filter pressure vs flow rate

[iam4iam]

Thanks, Mark. Still thinking about the "filter efficiency"--not efficiency of the filter itself, but of the filtering process--i.e. how much unwanted stuff is being filtered per unit of time, which would seem to be a function of the following:
1) filter effectiveness (ability to filter out particles), which would be related to the difference between actual filter pressure and "clean" filter pressure, since the filter becomes more effective at filtering as it gets dirtier) and
2) water flow rate.
It seems like this "efficiency" would be some sort of product of those two variables. Would flow rate be the more important variable here? The 25% recommendation would seem to imply that.

BTW, I have downloaded your spreadsheet. How does the data for Hayward SP2305X7 compare to the SP2305X7EE/SP2305X7EESP? (I have the former.)

 

[mas985]

Filters in general trap more particles as they get dirty at the expense of flow rate. But a small loss in flow rate is not a big deal.

As for too much flow rate, the higher the flow rate, the more likely debris can get pushed through the media (especially sand) and/or damage the media (cartridge and DE). So the flow rate limits of filters are important for that reason.

It seems like this "efficiency" would be some sort of product of those two variables. Would flow rate be the more important variable here? The 25% recommendation would seem to imply that.

The 25% has nothing to do with filter efficiency. The 25% mark is to prevent people from relying upon the manufactures recommendation of a 10 PSI rise before cleaning a filter. In the past, this has caused pool owners significant problems. 10 PSI is over 23' in head and for many pool systems, there is not 23' of additional head available in the pump's head curve so these pools basically wait forever for the PSI rise that never happens and in the mean time flow rate goes to zero.


But I think you are placing way too much importance on filtering. Filtering is more about aesthetics and has little to do with sanitation.

BTW, I have downloaded your spreadsheet. How does the data for Hayward SP2305X7 compare to the SP2305X7EE/SP2305X7EESP? (I have the former.)

The SP2305X7EE head curve is the same, only the energy use is different and not by much (~5%).

 

[iam4iam]

The 25% has nothing to do with filter efficiency. The 25% mark is to prevent people from relying upon the manufactures recommendation of a 10 PSI rise before cleaning a filter. In the past, this has caused pool owners significant problems. 10 PSI is over 23' in head and for many pool systems, there is not 23' of additional head available in the pump's head curve so these pools basically wait forever for the PSI rise that never happens and in the mean time flow rate goes to zero.

It certainly makes more sense to use a percentage increase than an absolute increase.

But I think you are placing way too much importance on filtering. Filtering is more about aesthetics and has little to do with sanitation.

I am not putting aesthetics ahead of sanitation. Assuming sanitation is a given, we turn our attention to the sparkle factor!

Now I have encountered a new head scratcher:
I decided to get some silicone based lubricant for the o-rings in my plumbing system. After lubing the pump connections and the pump basket lid this morning, the pump basket seemed to prime to the point of no air bubbles noticeably faster than before, which was no surprise. What was a surprise was that the filter pressure dropped from 11 PSI to 9 PSI. I have noticed that filter pressure sometimes varies a bit from one pump-on interval to another, which I (correctly?) attribute to variations in how the debris settles on the sand when the pump turns off, but never has the pressure dipped below 10. (10.5-11 has been consistent "clean" pressure after adding DE.) I know that low pressure is normally a sign of some sort of obstruction in the line somewhere, but I don't see how that is possible. I checked the skimmer to see if maybe there was something in the basket there that would be reducing the flow to the pump, but nothing. Any ideas, anyone?

PS Part of my lubrication project involved disassembling my multi-port valve and lubricating it. Now it turns so much more smoothly! That is the good part. The bad part is that the first time I put the pin back in the handle, I had it in the wrong position, so it was actually in the WASTE position when the lever was on the FILTER position. Thank goodness it was WASTE instead of CLOSED!!! Getting the pin back in was quite a chore. I had to get my son to push it in while I put all of my weight on the valve cover to compress the spring. (I hope the long PS didn't distract from the question about pressure loss.)

 

[mas985]

I am not putting aesthetics ahead of sanitation. Assuming sanitation is a given, we turn our attention to the sparkle factor!

Lack of sparkle is generally caused by something growing in the pool rather than debris.

Filtering removes debris from the pool surface via skimming and if you have a pool cleaner, it removes it from the debris that falls to the bottom of the pool. You might think filtering removes debris that is suspended in the water but that is not usually the case because in order for debris to remain suspended in the water, it must be smaller than about 2 microns and that is too small to be captured by any filter. All other debris either floats or eventually sinks to the bottom. Some of the debris may be temporarily suspended in the water while the pump is running but in that case, you are better off (i.e. faster process) allowing that debris to sink to the bottom of the pool and to vacuum it up than you are trying to remove it from suspension.

PS Part of my lubrication project involved disassembling my multi-port valve and lubricating it. Now it turns so much more smoothly! That is the good part. The bad part is that the first time I put the pin back in the handle, I had it in the wrong position, so it was actually in the WASTE position when the lever was on the FILTER position. Thank goodness it was WASTE instead of CLOSED!!! Getting the pin back in was quite a chore. I had to get my son to push it in while I put all of my weight on the valve cover to compress the spring. (I hope the long PS didn't distract from the question about pressure loss.)

This work could have also had an impact on the filter pressure.

 

[iam4iam]

Lack of sparkle is generally caused by something growing in the pool rather than debris.

Filtering removes debris from the pool surface via skimming and if you have a pool cleaner, it removes it from the debris that falls to the bottom of the pool. You might think filtering removes debris that is suspended in the water but that is not usually the case because in order for debris to remain suspended in the water, it must be smaller than about 2 microns and that is too small to be captured by any filter. All other debris either floats or eventually sinks to the bottom. Some of the debris may be temporarily suspended in the water while the pump is running but in that case, you are better off (i.e. faster process) allowing that debris to sink to the bottom of the pool and to vacuum it up than you are trying to remove it from suspension.

That makes sense. So that being the case, then, since I don't have a pool cleaner, the only thing my filter is doing is doing (when no organics are present) is removing debris from the surface? I would conclude that since there is a solar cover on my pool whenever it is not in use, I guess the only reason to run my pump would be to give my SWG sufficient time to add enough chlorine to the pool. If that is the case, then someone who sanitizes manually and has a pool cover and no automatic vacuum need only run their pump when adding chemicals, and only then for enough time to distribute what has been added.

So how big is algae? It seems to stay suspended in the water, but the filter catches it. Is it just that so much is suspended that it catches a lot, even though it would eventually fall to the bottom if the pump is not running but sufficient sanitizer is present?

 

[mas985]

So that being the case, then, since I don't have a pool cleaner, the only thing my filter is doing is doing (when no organics are present) is removing debris from the surface?

Yes

I would conclude that since there is a solar cover on my pool whenever it is not in use, I guess the only reason to run my pump would be to give my SWG sufficient time to add enough chlorine to the pool.

Exactly

If that is the case, then someone who sanitizes manually and has a pool cover and no automatic vacuum need only run their pump when adding chemicals, and only then for enough time to distribute what has been added.

Correct! There are some forum members who do just that and run only 30 min a day. But for most pools there are other reasons to increase pump time as we discussed before (SWGs, solar, heaters, lots of trees, etc).

So how big is algae? It seems to stay suspended in the water, but the filter catches it. Is it just that so much is suspended that it catches a lot, even though it would eventually fall to the bottom if the pump is not running but sufficient sanitizer is present?

There is a range of algae sizes and it differs if the algae is alive vs dead. Live algae tends to grow in chains and so will clog a filter in no time. However, dead algae can break apart and become smaller than what a sand filter can filter out of a pool. You will read many post about pool owners with sand filters that just completed a SLAM but are unable to clear the pool of the dead algae. However, once they add DE to the filter, it usually clears up right away. So because of this dead algae is most likely between 5-20 microns.

 

[iam4iam]

Thanks, Mark. You truly are a TFP expert! So by adding DE to my sand filter (which i currently do), have I essentially created a filter of comparable "filterability" to a DE filter?

There is a range of algae sizes and it differs if the algae is alive vs dead. Live algae tends to grow in chains and so will clog a filter in no time. However, dead algae can break apart and become smaller than what a sand filter can filter out of a pool. You will read many post about pool owners with sand filters that just completed a SLAM but are unable to clear the pool of the dead algae. However, once they add DE to the filter, it usually clears up right away.

This must be why it took my pool so long to clear when I opened it (too late!) the first year I owned it. That was when I found TFP, where I learned about BBB and DE!

 

[mas985]

So by adding DE to my sand filter (which i currently do), have I essentially created a filter of comparable "filterability" to a DE filter?

It definitely improves filtering. By how much? It probably depends on how much DE is used. But my guess is that it is closer to a Cartridge filter.

 

[JamesW]

The DE definitely helps. However, DE filters are typically 48 to 60 sqft whereas sand filters are about 3.14 to 4.9 sqft.

So, while the micron level improves, the filter can quickly get overwhelmed.

Using de in a sand filter should typically only be done for the final polish and not for early filtering.

 

[iam4iam]

The DE definitely helps. However, DE filters are typically 48 to 60 sqft whereas sand filters are about 3.14 to 4.9 sqft. So, while the micron level improves, the filter can quickly get overwhelmed.

Holy cow! There is seriously that much more surface on a DE filter? So it isn't just a matter of catching smaller stuff, it is a matter of catching more of it! i might have to look into getting DE filter! Are they as easy as sand filters to upkeep? Are there costs other than initial purchase to consider? (I know this thread has steered off original topic a bit now, but still related I suppose.)

Using de in a sand filter should typically only be done for the final polish and not for early filtering.

I have found on the few occasions I have encountered green that the DE speeds up the process dramatically. Sure I have to backwash more often, but that is because the pool is clearing faster. I guess if you are talking about a swamp, then that might be a different scenario.

 

[JamesW]

Cartridge filters typically have 200 to 400 sqft.

DE does best keeping the water clean vs cleaning dirty water.

As long as you follow TFP methods, DE typically only needs backwashing or take apart cleaning once or twice a year.