Purchase a 30% larger than spec N/G boiler, what effect will it have? 2

[thien2002]

I am going to purchase an AJAX boiler which is 30% more capacity than then RITE boiler because the AJAX salesman told me they do not have the 1,050,000 btuh so we have to purchase the 1,500,000 btuh. What effect will we have to use an oversize Natural Gas boiler. Please help ASAP!!!

 

[yogibear1]

This has to be a trick question, right? Are you thinking that due to a larger furnace volume you will have to use more fuel per pound of steam produced? I don't believe that will be the case. Well in theory, yes you will have to heat up a larger mass of water, and move more air and gas, initially, but you control the flow out, the steaming rate, so after initial warm up I think the fuel consumed per pouind of steam produced should be about the same as before, assuming efficiencies are equal between the two units. My opinion is not based on any particular experience or calculation, so I'll defer to others who can present a better case.

 

[thien2002]

This is not a trick question. What happen is that my Boiler shop requested a new boiler, they wanted to have Rite but Rite doesn't have the size they wanted, so they wanted Ajax, Ajax only has 1500000btuh boiler, but they spec called for 1050000 btuh. I do not understand if oversizing the boiler will have any negative effects steam supply process beside heating up larger volume of water. It seems like you got my point. I wish someone can tell me accurately cost comparison on energy, maintenance, efficiency, and risk. Thank you so much!

 

[unclesyd]

Just make sure the new boiler has the turndown ratio required for your needs and is easily controlled.

 

[kenvlach]

There are 2 negatives with using an oversized boiler:
1) Less efficiency. It is better to use a boiler running near 100% rather than one that cycles off and on. This is partly because there is a purge air cycle (required by law) at the start and end of every combustion cycle. I.e., the blower purges ambient air through the system and out the stack, wasting heat. I think about a 30 second purge at the start and end is required for a 1.5 Million BTU/hr boiler.

Boilers that operate at various turndown ratios (25, 50 & 75% for a small boiler) are better than cycling on and off, but you still have greater thermal mass and surface area to lose heat from, plus your boiler must be certified to meet the emissions limit at each firing ratio.

2) Air pollution regulations are more stringent for larger boilers, both the NO_x and CO limits and the frequency of testing. I have dealt with both Ajax and Rite, and both can supply boilers rated at say 990,000 BTU/hr to avoid the step up in regs at 1 Million BTU/hr. You need to talk to the right people, or maybe a vendor of multiple brands, e.g., R.F. MacDonald Co.:

http://www.rfmacdonald.com/

The South Coast AQMD regulations are on-line at

http://www.aqmd.gov/

Even if you are outside the SCAQMD, the other California APCD’s typically adopt the SCAQMD rules after a year or so (at least, Ventura & Santa Barbara do). Make sure to buy a boiler/burner combination factory pre-certified to meet the SCAQMD regulations (other districts will accept SCAQMD certs).

Our company recently had a Rite boiler overhauled, and we had it derated to < 1 Million BTU/hr. Estimated savings in fuel and testing > $2500/year.

Hope this helps,
Ken

 

[Fzob]

A few things you may want to keep in mind. Be sure to evaluate your auxiliary equipment; feed pumps, softeners, fans, fuel line sizes, etc. My experiance is if you install more capacity it will get used.

You don't say how if the new boiler is a replacement, or additional capacity, or what type of cycling it will be under, so its difficult to guess at what effects the larger size may cause.

In general if it is oversized you will lose some eff. It is probably a negligible amount. If it is a concern, you may be able to tune the boiler controls so it is at peak eff at 70%.

 

[thien2002]

Kenvlach, very valuable comparison. I didn't think about environmental impact on larger boiler vs. smaller ones. Fzob, this is a replacement. Our shop indicated to me our biggest problem will be the condensation on tubes and will have faster corrosion rate if not meet demands at 70% of time. You are all being so helpful. Thank you so much!

 

[smckennz]

Overcooling of flue gas is the only negative that I would be worried about. If your natural gas is low sulphur then this may not be a problem; in some places flue gas water content is deliberately condensed to improve efficiency, although this is generally on "once through" boilers.
If the sulphur content is appreciable, then reducing the gas path area to bypass some heat transfer surface will keep your back end / stack temperatures up.
The burner should be easily trimmed to reduce the output to your requirements. Modulating burners pay themselves off fairly quickly in variable load situations, and are fairly cheap anyway.

The larger thermal mass of the larger boiler may actually help you meet peaks.

Personally, I wouldnt be especially worried, assuming low sulphur. But make sure the supplier knows what maximum and minimum loads to set the burner up for.

Cheers

Steve