View Full Version : Pressure Calculations

Derek

03-09-2001, 01:27 PM

Here's one for those who found calulation of pressures in thin/thick walled cylinders a problem at college.

Take a cylinder (recovery) with any refrigerant at 20 deg C. The cylinder is totally full of liquid (no vapour space) so no easy use of PH charts here.

What will be the pressure for a rise in temperature of a further 20deg (OK 20K)for

a) R123

b) R22

c) CO2

Here's the number one failure mode for recovery cylinders I expect an almost exponential pressure increase but who has the numbers to prove it...

Enjoy!

Prof Sporlan

03-09-2001, 05:17 PM

If we were to consider R-22 at 20°C, NIST Refprop gives us a saturation pressure of 117.3 psig and a liquid density of 75.532 lbm/ft3. At 40°C, we get 207.3 psig and a liquid density of 70.452 lbm/ft3.

Say we have a 30 lb refrigerant tank, and for arguments sake, say 30 lbs of R-22 occupies 80 percent of the volume of the tank at 20°C. If if the tank were completely filled with liquid R-22, we would have: 30 / 0.80 = 37.5 lbs of refrigerant in the tank, which would give us an internal volume of the tank of: 37.5 / 75.532 = 0.4965 ft3. As we raise the temperature of the refrigerant, its density, of course, decreases. In this case, 37.5 lbs of R-22 at 40°C would require: 37.5 / 70.452 = 0.5323 ft3, or an additional 0.0358 ft3, or 5.16 in3. Since liquids are essentially incompressible, this additional space will come at the expense of the tank. The resulting pressures will be largely a function of the "elasticity" of the tank.

Derek

04-09-2001, 03:03 PM

Right Prof so all I need is a perfect state equation assuming a thick walled cylinder with minimal elasticisity before brittle fracture that relates the inverse efects of volumetric expansion to pressure.

Any cylinder manufacturers have that data?

Peter

04-10-2001, 01:50 PM

The pressure for all three will be atmospheric as a 20 degree rise in temperature will almost certainly have blown the bursting disc or worse still the cylinder if it is full of liquid

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