Calculation of Extra BTUs needed to increase steam psi

bob_50

Christian! bob

Steve Garson

Calculation of Extra BTUs needed to increase steam psi

This is a question for a real number cruncher:

We all know that lower steam pressure saves fuel.

Does anyone have the engineering knowledge to run the numbers to calculate how many additional BTUs of energy it takes to raise the steam pressure from 0.5lbs to 1.0 lbs to 1.5 lbs? Let's assume a boiler with 12 gallons water content, 1.1 GPM nozzle and 85% effiency.

Or maybe a relative percentage increase in BTUs.

I'll bet that this is something that everyone who find of interest!

PeterK

Heat calculation

Go here (on line steam tables)
http://www.efunda.com/materials/water/steamtable_sat.cfm

Enter the pressure that you want. Record the Enthalpy (Btu/lb). Do the same for the next pressure. The difference is the heat needed per pound. Multiply this by the water mass (lb) and that is the total heat needed. The time it takes is a function of the burner output (Btu/hr).

michael_15

I think

That the Burnham heating helper (which is a great thumbnail resource) has this sort of information already presented in a table. Google for "htghelper".

-Michael

Steve Garson

Calculation of Extra BTUs needed to increase steam psi

Thanks for pointing me to the formulas. Based on these calculations, it looks like running at 1.5 psi only uses an additional 1.4% fuel versus running at 0.5 psi, unless I am missing something. If I use 1000 gallons today with a pressuretrol at 1.5 psi, a vaporstat at 0.5 psi would save 14 gallons of oil, which wouldn't pay for the device. The calculations use 100 lbs water or 12 gallons.
BTU/LB 1.1GPM
PSI Enthalpy Heat Needed 1-Hr Oil Run-Time % Saved
0.5 1096.2 109,576 154000 0.712 1%
1 1105.8 110,536 154000 0.718 0.5%
1.5 1111.7 111,126 154000 0.722 0%

Does this logic make sense?

Steve

PeterK

calculations

I can't make sense of your numbers. You aren't really saving money by doing this. The calculations just show that higher pressure steam has more available heat (enthalpy) than lower pressure steam. The heat still goes into the space, which is what we want. I think if you run at alower pressure you will cycle more often. It's still a matter of oil-in heat-out. The steam is just a means to transfer the heat.

bob_50

Yo Steve

the steam tables are in PSIA. You gotta convert your boiler gauge pressure to absolute. Add 14.696 to your gage pressure. bob

Bob Morrison_3

Energy savings potential

Operating at a lower distribution pressure will reduce standby losses, because the distribution temperature will be reduced. However, if the losses benefit the space, then the savings is questionable or does not exist.

There is a notion that the boiler "has to work harder" to raise the steam pressure higher, so it must be using (wasting) more energy. As Cliff pointed out, the energy content per lb of steam is higher, but other than the standby loss potential, there's no impact on fuel to steam effiency per se.

Bob Morrison

michael_15

right. It's all in the chimney.

Ultimately, what goes up the chimney is the only lost heat.

-Michael

Fred Harwood

fuel savings and steam pressure

High steam pressure often indicates poor venting, which in turn means that the air in the system slows steam arrival at the radiator, which then requires the burner to run longer than it otherwise would with faster venting. In sum, better venting lowers pressure because steam more quickly begins to condense in the radiators and satisfy the thermostat. Fuel savings can be significant when the burner easily and quickly expands low-pressure steam into the radiators rather than wasting burner time building pressure to force air out of the system.

Steamhead (in transit)

Yes, there is a difference

With steam at low pressure, the steam is not compressed and can therefore fill the system quickly. Proper air venting makes this happen. If you have to compress steam at several pounds to fill the system, the burner must run longer to do it. This is where fuel is wasted.

Remember we're not running dry-cleaning equipment or steam turbines here. So the only pressure needed is that which overcomes the resistance in the pipes. Poor air venting adds resistance, so the pressure must be raised to make the steam move. This has the side effect of compressing the steam, which makes it move even more slowly. It's a Catch-22.

Vent your steam system properly and you decrease burner run-times, which saves fuel.

michael_15

However

I think this still boils down to what goes up the chimney. Because the run times are longer if the pressure is too high, more heat goes up the chimney, which is where the fuel is lost. If you, say, had an electric boiler, those longer run times actually wouldn't matter since the extra energy used to create the high pressure steam (and extra run time in making such high pressure steam move along the pipes) would still dissipate as heat within the home.

-Michael

Steve Garson

Nice thread of comments. My interest in all this was whether it is worth $180 to get a Vaporstat to replace my Pressuretrol on my new SGO-4. My system is vented extremely well, so I was curious if being able to operate at 8 oz versus 1.5 would save enough to warrant, but it doesn't look like it,

Steamhead (in transit)

Steve, if you have a Vapor system

then go for it. But if it's one-pipe, it might not be worth it. The pressure cutout is just a high limit- the venting will determine how fast the steam distributes and at what pressure.

On Vapor, you often have an inherent maximum pressure equal to the height of the "B" dimension. This is often less than a pound, and here the Vaporstat is always a good idea.

Christian Egli_2

It doesn't cost much to throw pennies out the window, indeed...

Pressure, temperature and energy content are related as described in the steam tables. As we know, it takes a monumental amount of energy to transform water into steam, it's no big deal because absolutely all of this energy is returned upon condensation of the steam. In fact, this makes steam the workhorse of heat transporters. It outdoes the other runners 1000 to 1.

The energy you pour into this steam first goes into the liquid as sensible heat then into evaporation as latent heat. Also, from the table, you'll notice the total enthalpy value keeps going up as temperature and pressure rise even though you've heated your water and you've already boiled it into a gas. What keeps on going is that some energy gets put into the pressure-volume-temperature relation for gasses, you're compressing the steam. That takes a little more energy but it is the stuff that is of great interest to power plants because that's how energy can be compressed into work. Power plants are not interested in heat, they want work.

Meanwhile, the energy from the raging fire gets converted to heat and work.

So far on this post, we've all correctly noticed that it does not take much energy to go up in pressure. Power plant operators count on this, they don't need to put much more energy into the steam once it is boiled to reach extra high pressures that are usable in steam turbines. Steam is fantastic that way.

When a compressed gas is needed, an effective alternative to running an air compressor is to use a steam boiler. It is not as much of a turn key operation but the energy costs are lower. Large forges where chunks of steel are hammered into giant doughnuts and whatever often use steam hammers just because electric air compressors are extravagantly expensive to run.

Steam hammers do work.

In the steam turbine you simply decompress the steam and harness the work.

In home heating we use a much smaller pressure for work. Only the work to move the steam to our radiators. We just need a bite size.

Now, even if it does not take much more energy to build more pressure, why do so if we don't need it? In home heating, any unnecessary pressure is dissipated in a useless and wasteful throttling process.

Throttling is what you do when slouching on the couch rather than working. A perfect waste of good energy. Pressure reducers and throttling valve are permanently guilty of this evil waste.

So should we still waste our pressure in home heating?

Wait, there is more to the story.

So, it does not take a lot more energy to raise steam to a higher pressure. That's totally true. But what else has raised as we were staring at the pressure gauge on our boiler? Turn around and look at the flue gas thermometer.

If you raise the pressure by 3 PSI, you will have raised the temperature of your heating medium by about 10 degrees F. Since boiler efficiencies are directly related to stack temperature, a 10 degree raise here will cause you a significant percentage decrease in efficiency.

How much, I don't really know. Modern boiler brochures show phenomenal boosts in efficiency with decreased stack temperatures. Any way, this efficiency is for you to loose. 1%, 2%...

Add these two factors together.

Even though you don't need a lot of energy to put into your higher pressure system, your heat transfer process is less efficient, so, in fact, you need more energy than just that called for by the steam table.

Your slightly less efficient operation applies to all the fuel combusted not just the little extra needed for extra pressure. This is where the significant losses occur.

How much does all that come up to? Not much, but if it costs you 1% in efficiency would you feel more inclined to installing a vaporstat?

Not sure yet? Let me go on a little more. I'm obviously on a roll.

There is a third factor.

It was mentioned that most everything your boiler does stays indoor anyway, so why should there be a mysterious heat loss with increasing pressure. Surely, it'll all washout at the radiator. So, heck, build pressure if you want you'll get it back.

Oh boy, you'll get it back. You get irretrievable wasteful inefficiency losses in return.

Remember the throttling process I described earlier. That's where the un-needed compressed pressure gets lost. And you think you get back some heat? at least some? maybe just a little? Nope, when you decompress things you get nothing but ... stunning cold coldth.

It's nothing magic. It's what the air conditioning units do all summer long. The boiler like the outside unit is where you build pressure and heat. In the summer, you promptly want to get rid of this heat. In a boiler, you obviously do not, you do your best at keeping it all. But as we realized earlier, running a boiler at higher stack temperatures is not the way to boost efficiencies and keep more heat.

Usually the extra heat you packed in while compressing your steam neatly offsets the cold created when decompressing the steam. Leaving you theoretically with a net zero. But of course, while your compressed steam was traveling with its extra baggage of heat, it lost some. It lost more than it would have lost under normal conditions just because it had the extra load of baggage. Remember, at higher temperatures you have higher temperature differentials and thus higher loss rates.

All the extra baggage you were keeping for decompression purposes and which you lost en route won't help you now. You have a net negative decompression factor.

How much does that come up to? I don't know. Oh a lot, I'm sure... undoubtedly enough to power the Titanic.

Numbers now.


Let's look at a 120,000 BTU/h boiler for a medium house. It boils roughly 100 lb/h of water. Let's figure the boiler logs 500 hours of operation per year. That calls for a steam production of 50,000 lb for the season.

Cooking the steam to 0.5 PSI calls for 1151 BTU/lb (from enthalpy steam tables). To produce the 50,000 lb we need thus 575.5 therms (57,550,000 BTU). Add a fix for the chimney, for 85% efficiency multiply by 1.15, we get 661.8 therms (A). All sounds realistic so far for a normal season.

Cooking the steam to 1.5 PSI calls for 1152.2 BTU/lb (from enthalpy steam tables). To produce the 50,000 lb we need now 576.1 therms. Add a fix for the chimney, for 85% efficiency multiply by 1.15, we get 662.5 therms (B).

Surprisingly little difference, no? Pressure comes easily.

The extra pressure takes up only (B-A) 0.7 therm or 70,000 BTU. Just around a whole dollar for a whole year of operation. We're not done yet.

A side note, these 70,000 BTU kind of represent the cost to move the steam heat around the house on its own power. Hot water heat with a pumped system burning 100W over the 500 hour season would consume 50 kWh. So water in motion is at least 5 to 10 times more expensive than steam in motion. For constant circulation think 20 to 40 times more money. The same argument steam hammer operators go through.

Back to steam pressure.

Let's look at the efficiency reduction issue. Taking the 576.1 therms and adding a new fix of 84%, multiplying by 1.16, we get 668.3 therms (C). The extra cost comes up to (C-A) 6.5 therms. Ten times more than through our first calculation. Real savings now. Pay back time for the vaporstat: 18 years.

Lastly, let's assume a horrible 5% drop in efficiency, 576.1 therms times a 1.2 fix, we get 691.3 therms (D). Now the extra cost comes up to (D-A) 29,5 therms. What would that represent about $50 per season. This might be significant but it is not much.

Were these the numbers you wanted? Anything not making sense?

After all this work and promises of shining efficiencies, we see that in reality it is much smoke and mirrors. Glossy efficiencies and conservation schemes have a true ring to them, but the results are often less than meaningful when you take into consideration the retrofit cost, like installing a new vaporstat.

I would say that what horrifies most of us when approaching an over pressurized boiler, is more often the fact that the system is plugged somewhere and someone's idea of a fix was to boost the pressure. You've got a boiler, packed with steam and trying to push it down a blocked main. Nothing going nowhere but to the birds and sky-high heating bills. Often pressure is just a symptom of poor boilering. These situations need to be fixed, here pay-back is quick.

Here is what you could do. Your pressostat is obviously on your mind why don't you play with it and see what happens. Try increasing the differential, if it is deductive, you'll lower your average boiler pressure. It will also increase ON times and this might make your operation more efficient.

Then, once your pressostat is broken, it'll be time to call for help and a vaporstat.

Hope this was fun.

paul lessard_3

very nice

thanks that was a good read. first time i heard of "poor boilering" I like it

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J.C.A._3

Like Paul...

I agree. Poor "boilering" is a new, but useful term.

Christian, you have WAY too much time on your hands!!!! However, I agree that either the returns are the problem, OR, it could be something as simple as CLEARING THE PIGTAIL to the pressuretrol.

I know it sounds simplistic, but think about it. All this talk of raising pressure is probably going to cause MORE problems than it will solve.

We're talking heating a home here. Pressure is the enemy!! It causes more problems than it will ever resolve. VENTING is the answer, being able to do it properly is the quest. Chris.

Christian Egli_2

A splash

Sometimes I need to clear out my own pigtail, what happens then is a post like this.

I did have a lot of time on my hands, but I notice you all had time to read, hmmm, must not be much going on TV these days anymore.

I'm glad you all enjoy the story.

Happy boilering!

J.C.A._3

Christian,

Actually, I was watching "The Godfather part 2" at the time I read the post. The TV was my partner when I was reading it.(good thing these posts don't show accumulated time reading a post!)

I did however turn down the volume to try to THINK about what you were trying to convey, and it DOES make sense to me, but again....so do some of Weezbo' posts!(abstract is only the beginning....)

First day off in weeks, I'm just listening to some Zappa and chillin out. Thank the Lord it's a day off! Chris

Tom R.

Wonderful explanation.....

For those who caught even a part of it - bad is bad, what difference how bad. If you do not need the pressure to perform work (a vacuum pump and/or proper line sizing will move the steam just as well as pressure) then you do not want to expend more fuel. Simple as that. BTW, replacing a faulty vaporstat is not a decision that shoud be based on payback. Without it, you are now using the high pressure cutout as the primary control. If it fails, you will be controlling pressure with the safety valve(if IT works). Buy the new vaporstat, clean what needs cleaning, farm out what you can't do yourself (or don't understand) to a pro. How much is peace of mind worth?