Friction Head Loss (Ft) Calculation for Closed Loop Systems

 
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lsayre
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Post by lsayre » Sat. Dec. 12, 2015 6:15 am

After all of that there must be an easier way to compute the effective ID for more than one pipe (zone) if all of them are of the same size, right?

Well, there is...

Simplified math method for the same diameter pipe in each zone:
-------------------------------------------------------------------------------------
2 pipes effective ID = the square root of 2 x single pipe ID
3 pipes effective ID = the square root of 3 x single pipe ID
4 pipes effective ID = the square root of 4 x single pipe ID
etc....

Those of you with individual circulators on each zone instead of zone valves have it so easy. Each zone is plotted individually upon its own circulators pump curve chart. You may well find that for more equal GPM of flow in each zone a different circulator will prove best for each zone. This is particularly more true if zones lengths vary greatly, or if you are mixing and matching radiant zones with baseboard zones, or perhaps if you have different pipe diameters for each zone. Using different types of circulators (different pump curves) for each zone eliminates playing with valves in an attempt to equalize things by changing/matching the friction head in each zone.

 
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Post by lsayre » Sat. Dec. 12, 2015 7:58 pm

A more precise way to compute the friction head in your circulator system loop is to install pressure gauges on both the inlet and discharge sides of your circulator.

The method is then to apply this simple formula:
Friction Head (Feet) = (Discharge PSI - Inlet PSI) x 144 /density of fluid (in lbs. per cubic foot)

Here is an example for a pressurized closed loop system and pure water at 180 degrees (weight = 60.57 lbs per cubic ft) with circulator discharge pressure = 15.5 PSI and circulator inlet pressure = 12 PSI.

Friction Head (Ft) = (15.5 PSI - 12 PSI) x 144 / 60.57 lbs.

Friction Head (Ft) = 3.5 x 144 / 60.57

Friction Head (Ft) = 8.32

This friction head if plotted onto the AquaMotion AM5 pump curve would indicate an actual flow of about 4.3 GPM.

 
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Post by Rob R. » Sat. Dec. 12, 2015 8:06 pm

Larry, I applaud your efforts on this but I think it will give the average reader a headache. I suppose if even a few people use the tools, it was worth it...but most will continue to buy whatever is cheapest, or whatever is the biggest that will still bolt up.

I have some old B&G literature on pump sizing somewhere, it would be interesting to see how the it compares to what you have come up with.


 
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Post by lsayre » Sat. Dec. 12, 2015 8:19 pm

I'll be satisfied if even one person finds it useful.

I would never have considered circulators in series to boost GPM flow and save big bucks. You sparked an interest there.

 
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Post by lsayre » Sun. Dec. 13, 2015 11:10 am

Rob R. wrote:...but most will continue to buy whatever is cheapest, or whatever is the biggest that will still bolt up.
Before they do that, there is an even easier way to do this for 3/4" copper. This method will almost assuredly oversize your pump, but it's easy. This is the method I would least recommend. But it is probably the method most often used.

For zones with zone circulators:

Step 1) Measure your zone loops individual total lengths (including through the boiler, from pump outlet to pump inlet).
Step 2) Multiply each zones length in feet times 0.06. This is each zone loops Friction Head
Step 3) Calculate the amount of radiating surface for each loop in BTU's. For 3/4 hot water baseboard this would be feet of baseboard x 450.
Step 4) Divide the BTU's you just calculated for each loop by 10,000. This gives you the GPM's of flow required for each loop.
Step 5) Find a pump curve (or pump curves) that give(s) you the required GPM's of flow at the measured friction head for each individual loop.

Example: Zone loop is 3/4" copper and hot water baseboard. The loop measures 130 feet total. The baseboards on this loop sum to 44 feet.

Step 1) 130 feet of run
Step 2) 130 x 0.06 = 7.8 feet of head
Step 3) 44 feet of baseboard x 450 BTU's per foot = 19,800 BTU's needed for this loop. Lets call it 20,000.
Step 4) 20,000 / 10,000 = 2 GPM.

Now find a circulator with a pump curve that delivers 2 GPM into 7.8 feet of head, and you have found your circulator for that individual zone loop.
Repeat for each zone loop.

The method for zone valves instead of zone circulators is only slightly different. Here you would do the following:

Step 1) Measure all of your baseboard footage across all zones. For hot water baseboards multiply by 450 for total BTU's needed by the house.
Step 2) Divide the above total BTU's by 10,000 to determine the GPM's of flow that your single circulator will require.
Step 3) Measure only your single longest zone for its total length in feet (including through the boiler, from pump outlet to pump inlet).
Step 4) Multiply your single longest zones length in feet x 0.06 This equals your total systems maximum friction head requirement.
Step 5) Find a pump curve that gives you the required total system GPM's of flow at the measured friction head for the single longest zone loop.

Example for 4 zones, single circulator with zone valves, the longest zone of which is 130 feet, and for a home with a total of 140 feet of measured 3/4" hot water baseboards.

Step 1) 140 feet of baseboard x 450 BTU's/Ft. = 63,000 total BTU's required.
Step 2) 63,000 BTU's / 10,000 = 6.3 GPM's of single circulator flow required.
Step 3) 130 feet for the single longest zone loop
Step 4) 0.06 x 130 = 7.8 feet of maximum system friction head to be overcome.
Step 5) Find a pump curve for a circulator that delivers at least 6.3 GPM's of flow into 7.8 feet of head, and you have found your single circulator.

Notice that for zone valves the total number of zones/zone valves does not ever enter into the equations. Neither does the total length of all zone loops summed together.

 
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Post by BunkerdCaddis » Sun. Dec. 13, 2015 9:37 pm

I know I for one will reference and use this information when I need it again. It would be nice to have a page in the knowledge data base with calculators and one for useful formulas, for a quick and easy reference. A while back someone had a formula for estimating the BTU of a stove by the area of the grate, I hunted but never did find it. :cry:


 
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Post by Rob R. » Mon. Dec. 14, 2015 5:23 am

lsayre wrote:I'll be satisfied if even one person finds it useful.

I would never have considered circulators in series to boost GPM flow and save big bucks. You sparked an interest there.
Works well and can be very inexpensive if you get some used 007's. Scottscoaled is the one that told me about it.

 
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Post by lsayre » Sat. Apr. 02, 2016 7:07 am

I've made some significant updates to my friction induced head loss calculator (spreadsheet) for circulators in closed loop systems, and it is to be found right here.

Why is this important? Only "friction" induced head is experienced by a pump in a closed loop system. There is no "height" induced pump head in a closed loop system.
Friction Head Loss Pipe.xls
.XLS | 17.4KB | Friction Head Loss Pipe.xls

 
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Post by Mikeeg02 » Sun. Apr. 03, 2016 9:26 am

lsayre wrote:Why is this important? Only "friction" induced head is experienced by a pump in a closed loop system. There is no "height" induced pump head in a closed loop system.
While I agree with this statement. It may also be useful to include the formula for the required system pressure, required to prevent cavitation when there is a "height" difference in the plumbing (which I cannot remember right now. I calculate it and then tested it too).

I always applaud your work, a usual I just did things the hard way. Researched, calculated, researched, calculated, in the cycle and then averaged my results before constructing my calculated answer which then was applied to pump curves an such and all proved worth it because I stay warm! This forum and the people in it are an invaluable group!

 
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Post by lsayre » Sun. Apr. 03, 2016 11:16 am

Mikeeg02 wrote:
While I agree with this statement. It may also be useful to include the formula for the required system pressure, required to prevent cavitation when there is a "height" difference in the plumbing (which I cannot remember right now. I calculate it and then tested it too).
Since 1 foot of water column generates a static pressure of 2.31 PSI I believe that the system pressure required to avoid cavitation is generally determined (ball-parked) by:

(Total Height of System in Ft) / 2.31 + 5 = Minimum Required (cold) System PSI

For a one story home this would nominally be 8/2.31 +5 = ~8.5 minimum PSI
For a two story home this becomes nominally 16/2.31 +5 = ~12 minimum PSI
For a three story home this becomes nominally 24/2.31 +5 = 15.4 minimum PSI
Etc...

Be sure to match your expansion tanks bladder pressure to your system pressure.

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