Heat exchanger

[Kilone]

I want to build a heat exchanger for use in a house.
My source of heat is quiet low, my aim. is to try to come up with a heat exchange to heat the water in the heating system.
What liquid could I use to bring the water up to a decent temp in a heat exchanger.
Is this possible or am i mad?

 

[Zoobie]

Before we can give you some real useful advice, please provide some details to better understand the problem/opportunity. What is the source of heat? At what temperature is it? What temperatue is the water in the heating system at? This will help scope things out a bit better. Then we can tell you if this is possible. To diagnose your madness however will require different expertise.

Why would you not just exchange heat directly with your heat source? Using another heat transfer fluid will not allow you to 'upgrade' your low level heat and it means that you will need to heat exchanger to get the job done (source to fluid and fluid to water). If your heat source is only a few degrees warmer than the water you want to heat than you will have a fairly large heat exchanger.

Without knowing more about what you are trying to do, I suggest that maybe a heat pump may be a possible alternative. Just google 'heat pump' for more info.

 

[Kilone]

Thanks Zoobie,

Another way of asking my question is:
If i heat a temp transfer liquid eg oil and run this through a exchanger to heat water, will it take less energy to heat the water in this way than to heat the water directly.
My heat source is 300watt DC elements,supplied from a wind turbine, and I am trying to heat the water the best possible way.
It is for the heating system in my house (rads), at the moment it is not possible for me to install a heat pump.
What ever temp I achieve will be then boosted with my oil boiler thus cutting down on the amount of time the boiler is in use (cutting down on oil useage)

 

[Zoobie]

Thanks for the update. I think I understand the problem a bit better. First of all, to raise the temperature a certain amount of a certain amount of water requires a certain amount of energy. I bet that made a lot of sense. The equation that describes this is:

Heat = mass x specific heat x temperature difference

Basically this means that it doesn't matter if you want to heat your water with an electric element or with hot jello pudding you will need the same amount of energy. Certain fluids will transfer heat more effectively but in this case I would say it is irrelevant (this will effect the size of your heat exchanger).

Besides added expense, using another heat transfer fluid will likely mean additional heat losses in your system. However, if the source of heat and the point of use are a great distance there may be an arguement for a more complex system. Another reason to look at a heat transfer fluid would be if your heat source and heat medium are not directly compatible. I won't elaborate much on this but things like skin temperatures, coking, corrosion, scaling could come into place in an industrial installation.

A few words of caution though with heating the water directly. Consider steam/pressurization, scaling of heating elements, corrosion and how this could affect your system. Otherwise, basically what you have is an electric water heater which have been used in homes for decades. In fact, maybe you could retrofit one of these for your situation.

 

[TBP]

I'd look at an electric hot water boiler powered off the windmill. Connect the piping in parallel with your existing oil fired boiler, and arrange the controls such that you can make either one the lead boiler. You could even consider an indirect domestic water heater with some excess storage capacity and possibly a mixing valve arrangement so you could throw heat into it when the wind is blowing, but there's no demand for space heating.

 

[Kilone]

This is exactly what I want to try,
My electric element will heat the water up to a certain temp( 25-30 degrees c, and the boiler takes it up to 60-70 degrees c, thus reducing oil usage.

 

[TBP]

Except on start-up from cold, your existing hot water heating system will never see water temps as low as 25 - 30*C. If you have entering water temps in that range for any length of time on a gas or oil fired boiler, the flue gases will condense. This will result in the fire-side of your boiler will corroding, unless it's specifically designed to be a condensing boiler.

There's more to this than just throwing some heat into water. I think you need to do quite a bit more reading, or find somebody who works with hot water heating systems for a living, and hire them. If nothing else, the electic hot water heating device needs to have have some safety interlocks, and above all, a properly sized and selected relief valve. You might try posting your question on the HVAC board.