Steam boiler sizing discussion

ChrisJ

I wanted to start a discussion about boiler sizing, and see what people can add.

I'm always learning, and there's often many ways to view something and I'd like to hear other thoughts and ideas.

What's the difference from the boiler's perspective of these two systems assuming no differences other than radiator section quantity? How will the behavior of the system change?

All vents are identical, all piping is identical. The only thing different are the number of sections. Keep in mind, the second system has substantially more EDR.

delcrossv

It's strange in the drawings that there is one riser for multiple floors. I usually see individual risers.

ethicalpaul

I'll jump in with what seems an obvious thought: there is no difference until some of the larger radiators start to fill with steam and have more area exposed to steam than the smaller system has.

At that point and thereafter, the larger system will generally have lower pressure than the smaller system.

ChrisJ

delcrossv said:

It's strange in the drawings that there is one riser for multiple floors. I usually see individual risers.

Let's assume they're all separate just for the sake of discussion. We just can't see them because they're behind the front ones, so to speak.

EBEBRATT-Ed

I am dumb. Don't understand what you are looking for.

Are you saying the heat loss of both spaces the same?
are the boilers the same size?

I am assuming all the piping is sized for the radiation connected.

ChrisJ

EBEBRATT-Ed said:

I am dumb. Don't understand what you are looking for.

Are you saying the heat loss of both spaces the same?
are the boilers the same size?

I am assuming all the piping is sized for the radiation connected.

The boilers are identical.
Piping is all identical, let's assume oversized for the smaller radiators, or, at least close enough they end up the same.

Everything is the same, except more sections and the piping is big enough to feed them.

From the boiler's perspective, what's different overall? How will the system behave when starting up cold, warm etc.

The heatloss is the same. Ambient temperatures are identical in both scenarios.

EBEBRATT-Ed

Since the boilers are the same size I will assume the first one is sized for the radiation and the second boiler is undersized for the radiation.

The second boiler will take longer to heat the radiation but will use the pic up factor to get the job done . I am assuming the extra radiation does not exceed the pic up load.

To my way of thinking the standard 1.33 pic upload is a little excessive but it depends on the job. Commercial jobs with very long runs may need the whole 1.33

I doubt that most houses do. Also the colder the outdoor temp the more the boiler runs so the piping and radiation have less time to cool between cycles and less pic up is needed.

Jamie Hall

Trouble is, steam systems are dynamic. So are we talking about steady state? During warm up?

I'll make a further assumption. All the vents are exactly the same. I'll also assume -- with an ear tuned for further comments -- that the system is a standard atmospheric system.

Now. The steady state condition is the easiest to work with, obviously. In that condition the boiler will be capable of producing a certain number of pounds of steam per hour. There is a certain total area of condensing surface, which is fixed, and therefore assuming further that the temperatures of the spaces in which the radiation is located is also fixed or rearly so, Then there are three possibilities: first, that that the boiler power is less than the condensing power of the radiation; that the boiler power is equal to the condensing power of the radiation; or that the boiler power is in excess of the radiation. In the unlikely condition that the boiler power is equal to the radiation demand, the boiler will fire indefinitely. There will be very small (although for the more precision minded measurable) pressure differences across the system, which will drive the flow of steam. Now if the boiler power is greater than the radiation demand, the same of very nearly the same pressure differences will exist but, because more steam is being produced than is being condensed, the overall pressure will rise; how fast depends on the imbalance of power. If, however, the boiler power is less, while the delta pressures will be similar, not all the radiation will be condensing at its capacity. What will then happen is that small differences in pressure across the system will tend to make some radiation condense less than others. Each radiator will condense just enough so that the total pressure difference from that radiator to the boiler is the same as any other radiator.

Keep in mind that these pressure differences are very small -- a few inches of water column pressure at most -- and while it would be amusing to speculate on what they will be, there are too many variables too poorly measured to do that. Note too that some radiators may have enough steam that they fill enough to close their vents. That will raise the pressure in that radiator to increase a tiny amount -- enough so that that radiator will see a slightly reduced flow (and hence reduced pressure drop in its feed lines) to satisfy the above pressure equality.

The dynamic start up condition is somewhat similar, in that each radiator will condense as much steam as it can obtain. Again, the volume of steam for each radiation will be controlled by the condition that the total pressure drop to the radiator is equal for all radiators.

Now you can influence which radiators respond more rapidly by altering the venting capacities. Basically, what you are doing is causing small pressure differences from radiator to radiator by controlling the resistance to air moving out of the radiator.

ChrisJ

@Jamie Hall Yes, normal single pipe atmospheric.

And I didn't name the dry return on the picture, it came that way so I don't want any "heat" over it.

trivetman

Theres a few factors which I think are important.  

1) is boiler sized to the large or small set of rads.
2) where is the thermostat.
3) does the small rad system provide enough heat output to keep rooms at temp.  

To keep this from spiraling out of control, I’ll assume the small rads are big enough and large are oversized for the house.

Under the above stated assumption its sized to the large and oversized to the small,  the system heats up in both cases.  It may cycle on pressure on the small rad system but does heat the house evenly.  In the large system,  the thermostat cuts the boiler when a room comes to temp.  It satisfies the thermostat quicker than the small rads and is probably more likely to overshoot the thermostat setting.

reverse the assumption and size it to the small rads, so undersized for the big,  that big radiator in the top corner far from the house doesn’t get its fair share of steam.  Too much steam is going to the oversized radiator in the bottom right.  So wheres the thermostat?   If its in the bottom right,  that top left room stays cold.  If the stat is in the far room at the top left,  the bottom right is a sauna.

ChrisJ

Ok now,
Let's assume we use a timer of some sort that will shut the boiler off when only 4 sections of the radiators are heated, approximately. Let's say the boiler runs for roughly 20 minutes after steam hits the main vents.

What will the behavior differences be between the two different systems?

trivetman

I’m think then it goes to @Jamie Hall  ‘s monologue of which rads fills fastest based on pressure differentials.  If the pressure of individual rads could be adjusted to be equal in each system,  theres no difference in behavior.  The four sections just heat up evenly across all radiators and then the boiler shuts off

ChrisJ

trivetman said:

I’m think then it goes to @Jamie Hall  ‘s monologue of which rads fills fastest based on pressure differentials.  If the pressure of individual rads could be adjusted to be equal in each system,  theres no difference in behavior.  The four sections just heat up evenly across all radiators and then the boiler shuts off

Why would they need to be adjusted?
What would cause any difference in this situation where all radiators are made up of the same sections, just different quantities?

trivetman

Per Jamies original post…too many variables to measure the pressure differentials from one radiator to the next.  This makes sense to me.  Something as simple as the additional friction from steam traveling in a long riser vs a short one might be one of those variables.

theoretically, venting could be adjusted such that the pressure differentials are eliminated.  In practice,  its probably impossible to fully eliminate them and make all radiators heat at exactly the same rate on each system.

but if you could eliminate these small pressure differentials,  i think the extra sections on the ‘oversized’ system are just there and don’t impact the system behavior….at least for your ‘heat the first four sections’ scenario 

ChrisJ

I think one of the things the above example may show is that the quantity of radiators and differences in piping could effect a system's behavior far more than the actual EDR of it.

For example if a system has 20 small radiators it could need a bigger boiler than one that has more EDR but only 8 radiators. At least when it comes to concerns about radiators being starved from others hogging all the steam.

Thoughts?

delcrossv

ChrisJ said:

I’m think then it goes to @Jamie Hall  ‘s monologue of which rads fills fastest based on pressure differentials.  If the pressure of individual rads could be adjusted to be equal in each system,  theres no difference in behavior.  The four sections just heat up evenly across all radiators and then the boiler shuts off

Why would they need to be adjusted? What would cause any difference in this situation where all radiators are made up of the same sections, just different quantities?

Distance from the boiler. Pressure drop needs to be equalized between the boiler and the rad vent. Then you'd get identical fill, given sufficient time, regardless of the number of sections. 

guzzinerd

I'll chime in since I just added a couple larger rads to my old system that was missing some originals.

Four of the original 28 cast convectors on my 2pipe steam system has been thrown out and replaced with small single tubed baseboard units.  All of them located in three living rooms (6-unit apts).  

Everything heated up to thermostat temp after the boiler cycled a few times (at 1.5psi).  Problem is that the living rooms with the low edr baseboards were still cold.

Last week I swapped out two of the baseboards in opposite ends of the building with the old cast rads I found recently.  The boiler now runs around 15min until it's cut off by the thermostat, no pressure, and the cold living rooms are now really warm.  There are still a couple of baseboard units installed but it feels really balanced now.

I haven't had a chance to time the boiler to temp from cold due to the recent cold spell (highs in the low 30s)  but will do as soon as it warms up during the day.

HVACNUT

@ChrisJ, I read your comment in another post about a heat loss calculation when sizing a steam boiler. I'm not experienced in steam design, but like most was taught to measure the rads and connected load. But I always wondered how the Dead Men sized them when they started from scratch. And there was the period of some flu or virus where the boiler could heat fine in the middle of winter with the windows open. And pardon me if that's blasphemous because it's probably all in one of @DanHolohan's books.

KC_Jones

I don't think people appreciate how small the differences are. They are so tiny as to have basically zero impact on system performance.

The difference between an 8 section radiator heating 4 sections and a 12 section radiator heating 4 sections is effectively zero, there will be no difference. Remember we are talking about the air moving out of the way not the steam. The air in the longer radiator only needs to go through the bottom connections to get to the vent. By the numbers, on my system, with my boiler (106,000 BTU gross output) 1 psi will move the steam through 3000’of 2” pipe, so to move the air through the bottom of the radiator ~1' more from one radiator to the other will equate to .00033” WC. I would suggest the vent is offering significantly more resistance than that, given that the hole in the vent is significantly smaller than the opening across the bottom of the radiator.

So, do people really think that .00033" of WC is making a significant difference? That's the argument being made here. For me, it isn't even about if you can measure it, we know we can't and, it doesn't matter based on the theoretical. Theoretical is where we always start, to see if it's worth real world testing, in this case real world testing seems pointless. The statement about a few inches of water column are wildly off and conjecture.

In both scenarios above the vent is the primary restriction so the losses across the bottom of the rad aren’t making a difference, for that reason, both systems above will perform the same.

Below is the formula I used to calculate.

dp = 0.0001306 q^2 l (1 + 3.6/d) / (3600 ρ d^5) (2)

where

dp = pressure drop (psi)

q = steam flow rate (lb/hr) 106

l = length of pipe (ft) 3000'

d = pipe inside diameter (inches) 2"

ρ = steam density (lb/ft3) .037316 @14.7 psi which is atmospheric at sea level

dp=0.0001306*106^2*3000*(1+3.6/2)/(3600*.037316*2^5)
dp=0.0001306*11236*3000*(1+3.6/2)/(3600*.037316*32)
dp=1.4674216*3000*(1+3.6/2)/(3600*.037316*32)
dp=4402.26*(1+3.6/2)/(3600*.037316*32)
dp=4402.26*(2.8)/(3600*.037316*32)
dp=4402.26/(3600*.037316*32)
dp=4402.26/4298.8
dp= 1.024068psi

I fully expect people to not believe this and scoff at what I posted, to that I say, show me the numbers that refute what I posted. Anything else is conjecture.

*edit: I had my steam density wrong, pipe is shorter, but my assessment doesn’t change*

ChrisJ

HVACNUT said:

@ChrisJ, I read your comment in another post about a heat loss calculation when sizing a steam boiler. I'm not experienced in steam design, but like most was taught to measure the rads and connected load. But I always wondered how the Dead Men sized them when they started from scratch. And there was the period of some flu or virus where the boiler could heat fine in the middle of winter with the windows open. And pardon me if that's blasphemous because it's probably all in one of @DanHolohan's books.

Yes I did mention heat loss but I feel it should be considered not that it should be the deciding factor.

The difference in the deadmen is they were sizing coal boilers which are a completely different animal.  I believe they were all grossly oversized for several reasons.

But it's also something I've brought up here several times.   I find it hard to believe anytime you wanted heat with steam and a coal or wood fired boiler that they had to light the biggest fire they could in that boiler or it resulted in uneven heat.  That doesn't seem realistic.

Id think you built a fire based on how much heat you wanted at that specific time.  But the general belief here is that a small or even medium sized fire would cause some rooms to not heat at all and others to overheat.

If a coal / wood fired steam heating system did that I don't think anyone would've bought one.

ethicalpaul

All my experiments and observations have taught me that pressure drop is not a thing in residential steam.

the pressure is low, the flow is low, and the pipes are big enough that there is no pressure drop within a residential steam system.

the pressure drop occurs at the vent orifice, that’s it