Balancing Supply and Exhaust Air in Lab 2

[remp]

Question in a Lab installation:

Design shows 6 rooms served from 1 off 100% freash air AHU. Each room has a supply air grille and balance damper of the main duct.
The rooms are designed to be at +30pa higher than atmosphere.
The rooms are also served by 1 off exhasut fan with an exhaust air grille and balance damper on each grille.

The question is when I come to balance the air in each room, should I do both supply and exhasut at the same time. How is this possible? Should they be done at the same time. Surely the supply will be influenced by the the exhasut and vica versa in each room.

Any ideas.???

 

[ChasBean1]

Hi billyg, not having terminal boxes makes life a little more difficult. If you have design flow values for each space, I would try to nail those down first. Balance either the supply or exhaust system first (your choice) so air is distributed proportionally. Then balance the other system. The total of the supply outlet volumes and the exhaust inlet volumes should be nearly equal to the design totals for each system. Deviation should then be handled by fan sheave adjustment. From that point, each space can be fine tuned by slight adjustment of volume dampers, or preferably by room sealing, door gaskets, and adjustable door sweeps.

Best of luck, -CB

 

[quark]

CB is right on and I need not put my comments here. However, as I struggled quite a bit with these type of systems (without VAVs) I would like to share some tips.

If all your rooms are to be maintained at +30 Pa, Just keep the main supply duct VCD open(say around 80%) and control the main exhaust duct VCD so that all rooms will be balanced at the same time. It is better if you decide your initial supply VCD opening with respect to your filter pressure drops vis-a-vis the fan performance. As the filters get loaded up, you just have to increase the supply VCD opening.

If your room pressures are varying, then acheive minimum common pressure required by adjusting main dampers and then fine tuning the individual room dampers.

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[SAK9]

Billy,

The sequence for balancing would be 1)adjust supply air volume in all rooms to design value 2)then adjust exhaust volume to obtain a pressure differential of 30 pa. 3)measure and record exhaust flow rates.

It is prefered to vary exhaust flow and keep supply volume to design value due to heat load requirements.The diffrence between supply and exhaust volume would depend on the room tightness.Another option would be to install pressure stabilizers in the wall.By adjusting the counter weight in the stabilizer damper,you can set the room to required pressure.Even if the air flow varies, the damper would self adjust to maintain the set pressure.

 

[Fzob]

Don't forget to consider the lab hoods (if any) as they can play havoc with the air balance.

 

[lilliput1]

You need to determine if the system would be constant volume reheat or VAV. Either way you should use air terminal boxes either CV or VAV per the system determined. If CV you still need the boxes to maintain CFM regardless of filter loading. You should have DDC controls to measure the supply air and control the exhaust for a fixed CFM offset which is equal to the differential that would result in the pressurization required + tolerance margin to account for the +/- error of equipment & allowance to not cause nuisance alarms. The air equation is Supply Air CFM + Transfer Air In CFM = Exhaust Air + Return Air + Transfer Air Out. To estimate the diferential required estimate all crack sizes & gaps around closed door. Calculate pressure drop as 1.5 X the velocity of the air through the opening. The factor 1.5 is based on using a coefficient of 0.5 for entrance loss to the opening + a coefficient of 1.0 for the exit loss through the opening. Use -- ,- ,0 ,+ ,++ , +++ o plan to indicate relative pressurization. Use 0.025" to 0.05" pressurization per stage of increment (example + to ++).

 

[CRG]

It sounds like the system may already be installed; however, you still may want to look on the web for Phoenix Controls Corporation. They make air valves for this specific application. Also, they have a lot of info on how installation and problems related to these types of systems.

 

[Accystan]

I'd check out the TSI website too. TSI employ closed loop control; Phoenix weren't at one time, just offset control really, but I'm not sure about their stuff now.

 

[MarauderX]

I'll second Phoenix Control valves for this application. Once you have everything set your balancing will be a breeze, even with lab hood exhaust changes.

http://www.phoenixcontrols.com/