[TRNSYS-users] increased capacity of air

[jeannieboef]

this can get complicated. If you had a CFD program you may be able to  
deturmine the "age of the air". in one hour all the air is seen not to  
have changed with 1 ACH. But thermodynamically, this is irrelevant.  
the exhaust air is not even considered. at each timestep air at  
enthalpy1 is injected into the zone (enthalpy or energy state is only  
a function of the mass temperature and fluid specific property the  
specific heat at constant pressure. It is assumed an equal mass flow  
of air leaves the zone control volume and although programs differ  
here, it is usually assumed the air leaving properties are equal to  
those. of the well mixed zone temp, humidity, etc, which again  
describe enthalpy2. So energy balance is massflow x (enthalpy1- 
enthalpy2).so really the question is how we come to the zone air temp.  
the zone air temp from the previous timestep is adjusted depending  
mainly on convection heat transfer from bounding surfaces, and here  
it's (air) capacity will also influence how fast it adjusts. the  
energy injection from the air is also considered (100% convective).  
exactly how and at which timesteps (with predictive correctors etc. )  
is software dependant and numerical solving is different. The capacity  
of the zone air influences how fast the zone air temp changes. The  
stored energy in zone air may increase or decrease.

Sent from my iPhone

On 18.11.2012, at 01:43, leen peeters <l.f.r.peeters at gmail.com> wrote:

> Hi all,
>
> I am simulating a low energy 18-zones building wioth mechanical  
> ventilation. I increased the capacity of the air in the zone by a  
> factor 5, as we generally do in TRNSYS simulations.
>
> Now I wondered, whether this is logical when considering the energy  
> losses through exhaust of the ventilation system: am I not taking 5  
> times the energy content? Do I have to adapt my ventilation flow  
> rates in order to compensate?
>
> I use a flow rate expressed in ACH as input to type 56.
>
> Leen
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