HEAT, WORK AND STEADY FLOW ENERGY EQUATION CLASS NOTES
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On September 13, 2013
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
(u_{1}+p_{1}v_{1}+V_{1}^{2}/2+gz_{1}) + q = (u_{2} +p_{2}v_{2} +V_{2}^{2}/2 +gz_{2}) + w_{s}
Since u_{1}+p_{1}v_{1} =h_{1} and u_{2} +p_{2}v_{2} = h_{2} =Enthalpy
Therefore steady flow energy equation becomes
(h_{1}+V_{1}^{2}/2+gz_{1}) + q = (h_{2}+V_{2}^{2}/2 +gz_{2}) + w_{s}
Where w_{s} is shaft work
(ii) STEADY FLOW ENERGY EQUATION FOR CERTAIN MASS, m
For m kg flow
m (h_{1}+V_{1}^{2}/2+gz_{1}) + Q = m (h_{2} +V_{2}^{2}/2 +gz_{2}) + 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