# Achieving Sonic Flow with Shop Compressors and Vortex Tubes

In summary, the conversation discusses the possibility of using a shop air compressor with a pressure of 120psi to achieve sonic flow in a vortex tube for spot cooling. The minimum pressure difference required for sonic flow is around 60%, and it is possible to reach this with a 120 psig system as long as the cooling air line is large enough to keep the pressure below 66 psig at the inlet of the vortex tube. The potential benefits and drawbacks of achieving sonic flow are also discussed. Additionally, the conversation mentions a helpful response from an Engineering Tips forum and provides a link to more information about choked flow. The conversation ends with a recommendation for the Engineering Tips forum.

I am wondering if I can use a shop air compressor (120psi) to achieve sonic flow? I am building a vortex tube (for spot cooling) and need a high velocity jet to form the vortex. What is the minimum pressure diff. required to achieve sonic flow of compressed air? Does varying the diameter of the orfice change the velocity if pressure is held constant? If someone could help me here or point me towards an equation that will answer my question that would be awsome.

Thanks

So I guess you answer your own questions around here. I posted on Engineering Tips forum and had 5 positive responses in 24 hours. Here is a very helpful responce I recieved.

Yes, what you are doing is possible. For air, the critical pressure is around 60% so for your 120 psig system, as long as your cooling air line is big enough to keep the pressure at the inlet of the vortex tube below about 66 psig (assuming sea level) then you should be able to get to choked flow and at that point it is just a matter of nozzle configuration to go above Mach 1.0.

I'm just not sure what you're gaining by going that fast. The device heats/cools the air due to heat transfer facilitated by the conservation of angular momentum between the inner (cooling) stream and the outer (heating) stream. By speeding up the outer stream aren't you decreasing the amount of time that this heat transfer can act? I've seen Ranke Hilsche Vortex Tubes with back pressure regulators on the outlet of the cold side to increase the heat transfer time, seems like you are going in the opposite direction.

Good luck with this. I've always been interested in a device that appears to violate the laws of thermodynamics (but doesn't really) and have designed a couple of odd applications to use them that haven't taken off yet (and probably never will).

David

Engineering Tips Forum was very helpful to me and I would recommend it to anyone whos questions go unanswerd here.

Here is some more usfull info I found by googling choked flow.

"The Basic Concept
A greatly misunderstood and misapplied notion is that of “choked flow”, also referred
to as “critical flow”. In gas flow through an orifice there is an occasion
where the gas velocity reaches sonic conditions. This occurs for air flow when the
absolute pressure ratio is .528, i.e. when the downstream absolute pressure (P2) is 52.8% of the upstream absolute pressure (P1)."

## 1. How can I achieve sonic flow with shop compressors and vortex tubes?

To achieve sonic flow with shop compressors and vortex tubes, you need to use a combination of both. First, you need to use a shop compressor to provide a high-pressure air supply. Then, you need to connect the compressor to a vortex tube, which will separate the air into hot and cold streams. Adjusting the control valve on the vortex tube will allow you to achieve sonic flow.

## 2. What is sonic flow and why is it important?

Sonic flow is the point at which the air or gas passing through a vortex tube reaches the speed of sound. This is important because it allows for the most efficient use of compressed air. At sonic flow, the cold air stream from the vortex tube can reach temperatures as low as -50°F, making it useful for various industrial applications such as cooling and drying.

## 3. Can any shop compressor be used to achieve sonic flow?

No, not all shop compressors are capable of achieving sonic flow. To achieve sonic flow, a compressor must have a high enough flow rate and pressure. It is recommended to use a compressor with a flow rate of at least 20 CFM and a pressure of at least 80 psi.

## 4. Can I adjust the temperature of the cold air stream from the vortex tube?

Yes, the temperature of the cold air stream can be adjusted by controlling the air pressure and flow rate going into the vortex tube, as well as the control valve on the vortex tube itself. This allows for customization and fine-tuning of the cold air stream to suit different applications.

## 5. Are there any safety precautions I should take when using shop compressors and vortex tubes for achieving sonic flow?

Yes, it is important to follow safety guidelines and precautions when using shop compressors and vortex tubes for achieving sonic flow. Always wear appropriate personal protective equipment, such as safety glasses and gloves. It is also important to properly maintain and regularly inspect your equipment to ensure safe operation.