What is glycol? and why is this compound indispensable in modern cooling solutions? Chemically, glycol is an organic compound in the diol family, with two hydroxyl groups (-OH) bonded to two adjacent carbon atoms. In the operation of industrial refrigeration system, glycol acts as a secondary refrigerant, absorbing heat from the product and carrying it back to the evaporator of the compressor system.
The two most common forms today are ethylene glycol (EG), with outstanding heat transfer performance, and propylene glycol (PG), with high food safety credentials. Mixed with water at 30% to 50%, the solution reaches a freezing point of -15°C to -37°C, meeting the demanding standards of the seafood processing and pharmaceutical industries. This article from Tan Long's engineering team analyses the composition in detail. water chiller your.
What is glycol chemically?
To understand what What is glycol? really is in engineering terms, we need to look at its molecular structure. It is an organic compound in the diol family — specifically an alcohol carrying two hydroxyl groups (-OH) on two adjacent carbon atoms. That structure is the core difference between glycol and ordinary monoalcohols such as ethanol, which carry only a single -OH group.
Those two -OH groups determine three important properties of glycol: complete solubility in water, a high boiling point (197°C / 387°F for ethylene glycol), and the ability to lower the freezing point of a water-glycol mixture through freezing point depression. When glycol dissolves in water, its molecules insert themselves between the water molecules, disrupting the ice crystal lattice and pushing the freezing point below 0°C.
Ethylene glycol (EG) has the formula C₂H₆O₂ and a molecular weight of 62.07 g/mol. Propylene glycol (PG) has the formula C₃H₈O₂ and a molecular weight of 76.09 g/mol. PG's carbon chain is one unit longer than EG's, which makes it more viscous and gives it roughly 10 to 15% lower heat transfer performance than EG at the same concentration.
Common industrial glycols
Each type of glycol is produced to meet particular technical standards and applications. Understanding What is glycol? through its chemical variants helps a business choose the right medium for its system. The table below summarises the key parameters of the four glycols most common in industry.
| Type of glycol | Formula | Boiling point | Toxicity | Main applications |
|---|---|---|---|---|
| Ethylene Glycol (EG / MEG) | CHOCOLATE | 197°C (387°F) | Toxic to humans and animals | Industrial secondary coolant, automotive antifreeze, PET fibre production |
| Propylene Glycol (PG) | CHOCOLATE | 188°C (370°F) | Food safe (FDA GRAS) | Food-contact secondary coolant, pharmaceuticals, cosmetics |
| Diethylene Glycol (DEG) | CH₄H₁₀O₃ | 245°C (473°F) | Moderately toxic | Solvent, polyurethane production |
| Dipropylene Glycol (DPG) | CH₁₄O₃ | 232°C (450°F) | Very low | Cosmetic fragrance, plasticiser, printing ink |
Ethylene glycol (MEG) in refrigeration systems
Ethylene glycol (or monoethylene glycol, MEG) is the most widely used secondary coolant in industrial refrigeration. MEG is a colourless, odourless, slightly viscous liquid with a density of 1.113 g/cm³ at 20°C (68°F).
MEG's technical advantage lies in its higher thermal conductivity and lower viscosity compared with PG, which reduces pressure loss in the pipework and improves heat transfer. At 40% by volume, a MEG-water mixture reaches a freezing point of -24°C (-11.2°F), suitable for cold stores holding -18°C to -22°C (-0.4°F to -7.6°F). At 50%, the freezing point drops to -34°C (-29.2°F), for deep-freeze stores.
MEG is toxic to humans if swallowed (LD50 = 4,700 mg/kg in rats), so it is used only in closed systems with no direct food contact. In seafood and food processing in Vietnam, MEG is commonly used for chilling process water where there is a barrier between the glycol water and the product.
Propylene glycol (PG) in food-contact applications
Propylene glycol is recognised by the FDA and the EU as a safe food additive (GRAS, E1520). That is why PG is chosen for refrigeration systems where a leak could reach food — pipework running through production rooms, or belt chilling systems in processing plants.
PG is about 40% more viscous than MEG at 20°C (68°F), which means greater pressure loss and pumps around 15 to 20% larger. At 40%, a PG-water mixture reaches a freezing point of -21°C (-5.8°F). At 50%, the freezing point is -29°C (-20.2°F).
Diethylene glycol (DEG)
Diethylene glycol (DEG, C₄H₁₀O₃) is a colourless, odourless liquid with a boiling point of 245°C (473°F). DEG is fully soluble in water and in most polar organic solvents.
In industry, DEG is a raw material for polyurethane resins and polyester fibre, and a solvent for printing inks and textile dyes. It is less common in refrigeration systems because the freezing point of a DEG-water solution does not fall as low as MEG or PG at the same concentration.
Dipropylene glycol (DPG)
Dipropylene glycol (DPG, C₆H₁₄O₃) has a longer molecular chain than propylene glycol and disperses fragrance well. DPG is fully soluble in water and many organic solvents. Its very low toxicity makes it a favourite in cosmetics, perfumes and skincare products. In manufacturing, DPG serves as a plasticiser and as a feedstock for polyester and polyurethane synthesis.
Glycol applications in industrial refrigeration
To understand the practical application of What is glycol?, we need to look at its role in indirect refrigeration systems. These work on the principle of two completely separate circuits. The primary circuit uses refrigerants such as NH₃ or freon to chill the glycol solution inside a heat exchanger. The secondary circuit then uses circulation pumps to carry the chilled glycol solution to the terminal units, or to cool directly..
This design brings two core benefits to plants in Vietnam. First, it keeps toxic refrigerants such as NH₃ away from direct contact with production areas.
Chilling process water in seafood plants
In seafood processing plants today, wash water and soaking water have to be held between 0°C and 4°C to control microbial growth under demanding HACCP standards. A water chiller using glycol does this by circulating the glycol solution through heat exchange coils immersed directly in the water tank.
Typically the glycol supply temperature is set between -5°C and -8°C, so the water in the tank reaches 0°C quickly without freezing locally. Tan Long is a leading supplier of process water chilling solutions with a flexible capacity range from 10 kW to over 500 kW, serving production scales from 2 tonnes to more than 50 tonnes of finished product per day for seafood businesses nationwide..
Distributed cooling systems in food plants
Large food processing facilities usually have several stages needing different cooling loads: the pre-processing area, the IQF freezer conveyor and the intermediate storage rooms. Installing a central glycol system optimises capital cost compared with fitting many separate refrigeration units. A central plant distributes the solution through insulated pipework to each point that needs it, which makes monitoring and refrigeration system maintenance simpler and more effective.
Temperature control in pharmaceuticals and chemicals
Particular chemical reactions and pharmaceutical storage processes demand an extremely tight temperature regime, typically between -10°C and 20°C. Circulating a glycol solution through the jacket of a reactor vessel or the cooling equipment in a pharmaceutical cold store allows engineers to control the temperature to within ±0.5°C. That is critical to maintaining the physical and chemical quality of the product throughout production.
Cooling electronics and data centres
In data centre cooling systems, glycol absorbs heat from the server equipment and carries it out through the cooling tower. In Vietnam's tropical climate, the glycol solution plays an important role in holding the supply temperature steady between 7°C and 12°C, protecting sensitive electronics from the constant swings in ambient temperature.
The relationship between glycol concentration and freezing point
A solid grasp of What is glycol? and of how to mix the concentration correctly is the decisive factor in both freeze protection and heat transfer performance. The mixing ratio affects more than just the temperature.
| Glycol concentration (% by volume) | Ethylene glycol: freezing point | Propylene glycol: freezing point |
|---|---|---|
| 20% | -8°C (17.6°F) | -7°C (19.4°F) |
| 30% | -15°C (5°F) | -13°C (8.6°F) |
| 40% | -24°C (-11.2°F) | -21°C (-5.8°F) |
| 50% | -34°C (-29.2°F) | -29°C (-20.2°F) |
| 60% | -48°C (-54.4°F) | -40°C (-40°F) |
The minimum operating temperature of the system must be at least 5°C (9°F) above the freezing point of the glycol solution for safety. For example, a system running at -20°C (-4°F) needs
A higher glycol concentration raises viscosity and lowers thermal conductivity, so never use a higher concentration than necessary. To measure the glycol concentration in a system, use a refractometer or a hydrometer — an important routine check in refrigeration system maintenance.
Glycol compared with other secondary coolants
To answer a question many investors have about choosing the optimal medium, we need to understand what What is glycol? really is alongside other secondary coolants. In cold storage design and construction projects in Vietnam, the three most common options are a glycol solution, brine (NaCl or CaCl₂) and plain water. Each has its own technical characteristics, set out in the comparison table below.
| Criteria | Glycol (EG 40%) | 23% NaCl brine | Plain water |
|---|---|---|---|
| Freezing point | -24°C (-11.2°F) | -21°C (-5.8°F) | 0°C (32°F) |
| Corrosivity | Low (with corrosion inhibitor) | High, corrodes steel | Medium |
| Food safety | EG: no; PG: yes | No | Have |
| Material cost | Medium | Low | Very low |
| Maintenance requirement | Check concentration every 6 months | Check pH and concentration regularly | Check water quality |
| Suitable temperature range | -5°C to -40°C (23°F to -40°F) | 0°C to -20°C (32°F to -4°F) | 5°C to 15°C (41°F to 59°F) |
Glycol is preferred over brine in the seafood and food industries because brine corrodes equipment faster and its concentration is harder to control accurately over time. Glycol with additives
Practical uses of glycol beyond refrigeration
Beyond understanding What is glycol? in freezing systems, this compound appears widely across many essential manufacturing sectors. Its excellent physical and chemical properties give glycol a role in:
- Textiles: Monoethylene glycol (MEG) is the key feedstock for synthesising polyester (PET) fibre. This application currently accounts for around 65% of total MEG consumption worldwide.
- Automotive: Ethylene glycol is the core component of engine coolant. Mixed with water at 50:50, it lowers the freezing point to -37°C (-34.6°F), protecting the engine.
- Pharmaceuticals and cosmetics: Propylene glycol is trusted as a solvent and humectant in skin creams, medicinal syrups and soft capsules, thanks to its high safety profile.
- Aviation: Propylene glycol solution is used to de-ice aircraft wings before take-off, keeping flights safe in cold climates.
A firm grasp of these technical properties not only optimises the production process but is the basis on which Tan Long's engineers advise on the right solution when installing water chiller or glycol heat exchange equipment specialized.
Technical safety rules for handling glycol solutions
Operating a refrigeration system safely with this medium comes down to three central concerns: chemical toxicity, concentration control and preventing food contamination. To understand What is glycol? in the context of safety, operators need to be clear on the following:
- Controlling toxicity and preventing leaks: Ethylene glycol (EG) is highly toxic if swallowed, with a lethal dose of around 1.4 ml/kg of body weight (equivalent to 98 ml for a 70 kg person). In a processing environment, an EG leak into food is an extremely serious incident. Systems using EG in food-contact areas must therefore be fitted with leak detectors, and the engineering team should give preference to.
- Scheduled concentration maintenance: The concentration of the solution in the system needs checking at least every 6 months. Automatic water make-up dilutes the solution over time, raising the freezing point and potentially cracking pipework through local freezing. The simplest refrigeration system maintenance method is a handheld refractometer, measuring the concentration directly at the sampling points.
- Waste handling and changing the solution: Used solution typically contains oxidation products such as glycolate and oxalate, with the corrosion inhibitors depleted. In practice, the solution should be replaced every 3 to 5 years, or once the pH falls below 7.0 (the solution turns yellow-brown). Waste glycol must be collected and disposed of under the applicable regulations..
- Personal protective equipment (PPE): When handling the chemical, technicians must wear safety goggles, nitrile gloves and a chemical-resistant apron. In the tank area, ventilation must work properly and an emergency eyewash station must be within 10 metres..
Tan Long advises on and installs glycol process water chillers for seafood, food and pharmaceutical plants in Vietnam, with more than 20 years of practical experience. Contact our engineering team for advice on selecting the right glycol and designing a system to suit your production process.
Glycol is most commonly used in indirect refrigeration systems — see the corresponding equipment: glycol water chillers.
Frequently asked questions about what glycol is
Are glycol and glycerine the same thing?
Glycol and glycerine are both polyols but differ in structure. Glycol is a diol (two -OH groups); glycerine is a triol (three -OH groups, formula C₃H₈O₃). Glycerine is more viscous with a higher freezing point than glycol, which makes it unsuitable as a secondary coolant for low-temperature systems.
Can automotive glycol replace industrial glycol?
Not advisable. Automotive glycol contains corrosion inhibitors formulated for non-ferrous systems (copper, aluminium), which are unsuitable and can cause problems with the materials used in industrial refrigeration — typically stainless steel and copper. Industrial glycol comes with an inhibitor package designed for the specific materials and operating temperature range.
Does glycol have a shelf life?
Yes. Glycol solution in a refrigeration system is replaced when the pH falls below 7.0, or after 3 to 5 years of operation depending on conditions. Check the pH with litmus paper or a pH meter at the glycol tank.
Frequently asked questions about glycol
What is glycol?
Glycol is a family of organic compounds used as an intermediate secondary coolant in industrial refrigeration. A glycol-water solution has a lower freezing point than water, so it can carry cooling at sub-zero temperatures without solidifying.
How do ethylene glycol and propylene glycol differ?
Ethylene glycol (MEG) transfers heat more effectively and is normally used in conventional industrial refrigeration. Propylene glycol (PG) is safer in food contact and is therefore chosen for food and pharmaceutical processing lines.
Why should food plants use propylene glycol?
Because there is always a risk of the secondary coolant leaking into the production area. Propylene glycol is less toxic than ethylene glycol, which limits the consequences if that happens.
What is glycol used for in a refrigeration system?
Glycol acts as the intermediate secondary coolant: the chiller cools the glycol solution, which is then pumped to the points in the plant that need cooling. This allows several areas to be cooled from a single central plant.
Can glycol be used in the pharmaceutical industry?
Yes. Glycol is used for temperature control in pharmaceutical and chemical production, where a stable, precise temperature has to be held throughout the process.
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With over 18 years of practical experience in directing, consulting, designing, and constructing heavy industrial refrigeration systems, ASRS automated warehouses, and IQF freezing systems across more than 29 major national projects.
