Steam boiler VS water boiler

[mfqd13]

Hi,

I'm searching a method to compare through calculations, the application of a steam boiler vs a superheated water boiler.
Of course there are advantages and disadvantages for each option, but how can we effectively compare this 2 options with calculations that can proove the choice.

Thanks!

 

[MJCronin]

marcos...

The choice between a hot-water boiler and a steam boiler involves the physical configuration of the heating system as well as the degree of temperature control required.

Each system has advantages and disadvantages:

You cannot compare just the "boilers".....you must look at and evaluate the capital and operating costs of the entire system.

Steam boiler system:

Advantages:

- Smaller steam pipe routing
- Longer distances between boiler room and users
- Higher temperatures can be achieved
- Lower operating (pumping)costs

Hot water Boiler System:

Advantages:

- Closer temperature control
- No water treatment costs
- Less corrosion than steam piping/condensate systems

There are other advantages and disadvantages...

Anyone else have information to share ???

My opinion only

-MJC

 

[macmet]

I'm not sure about 'superheated hot water' boilers, but steam boilers often need to have a different class operator(s). This can add significantly to operation costs.

Depending on the pressure of the steam, you could have a higher flue temperature so your overall efficiency will be slightly lower too.

 

[zdas04]

Terminology is making me nervous here. As I understand a "boiler" is used to create steam. A "superheater" is a device to add heat to steam in the absence of a steam/liquid interface--you cannot superheat liquid. A device to heat water to some value below the boiling point at system pressure is a heat exchanger, but not a boiler and certainly not a superheater.

The big difference in operating costs is the latent heat of vaporization. I need to add a LOT less heat to raise the temperature of water from (say) 120F to 260F if pressure is high enough to prevent boiling (say 35 psig, change in enthalpy is 140.8 BTU/lbm) than I have to add to get steam (say 34 psig, change in enthalpy 1079 BTU/lbm). I know that to add the same number of BTU's of energy to a sink I'll have to pump 10 times as much liquid as I would have to pump steam, but pumping water is pretty efficient.

The only time I can see that it really makes sense to make steam is if you have a use for the steam other than heat transfer. If you are just doing heat transfer then a pressurized water system has had the best economics every time I've done that arithmetic.

David

 

[TBP]

zdas04 - You add more heat to a steam boiler because of the latent heat of steam. But you get that latent heat back at the points of use.

 

[zdas04]

Its true that you add more heat per unit mass to a steam boiler, but the total heat added will be very close as long as you get 100% condensation of the steam at the point of use. Any steam that doesn't condense in the heat sink is going to waste the latent heat on the sink side, but you won't have to add it back in on the source side which might mean you are wasting more heat in a boiler than a water heater (because the burner is sized for 100% condenstation).

David

 

[Shmulik]

Steam system deserves a proper and liable design for mains condensate draining (considering start up and running loads) in order to avoid water hammer accidents. (Properly sided pressure relieve device is necessary as well).
Steam system will not tolerate design or maintenance mistakes. Consequence might be harmful.

http://www.kirsner.org/

 

[mfqd13]

Hi,

zdas04,
I'm Portuguese and when the water is heated above 100º we give a terminology, that i translated as "superheated". It means that the water is heated above it's "normal" conditions. But don't waste too much tipe fighting this misunderstanding.

Can anyone please give me an idea of my question? I know that is a complicated one, because we canot evaluate the boiler itself, but also all the system.

I'm asking this, because i'm dealing with a situation that the client is considering to change a steam boiler to a water boiler, because of the low requirements.

Thanks

 

[zdas04]

Marcosdias,
Your English is excellent and I believe we understand your question. I have encountered this "superheated liquid" term before and it always leads to confusion and generally leads to poor engineering. If I may labor the point, liquids are all either "sub-cooled" or "saturated". Gases are all either "super-heated" or "saturated". In the example I used above, at 127C and 241.3 kPa, water would be slightly sub-cooled even though it is well above the boiling point at atmospheric pressure. If this stuff was easy they wouldn't need Engineers to do it.

At the end of the day, the energy usage between a heated water system and a steam system for the same load would be the same as long as each system is properly designed and constructed. Energy usage would only vary by the relative system efficiency which can be a very small difference.

On the other hand, you have more precise control with a hot water system because there is no minimum system operating temperature--you will still add heat to a 10C space with 11C water, just not very fast. With a steam system, you must heat the system to the saturation temperature for the operating pressure so in low-demand conditions you will be adding a lot more heat to the steam system. Also, the water quality of a steam system must be very high to minimize scale, while a pressurized water system can tolerate lower quality very well.

My preferences will always tend towards a pressurized water system for simple heating because steam generation has some added maintenance, wear, and corrosion/scale issues that can make the life-cycle cost of the steam system much higher than a pressurized water system.

I hope this helps, if not maybe someone else can say it in a way that is clearer.

David

 

[MJCronin]

I would like to add...

In some states/counties etc, the addition of a steam boiler of a certain size means that an "operator" or "stationary engineer" must now be added to the annual cost.

It is not unusual for the cost of an operator to be the largest cost of operation

In the USA, the additional cost of that operator (24/7) outweighs the advantages/disadvantages of an one system over the other.

Consider this in you final evaluation

 

[LOUDOG]

You can have superheated liquid. In order for liquid water to turn to steam it needs a somewhere for the production of steam to start know as a nucleation point. Typically nucleation points are foreign material to the water or air pockets. HOwever if you have very pure water you can heat liquid water at atmospheric pressure past 100°C. However once the superheated water is disturbed it almost instanly turns to steam. (See youtube for sweet videos of this).

The reverse is also true for taking water to ice. You can supercool the water, then once the water is disturbed it will turn to ice. I have seen a bottle of water supercool to the point that when the water was poured onto a plate, it instantly turned into an icicle.

 

[zdas04]

Short duration parlor tricks are rarely an important factor in an Engineering analysis. Water at atmospheric pressure will not remain above the boiling point for long. Within a very short period it will either cool below boiling or burst into steam. Conversely, liquid water will not stay below the freezing point for very long before it becomes a solid. Phase change is rarely instantaneous.

I think we're talking about steady state liquid water that is sustained substantially above the boiling point of water at atmospheric pressure. The "steady state" and "sustained" terms in that statement preclude consideration of the parlor trick.

David

 

[LOUDOG]

Thanks for pointing that out David.

Marcosdias, if your client has low requirements you probably have to look at the burner turn-down on the unit. You will still get the same amount of heat out of the unit, whether or not it is water or steam.