Page 41 - Low Temperature Energy Systems with Applications of Renewable Energy

Principles and operation of refrigeration and heat pump systems 29

Mixture of water and ethylene glycol

To obtain such a mixture with a freezing temperature of 13 C, 75% water and 25%
ethylene glycol (by volume) should be mixed. This mixture has an increased viscosity
compared with pure water and results in decreasing the thermal power as well as in
increasing the hydraulic resistance.
Mixture of water and propylene glycol
To prepare such a mixture it is necessary to mix in a volume ratio of 70% water and 30%
propylene glycol. The disadvantage of such a mixture is the increased viscosity, which, as in
the previous case, causes an increase in the flow resistance. The drop in thermal power, how-
ever, is not as sensitive as in the previous case. The advantage of such a mixture is the lack of
chemical aggressiveness of the solution.
Mixture of water and ethanol (alcoholic solution)

To obtain such a mixture with a freezing temperature of e14 C, 75% water and 25%
alcohol are mixed (by volume). The increase in thermal power is insignificant, the advantage
being in moderate density. When doing calculations one should take into account that after
the mixing of 100 L of water and 100 L of ethanol, the total volume is 180 L. This mixture,
as opposed to others, does not require the use of neutral components since the use of partic-
ularly pure alcohol does not lead to system aging. In case of using conventional fermented
alcohol the reagent life time is about 5 years.
Mixture of water and CaCl 2
A mixture of 82% water and 18% calcium chloride salt (by weight) has a freezing tem-

perature of e13 C. The disadvantage is the drop in thermal power compared to pure water,
but it is partly compensated by a higher solution density. Owing to the strong aggressiveness
of the solution, however, the use of this solution is not recommended.

1.6.2 Evolution of refrigerants
The world’s refrigeration and comfort-control industries depend entirely on the deci-
sions of the global community regarding cooling agents. Several generations of refrig-
erants have already changed over the past few decades. Some substances, which until
recently were considered the most up-to-date and environmentally safe, have now been
withdrawn from use. Below we trace the evolution of working fluids for refrigeration
machines.
First generation e Everything and anything that worked. Conventional solvents
and other volatile liquids were the most commonly used cooling agents for the first
hundred years. In fact, the first generation of cooling agents included everything
that worked and was available. Almost all the first cooling agents were flammable
and toxic, and some were even chemically active. Accidents commonly occurred dur-
ing the operation of refrigeration equipment. Some companies vigorously pushed pro-
pane (R290) as a “safe, odorless cooling agent” over ammonia (R717). Accidents,
particularly in domestic situations, often provoked changes in design and new
regulations.
Second generation: Safety and durability. The second generation was marked by
the transition to fluorine compounds to increase the safety and shelf life of refrigeration
systems. Prior to this, early attempts to replace “ice boxes” with refrigerators, in which
either methyl formate (R611) or sulfur dioxide (R764) were used as a cooling agent,