FEATURE
INCORPORATING COLD CHAIN
Figure 2: Freezing temperatures of water
mixed with ethylene glycol.
also has the disadvantage of being toxic.
Both propylene and ethylene glycols
are miscible with water in all proportions,
which can produce freezing points far
lower than solid salt brines and without
the corrosion potential disadvantage of
high dissolved chloride levels. Also, since
the late 1980s, these two glycols have
become more generally available at lower
costs and have largely replaced solid
brines in water circuits, even when only a
few degrees of freezing point depression
are needed. Refrigeration circuits at much
lower temperatures, for example ice
cream and frozen food storage at -30°C or
even lower, require substantially increased
amounts of glycol component in glycol/
water mixtures.
Figure 2 shows how increasing ethylene
glycol content in a mixture with water
can reduce the freezing point of water
to as low as -60°C. Prices
of ethylene and propylene
glycol continue to rise in
accordance with global
prices of organic chemicals,
therefore only the minimum
amounts are added to water
circuits, particularly larger
volume circuits, to achieve
the required temperatures.
Also, replenishing glycol/water
mixtures due to leaks is far
costlier than adding salt when topping up
sodium chloride brines.
Propylene and ethylene glycol/water
mixtures for refrigerated liquid heat transfer
circuits have now become the norm
worldwide. Blends of substances with
water that have content/temperature
curves similar to Figure 2 are known as
eutectic mixtures, which undergo variations
in relative phases of the constituents
according to changes in temperature. How
this technology is applied in refrigerated
glycol/water mixtures is the subject of a
further article.
VAPOUR PHASE
The only substantial use of vapour phase
water in refrigeration plants has been in
steam jet cooling, which needs relatively
high-pressure steam to create a sufficiently
‘hard’ vacuum to enable it to be used for
cooling. Typical uses include industrial sites
such as power stations, pulp and paper mills,
Figure 3: Schematic diagram of a steam jet
refrigerating system.
sugar mills, chemical manufacturing, and
oil refineries, where a suitable steam supply
already exists for other purposes. Steam
jet coolers were extensively used during
the early 1930s for air conditioning large
buildings, but their popularity declined as
more energy-efficient vapour compression
and absorption systems were developed
and became commercially available.
Steam is passed through an ejector,
which creates a vacuum in a separate,
closed evaporator vessel containing
water. The vacuum in the vessel causes
some of the water to evaporate,
producing heat rejection through
evaporative cooling. The resulting chilled
water is pumped through the circuit to
air coolers, while the evaporated water
from the ejector is recovered in separate
condensers and returned to the cooling
circuit. In these circuits, the refrigerant is
water itself, so temperatures cannot go
below 0°C, and the majority is used for
comfort air conditioning of chilled water
running between 7°C and 13°C.
The single largest category and most
widely known refrigeration unit is the
domestic household refrigerator that, in
common with most other types of small-
sized units, has never and still does not
contain any water heat transfer circuits.
In fact, liquid water from condensation
of humidity on evaporator coils is often
a nuisance on the ubiquitous window
and wall-mounted split air-conditioning
units and has to be drained to forestall
the possibility of contamination by
microorganisms.
Another cooling method which does
not and never has required water in any
phase, is provided by dry ice: solid carbon
dioxide subliming directly to gaseous CO 2
at a temperature of 194.65 K (−78.5 C;
−109.3 F) at normal atmospheric pressure.
CO 2 , of course, is still being increasingly
used as a natural ‘green’ refrigerant.
A final comment regarding water and
refrigeration is that whether water is present
in solid, liquid, or vapour form, it is not
involved in any chemical reactions. Water is
just the carrier of other substances, either in
solution or dispersion. CLA
COLD LINK AFRICA • November/December 2018
www.coldlinkafrica.co.za
35