A large domestic component repair facility that specializes in re-building and testing hydraulic, electro-mechanical and fuel system components for the aerospace and aviation industries needed a solution to cool a fuel control valve for one of their customers. The control valve must accurately regulate fuel flow in response to the temperature. One of the tests required the valve to be cooled to -25°F plus or minus 2°, and the fuel flow measured.
The repair facility purchased Vortec’s model 308-35-H vortex tube for the application. Their engineering team built a test stand to attach the vortex tube to the control valve and monitor the temperature vs. flow performance. Initial testing of the apparatus showed that the -25°F desired temperature could not be obtained so the engineer contacted Vortec for advice. The engineer could only achieve 30°F temperature from the vortex tube. After some discussions, the Vortec application engineer determined that there were several issues.
1. The customer stated that there was ‘snow’ or frost exiting from the vortex tube. This is a sure indication that there is water or water vapor in the compressed air supply. Water vapor in the compressed air reduces the cooling effect because some of the sensible refrigeration is spent in producing latent refrigeration of the moisture. Even though the customer was using an in-line desiccant dryer and after-filter, the dryer was not rated for a low enough pressure dewpoint. The dryer and filter was also restricting air flow causing a pressure drop, leading to only 80 psig available at the vortex tube.
2. The test stand that connected the vortex tube to the control valve was placing a back pressure on the 10 cfm of cold air flow. This backpressure was significant enough to increase the cold air temperature by an estimated 20°F.
3. The third issue was a common error: the customer purchased a vortex tube that is designed to produce the maximum amount of refrigeration and not one that creates the lowest possible cold air temperature. All of Vortec’s “H series” vortex tubes are designed to efficiently produce the most cooling capacity (btuh) for a given volume of compressed air. All of the “L series” vortex tubes are designed to produce the lowest possible cold air temperature when they are adjusted to a low (10 to 30%) cold fraction. A simple generator change in the 308-35-H vortex tube converted it to a 308-35-L vortex tube and instantly it produced 10 degree F colder temperatures. Tests run at Vortec on the 308-35-L vortex tube using appropriately filtered and dried compressed air produced a 107F degree temperature drop, achieving a cold air temperature of -35°F.
In addition to the generator change, the customer made the recommended changes to their air supply and to their test stand. The results were dramatic-the cold air temperature dropped from 30°F to -35°F! This shows the importance of using the proper compressed air conditions and vortex tube selection for the application needs.