Aren Hellum

Amos Valenti wrote:Natalie: Our thinking was that if a stream of hot air was forced into a body of cold water, the temperature of the water would rise. How much and how fast are the variables. Im not sure about the relationship between the bubbles rising and the heat escaping but I see how one came come to that conclusion. The bubbles, in this case, benefit to our AP system by adding more oxygen into the water.

This is all correct, and perhaps I can add more details and some context to my prior comments.

1.) Heat transfer by the gas to the water is foremost determined by the temperature of the gas and its flow rate. There are some additional factors involving the bubbling that I'll discuss a bit later.

2.) In an open system of the type you have described, the back pressure on the pump moving the hot air has to work against the friction in the pipe and the depth of the water in which the open end sits.

3.) Pipe Friction: The friction in the pipe goes up as the flow rate goes up, and as the diameter of the pipe goes down. So moving air fast down a skinny pipe means that your pump will have to work much harder than pushing air slowly through a fat pipe.

4.) Immersion depth of open end: the deeper it is, the harder your pump must work. However, the deeper it is, the longer that your bubbles have to transfer their heat to the water. So there's a trade-off here. A closed system does not have to pay this penalty, but you won't get oxygenation, and you will need a pump that can handle whatever temperature gas comes BACK from the water, if you can't dump all the heat.

5.) Bubbles: smaller bubbles will transfer their heat more efficiently to the water, because they have a larger surface area/volume ratio. Also, smaller bubbles will take longer to rise for essentially the same reason (more resistance per buoyant force on the bubble). However, it effectively takes additional pump energy in the form of using a smaller pipe, or using a bubbler tip at the end. Again, a trade-off.

Your break-even in terms of input energy is that you need to get more heat energy from the water than you are using to pump the air, less whatever energy you save by not having to run a different oxygenation system.

Amos Valenti wrote:Hello everyone,

There is a lot of interest in heating water for different applications. In our case, the RMH is used for heating a greenhouse and we have always wanted to be able to safely heat the water in our fish tanks. Our solution is as follows:

Coiling a pipe around the outside of our drum (55 gallon stack). On one end of the pipe is a pump that pumps only air through the coil around the stack. The other end of the coil goes directly into our fish tank.

Does anyone think that the stream of hot air going into a mass of water will have any kind of significant impact on water temp?

From your description, I'm assuming that you've envisioned an "open loop" system, is this correct? That is, your blower takes in ambient (greenhouse or outside) air, and expels the hot gas into the tank? If this is the case, my suggestion is to use a closed gas loop, in which the same hot air runs from the barrel to the tank and back. If I've got your proposed design wrong, I apologize.

What would be even better, if you can arrange it, would be to use the fish tank water as part of the thermal mass. It would be possible to protect your fish in such an arrangement with a little math, but water sealing at high temperatures might be a little tricky. From your posts, it sounds like you've already got the RMH built and you're just trying to wring a little more out of it. If you can post/direct me to a picture of your setup, I might be able to be more helpful.