Refrigerant 718

News flash! A major spill of refrigerant R718 occurred on the Interstate last night!  The refrigerant soaked the highway and surrounding ground.  Luckily, in a few hours it dried up with no effect other than helping nearby plants and small animals.  Because, of course, refrigerant R718 is the designation for pure water.  Today we’ll consider advantages and disadvantages of using water as a refrigerant.

On the positive side, water might seem like an almost ideal refrigerant: it is non-toxic, cheap, abundant, and readily available and has a high latent heat.   One disadvantage for many applications is the relatively high freezing point.  Obviously, you couldn’t make a freezer that used R718 because the refrigerant itself would solidify.  

Could you use it for slightly higher applications, though?  Air cooled to near, 40 °F, say, might still be useful for air conditioning applications.
 

This table shows a comparison of cycles with an evaporator temperature of 40 °F for water, and  R134a, a more traditional refrigerant.  Both cycles are constrained to deliver 1 ton of cooling with an 80% efficient compressor. The coefficients of performance (COP, how much cooling is produced per unit work required)  is reasonable for both fluids, but the real problem with water can be seen in the pressures (0.1 and 1 psia for the evaporator and condenser, compared with about 14 psia for atmospheric pressure) and in the volumetric flow rate.  For water, the entire cycle would have to operate at a fairly high vacuum, and the compressor would have to handle very large amounts of very low density vapor.  These challenges could be overcome, but it is usually easier (cheaper) to just use a different refrigerant.

This table adds an additional column for water as a refrigerant operating at a much higher cooling temperature: 420 °F.  What for?  I can’t imagine—maybe cooling molten steel or something.  In any case, at that higher temperature, a refrigeration cycle based on water would work very nicely.