Enhance cryogenic gas production with a dual-stream vortex tube

The growing field of cryogenic applications underscores the importance of both technical and economical assessment of the available and the emerging methods of chilled gas production. The authors’ U.S.-based company that specializes in non-freeze pressure reduction of non-dried gas conducted a field study to evaluate its proprietary dual-stream, non-freeze vortex tube (VT) technology^a for the chilling of pre-cooled gaseous nitrogen (N₂) across a wide range of gas inlet temperatures and gas inlet-to-outlet pressure ratios. 

In the VT, high-pressure gas expands through the unit’s tangential inlet nozzles down to the delivery pressure. While in the VT’s cylindrical section, the rotating low-pressure gas undergoes energy separation—the Ranque-Hilsch phenomenon—forming two distinct streams: a cold stream located at the VT’s center and a hot stream situated between the cold flow’s outer layers and the VT walls. In the dual-stream VT^a used in this study, the cold and hot streams exit the VT separately through the dedicated outlets. A valve at the VT’s hot end is used to adjust the proportion (cold flow/Inlet flow) and, accordingly, the temperature of the exiting gas flows (FIG. 1). 

FIG. 1. Conceptual schematic of the VT^a. 

The intensity of the vortex energy separation is proportional to the ratio of the VT inlet-to-outlet pressures and is unaffected by the gas flowrate through the unit. The actual temperatures (T) of the VT’s cold and hot outlets are expressed in Eqs. 1 and 2: 

T cold = T inlet – ∆T Joule-Thomson – ∆T vortex-cold     (1) 

T hot = T inlet – ∆T Joule-Thomson + ∆T vortex-hot       (2) 

The principal mechanism of the vortex phenomenon is described here. An expanding compressed gas passing the VT tangential nozzles develops into a high-speed rotating body, or a vortex. The gas in the vortex cools as part of its total energy converts into kinetic energy. The angular velocity in the vortex is low at the periphery and very high towards the center. Friction between these zones forces the gas to rotate with the same angular velocity (as in a solid body), thus slowing the inner layers and speeding up the outer layers. As a result, the inner layers lose part of their kinetic energy and their total temperature decreases. The periphery layers receive the energy from the internal layers. This energy converts to heat through friction on the vortex tube walls. The inner (chilled) layers exit the VT through a central orifice in the unit inlet cross section. The outer (warmed) layers are discharged through a valve at a far (hot) end of a VT. This valve is used to adjust the flow proportion (cold flow/inlet flow) and, accordingly, the temperature of the exiting gas flows. 

In operations with non-dried natural gas, the Joule-Thomson temperature drop in the VT pressure-reducing nozzles and the resulting low/negative temperatures of the pressure regulated gas typically cause hydrate formation in a VT inlet and, accordingly, the unit freezes up. The proprietary ‘non-freeze’ design of the authors’ company’s VT^a uses the generated vortex heat to internally warm up the potential freezing, making the VT completely free from freezing. 

Experimental test setup. The VT test setup, shown in FIG. 2, included a pressurized liquid N₂ tank with a vaporizer, regulating valves to control gas flow and pressure, pressure and temperature gauges, rotameters, and a blending system to achieve and maintain the desired VT inlet gas temperatures and pressures. 

FIG. 2. A schematic of the non-freeze VT test set-up. 

Experimental results. FIGS. 3 and 4 demonstrate the actual temperatures of the non-freeze VT cold and hot outlets achieved in the test. The company’s VT^a was fed with chilled gaseous N₂ at inlet temperatures of –15°C and –30°C, respectively, with inlet gas pressures of 4 bar–8 bar (pressure ratios of 4:1 to 8:1) and an inlet gas flow split (µ) of 0.50–0.88 (here, µ is a ratio of the cold gas flow to the inlet gas flowrate). The magnitude of the operational parameters provides a detailed insight into the relationship between the intensity of vortex temperature division at cryogenic temperatures and VT operational parameters.  

The experimental data represented in FIGS. 3 and 4 demonstrates the VT’s robust performance in a broad range of different operating conditions. Key observations include: 

• The proprietary self-heating VT^a operates non-freeze even at the negative temperatures of its inlet gas flow. 

• At an inlet gas temperature of –15°C (FIG. 3), the VT^a cold outlet temperatures are significantly below the inlet gas temperature, with the temperature differential increasing as the inlet pressure rises. The hot outlet temperatures also increase with higher pressure ratios, confirming the effective energy separation. 

• At an inlet temperature of –30°C (FIG. 4), similar trends are observed, although the absolute temperature differentials are slightly reduced due to the lower starting temperature. The VT^a maintains efficient energy separation, producing cold and hot streams with substantial temperature differences. 

FIG. 3. Heating and cooling effects of the VT^a against the gas expansion ratio and the gas mass flow split as obtained from experimental data for N₂ at an inlet temperature of –15°C.  

FIG. 4. Heating and cooling effects of the VT^a against the gas expansion ratio and the gas mass flow split as obtained from experimental data for N₂ at an inlet temperature of –30°C.  

Takeaways. The study demonstrated the steady, thermally efficient performance of the non-freeze VT^a in operations with N2 at cryogenic temperatures. Key observations included: 

1. Energy separation: The VT^a produces hot and cold streams with temperatures significantly higher/lower than the inlet gas temperature. However, as the VT inlet gas temperature decreases, the intensity of energy separation at respective cold/inlet gas flow values (µ) decreases. 
2. Pressure influence: The inlet-to-outlet gas pressure ratio plays a crucial role in the intensity of energy separation in a VT. A higher inlet/outlet pressure ratio results in greater temperature differences between the cold and hot outlets. However, as the VT inlet gas temperature gradually decreases, the significance of higher inlet-to-outlet pressure ratios diminishes.
3. Flow influence: Greater VT hot outlet flows result in lower temperature differences between the cold and hot outlets. The cold/inlet gas flow rate ratio (µ) also significantly impacts VT^a performance. By adjusting the flowrates, the company’s VT^a can be fine-tuned to optimize the temperature differential between the cold and hot outlets.
4. Cryogenic performance: The non-freeze VT^a operates thermally efficiently at cryogenic temperatures, confirming its suitability for non-freeze applications in cryogenic engineering.  

Overall, the study confirmed that the non-freeze VT performs efficiently at cryogenic temperatures, with the inlet-to-outlet pressure ratio and cold/inlet gas flowrates being critical factors in optimizing its cryogenic effects. The ability to control these parameters allows for precise temperature management, making the authors’ company’s VT^a a valuable device in cryogenic gas applications without the risk of freezing.   

NOTE 

^a Universal Vortex's Non-Freeze Vortex Tube Technology 

ABOUT THE AUTHORS

Lev Tunkel graduated from the Moscow Oil and Gas University in Moscow, Russia with an MS degree in mechanical engineering and a PhD in fluid mechanics. He has more than 30 yr of experience in research and development, process design, and engineering in gas production and gas transmission operations. Dr. Tunkel pioneered the applications of the vortex phenomena for thermal conditioning of natural gas and other industrial process gases. He has served as Technical Director of Universal Vortex Inc. since its inception and has substantially contributed to refining the existing vortex phenomena applications and to the development of a new concept of energy saving and “green” vortex CNG and LNG technologies, as well as non-freeze vortex pressure reducers and vortex heaters. Dr. Tunkel holds more than 20 Russian and U.S. patents in the field, with publications in major professional magazines published in Russia and the U.S.  

Rajesh Patel is a seasoned energy professional with more than 30 yr of experience in the oil and gas sector, specializing in gas transmission and distribution, project management, design engineering, technical evaluations and business development. He combines technical acumen with leadership skills to drive operational excellence and innovation. Patel serves as the Head of Technical & Business Operations at Universal Vortex Inc. and has played a key role in expanding UVI’s global presence by developing customized energy solutions utilizing non-freeze vortex technology to meet industry-specific needs. He earned a BS degree in mechanical engineering and a postgraduate diploma in operations management. His career includes extensive international experience across multinational corporations, family-owned businesses and public-sector enterprises, with senior-level roles in four different countries.  

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