power plant ventilation

[delimashare]

Other than keeping a certain level of filtered air or strelized environment,why do we need a positive pressure inside a room which is equiped with electrical generators or motors.

 

[willard3]

So that nasties from the surrounding environment don't come into the room; the clean air comes out.

 

[abeerkhayyat]

Is there a possibility that the electrical equipments need a positive pressure to operate properly?

 

[EEJaime]

I don't know that you do need a positive pressure. Many of our generator rooms are equipped with entire walls of louvers to allow combustion air into the room, while the generator radiator fan expells air through the radiator which is normally ducted to an exhaust louver to get the heated air out of the room.

It is critical to get fresh air into the generator spaces in such a manner that the engines and generators recieve sufficient air to dissipate the heat, but that does not necessarily result in a positive pressure condition. When we have multiple engine-gensets, we often use fans to deliver this air, so in some cases that will be a positive pressure condition. But if the physical arrangement is such that a natural flow will work, then we will have a negative pressure relative to atmospheric. We sometimes provide filters in the louvers to minimize dirt infiltration. It depends on the location, project, etc....

Do you know of a code requirement or ASHRAE recommendation for positive pressure?

Thanks,
EEJaime

 

[delimashare]

The proposed arrangement is to install intake fans with filters blowing (x) cfm into the room(the x cfm is the amount needed to operate a vfd unit),while exhausting the same heated (x) cfm from the room through a duct and exhaust louver.Do we still get positive pressure in this way.
I will let you know about any Ashrae recommendation.In our case this is a request from the customer.

 

[rmw]

If the generator is air cooled its need for cooling air might be what is driving the requirement. A negative pressure in the room would reduce its capability to cool properly. Does the generator take air from the room and exhaust it from the building?

rmw

 

[cry22]

Who says you need positive pressure in a generator room?
Never heard of such a requirement.

 

[delimashare]

Ya rmw, it takes air from the room(through wall mounted)intake fans and exhaust it from the building.

 

[rmw]

Then I see that as being the primary reason for the positive pressure-to make sure that the generator never starves for air; to always make sure a positive pressure exists at the suction of the generator rotor fans. Without maintaining a positive pressure, if the filters became fouled, the airflow to the generator would be reduced which has serious consequences deep within the generator rotor and core.

rmw

 

[EEJaime]

That's a lot of air to supply to keep the positive pressure. For example, a standby 1500ekW, 1875kVA, 60Hz, genset like a CAT DM8260 requires 73278 cfm of airflow max at rated speed with a radiator. Also, 4573.3 cfm of combustion air inlet flow rate. With 7052 Btu/min of heat rejection to atmosphere from the engine and 3645.4 Btu/min from the generator, you need to move a lot of air through and out of the room very quickly. It is going to be a wind tunnel to keep the pressure positive with that volume of air. Hope you've got plenty of louver area.

 

[delimashare]

I have a chart for sizing ducts,but Iam looking for one for sizing grills and diffusers. Any recommendations please?

 

[ChrisConley]

delimashare - I recommend, starting a new Thread.

 

[delimashare]

let us talk about basics,and forgive me for this.
What is the meaning of applying positive pressure in a room in terms of figures?
Is it :
cfm on supply must be more than cfm of exhaust?
or
cfm on supply should be the same as cfm on exhaust but at a higher pressure.
or
is it both?
or use fans (where?) instead of natural flow?
Iam really confused.

 

[ChrisConley]

Applying a positive pressure means supplying a higher cfm than what you are exhausting.

For general pressure control a 10% variation in airflows will give you a positive or negative pressure.

 

[delimashare]

Thank you everybody. So Chriscorney how do I GARANTEE that I will supply more cfm than exhausted by sizing grills?
Let's say I need to exhaust 3000cfm,so I will use an intake fan of 3000+10%=3300cfm supply to achieve positive pressure.
Now how do I size my exhaust grill to guarantee exhausting 3000cfm ..not less not more?..taking into consideration the 3ooocfm(needed to be exhausted) we have three variables:the speed ,the static pressure and size of grill.
????How do I select the size of grill?

 

[berkshire]

Find the equivalent open area from the grille manufacturer, and size accordingly.
B.E.

 

[ChrisConley]

If you need to guarentee you are supplying more CFM than exhausting, you need two fans.

If you don't have an exhaust fan, then you will ultimately have 3000 cfm in, 3000 cfm out.