Some of the filters have pressure drop curves in their manuals but not all of them. If your filter does not have a curve in it's manual I might be able to estimate it based upon the type and size of filter. Also, the type of backwash valve has a big effect. You can find the manuals on Hayward's web site.
If this is an existing pool, you could just use the filter pressure to estimate total return head.
Yes we are.The exisring system is an "only skimmer system", with no main drain.Now that we have not the main drain,we are going to change it to an "over flow system".The surge tank we are going to use will have no main drain connection.The new system's head will consist ofressure loss in clean filter+ permissible pressure drop for the dirt+pressure drop in heater+static head (we will have only a little NPSH head,but nearly 6 feet discharge head)+pressure drop for pipes and fittings.
A system with no main drain is another matter I'm interested in.Please go to this topic and see what I have asked."Correct height of surge tank water...does it matter??? "By the way,they say the perssure drop in a clean sand filter is about 5-10 feet.
I'm still not clear as to why you need to calculate head. You may not really need to know head loss to determine the pump size if that is what you are after. If you are looking specifically for a low loss filter, a large cartridge filter will probably have the lowest head loss out of all three. If you go with sand or DE, the backwash valvecontributes quite a bit unless you can get a low loss version like the Jandy Never Lube. What ever filter you decide to go with, just make sure you get one at least one size larger than what you actually need. On the pump side, go with the smallest pump possible unless you really need the flow rate.
With a 20k gallon pool, I would suggest at least the s240t or any other 300+lb sand filter. The larger the filter and piping you use, the better flow rate you should be able to acheive, not to mention less backwashing
Thank you dmanb2b !
Why not mas985 ? It must be calculated for any pump.For any given flow rate you may find pumps with differenth heads.For example for an 80 gpm flow rate ,there are pumps with 10 to 60 feet head.The existing pump is not working and I'm not sure if it was working properly.We had not a heat exchanger,now we are going to have.In fact,you have better think of it as a new pool.
By the way thank you for your help and attention. The drop in pressure is typically less than 3 psi in a clean filter .(Scot Hunsaker, Filtration Fundamental,Project-specific needs determine commercial poolsâ€™ choice of filtration systems.)
My point is if you follow the simple rule of thumb of a small pump with a large filter, you will have an energy efficient pool with good filtering capability. The operating point of the pump with the plumbing might be of some interest but it really shouldn't be the key metric when determining which pump and filter to get. The more important factors are the size and efficiency of the pump and the size of the filter.
However, if you really want to determine head loss, you have understand that it is a very difficult and time consuming process. You must know the length and diameter of every pipe and a count of every fitting and valve even for the parts underground. Fittings and valves are the largest contributors to head loss and without an accurate count, you have to guess. Also, head loss in plumbing is dependent on flow rates which in turn are dependent on the pump's head curve so you have to solve both the pump head curve equation with the plumbing equation to find the unique operating point.
In my signature, there are some pump tools which allow you to estimate flow rates and energy costs for many different pool pumps. These tools use approximations for the plumbing system which will get you fairly close without having to know all the aspects of your pool plumbing.
Thank you mas985 for your useful discussion.
I tried to calculate the drop for a given pool piping system , about 350 feet long .( taking to account the equivalent length of fittings).For a flow rate of 40 GPM in a 2" pipe the friction loss rate is 6 percent,thus the drop in piping is 21 feet.Comparing to a 7 feet drop in a clean filter,is this 7 feet negligible? Of course I'm understanding now ,that because of relatively similar systems of plumbing and filtration ,the rule of thumb method as you say, is enough.