Re: [TRNSYS-users] mass balance error

[Jones Marcus <Marcus.Jones@ait.ac.at>]

I just solved a similar problem in balancing mass flow in a complex network with splitters and mixers. It's hard to follow your image in detail, but one important question would be; can you identify a single point in the network where all of the fluid passes through? ; Maybe P16 or P20? If so, you could impose the flow at one of those points using the "set flow rate" pump, and the change all other pumps to "flow rate as input". Then your system would balance itself naturally according to the mix/split fractions.

However, if your system does not have such a single "central" flow point, you must do something to set the splitter fractions yourself. In our case, it amounted to setting the splitter fractions sending the flow such that the fractional mass flow outgoing was equal to the fractional flow returning. This was done by dividing the individual flows by the total mixed flow in an equation. You really need to draw it out on paper and derive the fractions so that conservation of mass is preserved in each flow loop. I also suggest trying it on simpler cases if it isn't obvious.

For your second point, I don't think you can set a mixer to choose which stream to accept. This would require information being passed upstream of the mixer, (physically speaking this would be a pressure change). But TRNSYS doesn't model pressure driven flow. I simulate this using splitter fractions. I just noticed you're using type 11d. I don't know what this type is for - it doesn't conserve mass (generally)! I recommend the Tee-Piece instead...

Good luck, and I hope someone corrects me if I'm wrong in any of this!

Marcus


MARCUS JONES, MSc

Energy Department

Sustainable Building Technologies



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-----Original Message-----
From: Stefania Agosta [mailto:stefania.agosta@gmail.com]
Sent: October 29, 2009 5:48 PM
To: trnsys-users@cae.wisc.edu
Subject: [TRNSYS-users] mass balance error

Hi,

I'm using trnsys to model a part of an HVAC system. You can see in the
attached file the configuration of the circuit (ACS-project). The
purpose is to have the temperature in the "Bollitore-B1" equal to 50°C.

I have flow rates coming from the "Puffer" and from the "Boiler" and
going to the HX of the "Bollitore" passing through the "Pump - P10"
but it seems like  the mass balance of this circuit is not respected
(the total mass out of the pump is not equal to the sum of the two
mass flows entering the mixing valve)

AND

I would like to make the mixing valve working as a switching valve
(either the flow rate from the Puffer or from the Boiler should be
allowed) : how can I reach this goal?



I hope you can help me.

Best regards

Stefania Agosta