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The figure shows the flow around surgeons at the operating table below an LTDF
© TU Berlin, Hermann-Rietschel-Institut
Ventilation systems for maximum requirements
08.02.2018

The illustration shows the structure of the cleanroom system. This consists of two separate cleanrooms with different airflow systems and intervening air locks. Two protection concepts are used for the ventilation: one cleanroom is operated with a turbulent dilution flow, the other cleanroom with low-turbulence displacement flow.
© TU Berlin, Hermann-Rietschel-Institut

The figure shows the flow concept in the operating theatre with LTDF (left) and mixed air ventilation (right)
© TU Berlin, Hermann-Rietschel-Institut

Ventilating operating theatres energy-efficiently

How can the high energy consumption required for ventilating “clean rooms” in healthcare and industrial production be reduced while ensuring equal or even better protection against airborne particles and germs? This question was investigated by researchers from Hermann-Rietschel-Institut (HRI). They built a research laboratory with two cleanrooms. The results provide a detailed picture of the air flow processes and improved design principles for constructing and operating cleanrooms.

Every year more than 50 million operations take place in Germany. To optimally protect patients from germs, their concentration in the indoor air is kept as low as possible. Many production plants in the pharmaceutical, biotechnology, semiconductor, precision mechanics and food industries are also located in cleanrooms. Even small amounts of particles can influence the quality of the products. To ensure the required protective effect, ventilation systems have hitherto worked with very high air exchange rates. A better understanding of particle dispersion and flow behaviour could reduce the air exchange rate and thus the energy consumption. However, designers and users have previously lacked the necessary information.

Research laboratory with two cleanrooms

For the experimental investigations, Hermann-Rietschel-Institut (HRI) at Technische Universität Berlin set up a research laboratory with a 75-m2 area and two cleanrooms. To enable different operating modes to be simulated, the cleanroom system – including ceilings, walls and the system components – are modular and flexible. The air is supplied via a central ventilation unit and, in the case of the cleanroom with low-turbulence displacement flow (LTDF), through additional filter-fan units. The supply air volumes and speeds of the two rooms are freely adjustable within technically given limits. With these capabilities, approximately 90 per cent of real-world configurations can be modelled as “clean environments”.

Designing cleanrooms

In cleanrooms, turbulent dilution flow (TDF) is predominantly used. The clean supply air is supplied with a high impulse and turbulence intensity. As a result, it mixes with the room air and the concentration of impurities decreases. The systems work with air change rates of around 30 h-1. In practice, bypass flows and dead zones frequently occur which prevent optimal dilution of the room air. This can be prevented by suitably placing the supply and exhaust diffusers. As a result, the fans require less air and the energy consumption decreases.

When the highest purity is required, low-turbulence displacement flow (LTDF) is used. The air flows mostly from top to bottom and is extracted in the floor area. The supply air is introduced into the room at 0.45 m/s. This creates air exchange rates of up to 600 h-1. The scientists were able to show that a supply air velocity of 0.25 m/s is often sufficient to compensate for convecting heat sources equivalent in size, for example, to the volumes emitted by people. This corresponds to a power reduction of the fans of around 70 per cent. If there are no large heat sources in the room other than the personnel, a stable room flow can be ensured despite the significantly reduced supply air speed. Adapting the air volume to the loads in the room is the decisive criterion in making savings.

In addition to the heat sources in the room, the flow-mechanical properties of the floor slabs and the geometry of the double floor are of considerable importance. The investigations have shown that locally adapting open, cross-sectional spaces at the individual perforated plates in the room can prevent potential cross-contamination. The energy requirement is significantly lower compared with a very small perforation with a high pressure drop.

Operations without microbiologically contaminating patients

The current standards place high demands for ventilating operating theatres (OTs). It is important to protect the patients, especially in wound areas, from microbiological contamination.

For high-purity operating theatres, a laminar flow with no turbulence above the operating table is required as the flow pattern, which shields the surgical area like an air curtain. Such LF (laminar flow) fields enable the highest protection levels to be achieved on the operating table. However, if there are significant geometrical or thermal interfering bodies in the flow above the operating table, they prevent a directed outflow and the discharge of contaminants from the operating area. For example, the bent posture of the surgeons and surgical lights within the airflow may result in recirculation areas above the operating table. Taking into account that a large part of the airborne germs in the operating room comes from the facial areas of the surgical staff, germs can thus increasingly enter the wound field. With a realistic range of loads, this reduced protective effect corresponds to the level of a TDF with a significantly reduced air requirement.

The investigations carried out on operating rooms depict only a very limited range of the loads and configurations that occur under real conditions. For example, the measuring method used only covers airborne particles of about 0.3 μm in size. In reality, however, germs occur on particles in the room whose size significantly influences the sedimentation. Furthermore, issues relating to the thermal comfort and unwanted cooling of patients are also not taken into account. It therefore remains questionable as to which of the investigated flow forms shows the better protection effect and ventilation efficiency in accordance with the various requirements.

In order to clarify the issues mentioned, further investigations are needed. The “Energy-efficient ventilation in multifunctional operating theatres” follow-up project (running from 1 March 2017 to 29 February 2020) will continue the investigations and create integral protection concepts for different types of operating theatres.

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