Heat exchangers

[Joatmonbst]

Hello
I'm modifying our heating system. We have a log burner putting out approx 60kw into a large water tank (about 7500galls) The system appears to be corroding quite badly - I have used steel, S/Steel and copper + somewhere I have a leak so the water is being changed allot. Currently this water is being heated directly - the water circulates through the burner. I plan on putting a large copper coil inside the tank to heat it indirectly.
How do I work out how many m2 of copper to use? The formulas I have found look very complicated. But I think it boils down to 400w/m2 for each degree C difference?
I'm hoping that I don't need quite as much pipe work as that simple formula suggests.
Any help greatly appreciated.

 

[workerant]

don't we usually call tanks with 7500 galllons in it a swimming pool?

 

[Joatmonbst]

The tanks are actually old milk tanks(running at 60 to 80. We draw large amounts of heat over a period of 20 mins to warm milk up. A larger tank is also used for this purpose this is kept at about 28 to 29. This heat store happens to be about the right size and temp for a swimming pool.
Heating this tank/pool is achieved with an old s/steel plate exchanger.

 

[Norcon]

Why not use a suitable sized plate heat exchanger for the other tank as well instead of a coil?

30m of 22mm copper coil will transfer the energy you require.

Does the wood burner have a loading valve fitted?

 

[Joatmonbst]

Why not use a suitable sized plate heat exchanger for the other tank as well instead of a coil?

1, Because the tank water will be les turbulant and will stratify easily, I may have cold water at the bottom whiile the top is good and hot.
2, I will only need one pump instead of 2.
3, Probably cheaper unless someone gives me one.

Does the wood burner have a loading valve fitted?
Probably not as I don't know what that is! The burner is a Farm 2000 log burner.

30m of 22mm copper coil will transfer the energy you require.
Thanks - Is that from experiance or worked out. If so what is the formula - in it's most basic form.

Cheers[/quote]

 

[Norcon]

From the MI's............

A pumped shunt system should be included to prevent back-end corrosion, improve combustion and overall efficiency. A small domestic circulating pump can be used, controlled by a thermostat installed on the main return pipe. The thermostat should be wired to maintain the return temperature above 57°C. With accumulator systems the shunt is controlled by flow temperature (i.e. sensor adjacent to outlet).

Does your installation have the above?
A loading valve is another option. Search Laddomat and Esbe solid fuel products.

The coil size is from calculation.

Q=U x A X DeltaT
Where Q is the heat transferred in KW per unit time, A the area available for the flow of heat, and DeltaT being the temperature difference between the two fluids and U is known as the overall heat transfer coefficient which for copper coils in water is 0.4-0.57kw/m2.k.
That's it in its most basic form resulting in a heat exchanger capable of transferring roughly 82KW @70c DeltaT.
It would need to be designed in a certain way to acheive anything near that out put though!

If there is currently no method preventing low return temperatures to the boiler then why the need for any heat exchanger and just address the control aspect as regard to protecting the boiler.
This is probably where the corrosion is occuring!

The thing to remember about a coil is that its output reduces as the store is heated. In an ordinary boiler with a fixed kw output the boiler will cycle on/off on the electrical control stat so energy transfer is slowed down.
I would definately maintain the direct re-heat if at all possible. If not then a plate heat exchanger instead of the coil.

The benefit of the loading valve is a controlled thermal layering re -heat and the full output of the boiler (when connected directly) will not be compromised.

You should ideally contact Farm 2000 for advice before you proceed with a coil heat exchanger!

Which exact model do you have and what fuel are you burning?


How have you positioned the milk tanks? Have you stood these huge tanks on their end? Is this a food process heating the milk?
Milk is usually cooled at the dairy farm.
Just interested.

 

[Joatmonbst]

That's great information. I'm beginning to understand most of it now.
The Boiler is a Farm 2000 HT60 burning nice clean larch logs about 1m long.
It does have a shunt pump which I have just checked to make sure that it is working.
The tanks are milk tanks originally used for storing cool milk, they are mounted horizontally with a diameter of 2m. For this reason any highish volume pump will create some stirring.
They sit about 3m above the ground level of the boiler and I was using convection to circulate the water with a pump that boosted the water when it gets too hot. I did not realise that low temperatures assist corrosion however I don't think this is a problem. Our water is quite acidic as we are surrounded by peat bogs. One reason for putting in an exchanger is so I can treat a smaller volume of water - (I still have to find out the information about this)
It was very useful to find out about the minimum temp for the boiler as I was working with the assumption that it would be more efficient to have the water surrounding the burner as cool as possible. A high DeltaT.

I had found that formula before but did not really understand it. I do now thanks. The thermal heat transfer figures are a bit higher than the others I have seen. Does it depend a bit on the movement of water either side of the copper?

The milk is warmed up to make cheese. I use the same plate heat exchanger to cool the milk as well as heat it. When the milk is being cooled it is stored in the warm water tank (A small proportion of it anyway) It's then used to heat the milk again when the cheese is being made. The exchanger has 2 sections for the water one with warm water in the other is hot.
The whole system needs allot of tweaking to make it more efficient though. The tanks are simply the ones we had and are covered with 800 sheep to keep them warm! At the moment Only one 3000 gallon tank is used but I am unable to make any modifications to it as it's being used all the time. The other is 4500 galls and was going to be used to store iced water in , not enough whisky though as the distillery shut down for a year.

Thanks

 

[Joatmonbst]

The heat is stored in the warm water - not the milk. unfortunately Cleopatra does not bathe in the warm milk here

 

[Norcon]

The thermal heat transfer figures are a bit higher than the others I have seen. Does it depend a bit on the movement of water either side of the copper?

To quote from Coulson&Richardsons (Chemical Engineering Volume 1)

The value of the (Heat transfer) coefficient will depend on the mechanism by which heat is transferred, on the fluid dynamics of both the heated and cooled fluids, on the properties of the materials through which the heat must pass, and on the geometry of the fluid paths.

Other factors play a role also such as laminar/turbulent flow.
Many of the European Thermal stores have large diameter stainless steel ribbed coils in order to increase turbulence and enchance the heat exchange.

My calculation oversized the coil by about 20%.
That wasn't enough though.

Your boiler has an output of 120KW when using dry seasoned wood!
You may find a plate heat exchanger is as cheap to purchase as the fabrication of heat exchange coils.

All very interesting.

 

[Agile]

Based on my experience of my friends wood stove which is perhaps about 12 kW, it takes about 45 min to cut up ready felled wood to feed his stove for 2-3 hours warmth during the evening.

As this boiler is operating at 60 kW then that would be proportioned to four hours then reduced to about 90 min as your wood is used in longer lengths.

But thats only cutting the ready felled wood ! To include felling, hauling the wood and feeding the boiler it would almost a full time job if this is a continuous or regular operation.

Perhaps you buy in the wood but even so I would be surprised if gas or oil would not be more economical. If you had a large herd of cows then perhaps their waste could provide enough gas to fuel the heating? Apparently each cow can provide about 3 kWh per day!

Tony

 

[Agile]

don't we usually call tanks with 7500 galllons in it a swimming pool?

It sounds even better of you say its 34,000 litres!

 

[Joatmonbst]

It takes 30 to 40 mins about 6 days a week.
I don't know how long it takes to fell the wood about 3 men work the forests around here full time but then they cut down many thousands of tonnes every year.
Delivery to us takes 1 working day to deliver 125 tonnes.
The only gas is in small cylinders.
The wood costs about £20 a tonne + 20 to 30 mins to cut it up.

It's interesting you mentioned anaerobic digestion. It's not really to far from the topic of Heat Exchangers as they have to use very large ones.
We looked into it about 10 years ago. When we find 200 to £300,000 to heat the house it will be investigated again. AD systems need food to digest, our cows have already digested theirs. Supermarkets are a better source of this energy.

I think that the first digester has just gone on line to actually feed the methane back into the Gas mains, either that or it has got planning permission. A lot of plants get turned down because of local objection to the thought of a lot of odours wafting around. ironically one of the reasons we were considering it was to reduce the smell!!
The whole digester has to stay warm for 1 to 2 months Some of the heat is recovered with large heat exchangers.

 

[Joatmonbst]

Much appreciated Norcon.
Thanks

 

[Agile]

If the heat is only required for 30-40 minutes a day for six days a week then you seem to be producing a massive amount of heat burning all that wood which you have to pay for and cut up.

Are you sure it would not be cheaper and less labour intensive to use bulk gas delivered by tanker?

Tony

 

[Norcon]

I think what Joatmonbst meant was that 30-40 minutes is spent per day re-loading.

Not 30-40 minutes of heat!