Boilers in series or parallel

[Roktgr]

Looking at replacing one to two of our 100hp boiler with 2-3 50hp units. I'm wondering if boilers in parallel or series have the same effects as pumps do in the above configurations?

What would be the best method to run the 2-3 50hp boilers? They provide steam heating at a hospital and run ~26psi max in the winter, so the smaller boilers will need to be staged or modulate off each other.

 

[insult2injury]

Parallel.

How would you boil steam?

I2I

 

[Roktgr]

Yeah I guess you cannot run them in series. Should have thought about what the output were doing/going. My boss had this pretty confusing idea of how you could run them in series, but thats beside the point.

Running two 50hp ones @100% should equal one 100hp @100%.

Thank you for pointing that out.

 

[rmw]

Boilers in series.....NEVER say that you have heard it all.

rmw

 

[Roktgr]

Yeah well my boss is a civil, I tried to explain why you cannot run them in series but not sure he got it. Now he's convinced there is some formula to determine the equivalent output of running two smaller boiler instead of one larger.

I don't have much experience in facility engineering (just out of school) so I tend to listen to what the other engineers suggest and work from that. Anyways I'll see if I can get him straightened out. Thanks for the obvious.

 

[TD2K]

"Boilers in series.....NEVER say that you have heard it all."

LOL!!

 

[MJCronin]

Roktgr,

Let me get this straight....

Your boss is a Civil Engineer, who is running a department that is doing mechanical/power engineering.

You have recently discussed the merits of possibility of "running boilers in series".

Are there a lot of MBAs and "Certified Project Managers" running your organization ?

-MJC

 

[vesselguy]

MJCronin,

your message, "Are there a lot of MBAs and "Certified Project Managers" running your organization ?" is too funny but yet all too true nowadays. LOL.

Let's give the kid a break, he already said he's a new grad. At least he told us that, unlike some other members here that asks, "I'm designing/reviewing/SUPERVISING the design of a reactor and please tell me how to design a vessel?" Now that's scary!

 

[Roktgr]

Yes he's a civil engineer, but he also has quite a few other degree's and I think it's gone to his head. He is the facility manager at a hospital. He also tends to complicate the crap out of everything, he speaks very elaborately, using every 100 dollar word he can find. He tends to confuse the heck out of anyone he talks to, interesting guy to say the least.

So often times when he mentions something or asks a question I give him the benefit of the doubt that he has dealt with this before and has some knowledge about what he is asking, however I'm finding more often then not that is not the case. So I always go do my research and get all my facts straight before I give him a response, being I have not dealt with alot of systems. You know the one time I shout out idiot, he would be right .

Had a similar situation in where he wanted to move a recirculation pump to the outlet side of the heat exchanger and he all these theories to support it, however I didn't think that was correct but couldn't pull off the top of my head why or debate it with him. So I checked and it was in the correct placement to prevent cavitation, etc.

So in his typical fashion he had an elaborate idea of how these boilers could be ran in the above configuration, which didn't seem quite right to me but being the dumb new engineer being asked by a experienced engineer, I went along with it. So anyways I explained it to him and I think he understands.

But now he's asking me for engineering formulas and equations to prove that the two 50hp boiler will operate to the same output as one 100hp boiler. Multiplication maybe?

 

[vesselguy]

Roktgr,

Just curious, why are you refering a steam boiler capacity by the rating of its boiler feed water pump in horse power? I'm no expert on boilers but I do recall we usually describe the boiler by it duty in BTU and the corresponding mass flow of steam. To really help you, I strongly suggest you search for a book on steam boiler design and read. There is no good substitute unless you got an experienced engineer to lead you by the hand. Just googling I found these two for you.

http://www.amazon.com/Industrial-Boilers-Recovery-Steam-Generators/dp/0824708148

http://www.mhprofessional.com/product.php?isbn=0071475222

You need to find out from your boss on what is the motivation to use more smaller boilers. Is it reduction in capital cost? Operation flexibility during low turn down? If you don't understand the motivation it would hard to address his concerns. As others pointed out, you don't run boilers in series. Each boiler is rated for xxMMBTU to produce xxkg/hr of xx quality of steam. So it might make sense to buy 2 smaller boilers in combination to make xxkg/hr of steam to save on equipment cost or building size cost, for instance. There's formula for this.

Another suggestion is to phone up a boiler manufacturer and talk to their engineer. Any good boiler manufacturer will not mind talking to you about this kind of things to help you; after all they do want your business. Good luck.

 

[vesselguy]

Roktgr,
I forgot to mention, you could say that you can have a set of "boilers" work in series. Some boilers have a feed water preheat coils in the flue gas section to pre-heat the water to recover some heat and then pipes it in series to the radiant steam coil. So, if you say it is in series, you wont' be wrong. It could be possible your boss is thinking of having an exchanger or low duty "boiler" to preheat the cold feed water (still in water phase) and then send it directly into the next boiler to make steam.

You should really post this question in the heat transfer forum instead of in this forum. This forum is more for mechanical design of vessels and boilers.

 

[Roktgr]

Thanks, I will check them out. I'm referring to the boiler HP cause I don't know any better. Most of the boiler manf. list their boiler in HP rating which is the BTU output, correct?

Our boiler was installed in 1993 and has a design efficiency of 80%, so in to determine if we would replace or repair the boiler we were looking for a boiler with a greater efficiency. Unfortunately there were no records of what the boilers efficiencies actually are. I found one record from 2000 stating a efficiency of 78% given steam and fuel output and consumption. We do have a steam usage gauge but I guess it stopped working shortly after it was installed and has never been fixed. So I would approximate that we have maybe a 75% efficiency but thats just a guess based off the performance curves I have seen for a typical watertube boiler.

The idea for looking the 100hp boiler is to match our current setup as it provides for what our steam needs are. The current boiler can produce steam up to 150psi and on our coldest days we only use 26psi to get heat throughout our buildings.

Our boiler is used for steam which provides heating only. It's an old facility (1900's).

So our options are to use a vertical steam generator that are supposed to run ~85%+ through out the entire firing range. The other option is to use two smaller boilers so you can have a greater turn-down rate or modulation rate of the boiler, also giving more efficiency.

 

[Roktgr]

I understand the preheat but thats not quite the setup we were looking at. The water does have a preheat prior to entering the steam boiler now, and I will read through the other forum. I wasn't sure where to post this. It's looking like we will just replace the boiler with a similar sized steam generator, the two smaller boiler cost nearly 50k more and their efficiency is not significant enough to justify the extra $$ over the generator.

 

[insult2injury]

Boiler HP is common terminology.

1 Boiler HP = 33,475 Btu/h ---> which is the amount of energy required to generate 34.5 lbm/hr of saturated steam at atmospheric pressure from an initial saturated liquid state

I2I

 

[rmw]

The correct wording of the definition of BHP is "...from and at 212F" which in and of itself is exactly what insult2injury stated, not to take anything away from his post. That has some significance but it has been years-no decades since I thought of why it mattered and I can't remember. As I typed down below it came back to me so I came back up here and added this in. If you feed non preheated water to the boiler, then you pay an efficiency penalty for the portion of the fuel that serves to heat the non preheated water to saturation temperature. So in theory, only if you feed preheated water at saturation to a boiler at atmospheric pressure (for sea level) is it functioning at its rated HP. That is when your fuel input matches your steam output factored by your rated efficiency.

Said differently, if your feed water is not preheated to saturation, your fuel bill will appear as if it were for a higher HP or much less efficient boiler.

Also, BHP is in general used to describe fire-tube type boilers while water tube boilers are typically referred to by their steaming rate. The term correctly describes the size units you are working with. But don't sell fire-tube boilers short. I have seen some monstrous size fire-tube boilers used as start up boilers for super critical fossil fuel large utility boilers in the 500-1000 MW range.

And, yes, two 50HP boilers in parallel are the equivalent of one 100 HP boiler. All in all, given no other constraints, (duplication of support equipment and additional foundation and piping costs for example) I'd prefer the 2 X 50HP route rather than the 1 X 100HP route IF you have variable loads. If you tend to run pretty flat out all the time, I'd stay with 100HP. I'd rather have one boiler running flat out at 50HP or even down to 25HP than have a 100HP boiler running between 25 and 50 HP.

The efficiency of these boilers is a function of their design. There are 2, 3, and 4 pass (and who knows maybe more.) It is all a function of the laws of heat transfer and the temperature differentials and total surface area. That determines efficiency.

Have I paid enough penance for my frivolous remarks earlier?

rmw

 

[EngAbdelkader]

Hello my friends, well if you allow me to add some words to help our friend figure out the boiler issue, I will do.
first Roktgr, the issue that rmw is speaking about in his last post here is what is commonly called "economizer" so to preheat (if you prefer this word) the boiler feed water (BFW) before that it enters the water tubes.
The preheating process is done by passing the BFW stream through a device that is called "economizer" and that is working as a heat exchanger, heating the BFW by the exhaust hot gases coming out from the boiler. The purpose has been well explained (and much more than I can do myself) by rmw.

Than for the choice of 2x50 instead of 1x100 HP I would add another argument to what rmw said, which is having 2 boilers, your process is more safe as you have a "redundancy" and it's preferable that when not needed, one of the two boilers stays in standby. I can't assert that you'll multiply the life of your boilers by 2 but at least you'll preserve them better as they are not working all the time, let's say that the "life cycle" will be longer.

Hope it'll help.

 

[chicopee]

Isn't having a superheater analogous to having two boilers in serie? HUMMM...!

 

[rmw]

It's a stretch.

rmw

 

[Roktgr]

Thanks for the info, I'm familiar with economizers and either way we go they will have economizers. As for the two 50hp vs one 100hp that may come down to cost. The single boiler was priced at 98k and the dual 50 at 146k. I'm not sure we can justify the extra 46k. As for redundancy we will have an additional 100hp and 30hp boiler for backup. Both of the newer boilers are supposed to run at 85% fuel to steam efficiency.

 

[EngAbdelkader]

answering "chicopee"

NO my friend having two boilers doesn't mean (it's not analogous, using your words).
Here the issue was is it useful to have 2 boilers of 50 HP than 1 of 100 HP using horse power you'll understand easily that the issue concerns mainly the power (in the meaning of the steam produced).

If you don't mind I will explain briefly the superheating issue now:

heating the water up to 100 °c (212 °F)@ atmospheric pressure so 1 atm (14 psi) (sorry I am more used to speak in terms of SI units, hahaha) so you start producing steam that is at 100 °c and is called saturated steam, heating more the steam you'll produce what we call commonly "superheated steam" that has the characteristic of being less dense (greater specific volume if you want) and so easily transportable and as it is hot you'll get a better heat exchanger using it in a heat exchanger.

My explanation is a litle bit simplistic (I am not speaking about my english that is so so...) but I think it will help you figuring out the theoretical aspect of the superheating.

For the application (practical if you want) the steam is superheated generally in the same boiler that contains 2 parts, one where the hot water is transformed in saturated steam and the second part where the saturated steam passes through the tubes is superheated.

My final advise is to take a look at a Mollier chart to get an idea of what we are speaking about when we say "saturated" and "superheated"

bye