BTU capacity per pipe size

wolfie2132

I have seen this question asked many times on with a few answers but not what I am looking for. Does anyone have a link to a chart for what a size of steel pipe can deliver in btus. I need to find out how many btus a 3" steel pipe can deliver with a 40°F delta T. I think my pipe might be undersized on my boiler.

DanHolohan

The maximum (normally used) flow rate for 3" steel pipe is 140 gpm. If you're working with a 20-degree temperature difference between supply and return, that equates to 1,400,000 Btuh.

wolfie2132

Dan, thanks for the follow up! Is there a way to calculate this or a chart? I am looking for a 40°F delta T.

DanHolohan

I'm using a B&G System Syzer. At 40-degree delta T it will be 2,800,000

wolfie2132

Great! Thanks!

DanHolohan

Happy to help.

SWEI

Velocity limits will determine the maximum flow, which together with ΔT will give you the answer.

Three inch schedule 40 steel pipe flowing water at 95 GPM will result in a velocity of 4.1 FPS. That would be roughly 1.9 MM BTU/hr at a 40˚F ΔT (uncorrected for density change due to temperature.)

wolfie2132

Thanks SWEI! Maybe you can help answer this. I have a 3 MM input boiler that is supposed to have a 40°F Delta T with 120 GPM but I am not able to get it. They highest I can get is a 30° DT and I am wondering if the pipe to/from the boiler is too small.

wolfie2132

The draft, gas pressure, manifold pressure, gpm is all where it should be but I am not getting the btus out of the boiler. I am thinking maybe 3" isn't large enough. It's an atmospheric boiler.

bob_46

Two questions , have you clocked the gas meter and how do you know that you are circulating 120 GPM ?

wolfie2132

The gas meter has been clocked and it is putting 3MM btu into the boiler. The circ pumps were measured using DP and are at about 115+ GPM

SWEI

40˚F ΔT @ 120 GPM is roughly 2.4 MM BTU/hr, so 80% of the firing rate, which would be the nameplate rating for a 3MM BTU conventional gas-fired appliance.

Have you clocked the meter to see what the boiler is actually using?

wolfie2132

I have the gas company coming out next week to clock the meter again while the boiler is running. The first time, was by one of the contractors.

Ironman

wolfie2132 said:

Thanks SWEI! Maybe you can help answer this. I have a 3 MM input boiler that is supposed to have a 40°F Delta T with 120 GPM but I am not able to get it. They highest I can get is a 30° DT and I am wondering if the pipe to/from the boiler is too small.

If you're getting a 30* Delta T, then you're moving 25% more gpm than you would with a 40* Delta T. The pipe is not too small. The lower the Delta T, the more gpm. Btu's = (delta T x500) x gpm.

icesailor

Is there a hole in my understanding?

I thought the higher the Delta T at the same flow rate, the higher the amount of emission into the emitters. That if the flow rate was too high, and the Delta T number was lower, that less BTU's were being absorbed and released through the emitters. Isn't that what balancing cocks on circuits do?

If you can throttle the flow into the system, does the delta T go up?

The only thing I understand is if the boiler won't make temperature with the gas flow, the input is too low or the boiler is too small.

hot_rod

Where are you getting the info that the boiler is "supposed" to run at a 40°∆T ? What type of boiler, heat emitters and operating temperature are you running?

Gordy

I agree, there are a few unknowns to come to any conclusions.

Zman

I suspect there is simply not enough emmitor to absorb the btu's you are producing. Is the system underperforming or are you just trying to get the math to work the way you want it?
Thermal equilibrium is kind of a non negotiable thing.

wolfie2132

The system has 3 3MM boilers in one bank and 4 2MM boilers in another. Both connected to the same supply and return headers. Piped primary secondary. These heat a series of air handlers running 100% outside air. 70-75°F outside air. The system was designed to run on a 40°F Delta T. The problem is the 3 3MM boilers cant keep up with the load but 2 of the 2MM can.

icesailor

I'll bet you that if you sneak around carefully, you'll find some hidden queeb in the primary/secondary piping of the boilers.

I was called in on a cold Saturday to a museum that was just modernized with new heat and an addition. They had two large boilers. Each was supposed to be able to heat the building. One boiler had a burner issue that the service contractor couldn't figure out and it went on safety all the time. The high fire linkage was slipping. As long as it ran on the bad linkage boiler, it was OK. The good boiler could not heat the whole building. I fixed what was wrong and both boilers could run, or the one that could and would heat the whole building as designed. I checked out the piping, and found (in my opinion) one of these places where tee's can get too close together and one screws up the flow of the other.

Like the P/S system I found that was piped exact ally as drawn on the plan, with the bridge loops across the primary where the secondary connected and the secondary's were crossed on the bridge loops. So that if the 2" primary was crossing 180 degree water at the bridges, the return from the secondary was first and the bridged water was always cooled by the return being first. Are all the boilers piped together using a parallel reverse return? If not, all kinds of strange things can happen. If two boilers can do what 3 boilers can't, there's a piping issue somewhere. Figure it out and you will get the class star. Or the ones being paid the big bucks will say that you are stupid and don't know what you are talking about. Even if you show them you are right and they are wrong.

Get up for a good learning experience my friend. Water DOES care where it goes.

IMO.

SWEI

Perhaps the OP could post a few photos of the boilers, pumps, and associated piping.

Rich_49

If you are experiencing a 30* Delta in a system that was designed to operate at a 40* Delta than your boilers ARE KEEPING UP WITH the LOAD . If theyw ere not you would experience a higher than 40* Delta . Since you are seeing a 30* Delta they are producing more than the system is capable of transferring at this time . Since the system was designed for a 40* Delta remember that we are probably not at the Outdoor design temp either which could be the simple explanation of what is going on . Want a 40* Delta now ? See what you are flowing and decrease flow by 25% and you'll see your 40* Delta right now . Was the system designed to maintain a 40* Delta or to have a 40* Delta at design ? Bet it's the latter and you are worried about nothing . Besides changing flow rate the only way to effect the fluid Delta is to deliver more BTUs and the only way I know to do that is to increase the Delta between the emitter and reservoir . Not very hard to understand really . As Zman said thermal equilibrium is a kinda non negotiable thing .

Gordy

" These heat a series of air handlers running 100% outside air. 70-75°F outside air."

"The system was designed to run on a 40°F Delta T. The problem is the 3 3MM boilers cant keep up with the load but 2 of the 2MM can."

wolfie2132

To answer some questions. The system is designed to run these boilers at 120 gpm with a 40* delta T which would put out 2,400,000 btu per boiler. The boilers are running at about 115 gpm with a 30* delta T which means they are delivering 1,725,000 btu. I was just curious if the 3" pipe would stop the boiler from delivering the btus. Its a water tube atmospheric boiler.

Ironman

It's not the pipe, it's the boiler.

There's not enough difference between 115 and 120 gpm to really matter. The 10* difference in deltaT is huge - 25%.

It's not too much water, but too little fire being input. In other words, it looks like a fire side problem.

Is 2.4m the input or the output of the boiler? You must calculate from output, not input.

If the pipe were too small, you wouldn't have 115 gpm. Decreasing the flow would not lower the btu content; it would simply raise the delta T, not lower it.

If your numbers are correct, then they are proving there's not enough boiler output no matter what it's rating is.

Harvey Ramer

What is the return water temp to these boilers?

SWEI

I still want to know what the gas meter says it's burning. That plus the relevant CA numbers.

Gordy

bc3510 said:

wolfie2132 said:

wolfie2132 said:

To answer some questions. The system is designed to run these boilers at 120 gpm with a 40* delta T which would put out 2,400,000 btu per boiler. The boilers are running at about 115 gpm with a 30* delta T which means they are delivering 1,725,000 btu. I was just curious if the 3" pipe would stop the boiler from delivering the btus. Its a water tube atmospheric boiler.

It's an interesting situation you have there. While some conclude that it's not a boiler problem and must be an emitter issue and others are certain that the boiler is the culprit, the answer isn't available so quickly.

What must be ascertained is the supply water temperature.

If the system is designed for a SWT of 180F and the boiler is providing only 160F, the emitters cannot possibly deliver the rated output and the delta T will be narrower than design.

If the SWT was raised to the the rating for the emitter, more BTU's would be delivered to the space at the same flow rate and the delta T would climb back toward the 40F design point.

So, the question begs............what is the SWT.............and will the boiler achieve it?

If it will not achieve the SWT, then you've got a boiler problem. It will not make the 2.4M BTU for which it is rated.

Does the boiler ever reach high limit and shutdown? With a 30 degree DT, that must happen if the boiler is firing at its rating.



Of course, there is one other possibility:

The boiler makes the required SWT but the air temperature at the emitters is greater than design and the emitters cannot deliver the rated BTU's. The boiler would absolutely shutdown on high limit in this situation.

Since the OP is worried pipe size is to small I'm willing to bet that he's hitting SWT. Otherwise more flow would not help.

Need more facts.

SWT
RWT
At boiler, and emitters.
Gas meter readings
The 70-75*'outside air supply to AH is key i think.