Steam Control

TheRookie

Hello all,



I've been using this website as a resource for steam related questions most of my career as a Mechanical Engineer (which at this point has not been very long).  The knowledge base here is top notch. 



A little background:



I have a 30's era 15000 SF building that is currently heated by steam (around 6 psi maybe).  The terminal heating devices are the typical cast iron radiators; which are in great shape.  The type of system is a one pipe parallel flow with a wet return.  In the 80's when the old coal fired boiler was replaced, a designer added a condensate return assembly, I'm guessing because of the water cavity size difference between the new and old boiler.  I have been asked to design a heating upgrade and to improve energy efficiency.  Looking at several heating options for the facility and comparing them over a 20 year life cycle cost analysis, a new steam boiler was the bread winner.  Mostly because the existing infrastructure would be reused, cutting the upfront cost drastically.  This particular client has very stringent requirements on upgrading facility controls to BACnet over IP anytime new equipment is installed. 



Getting to the point::



I plan on replacing the cast sectional with potentially a flex tube, that remains to be seen.  I'll also plan on changing the condensate return system to an underground condensate pump configuration, I'm thinking this will operate much better than the tired hoffman receiver that is almost at the same level as the wet return piping.  I originally planned on installing TRV's at each radiator, however I've been toying with the idea of two-way valves at each radiator controlled off of a zone thermostat.  My weak link room can control the boiler firing.  I know my client would like the controllability.



Any thoughts or advice on my design choices?  They way I figure it, the key to sucess is surrounding yourself with people that have superior intelligence.  I'm thinking I've found the place.



Another couple of questions:



-Any advice on other energy conservatory measures for a system as such?



-I haven't measured the existing EDR yet, however if I compare what my calculated skin loss is to the size of the boiler, there is a substantial difference.  Pretty much the existing boiler is grossly oversized.



Just purchased "The lost art of steam heating" and "Every Darn Radiator", I am very excited to read these.  I'm sure my future steam knowledge will impress the ladies...



Thanks in advance!



Shawn

nicholas bonham-carter

15,000 sq/ft upgrade

i think you are right to keep the steam, and i am sure that i speak for all here when i say thank you for your complimentary remarks; however it is the cumulative years of experience, and enthusiasm which really solves problems, and answers questions here.

certainly, you must measure your edr, before selecting the replacement boiler. as you noted, there will be quite a difference between the capacity of a steam boiler, and the heat lost by the structure; however, if you measured the therms of gas burnt during heating, those figures would be closer.

the secret to effective, and comfortable steam heating with radiators is good main venting, at low pressure-a few ounces or so, attainable only with a vaporstat, monitored by a 0-15 ounce gauge.

is this building one floor or several? i would try to do without any sort of condensate pump or the like as it introduces the element of mechanical frailty into the system. my own 1,050,000 btu , 55 rad system on 3 floors is all gravity, and i can run it on a ups if the power goes down. most modern sectional boilers are designed to have enough water to fill the system with steam, without running dry, as long as there is no restriction in the return piping.

as far as control systems go, there is probably some steam boiler control from tekmar, which is internet-capable, but why bother? --nbc

steamer7676

steam control cool

i have done a couple of zone control systems on existing steam systems.  they worked pretty well and saved the customer some money. both were church/school buildings. they were both 2 pipe systems though. one building was broken into 5 zones as this was how the steam mains left the header. we haven't tried to break individual rads into zones but I'm sure people have done this plenty of times though it seems like it could get pretty costly . we used a Honeywell XL 10 and put sensors in each zone  and used the xl10 to control the mod valves. i think the range was pretty small so the valves didn't modulate much. we also used an out side air sensor to reset steam pressure. there again i dint think the reset was real great but with some tweaking we were able to Dial it in and the customer was pretty happy the boilers were 2 40s era kewanee fire tubes that were still in pretty good shape. there are some rules that need to be followed when putting in zone control but the LAOSH covers these like traps and vacuum breakers in the correct spot . we just finished a boiler install about a month ago and it had 2 zones on it similar type building a church/school. one zone reused barber coleman 8161 controller and mod valve and we replaced the other with a Honeywell 7300? i don't know what version of the top of my head and a mod valve the 7300 s are networkable and probably can be tied into a jace and put on the web . I'm not sure what communication protocol these are available in but they are the best tstat on the market easy to program easy to use they have modulation and or on/off control. this system works pretty good also there is a little water hammer when the zone opens up but its minimal . hope some of this helps you. keep posting updates i would like to know what you come up with.

nicholas bonham-carter

zone control needed for small bldg?

for that size building, all that is needed to heat the building evenly, and economically. is a single zone system. remember that the fuel used to recover temperature often equals that fuel which is not burnt during the setback. if you had a larger building, then maybe a zoned system would benefit. more likely a dual boiler system would be better.

do a search here for boilerpro's excellent treatise on sizing steam boilers. you may find that 2 boilers, with staged firing would give you the control you need.--nbc

will smith_4

Zoning

First thing I would do is pull the pigtails and clean them, then make sure the pressuretrol is working correctly. After that, figure out why the pressure is at 6#-I'd be willing to guess it's because the vents are not working. Check your main vents and your radiator vents. Set your cut-out to 1 1/2, your cut-in to 1/2, and see what happens. (Also helps to have an accurate gauge, of course)

As per control, I'd go with TRVs, with built in vents. Regarding sizing of the boiler, as you know, check your EDR.

If your client needs BacNet control, I'm sure they have a vendor they use to tie in to their systems. I'd let them decide how they want to integrate; it's up to them to determine what points they'd like to read and write to, and what components they need to do so. Just make sure you're in the loop so you know how to by-pass any BAS equipment on a zero degree day when it decides to go to go on vacation...

Brad White

Shawn, some thoughts

I have learned an awful lot on this site over the years in particular regarding what I would call "closed" steam systems, those operating in homes and where gravity and system pressure are the motive forces of return. But in my 30+ years on the design and engineering side, where you are, my dominant knowledge has been on the commercial, institutional and light industrial side.



One obvious difference is that the closed systems I described work best on what I used to think were inadequate pressures, ounces, not pounds, and the systems worked in cycles, not constantly, using typically high-mass cast iron to carry through between the cycles. Avoiding higher pressures has been a hard habit to break but once understood, the differences between system applications stands.



So to start the conversation: If you did not say that your terminal heating emitters are cast iron radiators, I would go on the assumption that the 6 psig you are dealing with is required for control valve authority reasons.  If so, that also means that your pressure must be maintained constantly because your demands can be anywhere at any time.



Conversely, in your case, because the system is all steam radiators, truly passive devices, low pressure again rules. It seems that you do not have any heat exchangers, air coils, unit heaters and unit ventilators, where the steam must be at the valves, at the ready.  Is this accurate? Any one of those and you are in need of the higher pressure and the radiators would need a separate cycling control valve.



Right now, I see you in a position to restore the system to it's original and presumptive low pressure, a pound or less may be possible. As a one-pipe system, yours is one of the larger ones I may have seen but the principles are the same.



What piqued my interest in the thread and prompted my response, is the use if a flex tube boiler rather than a cast iron boiler. Flex tube or any low water content boilers are what we would classify as "steam generators"- fast responding to meet a fast demand. The ability to cycle and maintain pressure requires much finer control (often in the main header rather than at the boiler itself or in addition to the boiler itself). That volume as a pressure buffer can ease the response times to manageable levels. Easier to maintain and control pressure in an 8 inch header vs. a 3 inch feeder or small steam drum you can see.



I see your system as operating on low steady cycles and absent a good large steam header as the controlled variable (met with several staged smaller boilers to meet demand or a very well controlled larger one). I am suggesting that a cast iron steam boiler with higher water content may be a better value. The water tube boiler median life is less than cast iron.  A middle ground, if you have the space, might be a Scotch Marine type boiler, but these have higher maintenance with all of those tubes to be cleaned, replaced and monitored.



Another facet to the cast iron boiler choice is water volume. You MAY be able to get rid of the boiler feed tank and return entirely by gravity if your dimensions allow and your pressures are held low. If you go with a flex-tube/water tube type with low water volumes, you will need the boiler feed unit and fine water level control. Volume is your friend. Elevation and gravity are your BFFs.



I think your rationale is sound regarding life cycle cost by the way. Your responsibility then is to tune the distribution, venting, get the pressure down and wring as much efficiency as you can out of it and shorten the ROI ahead of your projections.



I would be curious what your EDR count is and how this project proceeds!

TheRookie

Almost time for Full Steam Ahead

All,



Thank you very much for your comments.  My apologies for the delay in my response.  I (for whatever reason) didn't have internet this weekend and canceled my Sunday office visit due to the superbowl.  The half time show was mediocre at best.  Good game though. 



NBC to answer your question, this is a two story building with a full basement where my boiler is located.  Classic 30's commercial construction.  The project Doc's are really a piece of art.  No autoCAD in those days.  I've been pondering your comment regarding the amount of natural gas consumption (therms) of the building lining up with the buildings actual skin loss.  This must be due to the boilers actual run time?  I am apprehensive to eliminate the condesate return assembly as I have to expect that it was installed for a reason.  Maybe the sectional that was added during the 80's didn't have enough water volume to keep up with the steam demand?  The main reason this client prefers DDC is so they can monitor and adjust the building heating set points remotely.  Its a government entity, which really adds a whole new level of complexity.



Steamer7676:  The savings that you achieved for your client, where these quantified through energy bills or an active monitoring system?  Like a flow meter on the NG and such?  One thing that I would be curious about is how effective resetting steam pressure (based on OA) is, it seems to me that a steam system really doesn't have that much play on pressure? 



NBC regarding your second comment:  What size building would you expect to see zoning become beneficial?  Is a single zone still applicable even if this building has multiple stories?  One thing I can't quite wrap my head around is how steam system for light commerical use can effectively utilize multiple boilers.  Have you had much success with multiple boilers in steam system?



Will Smith:  Most of my steam work has been in the medical field where medium pressure rules.  Primarily because of the sterilization requirements of medical equipment.  I didn't realize that 6 psi was likely too high for this system, I guess that makes sense though, would it be safe to assume that the number of radiators was originally based around 1 psi steam?  Thanks for the tips on trouble shooting why my pressure is so high.  I wish this building was closer than 4 hours away so I could go tinker with it right now.  Insofar as the points that we will be monitoring, I unfortunately, will be in complete charge of that as this clients involvement in the design is typically minimal.    I don't forsee many points.  Boiler status, firing rate, steam pressure set point, maybe some points on the condensate reciever assembly, and then of course what my zone(s) are doing.  I think that economically speaking, TRV's are where its at for upfront cost reduction.  What do you think about looking at things over 20 years?  Life span of a ddc control valve compared to a TRV, etc..  I'll be sure that the end user has the ability to throw everything in Hand being as this climate is primarily heating (8176 HDD). 



Brad White:  As far as the steam set point and valve authority... I'd have to dig into that.  The first thing that comes to mind is that at some point a 100% OA air handling unit (wouldn't have been my choice) was installed and it does have a steam coil in it.  Which was set up with a two pipe system.  That could be why the pressure was bumped up.  I do not know of any other terminal devices that are not radiators.  The end user doesn't operate AHU because it is too noisy.  I almost could abandon that unit from this design.  Its redundant for heating and is there mostly for cooling.  Which this area doesn't have a lot of.  Sounds like I really should consider a cast sectional over a firetube mostly because I would like this system to be out of sight and out of mind for the end user.  Also, looking at how I'm going to even get the new boiler into the basement; it may be that we'll have to build in place.  I will absolutely pay close attendtion to the water volume as I really do think that there was some issues in the past with dry firing, I haven't verified this.    I can't wait to get the EDR of this facility as I am very suspicious that it wasn't even looked at when the current boiler was installed.  Basically using a sled hammer to sink a roofing nail.  I think the existing header is 6".  If I could get the system pressure decreased, as you said, there would be some saved dollars there.  Even better, if I can get decent pricing, there actually might be an ROI that is shorter than 100 years!



A random thought:



When should a designer look at variable pumping on the potential condensate return system?  My medical steam work dealt with very large facilities with a very diverse steam requirement, ideal for variable pumping.  This may not be a good application of that.  Just a thought. 



Thanks all for the comments and I'll post my progress once I can get my plate cleared and focus soley on this project. 

Brad White

More thoughts-

If you can dump the AHU and get back to pure radiator heating, you will be doing the owners a favor. Many radiator traps will not stand pressures higher than 3 psi for very long and you would need to reduce/regulate the pressure serving the radiators while preserving higher pressure to the AHU. If you can dump it, you are on the right path I think.



Yes on the EDR tally. Because you have the original drawings (love those!) keep them if you can. Good resource and the detail can be astounding and accurate.



As to your random thought on variable pumping- do you mean as a boiler feed approach or returning to a central receiver? Given the short run times (typically one minute to drain any receiver), the efficiencies and power usage are not of concern, just get the condensate back home.  If a constant flow of sorts, then one can map out an ROI. Maybe I did not understand the question or application? Let me know?

nicholas bonham-carter

condensate return pumping

if the condensate return was installed for a reason, then maybe it was done by knuckleheads. probably a messed up installation of the steam coil contributed to the mistaken view that there would not be enough water in the boiler to make adequate steam to fill the pipes and rads without running out of water. that and an increase in system pressure most likely interfered with the normal operation of vents and traps, causing dysfunction. if i am wrong, then thankfully the 2 lwco's you will have on the new one will keep the boiler safe!

if you look at the original plans you will see how well thought out the mechanical systems were in those days. people  [especially new building owners!!] would not have tolerated anything less than perfect temperature control. don't let the present condition of deferred maintenance affect the plans for the future renovations, except to improve, and return the systems to their original condition!

good luck in explaining all of this to members of some government department!!--nbc