Useful for Students of B. Sc (Ag.) and B. Tech. (Agril. Engg.)

1. Md. I. A. Ansari
Department of Agricultural Engineering
(e-mail: irfan26200@yahoo.com)
Renewable Energy and Green
Technology

2. Temperature
• Temperature is the degree of hotness or
coldness of an object.
• Temperature (T) is a thermodynamic
quantity related to the motion of
molecules.
• Temperature is a measure of thermal
energy.
• The SI unit of temperature is Kelvin (K)
and MKS unit is °C.

3. Temperature Scales
The three temperature scales most
commonly employed in engineering
applications are degree Celsius (ºC) scale,
degree Fahrenheit (ºF) scale and Kelvin
(K) scale.

5. Temperature ºC ºF K
Absolute zero -273 -460 0
Boiling point
of water
100 212
373
Freezing point
of water
0 32
273

6.  
8.1
32
 F
C
T
T328.1  CF TT
273 CK TT
273 KC TT
100
273
180
32
100



 KFC TTT
5
273
9
32
5



 KFC TTT
Temperatures Conversion Formulas
Note: TC, TF and TK are temperatures in Celsius, Fahrenheit and Kelvin scale.

7. • Suppose the temperature inside a solar
dryer is 55 ºC. What will be temperature in
Fahrenheit and Kelvin scale.
• F=1.8 x 55+32=131ºF
• K= 50+273=323 K

8. Heat
• Heat is a form of energy.
• The motion of molecules produces heat.
• The more motion, the more heat is
generated and accordingly there will be
temperature rise.
• Heat energy can be transferred from one
object to another due to temperature
difference.
• Denoted by Q.

9. Units of Heat
SI unit: Joule
1J = 1 N m = m2 kg/s2
CGS unit: erg.
1 erg = 10-7 J
A calorie is commonly defined as the amount of heat
required to raise the temperature of one gram of
water by1oC.
1 cal = 4.186 J

10. British Thermal Unit
• BTU is a heat quantity measure.
• BTU is the quantity of heat needed to raise
the temperature of 1 lb. of water by one
degree Fahrenheit.
• 1 Btu (British thermal unit) = 1055.06 J =
0.252 kcal = 252 cal

11. Rate of Heat Transfer
Rate of heat transfer is amount of heat
energy transferred per unit time.
Rate of heat transfer=amt. of
heat/time=Q/t
Denoted by q.
SI unit of rate of heat transfer: J/s or Watt
• SI unit of power: J/s or Watt

12. Flux
• Transfer of any physical quantity
passing through a unit area in unit
time is called flux of that quantity.
• Some examples of flux are heat flux,
solar flux, mass flux, etc.
• The SI unit of heat flux/ solar flux is
J/sm2 or W/m2.

13. Heat Flux
• The heat flux is the rate of heat transfer per
unit area and equation for heat flux is given
as:
• Also, heat flux =heat energy/(Area x time)
• Where q is rate of heat transfer, W and A
is heat transfer area, m2 .
• If A=6 m2 and heat flux=4 W/m2
• Then q=6 x 4=24 W
A
q
fluxheat 

14. Specific Heat
• Specific heat is the amount of heat required to
increase the temperature of 1 kg of material by 1°C
at a given temperature.
• It is denoted by Cp.
• The SI unit for Cp is J/kg K or J/kg °C.

15. Specific Heat
• The ability of a substance to absorb heat
energy.
• Different substances absorb heat at
different rates.
• The greater the mass of the object the
more heat is absorbed.
•

16. • For example, Cp is 4.18 kJ/kg · °C for
water and cp is 0.45 kJ/kg · °C for iron at
room temperature, which indicates that
water can store almost 10 times the
energy that iron can per unit mass.

17. Specific Heat Values Of Selected Materials
Materials J/kg.K J/kg.°C kJ/kg.K
Aluminium 887 887
0.887
Brass 920 920
0.92
Brick 841 841
0.841
Cast Iron 554 554
0.554
Clay 878 878
0.878
Coal 1262 1262
1.262
Copper 385 385
0.385
Glass 792 792
0.792
Gold 130 130
0.13
Ice 2090 2090
2.09
Iron 462 462
0.462
Oak Wood 2380 2380
2.38
Rubber 2005 2005
2.005
Salt 881 881
0.881
Sand 780 780
0.78
Sandstone 740 740
0.74
Silver 236 236
0.236
Stainless Steel 316 468 468
0.468
Water 4187 4187
4.187
Zinc 389 389
0.389

18. • The specific heat (Cp) may be expressed
by the following relationship:
Where:
Cp = specific heat at constant pressure, J/kg
K
Q = heat transferred, J
m = mass, kg
∆T = temperature difference, K
Tm
Q
Cp



19. hence 1°C change in
temperature
is equivalent to a change of 1 K.

20. Sensible Heat
The heat which causes an increase or
decrease in the temperature of a body
without changing its state is known as
sensible heat.
Suppose if the temperature of a material is
raised from 30 to 72 oC, the heat absorbed
to make this change is the sensible heat.

21. Addition or removal of sensible heat
results in a temperature change.
Addition or removal of latent heat results in
no temperature change.

22. Phase Changes
• There are 3 common state of matter: solids
liquids and gases.
• A phase change is a physical
change that requires a change in
heat energy
• Addition or removal of HEAT

23. • Latent Heat: The amount of heat
absorbed or given out by a unit mass of
the substance to change its state without
change of temperature is called latent
heat.
• The different types of latent heat are latent
heat of fusion, latent heat of vaporization
and latent heat of sublimation.
• All types of phase changes are presented
in different figures given below:

26. Latent Heat of Fusion: The amount of
heat required to covert a unit mass of the
substance from solid state to the liquid state
without change of temperature is called latent
heat of fusion. The equation for latent heat of
fusion or solidification is given by:
Where,
Qf = quantity of heat required or liberated, J
m = mass of material solidified or melted, kg
λf = latent heat of solidification or fusion, J/kg
Latent heat of fusion of ice = 335 kJ/kg
Latent heat of freezing of water= 335 kJ/kg.
ff mQ 

27. Latent Heat of Vaporization: The amount of heat
required to convert a unit mass of saturated
liquid to saturated vapour at constant
temperature and pressure is called latent heat
of vaporization. The latent heat removed or
supplied for condensation or vaporization can
be calculated from the relationship:
Where,
Qv = quantity of heat required or liberated, J
m = mass of a given material evaporated or
condensed, kg
λv = latent heat of vaporization or condensation, J/kg
Latent heat of steam at 100°C = 2257 kJ/kg
vv mQ 

28. Latent Heat of sublimation: The amount of heat
required to covert a unit mass of the substance
from solid state to directly vapour state without
change of temperature is called latent heat of
sublimation and reverse of this process is called
deposition. The latent heat of sublimation can be
calculated from the relationship:
Where,
Qs = quantity of heat required, J
m= mass, kg
λs = latent heat of sublimation, J/kg
Latent heat of sublimation=2838 kJ/kg
ss mQ 

29. Flow Rate
• The mass flow rate of a fluid at a cross
section is equal to the product of the fluid
density, average fluid velocity, and the
cross-sectional area.
• Mass flow rate: kg/s

30. Volumetric Flow Rate
• The volume of a fluid flowing through a pipe
or duct per unit time is called the volume
flow rate V and is expressed as
• Volumetric flow rate (Q):m3/s, cm3/s, l/s
• Q=V/t Where V is volume, t is time.
• Q may be calculated as: Q=AV where A is
C.S.A and v is average velocity

31. Heat content (sensible heat) of a material:
The heat content or sensible heat of a material is
given by the following equation:
Sensible heat = mass x specific heat x temperature
difference
i.e.
Where Q = heat content or sensible heat, J
m = mass, kg
Cp = specific heat, J/kg K
∆T = temperature difference=T2-T1, K
T1=initial temperature, K
T2=final temperature, K
 12 TTmCTmCQ pp 

32. • How much heat is needed to raise the
temperature of 30 kg water from 10 0C to 80
0C when specific heat of water is 4.19 kJ/kg
K?

33. Solution
• Q = m c  T
= 30 Kg x 4.19 kJ/kg 0C x (80-10)0C
= 8799 kJ

34. Find the amount of heat required for evaporating 2.8kg of water at
45 Cº? (Lvaporization =2, 3×106 joule/kg cwater=4190J/kg.Cº)

35. Rate of Change of Heat Content: The equation for
rate of change of heat content of a material is
given by:
• Where
q = rate of change of heat content, J/s or W
• m= mass flow rate, kg/s
• Cp = specific heat, J/kg K
•  T = temperature difference, K
• d=diameter of pipe,m
• v=velocity of flow, m/s
• ρ=density of fluid, kg/m3
TvCdTvCATCmq ppp  

 2
4


36. Heat Transfer
• The basic requirement for heat transfer is
the presence of a temperature difference.
• There can be no net heat transfer between
two mediums that are at the same
temperature.
• The temperature difference is the driving
force for heat transfer

37. Principle of Heat Transfer
 Conduction
 Convection
 Radiation

38. • Conduction heat transfer: Conduction
heat transfer occurs when heat moves
through a material (usually a solid or a
viscous liquid) due to molecular action
only. Conduction heat transfer is guided by
Fourier’s law which can be written as:
x
T
kAq




39. • Where, q=heat transfer rate, W or J/s
• A= heat transfer area, m2
• ΔT=temperature difference, K
• Δx=distance, m
• k=thermal conductivity, W/m-K or J/sm K
x
T

 = temperature gradient i.e. the temperature per
unit length of heat-transfer path. The negative
sign indicates that eat flow is always from
higher temperature to lower temperature.

40. • Transfer of energy takes place at the
molecular level.
• As molecules of a solid material attain
additional thermal energy, they become
more energetic and vibrate with increased
amplitude.
• These vibrations are transmitted from one
molecule to another and heat is conducted
from regions of higher temperature to
those at lower temperature.

43. 43
Thermal Conductivity
• Thermal conductivity: It represents the
quantity of heat Q that flows per unit time
through a mateial of unit thickness and
unit area having unit temperature
difference between faces.
• The SI unit of thermal conductivity is
W/m K.

44. • A high value for thermal conductivity
indicates that the material is a good heat
conductor.
• A low value indicates that the material is a
poor heat conductor or insulator.

45. Thermal Conductivity Values

46. • Convection: Convection heat transfer is
the transfer of energy due to the
movement of a heated (or cooled) fluid.
• It is of two types:
 Free or natural convection
Forced convection.

47. • Natural convection occurs when a heated
fluid moves due to the change in fluid
density.

48. • Forced convection occurs when the fluid is
moved by other methods (pumps, fans,
etc.).

49. • The governing equation for convection is:
• Where h is the heat transfer
coefficient (W/m2 K or W/m2 ◦C ), A is
the surface area, and Ts and Ta are
the surface of wall and ambient
temperatures, respectively.
 as TThAq 

50. Radiant Heat Transfer
(1) No medium is required for its propagation.
(2) Energy transfer by radiation is maximum when the two
Surfaces are separated by vacuum.
(3) Radiation heat transfer rate equation is given by the
Stefan-Boltzmann law of thermal radiation:
q: rate of radiant energy emission (W); A: area of emitting
surface (m2); T: absolute temperature; s: Stefan-
Boltzmann Constant = 5.676 x 10-8 W/m2-K4
4T
A
q s

51. Radiation which is given off by a body
because of its temperature is called
thermal radiation
A body of a temperature larger than 0
K emits thermal radiation

52. Radiation Heat Transfer

53. • Radiation = Radiates (heat escaping the sun)

55. Conduction, Convection and Radiation

57. • Solar radiation is often called short-wave
radiation, and atmospheric radiation is
often called long-wave radiation.
• Solar radiation has wavelengths mainly
between 0.3µm and 3µm; atmospheric
radiation has wavelengths mainly between
5µm and 50 µm