I think by "spill air", your boss meant air leakage. Since you have not identified the type of structure that the AHU is to service, it is hard to say how much one should allow, however, 10% of the total CFM would be a good starting point.
He probably meant exhaust if talking in reference to an economizer. This is simply the required exhaust one needs when bringing in 100% outside air during max economizer. It should be supply cfm minus building pressurization cfm.
My initial thought was duct leakage but I see it was regarding economizer mode. I would agree with MechEng about what your boss was speaking about although it's still not that clear.
However, when sizing an AHU one should always account for duct leakage, especially if reusing existing ductwork.
Mechengnpc,
So when you deduct the room pressurization to supply air, that value is the "spill air" - where you need to exhaust the spill air, at the return duct before or after the return fans?
Also how's the automation control of spill air, what set point or value to work/activate the spill air?
Hows this: there should be some applicable sections for you and everyone on here as a matter of fact.
SUPPLY FAN
The AHUs’ supply fan modulates its speed based on a supply duct static pressure setpoint of 2”WC. This speed modulation provides energy savings by using less brake horsepower when VAV terminal boxes are not calling for maximum airflow. During the heating season, VAV boxes will tend to drive their primary air dampers to a more closed position, enabling BMS operators to utilize supply air static pressure reset by changing the setpoint from 2”WC to 1.5”WC.
RETURN FAN
The AHU’s return fan modulates its speed based on a signal from the space static pressure sensor. This space static pressure sensor will enable the AHU to keep the area it serves at a slightly positive pressure of 0.05”wg (adj.). The sensor should be located in an open area, away from supply air diffusers, return grilles, or doors.
The return fan VFD will increase the speed of the fan if the space static pressure reading trends higher than the static pressure setpoint and slow the fan down when the space static pressure reading trends below the static pressure setpoint. A slight delay should be inherent in the sensor logic to mitigate fast reaction times due to rapid variations in pressure caused by doors opening.
OUTISDE AIR DAMPER (Normal Mode of Operation)
During normal operation, the AHU’s outside air damper modulates open or closed based on a signal from a return duct mounted CO2 sensor, with its setpoint set to 600ppm of CO2 (adj.). Upon a rise in CO2 levels above setpoint, the outside air damper will modulate open, allowing more fresh air to enter the space. Upon a decrease in CO2 levels below setpoint, the outside air damper will modulate towards the closed position.
RETURN AIR DAMPER (Normal mode of Operation)
The AHUs’ return air dampers shall open upon the start of the units’ fans, and remain in the open position during normal operation.
OUTSIDE AIR/RETURN AIR DAMPERS (Economizer Mode of Operation)
When the AHU initiates the economizer mode of operation, the outside air and return air dampers will modulate inversely proportional to maintain a mixed air temperature of 52 deg.F (adj.).
OUTSIDE AIR/RETURN AIR DAMPERS (Morning Warmup and Night Setback Modes of Operation)
During morning warmup and night setback modes of operation, the return air damper and outside air damper shall modulate to provide a constant minimum outside airflow as sensed by the outside airflow measuring station.
EXHAUST AIR DAMPER (Non-Economizer Modes of Operation)
The AHUs’ exhaust air dampers remain closed during non-economizer modes of operation.
EXHAUST AIR DAMPER (Economizer Operation)
When the AHU initiates the economizer mode of operation, the exhaust air damper will modulate inversely proportional to the return air damper. Damper operating positions are to be determined during balancing.
SUPPLY AIR DAMPER (All modes of Operation)
The AHUs’ supply air dampers shall open upon the start of the units’ fans, and remain in the open position during all modes of operation. Upon unit shutdown, the supply air dampers shall close. Upon an alarm from either the supply or return duct mounted smoke detectors, the supply air dampers shall close.
"EXHAUST AIR DAMPER (Non-Economizer Modes of Operation)
The AHUs' exhaust air dampers remain closed during non-economizer modes of operation." Could you please explain little more about this point.
- Can we use these control procedure in constant voleum systems?
317 This would only be applicable if the return fan is modulating to maintain space static. If you have a return fan that "tracks" the supply fan, the exhaust air damper would be set to modulate to maintain space pressure.
Constant volume systems would not need a vfd on supply fan, but the return fan could be set to modulate to maintain space pressure and the dampers would modulate as explained. Especially if you have exhaust fans kicking on and off you would want the return fan to modulate to react to this.
- How much percentage the VFD usually modulate of the nominal cfm?
- For the fresh air at normal operation, if the CO2 is under 600ppm (say 200ppm), what is the fresh air damper position would be (fully closed or partial open)?
- if all VAV boxes are 100% open,supply air fan in its nominal cfm (say 1000 cfm), return air fan modulate to keep a desired space pressure(say 900 cfm for sensor reading of 0.05" Wg),
fresh air (say 100 cfm), Co2 is under 600ppm,
Now,for some reason the Co2 became over 600ppm, how the control will respond to this situation to keep CO2 level.
- same question above, but the supply fan is not in its nominal cfm,I mean it will change either increase or decrease.
- VAV system is an instant CV system. I mean the CV unit give you 1000 cfm all the time, the VAV gives you 400, 500, ... up to 1000 cfm,depend on the conditions, but when it works on 500 cfm, it act like CV unit as long as there is no change in work conditions.
- if you have CV or VAV system with a return fan I don't think you will have an exhaust fan to kick on/off.
