air pressure sensor in apratment bldg laundry

[rmusa]

Laundry room with 6 dryers that exhaust to outside.
RTU supplies into room.
OA is modulated up as dryers come on.
Room pressure sensor is used to control the OA damper.
Pressure sensor to maintain 0.05" setpoint (adjustable) in room.

Is this the best way to control?

Current sensors on the dryers will make controls complicated, not designing for this.
Is setpoint of 0.05" good for this setting.

Will opening of door to laundry by users make controls difficult?
Need to maintain pressure in space as the door to the laundry in fire rated and has to stay shut when needed and open easily when needed. Pressure cannot be excessive in room.

What is best location to install the pressure sensor?

 

[Ziggypump]

How is this pressure sensor to work? Wouldn't a differential pressure sensor (2 of them) work better? This way you could make the room negative in relation to the corridor.

Opening the door will always void a pressure setting, as now there's a big hole to allow air to pass. Now that being said, there are at least three parts to this pressure problem, you are only indicating two in your question. The corridor needs to be taken into account too. Maintaining a negative pressure in the room means that when the door opens, the air will flow from the corridor into the room. This is good and the intent. The corridors of the building should always be positive pressure in relation to the adjacent rooms. That being said, take all three portions of the problem (the laundry room, the corridor, and the outside air) and determine if opening the door will cause the air to flow out instead of into the laundry room.

 

[lukaiENG]

If the OA is not conditioned, it may be simpler to run the OA at constant speed and install a barometric relief damper relieving out of the building.
This isn't the most energy efficient option perhaps but it will give good control.

I wouldn't do this if you are conditioning the OA as that would be too wasteful.

 

[rmusa]

This is top floor of 12 story apartment building. The 6 dryers add up to 3000 CFM. They are in laundry room. The adjacent room is user waiting area. next to waiting area is corridor/elevator lobby, with very minimal supply air from building system to the corridor (heating/cooling by FCU's). The other end of corridor leads to a door to roof deck (exterior). No makeup air available from corridor.

The RTU will provide 3200 CFM. Its a 10 ton unit and at factory minimum supply air. It does not dehumidify air but that is ok with user. Its setup like this since 1970.
The room does not have any space to build an enclosure behind the dryers.

1. When dryers are off 3200 CFM is supply air, 10% OA. 1600 CFM to laundry, 1600 CFM to waiting area. 10% OA will not cause any pressure problems. Return air will be ducted back to RTU and recirculated. Waiting room air is transferred into laundry room and then all air returned back to RTU through ceiling RA grille.

2. When dryers are running, they vent to exterior, directly through wall, each one with individual exhaust duct. No common header. The door between laundry and waiting room is not fire rated. The door between waiting and elevator lobby is fire rated. This door has to remain shut when not in use and easy to open in an emergency. The wall mounted pressure sensor will sense space pressure and try to maintain 0.05" SP setpoint with respect to corridor. As dryers are staged on the pressure is reduced in laundry and RTU OA is modulated up to balance. Instead of returning back to RTU, the air is exhausted through the dryers. The RTU only has supply fan and barometric relief. No return fan.

3. Can a barometric damper be added to laundry wall to relieve air if there is too much pressure in space. The door to corridor may become the path of least resistance and not the RTU RA duct or the dryer exhaust.

4. Is 0.05" setpoint sufficient? as used in labs and hospitals.

5. Is there an alternate way to control OA damper modulation? other than pressure sensors and current sensors?

6. OR Run RTU at 100% OA at all times and use barometric damper to the exterior for pressure relief and not use pressure sensor? It is run like this currently. Whole idea of pressure sensor is to modulate OA to save energy when dryers are not in use.

 

[Ziggypump]

OK, dumb question here, but if you are exhausting 3000 cfm (assuming all dryers are on) and only supplying 1600 cfm, even with increasing OA, where is the rest of the room make-up air coming from (1400 cfm)? You stated that the waiting and laundry rooms are separated by a fire door, which means the door needs to remain closed. Even with the undercut of the door you shouldn't expect more than 75-100 cfm of transfer air.

Now I'm not going to go into how I would design this. You're a big kid now and can figure this out on your own. In using a pressure sensor, calculate it, and make sure you can control this method. You should have plenty of books and access to the internet to figure this out. Read the code and make sure that the laundry room meets requirements due to supply and return air.

 

[lukaiENG]

So which door are you trying to maintain the pressure on? the fire door to the elevator or the door between the lobby and the laundry?
If it is the latter, this is not a fire rate wall, an a correctly sized transfer (sized for 1400 CFM) should be sufficient for preventing the door from sticking.
If it is the former then we are back to looking at some form of pressure control.

The issue I see with the pressure sensor idea is that pressure is hard to measure accurately at the level you are looking at, pressure sensors are also pretty susceptible to drift so it might work initially but in 6 months who knows.

Are you staging the OA damper? you state it changes from minimum position up "to balance". I assume this is staged.

Honestly, I would say that current transducers is the simplest way to go, you have a digital signal indicating when a dryer turns on and when it does, you index the damper to the next position. When a dryer turns off, you index the damper back a step. Simples!

 

[ChasBean1]

This sounds like a step above what you'd normally find in a building laundry room. I'd think typically for this application you'd want a 24" x 36" make-up damper for the room that would open if any one dryer runs. If your pressure scheme works, why not? Is there a problem? if so, I didn't see the concern defined in your post.

 

[lilliput1]

Let the AHU (for cooling and heating) handle just the minimum OA for ventilation. Provide louvered OA opening into the room with intake air gravity operated backdraft damper and automatic damper to shut off opening if all dryers off to avoid wind blown air from going in. This way the room will be in negative pressurization and you do not need complicated controls to bring in air. The OA opening should be located in the room close to the dryers but away from the dryer vents.