Why use a new steam system vs a hydronic hot water system?

[nuuvox000]

Just as the title suggests, I'm trying to figure out why you would design a steam system for HVAC when you can just use a hot water hydronic system. Steam seems to be way more expensive. Could someone enlighten me on this subject please?
Thanks!

 

[itsmoked]

Steam has a much MUCH faster response and you can do it with smaller hardware because it is more energy dense. That said, I'd avoid it unless it was proven necessary.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MJCronin]

Steam systems usually have a significantly higher capital cost, but the steam goes where it is needed without circulation costs

Pumped Hydronic systems incur operating costs year after year for circulation.

The cost of fuel and the combustion efficiency are extremely important for each, of course

MJCronin
Sr. Process Engineer

 

[GT-EGR]

Steam is a more compact heat source with less components to deliver, although it does come with return and distribution complications. It’s temperature also more useful for certain non HVAC processes.

But otherwise hot water is easier to manage and deal with modify and understand, and it’s operating temperatures allow for condensing boiler savings.

 

[BronYrAur]

Generally speaking, steam is "old school" for applications that strictly involve building heat. Most facilities that have steam heat also use it for humidification or process loads that require higher temperatures. In my neck or the woods (Chicago), steam is still around in many high rise buildings. Some still have vacuum returns (which few people understand). The steam is typically used for building heat and domestic water heating in those buildings. Retrofitting to hot water in those types of buildings would be very costly.

Other than that, I typically only see steam in hospitals, manufacturing plants, or in large campus settings where there are underground steam mains serving multiple buildings from a central plant. Even in those situations, I have de-centralized and put smaller boilers in individual buildings instead of the central plant concept.

So if you are just heating an office building or something like that, steam is probably not your best choice. What type of building are you working on here?

 

[TBP]

@GT-EGR: I've only seen condensing boiler used in in-floor heating or snow-melt applications, as they run at low water/glycol temps. Can you get outlet water temps suitable for an typical HVAC system out of a condensing boiler?

 

[BronYrAur]

They run more efficiently at low temps but they can run higher temps as well. It is a waster of money to use condensing boilers if your aren't going to take advantage of low temp, but you are not restricted (for all practical purposes) from high temps. Maybe there is a difference in the upper limit but not enough to prevent condensing boilers from reaching typical heating temps.

 

[TBP]

It was my understanding (although my info is several years old) that a condensing boiler that was running at above condensing temps actually had lower efficiency than a considerably less expensive to purchase conventional boiler.

 

[EnergyProfessional]

You only use it if you have it available or no other choice. Like a campus with a steam plant from a combined steam turbine. There is a lot of maintenance on the boiler, water-make-up etc. it really only makes sense, if at all, on a very large scale.

You will have a hard time finding experienced designers and more importantly operators.

Also look at boiler efficiency of modern condensing boilers vs. steam boilers and you will see that unless you have waste-heat (power plant), it is not fuel efficient.

 

[TBP]

I guess my questions are what is the maximum water outlet temp from a condensing boiler, that is actually running in condensing mode? Is that water temp high enough to allow the rest of the system to provide the heat required? I have little doubt that an entire system could be designed & built that would work well with low water temps, but what about retrofits?

 

[BronYrAur]

The return water temp is probably more important. having a low return water temp is desirable to promote condensing, but there is only so much temperature rise that the boiler can provide. And there is only so much delta-T that coils, etc. can efficiently transfer. If your heat transfer devices out in the system cannot give you a low enough return water, then a condensing boiler doesn't make sense. I know I am not being very quantitative.

 

[EnergyProfessional]

Condensing boiler efficiency depends on RETURN temperature. Look at manufacturer efficiency curves, which are based don modulation and RETURN temperature.

In retrofit, you still can run at lower temp since the design conditions don't take into account interior cooling loads (humans, electronics, lights) and there is quite some time where outdoor temperature is above design conditions. Supply temperature gets re-set based on OAT and valve position.

Even when not-condensing, a hydronic boiler should be more efficient than a steam boiler. Steam boilers produce steam at above boiling temperature. higher fluid temp = higher flu temperature. Higher flue temperature = lower efficiency.

Someone above made it sound like steam systems don't use energy for circulation. If that can be proven, I'm sure a Nobel price is justified. Because the energy to move a fluid and overcome the pressure drop has to come from somewhere. Steam doesn't move for free, it looses pressure, and with that enthalpy. Where does the pressure come from? From fuel. To move a fluid, you need energy.

The whole discussion is moot if we don't know the project conditions and size. We also don't know if the owner is open to both options. And if there is no experienced design engineer for steam, better to forget about steam. nothing worse than a steam system designed by someone who doesn't know how to. Hydronic is much more forgiving. Same goes for maintenance. Steam requires a LOT of maintenance.

 

[nuuvox000]

Thanks for the GREAT answers and discussion. Just FYI, this is only a question I'm thinking about; it's not for a particular project. I just recently saw that a steam system will be installed in a brand new prison where I live and I was curious as to why they chose this system.

 

[BronYrAur]

A prison will probably have a separate boiler plant building which will distribute steam underground to various buildings. The steam will no doubt be used not only for heating but for cooking and laundry facilities. Many of these require temperatures much greater than a practical water system can provide. Plus the distribution of steam would be easier than pumping around high pressure high temperature hot water

 

[nuuvox000]

Yes, you're correct. It looks like they are using it for at least cooking.

 

[EnergyProfessional]

I would disagree with the statement that a steam distribution system would be "easier". Hydronic is very forgiving and easy. Pipe sizing is well understood, gravity doesn't play a role (pitch etc. doesn't matter). You don't need steam traps (lots of selection to do)...
I'm not saying steam is extremely difficult, but there is much more to consider and it matters if it goes up or down. I recommend reading Holohan's books to understand what it takes and what problems you can have with steam if you don't fully understand (and most people don't fully understand steam)

 

[nglty]

Central plant makes sense in Prisons. Prisons staffs do not want any flame near inmates. Some inmates will bust anything to create trouble, diversion, etc. High temperature water would be even better for a campus type in the cold belt. Even DHW is generated from heat exchangers, not from gas-fired water heaters.
If the project is very cold climate, steam has proven to be more effective at keeping the building well heated with the high-degree heat from steam.
Condensing boilers at low-grade heat allow the use of heat recovery chillers.

 

[goutam_freelance]

Building heating requires low grade heat. Normal fuels have capacity to generate high grade heat, which can be partly converted to mechanical/electrical energy. So to be energy efficient one needs to have a Cogeneration plant generating energy in addition. Hot water can be generated by heating water using turbine exhaust or extraction steam.
The optimum choice will depend on heat load and dispersion area. Using hot water hydronic system has a clear advantage in this case.

Engineers, think what we have done to the environment !

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