Pressure drop as volume changes

In summary: It would be helpful if you could provide a link to a resource where you have covered this law in more detail.In summary, the author is trying to calculate pressure losses in a system. He has attached an image to show what he means. The first tank has a volume of 92.8 litres and the pressure is 155 psi when the shutoff valve is open. The second tank has a volume of 192.8 litres and the pressure is 74.59 psi when the shutoff valve is open. The author calculates the pressure in the larger tank to be 17.63 psi.
  • #1
David1
7
0
I am trying to calculate pressure losses. I've attached an image to show what I mean. Starting out tank 1 is 92.8 litres at 155 psi and when the shutoff valve is open the volume goes from 92.8 to 192.8 and so I thought the pressure would drop by the same factor that the volume increased by so giving 74.59 psi in each tank (192.8/92.8=2.078 so 155/2.078=74.59). This is then fed into a larger tank and using the same method as before I worked out the pressure would be 17.63 psi. Is this the correct way to calculate pressure changes?
 

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  • #2
Are the tanks allowed to thermally equilibrate with the room before they are separated? Or is the transfer made, and then the temperatures re-equilibrate with the room? Or are the tanks insulated so well that they don't ever re-equilibrate thermally with the room?

If the thermal equilibration occurs before the tanks are separated, then the final pressures can easily be calculated using the ideal gas law.
 
  • #3
The tanks haven't yet been positioned but the aim would be to position them next to each other. The aim is to conserve some air that would be otherwise be wasted so that it can be used to inflate a tyre. I hadn't really considered how temperature could affect this. Would it be better to allow the temperature to equal out before the valve is closed or close the valve and have each tank equal out themselves?
 
  • #4
David1 said:
The tanks haven't yet been positioned but the aim would be to position them next to each other. The aim is to conserve some air that would be otherwise be wasted so that it can be used to inflate a tyre. I hadn't really considered how temperature could affect this. Would it be better to allow the temperature to equal out before the valve is closed or close the valve and have each tank equal out themselves?
I'm assuming that the tanks are at the same temperature (say 20 C) before the valve is opened. As the gas transfers, depending on whether the tanks are insulated or not (and how quickly the transfer takes place), the temperatures in both tanks can change. And they could continue to change after the valve is closed. So, are the tanks insulated? And, if they are not, are you going to wait until the temperatures equilibrate with the room before you close the valve again?
 
  • #5
The tanks are not insulated. Would there be a way to calculate the time for the tanks to equal out temperature?
 
  • #6
David1 said:
The tanks are not insulated. Would there be a way to calculate the time for the tanks to equal out temperature?
Yes. But first, let's assume that enough time is allowed for the temperatures to equilibrate with the room while the valve is still open. Okay? If so, are you familiar with the ideal gas law and how it would apply to this system? (I assume the pressures you indicate are gauge pressures, correct?)
 
  • #7
Yes I'm working with gauge pressure. I haven't used the ideal gas law in a few years and so I may need some help applying it but am looking into it now.
 
  • #8
It looks like the numbers in your figure were calculated using the ideal gas law, assuming constant temperature.
 

FAQ: Pressure drop as volume changes

1. What is pressure drop as volume changes?

Pressure drop as volume changes refers to the decrease in pressure that occurs when the volume of a gas or liquid is increased. This is due to the decrease in density of the substance as its volume increases.

2. How does pressure drop affect flow rate?

Pressure drop has a direct impact on flow rate, as it is one of the factors that determines the rate of flow in a system. As pressure drops, the flow rate will also decrease.

3. What causes pressure drop as volume changes?

Pressure drop is caused by a decrease in the density of a substance as its volume increases. This can be due to factors such as temperature changes, changes in the physical properties of the substance, or changes in the flow rate.

4. How can pressure drop as volume changes be measured?

Pressure drop can be measured using a pressure gauge, which measures the difference in pressure between two points in a system. This can then be used to calculate the pressure drop as volume changes.

5. What are the practical applications of understanding pressure drop as volume changes?

Understanding pressure drop as volume changes is important in various industries, such as chemical engineering, where it is necessary to maintain a certain flow rate and pressure in a system. It is also important in the design and operation of pipelines, pumps, and other fluid systems.

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