Steam Boiler Staging

Mark Zoeteman

For a 50,000 sf school application, will a 100 hp steam boiler with staging or modulation reduce fuel consumption over a boiler with 1 stage (on/off)? I am not familiar with steam boilers. I do know that hot water systems that have increased burner turn down ratios will reduce burner cycling, wear, and fuel consumption as they match heating load better.

Unknown

Yes

There is a real benefit to either method. What boiler model do you have in mind?

Noel

Gary Fereday

Just like driving a car,

Matching the boiler output to the exact needs of the building out put will make for less fuel use. The cruise control on the car increases the milage since it is a more accurate judge than your foot is to the small increments it senses in the road.
Controls that can anticipate what is coming up, is even a better value, and if they control modulation & or stageing, savings in fuel will result. i.e. In the AM a good computer will know what the human body output will be when the Kids arrive at the school. The heating system will already have been set to provide for comfort in the first 15 minutes of that human output arriving. It would automatically adjust for the doors being held open. And realize that people put out a lot of heat. It would have taken into account the outside air temp.,and have the infiltration under control.
Does this make sense ? anyway Bigugh

Mark Zoeteman

I understand that with a hot water boiler system, reduced load will require less burner firing (that is heat can be added to distribution water loop at reduced firing capacity) since less heat is required at terminal units.
But with a steam system, there is no water, just steam. Can steam be produced at slower rate to match reduced heat loss from occupied rooms? So back to original question: Does it make sense to modulate the burner of steam boiler?

Unknown

You would control the steam in the rooms

individually, as with Thermostatic Radiator Valves at each radiator. The boiler modulation then would follow the load with a pressure controller that is variable, and a gas control that matches it.

Even without the valves, a steam boiler benefits from modulation as the pick-up losses fall off when the system warms up.

TRVs work great in big steam systems.

Noel

Tony Conner

Steam Boilers...

...like yours normally control off a pressure signal. If you don't have a modulating burner, the system will yo-yo on and off. It's either flat out, or nothing at all. If you can control the burner so that it modulates, it'll fire harder on a pressure drop (steam demand is up, valves have opened, or fans have come on), then throttle back as the pressure increases. If the load drops too much, the steam pressure rises, and the burner shuts down. As soon as the pressure falls enough, it'll fire up again. All big industrial boilers work like this. The actual room temps in the school would be controlled locally by steam control valves, starting and stopping fans, etc.

Floyd

Definetely 2 boilers with modulation...

Marc, here is a system much like you have described... this is an old hospital that has been transformed
to house a detention facility for troubled teens.
These boilers are 2- 30HP Byran steam boilers with modulating burners and a Honeywell step controller
to bring on the second boiler in case the one firing can't keep up.
Most of the time only one boiler runs and it just idles along... This facility is well over the 50,000 sf you have
we can see temps here as low a -20. Also, when I first took over the upkeep to these boilers they were running
around 10-13 psi. I have since cut then back and cut them back to around 2-3 psi!!! I talked to the maintainence man the other day and asked him how their running?? He stated that they have never been so quiet!!! Says that the one boiler just purrs like a kitten and the second one NEVER comes on!!!! Most days when I go down there the one boiler that is running is at or near low fire!!! And, get this... the fartherest run is over 600' from the boilers!!!!

The best part is that the gas bills have steadily decreased as I have fiddled with the controllers and set the pressures back, more than enough to overcome the price increases in gas costs!!!!

Do a check, but I bet you will find that your proposed boiler is way over sized.... at the very least I would install 2 smaller ones!!!!

Hey!!!! Steam IS fun!!!!! Have fun at it and enjoy your job!!!!!

Floyd

Jerry Boulanger

Don't get carried away..

with a high turndown on a modulating burner for low pressure steam boilers - there is a possibility that the boiler may condense at low firing rates. Check with the boiler manufacturer and ask what they recommend for a maximum turndown ratio.

Floyd

Actually....

Those boilers are a flexible tube boiler..... I'm not as concerned with condensation in them
as I am with water quality and overfiring!!!! Had to replace the tubes due to a lack of inhibitor
and bad water quality and the fire brick had to be replace due to overfiring........

Can see your point though depending on the situation and type of boiler.

Floyd