Engineering news
A cooling method that will enable freezers to reach temperatures as low as -180°C has been developed at the University of Brunel.
The technique uses cryogenic heat pipes, which transfer heat through the process of evaporation and condensation of fluid circulating in a sealed cavity, to transfer the cold temperature of liquid nitrogen.
Dr Hussam Jouhara from Brunel’s Institute of Energy Futures explained: “Unlike conventional cold storage using liquefied nitrogen gas, there is no need to physically transfer the nitrogen. The cryogenic heat pipe is literally just moving the cold.”
The system shows potential to be used in the medical sector to cool and store plasma made from donated blood. The team will collaborate with NHS Blood and Transplant Service to develop prototype freezers for this application.
Jouhara said: “We also see strong demand from facilities which store cord blood or eggs or sperm or other biological materials at very low temperatures.”
The pipes are also known as thermal super conductors or thermal ‘short circuits’ because they can transfer heat at high rates over relatively large distances with only small temperature differences between the heat input and heat output zones, Jouhara explained. They would require little to no maintenance as they have no moving parts.
Jouhara told PE: “A heat pipe operates when a temperature difference is imposed across its ends. The warmer end becomes the evaporator section where the liquid evaporates or boils by absorbing the latent heat from the heat source. The vapour then flows to the cooler end of the pipe where it condenses and gives up its latent heat; this end of the pipe becomes the condenser section.
“In this way heat is transferred from the warmer to the cooler end of the pipe and as only latent heat is being transferred this occurs at a practically constant temperature. The condensed vapour flows back to the evaporator section under gravity or capillary action if an internal wick structure is included, which closes the cycle. Heat pipes can be constructed to operate at working temperatures as low as -190°C and as high as 2,000°C.
The process could also use up to 50% less liquid nitrogen compared to traditional methods, making it a less expensive and greener solution. In terms of safety, the method would see gas tanks installed outside in the open air, limiting the possibility of fatal nitrogen asphyxiation episodes.
Gas and chemical supplier Air Products, which funded the research, is working with Jouhara to commercialise the new freezer technology.