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# Flue gas due point question

Member Posts: 200
Can anyone tell me what the flue gas dew point is for a gas boiler at 5600 ft altitude burning the gas shown in the following spread sheet? Flue gas CO2 is 8%, RH 30%, ambient air temperature 65°F.

<a href="http://www1.xcelenergy.com/webebb/html/Gasqualityzone.asp">http://www1.xcelenergy.com/webebb/html/Gasqualityzone.asp</a>

## Comments

• Member Posts: 7,265
edited February 2014
Flue Gas Dew Points

I don't think that the "dewpoint" works quite that way. Heated air can hold more pure water vapor than cold air. When the temperature of the air/water vapor drops to a point where the water vapor will condense out, water forms. But it depends on the temperature of the air. Air temperature at 100 degrees with 100% humidity has more water vapor that the same temperature at 90 degrees and 100% Humidity. But the 90 degree air holds less water vapor than the 100 degree air. But when the air temperature of the 90 degree air drops to 80 degrees, it has too much water vapor and can no longer hold it. It either loses it through clouds forming and them rain. If there is a colder surface than the outside temperature, the water vapor will condense on the colder surface.

As I understand it.

Read this. I's easier to read and understand than for someone to explain in detail. Because evaporation comes into it. Like trying to explain why you feel cooler on a hot and humid summer day wearing a cotton tee shirt than you would if you wore a synthetic one. Evaporation and cooling.

http://en.wikipedia.org/wiki/Dew_point
• Member Posts: 4,494
The theoretical

dew point for natural gas is 140 degrees (F). The relationship to the stack temperature is we want to not go below 275 degrees (F) in the stack as anything below that is condensing. Not sure if that is what you are looking for. If you are looking to go beyond that one of the engineers who post here could perhaps give you a very scientific breakdown.
• Member Posts: 7,137
Condensation

Are you trying to figure out when a condensing boiler will condense or when condensation will damage the flue?

As for the condensing boiler, it looks like a return water temp around 122.

The flue damage is more complicated.

This article may help. http://www.heatinghelp.com/files/posts/13375/AshraeCondensingtechnology.pdf

Carl
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 7,265
Dew Points & Spreadsheets:

There's nothing in that posted spread sheets that has anything to do with flue gas condensation. I guess your question has to do with "would be condensing dew point be higher or lower in Denver, CO at 5,000, above MSL". Yes because the atmospheric pressure is less. It effects the dew point. Your question really has to do with the "Phase Change" which is when the water vapor condenses to a solid which normally happens at about 140 degrees F with oil. In the Phase Change, there is an extra amount of BTU energy required to make the gas turn into a liquid or the liquid turn in to a vapor (steam). This extra amount of BTU's is "captured" by the boiler water from the flue and added to the efficiency of the boiler unit. Like steam heating. Water can be at 212 degrees F and be a liquid or 212 degrees F as a vapor. It takes a set amount of extra heat to make this "phase change" from a liquid to a vapor. When the steam gets into the radiator, that extra amount of heat energy is taken by the radiator and the steam turns to condensate.

Another example is frozen water on a lake. If the air temperature is 15 degrees, and the ice plate is 12", and you have a infra-red thermometer gun, shoot the ground in the parking lot. It will be 15 degrees. Go on to the ice. It is 15 degrees. Chop a hole in the ice and get to water. Shoot the water. It will be 32 degrees F. The ice freezes  and melts from the bottom. If you get Radiational cooling at night, and the surface temperature of the air above the ice is -10, the ice freezes rom the bottom. If the air temperature rises to 40 degrees, and the slab becomes 32 degrees and the ice melts from the bottom through the phase change. It can also melt from the top but the middle of the slab will be 32 degrees.

Because water boils at lower temperature at higher elevations like Denver, CO, (194 degrees F), the flue gas condensation levels may be lower. I've never seen it discussed. It makes sense.

But the flue gas of oil fired equipment is considered to be 140 degrees and gas equipment at 120 degrees. What's the issue?
• Member Posts: 200
Zman,

It is page 40 of the ASHRAE pdf you mention that got me curious about the condensing point of the flue gas in a natural gas boiler here in the Denver area. But the gas here has a Btu content of only about 850 Btu/cf as a result of the lower atmospheric pressure and the addition of air by Xcel. I suspect this may materially alter the condensing point (aka dew point). How can one set up outdoor reset for a cast iron boiler without knowing this?
• Member Posts: 7,356
Added air

should not change the dew point, just the air/fuel mixture needed for proper combustion (and of course the capacity of pipe & valves, etc.)
• Member Posts: 813
Air

Nick, the utility company does not add air to control but content . The reduction is due only to altitude .
bob
• Member Posts: 7,137
Good topic

Nick,

On a cast iron boiler, I would be more concerned about condensation in the flue at the lower temps (unless it is stainless). The flue is a tough one because it depends on how long it is and weather or not it is run in a conditioned space. I have seen flues rot out on boilers with 160 degree return temps. The flues were mostly run in unconditioned attics. The boilers were oversized and running short cycles.

I think it was Gennedy that said that  you can tell if the flue is condensing by looking to see if the plume is detaching from the vent termination. This make sense to me although I am not sure if it is commonly accepted.

Bob,

It is my understanding that the utilities derate (add air) to the fuel in order to make appliances run more efficiently at altitude. Do you have information that contradicts this? I am genuinely interested, if my understanding is incorrect.

Carl
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 200
Bob,

Here is an excerpt from a paper by Rosemarie Halchack, the gas quality engineer at Xcel:

"As noted, natural gas supplies on the Colorado Front Range are blended with air for the purpose of Btu stabilization. The original supply of natural gas to the Denver area contained high levels of nitrogen and was characterized by Wobbe factors of 1200-1250. As additional supplies with Wobbes of 1300-1350 became available from Wyoming, the issue of interchangeability became critical. In the late 1970s, the Institute of Gas Technology performed appliance testing and interchangeability calculations to determine that the new supplies could not be safely substituted for the existing base gas because of the risk of yellow tipping and incomplete combustion.

"The most feasible and economic solution to the interchangeability problem was to blend air into the new supplies to reduce the Wobbe to the required values. Air compressors have been installed at three stations supplying Denver and the surrounding areas. The quality of the gas-air mix is controlled using measurements of oxygen content and Wobbe Index. The air added is typically 5-8 percent of the gas flow, and the composition of the resulting mixture is thus significantly above the upper flammability limit.

"In other situations, Xcel Energy has solved interchangeability problems incurred by new gas supplies by reorificing entire distribution systems. The city of Fort Collins, located 70 miles north of Denver, was reorificed for higher heating value gas in 1968, and the remaining area north of Denver was reorificed in a "High Therm Project" in the early 1980s. In 1995 a reorificing effort in the mountain area near Vail was completed. Reorificing is a time-consuming and labor-intensive project that is undertaken only if no other alternatives are available. The company must enter all homes and businesses to install new orifices in the burners of all appliances and to adjust air shutters and manifold pressures to ensure proper combustion. Follow-up inspections are necessary to verify that the work has been properly performed and minimize the liability associated with improper adjustment."
• Member Posts: 813
Air

Nick and Carl , I stand corrected . Good information Nick . I worked in the mountains during the 80's on the west slope . When I moved there from Chi . I couldn't get a straight answer from anyone about altitude and btu's even the service men for RMNG . I finally got hooked up with the chief engineer for RMNG and he gave me the formulas and the base pressures for the towns on the west slope. He never mentioned WOBBE numbers . I don't think Rocky Mountain Natural Gas was that sophisticated at that time .
bob
• Member Posts: 200
Return temperature of 130°F looks safe

It looks to me like a return temperature of 130°F is OK in an atmospheric cast iron boiler at 5600 ft with CO2 at 8% or less, assuming 850 Btu/cf gas won't raise the condensing temperature from the theoretical 123°F.

My concern is not space heating, as those zones cycle between 150°F and 170°F. But my indirect can drop the return temperature to 130°F for quite a while, as our water supply is now about 40°F and falling.

Thank you all for the comments.
• Member Posts: 7,137
Flue gas

This came up in a post a while back.

Although any flue gas condensation is not ideal, If the boiler has has the opportunity to come up to full temp and dry out the flue before the heating cycle is over, the damage is minimized. I your case, it sounds like you may get some condensation when the DHW demand starts but by the time the cycle ends things are pretty hot and dry.

In the example I cited, the boiler was short cycling leaving a wet flue every time. This , I believe was  the cause of the flue corrosion.

Carl
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein