Vapor Pressure

[homerphish]

I konw that the formula to get the vapor pressure of a refrigerant from the temperature is

log P=A+B/T+ClogT+DT+ET^2+FT^3

Where A-F are constants dependent on the refrigerant.Does anyone know how to reverse this equation to get T in terms of P?

 

[LaSalle1940]

Tell me what refrigerant you're working with, and I can get you the constants. As you might guess, the constants are specific to each substance.

Unfortunately, that equation doesn't lend itself to an algebraic solution since it has transcendental functions (i.e., the natural log term). Your best bet would be to set up an Excel spreadsheet and use the "solver" function: you'd provide the constants and the vapor pressure, and in effect, solve numerically for the corresponding temperature.

 

[25362]

The Antoine equation: log p = A-B/(t+C)
is widely used.

If you are looking for a rough approximation then use:
log p = A-B/T that generally yields a straight line, and is helpful for interpolations. Perry VI, fig. 12.26, confirms that fact for about 23 refrigerants.


The above equations can be rearranged to yield, respectively:

t = [B/(A-log p)] - C

and

T=B/(A-log p)

t in deg C, T in K, p , the sat. pressure in mm Hg or otherwise depending on the source.

Lange's Handbook of Chemistry (McGraw-Hill) tabulates values of these constants for many compounds. So does the NIST Chemistry Webbook. Be careful when reading data since sometimes pressure and temperature are given in other units.

 

[sterl]

You can reverse the equation, but why bother?

Dupont and Allied Siganl and others publish the "accepted" constants and ranges of applicability for Equations of State for all the refrigerants they produce...

ASHRAE and the NBS published those for the OLD refrigerants quite some time ago...The ASHRAE publish was from 1969, and include both Thermodynamic and Thermophysical properties for the :then: convetional refrigerants.

 

[homerphish]

I need to reverse the equation, becasue I have 60,000 data points from a pressure sensor and I want to include the saturated temperatures for these pressures in my data to check the accuracy of the temperature sensors. THe only way to get this, aside from using a chart and individually inputing each temperature for each data point, it would be much easier for me to have the equation for T in terms of P so I can use it in Excel.