1. Heat Transfer
Dr. S.M. Mhatre

2. Heat transfer and its applications.
Practically all the operations carried out by chemical involve the production or absorption of energy in the form of heat .
Examples are ubiquitous:
heat flows in the body
home heating/cooling systems
refrigerators, ovens, other appliances
automobiles, power plants, the sun, etc.
Dr. S.M. Mhatre

3. Heat
A form of energy associated with the motion of atoms or molecules.
Transferred from higher temperature objects to objects at a lower temperature.
Dr. S.M. Mhatre

4. Temperature has only magnitude. It is a scalar quantity.
Although heat transfer and temperature are closely related, they are of a different nature.
Temperature has only magnitude. It is a scalar quantity.
Heat transfer has direction as well as magnitude. It is a vector quantity.
We work with a coordinate system and indicate direction with plus or minus signs.
Dr. S.M. Mhatre

5. Work done by falling weights = mgh
Calorie :The calorie had been defined as the amount of heat it takes to raise the temperature of 1 gram of water by 1 degree C.
James Prescott Joule use his device below to find out how much work you would have to do to create a calorie of heat.
Work done by falling weights = mgh
The Mechanical Equivalent of Heat was found to be 4.2 Joules of mechanical work per calorie of heat produced
4.2 J/cal
Dr. S.M. Mhatre

6. Task 1?
A 10 kg cinder block is dropped 50 meters. How many calories of heat will it develop if dropped into 1000 kg water?
SAT2: How much will the water’s temperature go up?
Pool of water
Pool of water
Pool of water
Dr. S.M. Mhatre

7. Heat Transfer
There are 3 ways that heat can move from one place to another:
radiation
conduction
convection
Dr. S.M. Mhatre

8. Dr. S.M. Mhatre

9. Modes of Heat Transfer qconvection qradiation qconduction
Conduction - diffusion of heat due to temperature gradient
Convection - when heat is carried away by moving fluid
Radiation - emission of energy by electromagnetic waves
qconvection
qradiation
qconduction
Dr. S.M. Mhatre

10. Typical Design Problems
To determine:
overall heat transfer coefficient - e.g., for a car radiator
highest (or lowest) temperature in a system - e.g., in a gas turbine
temperature distribution (related to thermal stress) - e.g., in the walls of a spacecraft
temperature response in time dependent heating/cooling problems - e.g., how long does it take to cool down a case of soda?
Dr. S.M. Mhatre

11. Task 2: Write three more design example related to heat transfer
Dr. S.M. Mhatre

12. Conduction Heat Transfer
Conduction is the transfer of heat by molecular interaction
In a gas, molecular velocity depends on temperature
hot, energetic molecules collide with neighbors, increasing their speed
Dr. S.M. Mhatre

13. H = the Rate of Heat Flow through a conductor
Unit:
Joules/sec or Watts
H = Q = k A T d
Temperature difference
Thermal Conductivity
thickness
Cross-sectional area
Dr. S.M. Mhatre

14. Fourier’s Law “heat flux is proportional to temperature gradient”
where k = thermal conductivity
in general, k = k(x,y,z,T,…)
units for q are W/m2
temperature profile 1
hot wall
cold wall x
Dr. S.M. Mhatre
heat conduction in a slab

15. Thermal Properties
Thermal Conductivity (k)
It is the term used to indicate the amount of heat that will pass through a unit of area of a material at a temperature difference of one degree.
The lower the “k” value, the better the insulation qualities of the material.
Units; US: (Btu.in) / (h.ft2.oF)
Metric: W / (m.oC)
Conductance (c)
It indicates the amount of heat that passes through a given thickness of material;
Conductance= thermal conductivity / thickness
Units; US: Btu / (h.ft2.oF)
Metric: W/ (m2.oC)
Dr. S.M. Mhatre

16. Thermal Resistance (RSI for metric unit, R for US units)
It is that property of a material that resist the flow of heat through the material. It is the reciprocal of conductance;
R= 1/c
Thermal Transmittance (U)
It is the amount of heat that passes through all the materials in a system. It is the reciprocal of the total resistance;
U= 1/Rt
Table 1 lists a few of the common materials and their thermal properties;
Dr. S.M. Mhatre

17. Dr. S.M. Mhatre

18. Dr. S.M. Mhatre

19. Task 3:
H = Q/T = k A T d
A steel slab 5 cm thick is used as a firewall, measuring 3 m x 4 m. If a fire burns at 800 C on one side of a wall, how fast will heat flow through the metal door. (The conductivity of steel is 46 Watts/m•K)
Dr. S.M. Mhatre