Understanding temperature dependence in pressure drop calculations - PEX vs copper

rossn · August 4

I'm doing some pressure drop calculations for some supply plumbing (pex vs copper) as there is low city supply pressure and over 100' to travel through many fittings.

For PEX, Uponor has tables for pressure drop (based on flow rate and tubing size), which are temperature dependent.

For Copper, I've used copper.org's manual for calculating pressure drop. However, I'm not seeing a temperature component included in their calcs.

Can someone clarify why copper.org would not have temperature as a variable in their equation? Or, if you know what temperature their equations are based on, that would also be helpful.

Many thanks!

Jamie Hall · August 4

Well now. Could you post a reference to a table which shows pressure drop based on flow rate and tubing size which shows it as varying with fluid temperature? I can't find such a critter, and I'd honestly be exceedingly surprised, as pressure drop is dependent on velocity and pipe roughness and only very weakly on viscosity, which in turn is only very weakly dependent on temperature… or, in other words, I can see no reason for the pressure drop to be dependent on temperature.

So — a link to the reference, please?

rossn · August 4

Sure, Jamie - Uonor's PDAM, friction loss tables in the appendix. See. p. 94 (at least in my older version). The friction loss differences are significant… maybe 50% higher at 180F than 40F.

They also outline dynamic velocity changes for water at temperatures on p. 89.

I'd have to dig into some fluid texts to refresh my memory, but believe there is a factor for fluid temperature. I can only think that copper.org assumes a temperature (presumably low) that would conservatively maximize losses?

Jamie Hall · August 4

Thanks. Never bothered with that… mostly, I suspect, as one tends to assume a relatively low temperature and not worry.

DCContrarian · August 4

Could you post a link to the Copper.org manual? I'll look it over and see if there's anything in it about temperature.

rossn · August 4

https://forum.heatinghelp.com/discussion/comment/1807429#Comment_1807429

Sure - a second set of eyes never hurts. Here's copper.org's Copper Tube Handbook

DCContrarian · August 5

So this is interesting, from the Copper.org manual:

"To avoid excessive system noise and the possibility of erosion-corrosion, the designer should not exceed flow velocities of 8 feet per second for cold water and 5 feet per second in hot water up to approximately140°F. In systems where water temperatures routinely exceed 140°F, lower flow velocities such as 2 to 3 feet per second should not be exceeded. "

hot_rod · August 5

depending on the mineral content, hotter water tends to be more corrosive, and higher flow velocities contribute. I think that is why the CDA and some plumbing codes have the velocity limits.

Same issue with constant,y recirculated, hot, chlorinated water in Pex. The listing number on the Pex tells the time allowed fore DHW recirculation. Although some Pex brands with the highest rating number still had failures.

Also over about 5 fps in copper it tends to get noisy, velocity noise.

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rossn · August 6

https://forum.heatinghelp.com/discussion/comment/1807433#Comment_1807433

Hi Bob - while your comment wasn't directly related my original question, you've peaked my interest. Hope I can pick your brain on this, relating to a challenge I have with my DHW distribution.

Would you anticipate issues using Pex with an on-demand style, higher velocity, recirc pump (ACT D'Mand), especially if the pump is placed at the far end (i.e. hot water is not returned down the return line)?
Is the 5fps being noisy specific to DHW, or will cold water traveling through Copper, between 5-8 GPM, also noisy?
I noticed the below comment from Uponor's PDAM, indicating DHW recirc return lines should only see 2fps. However, in my case I would not generally be sending hot water down the return line - so I'm not clear if that applies to me? I suppose I can call Uponor on that one.
Given your knowledge of the Pex recirc issue, what do you think I should do as it relates to tubing selection (3/4" copper vs 1" Pex)?

Background:

I have a situation where I have re-plumbed my main baths/kitchen DHW with home runs via ProPex. Those are fine. However, I have another (guest) 3/4 bath, a garage utility sink, and an above garage apartment (3/4 bath, small kitchen, clothing washer) that are quite far away (58', 78', and 93'-114' , respectively), by the way of the pipe, from the indirect… pretty contorted run. Hot water serving the area calculates as 6 WSFU's/10.6 GPM per IRC, but the guest bath will be used intermittently, and the apartment may not be rented all the time. I will also mention I have water pressure challenges… at the entrance to the home (280' from the meter), it's around 51psi, and the main line to the house needs an upgrade from the 1" copper. The pressure at the furthest (shower) fixture is likely to be in the mid-30 PSI range, when based on some practical-use flow rates for the home (less than the IRC calc). Water hardness: I don't have the numbers here, but the city test classified it as medium hardness.

My original plan: A few years ago I purchased a ACT D'Mand S3-103 recirculation pump. If you are familiar with the D'Mand product, you'll know it only circulates when either motion is sensed or when a button is pressed. At that point, it recirculates at a higher velocity than most other recirc pumps to deploy hot water. My plan, considering energy efficiency, was to locate the pump near that middle location (near garage slop sink). Originally I ran some 3/4 Copper for both supply and return lines, and then I came to know that the velocities and capacity of the copper was likely shy (and over-sized on the return line). I then came to the conclusion that a 1" Pex DHW supply line with a 3/4" return would be a more fitting setup. The copper has not yet been replaced.

From Uponor's PDAM manual for AquaPex:

Pump performance (3 settings)… likely I would not have head to account for, if placed in the garage. To get some head, I'd have to located it under a sink, in the apartment, and then get a recirc pipe to the same location.

Plumbing concept diagram of the east end of the home, with DHW indirect being the cylinder shown. This might be difficult to make much sense of.

hot_rod · August 10

Generally you will not hear excessive velocity noise in pex tube, only in metallic tube. And copper tube seems to transmit the noise beyond where the noise is generated.

A good example is an outdoor faucet running high gpm, you will hear the velocity noise throughout the home, with copper tubing.

With pex, the flow velocity, temperature, chlorine and time interval are all part of it.

Here is what Rehau states.

The pex failures in California were related to all these conditions lining up. High temperature, continuous recirculation, chlorine content and excessive velocity, from what the tube manufacturers claims. It seems some brands quietly reduced that flow maximum to 2 fps, I recall it being higher in earlier manuals? :)

With a demand system I don't see that short period of high velocity being an issue? But not knowing how long?? If it falls within these parameters.

Not all brands of pex have this rating, by the way. It seems the A pex are better for DHW recirc tolerance.

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Understanding temperature dependence in pressure drop calculations - PEX vs copper

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