Building Ventilation Question 2

[Krausen]

Hello All,
I do not normally work in HVAC, so I was hoping to get some direction from the forum on some basic ventilation concepts inside a small industrial building. Basically I have a small one-room Laboratory Shack (10'x24') with a stand alone fume hood inside a Oil & Gas facility. The shack will house a lab fume hood with a ducted ventilation system to atmosphere. The fume hood exhaust will only be turned on when performing lab tests inside it. The shack itself will also have a general ventilation exhaust fan mounted on one wall with intake hoods/louvers on the opposing wall to provide crossflow ventilation through the entire shack. The general exhaust fan will be turned on anytime anyone is inside the shack.

An issue has been raised on checking to make sure the general exhaust fan does not compete with or negate the ability of the fume hood to vent adequately. In other words, when both the fume hood exhaust & general exhaust fans are turned on, both must still vent adequately together. How would one go about analyzing something like this? Is it simply a matter of sizing one fan & system more than the other?

Thank you in advance!

Tolerance is the virtue of the man without convictions -GK Chesterton

 

[dbill74]

You need to look at both simultaneously and consider air flow rates in all situations. So long as your source of make-up air is sufficient to provide enough air for both fans they won't compete. Well... maybe IF the general EF inlet is within a foot of the face of the fume hood they might, but I have hard time imagining that being the case.

The real trick will be in managing the velocity through the hood which typially require a specific velocity rather than a CFM. This is so they can maintain their performance with the sash/opening at various positions. Although a VFD can provide varying CFM/velocity, the response time is slower than the hood needs. In these cases I've used venturi valves to control air through the hood.

 

[Lnewqban]

I agree with dbill74.
Verify that the free area of the intake lovers is big enough for the simultaneous demand of both fans.
If that area is restrictive, the volume handled by both fans will proportionally decrease.

"Engineering is achieving function while avoiding failure." - Henry Petroski

 

[Krausen]

Thank you guys for the tips. dbill74 is correct that the hood exhaust fan is governed by face velocity rather than SCFM. I need to have 100 fpm at the sash. The LABCONCO fume hood catalog states that to achieve the 100 fpm at the sash requires 1180 cfm @ 0.41 SP airflow. It doesn't state the blower motor size or anything else to achieve this. I'm not quite understanding their info here.

For general ventilation, I only need a minimum of 8 air changes per hour. The shack volume is approx. 2400 cf (24'x10'x10'-tall). My rough calcs say an exhaust fan that can do 320 scfm will achieve this. My question is on static pressure (SP) corresponding with the SCFM. How should I factor this into the fan sizing? I'm thinking I want to keep this fan size to meet only the minimum so it doesn't overpower the fume hood exhaust. Agreed?

Also, since the general ventilation is venting a hazardous area with hazardous vapors that are heavier than air, I've been told the exhaust fan should be mounted near the floor with the make-up air intake shutters mounted high on the opposing wall. Would you agree?

Tolerance is the virtue of the man without convictions -GK Chesterton

 

[Lnewqban]

Extracting 1180 cfm (almost four times the volume needed for ventilation) from that small room seems excessive for the ventilation louvers, especially if some of the hazardous vapors can be drawn around the operator and into the hood.

I would consider a dedicated make up duct just for the hood, discharging close to the curtain.
A linked damper would avoid back draft from the ventilation fan, impairing the low point pick up of hazardous vapors.

By your description, it seems that your hood requires a remote exhaust fan and that 1180 cfm could be for the maximum opening of the curtain.
For smaller openings, the speed, turbulence and noise may be excessive if the speed of the fan is not reduced accordingly.

If your ventilation fan is centrifugal, you could install it high and run a metal duct down with the intake opening located about 6" to 12" above the floor.
If axial, the low location may require protecting lab personal from accidental injury and rain from coming in.

"Engineering is achieving function while avoiding failure." - Henry Petroski

 

[chicopee]

Check out a Grainger catalog for air velocity meter and use it properly to get average velocities and CFM.

 

[Krausen]

Thank you for the helpful responses. I have another fundamental question - Since the goal is to ventilate heavier-than-air hazardous oil/gas vapors inside this building, shouldn't positive pressure inside the building be considered versus negative?

In other words, should the general ventilation fan/blower be supplying fresh outside air into the building with ventilation louvers on the opposite wall?


Tolerance is the virtue of the man without convictions -GK Chesterton

 

[dbill74]

Depends on the gas. Being "heavier-than-air hazardous oil/gas vapors" you really want the space (NOT building) negative to prevent pushing it into other occupied areas where people may be exposed and harmed by the gas.

You can have your lab negative relative to adjacent spaces and the building overall pressure positive relative to the outside. Int'l Building Code does allow air from other spaces in the building to act as make-up for the lab; not practical for your scenario due to volume involved, but something to keep in mind nonetheless. Bringing make-up air direct from the outside, you should consider a dedicated 100% OA unit to control humidity and temperature (depending on where you are in the world). Untreated air can cause other problems (i.e. mold).

For your lab space, the exhaust air should be greater than the supply/make-up air. Just make sure you aren't making the overall buidling pressure to go negative.

 

[Lnewqban]

I fully agree with dbill74's above post.

The only way to force the heavier-than-air gas through that low pick-up point is by vacuum effect.
If your ventilation fan pressurizes the space that contains that gas, it will be forced to leak out through any opening, crevice, door and window, besides that pick-up point, which would contaminate the surrounding spaces.

Selecting the point of discharge of the exhaust duct and the outlet speed should consider contamination of other populated areas, as well as predominant wind, windows and ventilation inlets.

"Engineering is achieving function while avoiding failure." - Henry Petroski