filters in AHU 4

[saeedplc]

Dear Experts,

I have a question regarding G4 filter (used in air handling unit)differential pressure switch adjustment please assist me;

As far as I know, each G4 filter has a initial pressure and final pressure that means if final pressure is reached
that filter should be replaced but the mentioned pressure is according to the nominal flow passing from the filter.
Now my question is that how should I adjust differential pressure switch if flow passing the filter is 10% of it's nominal flow
to know that filter is dirty?

Another question : Do you replace your AHU filters based on time or differential pressure switch alarm?

Regards,

 

[LittleInch]

I would suggest that you don't care.

Just change then when the DP exceeds your limits

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[saeedplc]

Thank you so much for the reply

But if I put the differential pressure switch as a criteria for replacing the filter then because of
the passing low flow from the filter compare to it's nominal flow at that time filter is much dirtier and may be
the differential pressure never give alarm?

Suppose the filter final pressure is 200pa/21000m3 so by passing the flow 2100m3 from it the differential pressure would be 20 pa !!!
even we consider the filter curve linear

Do you suggest I set the switch on final pressure value of the filter even the flow through it is only 10% of it's nominal flow?

 

[saeedplc]

Dear Experts

I will be appreciated to know your opinion

 

[LittleInch]

Yes, that's exactly what I suggest.

Just because, for reasons I can't work out, you want to flow at 10% of rated flow, that doesn't affect the set point of the filter.

So long as your system fan etc allows for the max DP of the filter up to the clogged DP, then what are you worried about?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[saeedplc]

Do you mean i should set the differential pressure switch to the filter final pressure value?

But i reckon the final pressure value is when the filter is clogged at flow about 21000 m3/h so if i adjust the differential pressure switch to 200pa but only at 10% of the nominal flow the differential pressure switch would not be activated even when the filter is clogged.

Am i wrong?

 

[pierreick]

Hi,
My experience with AHU in lime kilns is timer ,too much trouble to get DP sensors in good condition .
My 2 cents
Pierre

 

[LittleInch]

Yes, I think you're wrong.

But if you want to throw money away changing filters which aren't clogged that's up to you.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[pierreick]

Hi ,
To add to my previous reply , an extract of a paper published in Chemical engineering journal (July 1993) .
Good luck
Pierre

 

[moideen]

The data sheet of filter whether primary or bag filter specifies the pressure drop as clean, medium and dirty. The dirty indication is the alarm to clean the filter or replacement. If you have automation and the help of DP transmitter across the filter, then you can generate alarm point when crosses the “medium’’ as an informational waring and ‘’dirty’’ is for tripping the unit

The problem with the world is that intelligent people are full of doubts, while stupid ones are full of confidence.
-Charles Bukowski-

 

[EnergyProfessional]

Filter manufacturer will give a maximum pressure. Above that, the filter may collapse. then you obviously have a problem. So your alarm should be a bit below that value.

For VAV AHU, it is hard to measure how dirty the filter is just by looking at dP. Every once a while a mechanic just has to look at the filter to judge how dirty it is.

Some dirt is seasonal (cottonwood pollen), and also based on usage hours. So replacing just on time can be costly.

over time the operators will figure out how often they have to look based on their operation. AHU have other maintenance things to look at, so looking at the filter isn't too much of a burden.

 

[saeedplc]

Thank you so much for the replies

 

[MedicineEng]

I'm not sure how can you get 10% of the filter flow unless there was some after manufacture modifications to the unit.
Even if you drive your motor to minimum speed, you won't drive the fan down to just provide you 10% of flow.

Answering your question, when we started we had filter replacement based on time and after throwing money away for a couple of years, we changed to dP alarm and cut our filter changes by around 70%.

We do have VAV AHUs with variable speed drive but we just set the dirty dp setpoint as per manufacturer's recommendation and didn't actually care if the AHU is running at 60, 70 or 100% speed.

 

[saeedplc]

Thank you for the reply @MedicineEng
In our AHU the fresh damper is always 30 % and mixing damper is 70% and only in free cooling mode the fresh air damper become 100% so with this case how should i adjust the differential pressure switch across the filter G4 ?