Pressure Cycling Illustration

jpf321

Below is a chart of the effects of a well adjusted pressure control (cut-out @5oz / cut-in at 3oz (2oz diff)) ..



#1 you will notice that I attain pressure just a few minutes after my normal cycles .. normal cycle ~38m .. pressure built around 41min ..



#2using a "standard" of 33% off time during pressure cycling to arrive at the following ..

    pressure cycling for 63minutes of a 104minute t-stat cycle .. which equates to 21minutes OFF during pressure cycling portion of t-stat cycle .. if my pressure control had been turned UP, I would have burned 21mins more fuel than having it cycle...



   the question becomes, would my overall t-stat cycle have been shorter if there was no cooling for the 21min off time?



this speaks to "what is the optimum pressure cut-out/cut-in"?



<img src="http://content.screencast.com/users/JPF321/folders/Jing/media/94a91aea-f276-4ad5-aec7-c4f7fa82df52/2010-01-11_1017.png" />

lutorm

nice.

What are you using to get the data? That looks fun.

jpf321

Lascar dataloggers

Lutrom -- you can see in this thread here what dataloggers I'm using. http://www.heatinghelp.com/forum-thread/129007/quantifying-savings  .. i know that a few others have also ordered some .. please let me know if you decide to as well. 

Mike Kusiak_2

Cycling

Do you have a graph of radiator temp during the time period of cycling? That would pretty much tell you if the thermostat cycle would have been shorter if no cycling were occurring.



 Would be really interesting to decrease the sampling interval to see what the duty cycle is on pressure. Did you ever time the on-off periods or are you assuming 1.33 for piping and pickup?



Its good that you don't cycle on pressure during normal operation. Your long thermostat cycles look great for efficiency. I believe you lose a lot of efficiency when you rapidly cycle on pressure, not to mention all the additional wear and tear on the burner and boiler.

jpf321

rad chart

i have the rad data for that period...but haven't downloaded it .. will update above when i have it



i assumed 1.33 .. haven't measured pressure cycle timing



as u can see, i'm right on the cusp of pressure cycling (fire timewise) ... i guess tthe only thing that would change that is if i have a change in heat loss factor while making heat. (someone opens a window or holds the door open)

jpf321

pressure cycle timing....

fyi ... i timed my pressure cycles .. looks like it's right about on the money ..i timed a few cycles .. but roughly 45secs OFF 94secs ON .. that OFF includes a few second delay after the vaporstat clicks on .. so 1part OFF and 2parts ON

Mike Kusiak_2

Interesting

So it looks like your boiler is more oversized than you thought.



 According to Lutorm's Law, your duty cycle is about 2:3 or 66%. Taking the reciprocal of 2/3 you get 3/2 or 1.5, so your pickup and piping factor is 1.5 or 50% oversized. Have you thought about slightly downfiring the burner? With your high stack temp, you could afford to bring it down a bit and probably get a little more efficiency? 

jpf321

downfiring ..

sure i've thought about it .. but I don't touchy the fire.



regarding sizing .. the boiler is rated for 700sf of Steam and I only have about 400 installed. i call that 100+% oversized ..

Mike Kusiak_2

By the way,

My loggers just arrived tonight. I bought them from testequipmentdepot.com. Same price as direct from Lascar, but free shipping.



Too bad I can' t confirm any of your steam system results. They will be taking measurements on a hot water system.

FJL

On / Off

but roughly 45secs OFF 94secs ON .. that OFF includes a few second

delay after the vaporstat clicks on .. so 1part OFF and 2parts ON



- - Didn't you or someone else say the ratio should 1 off and 3 parts on?

FJL

My Bad . . .

I guess I misread your other post:  "in theory, that number was supposed to be 1unit off for every 3units .. or off for 33% of the on2on cycle..."



So if my boiler cycles off due to pressure and stays off for 45 seconds, during which time the pressure drops to the cut in setting, once it hits that setting and the boiler ignites, the boiler should take about 90 second to reach the cut out and cycle off.  That would be the ideal ratio between the amount of time the boiler is off  and the amount of time it will run before cycling off again due to pressure.

Mike Kusiak_2

Cycling vs. pickup factor

1 off and 3 on corresponds to the standard pickup and piping factor of 1.33 but his system is slightly more oversized according to his 1 off ,2 on ratio.

FJL

OK

Thanks.  I understand.  If a system is calculated with 1.33 pickup then the ratio should be 1-3?  

Mike Kusiak_2

Yes

Ideal 1.33 pickup and piping factor would translate to, for example a  1 minute off, 3 minutes on cycle. Or to put it another way, for a cycle that repeated every 4 minutes, the boiler would fire for 3 minutes and be off for one minute.

jpf321

wouldn't that be

wouldn't that be 25% of the cycle... 1part of 4



glad to hear you got your loggers, thanks for the free ship site .. i'm just about to post over in the $$$ thread for my enclosure test results...

Mike Kusiak_2

No

If you recall Lutorm's posting, the pickup and piping factor was derived to be the reciprocal of the duty cycle. In this case, 1 minute off 3 minutes on, the duty cycle is 3 out of 4,  75% or 3/4. the reciprocal of 3/4 is 4/3,  which equals 1.33.



Say for example we had a system which cycled at 1 minute on, 1 minute off. Duty cycle would be 1/2. Reciprocal would be 2 which would be the P&P factor, or the system was oversized by a factor of 2.

jpf321

got it ..

i was looking at the OFF time .. i should rather be looking at the ON time .. not 1 part of 4 but rather 3 parts of 4 ..



thanks .. and I do remember that discussion a while back .. but sometimes its hard to search the wall.

FJL

My Numbers

I went into the basement of my building at 6:30 a.m. and stop watched the times of the boiler cycling on and off due to pressure at the every end of the morning during recovering from the evening setback.  By this time, the boiler had been running for one hour.  I live in a four story, four unit apt building.  The first number is the time the boiler is on, the second number is the time between when the vapor stat cut out and then cut in, and the last number is the time it took for the boiler to ignite after the vapor stat had cut in:



2:23 - 0:36 - 0:07

2:28 - 0:38 - 0:07

2:21 - 0:37 - 0:07

2:40 - 0:37 - 0:07



Water in the site glass was clear and stable (not moving at all).

Mike Kusiak_2

Pretty close to "standard"

I did a couple of quick calculations on your numbers, adding the ignition delay to the vaporstat off period, for the total off time. Comes out to about 1.25 or 25% oversized, which is just slightly under the "by the book" piping and pickup factor of 1.33. Seems like the boiler was sized pretty well to the radiation.

FJL

Standard

The boiler was sized to the radiation in the building.  But the reality is that the ground floor resident gets too much heat for his liking and has some of his radiators turned off.  I also removed one radiator from my apartment.  So would it be true that my ratio would be closer to standard if all radiators were open and accepting steam?  Wouldn't more open radiators would extend the run time before the boiler cycles off and, thus, get me a ratio closer to 1.33, which is what I believed we had sized the boiler for?

lutorm

rofl

"According to Lutorm's Law..."



I have a law! :-)

Mike Kusiak_2

Adding more radiation

Adding more radiation would increase the run time relative to the off time, but since the formula for the P&P factor is reciprocal, it would actually indicate a reduction in the P&P factor, or amount of oversize. If you gradually increased the amount of radiation you would eventually reach a point where all the steam would condense in the radiation and the cycling would stop. At that point you would not build pressure and the P&P factor would effectively be one.



Keep in mind that this is a theoretical calculation which does not take all the real world factors into account. For example, we are assuming that all the steam is condensing in the radiators, when in reality some is condensing in the piping. Depending on how much insulation you have, some percentage of the heat is radiated by the piping. This could be as much as about 15% which is what is allocated for piping loss in the P&P factor. The calculations for EDR may also not be exact, and the boiler sizes available may not exactly match the calculated value.



Given these limitations, the fact that the P&P factor determined from the cycling data comes as close as it does to your EDR calculated boiler sizing is pretty encouraging.

FJL

In Theory

OK.  I guess I misunderstood the concept a bit, which is why I asked my question.  Just so I understand.  Increasing the run time reduces the P&P factor, which is the desired result.  I think I understand now.  In other words, ideally, the system would never turn off and would be at theoretical equilibrium, the point at which the pressure is equal to the amount of condenstation in the radiation (which I take the mean the radiators themselves and the piping).

Mike Kusiak_2

Yes

Yes, equilibrium is where you would want to be from an efficiency standpoint, but it might not be a practical, useable system, which is why the 1.33 factor is added. With no P&P factor, you might not be able to fill at the radiators with steam evenly and quickly, and venting would be critical. The system might heat slowly because from a cold start you have to heat all the piping to steam temperature before getting any heat to the rads. So, designers build into the system the 33% oversize factor to take care of these practical details. If you have a well running system that heats quickly and evenly with less than 33% oversizing, you are all the better off.



Take a look a Boilerpro's article in the resources section http://www.heatinghelp.com/article/11/Hot-Tech-Tips/1551/Taking-Another-Look-at-Steam-Boiler-Sizing-Methods-by-Dave-Boilerpro-Bunnell   for a good explanation of why you may not want to, or have to intentionally oversize with the customary factor, yet achieve good results.

FJL

One More Question

I am going to ask one more question about P&P.  I'm a bit confused about the way it has been used in this thread.

I'm familiar with the concept of P&P as a means of sizing a boiler.  You size a boiler based on the P&P.  The standard P&P ratio is 1.33. I understand that to represent the EDR of the radiators plus 1/3 of that EDR.

In your post, when you say that the boiler is "oversized" by 25%, by looking at the ratio between run time, and on/off time, are you saying that the boiler is 25% larger than it needs to be to satisfy the standard P&P of 1.33?  Or are you saying something else?  That the pick-up in such a situation is .25 instead of .33?

I hope that is clear question

Mike Kusiak_2

Confusion

Yes, I can understand your confusion as I have been using the terms "pickup and piping factor" and "oversize"  somewhat interchangeably, which is not quite clear.



In all cases I am referring to a boiler sized exactly to EDR not including P&P,  which can achieve the equilibrium condition where all the generated steam is condensed in the rads without building pressure.  So when I say  "25% oversize" it is in relation to this critically sized boiler,  not including the standard 1.33 P&P factor. A boiler sized including the 1.33 factor times EDR would be by this definition "33% oversized".



I hope I haven't confused you more by this explanation, but basically what I am saying is what you wrote in your last paragraph. If the percentage oversize is 25% as calculated from the on-off data, then the actual measured P&P factor of the system is 1.25 instead of 1.33

FJL

The Ratio

But there are other factors that could determine the ratio between the run time and off time.  My boiler could be "correctly" sized but short cycle because I've set the cut in/out too high.  Doesn't that make it difficult to determine the P&P by looking at the ratio?

Mike Kusiak_2

Not necessarily

It is true that there will be some effect on the ratio depending on the setting of the cut in/cut out, but for reasonable pressures, the results will change by only a few percent.



In fact the actual P&P factor is a function of the pressure, because as the pressure is increased the temperature of the radiators rises and they are therefore capable of emitting more heat. If you design the system for EDR plus 33% P&P, this factor is only valid for the pressure as designed, usually 1 PSI. If the pressure is increased, the radiators operate at a higher temp and are capable of emitting more BTU's . At a higher pressure, the effective P&P will decrease, until at some higher pressure the P&P will approach 1 and the rads will be capable of radiating the full BTU output of the boiler.  This will actually occur at a steam pressure of 11 PSI, where the rads will operate at 243 F and can therefore emit 33% more BTU's than at 1 psi where the original EDR plus P&P was calculated



Take a look at the original thread where this formula for ratio vs. P&P was derived:



http://www.heatinghelp.com/forum-thread/128792/More-boiler-cycling-thoughts



So as you say the result obtained from the ratio calculation will change with pressuretrol setting, but in reality it is only  indicating what the true P&P is at that particular operating condition.

FJL

Thanks

Thanks for your time on this thread.  I'm not sure yet that I fully understand all of the theory, but I will look at that link.  I guess the takeaway from all of this is that the ratio is encouraging and my system seems to be working pretty well and efficient.   I set my v-stat to cut out around 12oz and cut in at 4oz.  I don't have a low pressure gauge installed yet, but the needle on the installed pressure gauge shows it cutting out somewhere between one-half psi and one psi.  

jpf321

i tried this argument

i tried this argument once before .. see the thread Mike posted ... it turns out that it doesn't matter where you have your pressures set .. it's a ratio .. if you have it set for 10psi cut-out and 1psi cut-in .. then the ratio of off to on will still be the same. only much longer than a few seconds.