By Grant Laidlaw

What is added to the water in chiller systems is done so for a number of reasons including system performance.

Khulani asks:  Hello Mr Grant, we work with chiller plants and the chiller water uses a glycol. Can you tell us, what does this does in the system? Thank you.

Hello Khulani. Yes, glycol and other chemicals are added to chilled water systems in order to increase the performance of the air conditioning plant. By adding glycol to the water, we in effect lower the freezing point of the water. We call this mixture a glycol brine. There are other substances that you can add to water to form a brine, for example, salt – for a salt brine.

You use this in your vehicle as well. In your vehicle you add anti-freeze (add ethylene glycol or propylene glycol brine) to the radiator water in a certain percentage. This will lower the freezing point of the water and so, even if the outside air temperature is below freezing, the water will not freeze in your radiator.

If the water does freeze in your radiator the expansion of the water turning to ice will damage/burst your radiator. This is one of the reasons we add glycol to the chilled water in an air conditioning/refrigeration system.

When the optimal amount (percentage) of glycol is added to the water to achieve the lowest possible freezing point we call this a eutectic solution. A eutectic solution has the advantages over plain water in that we can realise increased system performance, have reduced refrigerant charges and gain protection against freeze-up and corrosion.

System basics

Refrigerating systems are often referred to as either direct expansion or indirect systems. A direct expansion system is one where the evaporator containing the evaporating refrigerant is directly in contact with the space or material from which the heat is being removed.

An indirect system is one where the refrigerant containing evaporator is used to cool some secondary medium such as water or brine. This secondary medium is then circulated through pipe coils to remove the heat from the space being refrigerated or is contained in a tank placed in the space to be cooled. These indirect systems are used in both refrigeration and air conditioning applications.

In order to compare, we find that a conventional direct expansion system includes a condensing unit usually located externally to the refrigerated spaces. When the refrigerated space requires cooling, a temperature controller instructs a solenoid valve to open, which allows for liquid refrigerant to flow into the evaporator and start the direct expansion cooling process.

A glycol refrigeration system is made up of sub-systems referred to as secondary loop systems. The first stage of the system is a direct expansion system that chills the liquid glycol.

The chilled glycol is then pumped to all refrigerated spaces. When the refrigerated space requires cooling a temperature controller opens a glycol flow valve which allows chilled glycol to flow through the cooling coils.

Benefits of glycol systems

Whilst glycol refrigeration systems have been used for some time, the cost of refrigerant as well as environmental impact of using refrigerants that are damaging to ozone and have high global warming potential has seen the popularity and benefits of glycol refrigeration systems increase significantly.

With the environmentally damaging potential of currently used synthetic refrigerants and with the new generation of refrigerants being flammable and/or highly toxic, it may be a better solution to restrict the refrigerant to a remotely located refrigeration plant and utilise a glycol solution to refrigerate areas where customers and consumers may be present.

Smaller refrigerant charge

In, for example, a supermarket application, traditional direct expansion systems circulate refrigerants through long pipe runs from the condensing unit through to the evaporators located within the cabinets in the shopping area. This type of direct expansion system utilises a fairly large refrigerant charge when compared to a chilled water system. More refrigerant piping means more opportunities for leaks and the greater amount of refrigerant in the systems means more refrigerant in the atmosphere when a leak occurs.

In the case of an indirect system, due to the refrigerant circuit being contained within the remote glycol chiller unit, the volume of refrigerant in the system is significantly reduced. In addition, the decrease in refrigerant volume can provide significant savings to the end user in the event of refrigerant loss.

Increased temperature stability

Unlike direct expansion evaporators used in conventional refrigeration glycol systems use fully flooded cooling coils. Direct expansion evaporator coils use only a percentage of the cooling coil in the cooling process whereas glycol system cooling coils are 100% filled with chilled glycol and therefore use 100% of the cooling coil in the cooling process.

This results in faster temperature pull-down time, increased temperature holding times, and improved recovery times after defrost, ultimately resulting in improved end-user satisfaction. In a glycol cooling coil the temperature does not fluctuate as much as in a direct expansion evaporator, improving installation temperature stability.

Due to sensible heat being utilised, should a plant experience a power failure, brine gives a further advantage of a slower increase in installation temperatures. Due to a reduction in the system complexity on a glycol system, servicing and down times are reduced.

Energy Costs

Glycol refrigerant systems have a faster temperature pull down time, longer off cycles and shorter on cycles. This results in overall energy savings. Whilst a glycol chiller will also increase capital cost, the increase in capital cost can be offset by the reduced energy usage.

Khulani, I hope that this answers your question with regards to the usage of glycol in refrigeration and air conditioning systems.