The quest for truly clean air in hospitals is rife with technical and financial challenges, but one company is making great claims for its new displacement ventilation solution.

Hospitals tend to have a large number of relatively small air handling units; each serving a specific function within the building. Special technical demands include hygiene, reliability, safety and energy related issues. Within this array, the air handling unit and the air distribution terminal devices are important parts of the ventilation system.

In a hospital environment there tend to be high concentrations of harmful microorganisms. Their routes to humans are either by physical contact or by airborne routes. In this environment they are particularly dangerous because of reduced immunity levels in patients.

The risk of being infected through the airborne route is a function of particle concentration. The chance of a particle that is carrying an organism falling into an open wound increases with particle concentration. By reducing the concentration we reduce the chance of infection.
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Four main factors affect the local concentration around a person in a room. Firstly, the concentration of particles would tend to increase with the rate of production of particles in the room. Secondly, the greater the proportion of supply and exhaust air quantity in relation to the size of the room. Thirdly, the level of filtration of the supplied air will affect the ability of the ventilation system to dilute the room air particle concentration. Fourthly, air turbulence and air movement in the room can transport particles so the method of air distribution will affect local concentrations.

The last three of these are attributes of the ventilation system that can be engineered to limit the effect of the first.

For example, air handling unit filters have the task of, as a minimum, limiting the concentration of particles entering the room from outdoors, but also of keeping the air handling unit components as clean as possible in order to reduce the risk of biological growth within the unit itself. Ideally, the air handling unit should not produce any dust itself, but that is more or less impossible where moving parts are concerned, for instance in the fan set. Systems and products to meet these engineering problems have been developed and, for example, Flakt Woods has some well-tested solutions that reduce the production of dust generated by the fan set. For instance, special flat belts can produce considerably less dust than normal v-belts and are more efficient at transmitting the power from the motor to the fan. Direct driven fans avoid the use of belts and both plug fans and axial fans can be offered.

The motor can also be mounted on the outside of the air handling unit casing to avoid mounting both the motor and the belt drive in the air stream. This involves the use of a special fan with an extended shaft. Care must be taken when engineering such a solution in order to increase the size of the shaft while avoiding the critical speed to compensate for the additional forces on the shaft. In addition, the fan and motor assembly must also be fully isolated from the building structure.

Final filtration can be used to substantially reduce the concentration of dust particles in the supply air and here the design of the filter frame and its installation can be decisive.

PERFORMANCE PRODUCTS

The standards used for determining the type of collecting efficiency of filters often differ between Europe and in the US. Even in Europe there are different standards in different countries. CEN EN 1822 gives a common standard within Europe and is based on the filters ability to collect the most penetrating particle size, MPPS, or in other words how good the filter is at stopping the particles that are most difficult to catch.

MIL Std 282, DOP 0.3[mu]m is used in the USA. Particles with a size of 0.3[mu]m are not necessarily the most difficult to catch so for any individual filter the test result will appear to be better, when tested in accordance with the US standard, than if tested to the European standard. As a result, care should be taken when comparing the performance data for HEPA filters from different manufacturers. Note that the collecting efficiency is inversely proportional to the air velocity. While air pressure drop is proportional to air velocity. That means that increasing the velocity (or reducing the overall filter area) may be a false economy since the while initial installation cost may seem lower, the operating cost and will be much higher and the filtration performance reduced.

HEAT RECOVERY

Either liquid coupled or plate heat exchangers are recommended for heat recovery in hospitals. The advantage of the liquid coupled system is that there is no risk of transfer of air and contamination from the extract to the supply side. The system also offers considerable flexibility since the supply and extract need not be near each other and multiple heat exchangers can be used on either or both of the supply and extract.