Pressure rise in closed system.?
Pressure rise in closed system.?
(OP)
Hello Guys,
This should be an easy one; I have a hermetic closed system with 20ccm3 water at 20C. Now I add heat until it reaches 90C, what is the pressure? What if the temperature reaches 120C..?
Thanks in advance!
This should be an easy one; I have a hermetic closed system with 20ccm3 water at 20C. Now I add heat until it reaches 90C, what is the pressure? What if the temperature reaches 120C..?
Thanks in advance!





RE: Pressure rise in closed system.?
Regards
RE: Pressure rise in closed system.?
RE: Pressure rise in closed system.?
1.00172(10^-3)m^3/kg
with that const spec vol, my forumulation yields
20C 0.14MPa which I approximate as one atm.
70C 49.5MPa
90C 79.3MPa
120 130.5 MPa
The KKHM formulation for properties are based on input of v and t. Therefore, I had to iterate to come up with the approximate specific volume.
Regards
RE: Pressure rise in closed system.?
I've always used the rule of thumb "about 100 psi change for every degree farenheit". From your table, the actual relationship doesn't look quite that linear, but for figuring if things will break it still looks ok.
David
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
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RE: Pressure rise in closed system.?
For liquids packed in rigid containers the developed pressure could be approximated by dividing the cubic thermal expansion α=(1/V)(∂ V/∂T)P by the isothermal compressibility κ=(1/V)(∂ V/∂ P)T. An example for water at 30oC:
(0.302×10-3/oC)÷(4.475×10-4/MPa) = 0.67 MPa/oC or about 6.7 bar/oC
The results are not linear because α increases while κ decreases with rising temperatures.
RE: Pressure rise in closed system.?
RE: Pressure rise in closed system.?
Ferlev, please note that what I said for the temperature effects applies to water. With other liquids, temperature increases generally result in a rise of both the thermal expansion and the compressibility.
RE: Pressure rise in closed system.?
RE: Pressure rise in closed system.?
RE: Pressure rise in closed system.?
But expanding a constant weight of water (w/o any evaporation loss) at 1 bar, you can use the cubic thermal expansion expression given by 25362 or just look up the density of water in handbooks. The CRC Handbook of Chemistry & Physics gives the following:
T, oC Density, g/cc Vapor Pressure, kPa
20 0.99821 2.3388
70 0.97778 31.176
90 0.96535 70.117
So, under 1 bar total pressure, 20 cc of water at 20oC would expand to (0.99821/0.96535)x 20 = 20.68 cc.