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Water is inexpensive, non-toxic, and available over most of the earth's surface.Liquid cooling offers higher thermal conductivity than air cooling. Water has unusually high specific heat capacity among commonly available liquids at room temperature and atmospheric pressure allowing efficient heat transfer over distance with low rates of mass transfer.
As the coolant system pressure increases with a rise in temperature, it will reach the point where the pressure relief valve allows excess pressure to escape. This will stop when the system temperature stops rising. In the case of an over-filled radiator (or header tank) pressure is vented by allowing a little liquid to escape.
Proper engine coolant and a pressurized coolant system obviate these shortcomings of water. With proper antifreeze, a wide temperature range can be tolerated by the engine coolant, such as −34 °F (−37 °C) to +265 °F (129 °C) for 50% (by volume) propylene glycol diluted with distilled water and a 15 psi pressurized coolant system.
New cars produced in the early 1990s contained the new refrigerant in their air conditioning systems. Owners of older cars with R-12 systems would either have to invest in an R-12 to R-134a conversion system, buy a new car, or find a qualified technician to recharge their older R-12 system. R-134a has been used in cars for almost 30 years.
Hot coolant passing through the heater core gives off heat before returning to the engine cooling circuit. The squirrel cage fan of the vehicle's ventilation system forces air through the heater core to transfer heat from the coolant to the cabin air, which is directed into the vehicle through vents at various points.
Finally, other concerns may dominate cooling system design. As example, air is a relatively poor coolant, but air cooling systems are simple, and failure rates typically rise as the square of the number of failure points. Also, cooling capacity is reduced only slightly by small air coolant leaks.
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A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corrosion of the cooling system.