Naturel Gas Question

bob_50

Tim, I don't believe connecting the regulator vent to the combustion chamber will eliminate or reduce the effects of a change in barometer. It just adds the blower pressure, regulator spring pressure and barometer. IMO the servo regulator was introduced because it cost less and is smaller. You don't see servo regulation on large burners.

I did some reading at Vissmann's site on the Vitodens. The Vitodens does compensate for changes in barometer. If I understand the sequence of operation correctly(the web site info could be more detailed) the whole burner operation is tied to combustion blower speed. The combustion blower speed is controlled by the static pressure differential across the heat-x and the demand for heat. Changes in barometric pressure are reflected in the static pressure developed by centrifugal blowers. I notice that the instructions say there are no adjustments on this burner. Steve have you done a combustion analysis on one of these burners? What kind of numbers did you get? I wonder how tight they set excess air? Jim it sounds like this burner has everything except built in CO analysis. bob

bob_50

Nat. Gas Question

After reading Timmies Wobbe thread I got to thinkin about burner adjustment and barometric pressure. I looked up some historical records on swings in atmospheric pressure and found 970 to 1040 hectopascals(don't you love that) or 28.6"Hg to 30.7"Hg or about 1 psi. Let's say the BTU content of Nat. gas is 1000 btu/cu.ft. at 14.73 psi I know a lot of utilities use that figure. That means one cubic foot varies in content from 970btu to 1040btu.. If we have a 100,000 input boiler the input varies from 97,000 to 104,000. This has nothing to do with things the gas Co. may be doing with the gas like substituting other gases or diluting with nitrogen etc. This is because your pressure regulator uses atmospheric pressure as a refrence. My question is when adjusting a burner useing new fangled, space age, super accurate combustion anal-izers how do you compensate for these swings in input? bob

bob_50

How do you spell Natural

(:

Steamhead

I'd set the burner up

for the same CF input that would be required at the high end of the BTU/cu ft range. This would tend to prevent impingement which would generate CO. If the BTU/cu ft value is not at its maximum, the only negative effect would be a little more excess air. If you're not sure what BTU/cu ft is being supplied that day, dial in a little more excess air like we do on oil burners. This will leave enough headroom for the inevitable variation.

Tim is the gas guru here, so I would check to see if he has anything to add.

jeff_51

you don't

during real hard demand, the gas co's do all kinds of things like mix in lp and air etc to keep up with demand. The best thing is obviously do the testing befor the heavy deman. You got to sell your cust on this of course. Service in the dead of winter, ya just do the best ya can and go back when the temp breaks and set it up. You will always have some variables. The old Lennox pulse was to persnickity for this which is one of the big reasons it failed. We had to much variation in the Jamestown N.Y. area in gas pressure to keep the things running. There may have been other reasons for it's failure (although some are still going) and there may be other brands out there, but the units I am familiar with seem to run just fine within the parameters of gas pressue that you get. That prolly doesn't help you much and there are smarter guys than me on thsi site

Jim Davis_3

Nothing New

These things have never really changed, it is just that someone is finally admitting it. When you add Flue size & height, room pressure,orifice size and altitude the question is how did anyone ever set anything correct? The answer is they didn't. As long as things run we think we did our job. Analyzers measure reality but if one does not know how to interpret this data they become useless also. For the past 20 years I have been accused of teaching service techs of setting gas pressure outside manufacturers specs. It looks like everyone else finally is figuring out why. But I never recommend any fuel adjustments until all variables that we can control are controlled, which means you might have to take a look at the non functional codes that must be followed. People that have been clocking meters and measuring gas pressure as a main tune-up parameter have left most equipment operating in poor condition to include safety and efficiency.

Unknown

The BTU content per cubic foot

has been a problem from day one in the gas industry. The obvious thing is that it is different almost every day. Before we had Electronic Combustion Analyzers we would clock the burner to get some idea of what it was burning, we would check the gas pressure and try to set it as close as we could to the rating plate setting recommended, we would also check all of the orifices to determine what size they were (with no meter on LP this is all you had), we would "eye ball the flame" (I know some of you will love that one)making air adjustments to get the flame to stabilize. What eye balling meant was with many years experience with looking at burner flames you could pretty much get the flame adjustment in the ball park. We wanted a blue flame as compared to a yellow one, but early on we knew even without electronics a blue flame was not a guarantee of no CO.

I had an advantage working for the utility because we knew everyday what the BTU content was and also if we had a mix, usually back in those days it was propane air. Today it is typically here in RI LNG. We always at the end of this clocking, and eyeballing we would get the Fyrite Fluids out and do an ORSAT (chemical analysis of the flue gases) we also used a draft guage to check draft, and thermometers placed in several locations in the stack. None of this was an exact science. Finally we would use various types of ampoules which changed color if there was CO based on how many squeezes you took on the aspirator.

Concerning adjusting regulators I have hesitated to post this in the past as I do not want techs to just start screwing the regulators on equipment up and down. The purpose of adjusting the regulator up or down should only be done if testing is being conducted as adjustments are being made. I should also note here that proper training is needed to know what you are doing. The adjustment of the regulator allows you to bring the equipment as close to design input and efficiency with the lowest CO possible. The regulator becomes the means to fine tune the equipment. The recommended pressure from the manufacturer was to me a point to start and then make small adjustments to get as close to design input as possible.

With electronic testing a whole new world opened up for adjusting. We can now make adjustments and see the immediate result of our adjustment by the analysis presented on the instrument. This has allowed us to fine tune equipment to a degree which was really not possible in the past. This should be done on every call on a piece of equipment.

When we had mixed gases we had to adjust equipment and then when we went back to regular natural gas we went back and readjusted the equipment. This was to say the least a pain.

Let it also be noted that air for combustion can be a problem as non mechanical means of supplying air are not always reliable. It is also important that when using mechanical means of air supply we have the system interlocked with the burner for safety.

I have said all this to now state that the higher BTU content of some imported gas presents a different problem to the contractor. First of all you will probably not be aware that it is being mixed with the regular pipeline gas. Second it may be so high in BTU content you will not be aware of it being a problewm until the equipment is sooted up. Back when propane air was being mixed with natural gas it usually only pushed the BTU content up to around 1100 BTU's per cubic foot. We set most of our burners in those days to 1050 anyway. This LNG we are talking about has a BTU content around 1300 to 1350. This means the equipment is going to be way over gassed. This will result in sooting, CO, and damage to equipment. I have already experienced this on some equipment in my area. I am working on some type of protocol for dealing with this as to adjustments and what we can do to set equipment at some point that will be safe and efficient. I plan to write several articles on the subject and welcome any input from all the experts out there.

Steamhead

As always I can't wait

to read your articles!

bob_50

How to compensate?

Steam,Jeff,Jim,Tim, thanks for the replies. Except for Steamhead I don't think youse guys get my drift. I'm in a customers house, I want to do the job right. I call the gas co. and ask what the btu content of the gas is and the gal says 1000 btu/cu.ft. at 29.92"Hg. The input of the boiler is 100,000 btuh. I put my manometer on the burner manifold, it reads 3.5"wc just like the rating plate says. I clock the meter and get 100 cu.ft. per hour. Based on what the gas co. told me thats 100,000 btuh. It's a stormy day and the barometer is 28.6"Hg. BUT I don't know that. I am really firing at 97,000 btuh. I get my shake bottles and my monoxer and I decide I'm gonna make this puppy real efficient and I set the co2 at 11% the o2 at 2% and the co at .02. The next day a cold front comes in and the barometer jumps to 30.7"Hg. But I don't know this, what I do know is I forgot and left my monoxer on the job, rats I have to go back. While I'm there the boiler fires and I notice the flame looks a little yellow. I check co2 again it's still 11%, I check o2 and get zip, co is .5 . Wow, how can this be. I check manifold pressure and it's still 3.5"wc, I clock the meter and it's still 100cu.ft./Hr. What I am unaware of is that with the change in barometer the boiler is now firing at 104,000 btuh. Nothing has changed except the barometer. Now I paid a fortune for these shake bottles and other test equipment not to mention the customer bought a "super duper" boiler. Steamhead says screw the efficiency CYA and dump a bunch of excess air in it! Unless you happen to set the burner up on a worst case day and know it, how else can you prevent this senario? bob

Bob Harper

35% overfiring?

Man, at 1350 BTU/ CuFt, you are looking at a 35% over fire rate if you assume 1,000 BTU/ CuFt for NG. Therefore, your typical 40 gallong DHW heater will be firing at 54,000 BTU/hr. while a 100,000 BTU furnace or boiler would be at 135K!!! Since everything from the clearances to combustibles to spill switches, temp. rating of wiring and controls to the venting are all based upon 1000 to 1050 BTU/ hr., not only do these parameters go out the window but we will be doing everything in our power as a Mfr. to transfer the liability to the utility.

It isn't just a matter of BTU firing rate being adjusted for the fuel composition in use at that time. These fuels all have specific properties and behaviors in atmopsheric injection burners and natural draft venting. With the attitude of the utilities, we would need to design orifices, regulators, air shutters, mixer tubes, burner ports and vent baffles, and vent stacks that all modulate to the input.

The only system I know of that can respond to dynamic changing conditions of fluid dynamics is the human body. Your blood vessels vasocontrict or vasodialate instantaneously to pressure demands and conditions. So, too does the heart, organs, and peripheral vascular resistance. If your feet get cold, this means blood is shunting to the major organs to keep them warm. Put on a hat, the cappillaries vasodilate, blood flows back to your toes and your piggies thaw out. There's no way in hell we can design equipment to respond to all these parameters RELIABLY. Remember, this ain't an Etch-a-Sketch we're installing. We're bringing fire and toxic gases into homes.

jerry scharf_3

self adjusting

Bob,

You bring up a great point about self adjusting systems. Would you trust a boiler that has exhuast gas sensors and adjusted the mix on the fly? How long could it run before the sensors needed replacing? For the high end modulating/condening boilers out there, this is plausible.

jerry

JohnWood1

Already being done!

IBC in BC Canada is making a wall mount boiler in the high 90's that has a similar setup to the closed loop system on the late model cars. Mass air flow sensor and O2 exahust sensor make "on the fly" adjustments. I talked to the owner of IBC and he said that he is struggling to keep up demand for his Canadian customers, so we will not see his technology in the states for a while........... sigh.

I think he would do well to licence the tech to some US mfgs. It is the only salvation for those areas that have a fluctuating BTU gas supply.

Steve Ebels_3

Just a thought

Wouldn't any sealed combustion appliance at least partially compensate for the barometric pressure changes? This would be due to the fact that it would always be "seeing" the same differential between intake and exhaust.

What got me thinking about this is Viessmann's "balanced flue" system in the Vitodens. That boiler seems to be nearly impervious to fluctuations in atmospheric conditions and/or gas pressure. I have personally seen Vito's running on WAY less than normal gas pressures and yet they stay running clean with no sooting or CO issues. Example: one particular LP fired unit that was operating at less than 7" input showed no sign of problems whatsoever. Why is that?

bob_50

Good thought

Steve, you jogged my memory. I am not familiar with the Vitodens but the Hydro-Pulse boiler had a special gas valve. The regulator instead of being vented to atmosphere had an equalizer tube that went to the air cushion chamber to insure the correct air fuel ratio. I wonder if Viessmann uses that same technique? bob

Unknown

Most new negative pressure valve

systems do not use atmospheric pressure to determine regulator setting. They are connected by tubing or some other means to the combustion chamber in order to work off the same atmosphere as the burner. Because they use the pressure created by the combustion air blower, which is also pressurizing the combustion chamber they have to work this way.

A regulator vent has always had to be sensing the same pressure as the burner. When we are talking about a pure atmospheric burner setup the regulator and the burner which is getting its air for combustion from the same room are at equal pressure. All the regulator does is sense downstream demand and adjusts to give a steady let us say 3.5" WC pressure. If there are changes in inlet pressure this should not affect outlet pressure. That is the whole idea of a regulator. Otherwise everytime pressure changed the burner would be going up and down. The amount of gas that flows to the burners is then controlled by the orifices downstream. Many of the modern dual seated gas valves have a sense port in the outlet of the gas valve which senses downstream demand and positions the servo regulator in the gas valve which in turn controls working gas pressure in the valve to position the diaphragm.

As for setting up the burner. I would still set the burner input to recommended rated input and really not worry about variables for now. Here locally we have used 1050 BTU's per cubic foot of gas at .6 to determine orifice size. Most equipment calls for 3.5" WC outlet pressure. This has served us well for many years. When you then fine tune with a combustion analyzer and adjust pressure to get your best analysis with the lowest Carbon Monoxide (under 100 PPM)you should be all set.

Some analysis figures that were posted here are not realistic for gas. CO2 of 8.5% to 9.5% is realistic and oxygen should never be below 4%, usually 5% to 7% is acceptable. Then you are working with burner design which takes into consideration Time, Temperature and Turbulence as some call them. Time being how long the air gas mix takes to come together in the burner and how long the flue gases are in the equipment for maximum wiping action. Thermal efficiency is more important than any other efficiency, but we use combustion efficiency to set the burners to as close to design capacity as we can. Temperature is keeping the gas or oil at its maximum burning speed and temperature for complete combustion and again maximum heat transfer. Turbulence is the wiping affect of the products of combustion as they move through the equipment. This all goes toward increasing the overall efficiency of the equipment. After that the system design comes into play.

Last of all you must test, and make sure you have adequate draft. In some cases draft may present a problem, excessive draft means more air in the burner area which lowers efficiency, curtain effect at the draft hood means flue gases may not escape up the flue. In these cases other alterations of the venting sysems may be needed.

I hope this helps a little bit. Most important however is to get proper training on combustion issues. I have been doing this for over 40 years and every burner set up is a new experience for me, I am learning every day.

Jim Davis_3

Self-Adjusting

For years industrial boilers have used automatic 02 trim systems to adjust for mechanical deficiencies. These controls allowed equipment to operate when something goes wrong without anyone knowing the difference. Recent articles have stated that useless you use Carbon Monoxide monitoring equipment with these controls unsafe conditions will go unchecked. With oil you would need continuous smoke(opacity meters) which have been around for a long time. Combustion analyzers measure the true reaction of fuel and air and have been the only means of setting equipment correctly. But if one does not know the operating parameters in the first place they don't help. The single biggest mistake everyone makes is setting equipment for input instead of output. Output can be measured while input can only be guessed at. In respect to barometric pressure it is correct to state that as it changes so will equipment operation. For over 20 years I have been teaching that if there is no barometric damper on the flue of a conventional appliance, you have no control of combustion. If you are not using mechanical combustion air you have no control of combustion air. According to Code 1 sq.in. of opening in a wall for combustion air will supply 345,600 cu.in of air for combustion. Once again nothing has changed but a lot of people are coming out of the closet. Bringing equipment up to proper operating temperatures will not create a fire hazard but it sures does reduce corrosion and condensation. By the way new FVIR water heaters operate hotter than any appliance I have seen in years.

Steamhead

Tim, the basis for my post

comes (as I'm sure you recognize) from our friend George "Firedragon" Lanthier's excellent book "Combustion and Oil-Burning Equipment". In it he describes finding the most efficient setting on an oil burner (where reducing the air supply any further would start to make smoke) and then adding excess air to bring down the CO2 by 1%. This gives a setting which will burn clean over the course of a heating season, despite changing oil and combustion-air temperatures and other variables.

It just makes sense to me that, until a better method is found, we can use a similar technique to avoid problems from gas-supply variations. Besides increasing excess air slightly, would it not make sense to set the input rate using 1100 or 1150 BTU per cubic foot if those were the maximum values expected? Most residential equipment is oversized so this should not cause under-heating problems.

bob_50

Steamhead

Steamhead I hope you don't think I was trying to single you out or put you on the spot. I believe your view is the most common in the industry and has been for a long time. bob

Unknown

bob, I understand what you are looking for

but it is not something that has been addressed on most design residential gas equipment. As much as can be done with residentail design equipment we attempt to do in the field. There are however so many changing varaibles to contend with.

As for servo regualtors they were developed to overcome some of the short comings of in line regualtors such as "droop" and "surge" problems. The reason you will not see servo regualtors on commercial equipment is two fold. One industrial and commercial equipment falls under a different ANSI (Z83.3) standard and also has insurance underwriter rules to follow. Two servo regulation is limited in the BTU capacity it can handle. On a lot of equipment today they have tied the regulator vent on the valve and the combustion air blower together to attempt as best they can to have a stable combustion chamber condition. Jim Davis pointed out that many set up burners relative to input only and size orifices, set pressure, provide fixed air openings and feel they have done the job.

I have had several phone calls asking what Jim means by setting equipment by output instead of input. Not that I can answer for Jim amd perhaps he will address that after reading my answer. To my way of thinking the only way to set a burner is to fire it to the maximum that will give the best and safest combustion analysis. Jim and I may work toward that a little differently but we ultimately reach the same solution I would hope. I size orifices and set pressure as close to rating plate setting as I can, I then will use air and gas adjustments (regulator screw adjustment) to obtain the best combustion analysis measured with electronic instruments. In some instances the pressure will be set above rating plate suggestion and if you clocked the burner on a given day it may be overgassed. It is however firing at its best design state possible. It has been my experience going all the way back into the 1960"s that a draft hood may present a problem with these adjustments. With some experimentation back then we discovered that replacing the draft hood with a barometric did solve these problems. We had a lot of experience with barometrics as we did a lot of coal and oil to gas conversions.

A lot of high end equipment is operating similar to the Vitodens as combustion blower speed is ramped up and down it controls gas flow from the negative pressure gas valve. I for one do not like burners which I can not adjust in the field. The reason is that different conditions as we have noted here change conditions.

If we are ever going to be totally safe in our burner set up we will need to have sensors developed that sense all aspects of burner operation and control gas and air to match conditions. That and sensors that if combustion reaches unsafe levels of CO it will shut the burner off.

I applaud all of the high end equipment that is reaching very sophisticated levels of operation. Let us however keep in mind that 99% of the equipment out there is not that highly developed. It is my job and Jim's to try and educate folks to safely set up that equipment. Also about 80% of it is forced warm air which presents a definite problem with possible CO issues.

Unknown

Steamhead,

I am looking at some of that as a possible solution. I have just not come to any conclusions yet.

Steamhead

No offense taken, Bob

this type of discussion is why we're here. With all that's happening these days I really don't know what we'd do without the Wall and other similar communities....