Fresh air for offices

The researchers have actually been able to show that increasing the supply air flow rate in offices may lead to an increase in performance levels by up to 4 % (Fig. 1). By far more interesting, however, are the findings regarding the satisfaction of room occupants since their expectations and general constitution greatly affect how they perceive the quality of the room air. The higher the supply air flow rate, the more people are satisfied, and then significantly (Fig. 2) – this is an immensely important aspect since motivation and well-being, but also the sick leave rates (reduction by up to 35 %), correlate to a large extent with employees' satisfaction.

And yet the modernisation of ventilation and air conditioning systems is often delayed since it incurs high investment costs. Increasing the fresh air rate is often delayed, too, due to the higher energy consumption.

The impact of good office air on a nation's economy cannot be denied: US researcher William J. Fisk carried out a cost-benefit analysis and has been able to prove that the economic benefit by far exceeds the investment into good room air quality. Studies suggest that if all office buildings in the US were refurbished (flow rate adapted to 10 l/s per person, room temperature of 23 °C max. in winter, further technical measures), the annual benefit to the US due to better performance in the workplace and fewer absences due to sickness would amount to USD 20 billion.

The most important criterion when deciding between an all-air system and an air-water system is obviously the cost, apart from the fresh air requirement [in m3/h.m2] and the cooling load [in W/m2]. If one takes into account how much space is saved, the differences in the life cycle costs for all-air systems and air-water systems are small. 

In rooms with many people, good air quality can only be achieved with a sufficient fresh air volume flow rate that is based on the number of these people. This is usually an argument in favour of a classical all-air system. 

Modern office and administration buildings contain much technical equipment and often have large areas of glazing. The heat emitted by the equipment plus the solar gain due to large windows may heat a room considerably without the air quality being impaired by contamination. 

An all-air system would require large volume flow rates for cooling such rooms and incur high energy costs for air treatment and air transport. In this case air-water systems and decentralised ventilation systems with an integral air-to-water heat exchanger are the preferred choice since the heating and cooling capacity of these systems is independent of the fresh air flow rate. An additional advantage of air-water systems is the fact that thermal energy is transported more efficiently by water than by air such that less energy, and less space, is required to provide the same heating or cooling capacity.

Renewable energy for ventilation and air conditioning.
State-of-the-art ventilation and air conditioning strategies play a significant role in achieving the goal of reduced carbon emissions. Just the existing technologies for exploiting renewable energy sources, such as solar energy, geothermal energy, free cooling, heat recovery and waste air, may suffice to contribute until 2020 approximately 9 % to the climate protection goal of the Federal Republic of Germany.Primary energy values for buildings with different energy performance:

The savings potential for office buildings is considerable when state-of-the-art ventilation and air conditioning systems are used and when outdated components and systems are replaced.

When selecting a supply air diffuser, the exact installation location is as important as the cooling load of a room. If one compares a room air conditioning system to a chain, air terminal devices would be the most critical links for thermal comfort. They are supposed to ensure that the occupied zone meets the most demanding requirements of thermal comfort: Room air conditioning is perceived as ideal if you cannot hear it or feel it.

Mixed flow or displacement flow ventilation?
With mixed flow ventilation, a uniform temperature and air distribution are quickly achieved. The supply air is discharged with a velocity of 2 to 5 m/s, mixes with the room air and dilutes in this way the contaminant concentration. Turbulent mixed flow ventilation can achieve higher air change rates and better room purging. It is the preferred solution for areas with a high occupancy level and for areas where the air conditioning system is also used for heating.

Displacement flow ventilation is characterised by low airflow velocities, low turbulence and a high air quality in the occupied zone. The supply air is discharged into the space with a low velocity and as close as possible to the floor; the result is a pool of fresh air over the entire floor area. The convection from people and other heat sources causes the fresh air from the pool to rise and create comfortable conditions in the occupied zone.

Inducing displacement flow.
An inducing displacement flow combines the advantages of both types of ventilation. The supply air rises in the near zone of displacement flow diffusers with a velocity of 1 to 1.5 m/s and mixes with the room air. Once the momentum of the jet dies out, the jet becomes a displacement flow. Due to the low levels of turbulence both inducing displacement flow ventilation and displacement flow ventilation meet the requirements of room air category A.

Central vs decentralised air conditioning systems.
Central air conditioning requires ductwork and hence space. This space is usually not availabe with refurbishments. 
In such cases the perfect alternative for both refurbishments and new builds are decentralised façade units. The fresh air connection is provided by ventilation openings in the façade system or external wall. Decentralised ventilation and air conditioning units are suitable for rooms with a depth of up to seven metres, depending on the room layout and furniture.