Evaporative cooling systems

Evaporative cooling systems are cooling systems in which water is trickled or sprayed, thereby releasing heat into the ambient air through the evaporation of the water. Evaporative cooling systems are often used in industry, for example, to cool buildings or to cool machinery and processes. A major advantage is that they require less energy than conventional compression refrigeration units. This is not only more environmentally friendly, but also saves money.

A recooling plant, also known as a cooling tower, is a system that removes excess heat from industrial processes or air-conditioning systems. Recooling plants work as follows: warm water is drawn from the process and cooled through evaporation before being returned to the circuit. In a cooling tower, the warm water is distributed over packing or fins, creating a large surface area that maximises contact with the air. The air flows through the tower either by natural convection or driven by fans, promoting the evaporation of the water and thus heat dissipation. Recooling plants are indispensable in many industrial sectors, as they provide an efficient and cost-effective method of regulating temperature. They are particularly common in power stations, refineries and large air-conditioning systems.A key advantage of recooling plants is that they dissipate large amounts of heat using relatively little water. This makes them an environmentally friendly solution.

A wet scrubber is a device that purifies air or gases by filtering out particles, dust or pollutants using a liquid, usually water. It works as follows: the contaminated gas is passed through a zone that is wetted with the liquid. The particles in the gas stream are captured by the liquid and removed from the gas stream. Wet scrubbers are particularly well suited to filtering out fine particles and gaseous pollutants that are difficult for dry filters to capture. They are often used in industry to control emissions and improve air quality, for example in the chemical industry, metal processing or waste incineration. Wet scrubbers can separate both solid and liquid pollutants. This makes them extremely versatile. They can even operate effectively at high temperatures and in humid conditions.

Cooling water can become contaminated for various reasons. During the cooling process, the water frequently comes into contact with various materials and chemicals that can contaminate it. On the one hand, substances dissolved in the water, such as salts and minerals, can accumulate, leading to increased water hardness. Furthermore, biological contaminants such as algae, bacteria and fungi can grow in the water, particularly when it is warm. Chemicals used for water treatment or to prevent corrosion and scale build-up can also leave behind residues. Furthermore, the water may absorb heavy metals or organic pollutants as a result of industrial processes. These contaminants can impair the efficiency of the cooling system and harm the environment if the water is not properly treated and disposed of.

Evaporative cooling systems are used to transfer process heat to the ambient air; they cool things by releasing heat into the air. They are used in many sectors where heat needs to be dissipated, for example in industry, power stations, refrigeration systems, data centres, all kinds of process plants, the retail sector, the catering industry, hotels and office buildings.

On the one hand, there are health risks for workers employed at these facilities. This concerns both risks posed by micro-organisms, particularly pathogens, and chemical substances such as the biocides used (disinfectants, etc.). Occupational health and safety legislation therefore requires a risk assessment to be carried out for these activities and appropriate protective measures to be put in place for workers.

In addition to workers, people in the vicinity of the recooling plant are also at risk of infection from pathogens if they inhale the aerosols emitted. According to current knowledge, this applies in particular to bacteria from the Legionella group, which have been shown to cause infections even several kilometres away from the source.

These microorganisms can lead to what is known as legionellosis. In its mild form, known as Pontiac fever, this infection presents with symptoms similar to those of summer flu. Estimates suggest an annual case count in the six-figure range. Even in the case of the severe form of legionellosis, which takes the form of pneumonia – known as Legionnaires’ disease – the Robert Koch Institute in Berlin assumes that there is significant under-reporting of cases and that the number of unreported cases exceeds 95 per cent.

Based on an evaluation of the evidence to date, it is estimated that of the approximately 800,000 cases of community-acquired pneumonia each year, around 4 per cent are attributable to Legionella. This amounts to approximately 32,000 cases per year, resulting in around 2,000 deaths. This clearly illustrates the current under-reporting and the scale of the problem. 

VDI 2047 applies to evaporative cooling systems and units in which water is trickled or sprayed, or may otherwise come into contact with the atmosphere. This applies where the circulating water is used directly as a cooling medium in the process, or where process heat is transferred via heat exchangers from a primary cooling circuit to a water cooling circuit. The guideline applies to systems with a thermal recooling capacity of up to 200 MW.

VDI 2047 is currently recognised as the state of the art for evaporative cooling systems. As a technical standard, it naturally has no legal force, but is used as a benchmark by regulatory authorities, experts and the courts. Numerous requirements of VDI 2047 are also set out in the 42nd Federal Immission Control Ordinance. This concerns the manufacture, design, construction, commissioning, use, operation and maintenance of these systems. It follows that all parties involved must apply VDI 2047 or, alternatively, equivalent measures. In the event of deviations from the requirements of VDI 2047, their effectiveness must be demonstrated. Ultimately, VDI 2047 is the safest solution, particularly for fulfilling operators’ obligations!

VDI 2047 is intended to ensure the operational safety of evaporative cooling systems. All aspects, from planning, installation and commissioning through to maintenance and operation, are intended to contribute to this. It is important to note that the choice of installation site now plays only a secondary role. Provided the intended hygiene requirements are met, the risks can be minimised to an acceptable level during proper operation. Even existing systems that do not comply with the structural requirements of this guideline may continue to be operated, provided the compensatory requirements of this guideline are met.

In order to achieve the objectives set out above, VDI 2047 specifies hygiene-related structural, technical and organisational requirements for the planning, design, construction, operation, maintenance and commissioning of recooling plants. With reference to other standards, the standard describes requirements for construction, the materials to be used and the necessary maintenance measures, including testing. Measures to be taken in the event of operational interruptions are also covered by these requirements. Another important area of focus is water chemistry and its influence on materials and operational behaviour.

VDI 2047 describes these requirements systematically and also addresses in detail the interfaces between the various sub-tasks. These details are, amongst other things, covered in the aforementioned VDI 2047 hygiene training course.

As part of the commissioning of evaporative cooling systems, an initial hygiene inspection and a risk assessment must be carried out in accordance with VDI 2047. Before filling the evaporative cooling systems, compliance with the design and construction requirements must be thoroughly checked to determine whether the system’s design features meet the specific requirements of the installation site, both from a hygiene and a technical perspective. Any serious defects identified must be rectified before the system is filled for the first time. This inspection may only be carried out by qualified personnel with additional qualifications in hygiene engineering.

A further testing requirement concerns the microbiological condition of the water in evaporative cooling systems. Every three months, both the total bacterial count and the Legionella count must be tested by a laboratory accredited to DIN EN ISO 17025.

The Federal Immission Control Ordinance stipulates a requirement for periodic inspections at five-year intervals, to be carried out by a publicly appointed and sworn expert in ventilation or refrigeration technology or by an inspection body in accordance with DIN EN ISO 17020.

VDI 2047 requires anyone working on evaporative cooling systems to have, in addition to technical background knowledge, an additional qualification in the form of hygiene training. The hygiene training in accordance with VDI 2047 (duration: 1 day) is designed to enable those involved to understand the necessary requirements for hygienic operational practices at all stages of working with such systems (planning, installation, operation and maintenance, including cleaning and disinfection) and to apply them in practice.