Pressure vs. Load

Med618

I'm still having a real problem understanding the whole load vs. pressure concept no matter how many times I read the materials here and in the Art of Steam Heating.



What is actually occurring in the boiler/burner which creates pressure and/or lowers pressure? When you raise the pressure, what is it that you're instructing the equipment to do? I understand pressure to be the compression of steam so what compresses it and why does it burn more fuel?



Also I notice in some of the posts that raising the pressure WILL get some of the radiators hotter. Why is this? Is it sending up steam for a longer period?



Any help you can give would be much appreciated.

Rod

Pressure

Hi - With regard to your questions: Ques- "I notice in some of the posts that raising the pressure WILL get some of the radiators hotter."  Ans-  This is due to bad venting . Trapped air in the radiator doesn't allow the steam to enter the radiator. Using more pressure compresses the air in the radiator which allows steam to partially enter the radiator and condense.

Since building pressure uses fuel, it would be far better to get  the air out of the steam's way than to use excess pressure to "muscle" steam into the radiator.

Keep in mind that the Empire State Building operates its steam system at a pressure of under 3 PSI so just because you have a steam system in a multi story build there is still no need for excess pressure

- Rod

icesailor

Pressures:

I'm not quite sure what you are asking, but maybe it is this.

Water boils at 212 degrees at sea level. In Denver Colorado, it is around 194 degrees. Because atmospheric pressure is lower. In Death Valley, CA, because it is below sea level, water boils at a higher point. Therefore, if a boiler is a closed system, and there is zero pressure, the water will boil at 212 degrees. If you raise the pressure to 12# PSI Gauge, it will boil at 244 degrees +/-. At zero pressure, the water turns to steam at 212 degrees. The steam vapor takes the place of 1200 cups of water, or, one cup of steam becomes 1200 cups of steam. Raising the system pressure will make the steam hotter but it will want to condense back to water at every opportunity. It takes a lot of extra energy to do this. If the system reverts to a vacuum, the water will boil at the lower temperature. The steam vapor will be below 212 degrees.

Another factoid about boiling water is this. When humans are trying to climb Mt. Everest, the atmospheric pressure is so low, and the boiling point of water becomes so low, that the water in food boils off before it can convert some starches into usable proteins that the body needs for nourishment.

As far as heating loads, perhaps what you are asking about "loads" is the ability of the boiler to generate enough steam into the system, to make pressure to heat the building. You need to fill a finite space with steam vapor to heat the building. If the boiler isn't big enough to make enough steam, the system can't be filled.

If you have ever used a pressure cooker to cook food, it is like if you turn the burner too low. It won't make pressure. Turn it up and the cap starts to wobble and maintain pressure. You are raising the temperature of the water in the pot, over 212 degrees. That's enough to cook the food more thoroughly and get it above 212 degrees. You can't get it as hot on a stove with a lid on it. You always hear stories about a pressure cooker blowing the contents on the ceiling. When you drop the pressure with the regulator and pressure inside the pot, the liquid water in the pot will immediately flash to steam when the lid is removed.

I hope that makes sense and helps.

MrDvorak

My try

Here is how I see it. Water + heat -> steam. The more heat is supplied (fuel burnt), the more water gets converted to steam. As the volume of the saturated steam in the system is almost constant at water boiling temperature (steam occupies space in tank above water level, pipes and radiators), more steam in the confined space can only lead to its higher density and pressure.

Med618

Still Confused

If I'm understanding correctly, pressure is created by more and more steam occupying the same fixed space.  So this is where I'm still confused.  When you say "raise the pressure" is that the same as saying "increase the amount of steam in the system"?  And if this is correct, then HOW is more steam created?  Is it simply a function of the burner running for longer and is this the reason more fuel is burned/wasted?



What about when the boiler is on a pre-set cycle depending on the outdoor temperature reading.  Is it possible for the cycle to end before the system has created enough pressure?  Not that you need that much pressure...

Dave in QCA

OK... I'll try to explain too.

This is interesting.  It is something that most of us working with steam understand, but when it comes right down to it, it is difficult to explain.



What makes steam pressure?  Burning fuel.  Period.

If you had a boiler that had not radiators and no piping, and all of the connections to the boiler were plugged or capped, and the boiler was filled up to the proper water line with water.   If you had such a boiler and you turn the burner on it was heat up the water and as soon as the water began to boil it would begin to build pressure... very quickly in fact.  If you turn off the burner, the boiling will stop and the pressure will begin to fall.



You could create this same scenario with a pressure cooker on the stove...   The fire provides the heat, which boilers the water... water turning into steam creates the pressure.



Now, confusion also exists with the pressure trol.  It does not make the pressure come up, but rather allows it.  The pressuretrol's job is to shut the burner off IF the pressure rises to the set point on the pressuretrol.  That is to say, to make sure that the pressure never gets any higher than 2 PSI for example, no matter how long the thermostate in your living room is calling for heat.   It is just like a high limit control and gas forced air furnace... which shuts the gas off if the temperature inside the furnace gets to hot.

Rod

Steam Pressure

Hi- I was just typing out an answer to you when Dave's reply came up and since I see he's given you a far better answer than I have, I erased my reply but wanted to include some links that maybe of interest to you.

Pressure - http://www.heatinghelp.com/article-categories/348/Pressure\



Tekmar 279 -    http://tekmarcontrols.com/hvacsystems/279.html

A boiler control that uses out door temperature to control the heat cycles. Read the I& O manual. It  is VERY interesting!



Boilerpro's  Article

http://www.heatinghelp.com/article/323/Boilers/1551/Taking-Another-Look-at-Steam-Boiler-Sizing-Methods-by-Dave-Boilerpro-Bunnell

A very interesting article written by Boilerpro , a very experienced Chicago Steam pro, who is on cutting edge residential steam technology.

- Rod

Wendell

Dave has got it strait,

but for backward minded thinkers like me, this might help...



Imagine it like this. Your boiler is capable of making a fixed amount of steam over a fixed amount of time. This can't be changed.



If your boiler is attached to nothing and is allowed to run unrestricted, there will be no pressure created (visualize a garden hose flowing water without a sprinkler attached).



When you add a restriction to the system, like a radiator, pressure will start to build. If the radiator is large enough to condense the expanded steam back into water at the same rate that the boiler is making steam, then there will be no pressure.



If more steam is being created than can be condensed, then pressure will start to build. Hence the need for a pressure switch to shut the boiler off.



When people mention "raising" or "lowering" the steam amount/pressure, they are simply referring to adjusting the switch. Some systems never trip (my converted vapor arrangement is set to 1/2 pound and never interrupts itself).



As mentioned in an earlier post, the higher the pressure, the higher the boiling point.



It's the increase in boiling temperature that costs you money. If you can make steam at 212, why would you want to add a couple of pounds and boil your steam at 220?



-Wendell



P.S.

To the pros, I made this all up as I went along… feel free to shoot down as needed.

Med618

Hmmm

Maybe as a layman I'm going to have to actually sit down with an expert and hammer this out because I'm still confused. I think what's throwing me off is the whole cycle concept.



In reading the tekmar documenation and based upon our own experience in our buildings(we use HeatTimer), the boiler cycles at varying lengths depending on the outdoor temperature. Colder days obvious have a longer cycle. I take this to mean that the burner is on for a longer period on cold days.



If pressure builds quickly, and if the pressuretrol shuts off the burner as soon as it hits lets say 2 PSI(pounds as my supers call it), won't that shut down the cycle prematurely?



Or am I mixing up the pressure cycle with the heating cycle?

Rod

Cut in and Cut out

When the pressuretrol reaches the cutoff setting (say 2 PSI) it will cut off the burner.

When the steam pressure drops to the cut in setting (say 1/2 PSI) the burner will go on again. This cycle will continue until the temperature setting on the thermostat is satisfied. The time period of the "cycle on time" and the "cycle off time" depends on how quickly the steam is condensing in the system.

Ideally the burner would be modulated so that after it establishes steam in the system, it would then cut back to maintain just enough heat to just balance the demand for steam in the system during that time period. Unfortunately they don't have modulated burners for small residential steam systems as yet. (Believe me we are all waiting for this to happen !!!)

As boiler/burner size rise, there are 2 stage and modulated burners available at the commercial level.

- Rod

MrDvorak

Negligent

Keep in my that it is the latent heat that heats your home, not the temperature differential. That is where most heat from burnt fuel goes. The difference in boiling temperatures is negligent in the greater scheme of things. It costs much more energy to make steam from 212F water than to get water to 212F. The extra mass of steam needed to reach the higher pressure is what costs more $$$.

Jamie Hall

Heating cycles vs. pressure cycles

That can be confusing.  And, Med618, you are not alone with the confusion...



Your heat timer -- common for larger building with multiple thermostats and users -- is used to control how long the heating system has steam available in it.  In a smaller installation, or a simpler one, the thermostat does the same thing.  That length of time, in turn, controls how long the radiators are or can be hot, and thus how much heat each one puts into the space.



The pressuretrol, on the other hand, controls how long the burner is on, and thus the average rate at which steam is being produced by the boiler.  No matter what you do, you cannot use more steam than the radiation in the building will condense -- any extra heat after that just raises the pressure, and is a waste of fuel (and money).  So what the pressuretrol does is sense when the pressure is starting to rise, and shuts the burner off until the radiation condenses the steam and the pressure falls again.  Then it turns the burner back on and makes a little more steam... and so on, until the heat timer or thermostat is satisfied.

As Rod said, it would be nice if there were modulating burners for boilers in smaller sizes -- but it is surprisingly difficult to design a boiler and burner combination which will modulate and still hold its efficiency, so they haven't happened yet.  If there were, the pressure sensor (it would have to be a different type) would just throttle the burner so that the heat in matched the steam requirement.  An analogy which might help a little would be driving a car -- suppose that you had only full throttle and off, instead of an accelerator pedal.  Then you would run the engine until you got up to some speed, shut it off and let your speed drop, turn it back on and bring your speed up again, and so on.  That's what we have with the pressuretrol...

crash2009

To me,

 a cycle is not only the length of time that the burner is on, but also the length of time the burner is off.  These two lengths of time make one cycle.



I usually time my cycles before anyone gets up, this removes the human factor from the equation.  The human factor includes such things as showers, dishes, doors, windows, and maybe even body heat.



For example: This morning it was 28 F outside with 0 mph wind.  The boiler came on at 6:31 am.  It ran for 16 minutes.  Then it shut off and stayed off for 65 minutes.  So todays cycle was 16 + 65 = 81 minutes long.

A comparison:  Friday it was 39 F with 12 mph north wind.  The boiler came on at 3:42 am.  It ran for 15 minutes.  Then it shut off and stayed off for 49 minutes.  Fridays cycle was 15 + 49 = 64 minutes long.

In both examples, their individual patterns will repeat themselves until something changes.  Just opening the front door and closing it right away might add a minute or two to your burn time, or subtract a minute or two from your off time.  I don't have any experience with a heat-timer or a teckmar.  The observations above are while using a plain old round Honeywell, with the anticipator set to 1.0



 

I have other cycles recorded from last winter.  I was stuck on 30 40 for the longest time.  Then we did a few improvements and started getting 20 50.  I enjoy turning the tables on the gas company.  There were a few cycles involving pressure.  One before and one after installing a vaporstat.  The outside temperature was 0 F and -15 F

Anyway thats the kind of cycle that I record each morning while I read the morning internet, and drink my coffee.

Dave in QCA

Cycles

MED618 question:     

In reading the tekmar documenation and based upon our own experience in our buildings(we use HeatTimer), the boiler cycles at varying lengths depending on the outdoor temperature. Colder days obvious have a longer cycle. I take this to mean that the burner is on for a longer period on cold days.



Reply: 

Generally, Heat Timer or Tekmar use a fixed cycle length.  The cycle consists of "on" time and "off" time.  If the cycle is set for one hour, a 10% heat cycle would have the boiler in the "on" status for 6 minutes and "off" for 54 minutes.  The "on" time does not start until after steam reaches the end of the main.  The establishing steam condition which begins when the boiler first starts to fire is not part of the cycle.  Yes, the burner will be on for longer periods of time on colder days.





MED618 question:

If pressure builds quickly, and if the pressuretrol shuts off the burner as soon as it hits lets say 2 PSI(pounds as my supers call it), won't that shut down the cycle prematurely?



Reply:

During the boiler "on" time, if the pressuretrol shuts the burner down because the steam pressure has gotten too high, it will not shut the cycle down prematurely.  The heat timer or Tekmar control, or any other thermostat for that matter, does not know, and really doesn't care what the pressuretrol is doing.  It doesn't make any difference.  The Tekmar is basically saying that the steam needs to be up for a certain percentage of the cycle. Once the steam is up and pressure begins to slowly rise, if the pressuretrol shuts the burner down on high pressure, the steam is still up.  The boiler is still boiling and making steam, even with the burner off, but it is cooling and the rate that it is making steam is slowing down very quickly.   As soon as the pressure drops to the "cut-in" point, the burner will come back on, but the steam has been up the whole time and the system has been heating.  In some conditions, the pressuretrol will cycle the burner off and on serveral times.  What is happening is the controller is calling for steam, the boiler comes on and the pressure rises.  Once the pressure gets to the "cut-out" on the pressuretrol setting the burner will shut off.  The burner will cycle of and the pressure will fluctuate between the "cut-in" pressure and the "cut-out" pressure setting on the pressuretrol.  This cycling on and off by the pressuretrol is all part of the "heat on" cycle.   When the heat controller "call for steam" ceases, the burner shuts off, the pressure drops to 0, the boiler stops making steam, and the entire system begins to cool down.





MED618 question:

Or am I mixing up the pressure cycle with the heating cycle?



Reply:

Yes.

Wendell

Ah...

so going from say 200 degrees to 212 degrees is relatively cheap, it's the going from 212 to steam that's costly.



Similar to the effect of evaporating water feeling cold on your skin... going from a liquid to a vapor takes energy with it?

MrDvorak

Yes

Exactly. Take a look at this table: http://www.engineeringtoolbox.com/water-thermal-properties-d_162.html



To heat 1kg of water from 0C/32F (extreme case) to 100C/212F takes 100*~4.2 -> 420kJ

To turn 1kg of 100C/212F hot water to 1kg of steam takes 2,270 kJ



The increase the temp. from 212F to 220F takes extra ~4*4.1 -> 16.4 kJ



To increase the pressure of steam by 1PSI, you have to convert about 9% more water to steam with about 200kJ for 1kg of water. Again, this energy does not get lost but it may lead to inefficiencies, extra wear, premature departure of parts etc.



Let me add one more note: this is just a mental exercise, not an assumption about absolute energy loss figures. In reality, the water does not get all converted to steam, only a fraction of its total volume does, but at the same time the conversion happens continuously when fuel is burnt and the longer steam gets produced, the longer water gets returned as a condensate that needs to be re-heated again. Heating excessive volumes of water just to reach boiling temperature certainly results in greater overall losses, as every owner of an oversized boiler knows.

Rod

The "Steam Train"

Here's an article by Dan on latent heat which explains it well.

http://www.heatinghelp.com/article/321/Steam-Heating-Basics/128/A-Steam-Heating-Primer

Dan calls it the "Steam Train" as it is steam that carries the heat from the burner and delivers it to the radiator.

- Rod