More boiler cycling thoughts

lutorm

Hi again,



It just got cold here and I've been watching my boiler pressure cycle when heating up from setback these past couple of days. You may remember I was wondering about what sort of cycling was normal in a post a couple of days ago. Then I had this realization:



<strong>The % of time the boiler is off when pressure cycling must equal (1 divided by) the pickup factor.</strong>



Do more knowledgeable people agree with me here? Here's the reasoning: When all the vents are closed and the piping is heated (assuming pipes are insulated, of course) then all the heat the boiler puts out into steam must go into condensation in the radiators. So if the boiler was exactly sized to the radiator load, it would run forever and never get the pressure up. If it runs 2/3 of the time and is off 1/3 the time (like mine) then the power output of the boiler is 3/2 of the power output from the radiators, because in the end all power produced by the boiler goes to the radiators. (Neglecting heat loss through insulation, etc, here of course.)That only fixes the percentage of the cycles, of course. The actual length of the cycles in minutes depend on how long it takes the boiler to get the pressure up, and that depends on the volume of the system.



Or is this all bs? ;-)

jpf321

At first glance

At first glance this looks correct if you have perfect pressure settings .. however, I think, that it will depend on your pressure settings .. if someone has a cut-out of lets exaggerate .. 2000psi, chances are that your boiler will also run forever .. and if you have a cut in of 10psi .. then you maybe off for a long time .. with your magic ratio cover such an extreme example .. I'm not sure ..



in my mind .. in an ideal situation .. the pressure controls are infinitely accurate and responsive...in which case, as soon as you sense "meaningful" pressure build .. maybe 2oz .. shut it down .. then once you sense anything less that than, start it up. the ideal world would exactly match the condensation and only condensation pressure lost .. water condenses in everything that isn't providing it more heat. condensation alone will rob pressure. therefore, you want to find the perfect pressure for condensation loss .. unfortunately, the tools which we have to sense and affect changes in pressure are pretty crude and rudimentary when we are dealing with the fine line of pressure lost due to condensation. every inch of anything that isn't supply heat is condensing steam (rates vary of course - rads are designed to condense fast, insulation is designed to condense slow)..



i'm starting to see why a certain length of main + riser run will require a certain amount of minimum (measured at the boiler) pressure, b/c pressure/steam is robbed along the way (again rates vary based on current temps and materials and heat-loss of the surface the steam encounter) .. i now have my cut-out at about 5oz (mainly b/c my v-stat is dreadfully bad below there) .. and my cut-in is as low a differential as possible .. hoping to attain the -- keep the pressure exactly right "there" at all times.



someone here once mentioned a lower stage burn in order to infinitely maintain a certain perfect pressure .. unfortunately, i think there are very few of us (steam users) that have the ability to drop to a lower burn.



so after my long drabble .. do i think you are on the money .. maybe .. but it maybe because you happened to be perfectly dialed in with your CI/CO pressures....



as always, i welcome corrections are criticisms

Mike Kusiak_2

Makes sense to me

Your analysis makes a lot of sense, but I think your resulting formula has an error.

If you substitute ON time for OFF time it seems to work out.



ON duty cycle = Percentage burner ON during cycling on pressure, divided by 100



ON duty cycle = 1 divided by pickup factor , or therefore



pickup factor = 1 divided by ON duty cycle



For example, let us take the case where when cycling on pressure, the burner runs for 3 minutes and stays off for 1 minute. On percentage is 3/4 or 75%.



pickup factor = 1/.75 = 4/3 = 1.33



So, if the burner runs 3 minutes out of the 4 minute total cycle, then the boiler is oversized by 33%, which equates to a 1.33 pickup factor.



As you point out, the absolute length of the cycle is not important, only the ratio of the burn time to the total cycle time.

lutorm

Maybe, maybe not.

I'm sure that everyone will howl here, but I'll say it: ;-)



Higher pressure does not, under the assumptions stated, lower the efficiency!



If the boiler has to run to raise pressure, that just puts more steam into the system, but there is no pressure work being done (we're assuming no leaking vents here, remember) so all that energy that goes into building up pressure will be available when the steam condenses. The cycles will be longer, but the duty cycle shouldn't change. Higher pressure does increase steam temp, which will increase heat losses through insulation, too, but we're ignoring that for now.



In fact, you should be able to measure the heat loss by closing all the radiator vents. The boiler duty cycle will now be set by only the heat loss through the piping. So, if you get a duty cycle of f1 with the radiator valves open and f2 with the radiator valves closed, then the efficiency of your system will be (f1-f2)/f1. This won't include burner efficiency, of course, or heat loss through radiator vents, but it should include heat loss through the main vents if they don't seal perfectly.



I just attempted to try this, but some of my radiator valves don't shut anymore...

lutorm

I agree

Yeah, you're right. I wasn't sure exactly how "pickup factor" was defined, but what you say is what I meant.

lutorm

Insulation

If someone's about to insulate their piping it would be interesting to try this before and after. Unfortunately I just insulated by system, and as interesting as I think this experiment would be, I'm not going to tear it all off... ;-)

jpf321

i think ...

i think the reason that low pressure is normally suggested is:

1) steam moves fastest at lower pressure

2) vents operate better at lower pressure

3) the latent heat curve at higher pressures is less steep than the initial one .. in fact I think it actually dips there at the beginning .. I know I saw a PDF of this (temperature/pressure/btu table) .. but I can never find them as quickly as Rod can ..



I can try closing all my rads .. but it will have to wait until morning when my kids aren't asleep. There is no argument that higher pressure steam has more latent heat, but I think the ratio of BTU to pressure is less attractive than at the beginning of the curve.

Mike Kusiak_2

Duty cycle vs. pressure

I agree that in itself higher pressure does not decrease efficiency, but the duty cycle will change.



Assume that the system is in equilibrium at a given pressure and duty cycle. If you now raise the pressuretrol setting, the pressure and therefore the condensing temperature will rise. The radiators will now reach a higher temperature at the new equilibrium pressure. At a higher temperature, the radiators will be able to emit more BTUs, so the energy balance changes and the duty cycle increases. If you increase the pressure high enough, the radiators will reach a temperature where they can emit all the heat generated by the boiler and the burner will run continuously at 100% duty cycle.

Big-Al_2

If you raise the pressure high enough

you will also raise the stack temperature and thus reduce the amount of heat available to make steam.  You will lose some combustion efficiency at higher steam pressure.

Steamhead

Uhhh, Lutorm

yes it does.



The reason is that as you build pressure you are compressing the steam. This is fine if you're doing work such as running a locomotive. But if all you're doing is filling pipes and radiators with steam, you want the steam to be as uncompressed as possible. This way you don't have to generate as much steam, so the system heats faster, the burner runs less and the efficiency of the entire system is improved.

Mark N

Bull-headed T

Steamhead



Lutorm has a bull-headed T in his supply piping. Would this causes his pressure to be high? I known it can cause problems with the water line.



Mark

Mark N

pick-up

The pick-up factor is the energy needed make steam and heat the pipes.  It is 1.33 times the net EDR.  As soon as the boiler shuts off the steam in the system collapses causing vacuum to form.  This causes the pressure to drop also the vents reopen letting in the atmoshere.  I would think you would want it happen a quick as possible so the boiler can fire to continue heating.  You should try to get your system to cycle on the tstat not the pressuretrol.  We went over this the other day in 1-pipe steam when the vents are closed the air in the rad expands because the vacuum formed by the condensing steam but because the vent is closed no steam can go to the rad because you can't vent the air.  What size boiler do you have and how many sq ft of rads.  An oversized boiler can cause high pressure.  It makes more steam than the rads can condense.



Mark

jpf321

latent heat graph....

Well I didn't find exactly what I was looking for in my post above .. but I did find a great chart which illustrates the latent heat of water from ICE to STEAM at different pressures...read the description to the right of the chart to fully understand it.

lutorm

Good point

You are right. Though I would think this is a small effect (for reasonable pressures, of course.)

lutorm

This thinking has nothing to do with my own system...

I should have pointed out (I thought I did) that I'm only speculating from what I know about thermodynamics here, I'm not saying anything specific about my own system.



In fact, I'm sure there are many "real life" imperfections that make my idea very oversimplified.

lutorm

Maybe...

I'm sure you're right that lower pressure is better. But I'm still not understanding where the energy goes. Building pressure should not cost energy beyond what you get back when you condense the increased amount of steam.



But the point about higher pressures meaning higher temperature equaling lower boiler efficiciency I believe. And I also believe that the mechanics are happier at lower pressure.



If you haven't figured out yet, I'm a physicist... But I work with nothing nearly connected to thermodynamics, so I'm just trying to understand what the basic underlying principles are. Things like breaking the vents aren't included in my world view... ;-)

lutorm

Well...

But my point is that since (most of) the pickup factor drops out after everything is heated and the radiators filled with steam, every boiler is oversized in relation to just the radiator load. So it seems that on design day, when the system is running full blast just to keep up, it must by definition cycle on pressure, not the thermostat, since the boiler can produce more heat than the radiators can radiate.

lutorm

Aha.

Wikipedia agrees with you:



http://en.wikipedia.org/wiki/Enthalpy_of_vaporization



That graph seems to indicate what you're saying. I didn't know that.

Mark N

Pick-Up

The pick-up doesn't really drop out, it is used up. The pick-up is equal to 25% of your boilers DOE heating capacity. The first 15min of BTU's you use are your pick-up. Hopefully you've got steam to all your rads in that time. I'll agree on the design day you would probably cycle on pressure. I have no idea for what day my system was designed for. Since I've lived in my house we've had days that are below zero and the system still cycles on the tstat. It just runs more often. I looked at your pictures on the web maybe next you should tackle that bull-headed T.



Mark

Steamhead

The only work that is done

in a steam heating system is that which moves the steam from the boiler to the radiators. Since the pipes are sized for a very low pressure drop, you only need a few ounces to do this. So there is no reason to build more pressure than that.



The latent heat- 970 BTUs per pound of steam- is what actually heats the building. Steam pressure has little or no effect on this.