Water Compressors: R-718

By Dale Yelnich

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The basics of water cooling stretch back into the dawn of humankind. Early humans swam in rivers or lakes to cool off, water quenching cooled metal alloys and steel to temper and harden it for use in weaponry, and as the industrial revolution began, water jackets around internal combustion engines allowed precise temperature regulation to increase durability and power.

When modern HVAC units were introduced in the early 20th century, water was studied as a coolant. It was found that water has a very high coefficient of performance, or COP, and it does, in fact, rank right up there with the best modern refrigerants. That seemed to suggest that when it was compressed into a vapor, which is how all modern refrigerants in air conditioning and chillers work, water would make an incredibly efficient and easy to obtain natural cooling source.

The problem is that, although water is easy to work with, completely non-toxic and has a high COP, the density of water vapor is very low. That means any viable system will have to compress the water vapor at extremely high pressure levels to allow heat rejection from the cooling process. To do this, the swept volume of a water vapor compressor has to be some 500 times higher than for a conventional refrigerant, which is daunting all by itself. The easiest and most straightforward way to accomplish that is to make a very large compressor impeller, in the ballpark of about 20 feet in diameter. Of course, precision balancing must be employed and alloys must be used to combat wear and tear, particularly from the extremely high tip speeds that are generated at the ends of the spinning blades. Needless to say, back in the early HVAC days, the technology was not available to to do this, and water vapor cooling was put on the shelf for decades to come.

Back in the 70’s, the research and use of titanium as a viable alloy for water vapor compression made this technology feasible. Granted, the rotor and therefore the impeller blades would still have to be huge, so this technology could only really be used in an industrial application. But as environmental concerns about dumping refrigerants into the atmosphere came to the fore, water vapor cooling ultimately promised a non-toxic and environmentally safe alternative.

The original compressors were fitted with centrifugal impellers. They were relatively easy to manufacture, they were effective as chillers, and energy wise, they were fairly efficient. Although more expensive to produce, as the cost of energy rose in Europe, and as environmental protocols were put into place, water vapor compressors began to become a practical alternative.

On the other hand, in the United States where energy prices are relatively low compared to their European counterparts, it did not make economic sense to make a switch to water cooling. Newer more modern environmentally friendly refrigerants were developed, that could essentially be used with standard compressor technology, and water vapor compressors were seen more as a novelty instead of a practical way to cool machinery or even residential households.

But that way of thinking is beginning to change.

The most important development in water pressurization techniques is the advent of the axial compressor. Axial compressors use the same design as modern jet engines. Instead of one giant rotor in a centrifugal compressor, an axial compressor has many banks of smaller spinning rotors, channeling the water vapor through stationary blades, that are arranged one after another in a row. By using many different rotating blades in an axial configuration, the swept volume is similar to that of a large centrifugal impeller in a considerably reduced size.

To further reduced the costs of manufacture, carbon fiber impeller blades are being tested as alternatives to expensive titanium. Carbon fiber is lighter, easier to work with and it costs less than the same amount of titanium that would be required. The newest designs would still be available only for commercial or industrial applications, but axial compressors can be engineered small enough to be effective for use in a residential cooling system.

As energy prices continue to rise and environmental concerns need to be met, the alternative may be in the wholesale implementation of refrigerant R718, otherwise known as water. It is the most benign, non-toxic, environmentally friendly refrigerant known, it is available everywhere and has been proven as a viable coolant since the dawn of humankind.

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