Converting backwash specs to tank size?

[EIR]

Looking at the kdf 55 literature. It states 15gpm/sqft for service flow and a 30gpm/sqft for baxwashing from the manufacturer.

How do I take those requirements and convert them to performance of various polyglass tank sizes?

I've seen recommendations for 6-7" diameter tanks to help with backwash requirements for kdf. I do not understand the correlation between the tanks diameter and height vs the flow specs.

Am I looking at the cross sectional area of the cylinder (a circle) (πr^2) or the side wall surface area (cylinder wall)((2πr^2)+h(2πr)) or neither?


Also in more general aspects, what effect does height have on baxkwashing filter tanks?

Mine are heading into my 4' crawlspace and am trying to design a system to meet flow. I might have to run tanks in parallel to meet maximum tank height vs flow demand.

 

[Reach4]

Am I looking at the cross sectional area of the cylinder (a circle) (πr^2) or the side wall surface area (cylinder wall)((2πr^2)+h(2πr)) or neither?

The first one-- cross sectional area.

The cross sectional area is the main thing, as I understand it.

You can dig a pit and set the bottom of the tanks in the pit.

A trap door in the floor above can make access easier.

 

[Sponsor]

 

[Bannerman]

The backwash rate is calculated on the water temperature and on the cross sectional area of the tank, not the height. As you have figured out, the cross sectional area is calculated by πr^2 to determine the gpm rate for the size of tank to be utilized.

Each media has specific backwash requirements which typically take into account the weight of the media. KDF is extremely heavy so the backwash requirements would be excessive and in a larger diameter tank (ie: 10" or 12"), maybe beyond the capabilities of the average well pump or residential municipal supply.

Here's a chart comparing various medias. The backwash rates indicated, are averages, not optimum rates. http://www.purewaterproducts.com/articles/backwash-chart

Standard tank sizes are:
9" X 48" for 1 cuft of media
10" X 54" for 1.5 cuft
12" X 52" for 2 cuft

The service flow rate will take into consideration the quantity of media and bed depth based on the standard tank dimensions for each media quantity.

Sometimes, a larger diameter but shorter tank (non standard) maybe utilized for a smaller quantity but the service flow rate supported may not conform to the usual rate due to the reduced bed depth. The backwash rate would need to be based on the actual tank diameter utilized, not the volume of media. (ie: 1 cuft installed in a 10" X 35" tank)

 

[ditttohead]

math for 8" tank

4x4x3.14/144x25
Pi R squared/a square foor (144)x backwash rate per ft2.

So a 8" diameter tank would have an approximate backwash rate of 9 gpm but this is highly dependent on the water temperature. Colder water requires lower backwash rates due to the increased density.

These rates are also recommendations and have a little leeway. A recommended rate of 30 GPM per ft2 is optimal but higher rates may be desirable for water of poor quality, slightly lower rates if the water is of excellent quality. Going outside of these guidelines does require a little thought and understanding as to how far you can stray away from the MFG recommendations.

 

[EIR]

I am specifically looking at the shorter squater tanks for both the kdf and carbon.

It's a one bathroom house with 2 people (eventually child in the future) with a standard tub shower setup. I can't see needing more than 5gpm max for us.

I understand I can dig a pit and sink them some but I have graded and installed a liner in the crawl already. I have 45-47" depending on the location to the bottom of the floor joists and then an additional 9" to the subfloor in the bays.

If the 35's aren't a real performance hit ID really like to try and utilize them.

 

[ditttohead]

Bed depth is important but... this gets into real wonky technical issues that are not really worth discussing in a forum setting. It is more of a classroom session discussion. Shorter tanks can typically be ok but some tweaking to the freeboards and backwash rates need to be considered.