Pressure sinewave in a steam heating system.

EhCanadian

How does a pressure sinewave in a steam heating system help in its function? (in particular re air venting). I know why the sinewave happens, and how a pressuretrol works. I would like to know WHY and HOW changing a system like this to constant steam pressure, may have disadvantages. Please provide a "Dan Holohan" type of answer, NOT a "Why would you do that?", "It would work the same", or a "Don't change anything." type of answer.

KC_Jones

The simplest answer I can think of (if I am understanding the question) is that you are burning fuel to do nothing.

Steam needs to move to heat the building. If you are maintaining a constant pressure in a heating only system, you are burning fuel to maintain the pressure, but not heat the building. This is especially true if the building isn't calling for heat, such as a mild day in spring or fall, but anytime the building isn't requiring heat (thermostat not calling) this would be true.

The lowest pressure possible that heats the building is the most efficient. And the required pressure is typically much lower than most people realize.

Jamie Hall

and see my answer on the other thread you started...

mattmia2

I think you are asking why the vaporstat or other pressure control cycles the burner on and off resulting in a varying pressure.

You mean a sine wave with a period of maybe tens of minutes.

If that is the question, the answer is essentially that conventional steam boilers aren't capable of modulating very much even with controls that would allow it, they would not draft properly so they would not burn cleanly and would clog up with soot and would spill products of combustion and possible actual flame in to the building.

Chris_L

Sinewave?

Here is what the pressure in one of my boilers looks like (single pipe steam):

The pressure does run pretty constant--until all the radiator vents close at which point it rises fairly quickly unit the pressure limit is met.

I am using a microcontroller to shut it off when the pressure reaches 10 inches of water, and keep it off for 14 minutes thereafter. But it would look much the same if the Pressuretrol controlled it--except that the pressure would rise much higher, and the off time would be much less.

PMJ

Chris_L said:

Sinewave?

Here is what the pressure in one of my boilers looks like (single pipe steam):

The pressure does run pretty constant--until all the radiator vents close at which point it rises fairly quickly unit the pressure limit is met.

I am using a microcontroller to shut it off when the pressure reaches 10 inches of water, and keep it off for 14 minutes thereafter. But it would look much the same if the Pressuretrol controlled it--except that the pressure would rise much higher, and the off time would be much less.

Great example. Thanks for sharing.

EhCanadian

Simply put the pressure builds the pressuretrol trips the boiler to stop firing the pressure drops the pressuretol signals to start firing and this repeats. This is what I meant by sine wave. I also am not looking for a "that's just how a pressuretol works.", I know how they work. I'm looking for a why cycle?

mattmia2

It isn't practical to manufacture a boiler that exactly matches the edr of the system so the control keeps the boiler from running away. Without it, the boiling would get out of control and ovrpresusrize the system and eventually boil the boiler dry. You also need to heat the water hotter to increase the pressure which decreases the efficiency of transfer of heat from the combustion products.

It would be possible to manufacture a modulating boiler that keeps a near constant system pressure that matches the rate of condensation of the steam but the size of the market and cost savings vs the cost of control and construction complexity wouldn't make it practical in residential applications.

KC_Jones

The answer is simple, you never want it to cycle on pressure. If it does there is a problem in the system. Either the boiler is too big, or the venting is inadequate or some other situation that is not allowing the system to take on the amount of steam being produced. The reality is many people seem to accept this situation as a function of steam and many of us can clearly show it is not.

The Pressuretrol or Vaporstat are supposed to be safety controls not operating controls. Personally I've never seen my vaporstat trip and mine is set to 6 ounces. I think I witnessed 5 ounces of pressure one time, but I had too much water treatment and was surging and foaming like crazy.

ChrisJ

EhCanadian said:

Simply put the pressure builds the pressuretrol trips the boiler to stop firing the pressure drops the pressuretol signals to start firing and this repeats. This is what I meant by sine wave. I also am not looking for a "that's just how a pressuretol works.", I know how they work. I'm looking for a why cycle?

Either poor venting so steam cannot get to the radiation fast enough to condense or the boiler is grossly oversized.

EhCanadian

I guess what I'm really kind of looking for is an answer along the lines of... If the system is a Vacuum Vapor steam system, when the pressure drops below atmospheric pressure, eventually enough air will leak into the system or gasses build up to block the steam flow and stop the movement of steam through the piping. So that the pressure has to cycle above atmospheric pressure every so often, to push the air or gasses that have formed or leaked into the system back out of the system, so the system can function again properly in its vacuum state. Or more such answers, along that line.

Jamie Hall

But it anything but a sine wave -- which is what confused me. Normal operation, well explained above. The influence of your microcontroller is plain. I do not see evidence in your graph in the longer first run (recovering from a setback?) that you are cycling on the pressure control, however; that would normally be a much higher frequency, but you are masking that with the fixed time hold off.

In a steam system (well, any system!) it is necessary that there be a pressure differential between the source of the working fluid (steam, in this case, and the source is the boiler) and the sink (the radiators, in this case) or the fluid won't move. The pressure does have to be above atmospheric to get the air and steam to move -- and deliver heat to the radiators. The system cannot operate in a vacuum unless you have a vacuum pump attached to all the vent locations; it will drop -- momentarily -- into a vacuum (as you observe) as the steam collapses when the boiler stops. This drop is rapid, again as you observe. If the system were well sealed, and all the vents were of the vacuum type, then it might preserve the vacuum for varying lengths of time; in a perfectly sealed system you would be at an equilibrium between the vapour pressure of the water in the boiler -- at whatever temperature it was -- and the vapour in the system pipes. In such a system, when the boiler was not firing, there would still be a small transfer of energy in the form of low temperature vapour from the boiler water to the radiators, assuming that the radiators were cooler than the boiler water.

The pressure at which the system cuts off is set by your microcontroller. Evidently you have determined in some other way that 10 ounces is optimal. I expect, studying your graph, that you could lower that to 5 ounces with some fuel savings and no loss of performance. You state that with the pressuretrol it would rise higher -- which is true, but only because the pressuretrol can't be set below around 1.5 psi. Vapourstats can be set much lower.

The question of whether it is preferable to use pressure or time as the modulation control for a steam boiler is, to put it, mildly, vexed, and has led to some rather heated (and inconclusive) debates. In that regard I would only mention that a vapourstat can be regarded as either a modulation control device (as I do) or a safety device -- but not both. If it is used as a modulation control device, then a pressuretrol should also be present as the safety device.

Might I ask what outside variable was different in the first run shown (about 20 minutes) vs. the others (perhaps 5 minutes, at most, each)?

Don't know if this helps much.

EhCanadian

Yes, I guess mathematically "sine" wave is the wrong term to use. I am speaking of the pressure cycle. Many boilers can and do run at a set pressure, but many cycle the pressure. I usually find there is a historic reason things like this happen that has been long forgotten, but originally had a purpose.

Jamie Hall

Please refer to my comments on your other thread. I can't keep two going at once.

There is an historic reason (as well as some sloppy engineering). Originally steam systems were all coal fired and the boiler firing rate could be -- and would be -- varied over quite a wide range, from almost nothing to really going along. On bigger installations, this was done by whoever stoked the boiler keeping an eye on the pressure gauge and varying the firing rate and damper settings to hold a reasonably constant pressure. Sometimes. Sometimes not so much, but that's a side story. On more sophisticated systems, there were arrangements of pressure sensitive devices (often variations on manometers) which controlled, directly, the draught on the fire, which in turn controlled the energy output of the boiler (provided someone put in some coal...). These worked splendidly well -- pressure could be held constant within an ounce or two for days on end (many such systems also had a mechanical thermostat, which would bias the draught). There were at least three problems. First, coal is messy. Second, it had to be stoked -- and while mechanical stokers worked well, they weren't easily adaptable to feedback control. Third, the efficiency on part fire was somewhere between poor and truly horrible. Along came oil firing. Much simpler, much cleaner, very desirable. Except... most oil burners, to this day, are either on at full song, or off (they don't have to be, but the wizardry to vary the firing rate while maintaining efficiency is neither simple or inexpensive, though this might change with the use of an array of digital sensors and servos). But full song was, usually, more than the radiators could manage. So... to allow the radiators to catch up, various pressure switches (Honeywell Pressuretrols and Vapourstats were by no means the only games in town) were used to sense the pressure rise when the radiators fell behind and turn the burner off. Much the same is true for gas fired boilers. Pressure, however, is not the only control strategy which can be used to match boiler output, averaged over time, to radiator capacity, and in the case or a grossly oversized boiler may not be the best.

EhCanadian

I know old systems using coal were often made to go into a vacuum so the the water could boil at 150f (70c), recovering as much heat as possible from the coal as it burned down to ash. For these systems cycling the pressure was designed in and required, and not cycling would cause the system to fail for reason described in one of my posts above. After coal went out of style, system were still designed to cycle (fire on to a set pressure reached, fire off until a set pressure drop) , but now they generally operate above atmospheric pressure. Other than the economics of boiler control costs, are there any components of any "above atmospheric pressure" systems that require this pressure cycle to function correctly?

Jamie Hall

Actually, if you study many of the vapour systems -- coal or otherwise -- you will find that they were open to the atmosphere -- no vent closures at all. And most coal systems did not run the pressure up and down, or at least no more than could be helped with hand firing; if they had automatic dampers, the pressure hardly varied at all.

There are no components that I know of which require a pressure variation to function properly.

EhCanadian

Thanks, for all the replies. The fact is all vapour vacuum systems absolutely did need a pressure variation to function. These systems can not, could not, would not function without this cycling. Also, the ball bearing (not cork) air vent(s) that some of these systems had, can only operate with a pressure cycle and can not operate with a constant pressure. I'm kind of hoping to hear from someone that already understands this and can grasp why, who could possibly provide a little more insight, beyond this.

Jamie Hall

Oh? How odd. Well, I guess you already know far more about vapour vacuum and related systems than I do. I wish you the best of luck in finding your answers.

gfrbrookline

@EhCanadian , your posts are very condescending to both the professionals and owners of steam systems. If you do not like how this forum works perhaps you would be better served finding a local engineer to answer your question.

EhCanadian

That was condescending. I am searching for information and trying very tactfully to reply to misinformation, while at the same time listening. I have liked all replays, but not heard from anyone yet that can provide new information. I tried to make it clear in my original post I'm not looking for replays from people do not understand the subject matter. These forums need to be for all, if this isn't a forum where individuals on the autism spectrum, or other, can ask questions and engage in dialog, Please let me know. Could we please keep personal attacks to a minimum. I do want answers and I will challenge misinformation, Please don't be offended.

ChrisJ

> @EhCanadian said:
> That was condescending. I am searching for information and trying very tactfully to reply to misinformation, while at the same time listening. I have liked all replays, but not heard from anyone yet that can provide new information. I tried to make it clear in my original post I'm not looking for replays from people do not understand the subject matter. These forums need to be for all, if this isn't a forum where individuals on the autism spectrum, or other, can ask questions and engage in dialog, Please let me know. Could we please keep personal attacks to a minimum. I do want answers and I will challenge misinformation, Please don't be offended.

What kind of steam system are you running?

EhCanadian

Chris J, 200,000,000+ btu. Mix of high pressure, low pressure., negative pressure, with equipment and buildings an piping ranging from about 100 years old to current year, with a mix of Coal, Natural Gas, Oil, Ammonia, Water, Steam, and "Other" equipment. For several years I, and others, have been trying to improve issue from this system that has been piece mailed together for a century. I realized that some of the oldest building may have been poorly modified at some or several times in the past. Just looking for a better understanding of some of these very old systems, to help trouble shoot or improve.

Jamie Hall

Well, me lad, you have managed to insult most of us, so I think you had best look elsewhere.

EhCanadian

Jamie, Thank-you. You did provide the most information on this question, and seemed to have a well rounded source of knowledge and understanding. I now understand how this forum works now quite clearly. I cannot challenge or question misinformation, ask hard questions, or seek difficult information. Long standing members can tell you to leave the forum, correct you, call you insulting, call you condescending, at the same time refer to you as Lad. I will try my best to follow these rules on this forum in the future.

Hap_Hazzard

There's a right way and a wrong way to challenge or question perceived misinformation. I can recall at least one time in the past week that Jamie challenged something I'd said, and he did it very humbly and politely, and it turned out he was right.

Coming from a theoretical science and engineering background, I often find statements by experienced professionals that are at odds with my understanding. If I can't reconcile the two positions, I usually ask why they don't jibe, and I usually learn something. So far I don't think I've made any enemies here, but I think people do understand what my background and perspectives are, and they know when to take me with a grain of salt, if not the whole shaker.

I think the diversity of experience represented here is what makes this forum so valuable, and as long as we respect each other and give each other the benefit of doubt, we're all the better for it.

EhCanadian

Thank-you Hh.
I did find most of the information I was seeking, by watching more Dan Holoham videos online. I will get a hold of a book or two of his to dig a little deeper.

nicholas bonham-carter

When these systems were first installed using a coal boiler, the steam was on a more or less constant pressure, as long as the coal fire was burning. No fluctuations were needed for the system to provide heat. The air in the system was driven out at the beginning of winter, and did not return until the fire went out in spring.
With the transition to gas or oil burners, there was no constant burn, as with coal, and the main vents became necessary for evacuating the air in one And two pipe systems, in addition to the pressuretrol. Because the new burners were “on or off” devices, there were naturally fluctuations in pressure, which occur as a result of the nature of the burner operation. These pressure cycles are only a result of the inability of the burner to modulate way down to a simmer, as happened before with a coal fire, and are not necessary for the delivery of steam to the radiators.—NBC

dopey27177

In my day (retired 16 yeas) we did not have the equipment to do a sign wave) I had to make a graph manually to see the system and boiler operation.

Here is what learned in small steam heating systems on off operation of a boiler (by vaporstat or pressure controller) was necessary due to financial reasons.

When getting into larger boilers where oil or gas burners could operate in a high fire and low fire modes constant steam pressure could be maintained. What has to be realized is in large one pipe steam systems a drop back pressure is needed so the air vent valves on the radiators could cycle.

The type of vent valve used on the radiators is generally used to set the high end pressure and of coarse the drop back pressure.

Example:

Hoffman # 40 vent valve
maximum operating pressure 10 psi
drop away pressure is 6 psi

what this means is the steam pressure needs to drop below 6 psi. for the vent valve to operate properly.

Jake

I am sending you chapter five from my book Steam the perfect fluid for heating and some of the problems. Available at Dan's
library, Barnes and Noble, Amazon and Dorrance Publishing in Pittsburgh Pa. www.dorrance

EhCanadian

I came across this You tube...  "GSMT - Dan Holoham, June 4th, 2019" 1:55 min to 7:15min more particularly 4:00min to 5:00min.

EhCanadian

Thank-you dopey27177 that was a lot of what I was seeking.

motoguy128

I don’t know, my “undersized boiler works just fine”. It can’t build any pressure so there no sine wave. It runs at <0.1oz.

nicholas bonham-carter

If you have TRV's on a one pipe system, then you may need some short-cycling, but not otherwise.--NBC