This weekend Carolina had a terrible problem in her apartment: the building’s radiator sprung a steam leak, and she doesn’t have a valve that can cut off the leak, so she’s having to depend on the building staff to not turn the boiler on. Whenever they forget and turn the boiler on, her bedroom fills with steam until her frantic phone calls succeed in getting it turned back off.
This led me to think about the problem of fluids for heat transfer in domestic life, and in particular problems of safety in the case of pipe failure. Water has the advantages of being nontoxic, inexpensive, having a large specific heat, and having enormous enthalpies of vaporization and fusion (at accessible equilibrium temperatures). However, it is somewhat corrosive, as a vapor it is somewhat dangerously overeffective (leading to explosions and burn hazards), and its limited temperature range and expansive freezing can cause problems.
Controlling heat flow is one of the major issues in quotidian human life. It gives us hot showers, cold refrigerators, dehumidification, warm houses in winter, cool houses in summer, fired pottery, cooked food, dried fruit, hot tea, warm beds, chilled sprains, and cool foreheads when we have fevers. Lack of air can kill you in minutes, lack of water can kill you in days, lack of food can kill you in weeks, and lack of sanitation can spread your diarrhea to your whole town, but lack of cool can kill you in an arbitrarily short period of time: milliseconds or less.
The major ways we control heat flow are through insulation, thermal mass, glazing, active heating, and convection. Convection is the one I’m focusing on here, because it allows the control of arbitrarily large amounts of power with arbitrarily small ones, given adequate available thermal mass and insulation. Small electric fans are a common way that we control enormous amounts of thermal power using very small amounts of mechanical power.
Candidate convection fluids as alternatives to water include air, glycerin, vegetable oil, propylene glycol, propane, mineral oil, sulfur hexafluoride, dimethyl sulfoxide, eutectic lead-tin mix, methyl ethyl ketone, d-limonene, Fluorinert, acetone, ammonia, turpentine, carbon dioxide, ethanol, difluoromethane, R-410A, tetrafluoromethane, fluoromethane (HFC-41), fluoroform, low-molecular-weight polyethylene glycols, and non-glycerin sugar alcohols (such as sorbitol, mannitol, maltitol, xylitol, erythritol, isomalt).
Air is nonflammable, among the least toxic alternatives, and has the widest working temperature range. It has very low viscosity, allowing it to be pumped easily, and has the lowest cost of any alternative, being free if you aren’t too picky about purity. Its major disadvantage is its very low density (1.2 kg/m³ = 1.2 g/ℓ = 1.2 mg/cc), which, combined with its fairly low specific heat (1.01 kJ/kg/K), requires torrential flow rates, large ducts (despite its low viscosity), and correspondingly high insulation costs.
Air works down to oxygen’s condensation point of -183°. It doesn’t have a sharply defined upper temperature limit; rather, its upper usable temperature limit is usually set by the corrosive effects of its oxygen on materials in the environment, which themselves do not have a sharply defined transition point but rather an Arrhenius relation. It becomes intolerably corrosive to carbon around 600° (anthracite’s glow point) or 700° (coke’s glow point) and to most metals in the range from 800° to 1500°, but does not corrode fully oxidized materials such as quicklime, silica, and zirconia, nor fluorinated materials.
Needless to say, air is used constantly as a heat transfer fluid.
Glycerin is very nontoxic — perhaps less toxic even than water and air — and has a fairly wide temperature range, in liquid form from 18° to 290°. It is fairly nonreactive, less corrosive than water. If used alone for domestic climate control, it would be likely to freeze in the pipes in normal use, so you’d probably need either a preheating system to liquefy it or a mixture with some other substance to lower its freezing point. Even at normal temperatures, it is fairly viscous, and a solvent might help with that too.
Glycerin’s autoignition temperature is 370°, and because its vapor pressure is very low, its flashpoint is 160°, so a glycerin spill is not a fire hazard under normal circumstances. It’s a byproduct of biodiesel production, resulting in a low price for non-food-grade glycerin of 2¢–5¢/kg. Here in Buenos Aires current prices seem to be about AR$76/kg = US$4.75/kg for drug-grade glycerin.
Ethanol is sufficiently nontoxic that people drink it recreationally (7000 mg/kg ORL-RAT LD₅₀), but this also makes it expensive, about US$4 per liter here in Buenos Aires. It’s considerably less corrosive than water, although it does dissolve many plastics, including some varnishes. It has an unremarkable heat capacity (0.11 kJ/mol/K, which at 46 g/mol works out to 2.4 kJ/kg/K) and water-like viscosity. It has an anomalously high thermal coefficient of expansion, leading to its use as a less-toxic substitute for mercury in thermometers.
Aside from the cost issue, fire hazards are probably prohibitive for wide domestic coolant use of ethanol by itself. Its autoignition temperature is 365°, but it is considerably more inflammable than this suggests because of its very high vapor pressure — 6 kPa at 20°, which leads to about a 6% concentration in air; its flashpoint is 16°, so a large ethanol spill in an inhabited area is almost certain to explode unless rapidly remediated. Unlike most organic solvents, it’s miscible with water, so remediation is possible just by dumping water on it.
Vegetable oils vary considerably depending on source, but the commonly-available ones are so nontoxic that they are used as macronutrients for cooking, although they are not quite as nontoxic as glycerin (which, incidentally, is easily prepared from them by transesterification or saponification). Sunflower oil is currently the cheapest I can find here in Buenos Aires, at AR$190/10ℓ = AR$19/ℓ ≈ US$1.20/ℓ ≈ US$1.40/kg; perhaps this is because Argentina is the world’s third biggest producer of it. Historically, soybean oil was usually cheaper (Argentina is also a major world producer of it) but I can’t find cheap soy oil here now.
Vegetable oils function over a relatively wide temperature range. Rather than boiling like most of the other chemicals discussed here, they begin to thermally decompose; because of its high saturated-fat content, sunflower oil is typically stable up to 230°, although refined forms can survive to 250°, nearly as high as soybean oil. It remains liquid down to -17°, and soybean oil down to -16°.
They are not inflammable (they will not support flames until well above the smokepoints mentioned above) and they are extremely noncorrosive, protecting metals, woods, and leathers from other kinds of corrosion. Because they are relatively poor solvents, they generally will not dissolve plastics, although they can plasticize some of the softer plastics such as polyethylene and polypropylene. If there is a pipe leak, however, cleaning the oil from the things that it has soaked would often be tricky, requiring detergents or even organic solvents (dry-cleaning).
Vegetable oil fires are definitely not to be extinguished with water — they won’t burn until they are hot enough to instantly flash the water into steam, which would aerosolize the burning oil and convert a mere fire into a huge explosion.
Propylene glycol is a very nontoxic† (33700 mg/kg LD₅₀) alcohol commonly used as antifreeze, as a solvent for drugs, and as a food additive; in the US it’s legal as up to 5% of an alcoholic beverage or 24% of a confection or frosting. Among its medical uses are direct application to human corneas to reduce edema. Cases of toxicity exist in the medical literature but generally result from continuous intravenous use. At about 50 centipoise, it’s not as viscous as glycerin, but it’s more viscous than water and ethanol. (Chemically, you get either of the propanols by hydroxylating propane, to propylene glycol by hydroxylating either of the propanols, and to glycerin by hydroxylating propylene glycol.)
It is antimicrobial. (Because hey, it’s only so nontoxic.)
It has an unremarkable specific heat of 2.5 kJ/kg/K and is commonly used as a heat transfer fluid, both alone and mixed with water.
It has a wide temperature range, not boiling until 188° and solidifying into a glass at -60°. It’s not much of a fire hazard, with a flashpoint of 220° due to its low vapor pressure (10.6 Pa at 20°) and an auto-ignition temperature of 700°, a higher temperature than anthracite. In fact, I think I’ve heard that it’s used as an antifreeze in industrial fire suppression sprinkler systems.
If exposed to air at high temperatures, it can oxidize over time.
Propylene glycol currently runs about AR$130/kg (US$8/kg) here in Buenos Aires; the most popular use seems to be mixed with glycerin to promote evaporation for vaping in e-cigarettes.
Like glycerin and ethanol, propylene glycol could, if it caught fire, be extinguished with water, because it’s miscible with water.
Propylene glycol is a relatively good organic solvent, so a spill of it might cause damage to plastics and varnishes; it’s commonly used as a “permanent” plasticizer. It can dissolve about 1% of its own weight in most vegetable oils, but is immiscible with hydrocarbons.
† Except to cats. Propylene glycol is toxic to cats.
Liquefied propane gas is commonly used as a refrigerant, mixed with butane; unlike most of the other heat transfer fluids surveyed here, we can take advantage of its enthalpy of vaporization, which allows the use of a much smaller amount of heat transfer fluid. However, propane is ridiculously inflammable.
Mineral oil, or paraffin oil, is a mix of alkanes of a relatively consistent chain length and low vapor pressure; it is relatively nontoxic, but indigestible, and is commonly used as a laxative. It is used as a heat transfer fluid in electric radiators, for high-voltage electric transformers, and in computers. Different grades have different freezing and melting points with different degrees of definiteness, but a typical melting point is -4°; it can survive slightly higher temperatures than vegetable oil, up to