When choosing a heat transfer fluid, there are many things
to consider to ensure optimum performance. There are four main categories of
heat transfer fluid:
- Water (including deionised water)
- Glycol/water combinations (ethylene glycol/water
or propylene glycol combination)
Water is one of the most efficient and highest quality
products available for heat transfer by theory, but freezes and boils,
providing a limited temperature range within which water alone can be used as a
heat transfer medium. Glycol and water combinations are able to offer very high
stability at higher temperatures and often require a lower start up temperature
than oils. Oils are often non-toxic, efficient, and cost-effective, performing
well at moderate to high temperatures. The benefits and drawbacks of each of
these heat transfer fluids will be discussed in a later section of this article.
PFPE, or Perfluoropolyether, is a type of fluorinated fluid
that can operate at both temperature extremes, with typical operating
temperatures ranging from -70ºC to 290ºC. PFPE is able to offer high thermal
stability, has good dielectric properties and is chemically inert, compatible
with metals, plastics and elastomers. However, PFPEs are expensive and have seeking
To select an efficient heat transfer fluid, you must first
know the minimum and maximum operating temperatures of your equipment. Fluid
with a lower operating temperature will offer protection from premature
degradation of your equipment when turning on. It is important to consider the
thermal stability, heat transfer efficiency, and expansion rate of heat
transfer fluids, and ensure that the fluid will meet the requirements of both
the process and chiller.
Several questions must be asked regarding the operating
conditions of the heat transfer fluid to ensure maximum compatibility.
- Is the fluid able to operate efficiently
throughout the entire process cycle, transitioning between temperatures
- What is the local climate?
- Will the chiller be located outside and so open
to the atmosphere?
- Is this a continuous process?
- Does this process require heat transfer fluids
with a food-grade rating?
To ensure that phase separation does not occur, it is
essential to consider fluid compatibility when replacing existing heat transfer
fluids. As it is incredibly challenging to completely drain fluid from a
system, any remnants of expired fluid that do not effectively mix with
replacement fluid could cause pump cavitation and unnecessary wear and tear on
other parts throughout the system. It is also important to ensure that there is
no contamination of heat transfer fluids during maintenance, as contaminants
are incompatible with most heat transfer fluids and will immediately degrade
their efficiency. When considering compatibility, it is also essential to
ensure that the heat transfer fluid is compatible with the construction
materials of your equipment to prevent leeching of particles into the fluid.
There are four crucial factors that should be compared when
choosing a heat transfer fluid:
- Heat transfer efficiency
- Temperature range
- Material compatibility
- Thermal stability
The heat transfer efficiency of a heat transfer fluid is
determined by several characteristics. An ideal heat transfer fluid will have:
- Low viscosity
- High density
- High thermal conductivity
- High specific heat capacity
With the right balance of these characteristics, a heat
transfer fluid will provide better heat transfer efficiency at a range of
operating temperatures and flow conditions.
Any impurities should be removed from the heat transfer
fluid to prevent fluid degradation and fouling on surfaces within the system.
This extends the life of heat transfer fluids and reduces the amount of
When comparing the thermal stability of heat transfer
fluids, it is important to remember that a product with higher thermal
stability will provide higher heat transfer efficiency for a longer period and
poses a lower risk of system damage or safety hazards when used in accordance
Consider how frequently heat transfer fluids need to be
replaced and whether a specialist waste removal service will be required to
safely dispose of expired fluids. Ensure that the cost of this has been
factored into the budget. It is also worth noting that although selecting a
heat transfer fluid with an extended temperature range will provide a safety
cushion, it can come at an enhanced cost, so discuss specific system
requirements with the supplier.
Take time to discuss the requirements of your chiller with
the manufacturer and take note of their heat transfer fluid recommendations.
Working with a heat transfer fluid supplier who can provide technical support
through the lifecycle of the product is beneficial, as fluid degradation will
In summary, choosing the right heat transfer fluid is
essential in protecting against accelerated degradation, machine failure,
increased maintenance, and decreased efficiency. Ensuring that fluids work in
synchronisation with the specifications of the chiller and that regular
preventative maintenance is undertaken will protect both the chiller and the
fluid from damage. Expert advice should be sought from the heat transfer fluid
supplier or chiller manufacturer to ensure that the right fluid is selected,
and a proper maintenance plan formed. This will ensure that the operation runs
with increased safety, efficiency, and reliability, with decreased downtime.
Should I use water as a heat transfer fluid?
Water can be used as a heat transfer fluid, meeting the
needs of most liquid-cooling applications. Tap or facility water is cheaply and
readily available, non-toxic and has a high thermal capacity. Due to the low
viscosity of water, it is easy to pump. Using good quality water is recommended
to minimise the potential for corrosion and to optimise thermal performance.
The qualities of ‘good water’ can be seen in the table below:
|Mineral ||Recommended Limit |
|Calcium ||<50ppm |
|Magnesium ||<50ppm |
|Total Hardness ||<100ppm |
|Chlorine ||<25ppm |
|Sulphate ||<25ppm |
If tap or facility water contains high levels of minerals,
salts, or other impurities, it is important to either filter the water prior to
use, or purchase filtered or deionised water. More sensitive applications may
require deionised water.
Deionised water is made by running source water through one
or more separate electrically charged resins, removing all, or most, of the
ions. Ions removed include sodium, calcium, iron, copper, chlorine, and
bromide. Removal of harmful minerals, salts, and other impurities can protect
the system from corrosion or scale formation, damaging machine health. ATC
supply the option to install in-line deionising cartridges, polishing the
recirculating water to better than 1 microsiemens/cm² or 10 megaohm/cm².
Deionised water has a higher resistivity than tap water.
Resistivity provides a measure of waters ionic content. It is worth noting that
as resistivity rises, as does corrosivity. Stainless steel or ABS piping is
required when using deionised water as a heat transfer fluid to ensure that
particles from plumbing materials are not leeched into the water circuit,
potentially causing fouling and blockages.
Conductivity provides a measurement of a fluid’s ability to
conduct electrical current. If resistivity is high, conductivity will be low.
As it is an excellent insulator, with very low conductivity, deionised water is
often used in the manufacturing of electrical components where parts must be
The relationship between conductivity and resistivity can be
seen in below.
|Conductivity (microsiemens/cm²) ||
Resistivity (megaohm/cm²) |
| 0.056 ||18 |
|0.063 ||16 |
|0.071 ||14 |
|0.083 ||12 |
|0.100 ||10 |
|0.133 ||7.5 |
|0.200 ||5 |
|0.500 ||2 |
|1.000 ||1 |
|1.333 ||0.75 |
|2.00 ||0.5 |
|4.00 ||0.25 |
|10.00 ||0.1 |
|20.00 ||0.05 |
|40.00 ||0.025 |
|80.00 ||0.013 |
|100.00 ||0.01 |
|200.00 ||0.005 |
|500.00 ||0.002 |
|1000.00 ||0.001 |
|2000.00 ||0.0005 |
|5000.00 ||0.0002 |
|10000.00 ||0.0001 |
There are three grades of deionised water.
Grade 1 water, or ultrapure water, is the purest form of
water available. This type of water should be used for advanced analytical
procedures and critical applications. It can also be used in applications that
require grade 2 water. Applications using grade 1 water include liquid
chromatography, gas chromatography, inductively coupled plasma mass
spectrometry (ICP-MS) and molecular biology.
Grade 2 water does not have the same level of pureness as grade
1 water, but still maintains high levels of purity. Although grade 2 water
cannot be used for applications requiring type 1 water, it can be used as a
feed water in the production of grade 1 water. Applications using grade 2 water
include general lab practices, electrochemistry, and general spectrophotometry.
Grade 3 water, or RO water, is water produced through
reverse osmosis. It has the lowest level of purity and is used for many basic
lab applications such as heating baths and media preparation. RO water can also
be used as feed water in the production of grade 1 water.
Please see the table below for the International
Organisation for Standardisation (ISO) requirements for the grading of
deionised water under ISO 3639:1987.
|Parameter ||Grade 1 Water ||Grade 2 Water ||Grade 3 (RO) Water |
| pH value at
||- ||- ||5.0 - 7.0 |
(µS/cm) at 25°C
||0.1 ||1.0 ||5.0 |
matter Oxygen content (mg/l), max
||- ||0.08 ||0.4 |
| Absorbance at
254nm and 1cm optical path length, absorbance units, max.
||0.001 ||0.01 ||- |
| Residue after
evaporation on heating at 110°C (mg/kg), max
||- ||1 ||2 |
(SiO₂), content (mg/l), max
||0.01 ||0.02 ||- |
As water, and deionised water, alone have no antimicrobial
properties, they are vulnerable to contamination. Microbial contamination can
be a difficult problem to remedy once it enters a system, as it causes growth,
leading to internal fouling and blockages. To minimise this risk, in-line UV
decontamination packs allow any growth to be prevented by passing the water
through a steel tube which contains a UV lamp.
When water passes under the ultraviolet light, the genetic code of
microorganisms is attacked, rearranging the DNA/RNA, meaning that the
microorganism is unable to reproduce or function.
However, water has a low boiling point and freezes easy, which
makes it unstable and difficult to manage under extreme temperature conditions.
ATC offer frost protection to protect chillers using water
as a heat transfer fluid, allowing the chiller to function in freezing
temperatures by altering the wiring and thermostat to prompt the pump to run
should the machine drop below +6°C.
To support chillers located in lower temperatures, Applied
Thermal Control offer a low temperature pack, allowing the chiller to operate
below 4°C. Addition of a low
temperature pack will allow chillers to be operated down to -15°C, although
heat transfer fluids containing glycol are recommended at temperatures.
offer a heater pack, making it possible to raise the operating temperature of
the chiller above 35°C.
Should I use glycol as an a heat transfer fluid?
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.
two types of glycol heat transfer fluid, ethylene glycol/water combinations and
propylene glycol/water combinations.
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.
glycol and water combinations as a heat transfer fluid has several desirable
- High boiling point
- Low freezing point
- Stability over a wide temperature range
- Low viscosity, reducing pump requirements
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.
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.
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 |
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
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.
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.
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
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
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:
- 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.
Should I use oil as an a heat transfer fluid?
temperatures, water ceases to become an effective heat transfer fluid. To
continue to use water as a heat transfer fluid it may become necessary to
pressurise the system, and substantial monitoring will be required to ensure
safe operation. Water can also cause corrosion within the system.
synthetic oils are suited for use as a heat transfer fluid at much higher
temperatures, and do not need to be pressurised until the top end of the range
heat transfer oil is capable of meeting all of the below factors evenly, the
following factors should be considered, alongside the specifics of the
- Low viscosity
- Good thermal stability
- High flash point
- Good heat transfer properties
- Ease of waste disposal
important that the viscosity of heat transfer oils is low, especially when
operating at the lower end of the temperature range, as this will affect the
operating conditions under which the chiller is able to function. If the oil
becomes too viscous under lower temperatures, the system will not be able to
start up, causing damage to the chiller. The operating viscosity of the heat
transfer oil affects the flow properties within the pipes. The correct
viscosity, combined with the associate optimum turbulent flow, enhances heat
stability of heat transfer oils makes a large contribution towards pump
efficiency and the safe operation of the heat transfer system.
important to ensure that high quality heat transfer oils are used. Heat
transfer oils are usually a mixture. It is essential that constituents with a
low boiling point are removed, as they will begin to evaporate during normal
operation of the chiller, their presence will reduce the viscosity and lower
the flash point of the oil. If the operating temperature of the chiller rises
with such constituents present, constituents with a much higher boiling point
are cracked, resulting in a high-viscosity substance with deposits a sticky
substance on pipes and surfaces.
selecting a heat transfer oil, it is important to look at the heat transmission
characteristics of the oil. The heat conductivity of the oil will give a good
indication of how well heat will transfer from the film coating the pipe walls
into the flowing heat transfer oil. The vapour pressure of the heat transfer
oil will indicate whether the system will be able to be run without
pressurisation. The thermal expansion-coefficient of the heat transfer oil will
indicate whether the oil is compatible with the size of the expansion tank.