Buffer tank for biomass boiler

Thick Farmer

Member
Location
West Wales
I want to put a buffer tank on my biomass boiler system.

Will a steel diesel tank be strong enough or will I need something more like a propane storage vessel?

Has anyone tried this before?
 

f0ster

Member
a diesel tank will not be tested for pressure, so unless it is going to be a vented system you cannot use it. they have to stand 3.5 bar max working pressure so the test pressure will be a lot more than that
 

Thick Farmer

Member
Location
West Wales
It's a sealed system. The boiler is 900Kw. I was thinking that fitting an expansion vessel would take up any slack and stop the tank blowing up? If not, I assume a gas tank would be ok and possibly not require an expansion vessel.
Thanks.
 

f0ster

Member
with a sealed system you will definitely require an expansion vessel, if you have pressure in the system when hot as soon as it cools a little bit the pressure will be gone causing a vacuum which will suck in air via the auto vents, there no way a diesel tank will withstand 3.5 bar, it will go pop, also a pressurised system is under pressure to raise the boiling point of the water, for each 1lb pressure you raise the boiling point by 1 deg. larger systems often have an automatic pressure set that keeps a min pressure automatically. for a 900kw system you are better off with a large buffer tank and an expansion vessel. it makes your boiler stop/start less often.
 

Thick Farmer

Member
Location
West Wales
with a sealed system you will definitely require an expansion vessel, if you have pressure in the system when hot as soon as it cools a little bit the pressure will be gone causing a vacuum which will suck in air via the auto vents, there no way a diesel tank will withstand 3.5 bar, it will go pop, also a pressurised system is under pressure to raise the boiling point of the water, for each 1lb pressure you raise the boiling point by 1 deg. larger systems often have an automatic pressure set that keeps a min pressure automatically. for a 900kw system you are better off with a large buffer tank and an expansion vessel. it makes your boiler stop/start less often.

That's more pressure than I had thought! Would I manage without the tank? I would only be using it every now and again for grain drying and surely it will take all day to heat the water in the tank if I go down that route? I'm not too fussed about stop/start as I'll be there feeding the dryer so might as well feed the boiler too!
 

f0ster

Member
a biomass usually has some sort of back end mixer so as to keep the boiler at a min temp of about 50 deg. it is essential to keep the boiler above this min temp to stop condensation inside the boiler which causes corrosion and shortens its life span, they also usually have a start up temperature for the boiler pump of about 50 deg so if the boiler drops below 50 the pump stops so as to maintain the min temp. a biomass boiler will usually heat up a buffet tank in about 3-4 hours if the buffer is correctly sized. if you have enough demand and a pump that can deliver the heat generated to the load then you might be able to get away without a buffer but it is far better if you do have a buffer, 900kw is a lot of heat and a pump that can deliver that amount of heat and keep the boiler cool to prevent it from start/stop all of the time will be very expensive, with a buffer tank it can be left hot when there is no demand due to the boiler will heat it up so as to be ready for immediate use. a buffer will stay hot for quite a long time if there is no demand on it.
 

f0ster

Member
it depends on what space you have available, any reasonable size will work but the bigger the better, you need to decide between size and cost and strike a balance. also how many m3 of water you are drawing off it has a bearing on size.even a 10,000 L is a monster tank, not everyone has the space. lots of biomass boilers can modulate their output down to about 30% so this makes things a bit easier if the buffer is a bit on the small size.
 

Still Farming

Member
Mixed Farmer
Location
South Wales UK
It's a sealed system. The boiler is 900Kw. I was thinking that fitting an expansion vessel would take up any slack and stop the tank blowing up? If not, I assume a gas tank would be ok and possibly not require an expansion vessel.
Thanks.
Gas tank !- only looks like an expansion vessel not work like one .
In an expansion vessel there is a rubber membrain between different pressure's to maintain it.
 
it depends on what space you have available, any reasonable size will work but the bigger the better, you need to decide between size and cost and strike a balance. also how many m3 of water you are drawing off it has a bearing on size.even a 10,000 L is a monster tank, not everyone has the space. lots of biomass boilers can modulate their output down to about 30% so this makes things a bit easier if the buffer is a bit on the small size.

I think one of his is a batch boiler so makes sense to have somewhere to dump heat.

Gas tank !- only looks like an expansion vessel not work like one .
In an expansion vessel there is a rubber membrain between different pressure's to maintain it.

@Thick Farmer likes living on the edge though :LOL:
 

f0ster

Member
if you have a batch boiler (ie log boiler) it is more important than ever to have a buffer tank, once lit you have a very large amount of energy producing heat and it has to go somewhere, with a pellet/chip boiler you do not have such a large quantity of fuel in the burner at any one time but with a log boiler you can have quite a large amount of energy burning and it must be dumped somewhere.
 

Thick Farmer

Member
Location
West Wales
if you have a batch boiler (ie log boiler) it is more important than ever to have a buffer tank, once lit you have a very large amount of energy producing heat and it has to go somewhere, with a pellet/chip boiler you do not have such a large quantity of fuel in the burner at any one time but with a log boiler you can have quite a large amount of energy burning and it must be dumped somewhere.

So if I was to run the just chip boiler and regulate the chip input to match the thermal draw from the grain dryer, I assume that (in theory) I would get away with connecting the boiler straight to the heat exchangers
 

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