Heat transfer fluid characteristics (process coolants, secondary refrigerants, glycols etc) have a direct and long-lasting impact on cooling and heating system performance, production output, energy consumption, maintenance, and down-time.

Efficient thermal energy transfer, minimal pumping-energy consumption, long-term preservation of pipework/system components, cost, and minimal environmental impact, all need to be taken into account when selecting the optimum fluid solution.

The effectiveness of HTF are governed by the following physical characteristics:

• Specific Heat Capacity
• Volumetric Heat Capacity
• Thermal Conductivity
• Density
• Viscosity – Dynamic and Kinematic
• Prandtl Number

Specific Heat Capacity - Cp (Joules per Kilogram Kelvin [J/kgK])
A high value of Specific Heat Capacity (Cp in Joules per Kilogram Kelvin [J/kgK]) is preferable as it affects the Heat Transfer Coefficient and Volumetric Heat Capacity.

Volumetric Heat Capacity - PCp (Kilo Joules / Meter-cubed Kelvin [kJ/m3K])
The Volumetric Heat Capacity (VHC) can be expressed as the Specific Heat Capacity multiplied by the Density, and has a direct influence on the volume flow-rate, V (m3/sec) needed for a given heating power Q (Watts) and temperature change ΔT (Kelvin).

The simple mathematical relationship can be written as (expressed as):
Q = V x PCp x ΔT

The VHC is then an index of the ability of a HTF to convey heat directly, by its flow and motion (turbulent or laminar) to a given point of heat-transfer, such as a heat-exchanger etc.

Thermal Conductivity - k (Watts / Metre Kelvin [W/mK])
Thermal Conductivity is defined by Fourier’s Law for heat conduction, as the negative rate of heat-flux and temperature gradient dT/dx over the thickness x and the Cross Sectional Area A:

The simple mathematical relationship can be written as (expressed as):
Q/dt = -kA x dT / dx

A high value of Thermal Conductivity is desirable, as it contributes to good heat transfer, decreasing the temperature differential (ΔT) between HTF and tube wall in heat exchangers. Whether those heat exchangers be tubular or plate, evaporator or condenser.

Density – P (kilograms / meter3 [kg/m3])
Within certain limits, the Density of a HTF will increase the VHC, but also increase the pumping load and pumping energy absorbed.

Viscosity
Dynamic Viscosity U (milli Pascal Seconds [mPa-s) & Kinematic Viscosity V (millimetres2 per second [mm2/s]) The Kinematic Viscosity of an HTF = Dynamic Viscosity divided by the Density.

Viscosity is a fundamental property in the Reynolds Number Re = wd/v, which in turn is essential to calculate heat transfer and pressure drop. Specifically, the Reynolds Number will determine whether the HTF flow is laminar, transitional or fully turbulent.

Prandtl Number
The combined value of the above characteristics can be represented and quickly compared using a single dimensionless value – the Prandtl Number.

The lower the Prandtl Number value the more thermally efficient the HTF.

Water is currently the most efficient HTF available, with a Prandtl Number of 7.0 at 20°C. In a heat pump system for example, the Prandtl Number values for typical HTF mixtures include:

Please note: All HTF listed below are mixed at ratios (strengths) to provide -15°C freeze-point protection.

33% weight Propylene Glycol mixed with 67% weight water at 20°C - 29.3
40% weight Glycerol (Glycerine) mixed with 60% weight water at 20°C - 27.9
25% weight Ethyl Alcohol (Ethanol) mixed with 70% weight water at 20°C - 24.0
™ mixed with 72% weight water at 20°C - 19.1
30% weight Ethylene Glycol mixed with 70% weight water at 20°C - 17.3
24% weight Thermox™ LVF15 mixed with 76% weight water at 20°C - 8.9
100% water at 20C, but will freeze at 0°C - 7.0

As mentioned above/earlier, water is the most efficient HTF on the Planet, but it freezes at 0°C and therefore is not suitable for use in sub-zero process systems, or where subjected to sub-zero ambient conditions.

In simplistic terms Thermox™ LVF15 – which is based on Potassium Formate salts – has the best thermal efficiency and Propylene Glycol–water mixtures have the worst.

 

HTF: engineering, safety, environmental and cost considerations

In addition to the thermal and pumping characteristics of any HTF, when making the appropriate selection for any closed-loop cooling or heating system it is also important to consider:

• Corrosivity, Corrosion, Corrosion Inhibitors and Fluid Monitoring
• Toxicity
• Biodegradability
• Biological susceptibility and durability (aka lifespan)
• Flammability
• Pumping energy demand and pumpability
• Cost and availability

Corrosivity and Corrosion
Unsuitable and uninhibited HTF can cause corrosion of metal components. The corroding part of a component is called the anode, which will tend to dissolve into the HTF and/or coat other metals of higher nobility. E.g., carbon steel will ‘sacrifice’ itself to copper.

Oxidation Corrosion, as caused by air and/or dissolved oxygen, will tend to dissolve metals evenly across their surface, and most often affects metals which do not form passive protective layers or are naturally resistant to oxidation corrosion.

The most susceptible metals to oxidation corrosion include magnesium, zinc (galvanised pipe), cast iron, ductile iron and carbon steel. Whilst iron and steel can be protected using the appropriate corrosion inhibitors, it is virtually impossible to prevent magnesium and zinc from corroding in a submerged aqueous environment, such as a flooded pipework system.

Galvanic Corrosion can take place when two different metals of varying nobility are ‘connected’ by a fluid, which acts as an electrolyte. E.g., HTF inside a geothermal or GSHP system. An electric potential difference is generated between the metals and the less noble (less precious) metal acts as an anode and dissolves, while the more precious metal acts as the cathode.

Pitting and Crevice Corrosion are typical in installations where metals protected by a passive coating, such as galvanised or some stainless-steel pipes. An increased local corrosion rate can occur where there is a flaw in the passive coating, when the potential difference is concentrated.

Erosion Corrosion is where metal and synthetic components are worn down over time, due to one or more of the following causes:

• High levels of entrained sediment in the HTF, acting as an abrasive inside pumps and bends.
• Cavitation is most often observed inside centrifugal pumps. E.g., worn impellers and volutes, but can also be found where there are severe changes in HTF direction, or poorly designed pipework etc.

To minimise corrosion, it is very important for HTF to be formulated with effective and proven inhibitors. Specifically, the inhibitor formulation should take into consideration the metals of construction, antifreeze type, susceptibility to air ingress and subsequent bacterial contamination, make-up water quality and effectiveness of pre-commission pipework flushing and conditioning (passivation).

Toxicity
The toxicity of HTF formulations becomes very relevant and important where that HTF might accidentally come into contact with humans, pets, mammals and aquatic life. Emphasis here on the word ‘accidental’ as we all recognise that no business or person would deliberately contaminate a water-source or consume a chemical.

Accidental exposure to HTF can occur for a number of non-deliberate reasons, including spillage, ground-loop failure (split pipe etc.), heat-pump connections weeping etc. As such it is important that the HTF be classified as non-toxic on the Safety Data Sheet (SDS).

Neat (undiluted) DTX is classified as being non-toxic. When diluted with water all chemicals become less toxic on a pro-rata basis, so when neat DTX is diluted with 72% volume water to achieve -15°C freeze-protection, it becomes even less toxic.

Biodegradability
To long-term prevent soil or ground-water contamination – in the event of accidental spillage or leak – it is important that HTF formulations completely biodegrade over time. I.E., they should not bio-accumulate.

Neat and dilute mixtures of DTX are classified as being 100% biodegradable and will not bio-accumulate. The five-day biochemical oxygen demand (BOD5) of a 28:72 mixture of DTX and water is 0.3 grams of oxygen for each gram of mixture. Water has a zero BOD5.

Biological susceptibility and durability (aka lifespan)
Our natural environment contains a myriad of different microbes, including bacteria, algae, fungi and protozoa, most of which are too small to be seen by the naked eye. Microbes live in air, water and soil and can multiply at exponential rates inside process cooling and heating systems if not prevented from doing so.

Hence, it is crucially important for geothermal and GSHP systems to be carefully sterilised before filling with HTF, and for HTF formulations to include chemicals which neutralise and/or supress all form of microbes.

DTX contains two biocides proven to provide long-term protection against microbes. As with corrosion inhibitor reserves, the ongoing function of these biocides should be periodically verified by means of the Fluid Monitoring Program, and where required additional biocides added.

Flammability
Ethanol is the most often used flammable HTF found in geothermal and GSHP systems, and whilst not often selected for UK installations, Ethanol is still regularly used in Scandinavia. The fire risks of using 20% to 30% Ethanol in solution with water are obvious and as such rarely acceptable to customers or their insurers.

HTF - Energy efficiency and cost
The combined HTF characteristics detailed in b) and c) above will directly impact the short, medium and long-term energy efficiency of associated hydronic and process systems. This will, in turn, effect the Coefficients of Performance (CoP), Running Costs, Return on Investment (ROI-Payback) and reduction in CO2.

Selecting poor quality or unproven HTF may save costs initially, but the long-term ramifications can be significant.

Alternative names for Heat Transfer Fluids

Various phrases and categories exist for Heat Transfer Fluids depending on which sector the fluid is being used in, for example:

  • RAC & HVAC: Glycol, secondary refrigerant, brine, inhibited antifreeze, working fluid.
  • Heat pump systems: Heat pump antifreeze, heat pump fluid, thermal fluid, glycol, brine. 
  • Solar hot water systems: Solar thermal fluid, solar fluid, solar glycol. 
  • Automotive: Antifreeze, coolant, glycol.

Hydratech: at the forefront of the heat transfer fluid industry

Since 1998 Hydratech have specialised in the formulation and manufacture of high-performance heat transfer fluids, based on glycols, brines, alcohols and refined vegetable extracts. Where required, bespoke formulations are often developed for unique applications.

Coca-Cola, Haribo, Papa John’s Pizza, Walls, McCain, Ocado, BrewDog, Mercedes, BMW, Arla Dairies, Ocado and Kensa Heat Pumps are just some of the companies benefiting from the numerous operating advantages of Hydratech's heat transfer fluids.

Hydratech invest heavily in research and development, to ensure they remain at the forefront of the heat transfer fluid industry. Their DTX glycol hybrids have been especially formulated for use in Process Cooling, Refrigeration, HVAC and Renewable Energy Systems - to exploit the advantages ethylene glycol has over propylene glycol, whilst delivering a non-toxic solution.

Benefits of DTX technology include; more efficient heat transfer, easier to pump (especially at low temperatures), less volume for the same freeze protection, and cheaper per litre.

Coolflow DTX™: a major step forward in heat transfer and pumping efficiency

Proven to reduce energy costs by >10% (as verified by Star Technical Services), Coolflow DTX™ is a revolutionary non-toxic secondary refrigerant, suitable for replacing propylene glycol in food and cold storage cooling systems.

Performance wise, DTX has very similar heat transfer and pumping characteristics to ethylene glycol and subsequently benefits from numerous operating advantages such as low viscosity, low dose rates and smaller systems and plant footprints onsite.

By retrofitting existing systems - moving from propylene glycol to Coolflow DTX™, customers have reported considerable CO2 savings and a noticeable reduction in running time and load on their primary refrigerant compressors, which they believe will considerably prolong the life span of their system.

Hydratech’s team of sales engineers, chemists and analysts can assist with all aspects of heat transfer fluid performance, running cost, environmental impact, compatibility and energy efficiency. By understanding your exact application requirements, the team at Hydratech can specify the optimum fluid solution and help your business improve system efficiency, extend system life and make considerable energy savings.