Media Head Loss During Air Scour & Combo Backwash 2

[riverhound]

Hi, I'm looking at sizing a PD blower for air scour in a gravity media filter with 18" of anthracite on 12" of sand. I have calculated the head loss from friction (pipe, bends, check valve, underdrain, etc.) and the static pressure from the water in the filter during the air scour only mode and combination air scour-low rate backwash modes. However, I have been told the media head loss is negligible for air scour and combo backwash modes. I am having trouble understanding this concept. It seems if the filter is designed for 6' of head loss (forward flow mode) the blower would see this "pressure" upon the filter air isolation valve opening and need to overcome it to properly clean the media. I would think the 6' or roughly 2.5 psi would need to be included in the friction and static water pressure calcs to accurately size the blower. Can anyone help!?! Thanks.

 

[bimr]

If you are using a combined air/water backwash, then the media will be fluidized and the pressure drop across the media will be about the same as the water head in the filter.

The forward flow pressure drop is the pressure drop of water flowing through a packed media bed. This pressure is not comparable to air flow through a fluidized media bed.

Rather than trying to calculate the air backwash pressure requirements, a more practical approach to obtain this information is to contact the filter manufacturer, who will ultimately guarantee the filter performance. Of course, it a good idea to doublecheck to confirm what the manufacturer says.

The blower discharge pressure should be around 15 psi. There is no little reason to spend much time optimizing the blower size as the blower units are small and only operate intermittently.

http://h2oc.com/pdfs/AECIAir.pdf

 

[riverhound]

Thanks for the response bimr...

In my case when backwash procedures start the water level in the filter is taken down to 6" above the media. At this point the air scour only mode starts with air flow equaling 4 cfm/sft of filter. This equals 850 cfm in our case. The blower runs for 5 minutes and then concurrent backwash mode begins. At this time backwash water is slowly introduced into the underdrain at a rate of 5 gpm/sft of filter which is close to 1050 gpm. The concurrent backwash runs until the water level hits the bottom of the troughs then the air is slowly removed from the filter underdrain and the blower shutoff.

I was told by the filter manufacturer that the head loss through the media during air scour only mode (non fluidized bed) is negligible because air is compressible and will find the path of least resistance. I understand this concept but thought there would be some pressure in the air only mode that was needed to "move" the bed so the air could reach the surface and scour. It seems this is not the case and the blower would only need to be sized based on the headloss from the pipe, fittings, underdrain, IMS cap and static water level. Correct?

In concurrent air/backwash mode the bed is also not fluidized yet. I believe the backwash would need to be closer to 8-9 gpm/sft of filter before it becomes fuildized completely. I would think the head loss through the media would be neglible in this case because the backwash water is helping to "move" the particles so the air can easily move through the media. However, head loss charts indicate the concurrent (air water) head loss through the undrdrain and IMS cap is greater in this mode. I was asked recently "if during the concurrent air-water wash when we have 5 gpm/sf going through the unexpanded bed, doesn't the blower have to overcome the backpressure in the water at the point of air introduction, and is not that backpressure greater than just that which would be predicted for water passing through clean media since the media is quite dirty." I would think the pump would only see the "backpressure" from a dirty filter since the air is just diffusing into the backwash water. Any thoughts or suggestions?

Couple of other things that might be useful. The filters in this plant have different depths (top of troughs to top of filter slab) and that is one reason we are looking at a PD blower. They are also relatively shallow...two of the filters are 10.50' deep and the other two are 8.41' deep. This give you an idea of what our static pressures are...the pressure loss in our air pipes is only 0.20 psi. Underdrain is roughly 0.35 psi. We are not anywhere close to 15 psi for total pressure loss in the system.

Thanks for your help and time bimr!!

 

[bimr]

I don't think that there would be a measurable difference in headloss between backwashing a dirty filter and backwashing a clean filter.

A filter bed will only operate successfully if the forward flow is adequate to compact the bed. If you shut off the forward flow, the filter bed will relax and allow particulates to penetrate the filter. If you shut off the filter, then the filter should be run-to-waste for a period of time to allow the filter effluent quality to reestablish.

I mention this because when the water level in the filter is taken down to 6" above the media, the filter would no longer be considered a compacted filter bed.

You made some typo in the 4th line:

"I was asked recently "if during the concurrent air-water wash when we have 5 gpm/sf going through the unexpanded bed, doesn't the blower have to overcome the backpressure in the water at the point of air introduction, and is not that backpressure greater than just that which would be predicted for water passing through clean media since the media is quite dirty." I would think the pump would only see the "backpressure" from a dirty filter since the air is just diffusing into the backwash water. Any thoughts or suggestions?"

The velocity to backwash a filter depends on the particle size and water temperature, but you should be able to expand the filter bed by approximately 10% with a backwash rate of 5 gpm/ft2.

Note that the maximum blower discharge head is only applicable against a closed discharge. The blower operating backpressure is determined by the resistance to flow because the pd blower is positive displacement. If the blower was operating against zero head, then a pd blower would never develop backpressure.

I agree with you that the maximum backpressure will primarily consist of the static water level that the blower will be starting against. If you are starting the pd blower against a water column of 10 feet, then a pd blower is generally necessary to force the water out and you need a pd blower with the capability of 10 feet plus the minor losses.

 

[QualityTime]

It has been a little while since i have designed a water treatment plant. But I will tell you this for a typical water plant:

1. A clean dual media filter will have a approximatley 3 ft headloss. We do a filter backwash when the dual media headloss is 6 ft. This number is different for different filter depths. Basically you do not want the water to go under negative pressure as it is filtering anywhere in the filter bed. Bubbles will start to come out of solution. As stated above by BIMR this has nothing to do any additional resistance your blower or backwash pump will see
2. At a full high wash backwash rate of 24 inch/min to 30 inch/min rise rate the anthracite and top layer of sand will be about 30-35%.
I have an old Hoffman Centrifugal Handbook. There is an example of how to calculate the back pressure on a blower when backwashing a filter. It will become clear to you when you read it. i would add 0.5 psi to your calculations to account for clogged IMS caps. I will try to post the example when i get the chance
3. The headloss across a filter when filtering has nothing to do with the headloss your blower or backwash pump will see. Those are two different things
The maximum static pressure your blower will see will vary. Typically it is NOT at the startup of the blower. The initial static head is the difference between the the lowest point of your air piping compared to the water level in the filter at that particular point in time. Usually the water level in the filter is filtered down to just above the anthracite just befofre a backwash is initiated. During an air scour backwash, the water level in the filter starts to rise from the antrhacite layer level towards the bacwash trough effleunt weirs. The maximum water hieght your blower will see will depend on when you turn off the blower. I am not a fan of allowing a concurrent air and water backwash to overlfow in the trough. You will get media spillover. The best way is to stop the blower when the water level reaches the backwash trough. Remember, the air pipe will be flooded with water at startup. The water in the pipe will then be evacuated by the blower as it is running. You will have air filters upstream of the blower suction. Head loss across the filters must be accounted for.
4. For your imformation I have installed the Leopold air water filter tiles with the IMS cap in a 44 usgmd green grass plant 15 years ago. I used a centrifugal blowers becasue they are much quieter.

 

[QualityTime]

I would also add that Leopold has published tables showing what the headloss is through their tile at different combinations of backwash and air flow rates

 

[QualityTime]

"2. At a full high wash backwash rate of 24 inch/min to 30 inch/min rise rate the anthracite and top layer of sand will be about 30-35%."

that should read


2. At a full high wash backwash rate of 24 inch/min to 30 inch/min rise rate the anthracite and top layer of sand will experience about a 30-35% bed expansion.

 

[QualityTime]

Hi, I haven't got time right now to find the book. I am in the middle of writing a proposal. I do have it though and may be able to post the blower sizing example for filter air scour. Hoffman Blowers published it back in the early 1970's. I still can remember the local Toronto Hoffman represntative giving me a copy for my personal use in the early 1980's. I checked on line at my old alma mater, the University of Toronto engineering library (...5th ranked engineering school in North America...just sayin...;-) ), and the proper information is:

Author: Stephenson, Revis L.
Added author: Nixon, Harold E.,
Title: Centrifugal compressor engineering; a handbook on the selection and application of multistage centrifugal compressors operating in the intermediate pressure range of one to twenty pounds per square inch,
Imprint: New York Hoffman Air & Filtration Division, Clarkson Industries [c1973]


I was also able to find one on line for sale for $22. Great investment...buy it or go to your local university engineering library:

http://www.amazon.com/Centrifugal-compressor-engineering-application-intermediate/dp/B0007F36IG

example

 

[QualityTime]

Hi All:

I found the Hoffman book! Go to the appended link for the sample calculations. There is no loss through the filter media to be accounted for.