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Chapter: Mechanical - Engineering Thermodynamics - Basic Concepts And Definitions

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Sensible and Latent Heat

It is known that a substance can exists in three phases namely solid, liquid and gas. When a substance is heated or cooled temperature of the substance increases or decreases respectively unless there is any phase change.

Sensible and Latent Heat

 

It is known that a substance can exists in three phases namely solid, liquid and gas. When a substance is heated or cooled temperature of the substance increases or decreases respectively unless there is any phase change. Quantity of heat added or removed to change the temperature by unit degree is known as specific heat. For solids and liquids same quantity of heat is required to cause unit degree rise for both constant pressure heating as well as constant volume heating as they are incompressible. But for gases there is appreciable difference in the quantity of heat required to cause unit difference in temperature between constant volume and constant pressure processes. Accordingly, they are known as specific heat at constant volume (CV) and specific heat at constant pressure (CP). Thus to increase the

temperature of m kg of the given substance by DT degree, amount of heat required is given by


Q =mCvDT at Constant Volume                        ...(2.5)

Q1 =mCPDT at Constant Pressure                 …(2.6)

If a certain single component system is undergoing phase change at constant pressure, temperature of the system remains constant during heating or cooling. Quantity of heat removed or added to cause the change of phase of unit mass of the substance is known as latent heat. For example latent heat of fusion of water is the amount of heat to be removed to solidify 1 kg of water into 1 kg of ice at a given temperature.

 

Let us consider a process of converting 1 kg of ice at -30°C to system to steam at 250°C at atmospheric pressure. We know that ice melts at 0°C and water evaporates at 100°C at atmospheric pressure.

 

For a constant rate of heating, if temperature at different instants are plotted we will get a graph as shown in Figure 2.9.


Figure 2.9 Illustration for sensible and latent heat        

The total heat required can be obtained as follows:      

          Q  = Qab + Qbc + Qcd + Qde + Qef ...(2.7)

          Qab = mCice (tb - tc)        ...(2.8)

Qbc   =  Latent heat of melting of ice at 0oC       

          Qcd = mCwater (td - tc)       ...(2.9)

Qde   = Latent heat of evaporation of water at 100oC  

          Qef = mCPSteam (tf - te)     ...(2.10)

Where Cice =Specific heat of ice        

Cwater = Specific heat of water 

 

CPSteam =Specific heat of steam at constant pressure

 

Reversible Adiabatic Process

 

A reversible process during which, the system and the surroundings do not exchange any heat across the boundary is known as reversible adiabatic process. For such a process, pressure and volume variation is governed by the law :

pV =constant                          . ..(2.11)

Where

 

Cp is the specific heat at constant pressure

 

CV is the specific heat at constant volume

 

Detailed discussion on these specific heats is presented in the next chapter.

 

A wall which does not permit the heat flow across it is known as adiabatic wall, whereas the wall that permits the heat is known as diathermic wall. In an adiabatic process the only possible energy interaction across the boundary of the system is work transfer to or from the system.

 

Displacement work involved in a reversible adiabatic process can be expressed as



Comparison between work and heat

 

l  Both heat and work are boundary phenomena, that is, they occur only at the boundary.

 

l       The interaction due to the temperature difference is heat and all other interactions are to be taken as work.

 

l      Both work and heat are path functions, that is, they are inexact differentials.

 

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