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separate boiler and superheater?

Posted by dullfig 
separate boiler and superheater?
February 01, 2015 01:05PM
Hi all:

It ads complication, but I was wondering if there would be an advantage to having separate boiler and superheater units. Are the heat fluxes always in proportion? For example, if you add water to the unit, does it absorb more heat leaving less for superheat?

At steady state everything works out. But how about during acceleration? Does that cause superheat to drop? Maybe if there was a separate superheater with its own burner you could control superheat better.

Just throwing it out there.

Dan
Re: separate boiler and superheater?
February 01, 2015 01:08PM
By the way, suppose you did have separate units, which would be the bigger one? Where is most of the heat going? Into boiling or into superheat?

Dan
Re: separate boiler and superheater?
February 01, 2015 01:10PM
Or maybe not two separate units, but two heat zones inside the same one, like an afterburner in a jet engine.
Re: separate boiler and superheater?
February 01, 2015 02:25PM
Hi Dan,

In naval parlance you are describing a "controlled superheat" boiler; these were common on WW2 warships, many old carriers and battleships had "M" type boilers with controlled superheat. The theoretical advantages were great but, in practice, they were difficult to operate. The amount of BTU required to raise superheat a degree isn't all that great compared to what you need to vaporize the water, this makes the control a bit "twitchy". As is common, when a property manifests itself it often turns out that other characteristics amplify the property. When adding more heat to an uncontrolled superheat boiler you may raise superheat but you also raise more steam, so the temperature swing is less drastic. The other issue is that temperature control involves greater time lags than pressure, you don't measure the temperature of the steam directly but instead you measure how much the steam heats the sensor element....depending on the sensor element the time delay between the steam reaching a given point and the sensor following suit can be significant.

"Uncontrolled superheat" boilers use a bit of art to keep the superheat within reasonable margins without adding active controls. Superheaters can be either "radiant" or "convective" depending on whether they are heated by passing combustion gasses or infrared energy produced by the flame. As it turns out, increasing the firing rate raises the temperature of steam flowing through convective superheaters and lowers the radiant superheater steam temperature....and vice versa if you decrease firing rate. Uncontrolled superheat boilers use radiant-convective superheater tubes (part of the tube is exposed directly to the flame and part only to the passing combustion gasses). The overall amount of superheater tube set the approximate superheat temperature while the convective to radiant ratio determines how stable the superheat tends to be under changing loads. Given the wide range of throttle demands imposed by naval operations, it is easy to see why uncontrolled systems became the preferred naval boiler....in addition to the simplified operating and maintenance demands accompanying the use of just a single furnace.

A "desuperheater" is probably the best alternative if you want to regulate superheat temperature closely but even a desuperheater will work better in a system having a radiant-convective superheater. In these boilers the superheater is built a bit oversized so that its natural tendency is to overheat during operation; a temperature sensor at the outlet regulates some method of cooling the superheater at a midpoint so that the superheater never approaches design temperature except right at the very end. The two typical desuperheaters either inject cool water into the superheater tube to absorb BTU while vaporizing the fluid or they work when a portion of the steam is bypassed through a tube in part of the boiler (usually the drum) so that the superheated steam gives up temperature to the saturated water. The first method is found in the Doble "normalizer"; this 'direct contact desuperheater' is simple in that a solenoid can deliver water directly from the feed pump or early economizer section. The second 'indirect contact desuperheater' has some advantage in that there is a natural feedback, as superheat is dumped into the drum more steam is formed and the pressure rise throttles the burner back accordingly. The Doble system is very effective because the subsaturated water absorbs so much heat so readily, this allows for a very compact mechanism. The indirect system is probably going to be a bit more bulky but it is also likely to react more smoothly. I've worked with both types of systems and they both do a great job.

Ken
Re: separate boiler and superheater?
February 01, 2015 03:33PM
Ken:

I see. So the normalizer preheats water that's going into the boiler anyhow so the heat is not lost. Do Doble's then have oversized superheaters and control final temp with the normalizer or is the normalize a "just in case"?

Dan
Re: separate boiler and superheater?
February 01, 2015 04:57PM
Dan,
The Doble normalizer does not preheat the water. They take feed water from the point where it enters the economizer and squirt it into the beginning of the superheater. The normalizer used to have it's own contact in the control box; but now is wired to come on whenever the second set of pumps comes on. Originally it was a just in case idea; but that got changed later.
They first depended on the two pumps-four pumps system with the compensator; but it was not enough to protect the superheater in heavy traffic and especially with hills. The slow engine driven pumps were the root of the problem.
Packy Nolan the Hills Brothers chauffeur asked Warren if they just couldn't squirt some water into the superheater to keep it from overheating and shutting the fire down because of over temperature. Squirt is not exactly the word he used, according to Warren.
Jim
Re: separate boiler and superheater?
February 01, 2015 05:06PM
Not sure of what you are getting at, but big ships like the Titanic had something like a 100 boilers and they would rotate according to the steam demand needed...so that every boiler was running at max. efficiency when it was on line. And you ended up using less coal to get that pound of steam.(except in their cases it was tons of steam) They also have reserve boiler so you could be working on one boiler while the others were online, dumping ash or whatever needed to be done.And never see a change even at flank speed.
Re: separate boiler and superheater?
February 01, 2015 05:12PM
Seems like feedwater purity would be critical to avoid depositing minerals in the superheater. I'd rather split the line after the superheater and pass one of the lines through the drum; a differential thermostatic mixing valve could combine both lines and provide a constant level of superheat; this would help avoid lubrication issues.

- Bart

----
Bart Smaalders [smaalders.net]
Re: separate boiler and superheater?
February 01, 2015 05:41PM
To make it even more freaky,if you are running a pressure firebox,that temperature would surge when you cut the blower
and and the reverse when you first start the blower again.Agree with Jim on everything else ,just pointing how hard control by temperature was.
Re: separate boiler and superheater?
February 01, 2015 10:22PM
Hi Arnold,

The Titanics propulsion systems are REALLY antiquated by today's standards. Something like 29 boilers putting out almost 60,000 peak HP at 24 kts. An Iowa class battleship (or Midway class carrier with the same engineering plant) was good for about 212,000 main engine shaft horsepower on just 8 boilers. Typically, all 8 boilers were kept on line at all times, yet the ship could go from barely making steerageway to somewhere around 34-35 knots and anywhere in between, on a moments notice --- which says a lot about the versatility and controllability of the boilers. The same boilers could be kept online for many thousands of continuous nautical miles. Realistically, those WW2 steam plants were about as capable as systems built right up to the end of maritime steam.....a Nimitz class nuclear aircraft carrier probably has about the same speeds from its 280,000 HP derived through 2 nuclear reactors each having 4 steam generator drums (go figure, 8 generators). The big improvement after WW2 was switching from the two furnace, controlled superheat, M type boiler to the single furnace, uncontrolled superheat, D type boiler....the D types were simply less manpower intensive and dependable. If we're going to draw a conclusion, I'd have to say that a rapidly circulating water tube boiler with moderate drum capacity and inherently stable "uncontrolled" superheat is not the worst model we could draw from....especially since we could increase the overall superheat capacity just a tad and toss in a desuperheater if we wanted to keep the temperature really stable.

Ken
Re: separate boiler and superheater?
February 02, 2015 09:34AM
A normalizer and a desuperheater are two different devices. The normalizer as used by Doble adds moisture into the steam cooling the steam, the whole purpose of a superheater is to get rid of all the moisture. The desuperheater used by the Navy passes the superheated steam line through the lower drum in a bank of tube much like a condenser and cools the superheated steam without adding any moisture to the steam.
Rolly
Re: separate boiler and superheater?
February 02, 2015 10:15AM
The following website confirms my definitions of direct and indirect contact desuperheaters (as does about 25 years experience with such systems in naval usage):

Engineering Design Encyclopedia

And Spirax-Sarco sells them under that name:

Spirax Sarco Direct Contact Desuperheater PDF

...."Spray type desuperheaters: A simple type of in-line desuperheater with cooling water injected into the centre of the unit via an atomizing nozzle sprayed in the direction of the steam flow"...

I had seen many direct contact desuperheaters (often called "inline desuperheaters) before becoming interested in steam automobiles and learning of the term "normalizer"; which appears to be a term the Dobles invented when they adopted this piece of hardware. The spray type desuperheater was certainly around long before they used it.

Ken
Re: separate boiler and superheater?
February 02, 2015 01:10PM
Ken
I am well aware of some of the miss used terminology as far as I am concerned. Cooling superheated steam was used for many reasons as you are well aware. Sending steam to the galley as well as running reciprocating steam pumps. No problem with wet steam.
As far as I am concerned the devices used that inject water into the superheated steam should not be called desuperheaters. The normalizer more appropriate used by Doble as far as I am concerned was to correct a poor design on the location of the superheater in the coil stack. Adding water to a superheater for a piston valve engine is just wrong. Any superheater in any generator should be located so as to put out a constant temperature required by the engine. In my opinion 200 F over saturated.
Used in turbines you just don’t want any water or moisture injected into the steam. All the supercritical plants I have worked on limited the temperature to 1200F max,
Rolly
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