Compressor Water Vapor and the effects of a PMV on Filter Usage

[Alan B]

I'm building some code that models the moisture vapor in the compression process. In particular it keeps track of the moisture that ends up in the molecular sieve water filter. This is pretty rough at this time, but it seems to be working and I'm getting some results that might be interesting.

One of my goals with this is to take my log of fills and estimate the usage of the filter cartridge. The cartridge I'm going to be using has an indicator, but I have to unscrew the top of the tower and pull out the filter to inspect the stripe. By running the log through this program I should be able to get a better idea when I need to inspect the indicator and change the filter. I will probably keep the compressor fill log in Google docs and read the data directly into the program (later).

I thought I'd post some preliminary results here to get some feedback on whether the program is producing results that agree with experience. Any comments?

So, let's review a few of the assumptions. I apologize in advance for the mixed up units. I seem to work in multiple units, as appropriate.
6.8 liter 4500 psi carbon fiber SCBA tank (about 72 cubic feet)
Filter with 70g of molecular sieve material that can hold 20% water by weight so 14g capacity (this is based on an estimate from the Coltri cartridge)
70F ambient air inlet temperature
20F temperature rise through the compressor
100% effective water separator (only vapor considered, 100% humidity at compressor output)
1900 psi PMV pressure maintaining valve (unless otherwise noted)

The program breaks down the fill into small increments and analyzes the air and water flow at each increment. No claims are made that the code is error free at this point, but it roughly agrees with some simple calculations. I'll probably find more errors over time, but hopefully it is close.

Experiment One - typical use cases
with a 1900 psi pmv fill the 6.8 liter tank from empty, 2000 and 3000 psi starting points and observe the water filter capacity usage (percent):
0-4500 psi, 3.1%
2000-4500 psi, 1.3%
3000-4500 psi, 0.7%

Experiment Two - value of the PMV at various pressure settings
Set the PMV to 0, 1900, 2900 and 4400 psi and see the effect it has on filter usage during a 0 to 4500 psi fill of the 6.8 liter tank:
pmv = 0, fill uses 9.6% of the filter's water capacity in a single fill when there is no PMV
pmv = 1900, 3.1%
pmv = 2900, 2.4%
pmv = 4400, 1.7%

[AlanMcD]

Are those experiments physical tests or tests of your math model?

Conceptually, everything looks about right, but I do question the 20 degree F rise in air temp - I would expect a good amount more than that, depending on the compressor you are using, and the air path out/after compression.

Assuming these are modeling results, they clearly show the value of the PMV on post compression filtration - but potentially also the importance of getting the discharge temp as low as possible to drive the condensation to happen.  You could also vary that parameter and see the impact - it will be quite large.

What kind of compressor do you use? Personally I run a Shoebox, and have done similar math to support the best filtration result on the output side of the shop compressor as I feed the Shoebox.  As a result, I run a much bigger desiccant filter on the input side that I would need on the output side.

[mobilehomer]

Alan, this article may help.

https://www.scubaengineer.com/high_pressure_compressor_filtrat.htm

[Alan B]

These calculations are based on data that I found here and elsewhere on the net. My compressor setup is waiting for an adapter to the filter so we're not quite going yet.

The only temperature the software uses is the temperature of the airflow leaving the water separator, and I've seen suggested values of 10-20C over ambient for that. 20F is on the low side there. The Coltri water separator is massive and the 1.1 SCFM pumping rate is low for this 115V model hence the estimate of ambient plus 20F. As you stated, increasing it will increase water filter consumption, as does filling in warmer temperatures, both of which would be interesting experiments to do with the model.

I have the Daystate LC-110 which is a Coltri MCH-3 "low consumption" with a 14.5A 115V motor. The 230V 14A model is rated to pump at about 3.4 SCFM. So this compressor runs slower and cooler.

[Alan B]

Alan, this article may help.

...not permitted link removed...

Excellent article. I found that before and it was quite helpful.

[Alan B]

Added more studies, cleaned up the code somewhat.

Format is grams of water vapor, percent of filter capacity, start-end psi, separator outlet temperature, pmv psi, comment

typical use cases
0.4 g 3.1 % 0 - 4500 90 F 1900 pmv initial
0.1 g 0.7 % 3000 - 4500 90 F 1900 pmv top off

temperature study std pmv
0.3 g 2.3 % 0 - 4500 80 F 1900 pmv initial
0.4 g 3.1 % 0 - 4500 90 F 1900 pmv initial
0.6 g 4.1 % 0 - 4500 100 F 1900 pmv initial
0.7 g 5.0 % 0 - 4500 110 F 1900 pmv initial
0.9 g 6.7 % 0 - 4500 120 F 1900 pmv initial

temperature study no pmv
1.0 g 7.1 % 0 - 4500 80 F 0 pmv initial
1.3 g 9.6 % 0 - 4500 90 F 0 pmv initial
1.8 g 12.7 % 0 - 4500 100 F 0 pmv initial
2.2 g 15.5 % 0 - 4500 110 F 0 pmv initial
2.9 g 20.8 % 0 - 4500 120 F 0 pmv initial

study of pmv settings
1.3 g 9.6 % 0 - 4500 90 F 0 pmv initial
0.4 g 3.1 % 0 - 4500 90 F 1900 pmv initial
0.3 g 2.4 % 0 - 4500 90 F 2900 pmv initial
0.3 g 1.9 % 0 - 4500 90 F 3900 pmv initial
0.2 g 1.7 % 0 - 4500 90 F 4400 pmv initial

[AlanMcD]

Nice data - thanks for sharing it!

This lines up well with something I posted a while ago on water vapor to help explain things in this complicated subject, in which I showed data for 1 cubic meter of air at atmospheric pressure.  The tank you are filling in this data, at 72 cubic feet capacity, is a little over 2 meters of air for a full fill, and the partial fills are proportional (~2/3 of a cubic meter per 1500 psi).

https://www.gatewaytoairguns.org/GTA/index.php?topic=139845.msg1407834#msg1407834

[Alan B]

Nice data - thanks for sharing it!

This lines up well with something I posted a while ago on water vapor to help explain things in this complicated subject, in which I showed data for 1 cubic meter of air at atmospheric pressure.  The tank you are filling in this data, at 72 cubic feet capacity, is a little over 2 meters of air for a full fill, and the partial fills are proportional (~2/3 of a cubic meter per 1500 psi).

https://www.gatewaytoairguns.org/GTA/index.php?topic=139845.msg1407834#msg1407834

That was a good discussion. Thanks for the link. It is good to link and re-read the good threads.

I ordered two filter cartridges for the Coltri and just realized that the one I measured a few days ago has an indicator that should be blue but is already white. I just checked the other filter and it is blue so I have at least one that is good.

I would like to try taking the exhausted cartridge apart and regenerating the media but I suspect there's no good way to put the cartridge back together. They do make rebuildable cartridges for the larger filters but not for this smaller size. I may try to fabricate something from the bad cartridge. I'm still waiting on a couple of parts to finish putting this filter online.

I did some more work on the code yesterday and added the pumped volumes, made the output cleaner and added a few more typical cases for filling various HPA bottles:


HPAsim 0.1 8/2019 akbiocca

Typical Use Cases
Liters H2Og Filt% Start  End Size DegF  PMV Comment
  2082 0.43  3.10     0 4500 6.80   90 1900 Fill 72 CF SCBA
   694 0.09  0.67  3000 4500 6.80   90 1900 Topoff 72 CF SCBA
   340 0.07  0.51     0 4500 1.11   90 1900 Fill 68 ci 4500
   161 0.04  0.28     0 3000 0.79   90 1900 Fill 48 ci 3000
    87 0.02  0.15     0 3000 0.43   90 1900 Fill 26 ci 3000
    43 0.01  0.08     0 3000 0.21   90 1900 Fill 13 ci 3000
    33 0.01  0.05  2000 3000 0.49   90 1900 Topoff AirForce

Temperature Study with Standard PMV
Liters H2Og Filt% Start  End Size DegF  PMV Comment
  2082 0.32  2.29     0 4500 6.80   80 1900 Fill 72 CF SCBA
  2082 0.43  3.10     0 4500 6.80   90 1900 Fill 72 CF SCBA
  2082 0.58  4.13     0 4500 6.80  100 1900 Fill 72 CF SCBA
  2082 0.70  5.03     0 4500 6.80  110 1900 Fill 72 CF SCBA
  2082 0.94  6.74     0 4500 6.80  120 1900 Fill 72 CF SCBA
   694 0.07  0.50  3000 4500 6.80   80 1900 Topoff 72 CF SCBA
   694 0.09  0.67  3000 4500 6.80   90 1900 Topoff 72 CF SCBA
   694 0.13  0.90  3000 4500 6.80  100 1900 Topoff 72 CF SCBA
   694 0.15  1.09  3000 4500 6.80  110 1900 Topoff 72 CF SCBA
   694 0.21  1.46  3000 4500 6.80  120 1900 Topoff 72 CF SCBA

Temperature Study without PMV
Liters H2Og Filt% Start  End Size DegF  PMV Comment
  2082 0.99  7.06     0 4500 6.80   80    0 Fill 72 CF SCBA
  2082 1.34  9.56     0 4500 6.80   90    0 Fill 72 CF SCBA
  2082 1.78 12.72     0 4500 6.80  100    0 Fill 72 CF SCBA
  2082 2.17 15.49     0 4500 6.80  110    0 Fill 72 CF SCBA
  2082 2.91 20.77     0 4500 6.80  120    0 Fill 72 CF SCBA

Study of PMV Settings
Liters H2Og Filt% Start  End Size DegF  PMV Comment
  2082 1.34  9.56     0 4500 6.80   90    0 Fill 72 CF SCBA
  2082 0.43  3.10     0 4500 6.80   90 1900 Fill 72 CF SCBA
  2082 0.34  2.40     0 4500 6.80   90 2900 Fill 72 CF SCBA
  2082 0.27  1.91     0 4500 6.80   90 3900 Fill 72 CF SCBA
  2082 0.24  1.71     0 4500 6.80   90 4400 Fill 72 CF SCBA

[Alan B]

I took the exhausted Coltri filter apart and measured the contained Molecular Sieve weight. Previously I had estimated the sieve quantity from exterior dimensions and published density. The wall thickness was more than estimated resulting in the estimate of dessicant being a little high. The measured weight of the saturated beads was 75g. This puts the dry weight at about 60g instead of the previous estimate of 70g. I changed this value in the software, which has the effect of increasing the percentages of filter usage by about 15%.

For those with "Gold" or other filters, how much sieve material do you end up fitting into the filter or the cartridge?

I added some code to analyze the fill log, and put in some fake data. The columns are a bit different for the log than the studies. That's at the end of the run below. There are flags to skip the output for either the log or the studies, both are included in the example.

I am looking at converting the Coltri tube into a refillable cartridge. It has an internal spring that makes it difficult to manually refill, but I have some ideas on how to make it easy to refill and still maintain spring pressure on the media. More on that later after I test some ideas.

Drying the saturated 13x media looks to be difficult with high temperatures required and cooling it without re-saturating with moisture. Using new media could lower costs over new filters by a factor of seven or more and be fairly simple. A pound of media is priced about the same cost as a new filter. Cartridges have a rated life limit of six months and a capacity of almost 100 topoffs of a 6.8 liter SCBA tank depending on temperature. For most changing or refilling the cartridge at six month intervals should cover usage.

One clear takeaway from these studies is that filling and topping off the SCBA tank is best done at the coolest available location and time to prolong filter life. Here that means early in the morning in the garage since we don't have air conditioning. The compressors that have the lowest temperature rise also have an advantage here, faster is hotter and consumes the filter media more rapidly.


HPAsim 0.13 8/2019 akbiocca

Cartridge contains 60.0 grams of 13x molecular sieve

Typical Use Cases
Liters H2Og Filt% Start  End Size DegF  PMV Comment
  2082 0.43  3.62     0 4500 6.80   90 1900 Fill 72 CF SCBA
   694 0.09  0.79  3000 4500 6.80   90 1900 Topoff 72 CF SCBA
   340 0.07  0.59     0 4500 1.11   90 1900 Fill 68 ci 4500
   161 0.04  0.33     0 3000 0.79   90 1900 Fill 48 ci 3000
    87 0.02  0.18     0 3000 0.43   90 1900 Fill 26 ci 3000
    43 0.01  0.09     0 3000 0.21   90 1900 Fill 13 ci 3000
    33 0.01  0.06  2000 3000 0.49   90 1900 Topoff AirForce

Temperature Study with Standard PMV
Liters H2Og Filt% Start  End Size DegF  PMV Comment
  2082 0.32  2.67     0 4500 6.80   80 1900 Fill 72 CF SCBA
  2082 0.43  3.62     0 4500 6.80   90 1900 Fill 72 CF SCBA
  2082 0.58  4.82     0 4500 6.80  100 1900 Fill 72 CF SCBA
  2082 0.70  5.86     0 4500 6.80  110 1900 Fill 72 CF SCBA
  2082 0.94  7.86     0 4500 6.80  120 1900 Fill 72 CF SCBA
   694 0.07  0.58  3000 4500 6.80   80 1900 Topoff 72 CF SCBA
   694 0.09  0.79  3000 4500 6.80   90 1900 Topoff 72 CF SCBA
   694 0.13  1.05  3000 4500 6.80  100 1900 Topoff 72 CF SCBA
   694 0.15  1.27  3000 4500 6.80  110 1900 Topoff 72 CF SCBA
   694 0.21  1.71  3000 4500 6.80  120 1900 Topoff 72 CF SCBA

Temperature Study without PMV
Liters H2Og Filt% Start  End Size DegF  PMV Comment
  2082 0.99  8.22     0 4500 6.80   80    0 Fill 72 CF SCBA
  2082 1.34 11.13     0 4500 6.80   90    0 Fill 72 CF SCBA
  2082 1.78 14.81     0 4500 6.80  100    0 Fill 72 CF SCBA
  2082 2.16 18.03     0 4500 6.80  110    0 Fill 72 CF SCBA
  2082 2.90 24.17     0 4500 6.80  120    0 Fill 72 CF SCBA

Study of PMV Settings
Liters H2Og Filt% Start  End Size DegF  PMV Comment
  2082 1.34 11.13     0 4500 6.80   90    0 Fill 72 CF SCBA
  2082 0.43  3.62     0 4500 6.80   90 1900 Fill 72 CF SCBA
  2082 0.34  2.80     0 4500 6.80   90 2900 Fill 72 CF SCBA
  2082 0.27  2.23     0 4500 6.80   90 3900 Fill 72 CF SCBA
  2082 0.24  1.99     0 4500 6.80   90 4400 Fill 72 CF SCBA

Usage Log (PMV 1900, Rise 20 F)
Date   Hours Liters  Filt% TotF% Start  End   Size DegF Comment
190801   1.1   2082   3.62   3.6     0 4500   6.80   70 Fill 72 CF SCBA
190802   2.3    833   0.99   4.6  2700 4500   6.80   70 Topoff 72 CF SCBA
190802   2.7  41644  49.55  54.2  2700 4500 340.00   70 Topoff 72 CF SCBA x50
190803  25.9    340   0.59  54.8     0 4500   1.11   70 Fill 68 ci 4500
190803  26.0    161   0.33  55.1     0 3000   0.79   70 Fill 48 ci 3000
190804  26.1     87   0.18  55.3     0 3000   0.43   70 Fill 26 ci 3000
190804  26.2     43   0.09  55.3     0 3000   0.21   70 Fill 13 ci 3000
190806  26.2     33   0.06  55.4  2000 3000   0.49   70 Topoff AirForce

[PCPhack]

Very interesting thread. Thanks for posting.

My compressor is also in the garage, but I have often considered running a small bit of tubing through the wall into the house, so that it can pull cool, dehumidifier air in to begin with. My compressor has a plastic inlet filter that would be easy to attach a hose barb to. This thread reminds me that I really ought to do that.

The 6 month cartridge life claim is interesting. As long as the cartridge is in a closed system, I don't see why it would saturate further while the compressor is not running. I would think it could easily last much longer. I know that mine last longer than that. After a fill, I run the compressor with the separator open for 5 minutes, shut it off, and then close the separator, so that at rest, the system is closed.

[Alan B]

Thanks for your comments. Good to hear experience that cartridges last longer. I don't know where the six month life comes from, they also mention 2 years for the sealed cartridge. They want consistently trouble free customer experience and to sell cartridges so they are likely conservative. Since they are primarily in the breathing air compression business I'd expect them to be particularly conservative. Even though this filter is 13x molecular sieve only, it is used for breathing air in a 2 cartridge system, and they also indicate both cartridges to be changed together.

Is the media volume of your setup similar to this small Coltri filter (about 28 by 160 mm in the bead column)?

I'm planning to operate the way you do, cooling down and drying the compressor with a few minutes of running out the open drain valve, then closing it. With the PMV being closed after the filter, and the drain valves closed, airflow is blocked from both ends and it would seem likely that the filter would not see moist air while sitting.

I don't think the inlet air temperature matters with all the other heat inputs to the air during the compression process. The critical temperature is the moisture separator. Operating an effective moisture separator in a cooling bath could really be helpful. That's the optimal place to cool the air.

If an effective moisture separator is being purged properly the extra inlet moisture won't make much difference, it will increase the moisture in the purge of course but won't affect the sieve filter usage.

[PCPhack]

I am not home right now to measure it, but my filter is the Alpha.

http://www.airtanksforsale.com/AlphaFilter/

[Alan B]

I added a Capacity Table.

This uses the PMV pressure of 1900 psi so is a bit conservative.

Temperature is of the moisture separator.
Water is grams per cubic meter.
Values based on formulas from the reference, they are very close to most charts, but some variation is noted.


HPAsim 0.14 8/2019 akbiocca

Cartridge contains 60.0 grams of 13x molecular sieve

Capacity vs Temperature (pumping 30 liters per minute)

  F   C  H2Og liters hours

 30  -1   5.6 281105 156.2
 40   4   6.6 235330 130.7
 50  10   9.4 166660  92.6
 60  15  13.2 118422  65.8
 70  21  18.4  84837  47.1
 80  26  25.4  61597  34.2
 90  32  34.4  45471  25.3
100  37  45.8  34170  19.0
110  43  55.7  28067  15.6
120  48  74.7  20938  11.6
130  54  99.0  15798   8.8
140  60 129.3  12088   6.7
150  65 166.5   9387   5.2

ref
hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/relhum.html

[Alan B]

I am not home right now to measure it, but my filter is the Alpha.

<unallowed link removed>

I looked for cartridge info on the AlphaFilter and didn't find any details. It seems likely that it is similar but it would be useful to figure that out for certain.

I made some progress wrenching on the filter plumbing. Parts are due in Thursday. Once I get it working and figure out how to best post photos on this forum perhaps I'll make a thread detailing this setup. It's slightly different than anything I've seen around, at least in some details.