Welcome to the solutions page

By Grant Laidlaw

Given that this month’s feature topic is chillers, one of the themes to emerge from the feature was a lack of adequate skills. Grant Laidlaw kindly agreed to address this topic.

GRANT LAIDLAW

Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy (ACRA) in Edenvale. He holds a Bachelor of Business Administration and an associate degree in educational administration. He has a National Technical Diploma and completed an apprenticeship with Transnet. He has dual-trades status: refrigeration and electrical. He has been involved with SAIRAC for over two decades and served on the Johannesburg committee as chairman and was also president between 2015 and 2018. Currently he is the SAIRAC national treasurer.

Many people ask for assistance in the understanding of theoretical and practical aspects of the industry. I will endeavour to enlighten. We are going back to basics as I have questions coming in that indicate that the basic understanding necessary to work in industry is not in place.

Hi Kyle, let us look at the dry cooler and then a basic chiller operation.Looking at the dry cooler, yes you are correct, dry coolers do not necessarily spray water to achieve the required cooling effect. A dry cooler is a cooling device that uses air to achieve the cooling effect and is based on the principles of sensible heat rejection.

For a dry cooler, thermal exchange is performed by drawing in external ambient air and passing it over tubes containing a cooling fluid (typically water or water-glycol mixture). To ensure efficient process cooling, a suitable difference in temperatures between the cooling medium and the air within the dry cooler must be maintained. A minimum difference of 5°C is usually sufficient. The cooled fluid is then circulated through a heat exchanger attached to an associated process. The excess heat transferred to the cooling fluid is then returned to the dry cooler and the cycle is restarted.

The outdoor unit differs from a cooling tower in that a cooling tower uses the latent heat of evaporation to cool the condenser water whereas the dry cooler simply blows cool ambient air over tubes that contain brine. The brine is contained in a closed loop circuit. The outdoor component is the dry cooler which rejects heat to the ambient air.

ADVANTAGES OF USING A DRY COOLER

• Easy installation and start-up
• Convenient separation of components allows operators to save vital floor space for other equipment
• Overall low operating costs after initial installation
• Dry coolers do not require a constant water supply, therefore supply and disposal concerns are non-existent
• The generated heat can be channelled directly into other processes, reducing energy waste and improving overall efficiency
• Dry coolers can be programmed to operate year-round, even in environments with low temperatures

DISADVANTAGES OF USING A DRY COOLER

• Additional costs of add-on components increase the total set-up expenditure
• Regular monitoring – or an automated chiller monitoring system – is required to ensure cooling fluid levels do not drop below required levels

DRY BULBTEMPERATURE

In order for cooling towers and dry coolers to work, the temperature of the water in the system needs to be hotter than the air temperature. Remember one of our basic laws states that heat always moves from a warmer body to a cooler body. However, air temperature has two parts: dry bulb temperature and wet bulb temperature.

Dry bulb temperature is the actual temperature – the temperature that the thermometer reads. This is the temperature we refer to when talking about the weather. Wet bulb temperature, on the other hand, is the temperature a thermometer would read if it was wet and had air moving over it. Wet bulb temperature is affected by the humidity in the air and will always be lower than the dry bulb temperature, except when the relative humidity is at 100%. Cooling towers work as long as the wet bulb temperature is lower than the temperature of the liquid running through it – a condition that is achievable in most of the country throughout the year.

Dry coolers work off the dry bulb temperature, meaning the outdoor temperature needs to be lower than what cooling towers require for effective heat transfer. As such, dry coolers are not as effective year-round as cooling towers. However, dry coolers are easier to maintain because the water is not exposed to air (closed loop), and they do not require water testing. Cooling towers are susceptible to degradation through fouling (getting clogged up), scaling (water-deposit build-ups), corrosion (gradual breakdown due to the environment), and require testing for the prevention of Legionnaire’s disease.

Occasionally, water sprays are used to increase the cooling effect by spraying the coil surfaces, thus providing additional capacity, utilising the water’s latent heat of evaporation. Kyle, let us move on to the basic operation of a chiller. Artificial refrigeration was demonstrated for the first time by William Cullen of Glasgow University in Scotland, in the year 1748. Thereafter, a continuous process of liquefying large volumes of gas was invented by the German engineer Carl von Linde, in the year of 1876. This invention eventually made chilling and refrigeration possible on a domestic and industrial scale and laid the groundwork for the development of the modern refrigeration industry.

In 1921, the first centrifugal water chiller was patented by an inventor named Willis Carrier. Before this time, chillers used a reciprocating compressor to move the refrigerant through the system. The main part of a centrifugal chiller is the centrifugal compressor. In a 1929 speech, Willis Carrier reminded his audience that “twenty-five years ago ‘air conditioning’ was an unknown quantity either in theory or practice.” He also made a forecast of the future when he said, “air conditioning and cooling for summer may become a necessity rather than a luxury, and we will look upon present times as marking the end of that ‘dark age’ in which there was but relatively little cooling for human comfort.”

Looking at how this all works we find, putting it simply, industrial chillers cool (chill) process fluids. Process fluids (typically water or a water/glycol brine mix) are used to cool people, machinery, equipment, food and such like. The process fluid (chilled water) absorbs heat from what is being cooled and then travels through the chiller where the heat is removed from the fluid and transferred to the ambient air in various different ways. Chillers are highly diverse in their application. They are used in industry for cooling processes, in the food industry, hospitals, shopping centres, office buildings and metal finishing plants. Their use extends to injection moulding processes, as well as cooling spaces in buildings, among others.

Chillers are sometimes preferred over traditional split systems or package units because water conducts heat better than air. This is also why water-cooled chillers are known for being more consistent and efficient in their performance and for having a longer lifespan than their air-cooled counterparts.

A dry cooler on the roof of a data centre. Image supplied by Grant Laidlaw

TWO MAINTYPES OF CHILLERS:WATER-COOLED AND AIR-COOLED

• Water-cooled chillers are common in medium and larger facilities, such as airports, hospitals and hotels. On the condensing side of the chiller, a condensing water circuit removes heat from the refrigerant in the condenser. The condenser water is pumped to the cooling tower which in turn passes the heat to the ambient air. The cooler water returns to the condenser to complete the circuit. Because the condensing water is exposed to the outside air, contaminates and sunlight – we use the term ‘open loop’.
• Air-cooled chillers utilising air-cooled condensers are more prevalent in small to medium sized facilities, where space and water may be limited. The costs to install and maintain these chillers are lower than that of their water-cooled counterparts that use condenser water and cooling towers or possibly evaporative condensers.

The chiller is generally located in the basement area within a plant room, which houses the chilled water pumps, as well as the condenser water pumps. The chilled water is pumped throughout the building via insulated steel piping normally rising up into each area that requires cooling from the system. In these areas there are air handlers that distribute the cooled air via ducts and diffusers depending on the way the air conditioning system has been designed and installed. Chilled water circuits are normally closed loop circuits.

The third circuit is the refrigeration circuit. The refrigeration circuit transfers heat from the chilled water circuit to the condenser water circuit. and completes the chiller described. The refrigeration circuit is the most technical part of how a chiller works. The refrigeration cycle uses the principles of thermodynamics to efficiently move heat from one area where it isn’t wanted, to another. In the case of chillers, heat is taken from the fluid being chilled and transferred to the ambient air. No matter the type of chiller, the main components are always the same for the unit to operate as per designed. The main components are similar in function to many air-conditioning and refrigeration systems.

TO REFRESH

• The compressor: The refrigeration cycle begins with the compressor, which takes low-pressure low to medium-temperature refrigerant in gas form from the evaporator and compresses it into a high-pressure high-temperature gas feeding it to the condenser.

• Condenser: This gas then flows through coils in the condenser. While in the condenser, air will flow over the coils or water will surround the refrigerant pipes and draw the heat from the refrigerant. As the refrigerant loses heat, it will begin to condense until all of the gas has condensed into a liquid where it now travels as a liquid to the expansion device.

• Expansion device: After leaving the condenser, the liquid goes through the expansion device which restricts the flow of refrigerant. When the high-pressure liquid goes through the expansion device, it enters the evaporator as a low-pressure low temperature mixture of liquid and vapour.

• The evaporator: The evaporator is where the refrigerant starts evaporating back into a gas. When the refrigerant evaporates it gets very cold and will absorb heat. It is in the evaporator that the chilled water will interact with the cold refrigerant. Heat is removed from the chilled water and transferred to the refrigerant. The refrigerant gas will return to the compressor and the cycle begins again.

Kyle, this concludes a basic description of the operation of a chiller and a dry cooler – hope that this helps with your understanding. Thanks to everybody for the overwhelming response. I receive on average over sixty questions per month and cannot publish all of them. But keep them coming, as I may answer you directly.

Looking forward to hearing from you,

Grant Laidlaw.

REFERENCES:

1. SETA training
2. ACRA