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[cool_hardware52]

I understand your explanation but the question I still have is related to the delivered air flow. At 100PSI the flow from the compressor is 4.3 CFM. As the pressure goes up to 4000PSI, the flow from the compressor is only slightly reduces to 4.1 CFM. With no PV installed the air flow is 4.3 CFM and with a PV installed the flow is still 4.3 CFM -- So how can the dwell time be extended when the air flow has not changed I am still filling an 80 cubic tank in 20 minutes with or without a PV. Are you telling me that dwell time has no relationship to air flow. Maybe I do not understand dwell time.
I have had this question for several years and still don't have a good understanding.
I know the PV increases the moisture condensation efficiency
Jim Shelden

Jim,

Lets assume we have a filter tower with an free internal volume of 1 cuft. and a pump that can produce ~1 cuft / min @ 207 bar.

What's the capacity of the tower at 1 ? 1 cuft.

What's the capacity of the tower at 10 bar? 10 cuft.

What's the capacity at 207 bar? 207 cuft.

How long will it take for the pump to push 1 cuft of gas through the tower if the outlet pressure of the tower is 1 bar? I minute

How long if the outlet pressure is 207 bar? 207 minutes. That means the filter media is in contact with the gas for 207 times longer than at 1 bar.

Breathing gas filters are not filters in the same sense as a coffer filter or automotive oil filter, but are really an absorption device, more like the box of baking soda in the back of the refrigerator to absorb bad odors. More dwell time is a good thing.

A PV will also reduce the pressure cycling of the towers.

Tobin

 

[Goferrin99]

I understand your explanation but the question I still have is related to the delivered air flow. At 100PSI the flow from the compressor is 4.3 CFM. As the pressure goes up to 4000PSI, the flow from the compressor is only slightly reduces to 4.1 CFM. With no PV installed the air flow is 4.3 CFM and with a PV installed the flow is still 4.3 CFM -- So how can the dwell time be extended when the air flow has not changed I am still filling an 80 cubic tank in 20 minutes with or without a PV. Are you telling me that dwell time has no relationship to air flow. Maybe I do not understand dwell time.
I have had this question for several years and still don't have a good understanding.
I know the PV increases the moisture condensation efficiency
Jim Shelden

Jim,

An important distinction is where the flow is being measured. A 4.3 CFM compressor has a 1st stage swept volume (or inlet flow) of 4.3 CFM. Its output flow is 4.3/100 CFM @ 100 Bar, 4.3/200 CFM @ 200 Bar etc

Dwell time is inversely proportional to pressurised air flow. Pressurised airfow is the compressors CFM rating divided by the operating pressure.

As a sanity check:
- a compressor producing 4.3 CFM/120 lpm of compressed air would fill an AL80 in 5 seconds
- a compressor compressing 4.3 CFM/120 lpm of air will fill an AL80 in 17 minutes.

Tobin's hypothetical compressor producing 1 cuft @ 208 bar would be labelled a 208 CFM compressor....!!!!

 

[frogman62]

Tobin,
I think I am understanding that the actual time that 1 molecule of air takes to get through the filter is not much different at 1bar than at 207bar but the pressure differential make the travel through the media 207 times more efficient. Air flow in and flow out is the same assuming 1 CFM. Is this correct?

I clearly understand the compressibility issue between a gas and the water/oil droplets and the pressure cycles on the filter.

On all my compressors I have used a PV for 30 years but for the reasons of water/oil condensation, pressure cycles and compressor cylinder back pressure. I have sized the moisture trap and filter based on compressor output volume and customer desire for longer time between filter changes.
My compressor packages have always had a grade E or NFPA1500 as required by most fire departments so I haven't even thought/worried about "dwell time"
At age 72 and after beginning to dive in 1954 I still have more to learn.

Jim

 

[cool_hardware52]

Tobin,
I think I am understanding that the actual time that 1 molecule of air takes to get through the filter is not much different at 1bar than at 207bar but the pressure differential make the travel through the media 207 times more efficient. Air flow in and flow out is the same assuming 1 CFM. Is this correct?

Whats the actual "water volume" of an al 80? About .372 cuft or 642 cu inches.

Same concept for towers.

(We all have plenty left to learn )

Tobin

 

[Zodiacdiverdave]

Good work lads, I think we have this all figured out. Set the back pressure to 2000-2400 is good, less is bad, too much is hard on the compressor.

Life is a learning experience.
ZDD