used in early mechanical refrigerators. Carbon dioxide (R-744) is used as a working fluid in climate control systems for cars, residential air conditioning, commercial refrigeration, and vending machines. Many otherwise excellent refrigerants are phased out for environmental reasons (the CFCs due to ozone layer effects, now many of their successors face restrictions due to global warming, e.g. the R134a). Heat pipes are a special application of refrigerants. Water is sometimes employed this way, e.g. in boiling water reactors. The phase change effect can be intentionally used, or can be detrimental. Phase-change materials use the other phase transition between solid and liquid. Liquid gases may fall here, or into refrigerants, as their temperature is often maintained by evaporation. Liquid nitrogen is the best known example encountered in laboratories. The phase change may not occur at the cooled interface, but on the surface of the liquid, to where the heat is transferred by convective or forced flow. Liquids Water is the most common coolant. Its high heat capacity and low cost make it a suitable heat-transfer medium. It is usually used with additives, like corrosion inhibitors and antifreeze. Antifreeze, a solution of a suitable organic chemical (most often ethylene glycol, diethylene glycol, or propylene glycol) in water, is used when the water-based coolant has to withstand temperatures below 0 °C, or when its boiling point has to be raised. Betaine is a similar coolant, with the exception that it is made from pure plant juice, and is not toxic or difficult to dispose of ecologically. Very pure deionized water, due to its relatively low electrical conductivity, is used to cool some electrical equipment, often high-power transmitters and high-power vacuum tubes. Heavy water is a neutron moderator used in some nuclear reactors; it also has a secondary function as their coolant. Light water reactors, both boiling water and pressurised water reactors the most common type, use ordinary (light) water. Some designs, e.g. CANDU reactor, use both types; heavy water in the nonpressurized calandria tank as the moderator and a supplementary coolant, and light water as the primary heat transfer fluid. Polyalkylene glycol (PAG) is used as high temperature, thermally stable heat transfer fluids exhibiting strong resistance to oxidation. Modern PAGs can also be non-toxic and non-hazardous. Cutting fluid is a coolant that also serves as a lubricant for metal-shaping machine tools. Oils are often used for applications where water is unsuitable. With higher