HEAT, WORK AND STEADY FLOW ENERGY EQUATION CLASS NOTES

HEAT, WORK AND STEADY FLOW

ENERGY EQUATION CLASS

NOTES

 

COMPARISON BETWEEN HEAT AND WORK

SIMILARITIES

Sr No	Heat	Work
1	Form of energy	Form of energy
2	Across a boundary	Across a boundary
3	Cross the boundary whenever there is a change of state of a body	Cross the boundary whenever there is a change of state of a body
4	It is a path function and hence is an exact differential	It is a path function and hence is an exact differential

DIFFERENCES BETWEEN HEAT AND WORK

Sr. No	Heat	Work
1	Heat is a cheaper and poor grade of energy	Work is a better grade of energy.
2	It can be produced anywhere by any simple means like rubbing hands.	This requires motion.
3	(i)Mainly it is produced by friction.(ii) It is also produced by burning .	Heat causes motion but a small work is obtained from a large amount of heat. Electricity also causes motion.
4	Only a fraction of it can be converted to work.	Work can be converted fully into heat and vice versa is not possible.

STEADY FLOW ENERGY EQUATION

Steady flow means there is no change with respect to time.  There is no change in the properties with respect to time. Steady flow is constant rate of flow of fluid throughout the control volume. The control volume of a thermodynamic system is  fixed area in space through which mass will enter the system and leave the system continuously. Energy and mass of the fluid will remain constant throughout. There will be zero boundary work.

(i)  STEADY FLOW ENERGY EQUATION ON UNIT MASS BASIS

For unit mass flow i.e. 1 kg flow

(u₁+p₁v₁+V₁²/2+gz₁) + q = (u₂ +p₂v₂ +V₂²/2 +gz₂) + w_s

Since u₁+p₁v₁ =h₁ and u₂ +p₂v₂ = h₂ =Enthalpy

Therefore steady flow energy equation becomes

(h₁+V₁²/2+gz₁) + q = (h₂+V₂²/2 +gz₂) + w_s

Where w_s is shaft work

(ii)  STEADY FLOW ENERGY EQUATION FOR CERTAIN MASS, m

For  m kg flow

m (h₁+V₁²/2+gz₁) + Q = m (h₂ +V₂²/2 +gz₂) + W_s

where Q=m q and W_s=m w_s

APPLICATIONS OF STEADY FLOW ENERGY EQUATION

It is applied in the analysis of

(i)                  Reciprocating compressor

(ii)                Rotary compressor

(iii)               Boiler

(iv)              Nozzle

(v)                Turbine

(vi)              Condenser

(vii)             Evaporator