Although
water alone is better at retaining and conducting heat from the associated
process, glycol has antifreeze properties, and is more suited to chillers that
are expected to function in low-temperature environments, where water alone
would freeze and cause obstructions within the chiller.
There are
two types of glycol heat transfer fluid, ethylene glycol/water combinations and
propylene glycol/water combinations.
Ethylene Glycol
Ethylene
glycol is the chemical used in antifreeze. Although similar, it is important to
never use automotive glycol as a heat transfer fluid in chillers due to the
presence of inhibitors specific to automotive processes that will cause fouling
within the chiller system, reducing the lifespan of pump seals and overall
efficiency of the chiller. Use of the correct inhibitors within
chiller-specific heat transfer fluids will prevent corrosion and prolong the
life of the chiller.
Ethylene
glycol and water combinations as a heat transfer fluid has several desirable
thermal qualities:
- High boiling point
- Low freezing point
- Stability over a wide temperature range
- Low viscosity, reducing pump requirements
When
preparing or selecting an ethylene glycol/water combination, it is important to
use the lowest concentration of glycol possible to meet the needs of the
process. The higher the glycol concentration, the lower the performance of the
heat transfer fluid. At a minimum combination of 25-30% ethylene glycol to
water, the ethylene glycol will also serve as a bactericide and fungicide,
protecting the chiller from microbial contamination.
A
recirculating chiller using a 30% ethylene glycol/water combination will result
in approximately a 3% reduction in performance. Although the thermal
conductivity of ethylene glycol/water combinations is not as great as water
alone, the added freeze protection, down to -15°C/5°F, can provide benefit both
during use and shipping.
The quality
of water used when preparing an ethylene glycol/water combination is important,
as the presence of ions within the water may cause the inhibitor to fall out of
the solution, leaving the system vulnerable to fouling and corrosion. The
minimum requirements for good quality water can be seen below:
Mineral | Recommended Limit |
Calcium | <50ppm |
Magnesium | <50ppm |
Total Hardness | <100ppm |
Chloride | <25ppm |
Sulphate | <25ppm |
Applied Thermal
Control stock CoolFlow, an industrial grade refrigerant antifreeze based on
ethylene glycol. CoolFlow is sold by ATC in three variations:
- CoolFlow EG
- (CoolFlow EG Pre-mix)
- CoolFlow 1
- CoolFlow B
All CoolFlow
products are classed as harmful if swallowed and may cause irritation to skin
and eyes. It is important to wear appropriate personal protective equipment
when handling chemicals, including goggles and rubber gloves.
Propylene Glycol
Propylene
glycol/water combinations offer many of the same benefits as ethylene
glycol/water combinations but are often selected in process that require a food
grade heat transfer fluid.
Applied
Thermal Control are an exclusive supplier of Hexid, a propylene glycol/water
combination. Hexid heat transfer fluid is optimised for temperatures from -45°C
to 90°C. This is beneficial for chillers situated outside or in non-heated
rooms.
Hexid is
fully compatible with system components, protecting even copper and aluminium
systems, preventing any leeching of ions into the circuit. Hexid is also able
to provide complete protection from freezing and algal growth. A trace biocide
is included within the fluid to prevent microbial contamination within the
system.
To ensure
that the system is running at optimum and the inhibitors within Hexid remain
effective, it should be replaced annually. Hexid is safe and easy to dispose of
as it is:
- Non-toxic
- Non-flammable
- Environmentally safe
- VOC-free (ozone benign)
Hexid is a
cost-effective heat transfer to protect the investments made in chillers at low
costs per bottle. It is also easy to store and remains stable for at least two
years when stored at ambient temperatures, in closed containers, away from
direct sunlight and other sources of UV light.