Re: [TRNSYS-users] increased capacity of air

[jeannieboef <jeannieboef@gmail.com>]

mmm...difficalt to get a "normalized" feel for this. The 5 times rule of thumb is probably for a average office and is there for stuff like furniture, softwall partitions, etc. In theory, capacity will flatten peaks and troughs, but won't change energy consumption...except in the effect on equipment partload efficiencies. 

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On 18.11.2012, at 10:55, leen peeters <l.f.r.peeters@gmail.com> wrote:

Yep, but as wqe assume an overdimensioned value of the capacity of the romm, indeed the inertia is wrong. I simulated on 3 minute base. The peaks in required thermal power are thus wrong. That is what I mean ... Is there a way to solve that? Maybe it is better if I use a mass flow rate instead of ACH?

On Sun, Nov 18, 2012 at 10:00 AM, jeannieboef <jeannieboef@gmail.com> wrote:

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.

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On 18.11.2012, at 01:43, leen peeters <l.f.r.peeters@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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