Heat exchanger idea

[Yule log]

Hello everyone, this is my first time to post on this great site.
My wife and I are hoping to build a new home in the near future. I would like to build an energy efficient home that is heated primarily with wood. We currently have a Jotul Castine wood stove that heats our 1700 sq ft home. Although I like our wood stove, I am ready to be done with the dust and smoke inside the house. I am very interested in wood gasification. As I look at different systems, heat storage seems to come up as great way to increase efficiency of gasifier units. I am interested in building non-presurized heat storage space into our next home.
Since I am always trying to find ways to save a buck when possible, I have been pondering economical ways to build heat exchangers. Although various forms of exchangers have been mentioned on this site, I have not seen anything on automotive radiators used as exchangers. Is there a reason for this? I would like to build a box type of tank and a large, truck type brass and copper radiator would fit crosswise in the tank. The Radiator Barn has 1980s Ford 460 truck radiators for $170. I would think that the large finned surface area of a truck radiator would work well as an exchanger. Any thoughts?

 

[Nofossil]

Welcome to the forum and to the boiler room.

I think that an automotive radiator would work fine. I have two concerns:

1) They're designed for operation with a relief valve at about 15 psi, while most boiler relief valves kick in at 30 PSI. Don't know the safety margin, but I'll bet that any radiator shop would have a good sense of the risk.

2) You'd want to make sure that you support it without having dissimilar metals touching. Galvanic corrosion is not your friend.

 

[jebatty]

I think the idea is good. As designed for a large movement of air, my concern would be that convection alone might not be very efficient to move the heat from the radiator into the tank. You might need a least a low flow of water through the radiator to get much heat transfer. Also, where in the tank do you place the radiator? If at the top to get desirable stratification, water convection would be minimal through the radiator as the upper water layer heats. If at the bottom, convection would be better but still a concern on sufficient flow through the radiator.

 

[Nofossil]

Convection is an issue. I should wait for someone who knows what they're talking about, but that's never stopped me before....

I think that you would want two radiators. One would be mounted near the bottom, and one near the top. Some sort of vertical rectangular duct could be installed above the lower one and below the upper one. They'd have to be offset so they're not directly above each other, and you'd want clearance between the sides of the ducts and the sides of the tank.

You could plumb them in series and use reverse flow, or plumb them independently. In either case, the lower radiator does the bulk of the work during tank charging, and the upper during discharging.

During charging, the lower radiator heats the water that it is in contact with, creating an upwards vertical flow in the duct above it. The duct ends a bit below the surface, so the rising hot water spreads out at the top, pushing cooler water down around the edges.

The reverse situation happens with the upper radiator during heat extraction.

I think it would work fine.

 

[MarcM]

Convection is an issue. I should wait for someone who knows what they're talking about, but that's never stopped me before....

I think that you would want two radiators. One would be mounted near the bottom, and one near the top. Some sort of vertical rectangular duct could be installed above the lower one and below the upper one. They'd have to be offset so they're not directly above each other, and you'd want clearance between the sides of the ducts and the sides of the tank.

You could plumb them in series and use reverse flow, or plumb them independently. In either case, the lower radiator does the bulk of the work during tank charging, and the upper during discharging.

During charging, the lower radiator heats the water that it is in contact with, creating an upwards vertical flow in the duct above it. The duct ends a bit below the surface, so the rising hot water spreads out at the top, pushing cooler water down around the edges.

The reverse situation happens with the upper radiator during heat extraction.

I think it would work fine.

I'd add, and maybe you're already thinking this, that it would be best to mount the radiator such that the fins are parallel to the sides of the tank to encourage convective flow around them (since that is what fins are designed for). I don't know if the ducts would be absolutely necessary... the thermal conductivity of water is small compared to its capacity to store and move heat via convection, and I think the convective flow generated either from top to bottom or bottom to top would have a low enough velocity that turbulent mixing and entrainment would not be big players.

I'd definitely be worried about galvanic corrosion however you choose to mount it- maybe attach it to PMMA (plexiglass) or polycarbonate feet with epoxy... both of those should bond and be nonreactive and withstand the heated water exposure.

And of course make sure you can get adequate flow through it. I'd do some trials first to see what kind of heat transfer rates at different flows and temperature differentials this type of setup would yield.

 

[Yule log]

Yeah, it sounds like we are on the same track. The attached file is what I had in mind. Layout is as viewed from the side.

If I may ask a question regarding system pressure...does the gasifier, supply line and heat exchanger have to operate as a closed loop or can it work at atmospheric pressures (ie have a vent to releive expansion pressure)?

 

[Eric Johnson]

I think you can run everything at atmospheric, but to avoid corrosion and other potential problems, I'd pressurize the boiler side. As I understand it, you have a nonpressurized storage tank with pressurized radiators immersed in it. My system usually sits on 12 psi, so I think you could keep it under 15 if you set everything up right. If the limit on the rads is 15, you might be able to put a 15 psi relief valve on the system instead of 30. But now I'm way out of my area of expertise.

 

[Stlshrk]

You may want to give the guys at everide radiator a call. They are one of the best aftermarket vendors for automotive rads out there. 800-828-4456 I think, google it to be sure. Anyway, they should be able to work with you to find something to work best in your custom application. You know, inlet sizes, deminsions, etc. Plus, they will ship it directly to your shop and save you the markup that someone local would have to charge you. Just a thought, a few bucks is a few bucks. Especially when gas is over $3 a gallon again (and likely forever)

 

[Stlshrk]

probably be 5 by next fall.