Effect of Fluid Expansion on Pressure vessel/chamber

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Discussion Overview

The discussion revolves around predicting the final pressure in a pressure vessel partially filled with silicon fluid and nitrogen gas when the temperature is increased. Participants explore the effects of thermal expansion of the silicon fluid and the behavior of the nitrogen gas under changing conditions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant describes the initial conditions of the vessel, including the volumes of silicon fluid and nitrogen gas, and poses a question about predicting the final pressure after a temperature increase.
  • Another participant suggests using the perfect gas law for calculations, assuming negligible evaporation of the silicon fluid and its incompressibility, but does not provide a final solution for the pressure.
  • A different participant notes the significant thermal expansion of the silicon fluid and estimates the volume change due to a temperature increase, emphasizing that this change should be included in pressure calculations.
  • One participant expresses concern that their experimental results differ significantly from predictions made using the perfect gas law and questions whether the compressibility factor (Z factor) might be influencing the results.
  • Another participant raises the possibility of moisture in the oil evaporating to steam, which could affect pressure, and questions the relevance of the Z factor in this context.
  • A participant provides experimental results, including initial and final pressures and volumes, and discusses the thermal expansion of the silicon fluid in relation to the observed pressure changes.
  • Further calculations are presented regarding the volume changes and predicted pressure ratios, with one participant noting that the increase in container volume due to thermal expansion has been neglected in their analysis.

Areas of Agreement / Disagreement

Participants express differing views on the adequacy of the perfect gas law for this scenario, with some suggesting it underestimates results. There is no consensus on the impact of the Z factor or the equilibrium vapor pressure of the silicon fluid, indicating ongoing debate and exploration of these factors.

Contextual Notes

Participants mention assumptions such as the incompressibility of the silicon fluid and the neglect of container volume expansion, which may affect the accuracy of their predictions. The discussion includes various calculations and estimates that depend on specific conditions and assumptions.

Tripoly
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Hi,
I have a vessel/chamber filled partially (25%) with silicon fluid (coefficient of thermal expansion is 0.00096 cm^3/cm^3/ °C) and the rest is filled with nitrogen gas at a certain pressure (P1) and temperature (T1).
my question is, how to predict the final pressure (P2) If I increased the temperature to a certain value (T2) ? because I will be having the effect of pressure increase from the gas due to temperature increase and the effect of pressure increase due to vessel/chamber volume reduction occurred by the silicon expansion
 
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OK, assuming that a negligible amount of the silicon fluid evaporates, and that the silicon oil is incompressible, we can just use the perfect gas law.

Initial : ##P_1V_1=n_1RT_1##
Intial quantity of Nitrogen ##n_1=\frac{P_1V_1}{RT_1}##
Nitrogen is conserved, so ##n_2=n_1##
Volume is conserved so ##V_2=V_1##
Final: ##P_2V_2=n_2RT_2##

The last step, solving for ##P_2##, I leave for you.
 
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That silicone oil has significant thermal expansion. A 100 deg C temperature change will change the volume of oil by 0.00096 X 100 = 0.096 = 9.6%. Since the oil fills 25% of the volume, the gas space volume change will be about 25% of that, or 0.25 X 9.6% = 2.4% for a 100 deg C temperature change. The pressure change due to volume change is smaller than the pressure change due to gas temperature change, but is still large enough that it needs to be included in the calculation.

I say about 25% because the percent fill changes as the volume of the silicone oil changes.
 
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anorlunda said:
OK, assuming that a negligible amount of the silicon fluid evaporates, and that the silicon oil is incompressible, we can just use the perfect gas law.

Initial : ##P_1V_1=n_1RT_1##
Intial quantity of Nitrogen ##n_1=\frac{P_1V_1}{RT_1}##
Nitrogen is conserved, so ##n_2=n_1##
Volume is conserved so ##V_2=V_1##
Final: ##P_2V_2=n_2RT_2##

The last step, solving for ##P_2##, I leave for you.
anorlunda said:
OK, assuming that a negligible amount of the silicon fluid evaporates, and that the silicon oil is incompressible, we can just use the perfect gas law.

Initial : ##P_1V_1=n_1RT_1##
Intial quantity of Nitrogen ##n_1=\frac{P_1V_1}{RT_1}##
Nitrogen is conserved, so ##n_2=n_1##
Volume is conserved so ##V_2=V_1##
Final: ##P_2V_2=n_2RT_2##

The last step, solving for ##P_2##, I leave for you.
I tried that method to compare it with my experiment. this method is under estimating the results (huge difference) do you think that the Z factor is the reason or what else may affect the results ?
 
Might there be moisture in the oil that evaporates to make steam?

I'm not sure what you mean by Z factor.

I'm going to ping an expert @Chestermiller , maybe he can give better help.
 
anorlunda said:
Might there be moisture in the oil that evaporates to make steam?

I'm not sure what you mean by Z factor.

I'm going to ping an expert @Chestermiller , maybe he can give better help.
The Z factor is the "compressibility factor" which represents deviation from ideal gas behavior. I would guess that the pressures in this system are not high enough for significant deviation from ideal gas behavior. What is the value of P1?

There is another factor that needs to be considered. What is the equilibrium vapor pressure of the oil at the 100 C higher temperature? This will add to the ideal gas pressure of the nitrogen.
 
@Chestermiller
experimental results as follows:
initial pressure at 60F is 1,479 psig
finial pressure at 150F is 1,972 psig
Total volume of silicon (occupying 75% of chamber) 240.2 cc
Total volume of the chamber is 320.3 cc
thermal expansion is 0.00096 cm^3/cm^3/ °C
I do not know about the equilibrium vapor pressure but this is the silicon details that I am using
https://krayden.com/technical-data-sheet/dow_510_tds/
 
Last edited:
The absolute pressure ratio is 1.33. The absolute temperature ratio is 1.163. For a 90 F temperature increase, the increase in silicon oil volume is (0.00096)(90)/1.8=0.048 of the initial volume. So, the change in volume of the silicon oil is (240.2)(0.048)=11.5 cc. The initial volume of air is (320.3-240.2)=80.1 cc. So, the volume of air at the higher temperature has decreased to 80.1 - 11.5 = 68.6. So the volume ratio is 0.856. So, assuming Z = 1, the predicted pressure ratio would be 1.163/0.856 = 1.36. This is pretty close to the observed 1.33. What has been neglected here is the increase in volume of the container as a result of thermal expansion.
 
Chestermiller said:
The absolute pressure ratio is 1.33. The absolute temperature ratio is 1.163. For a 90 F temperature increase, the increase in silicon oil volume is (0.00096)(90)/1.8=0.048 of the initial volume. So, the change in volume of the silicon oil is (240.2)(0.048)=11.5 cc. The initial volume of air is (320.3-240.2)=80.1 cc. So, the volume of air at the higher temperature has decreased to 80.1 - 11.5 = 68.6. So the volume ratio is 0.856. So, assuming Z = 1, the predicted pressure ratio would be 1.163/0.856 = 1.36. This is pretty close to the observed 1.33. What has been neglected here is the increase in volume of the container as a result of thermal expansion.
Great! many thanks
 

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