Measuring edr:When to include pick-up

vaporvac

Hello,

I'm trying to size a new boiler and thought I had it all figured out correctly until I  reviewed some prior posts which talked about the pick-up factor already being included in the edr boiler rating. So now I'm confused.  I had taken my edr as calculated with the radiation reference tables and then multiplied that by 240 to arrive at the Btu net(1.33)=gross ouput.

In my case that is 1,460(240)=350,421(1.33)=466,060. Therefore, if my boiler is rated 84% efficient it needs a gross input of 554,933.33.  Is this correct?

I also wonder if someone could help clarify what a turn down ratio means in reference to a power burner and if all pbs have one? Still trying to make steam here in Cincy, so thanks for the help.

Colleen

RDSTEAM

??

why are you multiplying by 240?? get the EDR of each rad, add them together and multiply by your pick up factor of 1.33

Mike Kusiak_2

Calculations

Yes, your calculations are correct, but you are making it more complicated than it actually is. The net steam rating of the boiler already includes the 1.33 pickup factor, so simply compare the new boiler's net steam rating in sq ft to the amount of EDR you measured for your rads.



The turndown ratio is the amount the burner can be downfired from its full fire rating. If the burner can be downfired to 20% of its maximum, then it has a 5 to 1 turndown ratio. This ratio is usually specified for modulatiog burners, whose firing rate can be manually or automatically adjusted.

BobC

Nameplate net steam sq ft = radiator sq ft

I agree with Mike, match the square feet of steam the boiler is rated for with the square footage of radiation that it is feeding. Also insulate all the piping in the cellar.



Bob

Paul_11

What kind of building is this?

If you are thinking of installing a 500,000+ BTU boiler this must be a large building.

Is this  a single family or a multifamily building.

What is the square footage of the building?

There are many factors that go into sizing a steam boiler?

Connected load and heat loss are the two most important.

Square footage of the building has a lot to do with your heat loss.



Paul

.

vaporvac

I think I've got it, but...

Thanks for all of those replies.  I initially went through all of the calcs as my current boiler doesn't give the edr and I wanted to see how much over-sized it was. It was the boiler pick-up that was confusing me a bit, but now I understand that the edr just gives the output for the radiators themselves, while the boiler rating includes the pick-up factor to heat those rads and the pipes etc. leading to them.

However, do I still need to up the rad edr by 1.3 as suggested by RDSTEAM to compare it to the boiler edr or not?  I'm sure I am making this more difficult than it is, but that post confused me again.  Sorry.

Colleen

Mike Kusiak_2

No

If you are comparing the net steam rating of the boiler in sq ft to the measured EDR you don't multiply by 1.33, because that factor is already included in the net rating.



If you only have a gross output rating of the boiler (BTU or otherwise) them divide the gross rating by 1.33 to get the net rating, and then compare it to the EDR of your radiators. Your original calculation in your first posting was correct.

Jim Pompetti

EDR

We always do an EDR ,but recently (2 cases) the boiler had 2 stage burner, just for fun I set both boilers to low fire app. 60% of the EDR . They ran all winter , we had a cold one, with no complains . Although the boiler we use , had a larger water content, which we needed because of the size of the bldg . Many of the boiler are so over sized to begin with .after the EDR our replacement are 30 to 40% smaller, then the existing boiler . Any input?

Steamhead

Are these

one-pipe or Vapor systems? 

vaporvac

In answer to Paul...

Yes, it is a big building...about 7200 sq ft, not including the basement and other subterraneanareas.  The edr does include the ceiling hung rads for the basement and garage, although we've never used them.  They add about 98sq ft. to the number, but excluding them doesn't seem to change the boiler options. We're well caulked and insulated/vapor-barriered in the attic and basement with interior storms in the most "windowed" rooms and plans to add more throughout. Not sure what the air exchange or heat loss number is...still fairly high I'm sure. However, I did not take any of this into consideration in my calculation; I only used the attached radiation.  Hopefully it won't be too much as the current boiler only seems to be over sized by about 10%.  That probably explains the radiation reference tables given to us by the prior owners,  In addition, two bays of interior dividing French doors were removed from the foyer at some point, so until they're replaced a lot of heat escapes up the stairway and landing.   I'm hoping this gives me the "right " number. 

Thanks again everyone for the clarification

Colleen



.

Jim Pompetti

STEAMHEAD

Both systems were 2 pipe .One we installed the other was years old

Brad White

Not to confuse anyone but...

I agree with the advice that the net EDR boiler rating is usually the correct one to match to the standing radiation. The pick-up factor is included. The net steam BTUH output (EDR x 240), is a sometimes surprisingly small number compared to your input (and compared to hot water systems).



As with any rule, there are exceptions. I will depart from the 1.33 factor and size a boiler from scratch if the piping is larger than it needs to be (some old systems have 8 and 10 inch mains, working on two ounces of pressure, but still have to be filled). These pipes sometimes are not insulated and are inaccessible for insulating for whatever reason.



Another condition I have come across is in a church or building seeing sporadic use and a cold start takes on a deeper meaning. Some of these buildings get down into the 40's between uses. That is a pick-up factor personified.



In these and similar conditions or when the outcome is not certain, I will measure and assess my radiation then estimate the piping, total lengths by each size.  Factors from tables will readily convert the piping to EDR. I add that in and see how it compares to the factory net selection.



A factor of 1.5 might well be in my future, it may drive the boiler up a size, but at least I will know. For what it is worth, I never had to apply a factor greater than 1.5, although such conditions may well be out there.

Mike Kusiak_2

Back calculating EDR?

Brad, have you ever sized a steam boiler by taking measurements of the existing boiler as far as firing rate and efficiency, and back calculated the effective EDR of the rads and piping?



If actual physical measurement of the emitting surfaces is difficult or impossible, how close do you think you can get to to accurate sizing by this method? If with proper venting the system maintains a constant low pressure during continuous firing without hitting the pressuretrol limit, then the steam generation rate should be equivalent to the actual condensing rate of the piping and radiation. Then the optimum replacement boiler size could be reverse calculated from the existing firing rate and measured boiler combustion efficiency. This seems to me like a valid method if the situation involves concealed piping and emitters whose actual output may be positively or negatively affected by their enclosures.



What do you think?

Brad White

I agree with that approach, Mike

If you can determine the fuel flow and combustion efficiency, I think that is an essential way to size a new boiler when no nameplate data is existing.



What I like to do is triangulate, without making a huge project out of it. Find two or three means of getting to the same point and know how close you may be.



To give you one example, a couple of years ago, we designed the replacement of three 1925 Mills 350 boilers. One was shot, out of commission and was, for our entertainment, retrofitted with about the largest atmospheric tube burners I have ever seen. Never did see that one lit up that I recall. The other two boilers had regular burners, an IC and another I do not recall.



We had Mills data, even if new vs. the old coal fired days, knew the sections and that two carried the building. What we did not know was by how much was the second boiler needed. So we did clock the gas meter on a cold day, a snapshot in time, granted. We looked at the fuel bills, noting that the top floor was now served by a gas-fired rooftop unit. (The building was an old warehouse converted to offices and other functions including a lab.)



We had the original 1925 drawings with EDR loads, since modified plus there were many in covers behind casework, so unseen.



So in the end we calculated a heat loss for certain known spaces and compared how closely they sized the radiation to the spaces in a few areas and extrapolated that across the building. Came up with a number.



Took off the piping and how well insulated. Got a number there.



We took the fuel data and extrapolated that for a year normalized for degree-days. Got a number of some use there.



Used the Mills data with anecdotal observations that one boiler would carry the building down to so many degrees outside before the second boiler filled in.



Subtracted the areas where radiation was removed and the RTU served the space. Added back a heat exchanger for a local HW system. Tied that in to the various numbers.



Basically took about five different tacks and reconciled them to a common number within ten percent.



The only measure of success is in operation, with one boiler carrying the building down to the mid-teens and the second one staging in.



Still do not know the real EDR number. Maybe this is the engineering equivalent of base jumping?



But we certainly exercised due-diligence and the Owner agreed with our methodology, understanding the conditions.

BobC

Experience wins.

i wiuld call that empirically flying by the seat of your pants. I've done that several times in electrical engineering and it is often more accurate than just sitting down with the slide rule because you usually don't have all the numbers at hand.



That is where experience and a good eye trump all else.





Bob

Mike Kusiak_2

Optimizing boiler size

Thanks for the input regarding different methods of determining proper boiler size. It seems that the usual prevailing wisdom is to only use physical radiator measurements to determine EDR, rather than alternate methods which actually may give a more accurate result. In any case it is good to have an alternate method to confirm your calculations.



This all comes to mind as I am presently a volunteer advising a church on the replacement of their boiler. The church has received two estimates, with widely different size boilers specified. As you mention in your previous posting, churches are a special case, where normal operation is usually recovery from setback on Sunday morning.



 As found the boiler, a slowly leaking 60 year old Smith Mills 2000L was severely underfired, with recovery from 50F taking more than 4 hours. Maintenance has been done by the oil service company and as been haphazard at best. We have since increased the firing rate from 2.5 GPH to 3.5 GPH, resulting in a much shorter recovery, and steady state operation at 8 oz pressure with no pressure cycling during recovery. To my way of thinking, this is the optimal steam generation rate for the system, resulting in good pickup, yet not resulting in long periods of pressure cycling at the end of recovery. A graph of the present optimized operation during recovery is posted below.



My recommendation will be to specify a boiler with a capacity corresponding to the present parameters of operation, adjusted for the likely higher efficiency of a modern boiler. I have purposely not examined the proposals, and do not know the actual sizes of the replacement boilers specified, so it will be interesting to see how my recommendation compares.

Brad White

Prevailing Wisdom

Well, not the only way, but certainly the most reliable is to measure the EDR. As we agree, one cannot always do that, so we get a bit creative.



My only caution and this is a minor one, is to nail down the correlation between firing rate and steam output. Newer boiler should be more efficient, all the better, so the same firing rate should, at the same or higher efficiency, produce the same if not more steam in lbs. per hour. Cannot fault that even if it is nice to know what the connected radiation is.



As Bob C. said, experience trumps theory every time.

The old adage of the bumblebee is true- "aerodynamically speaking, it cannot possibly fly, yet buzzes around happily in his ignorance."