Code Required Combustion Air

[BronYrAur]

My situation falls under Chicago Code, but I would appreciate input from anyone who has run across this. I am sizing up combustion air for a large boiler plant - probably eight (8) 700BHP boilers. It will be an N+1 design. Two questions:

1. Do I need to size the combustion air for that "Plus 1" boiler even though it is not supposed to run unless another one fails?

2. The boilers will run on natural gas; however, they also have oil as a backup. I am delivering combustion air by mechanical means. The Chicago code requires 0.35 CFM per 1,000 BTUH for gas, and 0.4167 CFM per 1,000 BTUH for oil. Do I have to size the combustion air for oil (which is larger) even though the oil will only be used in an emergency if the gas supply fails?

Thank you for you help.

 

[cdxx139]

Not sure if required (likely for the oil), but it is smart to size for ALL boilers and at worst case.

What are you relaly saving otherwise? Some inches on a louver, the next size up heater?

The worst case scenerio would be a big change becasue an inspector can always say "someone" can turn all boilers on, even though their not supposed to.

knowledge is power

 

[BronYrAur]

It's actually pretty significant. I am between 10,000 and 11,900 CFM per boiler depending on if I use the natural gas or oil numbers. They are tempering the air to try to maintain 85 deg in the boiler room, so we are talking a lot of cooling load as well as heating.

 

[cdxx139]

I guess its all relative. A typical boiler room, a couple of inches for a larger louver shouldnt be a big deal. Your size mechancial room, a couple of feet larger louver is probably not a big deal either with the size of the porject you are working on, but I digress.

Sorry, not sure of a specific code section. But I relate this to the gas code. You need to size gas piping for all equipment running at max at all times. Even if the equipment runs just in emergencies.

Another option is you can potentially use transfer air as a portion of the combustion air (as allowed by code) If you locate the transfer above the main entrance door, at least you can keep that area conditioned, even if other portions are not.

Im going to stick with my original thought. Ive been burnt to many times in CA, trying to save chump change (again relative) in first cost, and then pay 5x that to fix it later.

Good luck

knowledge is power

 

[CarolinaPE]

Being that you cannot guarantee they will not fire all boilers up on oil I would go with the higher rate.

 

[EnergyProfessional]

You are cooling the combustion air??? Have you explored sealed combustion?
Obvioulsy you have to size for all boilers. Safety and common sense requires there will always be a point whne the +1 boier could be on.

As for NG or oil, will you have oil tank and oil piping isntalled and ready to go in case you need it? then yes you shoudl size for oil. If the boilers are jsut made to use oil, but you don't isntall any oil-piing and tank, I assume JHA woudl allow NG sizing. You shoudl ask them for clarification.

 

[BronYrAur]

It's a hospital with oil lines installed and ready to switch over at a moment's notice. I knew the answers to my questions ..... I just didn't like the answers.

I'm not specifically cooling the "combustion" air, but I am trying to maintain 85 deg in the boiler room. I may have on the order of 100,000 CFM of OA required, which could be 95 deg. That, plus the heat rejection of the boilers will be a significant cooling load. They only way to temper the room is to cool the makeup air.

I don't think sealed combustion is an option, but I will ask Cleaver.

Thanks all for your help.

 

[MintJulep]

Trying to cool a boiler room - wow!

How many boilers are running when it's 95 degrees outside?

 

[BronYrAur]

We are thinking four 700 horsepower boilers.

 

[317069]

what about direct vent boilers

 

[EnergyProfessional]

Select a better boiler that has sealed combustion. Sealed combustion is pretty much standard nowadays, it is not 1892 anymore. Let me guess, they are not condensing either and have a turn-down of 5:1?

I have the feeling whoever is helping you on this gives you bad advice. I have never heard of someone proposing to cool the combustion air. I've seen it in power plants fro gas-turbines to boost power (at expense of efficiency), but not for boilers. and boilers would love warmer air.

 

[ProcessHVAC]

The original question: size for worst case scenario, nothing more embarrassing than inspector from AHJ showing up late in construction requesting you to core wholes to existing walls( been there, at that point the Owner "forgets" a lot of design agreements) why would you want to cool combustion air? Not really looking at the details of your application so I don't know why boilers run full on during summer; but combustion air goes to, well, combustion (i.e. gets hot)

 

[Drazen]

if you don't want to burn lot of money, why don't you size for heat rejection and you will likely have plenty of makeup air reserve.

 

[tys90]

I'm guessing direct vent/sealed combustion is not an option in this scenario. Since it's a hospital, I'm guessing they are steam boilers. Good luck finding a direct vent high capacity steam boiler with dual fuel (oil) capability. If you do, let us know. I've had a lot of difficulties finding sealed combustion, oil burning capable condensing hot water boilers, let alone steam boilers. I only know of one manufacturer I would trust who does it.

Anyways, to answer the question, size for worst case on your intake sizes and airflow. But for cooling, I think it's insane to cool boiler rooms to 85 in Chicago. If you must do something, I like Drazen's idea of only sizing for heat rejection for expected summer operation (your four 700 HP boilers plus whatever else is in the room).

 

[urgross]

Installing and conditioning a control room for the operators, using separate dedicated outside air ventilation for boiler air changes, and hard ducting combustion air might be a suitable alternative. I'd definitely recommend sizing for #2, not just for NG. I'm fairly certain CB has integral VFD combustion air fans for the 700-HP sizing. If not, maybe check out York-Shipley. If you hard duct to the combustion air intake, you could use heat recovery to increase temperature with no impact to boiler room temperature, and maybe even use something fancy, like transpired wall/roof to pre-heat combustion air or make-up water.

Not familiar with Chicago area codes, but I'd bet that full capacity burning #2 will be basis for the air permit.

 

[BronYrAur]

Thanks. There is a conditioned room now for the operators, but they will be loosing it to make room for more boilers. That's how this whole situation came about. The boilers have economizers to heat the feedwater. The convective and radiative losses from the boilers themselves reject a lot of heat into the boiler room, which is the primary complaint from the operators. the existing makeup air units are not large enough, which makes the heat problem worse. They will be doubling the boiler capacity, so I am trying to provide enough new makeup air for all the boilers. I may then try to use the existing makeup air units a re-circulating cooling units.

 

[cry22]

I've done Facility condition assessment for 12 VA hospitals in the mid-west, including two in Chicago (NW and Hines), not one of them had cooling, and no one ever complained.
heat comes with the job in a boiler room, just like cooks in a kitchen.

You could give them may be an air washer on the OA intake (evaporative cooling) - and you get what you get -

http://www.munters.com

Especially since cooling form 95 to 85 will occur only a few hours in the year in Chicago, does not make sense to cool 100,000 CFM for a few hours.

 

[RossABQ]

I did a steam boiler room with design summer temps of 104F, 8 @ 1,250 HP dual-fuel (NG/#2) units with economizers. Yeah, it gets warm in there. We provided "cooling" (OSA at no more than 85F) directed at the boiler local control panels, the only thing in the room with a real temperature limit. Since that is also where the operators were likely to linger, it served a dual purpose, but no attempt was made to provide "comfort cooling". Boiler operators are used to it, and I believe current energy codes prohibit it.

As far as combustion air requirements, this was also an N+1 plant, and the owner's practice was to run N+1 at all times, because you don't just flick a switch and get steam out of even a warm boiler. This plant could stand no more than a 10% pressure sag due to process requirements. That doesn't mean all the installed boilers are ever drawing full air requirements, but as stated above, the AHJ's assertion was that nothing prevented them running all 8 flat out. Similar situation for emissions permits, they were based on all 8 running.