Vapour compression and vapour absorption refrigeration systems are two commonly employed conventional systems in almost all the major applications of refrigeration and air-conditioning. However, environmental problems such as ozone depletion and global warming caused due to CFC refrigerants have compelled us to look for other non-conventional systems. Vortex tube is one of the non-conventional systems where a natural substance like air is used as the working medium to achieve refrigeration. The experimental investigation shows that the carbon dioxide gives higher temperature drop than air and nitrogen. Maximum cold temperature drop is obtained at cold mass fraction of 60%. And the optimum geometrical parameter are L/D = 17.5 and diameter of cold end Dc = 4 mm.
Background
The refrigeration and air-conditioning industry is in an unprecedented transition phase, caused by environmental concerns with the impacts of refrigerant emission. To combat the twin menace of ozone layer depletion and global warming caused by synthetic refrigerants, there is increasing interest in environmentally safe alternative cooling solutions, based on the natural substances.
Vortex tube is one of the non-conventional systems where natural substance such as air is used as working medium to achieve refrigeration. Vortex tube has been used for many decades in various engineering applications. The vortex tube is interesting for new energy and refrigerating engineering as an experimental object with high development potential and as industry product with a quickly widening, unique combination of technological and operation properties because of its compact design and little maintenance requirements, it is very popular in heating and cooling processes.
Vortex tube refrigeration system
The vortex tube is a device without moving mechanical parts, which converts a gas flow initially homogeneous in temperature, into two separate flow of differing temperatures. The vortex tube contains the following parts: inlet nozzle, a vortex chamber, a cold-end orifice, a hot end control valve and a tube (Fig 1). It separates compressed gas stream into a low total temperature region and a high one. Such a separation of the flow into regions of low and high total temperature is referred as the temperature (or energy) separation effect.
When high-pressure gas is tangentially injected into the vortex chamber via the inlet nozzles, a swirling flow is created inside the vortex chamber. When the gas swirls to the centre of the chamber, it is expanded and cooled. In the vortex chamber, part of the gas swirls to the hot end, and another part exits via the cold exhaust directly. The part of the gas in the vortex tube reverses for axial component of the velocity and move from the hot end to the cold end. At the hot exhaust, the gas escapes with a higher temperature, while at the cold exhaust, the gas has a lower temperature compared to the inlet temperature. This was first discovered by Ranque in 1933 and by Hilsch in 1947. In memory of their contribution the vortex tube is also known as Ranque Vortex Tube (RVT), Hilsch Vortex Tube (HVT) and Ranque-Hilsch Vortex Tube (RHVT).
The use of vortex tube for small capacity applications is always justified if the compressed air is readily available. The vortex tube has number of features that make it attractive for industrial applications. Firstly, it has no moving parts and also quite reliable. Secondly, it requires no external power such as electricity or flames in order to operate, making it a comparatively safe system to achieve heating or cooling. The vortex tube is therefore ideal for use in environments where maintenance is difficult or where safety is critical.
A RHVT has the following advantages compared to the normal commercial refrigeration device: simple, no moving parts, no electricity or chemicals, small and lightweight, low cost, maintenance free, instant cold air, durable (because of the stainless steel and clean working media), adjustable temperature. But its low thermal efficiency is a main limiting factor for its application. Also, the noise and availability of compressed gas may limit its application. Therefore, when compactness, reliability and lower equipment cost are the main factors and the operating efficiency becomes less important, the RHVT becomes a nice device for heating gas, cooling gas, cleaning gas, drying gas, and separating gas mixtures, deoxyribonucleic acid (DNA) application, liquefying natural gas and other purposes.
Vortex tubes are categorised by their main technological and design features: flow configuration, the method of heat supply (removal), and how removal of low-pressure gas streams is organised. For the positioning of the cold exhaust, there are two different types: counterflow vortex tubes and parallel flow (uniflow) vortex tubes. Vortex tubes are classified as uncooled (adiabatic) and cooled (nonadiabatic) according to the method of heat supply (removal). On the other hand according to how removal of low-pressure gas streams is organised vortex tubes are called as dividing vortex tubes, self-evacuating vortex tubes and vortex ejectors.
An in-house facility is developed to carry out the experimental investigation of the vortex tube using different working substances; air, carbon dioxide and nitrogen. Vortex tubes of three different configurations i.e. 12.5, 17.5, 22.5 are selected and also the cold orifice diameter is 3mm, 4mm and 5mm are tested. A series of experiments are carried out to evaluate the performance of the system and to optimise the geometrical parameters.
The schematic diagram of the experimental test facility is shown in Fig. 2. Compressed air from the compressor (1) passes through the control valve (4) and pressure regulator filter section (5) and enters in the vortex tube (9) tangentially. To ensure the tangentially entry of the compressed air in the vortex tube to have proper swirling of the air special care was taken. The compressed air expands in the vortex tube and divides in to cold and hot streams. The cold air leaves the cold end orifice (11) near the inlet nozzle (10) while the hot air discharges the periphery at the far end of the tube i.e., hot end (12). The control valve (needle valve) controls the flow rate of the hot air (8). Two rotameters (7) measures the mass flow rates of the hot and cold air. Thermocouples numbered (6) measure the temperature of the leaving cold and hot air in the vortex tube. The pressure of inlet gas is measured by pressure gauge (2) and the temperature of inlet gas is measured by thermocouple (6). To investigate the effect of geometrical parameters on the operational characteristics of vortex tube, vortex tubes with different tube sizes and different cold end sizes has been constructed and tested. Photograph of the experimental test facility and the three vortex tubes of having different configurations are shown in Fig. 3.
It is observed that for each L/D ratio, initially cold end temperature drop increases to maximum at an optimum value of cold mass fraction of 60% (Fig. 4). Maximum value of cold end temperature 29°C is obtained for L/D ratio 17.5 at 60 % cold mass fraction while with L/D ratio 12.5 and 22.5, maximum cold end temperature drop values are about 26°C and 24°C, respectively.
The highest temperature drop is experienced is 32°C at the inlet pressure 5 bar while 29°C and 26°C temperature drop were obtained at 4 bar and 3 bar pressure supply, respectively. The result shows that CO2 produces higher cold end temperature drop than air and nitrogen. At pressure of 4 bar cold temperature drop for CO2 , nitrogen and air are 23°C, 18°C and 20°C, respectively (Fig. 5).
Conclusions
Vortex tube refrigeration can be an effective non-conventional cooling technique to develop cooling effect. The vortex tube has a number of features that make it attractive for industrial applications – and ideal for use in environments where maintenance is difficult or where safety is critical. It is observed that the cooling effect produced by the vortex tube depends on properties of the gas, molecular weight and specific heat ratio.
