Should I use water as my 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 electrically isolated.

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 25°C - - 5.0 - 7.0
 Conductivity (µS/cm) at 25°C 0.1 1.0 5.0
 Oxidisable 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
 Silica (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.  

ATC also offer a heater pack, making it possible to raise the operating temperature of the chiller above 35°C.

Applied Thermal Control Ltd.
39 Hayhill Industrial Estate, Barrow upon Soar, Leicestershire, LE12 8LD.
Telephone: +44 (0) 1530 83 99 98
E-Mail: sales@app-therm.com
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