Evaluate the specific enthalpy - thermodynamics, Mechanical Engineering

Evaluate the specific enthalpy - thermodynamics:

Steam is being generated in a boiler at a pressure of 15.25 bar. Calculate the specific enthalpy when

(i)  Steam is dry saturated

 (ii)  Steam is wet saturated having 0.92 as the dryness fraction, and

  (iii)  Steam is already superheated, the temperature of steam being 270°C.

Sol.: Note. Specific enthalpy means enthalpy per unit mass. From steam table which is given below, we get the following data:

 

Absolute pressure (P)

bar

Saturation temperature (t) ºC

Specific enthalpy kJ/kg

Water (hf)

Latent heat (hfg)

15

15.55

198.3

200.0

844.6

852.4

1947

1941

Now  15.55 - 15.25 = 0.30 bar

15.55-15 = 0.55bar

For difference of pressure of 0.55 bar, difference in t (or ts) = 200 - 198.0 = 2.0°C For difference of pressure of 0.30 bar, difference of

t = (2/0.55) × 0.30 =1.091°C. Corresponding to 15.25 bar, perfect value of

t = 200 - 1.091 = 198.909°C

For difference of pressure of 0.55 bar, difference of hf (which is heat of the liquid) = 852.4 - 844.6 = 7.8 kJ/kg For difference of pressure of 0.30 bar, difference of hf  =   (7.8/0.55) × 0.30 = 4.255 kJ/kg. Corresponds to 15.25 bar, exact value of

hf  = 852.4 - 4.255 = 848.145 kJ/kg.

Again, for difference of pressure of 0.55 bar, difference of hfg (which is latent heat of evaporation)

= 1947 - 1941 = 6 kJ/kg.

For difference of 0.30 bar, difference of

hfg  = (6/0.55) × 0.30 = 3.273 kJ/kg. Corresponding to 15.25 bar, perfect value of

hfg = 1941 + 3.273 = 1944.273 kJ/kg

[Greater the pressure of steam generation less is latent heat of evaporation.]

The data computed above are written in a tabular form as below:

 

Absolute pressure (P)

bar

Saturation temperature (t) ºC

Specific enthalpy kJ/kg

Water (hf)

Latent heat (hfg)

15.25

198.909

848.145

1944.273

 

(i) When steam is dry saturated, its enthalpy can be given by

HDry = hf  + hfg  kJ/kg

= 848.145 + 1944.273 = 2792.418 kJ/kg                                           .......ANS

(ii) When steam is wet saturated, its enthalpy can be given by

Hwet = hf  + xhfg   kJ/kg

= 848.145 + 0.92 × 1944.273 = 2636.876 kJ/kg                            .......ANS

(iii) When steam is superheated, its enthalpy can be given by

Hsup = hf  + hfg  + Cp(tsup  - ts) kJ/kg

Posted Date: 10/15/2012 7:31:33 AM | Location : United States







Related Discussions:- Evaluate the specific enthalpy - thermodynamics, Assignment Help, Ask Question on Evaluate the specific enthalpy - thermodynamics, Get Answer, Expert's Help, Evaluate the specific enthalpy - thermodynamics Discussions

Write discussion on Evaluate the specific enthalpy - thermodynamics
Your posts are moderated
Related Questions
For the beam shown below, we need to determine: (i)  the support reactions R L and R R (ii) the shear force and bending moment diagrams (iii) the maximum stress in the b

Internal combustion models of engine: The engines which develop power by combustion of fuel within the engine are called internal combustion engines. The examples of internal


Track Renewal: Need for Track Renewal Normal maintenance is able to keep up the track standards only to a limited extent. The track gets overaged or the speeds and tr

There are various methods to visualize the design process which can be summarized as given below: a. An automated drafting tool and presentation of graphics. b. A means

Heat distribution in spot welding Besides the resistance at the interface, there are six other resistance through which the same welding current is passed. They are: Res

Distribution of shearing stresses on l--sections and miscellaneous sections

Determine the maximum bending moment: A 12 m span simply supported beam is carrying a consistently distributed load of 2 kN/m over a length of 6 m from the left end and point

Contrary to the misconception that the growth in automobile industry has catered only to the higher income-stratum of society by producing mostly passenger cars, the fastest growt

Q. Describe about Water disposal system? Disposal water from the Evaporator Package is cooled down to 50°C by the Disposal Cooler and is sent to Disposal Water Tank(s). Disp