And also why can’t we use nitrogen instead of coolants ?
In: Chemistry
There’s something called a refrigeration cycle. It’s a series of processes that you can do to a liquid or gas to make it cool something else. You can look up what it is later. For this cycle to be possible, certain properties are required. Refrigerants are the go-to materials today. If I recall correctly ammonia shows these properties, water doesnt.
Coolants absorb heat from these types of machines to keep them cool. Then the warmer coolant can carry the heat away to a cooler area. When it hits the cooler area, the heat will leave the coolant. You can then cycle the coolant back to the hot area to repeat.
Liquid nitrogen is sometimes used as a coolant or refrigerant in small scale. But it’s unstable at room temperature because it’s going from liquid to gas, which would cause expansion issues in a line trying to absorb heat. It would require extra insulation, an extremely low ambient temperature to keep it in liquid form, and would cost more, which is not super useful in most circumstances.
Coolant is just something which can be pumped onto or through some equipment to prevent it overheating.
In a car engine, heat needs to be removed from the engine and transferred to the air. A coolant is used to transfer heat from the engine to a radiator, which then transfers the heat to air. In a car engine the coolant is water, with some additives to prevent corrosion and prevent the liquid freezing in cold temperatures.
Coolants can be used in other machines, as long as they are suitabke for the process. Gases can be used as coolant, but don’t transfer heat as effectively as liquids like water.
Aircraft engines used cold air as a coolant. Cold air is pumped into very hot parts, like turbine blades, in order to prevent them overheating.
Air conditioning is a completely different process. This is refrigeration which typically uses a process called phase change cooling. This uses the principle that when a liquid evaporates it absorbs a ton of heat, and that when a gas is compressed it may liquify and release that heat.
Phase change cooling requires that the substance used liquifies or evaporates at suitable temperatures and pressures. These substances are called refrigerants. Different refrigerants are needs for air conditioning, refrigerators and freezers because of the different temperatures needed.
Nitrogen is not suitable for phase change refrigeration as it does not change phase at useful temperatures and pressures. Car in dioxide can be used, but the pressures needed are extremely high making systems difficult to build compared to synthetic refrigerants like hydrofluoroolefins or hydrofluorocarbons.
so, the short answer to the second question is we could, and do: its called air cooling (given that the atmosphere is 70% nitrogen by volume). pure nitrogen gas would be a significant increase in expense but limited additional capability. liquid nitrogen would come with massive weight penalties related to the extensive refrigeration required to keep the nitrogen down at -200c, and deal with the state change of the nitrogen turning back to gas….at which point you might as well use that refrigeration capacity to just directly cool the engine and stop mucking about with liquid nitrogen.
as to how of liquid coolants: they work by running a liquid, which is usually but not always water* through the hot parts to the engine. this heats up the coolant, and takes heat out of the engine block itself. the hot coolant is then moved to a radiator system of some sort, with a lot of small diameter pipes that maximise the effective surface area of the coolant, and then the heat is transferred out of the system into the air. the cooled coolant is then fed back into the engine block.
why? its smaller, which is vital in cars and such where space is at a premium, and while water is heavy, a liquid cooled system can often be lighter than a air cooled system for a given heat load, as side effect of that reduced size.
*most cars run on water with some additives like anti-freeze, but you can get systems that run on more exotic working fluids if the specific engine requires it. Their are some nuclear reactors that have a molten salt working fluid, for example.
Coolant in ACs is a speical gas/liquid that changes phases from liquid to gas at room temperature and back depending on the pressure. This phase change takes some energy which comes from the air around the cooling coil, that heat is then released at the point where you want to put it, so outside where the coolant is forced to change back to its liquid form with a compressor. For automobiles its just water running near hot parts and cooling down in a circle and you could in theory use nitrogen but its just very impractical.
Coolant in a car engine works differently than coolant in an air conditioner
Coolant in a car engine is basically just running water over the hot parts, and then blowing air throw the hot water to cool the water back down, and repeating the cycle. It’s only a special coolant and not water because water will freeze, and automotive coolant freezes at a much lower temperature. In fact, you can just use water if you don’t have anti-freeze and the weather will not be cold
Air conditioners work differently, by compressing a gas or “decompressing” a gas it will either get hot or cold. This is the refrigeration cycle. The specific properties of the gas will make it absorb or release more heat, but it may have other tradeoffs such as being expensive or bad for the environment
The problem with using nitrogen or extremely cold liquids for cooling for machinery with moving parts is that they will freeze and not work.
Liquid nitrogen or liquid helium is really good for cooling of things that don’t need to move, like magnets.
For refrigeration and air conditioning or heat pumps, the ideal refrigerant is one that can easily be converted from liquid to gas in the temperature that it will be used in and within safe pressure for its use case.
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