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Q: Boiling point of water vs pressure > 1 bar ( Answered 4 out of 5 stars,   3 Comments )
Subject: Boiling point of water vs pressure > 1 bar
Category: Science
Asked by: ivandelsol-ga
List Price: $9.50
Posted: 01 Aug 2003 16:28 PDT
Expires: 31 Aug 2003 16:28 PDT
Question ID: 237993
Can you give me an equation or graph, solving the boiling point of
water for pressures increasing above one atmosphere.  Particularly
interested in solar water heaters which may have check valves between
the city water supply and the solar water heater

Request for Question Clarification by maniac-ga on 01 Aug 2003 18:43 PDT
Hello Ivandelsol,

It may be easier to recommend
 - an Excel add in OR
 - another application
that would calculate the thermodynamic properties of water / steam at
a range of pressures. If this would be acceptable, please let us know.
If so, please also indicate the type of computer / OS you would be

Subject: Re: Boiling point of water vs pressure > 1 bar
Answered By: haversian-ga on 26 Aug 2003 19:16 PDT
Rated:4 out of 5 stars
Hello ivandelsol-ga,

A brief outline of the problem is as follows:

The hotter a given body of water is, the more it wants to be a gas. 
However, the higher the pressure, the more the gas wants to condense
into a liquid.  Those two oppose each other, but not linearly. 
According to this page on non-linear regression ( ), the formula for boiling point as a
function of pressure is given by Clapeyron's Equation:

Temperature = -3200 / log(Pressure/1700) - 459.7

Temperature in degrees Farenheight; pressure in PSI

For more information about the vapor pressure of water, and boiling:

For more information about the The Clausius-Clapeyron equation:

Search strategy:
boiling point of water at pressure
boiling versus pressure
Clapeyron equation
Clausius-Clapeyron equation

ivandelsol-ga rated this answer:4 out of 5 stars
Well done.  The only thing missing is the EXCEL spread sheet.  But I
can do that, no problem.  Thank you.

Subject: Re: Boiling point of water vs pressure > 1 bar
From: atozxrod-ga on 01 Aug 2003 19:16 PDT
I found a table that shows the vapor pressures (boiling point) [mm HG]
of water at various temperatures in 20-degree Celcius increments. It
is on page 13 at the following URL:

This table could be used to estimate the boiling point of water by
interpolation and unit conversion.

For example, supposed you wanted to know the boiling temperature
(vapor pressure) of water at 3.5 bar.

Since the table is given in mm Hg, conversion of 3.5 bar is necessary.
1 bar is 700 mm Hg.  Therefore 3.5 bar is 2400 mm Hg.

From the table:

the vapor pressure of water at 120 degrees C is 1489.14 mm Hg
the vapor pressure of water at 140 degress C is 2710.92 mm Hg

Therefore the temperature at which water boils (vapor pressure) at
2400 mm Hg is somewhere between 120 degrees Celsius (1489.14 mm Hg)
and  140 degrees Celsius (2710.92 mm Hg).  But where?

*Assuming* a linear relationship between these two values, it can be
found thus:

The difference between 1489.14 and 2710.92 is 1221.78.
The difference between 1489.14 and 2400.00 is 910.86.
910.86 is 74.5519 percent of 1221.78.

Therefore, the temperature at which water boils at 2400 mm Hg (3.5
bar) will be 75.5519 percent of the difference between 120 and 140
degrees celcius.  Since that difference is 20, and 75.5519 percent of
that difference is 15.11, the temperature is 120+15.11, or 135.11.

Hope this is helpful.
Subject: Re: Boiling point of water vs pressure > 1 bar
From: redhoss-ga on 02 Aug 2003 06:12 PDT
You may have a serious problem. If you do have a check valve installed
between your water heater and the water main, you should have a
thermal expansion tank in your system. The T&P valve on your water
heater is not designed to operate on a daily basis. They are a safety
valve only and should be inspected and replaced on a regular schedule.
The thermal expansion tank allows the water in your system a volume to
expand into when expansion occurs due to heating. Water heaters can
explode with considerable force if there is no way to relieve the
Subject: Re: Boiling point of water vs pressure > 1 bar
From: gasser-ga on 24 Aug 2003 13:51 PDT
This relationship requires a complex polynomial, but you can use the
following simple formulae within the prescribed ranges.

For 14.7 psi abs(1 atmos) - 24 psi abs use:-
Y = 52.68logX + 70.3

For 24 psi abs - 100 psi abs use:-
Y = 52.68logX + 0.19X + 65.8

X is steam pressure in psi absolute
Y is steam temperature in degrees Fahrenheit
Logarithms are natural(base e).

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