Possible Soluable Core Materials for Cu casting

Hi,

this is done since many years with aluminum core for brass castings (?) or electrolytic deposition in production of microwave-waveguides.

Ask a precision casting shop - they are used to your dimensions and no longer really expensive. They have the lost mold procedure - nearly any shape with nearly any inner shape and thin walls can be done.

Also the big suppliers of CuNiBe or CuCoBe will offer their casting capabilities.

So etching is not necessary. But if you insist on etching: try iron or steel (hollow to allow the etchant to enter and provide a large area to speed etching), etch with hydrochloric acid or sulfuric acid or electrolytically by passing a current and using an ordinary salt or nitrate solution.

Why did you decide on this type of heat exchanger?

If you spray the liquid air into "warm" gaseous air the process may be simpler?

RHABE

Hello rhabe: You asked "Why did you decide on this type of heat exchanger?"

It seemed like the easiest way to confine the gaseous air produced when evaporating liquid air for use with a compressed air motor considering the problem of frost accumulation on the heat exchanging surface.

Considering low cost and simplicity, the best solution I could come up with was to spin the heat exchanger fast enough that the ice and frost forming on the surface from the humidity in the air were thrown off by centrifugal force.

The spinning heat exchanger forms the body of a reaction turbine resulting in a very simple and inexpensive design, and one requiring no lubrication.

There are a lot of variables in the adhesion force of ice and frost to teflon,(the copper will be teflon coated) but it seems that there is a relatively narrow speed range where it is practical to operate before the pressure of the liquid air generated by centrifugal force in the passages ruptures the heat exchanger body, or the body itself disintegrates. The passages need to be as long as possible to present the maximum surface area for maximum heat conductivity.

There may be better solutions, but I can't think of any.

bill_michaels

You need to determine the number of liters per second of liquid air which need to be evaporated and brought up to room air temperature. For this you will need to find the latent heat of vaporization of liquid air plus the specific heat of the gaseous air produced times the temperature you will raise it to for discharge. That is the heat the air side must bring into the heat exchanger. You will then need the area of the heat exchanger surface, the thickness and what it is made of. This will allow you to compute the volume of air blown onto the outside for each liter of liqud air boiled off. You will also need a turbulence correction because when the liqud air boils off the surface it will leave a dry surface behind which you will need to flood with fresh luqid air by some form of stirring.

G forces from the liquid air inside will help with this, but may not be a complete solution, so an active method may be needed to prevent boolied off air from reducing heat transfer.

The calculations for this are complex, and the risk of accidentally constraining liquid air in a fixed closed space = bomb make this a procedure that requires an experienced engineer

A 60 inch high 9.25 " diametersteel compressed air cylinder holds 110 pounds of water and weighs 135 pounds empty. has a capacity of 1.76 cubic feetand Compressed air at 2000 PSI has a SG of about 9 so it will hold about 15 pounds of compressed air.

Liquid air has an SG of 0.87 = ~ 96 pounds of liquid air. so when it evaporates in the bottle it will reach over 12,000 PSI, more or less, as it departs from an ideal gas at this pressure. Needless to say, the safety disc will rupture and vent it, or it would explode like a bomb.

Hi,

I agree totally that this is a complicated task, dangerous and will need some official approval as any pressure container.

I would try to simplify in not using boiling liquid but thin layers or droplets only sprayed on the surface of the heat exchanger. Thin layers can also be created by centrifugal forces on conical rotors either with a smooth surface if fed by a sufficiently small amount or with a finely threaded surface where the liquid is held in the bottom of the grooves by capillary forces.

This approach would prevent the dangers of pressure buildup and provide a near 10fold increase in heat tranfer.

We did something similar like the threaded surface with ball-bearing oil - much lower flow rate, much higher viscosity, hopefully nearly no evaporation.

I also doubt on the usefulness of Teflon. Any airplane deicing system is relying on rubber. So very likely one of the fluorinated synthetic rubbers will work.

RHABE

"I also doubt on the usefulness of Teflon. Any airplane deicing system is relying on rubber. So very likely one of the fluorinated synthetic rubbers will work."

In my limited experience, diamond-like coatings (deposited with a plasma gun in a "vacuum"), will not allow ice to adhere. I wanted to coat the leading edges of helicopter rotors to make them ice-proof, but never got the chance to test it.

A process called 'electroforming' is used for making tools for moulding. 'Electroless nickel' is used to coat virtually any material. For example some waxes are formed to the required shape and then coated with graphite coating to make it conductive and put into a chemical bath. After the plating operation the wax can be melted out. Electroplating can also be used - there are some companies in CA that specialise in these techniques.

I am not sure of the properties of this material under the temperatures of liquid air but check on google under those techniques to see if they suit you.