OF POOL BOILING
STAGES OF POOL BOILING
• Free convection boiling
(OA)
• Nucleate boiling (ABC)
• Transition boiling (CD)
• Film boiling (DE)
STAGES OF POOL BOILING
• Curve OA, represents free convection
boiling.
• When we light the burner, heat transfer takes
place between the vessel and water due to free
or natural convection.
• Thus the value of heat flux and ∆Texcess starts
increasing, and when the temperature of water
reaches its saturation temperature, i.e. when
∆Texcess attains positive value, then boiling
takes place.
• This stage of boiling is known as free/natural
convection boiling.
• Curve ABC nucleate boiling consist of two parts i.e. AB and BC.
• Curve AB is called liquid entrainment:
• Due to continuous heating, the value of ∆Texcess is increases and the
bubble are formed at the bottom of surface of the vessel.
• These bubbles moves upwards but they collapse after covering some
distance in the water.
• Raising bubbles carry some water along with them.
• Disturbance caused by Liquid Entrainment in water, increases the
heat transfer coefficient, so heat flux also increases.
• Curve BC is called critical heat flux.
• As ∆Texcess further increases, bubbles start forming at a faster rate.
• These vapor bubble moves upward and merge to form numerous
continuous columns of vapor in the liquid.
• These bubbles in column then move upwards to the free surface,
where they break up and release their vapor contents.
• Due to this the heat flux attains maximum value known as critical
heat flux.
• Curve CD called transition boiling.
• When the value of ∆Texcess increases beyond the
critical point, the heat flux start decreasing.
• This is because of large fraction of the heated
surface of vessel is covered with a vapor film.
• This vapor film acts as an insulator as its thermal
conductivity is lower than water.
• Thus, the value of heat flux attains its minimum
value.
• Point D corresponding to the minimum value of
heat flux is known as Leiden-frost point.
LEIDEN-FROST POINT
STAGES OF POOL BOILING
• Curve DE, represents film boiling.
• After the transition phase, the ∆Texcess
further increases due to which the
vessel surface is completely covered
by continuous stable vapor film.
• Because of high temperature the
radiation heat transfer takes place
between heated surface and the water
through vapor film.
• Thus, this stage is known as film
boiling.
26
Heat Transfer Correlations in Pool Boiling
The most widely used correlation for the rate of heat transfer in the nucleate boiling regime
was proposed in 1952 by Rohsenow, and expressed as
Example:
Water is boiled at a temperature of Tsat = 160°C by hot gases flowing through 50 m long , 5 cm outer diameter a
mechanically polished stainless steel pipe submerged in water whose outer surface temperature is maintained at Ts =
165° C.
The rate of heat transfer to the water,
the rate of evaporation,
the ratio of critical heat flux to current heat flux,
and the pipe surface temperature at critical heat flux conditions are to be determined.
Assumptions 1. Steady operating conditions exist. 2 Heat losses from the boiler are negligible. 3 The boiling regime is
nucleate boiling since ΔT = T s −Tsat = 165- 160 = 5 °C which is in the nucleate boiling range of 5 to 30°C for water.
(a) Assuming nucleate boiling, the heat flux can be determined from Rohsenow relation
The heat transfer surface area is A =
The rate of heat transfer during nucleate boiling =
(b) The rate of evaporation of water is determined from
(c) For a horizontal cylindrical heating element, the coefficient Ccr is determined from (L =0.025)
Then the maximum or critical heat flux is determined from
(d) The surface temperature of the pipe at the burnout point is determined from Rohsenow relation at the critical heat
flux value to be
CONDENSATION
• Two forms of condensation:
• Film condensation
• Dropwise condensation.
DROPWISE CONDENSATION
• One of the most effective mechanisms of heat transfer, and
extremely large heat transfer coefficients can be achieved.
• Small droplets grow as a result of continued condensation,
join into large droplets, and slide down when they reach a
certain size.
• No liquid film to resist heat transfer.
• As a result, heat transfer rates that are more than 10 times
larger than with film condensation can be achieved.
• Large heat transfer coefficients enable designers to achieve
a specified heat transfer rate with a smaller surface area.
FILM CONDENSATION
• The condensate wets the surface and forms a liquid film.
• The surface is blanketed by a liquid film which serves as a
resistance to heat transfer.
• Entire surface is covered by the condensate, which flows
continuously from the surface and provides a resistance to
heat transfer between the vapor and the surface.
• Thermal resistance is reduced through use of short vertical
surfaces and horizontal cylinders.
• Characteristic of clean, uncontaminated surfaces.
CONDENSER
Factors affecting the selection of a Condenser
Capacity of the refrigeration system.
The type of cooling medium available.
Corrosion.
Mass flow rate of refrigerant
The type of refrigerant used