Modulating exhaust air damper from supply fan inlet sp

[MechEngNCPE]

Ok, so I just wanted to post this to get a feel of what the community thinks on economizer relief/exhaust damper modulation when the ahu has both supply and return fans on vfd's, the supply vfd modulating off supply duct static pressure, and the return vfd modulating off space static. During normal operation the OA damper modulates off DCV control from a CO2 sensor in the return duct. During economizer operation the OA damper and RA damper modulate to provide 55 LAT. I have the exhaust air damper set to modulate to provide a -0.2" supply fan inlet pressure. We have airflow monitoring stations on OA, RA, and SA for the BMS to view and also set an OA airflow if they want. Does anyone see anything wrong with this setup, besides it being a controls nightmare? Oh, and this is VAV setup with about 40 DDC VAV's.

 

[rogzog]

What about the building static pressure? Is it controlled? Couldn't you use the exhaust to control the building static? Too much OA and no relief and the building outside doors might not close correctly or too much relief/exhaust with not enough MUA and you might have to pull like hell to open outside doors.If the buildings other exhaust systems are independent of his AHU and controlled with a light switch and not associated with the unit then what? If you modulate the relief/exhaust based on building pressure it won't matter. The building pressure sensor should modulate the relief/exhaust to maintain pressure setting regardless of O/A inlet or other exhaust systems. I have seen systems set up that way. You could use the control point for the supply fan inlet pressure to monitor building pressure instead.

ROGZOG

Just my thought based on what I gathered from your description of the system.

 

[rogzog]

Sorry, my bad, I re-read and see that the RA fan is linked to the building static, but I still wonder if you might still run into problems controlling the overall static pressure in the building related to the outside pressure.

ROGZOG

 

[MechEngNCPE]

Yea the RF VFD will take care of building static, im just wondering about the exhaust damper modulating to provide a constant fan inlet at -0.2"wg.

 

[cry22]

I ma not a big advocate of having a variable controlled through the middle man.
The space static is intended to control just that: "Space Static", not a VFD, a RF or whatever. Is the space static controlled in your scheme or you are just using it to control the RA VFD?
For me, the space static should control a relief damper between the space and outside or say a return air damper connecting to a returnair shaft NOT a RA fan VFD, i.e a control variable acting on its actuator directly.
When you control the RA VFD, nothing says that other floors are having proper air balance.

Now let's compare your scenario to the standard control system - RA Fan VFD tracks SF VFD and maitains air flow difference calculated by AFMS and building exhaust.
If your RF VFD does not track the SF, then your RF could pull may be more air from other spaces (floors without the static sensor)than SF air flow when SF is turned down.

Unless I am missing something in the stated problem.

 

[MechEngNCPE]

The return fan modulates its speed to maintain a space static pressure of 0.05". If the supply fan slows down, then pressure will decrease, causing the return fan to slow down as well.

 

[MechEngNCPE]

Cry, when have you come across a scenario where a supply fan and return fan have the same fan curve and can "track" effectively? RPM's dont relate linearly to cfm's. Especially on two different size fans.

 

[rogzog]

I am trying to understand how the RA fan can control the building pressure instead of the relief/exhaust fan/damper.

Lets use a particular condition, say the unit is in economizer mode taking in outside air. As the unit brings in OA the building pressure goes high, the building pressure sensor increases the RA fan speed to try to lower the pressure in space, so the pressure at the inlet of the supply fan goes high and the relief/exhaust/damper or fan opens or comes online to try and maintain -.02” at the supply fan inlet and relieving building pressure.

Is that correct? I see the problem that you are trying to maintain a negative pressure at the fan inlet but the relief damper is in the negative space and will never relieve anything unless you have a powered exhaust/relief fan. It needs to be in a positive space to relieve. If you have a relief/ exhaust fan, then you have to control it also to slow the supply fan and allow the exhaust to balance the building pressure.

IMHO it is too complicated therefore more cost. It can be done a little simpler unless you have some particular reason to hold the SA fan inlet at -.02”.

I must be missing something here.

ROGZOG

 

[MechEngNCPE]

No, you hit the nail on the head - its too damn complicated, so I'm going to track the supply fan using airflow measuring stations to hold the difference in cfm to provide pressure, and have the exhaust damper modulate to hold building pressure. Does that sound like a simpler solution?

 

[rogzog]

Yes, it does.

ROGZOG

 

[cry22]

and another "Yes, it does"
Cry

 

[MechEngNCPE]

Thanks guys, but I went with the rf vfd modulating to take care of space pressure, and the exhaust damper closed during normal operation and modulating inversly proportionate to the return damper during economizer operation.

 

[317069]

I don't think it is a problem with this control as long as the return damper will never be 100% closed,
but you said "During economizer operation the OA damper and RA damper modulate to provide 55 LAT"
could you please give more explenation for this point

 

[rogzog]

Here is some information on this subject, maybe it could be helpful. It is from Penn State found on a web search. It is quite detailed and covers all situations, I think. Interesting reading. ;p

ROGZOG

 

[rogzog]

Aaaargh! The file didn't show up in my last post, I will try again here.

 

[MechEngNCPE]

We have a dual enthalpy economizer cycle, so the return and outside air dampers are modulating to provide 55d lat and the return air damper will only be closed if the exact temps and enthalpys are present to provide the correct mixed air temp.

 

[317069]

at economizer cycle, if the Return damper was 100% closed and return fan is runing then what building parameter will effect on supply fan inlet pressure to make exhaust damper to open

 

[MechEngNCPE]

I'm not using fan inlet static setpoint anymore, just keeping the exhaust damper closed during dcv and during econ letting the exhaust damper modulate inversly with the return damper, so the OA/RA/EA dampers all modulate when the economizer cycle is trying to maintain 52 mixed air temp. (3 deg for fan heat)

 

[317069]

one more thing, I know that the return fan is suitable to constant voleum appilaction more tha VAV applications which is preffered to use exhaust fan
why did they choose this mode of control for VAV application

 

[MechEngNCPE]

It was an ahu replacement with upgraded controls including vfd's and dcv. I believe this is the optimal way to set up the units. The return ducwork is extensive so that drives the return fan setup.