DaDaDadeo
Member
The IAPWS-IF97 steam table is used as a measuring standard for the correlation between steam temperature and pressure. The calculation formula(s) for PLCs can be elusive. There are formulas such as the Antoine and Clausius-Clapeyron, but these deviate too much from the IF97 curve. EngineeringToolbox.com has one that may be accurate enough for boilers and pressures less than 15 Bar.
Below are different formats of the same formula (Remove the "*14.554" for Bar units):
RSLOGIX 500 (F8:0 = Float Tag Temperature in C)
(((2.71828**((77.345+(0.0057*(F8:0+273.15)))-(7235.0|(F8:0+273.15))))|((F8:0+273.15)**8.2))|100000.0)*14.554
RSLOGIX 5000 (SBR_Input_REAL = Float Tag Temperature in C)
2.71828**(77.345+(0.0057*(SBR_Input_REAL+273.15))-(7235.0/(SBR_Input_REAL+273.15)))/((SBR_Input_REAL+273.15)**8.2)/100000.0*14.554
Float Script (IDEC)
// "@" is for temporary storage
@1 = [IN_TEMPERATURE] +273.18 ;
@2 = ( ( 77.345 + ( 0.0057 * @1 ) ) - ( 7235.0 / @1 ) );
@2 = EXP(@2);
@3 = POW( @1 , 8.2 ) ;
[OUT_PRESSURE] = ((@2 / @3 ) / 100000) * 14.554;
EXCEL (A2 = Cell with Temperature in C)
=(2.71828^(77.345 + 0.0057*(A2+273.15) - 7235 / (A2+273.15) ) / (A2+273.15) ^8.2/100000)*14.554
Reference: https://www.engineeringtoolbox.com/water-vapor-saturation-pressure-air-d_689.html
Below are different formats of the same formula (Remove the "*14.554" for Bar units):
RSLOGIX 500 (F8:0 = Float Tag Temperature in C)
(((2.71828**((77.345+(0.0057*(F8:0+273.15)))-(7235.0|(F8:0+273.15))))|((F8:0+273.15)**8.2))|100000.0)*14.554
RSLOGIX 5000 (SBR_Input_REAL = Float Tag Temperature in C)
2.71828**(77.345+(0.0057*(SBR_Input_REAL+273.15))-(7235.0/(SBR_Input_REAL+273.15)))/((SBR_Input_REAL+273.15)**8.2)/100000.0*14.554
Float Script (IDEC)
// "@" is for temporary storage
@1 = [IN_TEMPERATURE] +273.18 ;
@2 = ( ( 77.345 + ( 0.0057 * @1 ) ) - ( 7235.0 / @1 ) );
@2 = EXP(@2);
@3 = POW( @1 , 8.2 ) ;
[OUT_PRESSURE] = ((@2 / @3 ) / 100000) * 14.554;
EXCEL (A2 = Cell with Temperature in C)
=(2.71828^(77.345 + 0.0057*(A2+273.15) - 7235 / (A2+273.15) ) / (A2+273.15) ^8.2/100000)*14.554
Reference: https://www.engineeringtoolbox.com/water-vapor-saturation-pressure-air-d_689.html
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