8.3.1 Introduction

One way to reduce the use of outside air—which needs heating, cooling, hu­midifying, dehumidifying, or cleaning—is to recirculate the air. Other ways to save energy for a ventilation system should preferably be used, such as the use of a heat exchanger, equipment that uses less energy, use of less air, etc. The main difference between other energy-saving processes and air recirculation is its influence on the contaminant concentration. This is a valid point for both industrial premises and offices and living rooms. Recirculation always includes some cleaning of the air. If the air is not cleaned, the process is seen as transferring instead of recirculating the air.

Through the years many reports have been published on air recirculation. Mostly they have treated the problems in offices,1 living rooms,2-6 and schools.7 There have been many reports on air recirculation, which have concentrated on the use of some specific air cleaning system or on possible health risks.8’9

From the literature it is evident that air recirculation has been used and is used for many different types of ventilated areas. For this reason there have been some large investigations into the advantages and disadvantages of using air re —

Other difference with central systems is that their performance is heavily influ­enced by the exhaust hood. If all contaminants generated at a process are captured by the exhaust hood, the performance is similar to a central system, but since this is a very rare phenomenon, the resulting concentration in the room is influenced by the capture efficiency (see Chapter 10).

The following equations separately outline calculating contaminant con­centration inside a room with central and local recirculation. The assumptions for the room are that it has one main ventilation system with supply and ex­haust air and that the contaminant concentration is the same in the whole vol­ume (except very close to the contaminant source or in the ducts, etc.). The contaminant source is steady and continuous. The model for local ventilation assumes also one main ventilation system to which is added one local exhaust hood connected to a local ventilation system (see Chapter 10) from which all the air is recirculated. In the central system the number of inlets and outlets could vary. The flow rates are continuous and steady.

Different models have been published using two or more zones,10’28’29 or where a mixing constant is used.30 Models also exist for multiple ventilation systems in a room or a building,9 for varying source rates,13’31“32 and where local and central recirculation take place at the same time.13’29 31

The most important part of a recirculating system is the cleaning device. Different types of air cleaning devices are described in Chapters 9 and 13. The cleaner must be suited for the contaminant to be collected and its concentra­tion. This sounds obvious, but is very often forgotten. Cleaners for aerosols (dusts) have efficiencies depending on particle size; for gases, efficiency varies with concentration and type of gas. The efficiency also varies with time, both for particle and gas cleaning systems. For mixtures of gases (vapors) and parti­cles, such as oil mist, it is usually not enough to separate the particles (drops) from the air; the gases must also be collected.

Please note that filtering in dust collectors, vacuum cleaners, etc. does not usually collect the smallest particles. A good rule is not to recirculate such air.

The most common equipment for cleaning recirculated air from particles is fabric filters, mechanical collectors, electrostatic precipitators, and cleaners and wet collectors.33 For cleaning of recirculated air from gases, absorbers and ad­sorbers such as activated carbon, sometimes with impregnation for specific gases, and impregnated alumina are most common. The performance of differ­ent air cleaning equipment is described in many textbooks and handbooks. 34 The models described in the following use only one parameter for the cleaning efficiency, which is thus a simplification that must be kept in mind when using these models. This works quite well as long as the efficiency value is the smallest one—e. g., the efficiency for the most penetrating particle size or the efficiency for the most penetrating gas concentration.