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[EnergyPlus_Support] Re: How to calculate Fan Pressure Rise



Earth Tubes normally have a diameter of 100 - 200 mm? To move any reasonable amount of air you need a lot of those in parrallel. Lenth of tube say 30 m. Air velocity should be slow as possible, let's say 3 m/s. 

Q=VA
= (3)((0.1^2)(3.1415)/4)
=... m3/s

Assume a fire damper pressure drop 20 Pa at these velocities at the exit of pipe into zone (unfiltered). Assume Xx? (maybe 0.5 Pa per meter, but over 30 m that's 15 Pa (you calculated a believable 8 Pa), see fluid mechanics texts or use a air slide rule)  Pa drop over straight pipe and 3 Pa for every bend. If you have a weather grill, another 5 Pa. Also, depending on the shape of the entry and exit orifaces, there are resistances to be accounted for. see Cv or Kv velocity flow coeff.

Put on a 1.2 margin and very quickly your fan needs to give >50 Pa. 

Still peanuts, but you must create this pressure differance across each of your very many 100 mm x 30 m pipes. Do you put a fan on each pipe?...probably not. If the nodes come together at the start (the outdoors) and the end (a vacume chamber exhausted by fan) you need to add the resistances in parallel, i.e. 1/Rtot=1/R1+1/R2+...




--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "wasilqayyum" <wasilqayyum@...> wrote:
>
> I saw the EnergyPlus Engineering Reference documentation which talks about the "fan pressure rise" (pg. 676), but I have no idea how to go any further. Any thoughts on that?
> 
> If someone can please guide me it will be very much appreciated. 
> 
> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, YuanLu Li <yli006@> wrote:
> >
> > 
> > I was wondering sometime why the head required is so low for the tube alone. I think, the simple calculated low value assumes that there are no restrictions at both ends of the tube.  The pressure required to force the air into the zone is not taken into account. Calculated fan power can also be very low, probably because the blade size, surface friction, etc. were not taken into account.  All these are hidden in the efficiency value of the fan.
> >  Dr. Li   
> >  To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> > From: wasilqayyum@
> > Date: Fri, 1 Jul 2011 19:48:50 +0000
> > Subject: [EnergyPlus_Support] Re: How to calculate Fan Pressure Rise
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
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> >  
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> > 
> >   
> > 
> > 
> >     
> >       
> >       
> >       Thanks for the answer.
> > 
> > 
> > 
> > I do infact know how to calculate pressure drop etc. But I get very low results like 8 Pa (just the horizontal section of earth tube). Whereas, the values often used is around the 400 Pa range. So my really value has left me confused.
> > 
> > 
> > 
> > --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "Jean marais" <jeannieboef@> wrote:
> > 
> > >
> > 
> > > I have never used this object. I assume that it is the same as the hvac fan objects, meaning that a direct relationship exists between air flow Q, total efficiency, nominal rated pressure rise dP and Nominal Rated Power E.
> > 
> > > 
> > 
> > > To calculate dP you need
> > 
> > > Power (W)
> > 
> > > Flow  (m3/s)
> > 
> > > Total Efficiency ()
> > 
> > > 
> > 
> > > If you don't have Power and you really must calculate the pressure drop in your pipes at a specific mass flow, then you'll need to dust off the old fluid mechanics text books, find the section on Reynolds numbers and flow in rectangular or circular ducts and hand calculate it. Or find some other program to do it.
> > 
> > > 
> > 
> > > I don't know if E+ has something on their site to do this? Basic slide rule stuff. I think one can simulate it as well, but I've never done it.
> > 
> > > 
> > 
> > > --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "wasilqayyum" <wasilqayyum@> wrote:
> > 
> > > >
> > 
> > > > Hello,
> > 
> > > > 
> > 
> > > > There is a field in zone earth tubes called "Fan Pressure Rise". Can someone please tell me how to calculate this value?
> > 
> > > > 
> > 
> > > > Thanks.
> > 
> > > >
> > 
> > >
> >
>


--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "wasilqayyum" <wasilqayyum@...> wrote:
>
> I saw the EnergyPlus Engineering Reference documentation which talks about the "fan pressure rise" (pg. 676), but I have no idea how to go any further. Any thoughts on that?
> 
> If someone can please guide me it will be very much appreciated. 
> 
> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, YuanLu Li <yli006@> wrote:
> >
> > 
> > I was wondering sometime why the head required is so low for the tube alone. I think, the simple calculated low value assumes that there are no restrictions at both ends of the tube.  The pressure required to force the air into the zone is not taken into account. Calculated fan power can also be very low, probably because the blade size, surface friction, etc. were not taken into account.  All these are hidden in the efficiency value of the fan.
> >  Dr. Li   
> >  To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> > From: wasilqayyum@
> > Date: Fri, 1 Jul 2011 19:48:50 +0000
> > Subject: [EnergyPlus_Support] Re: How to calculate Fan Pressure Rise
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> >  
> > 
> > 
> > 
> >   
> > 
> > 
> >     
> >       
> >       
> >       Thanks for the answer.
> > 
> > 
> > 
> > I do infact know how to calculate pressure drop etc. But I get very low results like 8 Pa (just the horizontal section of earth tube). Whereas, the values often used is around the 400 Pa range. So my really value has left me confused.
> > 
> > 
> > 
> > --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "Jean marais" <jeannieboef@> wrote:
> > 
> > >
> > 
> > > I have never used this object. I assume that it is the same as the hvac fan objects, meaning that a direct relationship exists between air flow Q, total efficiency, nominal rated pressure rise dP and Nominal Rated Power E.
> > 
> > > 
> > 
> > > To calculate dP you need
> > 
> > > Power (W)
> > 
> > > Flow  (m3/s)
> > 
> > > Total Efficiency ()
> > 
> > > 
> > 
> > > If you don't have Power and you really must calculate the pressure drop in your pipes at a specific mass flow, then you'll need to dust off the old fluid mechanics text books, find the section on Reynolds numbers and flow in rectangular or circular ducts and hand calculate it. Or find some other program to do it.
> > 
> > > 
> > 
> > > I don't know if E+ has something on their site to do this? Basic slide rule stuff. I think one can simulate it as well, but I've never done it.
> > 
> > > 
> > 
> > > --- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "wasilqayyum" <wasilqayyum@> wrote:
> > 
> > > >
> > 
> > > > Hello,
> > 
> > > > 
> > 
> > > > There is a field in zone earth tubes called "Fan Pressure Rise". Can someone please tell me how to calculate this value?
> > 
> > > > 
> > 
> > > > Thanks.
> > 
> > > >
> > 
> > >
> >
>




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