What is a chiller's circulating fluid? The main types and how to choose them, and how they differ from similar terms such as cooling water

1. What is the circulating fluid in a chiller? Its mechanism and role

First, let's understand what circulating fluid in a chiller means and how it functions in a chiller.

■ Meaning of circulating fluid and how to use similar terms

Chiller circulating fluid is a general term for the liquid that circulates between the chiller, heat source, and equipment.

Because there is no clear definition, it is often referred to by other names such as "circulating water," "chiller water," and "cooling water," and some people may find this confusing when looking at unfamiliar specification diagrams and system diagrams.

For example, circulating fluid and cooling water are sometimes used synonymously, but from the perspective of the chiller, it is sometimes written as "circulating fluid" as "a liquid that circulates starting from the chiller," while from the perspective of the heat source, it is sometimes written as "cooling water" as "a liquid that cools."

In this article, we will explain each liquid from the perspective of the chiller.

<Definition in this article>

■ Chiller mechanism and circulating fluid function

The mechanism by which a chiller cools an object is to first cool the circulating fluid inside, then exchange heat between the now low-temperature circulating fluid and the hot object, cooling it. The circulating fluid that has been warmed by the heat exchange returns to the chiller and is cooled again inside.

<Chiller structure diagram for Air-cooling type chillers>

*Here, circulating fluid is described as circulating water.

There are also many types of circulating fluids, including water-based fluids such as tap water and ion-exchanged water, antifreeze-based fluids, alcohol-based fluids, and fluorine-based fluids. Check their characteristics and choose one that suits your application and company standards. We will explain this in more detail later.

2. Main types and characteristics of aqueous circulating fluids (circulating water)

There are various types of circulating fluids for chillers, including water-based, antifreeze-based, and alcohol-based, but water-based circulating fluids are generally used for temperature control near room temperature (up to 10°C).

The main water system circulation fluids include tap water and purified water, both of which have low viscosity and high heat capacity, allowing you to get the most out of the cooling capacity of circulation pumps and chillers.

■ Tap water

Tap water is the most easily available, has stable quality and quantity, and contains chlorine, making it less likely to spoil.

However, water temperature fluctuates greatly with the seasons, making it unsuitable for low-temperature environment. Also, because it contains impurities such as minerals, it has a higher electrical conductivity than deionized water, so care must be taken.

■ Purified water

Purified water is highly pure water that has been purified by removing impurities from tap water through distillation, filtration, ion exchange, etc. Because it has few impurities, it is less likely to produce scale and has low electrical conductivity.

Purified water can be broadly divided into four types depending on the purification method: distilled water, ion-exchanged water, RO water, and pure water.

①. Distilled water

Distilled water is purified by boiling tap water and cooling the steam. In addition to removing impurities, it also removes organic matter that can lead to the growth of microorganisms, making it less likely to spoil.

② Ion-exchanged water (deionized water)

Ion-exchanged water, also known as deionized water, is water that has been purified by passing tap water through a filter made of ion exchange resin. The ionic components in the water are removed, resulting in water with the lowest thermal conductivity.

③. RO water

RO water is tap water that has had impurities removed by passing it through a reverse osmosis (RO) membrane. It can remove even minute viruses that would remain in conventional filtration, and is also expected to be effective in preventing scale and corrosion caused by inorganic ionized substances.

However, each manufacturer may have their own standards for the impurity concentration of purified water, so be sure to check the contents carefully to make sure it is suitable for your purpose and use.

④． pure water

Among purified water, water with high purity is called pure water. Because impurities have been removed, impurities dissolve easily in it, and it is also used as water to wash precision equipment.

Among pure water, water that is completely free of impurities and is as close as possible to the following values is called ultrapure water.

• ・Electrical conductivity…approx. 0.055uS/cm (25℃)
• ・Specific resistivity: 18.24 MΩ cm

3. Main types and characteristics of low-temperature heat transfer fluid (circulating water)

When setting the circulating fluid of a chiller to 10°C or below using a low-temperature constant temperature water bath, a low-temperature heat transfer fluid should be used instead of a water-based fluid.

Alcohol-based circulating fluids such as methanol, which are easy to handle, have long been used, but an increasing number of companies are avoiding them due to the risk of ignition, and safety alternatives such as Nybrine® and Etabrine® are becoming mainstream.

■ Methanol/Ethanol

Methanol and ethanol are two representative alcohol-based circulating fluids, and are used at temperatures below 10°C with concentrations below 60 wt%.

They are inexpensive, easy to obtain, and have good fluidity even at low temperatures, allowing circulation pumps and chillers to perform to their full capacity. However, they vaporize easily and have very low flash points (12°C for methanol and 13°C for ethanol), so care must be taken with regard to temperature and fire. They also have an odor, so ventilation is important.

■ Ethylene glycol

Ethylene glycol is a type of alcohol that lowers the freezing point when mixed with water and is well known as antifreeze. It is inexpensive, easy to obtain, tasteless and odorless, and has a high flash point of 121°C, making it very safety to handle.

The viscosity of this oil increases as the temperature drops, so it is important to dilute it to suit the temperature, such as adjusting the concentration to between 10wt% and 30wt% in the temperature range of -10°C to +10°C.

In addition, when added to ion-exchanged water, it increases the resistance of the fluid, making it a valuable circulating fluid with high insulating properties.

Nybrine® Z-1

Nybrine® Z-1 is a medium that contains ethylene glycol as its main ingredient, with added rust inhibitors and preservatives. It is a medium that is compatible with metal piping, and overcomes the drawback of ethylene glycol, which is highly corrosive.

Furthermore, it can be used in a wider temperature range than ethylene glycol, from -40°C to +60°C, and has other advantages, such as no flash point and not violating the PRTR Act.

■ Nybrine® NFP

Nybrine® NFP is a heat transfer medium with enhanced antiseptic and anti-rust properties. It is a highly safety medium that uses propylene glycol instead of toxic ethylene glycol to meet food-related applications.

Like Z-1, it has excellent corrosion resistance to metals and is non-flammable. It can be used in a wide temperature range from -40°C to +110°C, and can be used not only as refrigerant but also as a heat transfer medium.

Etabrine® EC-Z type

The main component is alcohol-based ethanol, but because it is a low concentration of less than 60 wt% and has a high flash point of 23.8°C, it is not considered a hazardous material under the Fire Service Act.

It is an easy-to-handle medium because it has low viscosity and changes in viscosity little even in the sub-zero temperature range.

4. Key points when selecting a chiller's circulating fluid

As mentioned above, there are various types of circulating fluids for chillers, but the type of circulating fluid to use should be selected taking into consideration the following points.

• ・Set temperature
• - Electrical conductivity
• ・Insulating
• ・Availability
• ・Hygiene
• ・Company regulations and safety

For example, tap water is used to control temperatures around room temperature, while glycol- or alcohol-based heat transfer media are used to control temperatures below 10°C. However, when safety is taken into consideration, it is best to avoid alcohol-based media, which have low flash points.

Additionally, it is recommended that chiller circulating fluid be replaced approximately every six months to prevent corrosion and scale buildup inside the piping, as well as the proliferation of microorganisms. If it has been six months since the last replacement, select a new circulating fluid taking the above points into consideration.