fan selection based on static pressure

[delimashare]

Hi Everybody
Sorry to take you back to thread 403-43928 by VEEKRISH AND TO THREAD 403-31557 by juan2025...My question is :
I want to pressurize a building by adding an intake fan and Iam calculating the fan cfm based on equation:
P1V1=P2V2
P1=ATMOSPHERIC PRESSURE
V1=VOLUME OF BUILDING
P2=STATIC PRESSURE TO BE ACHIEVED(O.5")im my case
V2=OVERALL VOLUME OF AIR NEEDED.
Now don't we need T1 AND T2 ?Why it is not P1V1/T1=P2V2/T2?
The temp. of air doesn't count here?
Thanks

 

[trashcanman]

Nope. Size the fan on the amount of leakage you expect. If the building is leaking 1000 cfm, size the fan for that + 10%. You can see that it is really quite simple. Temperature is negligible, especially since the building's temperature will soon = outside air temperature with all the air you will be pumping through it (unless you have a really robust A/C unit).

 

[delimashare]

Thank you trashcanman..but please try to calculate with me.Suppose the building volume =1500ft3 and the ambient temp.=90 and the internal temp =75F( to be acieved by A/C UNIT).NOW taking temp into consideration and applying to the formula:
P1V1/T1=P2V2/T2
10000*V/90=10010.16(eq.to 0.4"s.p.)*1500/75 GIVES V=1801.8 FT3
1801.8-1500=301.8 FT3
fan should be 301.8 +leakage+10%safety factor
but if we don't take temp. into consideration:
P1V1=P2V2
10000*V=10010.16*1500 GIVES 1501.5 FT3
1501.5-1500=1.5 FT3 compare it to the 301.8 doesn't that make quite a difference?????

 

[quark]

You should be careful to work with absolute units. Convert the temperature to absolute units and then you will get about 40 cu.ft more.

The reason why you have to ignore the temperature is the air coming inside the room is always at 75F. The pressure builds up as you go on pumping air inside. So, 1500 cu.ft of air flows in (cool air as it comes from AHU) and room will be at atmospheric pressure. If you pump in more air (at 70F) then pressure rises. You are not allowing the air to heat up for sufficient time as you are continuously renewing it.

Once you provide 1.5 cu.ft of extra volume inside the room, the room pressure builds up to 0.4" but then since the room pressure is more than the surroundings, air leaks out into low pressure areas. The leak rate is quite significant than the quantity required to pressurize a completely sealed room. That is the reason you need not use the PV = C formula.

 

[hollandhvac]

My opinion here is, that you can not use these formulas like PV=contant or PV/T= constant or even PV/T=Rs*m. These formulas are for closed systems and yours is leaking.

 

[delimashare]

Thanks quark and hollandhvac
quark why did you assume air coming inside the room is always at 75F .I want to clarify that air is entered through a pressurization fan and so the air entered will be at ambient temp. until it is conditioned by the AHU. and I don't understand your second point about not allowing the air to heat up. This second point doesn't support the idea of taking the temp. into consideration in our calculations?
quark what is your reply about hollandhvac comment that we can't use these formulas in this case.This is really an issue.
Regards

 

[willard3]

You must have a control (closed) volume for pv=nrt to work. The reason it is called the perfect gas law is that it doesn't work very well for imperfect gasses, which air is.

 

[Zesti]

The equation that applies to a leaking building would be:

dP = 1/2 x density x v^2 x z

for each and every leak in the building...

The pressure-difference dP will result in a flow of air through the leak at flow-velocity v with resistance factor z put in to take into account the shape/form of the leak.

In other words: Impossible to calculate...

All you can do is make an estimate on the leak or run a test with a fan you have at hand and measure the airflow and pressuredifferece it creates inside the building and then scale up to the required pressure-difference using dP=1/2 x density x v^2

 

[KiwiMace]

Or you can do a blower door test, or more simply look up data on blower door tests done on buildings with similar construction.

Read this to get started. Everything you ever wanted to know about infiltration:

http://epb.lbl.gov/Publications/lbl-35173.pdf

 

[quark]

At any instance, the volume of air contained in a pressurised room, under steady state conditions, can be calculated by using PV = C and there is absolutely no reason as to why you shouldn't use it. What HollandHVAC says that the formula is inappropriate to calculate leak rates.

Secondly, pressure is not built up in the fan but in the room as you restrict the return air flow (once the room is pressurised then fan considers this as system resistance and then develops the required static). As the room entry temperature will be more or less constant, you can ignore temperature effect.

 

[Zesti]

> Read this to get started. Everything you ever wanted to know about infiltration:
>

http://epb.lbl.gov/Publications/lbl-35173.pdf

Note how Equation 8 on page 13 of the above mentioned document contains a reworked form of the equation I mentioned in my earlier post (dP = 1/2 x density x v^2 x z).

 

[Zesti]

quark wrote:

> At any instance, the volume of air contained in a
> pressurised room, under steady state conditions,
> can be calculated by using PV = C

The volume of air will be constant as I do not think the room will be changing size.

The amount (mass) of air inside the room will have to change in order to raise the pressure inside.

Questions to delimashare:
- does the building consist of individual rooms or is it one large space?
- do you need to control the pressure?


PS: I just glanced over the other two threads the original poster mentioned: boy, this topic has evoked some discussion in the past! Interesting to see how people can end up thinking totally the wrong way about something with respect to the physics involved.

 

[quark]

If you want to be precise, I am talking about the standard cubic feet(volume of air reduced to 14.696 psia pressure) and not actual cubic feet (room volume at given pressure). The reason why I didn't say it specifically is that it can cause more confusion to the OP.

In layman's terms, a 1500 cu.ft room should be filled with approximately 1545 cu.ft (at atmospheric pressure) of air to raise the room pressure to 0.4" wg when there are no leaks.

If you feel something wrong with the above threads, you can start with your views in a separate thread.

 

[quark]

Please read you can start as please start

 

[Zesti]

My comment on the other threads was merely a sign of my amazement that the topic resulted in such a lengthy discussion whereas in my personal opinion the practical problem of putting pressure in a room is a rather trivial one. I'm sure you will agree that any fair discussion of different views is a good one.

> In layman's terms, a 1500 cu.ft room should be filled
> with approximately 1545 cu.ft (at atmospheric pressure)
> of air to raise the room pressure to 0.4" wg when there
> are no leaks.

I agree, it can be helpfull to put things in terms that are easy to understand.

However, the amount of air needed to pressurize a room is in my view a piece of information that is irrelevant to the OP and the problem he has to solve.

My startingpoint is usually a difference between supply- and extraction airflow of 1 or 2 times the roomvolume, so 1 or 2 AirChanges per Hour.

And you have to make sure the buildingcontractor does his best to minimize leaks in the room/building.

 

[quark]

There is nothing to disagree with you and you are correct. Volumetric increase in room volume is insignificant and it is already mentioned above by me and other posters.

Regarding other threads, it was my sincere request to know your views and no hard feelings from my side. There is a problem posting my replies today and so I replied in haste. That is why it took me a second post to correct it. Why I said a separate thread is because the earlier threads are already locked.

Individual experience and knowledge are what making these forums so lively. We all benefit from each other.

 

[Zesti]

Quark,

> Regarding other threads, it was my sincere request to know your views and no hard feelings from my side.

I see. To be honest, I was a bit unsure myself whether or not I had stepped on anyone's (quark's?) toes when I made my remarks on the other threads. If so, it was of course unintentional and I appologise.

> Individual experience and knowledge are what making
> these forums so lively. We all benefit from each other.

I could not agree more !!

Now all we need to know if all this was any help to the OP...

delimashare, please let us know

 

[delimashare]

Thanks to all of you.I really need some time to go all through your replies in this thread and in previous ones.
Write to you soon.

 

[delimashare]

I am really glad with all this feedback. Thank you all very much.

Actually the application (function) of the building is highly technical. There are electrical equipments running in this building, which explains my worries and concerns. Calculations here are very critical.

Zesti .About your questions, here is my answer:
1. The building consists of three different buildings(one of them is the building we are talking about) .Those 3 buildings are attached to each other in series, and there is a door connecting building 1 to 2 and building 2 to 3.
2. The control of pressure is of high importance here.

Now. The information you all provided is very helpful. Still I would like to go further and ask you kindly for more support.

Quark.
1. Where did you get the 14696psi from?
2. Your comment about not allowing the air to heat up for sufficient time makes me think:
What is the role of the A/C unit here? I need the room to be maintained at 75F all the time.
Specially that there are huge amounts of air supplied to(and exhausted from) the building, OTHER than the air supplied by the pressurization fan.

Zesti.
1. I am not sure if I understand your point about allowing 1-2 air changes/hr as a difference between supply and extraction.
Let’s calculate
2air changes/hr * 1500ft3(room volume) =3000ft3/hr.
So, based on existing EQUAL supply and exhaust in the building, the capacity of our pressurization fan should be around 3000ft3/hr?

KiwiMace
Frankly saying I had no chance to read (The use of blower door data). I WILL for sure and thanks.

I hope I will come up with a summary after all and share it with all of you.

Regards
delimashare

 

[ChasBean1]

Quark, can you believe it was seven years ago when we were talking about this?

delimashare, you referenced those threads - did you read through them? I'm just curious. If you did, I'd hope you woundn't be sizing a fan based on P1V1=P2V2...