Page 55 - DJJ20063- Thermodynamics 1
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DJJ20063- Thermodynamics 1
ii. Double Interpolation
In some cases a double interpolation is necessary, and it’s usually used in the Superheated
Steam Table. Double interpolation must be used when two of the properties (eg.
temperature and pressure) are not tabulated in the Steam Tables. For example, to find the
o
enthalpy of superheated steam at 25 bar and 320 C, an interpolation between 20 bar and
o
o
30 bar is necessary (as shown in example 2.9). An interpolation between 300 C and 350 C
is also necessary.
Example 2.9
o
Determine the specific enthalpy of superheated steam at 25 bar and 320 C.
Solution to Example 2.9
An extract from the Superheated Steam Tables:
o
t( C) 300 320 350
p(bar)
20 3025 h1 3138
25 h
30 2995 h2 3117
o
Firstly, find the specific enthalpy (h1) at 20 bar and 320 C;
At 20 bar,
T
350 h − 3025 3138 − 3025
1 =
320 − 300 350 − 300
320
300
h = 3070 2 . kJ/kg
1
h
3025 h1 3138
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