[TRNSYS-users] wet bulb temperature above maximum wet bulb temperature in the psychrometric routine

[David BRADLEY]

Dear Ahmadu,
   It looks as if your simulation is going in the right direction but as 
though there are a lot of small errors. I don't have the *.bui file for 
your building so I cannot run the simulation. However, in taking a look 
at some components, here is what I noticed:

- the equation for the return temperature from the building is 
incorrect. It is Tout = Tin+(Qload/4190*mdot) and should be Tout = 
Tin+(Qload/(4.190*mdot)). The cooling load output of Type56 is in kJ/h.
- the pump in that same loop has a rated flow rate of 100 kg/h, which is 
unrealistically small. This pump sets the maximum rate of flow to the 
load to 100 kg/h and will result in a VERY large temperature increase. 
In fact, the same is true of all the pumps in the system. If you aren't 
sure about pump sizes, try to get an idea of what kind of temperature 
rise you want across a given branch of the system and determine the flow 
rate that would obtain such a temperature rise.
- there is no thermal capacitance on your absorption chiller's cooling 
water loop. I think you need to put a tank in that loop.
- as in a previous email that I sent to the list today, make sure that 
your tanks are large enough to hold more than one timestep of fluid in 
them.
- your time step should be on the order of 1 to 5 minutes, not an hour. 
TRNSYS can only make one control decision per timestep and systems that 
can only change their behavior once per hour rarely converge.
- on both of your Type4 tanks, go to the output tab and enter "4" in the 
field that asks how many nodes there are in the tank besides the top and 
bottom nodes. There is an inconsistency in your tanks as to how many 
nodes they are modeling.

I am afraid that this is not an exhaustive list. I would recommend that 
you back up to the point where you have just the building giving loads 
to the equation labeled "Equa." Give a constant flow of water to that 
equation and make sure that you are seeing somewhere between a 5 and 10C 
temperature rise in the water at the peak load. Next, add pump 
"Type3b-5" and control the flow through "Equa." Add one component at a 
time and each time you add a new now, attach a type65 to it and plot 
some output or other to make sure that you fully understand the results 
that you are seeing.
Kind regards,
  David



On 10/29/2012 08:09, Talib Ahmadu wrote:
> Dear all, I am working on an absorption cooling system using trnsys 
> 16. each time I simulate it gives an error. "the utility 
> psychrometrics which calculates the thermodynamic properties of moist 
> air has been called with a wet bulb temperature above the maximum wet 
> bulb temperature for the given pressure". All efforts to address the 
> problem have not worked out. Can anyone help me out on this. Pls I've 
> attached the tpf file. Thanks.
>
>
>
> _______________________________________________
> TRNSYS-users mailing list
> TRNSYS-users at cae.wisc.edu
> https://mailman.cae.wisc.edu/listinfo/trnsys-users

-- 
***************************
David BRADLEY
Principal
Thermal Energy Systems Specialists, LLC
22 North Carroll Street - suite 370
Madison, WI  53703 USA

P:+1.608.274.2577
F:+1.608.278.1475
d.bradley at tess-inc.com

http://www.tess-inc.com
http://www.trnsys.com

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