Buffer tank or no buffer tank

Tinman

Because I don't like asymmetry. : ) I just split the load and it gives me a 20:1 instead of a 10:1. The redundancy gives me a backup too.

Tinman

Cascaded boilers don't have to be the same size. At least, the ones I use don't.

Paul48

Can you explain the doubling of the TDR? Honestly.....It seems like "new math" to me. I don't understand it.

Tinman

Are you playing me?

If you have a total 400k firing rate and only one of the boilers is firing at 20k, 20:1.

Tinman

In effect, that is.

Paul48

No....It was something stupid that was alluding me.

Zman

I like the boilers to be symmetrical for most applications. It allows you to run them with rotating lead/lag without any imbalances.

I did look at a job that had (3)-2 million BTU fin tube boilers. It needed to have the capacity in order to run a big commercial snow melt. The problem was that it had a minimum turndown of 500,000. Whenever the snowmelt was not calling, the small fancoils heating the building would short cycle the boiler to death. Literally a 2 million BTU boiler running a 30 second cycle. It would barely get a stable flame then shut down.

On that job, I thought a small modulating boiler as a fixed lead boiler would have been a better solution.

They decided to leave it the way it is. I get a nice chuckle every time I drive by and see a service truck changing parts.

hot_rod

If the load is 326,000 and the largest size available in that brand or model is 199,000 input, about 163,000 output, what other approach?

I think 199,000 keeps you under ASME requirements maybe. It does in tanks I know.

tekmar had maybe still does a unique staging control for mod cons to keep them in the highest efficiency range and rotate them. Probably the boilers have that cascade in their controls now?

Paul48

hot rod said:

If the load is 326,000 and the largest size available in that brand or model is 199,000 input, about 163,000 output, what other approach?

I think 199,000 keeps you under ASME requirements maybe. It does in tanks I know.

tekmar had maybe still does a unique staging control for mod cons to keep them in the highest efficiency range and rotate them. Probably the boilers have that cascade in their controls now?

Certainly the approach would not be to get a bigger hammer to make the square peg go into the round hole. You use equipment that fits the need. Why all the talk of "staging", "rotating", "cascading" , etc.? Why is any of that necessary? Why not just run your small zones off a smaller boiler? Am I missing something else?

Tinman

Yes. Why buy another boiler that adds nothing to the upside of the equation? A buffer tank is a fraction of the cost of the mod con alone - now add in the near boiler piping, the flue, electric, drain, parts that can and will fail or need maintenance. You've just spent 4-5 times the amount of money you would for a buffer tank and gained nothing.

Tinman

A buffer tank also provides your system with hydraulic separation.

Paul48

@Stephen Minnich
199/199 vs. 285/110
Where did all that come from?

Tinman

Knock-knock?
Really?

Paul48

Sorry.......It's early You could have said the same thing about my question re: TDR. I took that off. Sometimes I forget I'm not face to face with someone.

hot_rod

Stephen Minnich said:

A buffer tank also provides your system with hydraulic separation.

As well as excellent air remove, dirt separation, and add a magnet into the tank for magnetite removal.

Caleffi sells the brass dry well and magnetic rod to add mag separation to tanks or seps.

CMadatMe

I'd say 98% of mod con installed systems need a buffer tank only because boiler manufactures used Fixed Speed Boiler Pumps to keep costs down and to keep the piece of mind that any joe blow can install it with less headaches for them.

EBEBRATT-Ed

For a boiler I shoot for 15-20 min minimum run time if I can.

@hot rod is correct about buffer tanks for chilled water. CW systems usually only use a 10-12 deg Delta on the water so chillers will really short cycle. Carrier recommends a minimum loop volume of 3gpm/ton for comfort cooling and 6gpm/ton for process cooling. Trane and others do the calculation differently but the answer come out about the same.

I have seen several very expensive (like $50,000.00 ) large chillers self destruct on short cycling or inadequate oil return (because of short cycling) because the original designer neglected to account for adequate loop volume.

And it's not just loop volume, it's "active loop volume". The tendency for energy savings is to use all two way control valves and delta P the pump on commercial jobs. But two ways isolate the loop volume so it's not available.

Best bet, sprinkle a few three ways around the system to keep the water active