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

 

[quark]

CB,

Welcome back. I agree, time is flying. I remember that I had jet black hair that time and now it is half grey (chromatically), though not sure about how much wisdom I picked up all these days.

BTW, I sincerely feel this forum is missing you. Seems you have been busy for quite some time.

 

[walkes]

Just from my experience, in-line with the previous posts, when trying to pressurize a space, the airflow design (differential between supply and exhaust)is usually based on a guess on the amount of leakage.

The differential is confirmed after the room has been constructed by testing leakage with a blower door set-up.

The differential airflows are adjusted based on the findings of the blower door test.

 

[Zesti]

> 1. The building consists of three different buildings
> ..
> and there is a door connecting building 1 to 2 and
> building 2 to 3.

If the doors are relatively "airtight" and are kept closed as much as possible then you only need to consider the one building you need to pressurize.

> 2. The control of pressure is of high importance here.

In that case I would take a fixed amount as the supply to the building and put a VAV-system in the exhaust.
Frequency-controlled fans on both AHU's. The VAV-valves would be controlled by a differentialpressure-sensor.
The frequency-control on the fans would be regulated to a constant pressure in the duct.

> What is the role of the A/C unit here?
> I need the room to be maintained at 75F all the time.

If by A/C-unit you mean the AHU: like I said, maintain the supply at constant flow, vary the supply-temperature to control the room-temperature.

> Specially that there are huge amounts of air
> supplied to(and exhausted from) the building,
> OTHER than the air supplied by the pressurization fan.

Are you talking about a separate fan just for pressurization? You do not need that if you already have an AHU for ventilating the building.

> 1. I am not sure if I understand your point about
> allowing 1-2 air changes/hr as a difference
> between supply and extraction.

Like Walkes says, you need to guess at the "leakiness" of the room/building. 1-2 air changes/hr is just my first guess.
If, for instance, you are already using 6 ac/hr for the ventilation/temperaturecontrol of the building then this could all end up leaking from the building at the desired pressure. Then there would be nothing left to do for you exhaust fan. If 1 or 2 air changes/hr is the leak-flow from the building then your exhaust system needs to take care of the remaining 4-5 ac/hr.
The calculation you make is indeed what I mean.

It's like this:

^ ^ leaks (unknown)
| |
------------------
fixed | room/building | variable
air supply --->| |---> exhaust
(AHU) | pressure build-up| (AHU)
| |
------------------

 

[Zesti]

Oh, the picture I tried to make looked better when I was typing it...

 

[ChasBean1]

Quark - thanks for the positive note! I have been busy but try to stop in and keep involved when I can.

Walkes I agree w/your last post.

Zesti, I've yet to see a VAV exhaust matched with a CAV supply, but it doesn't mean it can't be done. Would probably depend which is more critical (supply or exhaust).

 

[Zesti]

@ChasBean1:

> I've yet to see a VAV exhaust matched with a CAV supply,
> but it doesn't mean it can't be done. Would probably depend
> which is more critical (supply or exhaust).

I'm afraid I don't quite understand what you mean.

The VAV on the exhaust is controlled by the measured room-pressure.

I would want CAV on the supply-side because supply is based on, for example, heatload, number of people in the building, some health/safety requirement etc.

Anyway, main point being that the exhaust flow is controlled according to the room-pressure.

 

[delimashare]

Thank you all very much.

ChasBean1. I understand that leakage has the biggest effect on pressure . I am not sizing the fan based on p1v1=p2v2. I just don't want to miss anything in my calculations. No matter how small it is.

walkes ,KiwiMace, Zesti. The application of BLOWER DOOR TEST and its relation to the (guessed ac/hr) are clear to me now.


ChasBean1 and Zesti. Before proceeding in discussing the CAV and VAV,I would like to explain that my intension was to use a separate pressurizing fan in order to be able to pressurize the building in case of gas leakage while all other equipments including the A/C(A.H.U.)are controlled to stop running.

Talking about which is more critical here supply or exhaust. I would like to clarify that there are electrical equipments in the room which needs a fixed rate of air supply (specified in data sheet) in order to run properly.

I feel that Iam giving you the information in pieces. I apologize. I was thinking of simplifying things.
Is there anything I can do to give you a better idea about the case.

 

[ChasBean1]

I don't know where to go from here. Size your supply to meet heat loads and fresh air flow per occupant needs; have your exhaust track about 10% lower initially by some means. That value can be adjusted as you see fit for pressure control. Also consider sealing and door gaskets and sweeps to get a certain DP if offsets become too large during adjustment.

 

[delimashare]

Thank you ChasBean1
I will calculate and come up with real figures.
Please What do you mean by (heat loads) in your sentence (Size your supply to meet heat loads and fresh air ...) .The pressurizing fan here is A SEPARATE fan; it not the HVAC fan.
Do I understand you correctly?

ChasBean1 , Zesti
Back to the VAV and CAV any more explanation!

Is it possible to add to the system :
1. a space pressure sensor controlling a relief fan with a frequency drive?
OR
2. to add a barometric relief damper? and which is better ?and why?

You are all challenging me.Thanks

 

[Zesti]

Do you need the pressure in the building to be at a required minimum level at all times?

I mean, if the level of pressure is important, you need some sort of control.

You say you have separate systems for ventilation/AC and pressurisation.

The amount of air going through the AC-system will vary over time when the filters get more and more dust in them.
This will influence the pressure of the building and the separate pressurisation-system needs to be able to adapt.
Even more so if this systems needs to work when the AC is switched off.

If all you are going to have is a supply-fan for pressurisation then you need it to be frequency-controlled according to pressure in the building.