MEASUREMENT OF QUALITY OF STEAM

Measurement of Quality of Steam

For pure substances –in-dependant properties such as pressure and temperature are needed (if pressure is known, temperature can be found)

We need specific volume, enthalpy or quality of mixture(liquid & vapour)

For measuring specific volume of mixture (quality) calorimeters are used.

a) Separating Calorimeter

b) Throttling Calorimeter

c) Separating & Throttling Calorimeter

d) Barrel Calorimeter

e) Electric Calorimeter

a) Separating Calorimeter:

Fig

X = M/ (M+m)

M=Mass of dry steam read from gauge of condenser

M=Mass of water separated in specified time interval

x= Dryness fraction or quality of steam

b) Throttling Calorimeter

Fig

x1= (h-hf1)/ hfg1

Let P1 = Initial Pressure

x1 = Dryness Fraction

hf1 = Enthalpy of water at P1

hfg1 = Enthalpy of vaporization

Cps = Sp heat of superheated steam after throttling..kJ/ kg

Tsat = Saturation temp at P2

During throttling process, the enthalpy remains constant.

Enthalpy of steam at P1 before throttling= Enthalpy of steam at P2 after throttling

hf1 + x1 hfg1 = hg2 + Cp (T2-Tsat)

x1 hfg1= hg2 + Cp (T2-Tsat) - hf1

x1= hg2 + Cp (T2-Tsat) - hf1/ hfg1

= h- hf1/ hfg1

h = sp enthalpy at state 2 from superheat read from table

CLAUSIUS STATEMENT OF SECOND LAW

▀Clausius Statement of second law:

It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a system at low temperature to another system at high temperature.

Clausius statement points-out the essence of work input required in Refrigerators & Heat pumps

Fig

Heat can’t flow from low temperature to high temperature systems.

COP=Q/W=Q/0=∞.............(impossible)

It is impossible to construct a refrigerator, which operates without an input of work.

Fig

CLAUSIUS STATEMENT

▀Clausius Statement of second law:

It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a system at low temperature to another system at high temperature.

Clausius statement points-out the essence of work input required in Refrigerators & Heat pumps

Fig

Heat can’t flow from low temperature to high temperature systems.

COP=Q/W=Q/0=∞.............(impossible)

It is impossible to construct a refrigerator, which operates without an input of work.

Fig

ENTROPY

Entropy

Entropy represents the energy in the system which couldn't be utilised due to some deficiency of the system. As we know that when we burn oil, only part of heat energy is utilised and rest goes un-utilised. This resents that efficiency of system is not equal to input of heat energy into Heat-Engine and it is always less than heat input. This is as brought out in 2nd Law of Thermodynamics.

Whether it is a case of Heat Input (heat engine) or Heat extraction (Refrigeration), the system output will be < total input. The most ideal case for thermodynamic cycle is Carnot-Cycle. It is defined as follows:

"Entropy" is defined as a measure of unusable energy within a closed or isolated system (the universe for example). As usable energy decreases and unusable energy increases, "entropy" increases.

Entropy is also a gauge of randomness or chaos within a closed system. As usable energy is irretrievably lost, disorganization, randomness and chaos increase.

For more details refer my scanned hand notes.

Teacher's notes