Town Garage/ Coal Boiler Project

I think the btu content as received is more like 12,250 btu/lb, and the btu's you get from the feed rate need to be adjusted downward for the percentage of unburned coal in the ash (typical allowance around 10%). Those two factors should help the numbers line up.

Mike

Mike, the literature I have actually calls out the btu/lb as Scott had it listed. I don't disagree that you may want to hedge that a little, but based on what I see they used 13,000 BTU/lb to come up with their output figures.

The literature I have provides a set of correction factors to use to adjust output ratings for actual coal heat content below 13,000 btu/lb. It looks like you should reduce the published ratings by about 6%.

Mike

How do you come up with 6% ? They could easily have that much buffer in whatever they used for an efficiency figure?

I do not expect they sold these boilers for commercial applications with much of a chance that they would fall short of the mark...but it would be good to know what the intended coal source is and how it stacks up to their cut sheet.

So I guess the short answer is there are other factors, and believe the manufacturers published ratings

Rob, Van Wert appears to compute the adjustment factor as (13000/actual btu per pound), so 13000/12250 = 1.06122. The values computed by VW are in the last row of numbers on the 10th page of the 15 page VW manual circulated as a pdf a few years ago.

ScottB, I think the short answer is that they show you how to adjust the ratings for less heat in the coal, and the adjusted numbers are what you should count on.

Mike

Some manufacturers are more transparent than others with their ratings and what they base it on. I think Van Wert probably wins in that regard, but most of the old stoker boiler companies were pretty clear on what they expected the unit to do. I have been on this forum nearly 10 years, and I have yet to see someone come up short with a EFM/AA/AHS/Van Wert that was matched to the heat load, and correctly installed.

I agree that if you get a TT load of poor coal the output of the boiler will be reduced. Buy your coal from a reputable source that routinely tests their product, and do not choose a boiler with a NET hot water rating below your calculated heat load.

My guess would be VW's lawyers put that in precisely to protect themselves against somebody using lower-grade coal and trying to hold VW accountable for failure to achieve the rated output. I agree it produces transparency, and almost certainly greater accuracy in the capacity estimate.

Mike

I don't mean to interrupt, but how does giving an inflated BTU value per pound of coal give you better "real world" heat estimates? I'm confused.

Using the 13,000 btu/lb provides an "absolute maximum"-type of capacity number, and the adjustment VW provides enables that number to be tailored to the actual heat content of the coal used. In past discussions of the rationales for having excess boiler capacity, coal that falls short of expected btu content has been one of the reasons. Here VW has been open about the btu content needed to achieve the rated output, and the actual output you are likely to get with lesser coals.

Mike

I see now, thank you

You're welcome.

Mike

ScottB wrote: ↑

Tue. Dec. 05, 2017 12:44 pm

When reviewing the Van Wert literature I noticed the net BTU ratings were 15 - 18% lower then what I had calculated using the the specified feed rate of the boiler (BTU/ hour Net = (feed rate/ hour x Coal BTU(13000) x efficiency ( 0.80))/ pickup factor(1.15). Is there another factor I'm not considering. Also, anyone have specs for larger EFM units, 900, 1300 specifically. A recent trip south to see a couple of installations has sold me on the idea of going with a refurbished underfed unit like a Van Wert, or maybe a large EFM unit, depending on what is available that would meet our needs in our time frame.

I don't think a 900 or 1300 is going to get you anywhere close to 500k btu/hr gross output. Somewhere I have a sheet that gives those specs, but until someone finds that, for a 900 (named for its rated net steam capacity) you can make a pretty good estimate of gross output as:

900 sf net steam capacity x 240 btu/hr/sf steam x 1.33 steam pickup factor = approx. 287k btu/hr.

For a 1300, the same exercise yields:

1300 x 240 x 1.33 = approx. 415k btu/hr.

You can use the kinds of numbers you already have been using to estimate parameters for a 500k gross output unit. Going with your 0.80 efficiency factor, I would estimate the coal feed rate you need as:

(500,000 output btu/hr)/(0.8 efficiency) = 625,000 input btu/hr
(625,000 input btu/hr)/(12,250 btu/lb AR) = 51 lb/hr burned
(51 lb/hr burned)/(0.9 lb burned per lb fed) = 56.7 lb/hr feed rate

To do that with an EFM underfed stoker, you are talking about a 60R or 85R conversion unit mounted in a fairly serious boiler. This is starting to feel more like home now.

Mike

ScottB wrote: ↑

Tue. Nov. 21, 2017 11:20 pm

The heat loss calculation indicated the BTU loss/ hour to the outside environment at the design temp. For my calculation I used a design temp of 80 deg. F. I'm thinking to make up for the BTU loss at my design temp, the boiler would have to have a gross output that was greater then my calculated loss multiplied by the system pickup, or 340k BTU x 1.15. If my loss calculation is in the ballpark, a boiler with a gross output close to 500K BTU or greater should satisfy the requirement with some margin.

Why did you use 80 degrees? That seems pretty warm for a truck shop and office. What did you allow for an outside temperature?

Pacowy wrote: ↑

Wed. Dec. 06, 2017 11:33 pm

I don't think a 900 or 1300 is going to get you anywhere close to 500k btu/hr gross output. Somewhere I have a sheet that gives those specs, but until someone finds that, for a 900 (named for its rated net steam capacity) you can make a pretty good estimate of gross output as:

900 sf net steam capacity x 240 btu/hr/sf steam x 1.33 steam pickup factor = approx. 287k btu/hr.

For a 1300, the same exercise yields:

1300 x 240 x 1.33 = approx. 415k btu/hr.

You can use the kinds of numbers you already have been using to estimate parameters for a 500k gross output unit. Going with your 0.80 efficiency factor, I would estimate the coal feed rate you need as:

(500,000 output btu/hr)/(0.8 efficiency) = 625,000 input btu/hr
(625,000 input btu/hr)/(12,250 btu/lb AR) = 51 lb/hr burned
(51 lb/hr burned)/(0.9 lb burned per lb fed) = 56.7 lb/hr feed rate

To do that with an EFM underfed stoker, you are talking about a 60R or 85R conversion unit mounted in a fairly serious boiler. This is starting to feel more like home now.

Mike

First, I'm not sure why you are talking about steam. From the start I have been referring to a hot water system. The Van Wert VA2400 is rated at 529.3 net BTU at a feed rate of 60 lbs /hr. This output would exceed the total BTU heating capacity we now have in the current structures (510K btu). To be more specific, in the building that will remaining, we have a 230k btu furnace and I have confirmed the propane units add up to 50K btu (10k + 15k +25K). That totals 280k btu. That would leave 250k net BTU to handle the new 6000 ft very well insulated structure. We have dropped the idea of heating wash water so that's no longer a concern since we already have and use diesel powered units. From my observation over the last 6 years, our use of DHW is very minimal, much lower then a typical household, just used for hand washing. Since the availability of the large Van Wert is in doubt, I have asked in a previous post for specifications for the larger EFM models.

Regarding specifications, I'm inclined to believe and go by the manufacturers ratings, as long as I diligently size based on a realistic heat loss calculation, and prior heating system sizing of existing structure. I see no need to derate the manufacturer ratings any further. On the subject of heat loss calculation. I believe my previous stab at a heat loss calculation was realistic for the loss through walls, ceiling, windows, and doors, but I now feel that I underestimated air infiltration in the garage portions of the structure. I've not found a lot of information that deals with that.