New here. Quick question on Delta T on main header 100ft from structure?

fastline

We have a LOT of technologies going into a new area but I am going to try to keep this brief just so we can stay on subject. We have a large building going in, and a smaller "mechanical shed" that will house solar batteries, pressure tank, air compressor, etc. This shed will be 100ft from the main building, and we would like to send all heat that we will use in the radiant concrete floor system in the main building, through a pair of 1" insulated pex pipes. Issue is I need to ensure we can get the BTUs transferred with that size. 1.25" pex is way more expensive.

What we are looking at is a common water/water exchanger in the mechanical shed where we will extract heat from a large generator when running, solar waste heat, maybe even oil burner, but that heat needs transferred to the main building. I think right now our target "floor deltas" will be between 15-25F, BUT I am curious if we can use mixing valves so we can push the delta T higher in the headers between the mechanical shed and main building? In trying to plan for expansion, we could probably use 250K btu at full design capacity, though it would be extremely rare! I am trying to tap the 1" pex out at 15gpm due to the head losses, and maybe target a delta of 35F or even more in the header? Because of the 100ft distance in a trench, running the header hot will lose some energy so I know this will be a balancing act. I think running at 140-150F in the header might be a safe a reasonable target.

If someone knows of some reasonable radiant software so I could peck this out for you, that would work too. We are not really "shooting from the hip" here and do work in engineering, but this is sort of a test bed for a few different techs.

EdTheHeaterMan

there are a few things to think about. If this is a closed system, then you will want to use oxygen barrier PEX, and the higher the water temperature in the tubing the less effective the O2 barrier will be. That fact is different with different PEX products. PEX/Alum/PEX might be a better choice. The second thing to look at is the System Syzer Wheel that Gil Carson used when he had to figure this stuff out years ago.




I think there is an app for that now. https://apps.apple.com/us/app/bell-gossett-systemsyzer/id491524114#:~:text=Description,hundred thousand engineers and tradesmen.

Jamie Hall

Can you place your mixing valves and floor circulators in the main building? Then you can run as high a temperature on your supply line from the equipment shed as hot as you like, and accept a greater delta T. There is a practical maximum on PEX -- your really shouldn't go over 180 -- but if you have a supply from the equipment shed at, say 180, that gives you the potential for a 80 degree delta T, mixing that 180 down to whatever you need in the floors. That lowers your flow rate down to about 3 gpm for the 1 inch... which is quite manageable -- or 120,000 BTUh.

mattmia2

1" pex is roughly the same size as .75" copper pipe. If you are using a low temp source like solar and a low temp emitter like radiant, i would not skimp on the volume you can move. The trench will cost far more than the tubing. I would also leave a sleeve that you can pull future pipe through in the trench, especially if you have plans to add on in the future.

EBEBRATT-Ed

155-120 or a 35 degree delta is what you need at 15 gpm.

I don't think 15 gpm will work well on 1" pex. Your going to need some kind of pump to do that

Dave H_2

What you are thinking of is what is called an variable speed injection mixing/piping or minitube injection. The magic is the large delta T across the injection legs of the piping (see pic, the piping in the middle would be 100' of pex)


As with almost any system, you have to "Do the Math".

On page 3 of this instruction manual, it show how to that math.

Enjoy

fastline

Thank you guys for the replies! Other sites were not very active so I appreciate it. When I say, "shoot from the hip", I don't mean we are truly driving blind. Being in engineering, I have the science and math down, but I also realize this has been done for decades and certain things like concrete and soil thermal conductivity is something I can probably lean on from other's experiments.

Regarding 15GPM in 1" pex, I noted that as an extreme limit due to the head loss and turbulent flow. I would think on the main underground boiler loop, I should target the upper limit of laminar. As well, I realize I could "shoot the moon" regarding the boil line temp, but being in the soil, the BTU loss in the trench will be a function of running temp, so I am just trying to balance that.

As far as trenching, I own a very large excavator so the trenching is at the cost of fuel. I already have 7 other conduits going in this trench! 2 electric, 2 domestic water, 1 compressed air, 1 communication, 1 spare. This trench will be stacked out! lol But it will be 4ft wide and this is not my first rodeo so not too worried about all that. One concern I have is hot water pipes in there with domestic water and my electric. I am realizing I will have to certainly insulate the boil pipes and create distance between them and everything else or I could end up with a shot of hot water on cold taps at times. More concerning is a derate on my wiring. Max practical limit I want to see is 75C. Wire is rated to 90C, but it should never be forced to run that hot.

As to heat sources. A large 3P generator will be onsite and run only when 3P loads are to be run, which won't be 24/7. The idea is to create a water/water exchanger so multiple heat sources can be experimentally deployed. The generator would have a coolant temp of 180-210F, which is too hot IMO, but may be needed to achieve proper BTU heat flow for say 150F boiler line temps. What I am trying to achieve is a design where the PEX is protected against overheat.

I should further mention that this building is ALSO getting a 5T geothermal heat pump so heat will not be purely dependent on the floor tubing. The place is also getting a starting solar system of 15kw, with 50kwh of battery storage. It will take some testing/tuning to determine how to best optimize the power sources, but obviously priority will go best use of the solar first. The heat pump will effectively run free. It also has a desuperheater but I have not found an optimal to take advantage of it. It would be nice to have a water/water exchanger in this unit

hot_rod

This is the best tube calculator I have found. The PPI website, Plastic Pipe Institute It allows you to enter fittings, temperature, flow . 10 GPM would be the high side for 1" pex at 5.5fps velocity.

https://www.dropbox.com/s/kfzermfcqgxskoq/Screen Shot 2021-01-27 at 12.46.47 PM.png?dl=0

The Engineering ToolBox site has calculators for other types of pipe.

Zman

To run 15 GPM through 1" pex, you are going to need a circulator that costs ~$1,500.
To run 15 GPM through 1 1/4" pex, the circ will cost ~$500
To run 15 GPM through 1 1/2" pex, you are looking at a ~$300 circ.
The cost to operate the circs probably ranges from $50 to $500 annually.

I think you should reconsider your concerns about the cost of the tubing.
It looks like the 1 1/2" pex will cost you about $600 more than the 1".

mattmia2

Pex is rated something like 180 degrees at 100 psig. It is over 200 degrees at 30 psig, but code in the use doesn't account for that, so I wouldn't worry about temps under boiling occasionally hitting it.

As far as the wire goes, you can get pex jacket wire that is much tougher and more heat resistant than thhn (xheu i think). I would be worried about temp at the terminations, much less so in the middle in a trench. Unless you have continuous loads that are a good fraction of its capacity, it won't heat in any measurable way from resistance anyhow.

EBEBRATT-Ed

I agree. 15gpm is over the top for 1"pex