A look at steam efficiency - (was in boiler temp thread)

From Christian Egli...

I hope people don't mind - Christian's reply to my somewhat tounge-in-cheek post in the boiler temp thread got me thinking about steam system efficiency and I didn't want the discussion to get lost in the other thread.

Here's Christian's post about equating steamer run time to equivilent supply temp for a hot water system:

"There is more to life than numbers and how hot water compares to steam, but I don't know any better.

To expand a bit on BC's numbers, he was at 212F for 7.5 hours, while the rest of the time the steam was flat. Stone cold radiators. (Note how this is drastically different from what hot water is - Most have posted water temperatures of above 100F, which in constant circulation, will have presumably been kept that hot all day long)

While steam comes at 212F, this is not the average system supply temperature; oppositely, hot water supply temperatures are (within a bracket) the average values. Can we compare apples to oranges? Yes we can.

BC's system ran for 7.5 hours. In his whole day, there were yet another 16.5 hours that ran cold, and steam goes cold and flat within minutes (this is a source of great efficiencies). We can now compute the average system temperature for BC's system, let's guess his home is at 68F (radiators under draughty windows would be colder yet).

(212F * 7.5h + 68F * 16.5h) / 24h = 113F weighted average

113F averaged supply temperature for steam is surprisingly low considering BC lives somewhere where it was only 6F. In comparison, Al also at 6F, reported 128F. (Technically, this 128 is more of an instantaneous morning number, and I'd expect the average supply temperature for the whole day to be lower, possibly 113, who knows)

Steam heat grabs you by the lapel, gets you out of bed in the morning and shakes you until you're cooked. You feel the heat and it's addictive. At 113F, it certainly isn't crazy at all, just different.

What is modulated in a temperature space in a modern hot water reset is equally modulated in a time field for hot air and especially hot steam. Same difference.

Was this a good splash or what?"

Another piece of the puzzle

This shows a little of what I'm talking about - there is a significant lag between the boiler run time and the radiators heating up the room. The bottom sensor is close to the boiler stack, basically any time the temperature is increasing the boiler is on (I have verified this with my hourmeter). The top sensor is at my thermostat (either it reads a bit low, or the thermostat reads high, it is set at 68F during the day).

I highlited the boiler on times, you can see that the room temp keeps heating up long after the boiler shuts down. I'll try to get some radiators instrumented tomorrow and post the results.

Also notice the nice rapid warm up from the nighttime setback - one of the things I love about steam.

Cast iron radiation

evens out the temp distribution time. Fast "charge" into the radiator, slower discharge into the room. Low temp hydronics match charge/discharge times. So don't forget to introduce a time constant for cast iron radiation with intermittent firing. The proof is that a 100% efficient boiler would somehow waste 50%. Reality is, that's obviously not happening. You're somehow eliminating from the equation all the heat eminating from the radiators after boiler firing has ceased.

Heat exchanger manufacturers usually stipulate that greatest heat transfer efficiency occurs with high delta T's and high delta P's (temp and pressure differences). So analyzing the entire heating system from fire to heated space, steam does quite well, thankyouverymuch. Delta T from fire to water within the boiler is somewhat less than with forced hot water (in non-condensing mode-more on that later), but is extremely high at the heat transfer surface (interior walls of the radiator), typically (215-100)+latent heat. Steam's total heat is around 1,000 BTU/lb. Also, pressure drop is HUGE at the laminar surface at the point of condensation (don't forget that water drops in volume nearly 1,700 times in changing back from a vapor to a liquid).

Thermal transfer properties of steam are nearly ideal. Low mass, low pressure, high thermal content.

Once phase change is introduced (i.e. heating with latent heat) the formulas have to take into account the initiation, or priming of the process. In typical (non-vacuum induced) steam systems, the boiler must develop the POWER of distribution before the POWER of condensation can take over. This is why there's a pick up factor. The pick-up factor is a good indication of how oversized (and less efficient) the system can become once condensation is established in each of the radiators. Efficiency ratings hold this against steam systems, perhaps with good reason.

"Good reason" because the problem can be solved so easily. Simply remove the required pick-up factor from the firing rate once steam condensation is established.

It seems that when boiler efficiencies are rated (outside the application) this power factor is never considered. Sadly, neither do the boiler manufacturers consider it since we are not given the option of staged firing in the typical steam boiler. When I encounter commercial steam heating plants with just two stage firing, its shocking how much of the time is actually spent at low fire. The best is a comfortably oversized boiler with an adjustable low fire for field "dial-in" to match boiler output to radiation load.

Having this over capacity at will is a wonderful thing, IMHO, since a modern steam heating system could be designed with radiators sized smaller than your region's outside design temperature would indicate. The steamer could kick back into high fire and selectively (based on outside temp) increase the pressure to increase the output of the radiators during extreme weather conditions only. You'd probably see this happen maybe once a year. The rest of the time you are enjoying lower energy bills. At the end of the season you'd be very far ahead. In the meantime all we can do with existing systems is balance them for similar effect, and lower the firing rate with either a carefully placed aquastat at a riser or main (a la Boilerpro, Gerry Gill, Steamhead) or carefully tuned vaporstats. And it works.

Steam is competitive with modern hydronics because circulation is just what steam wants to do naturally, and is naturally attracted to the coldest surfaces. This is why you can heat, for example, a 36 unit 40,000 square foot 4 floor apartment building evenly (with proper venting!) with NO zone valves, NO electronic controls, NO circulating pumps, NO room thermostats, NO mixing valves, NO blowers, NO radiator bleeding, NO expansion tanks, NO freeze protection, NO static head pressure. Just automatic vents. Which are cheap in comparison.

I will take this opportunity as a steam heat advocate to take some lumps from the hi-tech hydronics guys who are justifiably proud of the truly remarkable strides they've made in fuel conservation technique. Steam has been soooo stuck in the post WWII intermittent firing, cast iron boiler, oversized, fuel-is-cheap world. Ironically, steam heat fuel conservation was developing into a high art in the hand fired and stoker fired era. Even some of the gas boiler manufacturers were trying for economy with clever design and firing rate/damper modulation. But the clever ideas never survived the post war fast and cheap mindset. Hydronics at the time didn't fare much better. And everything had to compete with forced air in first cost. But along come the new modulating condensing hydronic boilers, and steam boilers (since they're for "replacement only") have been caught with their pants down for a few years. Finally we're seeing good stuff coming along for our steam systems.

The only area of contention is that the steam temp is high enough that operation in condensing mode and related fuel savings are not possible. I disagree, since discrete devices have long existed to extract waste heat from industrial steam boilers. Preheating return condensate is a great place to put it. Condensing economizers exist for this purpose. Combustion air preheating exists and is useful as well. Small packaged versions of these things would put steam ahead of the pack, but we won't likely see them soon, if ever. They would be separate packages of sufficiently high cost that the payback period would be very high, and ROI might be non-existent in the life of the product. A familiar arguement in hydronics world.

-Terry

steamhead,

it seems to me that the only reason any of these thing have happened in Canadia (my Canadian friends hate it when I do that) and the EC specifically is that its

--Government Mandated-- (imagining the voice of God works well here).

When some of those rules start heading our way in the next couple of years, we may see positive changes in home steam boiler design.

Just another reason that I've told clients to hold off on a new gas boiler for now. Obviously I only advise this if their old beast is operating moderately efficiently and absolutely safely. These boilers I'll go ahead and do complete waterside and fireside cleanings, and combustion adjustments.

My only fear is that some government people incapable of systems approaches and no-common-sense, think-inside-the-box, paint-by-numbers nitwit bureaucrats will push for the conversion of all steam heating systems to forced HW with the HVAC industry in gleeful support. And I truly believe its a real threat, given the prevailing views of the HVAC industry and the general nature of bureaucrats.

-Terry

awesome thread

The more I learn from these guys the more I get excited about touching more steam work. It reminds me of when I started getting into hydronic radiant many years back.

Looks like steam may have a bright future!

Cosmo

That's a management company problem

they were probably the low bidder when the previous contract ran out. We all know about low bidders. The building owners will get what they deserve.

Come to think of it, I've never had to replace a cracked steam boiler that had a probe-type LWCO on it. All the ones I've done had floats. I'm sure probes go bad too, but personally I haven't had to replace a boiler because of a bad probe.

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Very true

This is in no way an optimized system. It is vented and balanced pretty well, but the boiler is sized for about 750ft2 of radiation and there is about 300ft2 currently turned on. The boiler is drastically downfired, but it is still almost double the firing rate it really needs. It needs to be replaced and it will be intersting to see how much more efficient a properly sized unit will be.

In reviewing this

"drastically downfired" keeps coming back to haunt me. You can only downfire so far before the firebox becomes a cavern in which the flame gets lost. I hope that the downfiring was done in the context of a running combustion analysis so that excess air is not extreme. If draft is not properly controlled as part of the downfiring, you've lost a tremendous amount thermal transfer efficiency regardless of the combustion efficiency numbers.

Is this why so many radiators are closed-- fuel consumption is totally out of control?

I'd bet that a boiler sized for the total load of all the radiators open that could modulate downward (or a dual boiler setup described elsewhere) after steam is established would use less fuel with all the radiators open than your current boiler with half of them shut off.

\"And none of them is getting any younger\"

Hey! I've been 39 for the last six years. What about that?

Seriously, the expertise on steam systems is minimal.

But you mention hydronics in general. I think this is true also. I do steam, but I get referred to simple, classic pumped HW systems with a few zones more and more frequently and there they are all screwed up and not heating properly--just like the steam systems. The problems are different but no less maddening in their origins in "service work gone wrong." And the solutions are simple if you've bothered to read a book (or this site) at all. The trail of destruction is remarkable.

I hate to think the only heating system that will succeed is one that never requires service of any kind until it fails completely in 7 or 10 years and gets wholesale replacement. Just like the consumer electronics industry. Not much comfort there.

True story: A camera shop I once frequented had a cartoon taped inside the glass counter. It was "Shoe" by Jeff McNelly.

A guy goes into a camera shop and says "I need a totally idiot proof camera."

The shop owner says, "Here. This one does everything automatically. You don't have to think. Completely idiot proof."

The customer leaves with his new idiot proof camera.

The shop owner says, "I don't know how long I can stay in business selling cameras to idiots."

My camera shop's out of business.

-Terry

I'm using Dallas 1-wire sensors

I'll try to take some pics tomorrow, but it is nothing fancy, just Dallas 1820 and 1822 sensors (http://datasheets.maxim-ic.com/en/ds/DS18B20.pdf) hooked together with an assortment of cat5, telephone wire, etc, whatever is handy at the time. They seem pretty tolerant of sloppy wiring! They are robust, cheap, and you can string dozens of them along the same 2 or 3 wire cable.

I'm planning on getting a WEL from Phil Malone (http://www.ourcoolhouse.com/WEL/) when finances allow. Now the 1-wire network is hooked into my weather station software running on an old PC, which allows logging but nothing fancy.

The system ain't pretty, but it gives me tons of sensors with +/-2deg accuracy and I have less than $50 in the whole thing.

Looks like you're in the market

for a Burnham Mega-Steam boiler!

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"