Boiler > BTU > Exchange > Storage: Oh My!!!

[ejhills]

Hello All-

The question is probably simplistic but regardless......

The Boiler is 138,000 BTU/hr
The Storage Tank is round 250 gallons and say 280,000 BTU.....

What size heat exchanger would be ideal?

Ed

 

[Hunderliggur]

1 BTU is the amount of energy required to raise the temperator of one pound of water one degree F. Water is about 8 pounds/gallon, so your 250 gallon tank is 2000 pounds of water. Assuming best case that you have a low temperature radiant system, usable temps MAY be as low as 110F to 120F. You max storage temp is going to be in the 180F-190F range, so your total maximum temperatur differential is 80F (190-110), which represents 160,000 BTU. More realistically, you should probably figure on 60F (180-120) which will give you 120,000 BTUs storage. If it is a pressurized storage tank the boiler water can be circulated into the tank with no heat exchanger required to more the energy fom boiler to storage. If it is a closed hot water heat system there is not extra heat exchanger required, just the distribution units (fan coil, radiant floor, baseboard, etc.). Take a look at the simplest pressurized storage sticky for more info.

 

[ejhills]

The storage is open.... a used oil tank only capable of 5 psi

So I'm thinking of something in a braised plate. The size is my main concern.

What are the effects of a boiler pushing 140 k at a storage tank of this small size through a hx of whatever size.....

ed

 

[Hunderliggur]

The choice of the open storage group here on the list is coppper tube in the tank 100 - 200 LF. I think nofossil has this setup with specs on his website.

 

[in hot water]

The HX is sized by the amount of energy you want to transfer and how quickly you want to transfer. If you go to the HX manufacturers websites they will have a program or design help to size for you.

Typically temperatures and flow rates (speed) are what they will ask. Of course larger HX with more surface area move more transfer and work with close approach temperatures where both A and B side are near the same temperature. But no sense in grossly oversizing $$ if you can provide the info an exact size hX can be selected.

A plate style HX with two pumped flows is easy to model and size.

But copper coils in a tank are much harder to define. The size, shape of the tank, mixed or still fluid in the tank, etc. There are some fairly accurate sizing formulas, based on years of experience, available from the folks that build storage vessels with coil HX. But for a homemade tank of a different design, start with the rules of thumb available in different threads on this site.

hr

 

[Nofossil]

I'd suggest that the proposed storage is WAY too small to be useful. in my never-so-humble opinion, the primary benefit of storage is to let you build one fire per day (or longer). In order to do that, the storage has to hold a significant fraction of a day's average heat load.

Mine is just barely big enough to be helpful. Keep in mind that with an unpressurized tank, the effective storage is less because you have a hard time transferring enough heat through the heat exchanger when the tank is near the boiler outlet as well as when the tank is near the baseboard return temp.

I have 880 gallons of storage. I figure that my maximum usable storage is about 360,000 BTU (170 degrees to 120 degrees). Practical storage is more like 300,00 or so.

Here's how a typical day works out:

I need about 15,000 BTU/hr on an average winter day plus 60,000 BTU for DHW and the hot tub - total of 420,000 BTU.
My EKO is rated at 80,000 BTU/hr, but actually delivers an average of around 60,000 BTU/hr over the course of a burn.
That works out to a burn time of about 7 hours on that 'average' day - that gives me my 420,000 BTU.
Of that, 60,000 BTU goes into DHW and the hot tub, and 100,000 goes into heating the house during the time the boiler is running.
That leaves 260,000 that goes into storage and heats the house for the other 17 hours.

I try to have the fire end with around 300,000 BTU in storage so that I have some flexibility about when to start the next fire. If I think I can skip a day, I'll 'push' the storage temp a bit higher.

 

[Hunderliggur]

Thanks nofossil - I knew you coul explain it better. Your new(er) graphs are really good. What package do you use for the graph (something flash based it appears).

 

[Nofossil]

Thanks nofossil - I knew you coul explain it better. Your new(er) graphs are really good. What package do you use for the graph (something flash based it appears).

I've been playing with OpenFlashChart - works pretty well. I have other cool stuff in the works too.