[TRNSYS-users] Problems in using humidity ratio

[David Bradley]

Jace,
   Make sure that wherever possible, you are passing around the 
atmospheric pressure to the components. Often times the air is not 
saturated if you know the pressure but is saturated if you assume that 
pressure is 1atm. The rest of the answer depends a bit on what kind of 
weather data you are using. If you are using a "standard format" weather 
file such as TMY2, meteonorm, etc. then the values should be corrected 
for pressure. The other piece of advice that I would give is that 
wherever possible, try to pass relative humidity around instead of 
humidity ratio. The humidity ratio values are so small that when you 
apply the convergence tolerance of 0.001 or better 0.0001, even a tiny 
tiny change in humidity ratio can begin to cause some convergence 
issues. If you use RH, the values are less sensitive.
Best,
  David


On 6/16/2010 01:15, LE GALL, Jace wrote:
>
> Dear All,
>
> I am currently using TRNSys to model an HVAC installation but when 
> using the weather data from humid regions such as Singapore or Hong 
> Kong, I have a recurrent message on different units such as Type52b 
> cooling coil, Type667c heat recovery wheel and Type88 single zone model.
>
> Message : The referenced UNIT as called the PSYCHROMETRICS routine one 
> or more times during the simulation withan enthalpy greater than that 
> for saturated air and the given dry bulb temperature. The air was 
> assumed saturated and the enthalpy set for this condition.
>
> I assume that this error leads to more errors in the final energy 
> given as a result and eventually, even manage to crash my simulation 
> when I lower the time step too much, as the number of messages is 
> limited to 500. I based my calculations on the hypothesis that the 
> humidity ratio asked in the different units is the absolute humidity 
> ratio with is kg of water / kg of dry air.  I have isolated Type52b 
> cooling coil to search more deeply my possible mistakes (constant 
> humidity ratio of 0.02kgw/kga and wet bulb temperature varying from 27 
> to 33). According to my psychometric chart (at sea level), the air 
> shouldn't reach saturation but TRNSys tells me the opposite.
>
> Has anyone encountered a similar problem in a simulation and was able 
> to solve it?
>
> Best Regards,
>
> Jace LE GALL
>
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***************************
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
bradley at tess-inc.com

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

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