Forcing air around stack to heat water

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?

Hi amos; How big a pipe are you using? If you are running your rmh enough it should have an effect. The problem I see, is it taking all winter long to bring the water temp up. Keeping your greenhouse air temp up will probably do more. I am also trying to learn how to safely heat water, why not ,we have all this heat!

Hi Thomas! The pipe we plan to use is 1/4in stainless tubing. My stack is just shy of 5' from the ground so the pipe will cover a lot of hot surface. Sometime this month we are going to start experimenting and I will be sure to post results.

amos; Have you checked barrel temps? mine will hit 900 degrees on the side of the barrel when the dragon is at full roar. Seems like that 1/4" pipe will be a scary bad burn if you bump it! Also I think that 1/4 may be way small , your trying to move non pressurized air , think big. You also may want to have a way to keep your fish back from super hot pipe ? Tom

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.

Thomas: Our thinking behind using the 1/4" pipe was that we could expose more surface area of pipe to the stack aaaand someone was kind enough to donate a good length of it to our project

I left out a detail in the OP. The water that we plan to warm is a reservoir that houses the pumps in our AP system and not the actual fish tank.

Safety is always a concern when dealing with extremely hot surfaces and we will take the necessary steps to ensure no one is hurt. In this case we plan to wrap the stack tightly with the pipe and bury the hot air line in the ground as it travels to the reservoir.

Aren: You are correct. The system we had come up with was designed as an "open" system. The extra bonus to directing a stream of air into our reservoir is that we get the extra aeration into our water.

Since you mention the closed loop my mind immediately started projecting images of what that might look like and started to have other thoughts.

I wonder... Would the tubing act as a traditional heating element that would be found in a residential water heater? Would using a closed system eliminate the need for a electric pump? Would the "pump" then be the rapid cooling of the gasses as the hot tube entered the cold water?

Our original plan was to use the water as a thermal mass. In our case we have only come across a few solutions in the wild for using a RMH to safely heat water that we feel comfortable implementing. Our working budget and general safety issues prevent us from experimenting with a homemade option.

Somehow I remeber posting pictures on these forums but cannot find that option? I will try to get some pictures or a video of the proposed plan.

I am a bit new to RMH, but it seems like the air bubbles would rise to the top of the water too quickly to effectively change the temperature of the water.

I would lean more toward affixing the copper tubing to an appropriately heated area of the mass and circulating the water through it.

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.

As a side note: Copper should always be avoided in an AP system because it can have a negative impact on the fish.

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.

http://donkey32.proboards.com/thread/1096/hot-water

Amos,

It is good to avoid heating the fish tank water directly.

We developed the plumbing to circulate hot (distilled) water into a SS spiral in the fish tank. See Fish Sweaters at Greenlife Aquaponics.

Instead of the gas water heater we are using as the source of hot water, you could use a copper spiral in an open tank heated by your rocket heater. Then, you wouldn't have to worry about air bubbles transmitting the heat to the water.

Cindy

What about using the bell concept from masonry heaters/RMHs etc. but under water to catch the bubbles and transfer more heat?

I imagine a two part bell. The rising bubbles are caught by an upside down funnel shape, directing the air into the actual bell, it cools a bit and exits the bottom of the bell (not out of the upside down funnel). Separate entrance and exit points.

Here's a crude drawing-



The funnel might need to have a much longer spout, allowing a significant pressure difference between the bottom of the funnel and the bottom of the bell, so the air will flow as desired and not just bubble up around the funnels edges. I'm not sure if the concept will even work at all unless the funnel holds a larger volume of air than the bell.

1. air is not a good conductor of heat; metal and water is a better conductor.
2. If you try to force your exhaust through water, it will back up and you will have smoke coming back out of the feed tube.