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DaBit
16-01-2004, 03:19 PM
I am just wondering how well a standard 3A molecular sieve drier works at liquid temperatures of -40C/-40F or lower.

The molecular sieves are supposed to capture water molecules in their 3A wide 'cloves', but I am not sure how well this process works when the incoming liquid is -40C/-40F, as would be the case in a cascade low stage.

chemi-cool
16-01-2004, 03:33 PM
"when there is a doubt - there is no doubt"

ask the manufacturer.

chemi

Prof Sporlan
17-01-2004, 05:14 AM
Perhaps it is better to view the performance of filter-driers with respect to the refrigerant's affinity to moisture. For example, saturated liquid R-134a at 100°F will have a greater affinity for moisture than R-12 at the same conditions. Therefore, a well designed refrigerant filter-drier, e.g., Sporlan's Catch-All :), will be able to remove more moisture from liquid R-12 than from R-134a, all things being the same.

You may find it interesting that a refrigerant's affinity to moisture decreases at lower temperatures. Lower temperatures will increase the filter-drier's ability to remove moisture.

Gary
17-01-2004, 12:18 PM
That's interesting, Prof. It would seem then, that the ideal (coldest liquid line) spot for a drier would be at the metering device inlet, particularly if the system has a suction/liquid heat exchanger.

Prof Sporlan
17-01-2004, 03:35 PM
That is correct. But, of course, if one has an adequately sized filter-drier in the first place, the resulting ppm moisture level in the system will be well into the "safe" range regardless of the temperature of the liquid line.

DaBit
19-01-2004, 12:21 AM
And this is also valid for way-below-freezing temperatures?

OK, then I will put the drier just before the cap tube. As usual :)

Gary
19-01-2004, 10:45 PM
OK, then I will put the drier just before the cap tube. As usual

On the close coupled (short liquid and suction line) systems you are familiar with, it doesn't matter. Most commercial refrigeration systems, however have their condensing units in a remote location, using very long suction and liquid lines.

DaBit
20-01-2004, 09:30 AM
If a regular drier would not perform as it should do at low temperatures, I would have desuperheated the discharge gas to room temperature, and put the drier between the desuperheater and condenser. A regular 1/4" x 1/4" solder-in drier like Sporlan's Catch-all has more than enough capacity and little enough flow resistance to dry the high pressure refrigerant vapour.

herefishy
20-01-2004, 03:39 PM
Originally posted by DaBit
If a regular drier would not perform as it should do at low temperatures, I would have desuperheated the discharge gas to room temperature, and put the drier between the desuperheater and condenser. A regular 1/4" x 1/4" solder-in drier like Sporlan's Catch-all has more than enough capacity and little enough flow resistance to dry the high pressure refrigerant vapour.


Errrrrr, the device is called a "liquid line" filter-drier, not a "hot gas" filter-drier. The velocity of the vapor is much higher than that of the liquid. From my personal experience (after having followed an errant technician on a package unit) a filter-drier in the discharge line creates a high PD, and results in excessive head pressures (of course depending on the size of the drier, but one of the typical recommended size for the application - in the liquid line)

Just speaking from experience............ my observations could be wrong.

The erroneous application resulted in several humorous remarks regarding the proper application of a hot gas filter-drier! My co-worker at that time was not amused by being the butt of my jokes. Particularly when he found the component on the boss' desk with "hot gas drier" written on it in marker pen. LOL

Cheers

DaBit
20-01-2004, 04:07 PM
Originally posted by herefishy
[B]Errrrrr, the device is called a "liquid line" filter-drier, not a "hot gas" filter-drier. The velocity of the vapor is much higher than that of the liquid.

Sporlan's bulletin 40-10 says (on page 25)
'THE CONSTRUCTION OF THE SUCTION LINE FILTER-DRIER is not
significantly different from the standard liquid line filter-drier'

Now, why can't we use a drier in the discharge line, while we can in the suction and liquid lines? Usually the temperature is way too high there to allow the filter to do it's job properly. The high temperature would severely limit the drier's ability to remove moisture.

The other good reason not to use a liquid line drier in the discharge or suction is pressure drop.

The last good reason is that density in a vapour stream is lower than in a liquid stream, thus the filter works slower.

And that's about it, right (if I'm wrong you can stop reading here ;) )?


Thus, if we intend to use a liquid line drier in the discharge of a cascades' low stage, we can do so if we fullfill a few requirements:

- Our temperature must be low.

This is easily done by desuperheating the discharge vapour. Something you actually always want since it reduces load on the high stage.
Please note that this cannot be done in a regular system; we would condense the refrigerant instead of desuperheat it..

- Pressure drop must be low.

The lowest capacity 'normal' (not pencil) filter/driers such as Sporlans C-032-S or Danfoss' DU can do about 3kW (1 ton) with 1psi pressure drop, when used as liquid drier. If we used such a filter in the discharge line, capacity would drop tremendously. But not enough to get into trouble with the very low (150-200W or about 1/16 to 1/8 tons) capacity systems we use in CPU cooling.
This can be proven by comparing discharge vapour density and liquid density (which I leave as an exercise; no Coolpack here at the moment).


I expected that a filter/driers efficiency would be impaired when used below freezing, which is why I considered mounting the filter in the discharge line since that is the only above-freezing spot in the low stage piping.

Luckily, this is not the case so I happily continue using the filters in the liquid line.

Prof Sporlan
21-01-2004, 02:42 AM
So many points to address while the Prof is enjoying a fine Belgian style Trippel ale. But he shall attempt his best… :)


From my personal experience (after having followed an errant technician on a package unit) a filter-drier in the discharge line creates a high PD, and results in excessive head pressures (of course depending on the size of the drier, but one of the typical recommended size for the application - in the liquid line)
Correct. Pressure drop across the filter-drier is a function of the refrigerant’s density. A filter-drier will have a lower pressure drop in the liquid line versus than the discharge line given the same refrigerant flow rate.


Now, why can't we use a drier in the discharge line, while we can in the suction and liquid lines?
One can use a filter-drier, provided it is properly sized, in the discharge line. Two caveats here: (1) the filter-drier should be a core type, e.g., Sporlan Catch-All. It should not be a desiccant bead type. Compressor pulsations could tear apart these driers; (2) On light commercial or larger equipment, say greater than 5 tons, there are economic, sociopolitical, and some common sense engineering reasons not to do this… :)

You may be interested to know that York has applied Catch-All filter-driers to some of its residential heat pump systems in the common discharge line.


The last good reason is that density in a vapour stream is lower than in a liquid stream, thus the filter works slower.
This gets us into an interesting subject… the refrigerant’s affinity to moisture in its vapor state compared to its liquid state. Interestingly, there is substantial variance here, depending upon the refrigerant. As the Prof recalls (keep in mind Belgian ale), the affinity of R-12 vapor to moisture is much greater than in its liquid state. Thus the filter-drier is less effective with R-12 vapor. With R-22, we don’t have quite the variance.


The lowest capacity 'normal' (not pencil) filter/driers such as Sporlans C-032-S or Danfoss' DU can do about 3kW (1 ton) with 1psi pressure drop, when used as liquid drier. If we used such a filter in the discharge line, capacity would drop tremendously. But not enough to get into trouble with the very low (150-200W or about 1/16 to 1/8 tons) capacity systems we use in CPU cooling.
Very good grasshopper… :)

DaBit
21-01-2004, 08:54 AM
Originally posted by Prof Sporlan
Correct. Pressure drop across the filter-drier is a function of the refrigerant’s density. A filter-drier will have a lower pressure drop in the liquid line versus than the discharge line given the same refrigerant flow rate.

Prof: I'm just curious. What happens with the pressure drop if we assume equal velocity instead of mass in both the vapour and liquid scenario?


Compressor pulsations could tear apart these driers;

You assume heavy compressors :)
Pulsations after the oil separator are neglectible in my system.


You may be interested to know that York has applied Catch-All filter-driers to some of its residential heat pump systems in the common discharge line.

Why?

Gary
21-01-2004, 09:59 PM
Unlike the liquid or suction lines, in the discharge line if the drier plugs, the valves inside the compressor break, and you get to buy a new compressor.

Prof Sporlan
22-01-2004, 02:36 AM
Prof: I'm just curious. What happens with the pressure drop if we assume equal velocity instead of mass in both the vapour and liquid scenario?
The math gets a tad cumbersome looking at it this way. Say your liquid has 100 times the density of vapor. If they are flowing at the same velocity, then you are flowing 100 times the liquid mass. But flow rate thru a filter-drier is approx proportional to the square root of both density and pressure drop.

York has applied Catch-Alls in the common discharge line for a number of reasons, a couple: (1) it is less expensive than a bi-directional filter-drier; (2) provides protection for the reversing valve.

RogGoetsch
22-01-2004, 07:21 AM
I'm not too concerned with speed as much as with final moisture level.

Since mass flow rate is the same, vapor or liquid, past any point in a system in equilibrium, it seems with the Prof's info that the drier would be more effective in the liquid line, but is that difference significant? I doubt it.

Gary Collins
26-01-2004, 03:34 AM
A 2 stage cascade system with 404A or the old R-502 in the high stage system typically will sub-cool the low stage liquid R-23 or similar refrigerant to 10F to -15F. The liquid line drier will work at those temperatures however the low stage system should be evacuated to 100-250 microns prior to charging to fully dehydrate the system. It will take some time to acheive that level of vacuum but the reliabilty factor is worth it. A filter drier that has water absorbed in it and then is exposed to below freezing either refrigerant or ambient temperatures could possibly mechanically breakdown the filter core by the expansion of the freezing moisture, but that would be a very wet system with other water related problems