Thermal Energy Can Be Divided to Tow General categories:
Heat
Stored Internal Energy Of Substance
1- Heat
Heat is not state of the material, it's the transfer of Energy Caused By Temperature Difference.
When heat is passed from one body to another the internal energy of the bodies Change.
Heat is Energy in Transit. Recall the First law from Thermodynamics.
ΔU = Q – W
2- Stored Internal Energy
Is the amount of Energy Processed By Substance it Can Be Considered to be the total molecular Energy of Body
3- Heat Transfer
Heat transfer is the transfer of Heat affected by a temperature difference
4- Latent Heat of Vaporization
When matter changes from liquid to vapor or vice versa, it absorbs or releases a relatively large amount of heat without a change in temperature.(970 Btu)
5- British Thermal Unit " BTU"
Is the quantity of heat needed to raise the temperature of 1 lb. of water one degree Fahrenheit.
6- Specific heat :-
Amount of heat needed to increase the temperature of a pound of a particular substance 1f ( the unit specific heat is BTU per pound per degree Fahrenheit ( BTU/lb/f ) .
7- Latent heat :-
Heat transfer can occur with no temperature change taking place this heat transfer is referred to as latent heat transfer .latent heat transfer results in phase change , that is a substance changes from solid to liquid , liquid to solid , liquid to gas or gas to liquid .
8- Sensible heat
When an object is heated, its temperature rises as heat is added. The increase in heat is called sensible heat. Similarly, when heat is removed from an object and its temperature falls, the heat removed is also called sensible heat. Heat that causes a change in temperature in an object is called sensible heat .
Mention the Heat Transfer Mechanisms (Modes )
Conduction
Conduction involves the transfer of heat by the interactions of atoms or molecules of a material through which the heat is being transferred.ρ
Convection
Convection involves the transfer of heat by the mixing and motion of macroscopic portions of a fluid.
Radiation
Radiation or radiant heat transfer, involves the transfer of heat by electromagnetic radiation that arises due to the temperature of a body.
Conduction
Conduction Is The Heat Transfer Between Two Substances By Direct Transferring Of Molecular Kinetic Energy . Conduction Occurs When A hot Substance Comes Into Contact With A cold
Substance : Heat Is transferred From Hotter Substance Has A higher Temperature Than The Colder Substance , The Substance With A higher Temperature Has Molecules With Higher Average Kinetic Energy Than The Substance With The Lower Temperature .
Heat transfer rate by conduction speed related to four factors :-
1. Thermal conductivity of the substance
2. The surface area over which the heat is applied
3. The thickness of the material subject to the heat transfer
4. The temperature difference across the materials
Fourier‟s Law of Conduction
Q = k A ΔT/L
Q = rate of heat transfer (Btu/hr)
A = cross-sectional area of heat transfer (ft2)
Δx = thickness of slab (ft)
Δr = thickness of cylindrical wall (ft)
ΔT = temperature difference (°F)
k = thermal conductivity of slab (Btu/ft-hr-°F)
Convection
Convection heat transfer involves fluids, either liquids or gases in motion
Convection heat transfer occurs in two parts :
1. Heat transfer into or out of a fluid by the process of conduction
2. The movement of the fluid transports the heated fluid, as well as its thermal energy, to another location
Convection either natural or forced (using fans, pumps or other devices).
Also the increased turbulent of the forced flow will increase convective heat transfer rate as a result of increased heat transfer coefficient.
Newton's Law Of Cooling
Q= h A ΔT
Q = rate of heat transfer (Btu/hr)
h = convective heat transfer coefficient (Btu/hr-ft2-°F)
A = surface area for heat transfer (ft2)
ΔT = temperature difference (°F)
Radiation
Is the energy emitted by matter in the form of electromagnetic waves (or photons) as a result of the changes in the electronic configurations of the atoms or molecules.
Unlike conduction and convection, the transfer of energy by radiation does not require the presence of an intervening medium .
All bodies at a temperature above absolute zero emit thermal radiation.
Black Body Radiation
A body that emits the maximum amount of heat for its absolute temperature is called a black body. Radiant heat transfer rate from a black body to its surroundings can be expressed by the following equation.
Q= ðAT4
Q = heat transfer rate (Btu/hr)
ð= Stefan-Boltzman constant (0.174 Btu/hr-ft2-°R4)
A = surface area (ft2)
T = temperature (°R)
Two black bodies that radiate toward each other have a net heat flux between them. The net flow rate of heat between them is given by an adaptation of Equation
Q= ð A(T14-T24)
A = surface area of the first body (ft2)
T1 = temperature of the first body (°R)
T2 = temperature of the second body (°R)
Emissivity
Real objects do not radiate as much heat as a perfect black body.
They radiate less heat than a black body and are called gray bodies.
To take into account the fact that real objects are gray bodies,
so the previous equation will be modified to.
Q=Ɛ ðAT4
where:
Ɛ = emissivity of the gray body (dimensionless) Emissivity is simply a factor by which we multiply the black body heat transfer to take into account that the black body is the ideal case. Emissivity is a dimensionless number and has a maximum value of 1.0.
Example
The heat transfer that takes place in boiler furnace is a good example of the three modes of heat transfer
Conduction : heat transfer through the tube walls to the water in the tubes .
Convection : heat transfer from the hot combustion gases to the water in the tubes
Radiation : burning fuels transferring heat to the combustion gases and the tube walls
Some Notes
Heat ALWAYS flows from hot to cold when objects are in contact or connected by a good heat conductor. The rate of heat transfer will increase as the difference in temp between the two objects increases
Cold objects have less heat than hot objects of the same mass
The mass of the object remains the same regardless of the heat content
Changing the pressure on a liquid or a vapor changes the boiling point.
Each lb. of pressure above atmospheric pressure, raises the boiling point about three degrees Fahrenheit.
When a vapor is compressed, its temperature and pressure will increase even though heat has not been added
Convection : Occurs only in liquids, gases or vapors
The transfer of heat by the circulation of a liquid or a vapor (like cooling system)
Heat flows from a hot surface to a surface containing less heat.
Heat rises. (Like on a stove)
Radiation : The process that moves heat from a heat source to an object by means of heat rays without the medium becoming hot.
Works on the principle that heat moves from a hot surface to a surface with less heat.
Does not require air movement or anything in between the source and component. (Like rays of the sun)
Conduction : Heat is transferred through a solid and gets the solid hot. (molecules get hot than they in turn give motion to nearby molecules and they get hot too)
Different solids conduct different amounts of heat in a specific time. (copper vs. glass)
Factors affecting heat transfer between fluids
1. Types of fluid or substance , e.g. liquids are better than gases .
3. Thickness of materials , the thinner material , the faster heat will transfer .
4. Surface area , the greater the contact surface area , the more heat transfer takes place
5. Gravity of fluids , the tighter a fluid , the fester heat will Transfer
6. Turbulence of fluids , the greater the turbulence the better heat will transfer .
7. Temperature difference , the greater difference in a fluid temperature, the faster heat will transfer, the greater driving force causing heat transfer
8. Thermal conductivity, every substance has a definite thermal conductivity which affects the amount of heat transferred. Metals are good conductors while wood and carbon are very poor conductors .
9. Velocity of the fluids in the tube, the velocity effect on fouling, with higher velocities reducing the possibility of scale or dirt deposits on tube .as the velocity increase the heat transfer rate Increase .
10. Direction of flow of the liquids exchanging, When using identical equipment with equal rates of flow, the one with counter current flow and the other with parallel ( co-current ) flow , the final temperature will be higher with counter current .
11. Amount of liquids, The effect of higher flow rates and thinner film result in higher heat transfer rates as shown in the figure below .
12. The effect of fouling or deposits on the walls o f heat exchanging decreasing heat transfer rate
Types of fouling
- Scaling
- Particulate fouling
- Chemical reaction fouling
- Corrosion fouling
- Biofouling
- Freezing fouling
- Air or gas blanketing
Function of Heat Transfer Equipment
A- Cooler: Reduce the temperature of a liquid or gas using water to remove heat
B- Condenser: Remove Heat from a gas, changing it to a liquid
C- Vaporizer: Add heat to liquid, changing it to a gas
D- Evaporators: Employed for concentration of the solution by evaporation of water
E- Reboiler: Provide heat as latent heat to liquid in the bottom of a distillation tower . the heat may be supplied by either stream or a hot process stream
F- Chiller : Cool a liquid or gas using a refrigerant instead of water
G- Exchanger: perform two functions. They can heat a cold process stream by using a hot process fluid, or they can cool a hot process stream by using a cold process fluid
What's the Difference between Heat Transfer and Mass Transfer? Mention examples for each
Heat transfer is the motion of thermal energy in transit due to a temperature difference. That is, whenever there is a temperature difference in a medium or between mediums, heat transfer must occur.
There are three main modes of heat transfer: conduction, convection, and radiation .
Similarly, mass transfer is mass in transit due to a concentration difference. Therefore, if there is a difference in the concentration of some species in a mixture, mass transfer must occur. Just as the driving potential for heat transfer is a temperature gradient, a species concentration gradient is the driving potential for mass transport of that species.
The analogy of heat transfer to mass transfer is correlated to the individual modes by which they are influenced. Mass transfer by due to fluid flow relates to convective heat transfer, and mass transfer by diffusion is analogous to conduction HT.
- Mass transfer Example: Distillation Column
- Heat Transfer Example: Heaters etc.