CHILLERS
A short history: evaporative cooling
Sami Ure of Ecomesh looks at the history of evaporative cooling and explains how it fits into current and future trends.
E
vaporative or adiabatic cooling is a concept that has spanned across the ages. It is recorded throughout the pillars of time that many ancient civilizations utilised the natural effects of water and evaporation to cool both its people and buildings.
The Ancient Egyptians are known to have used porcelain clay pots to collect water and had their slaves fan the cooler air into their vicinity. In a similar yet slightly more advanced manner, the Romans are noted to have engineered their aqueducts to circulate water throughout the walls of their houses, hence maintaining a cooler air temperature.
Meanwhile, the Persians engineered dedicated building structures which featured a tower in each corner. At the bottom of these towers was a water reservoir, whilst at the top lay a specially designed roof. The air from outside was captured by the roof’s design and funnelled down and across the water reservoir below, cooling the occupants within the building. These accomplishments throughout time are truly amazing and offer an insight into the world’s first true HVAC applications.
Cooling represents without a shadow of a doubt one of the main sources of power consumption within any modern building application. With an ever-increasing demand from both businesses and individuals alike, many buildings and facilities are being pushed to their technical cooling limits. Increasing energy prices, government regulation, and the potential consequences associated with a complete system shutdown or cooling failure, has led to a new breed of building services, whereby systems must be far more wisely designed and back-up facilities put in place.
Even with the smartest possible design, however, it is sometimes unavoidable that issues such as heat surges, fluctuations in wind direction and installation environment can affect a HVAC system’s efficiency. There are many cases in which several chillers are packaged in tightly together, leading to a domino effect of heat being transferred from one unit into another. This creates an artificial ambient, far higher than the actual ambient
26 August 2018
outside. In such conditions, it is an inevitability that within a matter of time an overheating issue arises and the compressors simply shut down, leading to a complete cooling failure. In response to these issues, a multitude of methods and technologies have arisen that target evaporative cooling as the solution.
Evaporative condensers and cooling towers were once seen as the best and most economical solution to this issue, but they have come under increasing scrutiny and regulation in recent times as the risks of legionella become ever-greater if good maintenance is not ensured.
Although well-maintained cooling towers operate effectively, many sites unfortunately do not receive the adequate levels of maintenance to ensure safe operation. This is without a doubt one of the primary reasons we can see a significant rise in the adoption of a number of alternative adiabatic technologies such as plastic drift eliminators, rigid or paper mats, direct water spray on to the coils and various forms of fogging systems. High-pressure water fogging systems have recently been designed and installed as a method of cooling the condensing coils directly. Although this concept does provide efficient cooling, it has a multitude of glaring negatives. The water consumption for these systems is significantly high, enhanced further by the fact that the majority of the water is wasted.
Secondly, because this system sprays water directly onto the aluminium and copper coils, advanced coil protection strategies need to be adopted, otherwise it is inevitable that scaling and corrosion will become a serious issue. The final disadvantage with this system is that because there is a requirement for highly atomised water, a UV filter or other form of water treatment must be installed to ensure a high level of water quality. When you combine a requirement for high quality water and the risk of a corroding coil, the level and cost of maintenance for these systems becomes so high that any energy savings you receive from the evaporative cooling have the potential to be wiped out.
Another system designed to tackle the issue of high ambient is a Wet-Pad system. This design features cardboard or plastic pads placed directly in front of the condensing coil. Water is then dripped throughout the pads, allowing for the adiabatic evaporative cooling to take place. Although this system uses less water than an atomised system, it faces a significant issue related to dust and dirt clogging the pads. Once this happens, it is unfortunately the case that a significant pressure drop can arise, leading to decreased efficiency and eventual compressor shutdown. One further issue is maintenance; these pads require replacement on a regular basis, which removes the convenience and ease-of-use factor.
The final adiabatic technology currently available is a mesh and frame-based system, designed in such a way that the mesh protrudes away from the chiller coils at a 45° angle. This system sprays water at a low pressure of only 3.5 bar, away from the condensing coil and onto the purpose-built plastic mesh. A second layer of mesh is installed along the back-side of the frame, in order to negate any potential water spray back to the coil. All of the evaporative effect is generated along the mesh material itself, cooling the air passing across and into the chiller. This system reduces the risks associated with legionella by combining the total waste principle, no water storage and a low pressure water spray. The mesh itself also acts as a self-cleaning filter by combining a dual-layered mesh design; the layers rub up against each other and break up any calcium deposits or debris. This system is also designed to work in conjunction with an advanced control management system, intended to control water spray duration and frequency and generate flush cycles to avoid dead legs of water. The future of evaporative cooling has never looked stronger, with multiple companies and technologies being established yearly, and industry acceptance of the concept with regards to chillers growing exponentially. We are in an exciting time period for HVAC efficiency and hopefully the increasing levels of competition can spur yet further innovations and improvements.
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