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HEAT
Heat transfer ways:
Heat is transferred from higher temperature to lower temperature through 3 ways:

1- Conduction:

In Fig.1.1, heat is transferred from hot plate T1 to cooking pot bottom T2 by conduction, T1 > T2.
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Fig.1.1 Conduction heat transfer

2- Convection:

Also in Fig.1.1, heat is transferred from T2 to T3 through conduction between pot bottom and the lower layer of fluid (which in direct contact with pot bottom) – T2 > T3. Just heat is transferred T3 (layer temperature) will be increased, accordingly the liquid density for this layer will be decreased and liquid will go up (because of lower weight) & replaced by another higher density layer, again the new layer temperature will be increased, will go up & replaced by a new layer. This process will be repeated in a continuous manner to transfer heat to all the liquid, this process is known as Natural convection.

Natural convection is negatively affected by particles in fluid & fluid viscosity, which require an external device to create liquid current and accordingly transfer heat to all the liquid. For example, some additives can increase soup viscosity; a stirrer should be used to create motion between soup layers, otherwise, lower layer will be scorched, while higher layers are not yet heated properly. Transferring heat in this way is called Forced convection. Examples for external devices are stirrer in a cooking pan, water pump in a central water cooler and fan in a convection oven

3- Radiation:

Heat transfer through radiation is different than conduction & convection, that it doesn’t need transfer medium to travel through it such as air, water or metal. Heat is transferred through waves from the higher temperature source to the lower temperature receiver. Examples for heat transfer through radiation are salamanders, rotisseries, potato warmers, etc.
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Fig.1.2 Heat transfer through radiation

First law of thermodynamics:

First law of thermodynamics states that “Energy can neither be created nor destroyed“, this is the basic law for all energy sciences, through which you

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Fig.1.3 Cooking process 

can understand energy behavior & efficiency for any energy system (heating, cooling, etc).
As a simple example, in a soup cooking process (Fig.1. 3 ), the energy or power is lost as follows:
P = Electric heater power in KW.
P1 = Heating power lost from hot plate to the surrounding in KW.
P2 = Heating power transferred from hot plate to metal pot in KW.
P3= Heating power transferred from metal pot to soup in KW.
P4= Heating power lost from pot to surrounding in KW.
P5= Heating power lost through heat transfer from soup surface (un-covered
       pot) to surrounding air in KW – (part of P3).
P6= Heating power lost through evaporated water (steam) transfer to
        surrounding in KW – (part of P3).
 
So, from first law of thermodynamics:
P = P1 P2 P3 P4 P5 P6
 
And actual useful heating power in cooking process = P3 – (P5 P6)
And impossible to have:
P < P1 P2 P3 P4 P5 P6     (Energy can not be created)
And also impossible to have:
P > P1 P2 P3 P4 P5 P6     (Energy can not be destroyed)

Efficiency:

As a general rule; efficiency (η) for any system can be simply defined as the ratio between “What we get” on “What we pay for” multiplied by 100 to get result as a percentage (%), or

        What we get
η = ————————   x 100
        What we pay for
 
Applying this on above mentioned example to get
 
      P3 – (P5 P6)
η = —————— x 100
              P
Efficiency (η ) for gas cookers is around (30 to 55 %), while for electric heating cookers is around (50 to 65 %) and for electric induction (see electrical engineering basics) cookers is around 80 to 90 %.
Note:
Above mentioned efficiencies are approximate values for guidance only and may vary from manufacturer to another. Manufacturer should be checked for actual equipment efficiency

Sensible & latent heat:

When we add heat to water inside a pot (Fig.1.4), water temperature rises inside the pan. This energy is called “Sensible heat“, where it is sensed as water temperature rise. As higher the heater or burner power

 

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Fig.1.4 Water boiling 

, as faster the water temperature rise. When water temperature reaches 100 ºC, water start to boil and evaporate. Although the heater is still heating the water, the thermometer reading stay at constant temperature which is the boiling point at 100 ºC. The heat added to water after reaching100 ºC is used to change water from liquid phase to gas phase (steam), this energy does not increase temperature and is called “Latent heat“. If we continue heating, all water will be evaporated with constant temperature 100 ºC, for that when we boil food, it is recommended to start at maximum level of heater or burner power, and just water start to boil we have to reduce heater power where it will be adequate to keep water boiling at 100 ºC (which is maximum cooking temperature in food boiling under atmospheric pressure), and will save energy and reduce water evaporation which means water saving also.

 

Boiling under pressure:
Water boiling point is affected by surrounding pressure, in sea water level the atmospheric pressure equals 1 bar (1 atmosphere), at this pressure water is boiling at 100 ºC. If surrounding pressure is increased, water boiling point will be increased and vise-versa, i.e. at higher pressures than 1 bar, water boiling point will be higher than 100 ºC. This is the basic idea of pressure cooking pans which reduce cooking time. Pressure cooking pans have sealed cover to prevent steam escaping from pan which increases pressure inside the pan and accordingly boiling point. This will increase water temperature (e.g. to become 110 ºC), which speed up cooking of food in less time than ordinary or pressure-less boiling.
 
 
Calorific value:
Calorific value is simply the amount of energy stored in a unit mass of fuel (solid, liquid or gas), or in other words calorific value is the amount of energy released during combustion of 1 kg of fuel. The following table shows some types of fuels and their approximate calorific values in MJ/kg (MJ = Mega joules):
 
Fuel type
Calorific value (MJ/kg)
Butane
46
Propane
46
Natural gas
39.6 (MJ/m3)
Gasoline
44
Diesel oil
41
Grease
41
Charcoal
35
Wood
18