Coolants are heat transfer media which are intended for use below 0°C. Due to its high specific heat capacity, water is often used even below 0 °C. However, pure water already freezes at 0 °C, i.e. the polar water molecules arrange themselves into ice crystals. To be able to use the good heat transfer properties of water at lower temperatures, this order must be disrupted.
About glycol as a component of a freezing-point lowering mixture with water
As mentioned above, to be able to use the good heat transfer properties of water at lower temperatures, the natural properties of water below 0°C must be disrupted. The addition of special cooling brines or anti-freeze agents such as glycol is suitable for this purpose. In this way, operating temperatures of up to -55 °C can be achieved.
As previously stated, water is an excellent coolant but with “side effects”: When combined with oxygen and other air components, water is corrosive. And at temperatures below zero, it loses its flowability. Chillers, particularly their piping and heat exchangers, react very sensitively to both of these effects. By adding suitable additives, these problems can be brought under control. When mixed with suitable additives, the required properties of water can be used and yet the risks can be reliably excluded.
Substances like glycol combine frost protection with anti-corrosion features. This is because glycol is enriched with so-called corrosion inhibitors. These shield the metallic materials in the refrigeration circuit against aggressive influences. At the same time, the flow properties required for each application are guaranteed even at very low temperatures. Such flow properties can be achieved by proper blends of water and glycol. All mixtures are non-flammable, only slightly hazardous to water and easily biodegradable.
What do we need to know about glycol?
The function of glycols in a glycol-water mixture is to lower the freezing point. However, not only the freezing point is lowered, but glycols have an additional effect in water, as can be seen in the figure on the right using the example of monoethylene glycol. The mixture does not freeze at the specified freezing point, but first forms ice slurry. From the crystallization points (on the blue line) onwards, small spherical ice particles are formed which remain mobile, so that the ice slurry does not have an explosive effect on metallic plant components. If the temperature drops below the pour (solidifying) point, an explosive effect is possible.
Ice slurry consists of a mixture of small ice particles (0.01-0.5 mm), water and a substance or product that lowers the freezing point. Ice slurry is liquid to viscous and pumpable. It is produced at temperatures close to the specific freezing point or pour point of a liquid and, due to its aggregate state, has no explosive effect as long as it is able to flow freely.
Glycols used are mainly ethylene glycol and propylene glycol. Ethylene glycol has the best physical properties when used as an antifreeze in heat transfer media. However, when glycol is used in cooling systems in food processing or if the refrigerant can come into contact with drinking water, only propylene glycol (1,2-propanediol), which is approved in the EU as a food additive, may be used.
A bit of chemistry: Ethylenglycol (EG), also Mono-Ethylenglycol (MEG), is the simplest dihydric (bivalent) alcohol and has the chemical name ethane-1,2-diol. It is therefore the simplest diol.
1,2-propanediol (1,2-propylene glycol) is a clear, colourless, odourless, and strongly hygroscopic liquid. Propylene glycol belongs to the polyvalent alkanols. It is miscible with water in any ratio, but not with fatty oils.
Pure glycols, in combination with oxygen, form very aggressive substances towards metals. For this reason, so-called corrosion protection inhibitors are added. In addition, at the high speeds of the pumps there is a risk of foam formation, which can lead to a disruption in the flow rate. To combat this, foam-inhibiting additives are also added to glycol.
In order for these additive substances to do their job, minimum concentrations must be maintained. For ethylenglycol-water mixtures, these are 20 % by volume, and for propylenglycol-water mixtures 25 % by volume. This also applies if the addition of glycol allows much lower temperatures than the design of the refrigeration system may require. If this is not considered, the long-term corrosion-inhibiting effect is no longer guaranteed. There is also the risk of microorganisms forming in the system, whose decomposition products produce acids which may attack metals and can lead to blockages in filters and poorer heat transfer coefficients in the system.
So the minimum concentrations mentioned above must not be undercut.
Mixtures of ethylenglycol and water offer the following typical lowest temperatures for a couple of concentrations:
20% – 9° C
25% -12° C
30% -16° C
40% -20° C
45% -31° C
The highest concentration shall not exceed 58% because relevant physical and chemical properties make the use of such mixtures difficult.
For ethylenglycol the most important technical data at 20°C are
Density 1.13 g/cm³
Vapor pressure 0,053 mbar
Specific heat capacity 2.35 kJ/(kg ∙ K)
Dynamic viscosity 21 mPa ∙ s or 21 kg/(m ∙ s)
and a boiling point of 197°C. Note that these figures may vary for glycol products from different suppliers.
Let us look at the following example:
A plant has been designed for a capacity of 100 kW, ΔT = 5K and a glycol percentage of 20% ethylenglycol. This allows frost protection down to -9°C. From some tables we read
ρ (at -9°C, 20% vol-%) = 1,038 g/cm³
cp (at same conditions) = 3,85 kJ/(kg ∙ K)
We calculate a volume flow of 5 m³/s or for the abovementioned conditions.
How large is the degradation in performance if we would like to run the unit down to -29°C?
This requires 44 vol-% of glycol and the related capacity for the same volume flow of 5 m³/s can be calculated with these values from the table
ρ (at -29°C, 44% vol-%) = 1,088 g/cm³
cp (at same conditions) = 3,13 kJ/(kg ∙ K)
and find a capacity of 85,1 kW which is a degradation of about 15% due to the lower specific heat capacity of glycol.
A brief discussion of use for service interventions. The first thing is determining the required frost protection and thus the percentage of glycol in the water circuit as well as the plant volume. Because of the stance that glycol is usually purchased in kg due to the much higher transportation cost in case of already mixed with water.
Let us assume that the required frost protection is for -20°C and the chiller/plant has a volume of 5.000 litres. To keep the water liquid we need glycol at a volume % of 34%.
The density of the glycol/water mixture can be calculated from the mass-% of glycol in the system
wglycol = mglycol / (mglycol + mwater) = vol-%glycol ∙ ρglycol / (vol-%glycol ∙ ρglycol + vol-%water ∙ ρwater)
vol-%glycol = 35%
ρglycol (at -20°C, 34% vol-%) = 1,065 g/cm³
we find for the mass-%
wglycol = 36,52%
The density of the water/glycol mixture is ρ = 1,04 g/cm³. So, the total mass of the water/glycol mixture is
mass = 5.000 litres ∙ 1,04 kg/m³ = 5.200 kg
The mass fraction of glycol therein is
mass fraction = wglycol ∙ 5.200 kg = 36,52% of 5.200 kg =1.900 kg
There are many other aspects which need to be considered when dealing with glycol in a system. This brief report summarizes necessary information for an initial and better understanding of the subject matter.