Pressure measurement for Biofilter

[swazimatt]

Hi
I have taken over a project where the original engineer has moved on. The project is a wastewater pump station with a fan blowing foul air from the wetwell through a biofilter

The design report and other design documentation states the following:

Media lifetime may be 5 to 8 years or longer. During this time the
media decomposes and becomes denser, which reduces the porosity
(air space in the media) and increases the pressure needed to move
the air through the biofilter media. As the airflow rate through the
biofilter increases, the force needed to push the air through the
media increases. This force is measured as the static pressure
difference from the inlet side of the biofilter to the atmosphere.
Static pressure also can be thought of as the resistance to air flow
through the biofilter material. Resistance to air flow is fundamental
to all ventilation systems and is typically reported in inches of
water. Static pressure (pressure drop) between the inside and
outside of a mechanically ventilated livestock building without a
biofilter ranges between 0.04 and 0.12 inches of water (H2O).

I understand that this pressure drop (10mbar)was intended to be measured between the suction side of the vent and downstream of the fan and any fluctuations would result in an alarm being set off to adjust the fan, locate bypass through the media or replace the biofilter media. The drawings were prepared incorrectly with the pressure sensor tappings both being downstream of the fan, effectively measuring the same pressure.
From the text above i understand that the measurement should be measured between the fan (vent downstream of the fan) and the other side of the biofilter (atmospheric pressure) as this is the resistance to blow through the media

My question is would we be able to measure just the gauge pressure in the vent downstream of the fan. Gauge pressure being measured relative to atmospheric would show that the pressure in the vent is targeted at 10mbar (100mm H2O)?

The media is basically bark with lime chips so with time this will degrade and either block up (pressure increase)or the foul air may create shortcuts through the media which would result in pressure drop
(please do not say we should have used a bio-scrubber or other proprietary product, you will just be preaching to the choir!)

 

[HVAC-Novice]

Schematic?

 

[swazimatt]

 

[HVAC-Novice]

By schematic I more meant a schematic how everything is arranged schematically. it is a bit hard to tell from the actual construction document.

Usually for a filter we use a single gauge measuring up and downstream of the filter. You just run tubes to the appropriate locations. in theory using 2 separate gauges can work. but each gauge has in inaccuracy and those can add up if the one happens to to show + 1 ", the other is -1" for example. If you use only one gauge, you don't add errors, and if airflow is zero, you see it being zero (and if not, you know it is wrong). You kind of need to have both gauges at the same location anyway... so I don't see how that would make things simpler.

 

[swazimatt]

I am a civil engineer so forgive my schematic.

But from what you are saying you would measure either side of the filter. In our case the "other side" of the filter is atmospheric so if we only measure gauge pressure in the duct between the fan and filter it is effectively measuring the difference between vent pressure and after filter (atmospheric pressure)

The sensor we are proposing has relatively large steps in measurement (20mm water) so once the fan is set it will only set of an alarm or change fan speed when the pressure drops below 80mm or over 120mm

 

[HVAC-Novice]

Range determines accuracy. If you do a differential pressure, you only need the range at dirty filter.

Make sure flow is the same as that changes pressuredrop. Is that flow pressure-independent and pump speed controlled by a flow meter?

Your original setup may have some merit. If the pump runs at constant speed, that pressure will increase with less flow dud to dirty filter.

 

[swazimatt]

Thanks.

The more i think about it, it must activate an alarm, not the fan. If the filter gets blocked the pressure will increase, if the sensor automatically controls the fan it would just keep slowing it down to match the pressure and there may be zero flow through the filter. The converse also applies for bypass flow, this will just keep increasing the fan until the pressure target is achieved and we have a fountain of bark being blown out the filter

 

[HVAC-Novice]

If you want to adjust fan or pump speed to account for filter degradation, you need to use a flowmeter and control based on that.

Hard to make a general statement if we don't know what the rest of the system does. Is water flowing or supposed to flow all the time, or does it show up intermittently?

you need to look at the whole system before changing parts of the controls.

 

[swazimatt]

no water, just foul air. A biofilter is (IMO) a very primitive filter with multiple opportunities for failure

 

[danw2]

>My question is would we be able to measure just the gauge pressure in the vent downstream of the fan?

Yes, but frequently gauge pressure (GP) transmitters (one pressure port; internally vented to the atmosphere) are not available with a sensing range as low as differential pressure (DP) transmitters (high side port and low side port).

So pressure measurement in a very low 'draft' range are frequently done by DP transmitters because the DP's sensing range matches the application requirement. The low side port is left open to the atmosphere (or plugged with a scintered filter to avoid having insects make a home in the low side sensing cavity.)