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If an ideal gas flows at a flow through a pipe from inlet to outlet, there is a pressure increase, and a temperature decrease, what is the density at the outlet compared to inlet?.
No actual figures, just a theoretical Q in my coursework.
I'm thinking Density remains the same? assuming pressure increase is offset equally by temperature drop...
Your gas is going to have a hard time flowing towards higher pressure.
Density of C3 and C4 is usually measured in Vac at 15 deg C for ships bills of laden so I assume density changes dependant on pressure.
Sorry, typed it arse for tit! Pressure decrease, temp increase!.
PV/T = constant is all I can recall
How enbarassi g
As ot flows and the pressure drops you will get a decrease in density and an increase in velocity as the gas expands.
Pressure increase would increase the density; same amount of gas in smaller space. Temperature decrease also increases density (cold air sinks).
Also, since they don't give you any numbers, the changes must be in the same direction. If they had opposite effects on density, you'd need the numbers to work out which was dominant.
Hmmmm...
There is a correction factor used to take account of this. For most gas users this is a standard factor of 1.022640 IIRC.
Pressure drop will tend to decrease the density and the temperature increase will tend to do the same. In real life scenarios like this the pressure change tends to dominate as the temperature changes are generally relatively small once converted to Kelvin.
The mass flow must be constant. If the cross-sectional area and velocity at the exit is the same as at the inlet, the density must be equal, surely.
Makes sense to me gonefishin.
The forum appears to have eaten my serious answer?
Anyway, the problem is underspecified for an ideal gas. To work out the exit temperature you need to know the inlet conditions and the outlet pressure.
PV=nrT
Volume in this case becomes cross section area x velocity.
In a normal pipe flow calculation you actually assume the change in pressure is negligible along the pipe in order to calculate the pressure drop and take the outlet condition as the starting point as generally this is the more defined of the two (a control valve upstream would be setting the pressure at the inlet to a system some distance away).
Depending on the conditions this may well be the speed of sound and you get into the realms of compressible flow. This would not happen on shorter pipes transporting gas within with a refinery for example, but may be relevant in pipelines or in systems relieving to atmosphere. These systems also tend not to be isentropic.
If you take a non ideal gas it's actually easier as you get a number for the ratio of specific heats. (Cp/CV). For air (mostly nitrogen ) this is 1.41 iirc, and can reasonably be estimated by the type of molecule (nitrogen has a triple bond, meaning fewer degrees of freedom meaning it tends to get hot as it can't absorb energy). This then allows you to calculate by how much the temperature drops as the gas expands or increases as it is compressed.
One very important application is in commissioning a plant it's often filled with inert nitrogen first to flush out any air. This has actually lead to explosions as when the process fluid is introduced and pressurised nitrogen gets compressed and trapped in pockets which get incredibly hot, causing the process fluid to react (e.g. butadiene is a chemical used to make synthetic rubber, and undergoes polymerisation at high temperatures, affectionately known as popcorn polymerisation due to the appearance of the expanded polymer when the pipe bursts).