6 replies to this topic

[aliadnan]

Hi
I have a query. What will be the pressure of the resulting stream when two streams are mixed together having different pressures.
Suppose we have X amount of stream A having pressure P1 & temperature T1 and Y amount of stream B having pressure P2 & temperature T2.
Then the resulting pressure should'nt be equal to

P1*X + P2*Y = P (resulting presure of stream)

And the resulting temperature should'nt be

T1*X + T2*Y = T ???

Please comment on that

In one of the threads in this fourm I have seen that someone saying that there is a heuristic which states that "When mixing two fluid streams, the pressure of the exit stream will be equal to the pressure of the stream with the lower pressure".
If it is correct then can anyone explain it be me why this is soo.

Regards

[Ankur Shah]

yes that is true what that person said about mxing two streams... btw the temperatures will depend on the ideality of the system.

At one point of time, while simulating the mixing of two streams which were both at 150 C (of propene and water resp), i got the exit temperature of 180 C!!! that showed a significantly non-ideal system. So depends on what type of system you have. Since there can be +ve and -ve deviations, you cant even say whether there would be an increase or decrease in the temp. Hence, highly system dependant.

Ankur

P>S: I was the one who talked abt mixing two streams earlier

[aliadnan]

Hi
I hope to get few more comments on my query because it is still not much clear to me. Some comments from the experienced engineerings.

Regards

[MrShorty]

For the pressure part of the question: Fluid flow is always from high pressure to low pressure (anytime we want flow to go against the gradient we employ a pump or compressor to increase the feed pressure). Not sure that this is a complete explanation for "the pressure of the exit stream will be equal to the pressure of the stream with the lower pressure."
Consider filling a tank with two water streams. Stream one is at some Pressure P1 and stream 2 is at a higher pressure P2. As long as the pressure in the tank is lower than either P1 or P2 the two streams will continue to flow into the tank. Once the pressure reaches P1, flow 1 will stop flowing. If the pressure in the tank continues to increase, the contents of the tank will start to back flow into stream 1. Basically, if we insist that all feeds are flowing into the system, the system pressure cannot be greater than the lowest feed pressure, or that stream will not be flowing into the system (hope that makes sense).

As for the final temperature of a mixed stream. Thermodynamics allows for some rather complicated scenarios with multiple phases and such. Consider a simple two liquid stream case like above, so there's only one phase (visualize filling a bathtub). We take a certain amount of hot water and a certain amount of cold water and mix them together and the final temperature is in between the temperature of the two streams. I remember doing these kinds of problems in high school chemistry (given mass m1 of water at a temperature T1 and mass m2 of water at T2, mix them together, assume no heat loss to the surroundings and what is the final temperature).

Of course, if there are multiple phases (eg liquid in equilibrium with vapor) and multiple components (water and methanol), then the relationship between temperature and pressure will depend on the VLE behavior of the system.

In conclusion, I suggest reviewing a few basic concepts:

Fluids flow from high pressure to low pressure
In simple cases, heat flows from high temperature to low temperature.
Remember that heat and temperature are not synonymous.
If phase equilibria are involved, then review some basic thermodynamics of phase equilibria (Gibb's phase rule, activities and activity coefficients, fugacity and fugacity coefficients, equations of state, etc.)

hope that helps.

[aliadnan]

Hi
First of all I will thank Mr. Shorty for a very nice reply which really helped me in understanding. Now secondly I would like to have comments on ChE 's reply because what he said is quite different than Mr. shorty's reply. I am looking forward for more comments on that.

[djack77494]

Aliadnan,
The systems discussed by MrShorty and ChE are very different from one another. MrShorty explains (I think quite well) how two flowing streams combine. The outflowing stream is at a pressure lower than either inflowing stream as hydraulics demands in order to obtain flow. (There appear to be some exceptions to this, such as eductors, but they still meet the above statement if one adds in the dynamic [velocity] pressure to the static pressure.) The post from ChE imagines a closed system. There are not two streams joining together to generate a third (ouflowing) stream. Instead there is one stream which flows from a higher to a lower pressure. The volume is fixed and so the resulting pressure is the average fo the two initial pressures. Both scenarios are true as described, but they are very different.
Doug