Found a bit of encouraging info about using a roots-type blower as the engine in this discussion in the "oil collection" forum, they are using the 175 CFM roots blower with 2-inch in and out fittings to create both suction and pressure for a 500 gallon oil collecting super sucker tank. He indicates that he sometimes uses the pressure or suction in the tank to spin the roots blower for starting the 11HP Honda gasoline engine that is used to turn the blower. Apparently the blower only needs around 12 pounds pressure across it to easily spin the gasoline engine even against compression, sounds promising.

He also says that trying to make more than about 12 pounds pressure with this blower will stall the 11HP gasoline engine so the 175 CFM size of this blower is probably WAY larger than is needed, or even wanted, if intending to produce only a couple killowatts of power, this size blower would likely be capable of making 25-30KW quite easily if the rest of the system could vaporize and condense that much refrigeration fluid.

Tracking roots blowers on ebay it looks like they can be had for $150 to $200 bucks regularly. I will research these blowers a bit more and try to deturmine how to correlate there model numbers with the cubic feet of air they move and the pressure and power needed. These blowers seem to be listed as "Dresser Roots blowers"? I suspect these blower units will need to have there seals improved to handle the higher pressures that will be generated by the frig fluids used in this generator concept but it should be doable. If all else fails the entire blower and generator could be sealed inside an additional external tank to contain any freon leaks.

Tim: my engineering thesis project was a gas turbine using Roots blowers for both compressor and turbine stages. It managed to reach 6% thermal efficiency at roughly 300 hp and 35,000 rpm. It consumed gasoline, diesel and propane at a prodigious rate, and sounded absolutely amazing. I was pumping lube oil through the rotors as coolant. It could run steadily at about 10 hp without overheating and seizing, burning a pound of propane each minute. Think about that - a one pound propane tank is about half as big as a lawnmower tank, yet only holds 2 minutes worth of fuel. A pint can of gas/diesel mix would idle it for 5 minutes under no load, or be consumed in 30 seconds at full load (about 20 psi). HP was nearly linear with RPM, at constant chamber pressure.

Roots blowers are terribly inefficient as air motors, and also inefficient as compressors at any significant compression ratio. They operate entirely as displacement motors, not expansion motors, so they lose all potential expansion energy, along with most of the thermal energy (most of the expansion of the gas takes place after leaving the turbine stage). The seals are cast iron split rings on the shafts, and work pretty well to keep gas from leaking into the gear case. The rotors operate with a little clearance at the ends and no side seals. In other words, they leak. At high speeds the leak rate becomes close to a constant. At low speeds it's significant at higher pressures (mine operated at less than 20 psi, and made 10 hp at between 4-6 psi).

When used as a compressor, the moment the lipseal of the lobe passes the edge of the outlet port (on the "high pressure" side of the compressor) high pressure air rushes back into the advancing rotor cavity. The rotor then has to push that air back out against the high pressure, a second time. I wanted to try adding a reed valve to the outlet, to reduce this "backflow", but never found the time. Besides, I figured the world had enough ways to burn limited natural resources without a new one. Still, the experience taught me a lot, and I still dream of developing it some more. I have in mind to build a wood-burning turbine.

The design is well balanced and capable of very high speeds without vibration. As an air motor it's capable of very high hp for very light weight and small size, but low efficiency. If operated from stored compressed air at low pressure (say, just a couple psi), they would be one of the most efficient ways to convert stored power into shaft power (i.e. a generator).

Note that they will pump non-compressible fluids (oil, water) quite efficiently, functioning exactly as a gear pump, but with much larger pulsations.

Cheers,
JohnO

Thanks johno, I just did a web search for Roots blowers and found the DRESSER website, apparently Dresser now manufactures the blowers under the ROOTS trade name. The smallest blower moves 10 cubic foot of volume at 1100 rpm, still WAY too big for this application, the specs also say the case of these blowers should not be pressurized with more than 25 pounds pressure across it, sort of eliminates it for use as the motor here due to the ambiant system pressures of the freon being at best something like 50 pounds pressure, probably more like 150 pounds.

Next search will be for "screw compressors", from johno's info I assume these are also not "expansion" devices but only "displacement" devices but they are at least available, but expensive. Being used as air compressors I assume they will accept a much higher air pressure accross them?

I will have to think a bit about the differance between an "expansion" device verses a "displacement" device. I thought the differance had more to due to the way the device is used rather than the device itself, if I inject a pressurized liquid that then expands as a gas I would think you would have an "expander" device, if I inject already expanded high pressure gas I would think you would then be using it as a "displacement only" device?

From reading lots of info about existing Organic Rankine systems I had not actually thought to deturmine if they were injecting liquid or gas into there motor device, I assume the large commercial systems that use actual turbine devices inject a liquid that expands as a gas through the turbine but can't say for sure. The commercially available 4KW ORMAT orc generator animated displayed HERE shows the fluid being heated until it expands into a high pressure gas, the gas is what is passed through what looks like a very inefficient short bladed single stage type turbine, but this animation may just be very simplified for clarity, would the turbine as shown be an "expansion" device or a "displacement" device.

It would seem that the efficiency of the device would be based largely on how long the working gas is doing work on the blades or pistons of the device, the more pressure that is dropped as the gas expands inside the device while doing work would seem to indicate a greater efficiency? this would seem to make a turbine with many increasing diameter blades the more efficient, or even a piston motor with increasingly larger diameter pistons, except that the exhaust stroke of the piston is purely wasted energy. I begin to have a better understand of why a rotary device that has an expanding diameter, and that has the working fluid flowing continously from one end to the other, would be the most efficient. Unfortunatly, turbins are very high speed devices (also not available in my junk) and I don't have access to an acceptable gearbox to use with one, looks like I am back to using a piston pump as my engine, if I heat the pump with solar heated hot water and inject liquid freon into the cylinders it would seem that this would make this an "expander" since the fluid expands into a gas inside it, if I injected high pressure gas into it, like a simple steam engine, would it then would be considered a simple "displacement" device? Would increasing the piston stroke length increase the efficiency? It would seem that injecting liquid that expands into a gas inside the device would be the most efficient of the two methods, and the efficiency would depend on how much of the working fluid's energy is transfered to rotary motion before it is exhausted from the unit.

Tim,

The way it looks to me is this is kind of an analog to steam.

The liquid refrigerant is first heated, creating gas pressure and that pressurized gas is sent to the expander-device inlet.

As motion is imparted to the expander shaft by the pressurized gas acting on it's blades (or piston), the gas pressure is reduced because it is allowed to expand to the next stage of the engine or to it's outlet.

A low-power recirc-pump is usually used to send the exiting gas to a condenser, where it changes-state back into a liquid and the cycle is repeated. The recirc-pump also serves to promote a lower pressure at the expander outlet than would be the case without it, insuring the continuation of the process (pump speed is, normally, adjustable until an optimized recirculation is attained in the loop).

I suppose you could flash-off the liquid at the entrance to the expander, but it doesn't seem it would have as much of an opportunity to build pressure if it was used that way and might also allow some liquid to enter the expander, taking up room and leading to further inefficiency.

In comparing displacement-engine and expander, it seems to me the first term has to do with the engine-type and the second has to do more with the process it might be used in.

In any case, that's the way I've looked at over the years but it's not to say it's the only way or even the right way...just my way.

Tony