Garn - stratify? output?

[jebatty]

Question 1: to what extent does the Garn stratify in normal operation? Question 2: is my estimate of average realistic output reasonably on target?

I'm volunteering some time for a youth education organization which has installed a Garn WHS 3200 (3200 gal). We're working out some issues in their system, as it also involves 8 LP staged boilers, and now also a Garn and a Wood Gun. I'm estimating realistic, average btuh output at about 570,000 (950,000 btuh burn rate x .75 = 712,500 btuh output x .8 = 570,000 btuh average output). At a delta-T of 20F, gpm = 57. At this flow rate, to what extent if any does the Garn stratify? How good is my estimate of realistic, average output?

Thanks.

 

[TCaldwell]

jim, hopefully heaterman can answer, i have no experience with a 3200, with my 1900 witch is basically a 2000 gal capacity and a smaller combustor design i have recorded btu outputs over 770kbtu for limited time periods with the right wood, i would say the outputs you are computing might be on the modest side. also the garn is able to tolerate large delta ts , the percentage of return water with respect to the total volume of water in storage is miniscule, it is not uncommon to see a 25/40 deg delta t at the hx, allowing a lower gpm.

 

[heaterman]

I have not had the pleasure of working with a 32 yet so I'm jealous JB. I think I'll pout for a while........


There that's done.

If your flow rate calcs are correct you would be turning the tank over once every 56 minutes. My guess is that you will see some pretty decent stratification. Talking with Martin a couple evenings ago and he made the comment that depending on the heating system it's connected to and of course, flow rates, they have measured temp differentials as high as 50-60* occurring in only 2" of water column height. That's an extreme example but goes to show that a pretty significant thermocline can be established in the tank. He also said that once a fire is established any of their models regardless of size will turn over the whole tank in a matter of minutes. You can actually see the water flowing in the tank.

As far as firing rate goes, it's true that much depends on the wood quality and quantity but I've found their ratings to be more of an average over the entire burn cycle than a peak rating. As an example, we put a 2000 in a prison last year that housed about 400 inmates. The gas boiler firing the domestic hot water had a rating of 850,000 btu while the Garn is rated at 425,000 (fuel dependent) according to the installation manual. They found that in all but the most extreme load conditions the gas boiler would not fire. Extreme being 30-40 showers on at one time. Now to qualify that, being that they had a "captive" source of labor (pun intended) there was an inmate sitting there in the boiler room firing the Garn basically as needed 24/7 so the burn was constant and the output probably exceeded the factory rating by a fairly large margin. My guess from talking to the prison maintenance foreman and a back of the napkin type calculation is that it was doing close to 550 - 600,000 continuous. They were burning 4 pulp cords of wood a month through it.

One more thing. The 3200 is rated at 950,000 into storage per hour so that would be a net or actual input rating. No deduction for efficiency.

Also, the "fuel dependent" nomenclature in the manual is defined as follows. "Figures based on split 24" oak with 15% moisture and reloading once per hour."

 

[jebatty]

One more thing. The 3200 is rated at 950,000 into storage per hour so that would be a net or actual input rating. No deduction for efficiency.

The spec is "burn rate 950,000 btuh." I interpreted that to mean wood burned, not btu's into storage. OTOH, if Garn is conservative in their ratings, then I do need to up the estimates. In the application in which I am involved, not likely that continuous high firing will be provided due to limited staff, and Garn will be allowed to burn down at night, as well. I anticipate periodic daytime addition of wood but not highly attentive tending.

So far the operator has been reluctant to drive the baby above 170F (it even took urging to get the operator to go above 150F!). I've assured the operator that 190F is a reasonable high limit for operation, although some Garn users say they push 200F.

On the plus side, the heat exchanger was sized for 950,000 btuh at 60 gpm and delta-T=30 and approach temp of 10. I still don't want to provide advice based on a too conservative estimate of real world performance, so further thought is appreciated.

 

[heaterman]

One more thing. The 3200 is rated at 950,000 into storage per hour so that would be a net or actual input rating. No deduction for efficiency.

The spec is "burn rate 950,000 btuh." I interpreted that to mean wood burned, not btu's into storage. OTOH, if Garn is conservative in their ratings, then I do need to up the estimates. In the application in which I am involved, not likely that continuous high firing will be provided due to limited staff, and Garn will be allowed to burn down at night, as well. I anticipate periodic daytime addition of wood but not highly attentive tending.

So far the operator has been reluctant to drive the baby above 170F (it even took urging to get the operator to go above 150F!). I've assured the operator that 190F is a reasonable high limit for operation, although some Garn users say they push 200F.

On the plus side, the heat exchanger was sized for 950,000 btuh at 60 gpm and delta-T=30 and approach temp of 10. I still don't want to provide advice based on a too conservative estimate of real world performance, so further thought is appreciated.

Sounds like in your application a little "CYA" factor would be in order considering the variation in firing times. Better to err on the conservative side than be overly optimistic.

How exactly is are all those heat sources interfaced together?

 

[jebatty]

How exactly is are all those heat sources interfaced together?

Will provide info later; right now writing up an evaluation of the as-installed design for the Director of the organization, to be first reviewed by the organization's consulting engineer. Had a long meeting today with two engineers for the organization and the contractor. I'm on board only because I'm the only one with real world wood burning experience, although only with the Tarm Solo Plus 40, and this system is the WHS3200, plus the 500,000 btu WoodGun, plus 8 LP boilers.

 

[heaterman]

How exactly is are all those heat sources interfaced together?

Will provide info later; right now writing up an evaluation of the as-installed design for the Director of the organization, to be first reviewed by the organization's consulting engineer. Had a long meeting today with two engineers for the organization and the contractor. I'm on board only because I'm the only one with real world wood burning experience, although only with the Tarm Solo Plus 40, and this system is the WHS3200, plus the 500,000 btu WoodGun, plus 8 LP boilers.

Sounds like hydronic pornography to me Love to have my fingers in that collection of heaters

 

[Jim K in PA]

The spec is "burn rate 950,000 btuh." I interpreted that to mean wood burned, not btu's into storage. OTOH, if Garn is conservative in their ratings, then I do need to up the estimates.

You can take GARN's burn rates as input ratings, and you can take them to the bank!

In the application in which I am involved, not likely that continuous high firing will be provided due to limited staff, and Garn will be allowed to burn down at night, as well. I anticipate periodic daytime addition of wood but not highly attentive tending.

Jim - is there reasonable training of the operators happening? The GARN is "high firing" whenever it is running (fan on). There is no idling or modulation of the burn in a stock GARN. I am also unfamiliar with the WHS3200, so do not know if its controls are different than the WHS2000 (which I have). On the 2000, the fan timer essentially controls the firing, but only if there is fuel in the fire box. If untrained operators turn the fan timer to 5 hours and only 1 load of wood is fed into the fire box (no reloading), then the fire will liekly burn out long before the fan shuts off, and the unit will then start to cool itself by shedding heat out of the flue.

So far the operator has been reluctant to drive the baby above 170F (it even took urging to get the operator to go above 150F!). I've assured the operator that 190F is a reasonable high limit for operation, although some Garn users say they push 200F.

What is the concern about higher temps? The GARN is a closed, but unpressurized system, so the temperature of the storage water has no correlation to any safety concerns (unless you boil it and overflow water is pouring into the room). As outside temps get colder, I often run my 2000 up over 200, but I usually shoot for the 190-200 range. It gives you more stored Btus to draw from.

What temp are they drawing the unit down to now?

On the plus side, the heat exchanger was sized for 950,000 btuh at 60 gpm and delta-T=30 and approach temp of 10. I still don't want to provide advice based on a too conservative estimate of real world performance, so further thought is appreciated.

I would love to see some pics of this setup. If you are allowed to share them.

Keep us posted.

 

[jebatty]

Jim K - really appreciate your input. I will adjust my comments to the organization based on Garn providing 950,000 btuh to the system on high burn. I think I'm beginning to understand well the operation of the Garn and soon the same for the WoodGun. I will recommend mfr or other qualified professional training of the operators, but my bet is that they will avoid this due to probable cost and continue to look to me for guidance. At this time I don't think I will be far off on proper operation.

Reluctance to "burn the beast" is based on ignorance plus some really bad advice from the HVAC contractor with no experience with Garn/WoodGun burning knowledge/experience. The org's operators will come around.

The system still is in the "not-accepted" state, so "testing" is still taking place. Contractor already has agreed to make one critical change and I am bringing up other important "as-built" design issues for consideration by the org's director and other consultants before pursuing them with the contractor.

Will do the best I can, with the org's permission, to provide additional info, once the system is accepted. As not uncommon with 501(c)(3) org's, with govt funding, grants, reporting, and govt involvement, there are "political" issues in all of this, so I need to be a bit sensitive.

Thanks, again.

 

[Como]

As far as firing rate goes, it's true that much depends on the wood quality and quantity but I've found their ratings to be more of an average over the entire burn cycle than a peak rating. As an example, we put a 2000 in a prison last year that housed about 400 inmates. The gas boiler firing the domestic hot water had a rating of 850,000 btu while the Garn is rated at 425,000 (fuel dependent) according to the installation manual. They found that in all but the most extreme load conditions the gas boiler would not fire. Extreme being 30-40 showers on at one time. Now to qualify that, being that they had a "captive" source of labor (pun intended) there was an inmate sitting there in the boiler room firing the Garn basically as needed 24/7 so the burn was constant and the output probably exceeded the factory rating by a fairly large margin. My guess from talking to the prison maintenance foreman and a back of the napkin type calculation is that it was doing close to 550 - 600,000 continuous. They were burning 4 pulp cords of wood a month through it.

One more thing. The 3200 is rated at 950,000 into storage per hour so that would be a net or actual input rating. No deduction for efficiency.

Also, the "fuel dependent" nomenclature in the manual is defined as follows. "Figures based on split 24" oak with 15% moisture and reloading once per hour."

I looked at a 3200, but it is cheaper to go with 2 x 1500's and it gives you more flexibility.

4 cords is say 80m btu's net assuming they are using hardwood, is it 4 cords a week not a month?

 

[Como]

How does the Garn compare to the Wood Gun?

Is there any storage for the Wood Gun?

The first time I have come across a direct comparison situation.

I have pretty much decided to go with the Garn, the Wood Gun was second choice. Of those available.

 

[heaterman]

As far as firing rate goes, it's true that much depends on the wood quality and quantity but I've found their ratings to be more of an average over the entire burn cycle than a peak rating. As an example, we put a 2000 in a prison last year that housed about 400 inmates. The gas boiler firing the domestic hot water had a rating of 850,000 btu while the Garn is rated at 425,000 (fuel dependent) according to the installation manual. They found that in all but the most extreme load conditions the gas boiler would not fire. Extreme being 30-40 showers on at one time. Now to qualify that, being that they had a "captive" source of labor (pun intended) there was an inmate sitting there in the boiler room firing the Garn basically as needed 24/7 so the burn was constant and the output probably exceeded the factory rating by a fairly large margin. My guess from talking to the prison maintenance foreman and a back of the napkin type calculation is that it was doing close to 550 - 600,000 continuous. They were burning 4 pulp cords of wood a week through it.

One more thing. The 3200 is rated at 950,000 into storage per hour so that would be a net or actual input rating. No deduction for efficiency.

Also, the "fuel dependent" nomenclature in the manual is defined as follows. "Figures based on split 24" oak with 15% moisture and reloading once per hour."

I looked at a 3200, but it is cheaper to go with 2 x 1500's and it gives you more flexibility.

4 cords is say 80m btu's net assuming they are using hardwood, is it 4 cords a week not a month?

Week is correct. At least that is what I was told. Where the maint guy was getting his info from would probably be the inmates so............I'm just sayin..........

1 cord of the well seasoned red oak they were burning probably held close 32-34,000,000 btu. The 20,000,000 figure is per ton whereas a cord of red oak weighs about 3000-3300 pounds.

So let's crunch the numbers on that and see if the inmates were telling the truth. Maybe it'll give a clue as to the output they were getting from the Garn.
Assuming they burned an honest 4 cords we'll call that 132,000,000 btu. At 80% efficiency that yield would be 105,600,000 net. If you divide that by 168 hours per week you get 628,571 btu per hour. That jives with what I was estimating the output to be. ...........Well I'll be jiggered............