Bastien,
The combination (w,RH) is indeed unusual and the programmers of Type 33 did
not expect it. If you can do the calculations off-line I would suggest EES
or a similar program as well, but there is also a way to calculate the humid
air enthalpy from w and RH in TRNSYS. It uses Type 33 (Psychrometrics) and
Type 58 (refrigerant properties).
1. In an equation block, calculate Pvsat from w and RH using their
definition:
Pvsat = w * P / ((0.622 + w) * RH)
(note that RH should be from 0 to 1 in this equation, not in %)
2. Input Pvsat (in kPa) and a constant quality into Type 58. You will get
the saturation temperature (i.e. Tdb, the dry bulb air temperature of your
mixture) as an output
3. Input the temperature from Type 58 and RH into Type 33 and you will get h
(using RH is safer than w because small errors
In w and Tdb could lead to unfeasible conditions)
This obviously does not work for w or RH = 0 and will lead to larger errors
for very dry conditions. It also assumes that you know the total pressure P.
If you don't you can assume it is 1 atm (or the average pressure at the
location), but the errors could be much larger. P affects Pvsat directly so
Tdb and h will be affected (you can see that by yourself if you replace P
with 1 in the equation for Pvsat in the attached example).
I am attaching a simple example that reads RH and w from a weather file to
demonstrate the process. You need to watch your units and use a trick to
avoid divisions by zero in the equation for Pvsat.
Michaël Kummert
ESRU, University of Strathclyde
From: David Bradley [mailto:bradley@tess-inc.com]
Sent: 11 February 2009 20:25
To: Bastien GARBAY
Cc: trnsys-users@engr.wisc.edu
Subject: Re: [TRNSYS-users] Psychrometrics
Bastien,
Type33 assumes that you know the dry bulb temperature. You will probably
need to use some other tool (EES, perhaps) to get the temperature based on
the humidity ratio and relative humidity, then use Type33 to get the
enthalpy.
Kind regards,
David
Bastien GARBAY wrote:
Dear TRNSYS users,
I would like to know, if there is a Psychrometrics model, which calculates
the enthalpy of air in function of two others parameters: humidity ratio
and relative humidty of air.
Thanks for your answer.
Bastien Garbay
Cabinet SIDLER / ENERTECH
Quartier Bourgounion
F-26160 FELINES SUR RIMANDOULE
tel-fax : +33 (0)4 75 90 18 54
Courriel : garbay@enertech.fr
Site : www.enertech.fr
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