Book - School of Science and Technology

398 Air-conditioningsensor as an energy saving feature to shut-off the air supply at the workplace when theoccupant is away from the desk.Displacement ventilationIndoor pollutants are diluted by `mixing' with outside air when traditional methods ofventilation are employed and the same principle is adopted for dealing with heat gains andlosses. An alternative approach is to introduce the supply at one position, and at lowvelocity, such that it moves in a single direction through the room using a piston effect totake the pollutants, including thermal effects, with it. In such systems, supply air isintroduced close to the floor at a fewdegrees belowthe room design temperature allowingthe input to flowacross the floor forming a `pond'. Heat sources within the room produceupward convective currents resulting in a gentle upward air flow towards high levelextract positions. For effective operation, the air in the space should not be subjected tocontinuous disturbance by rapid movement of occupants neither should there be highrates of infiltration nor down draughts due to poor insulation.A number of limiting performance and comfort factors impose restrictions upon the useof displacement systems.* The warm and often polluted upward air flow spreads outbeneath the ceiling and, since the lower boundary of this layer should be kept above thezone of normal occupancy, application is limited to rooms with high ceilings. Comfortfactors which limit the temperature difference between head and feet determine that theair supply temperature should be in the range 18 to 20 C for seated sedentary occupations,and this, in turn, restricts the cooling capacity of the system to 30±40 W/m 2 . Airsupply terminals should be selected to achieve a uniform air distribution pattern acrossthe floor whilst keeping air velocity low, not exceeding 0.4 m/s close to the point ofdischarge. Terminals of suitable pattern for this application are illustrated in Chapter15, p. 424.Room air-conditioning unitsDeveloped mainly for computer room applications, such units have been produced inrecent years to provide close control of both temperature and humidity over wide rangesof load variation. The units may be self-contained, that is with an integral refrigerantcompressor, or be served from a central chilled water source. Self-contained units mayhave remote condensing units or be connected to a water cooling circuit. Heating may bedirect electric or by coils served from a central heating circuit.Humidification is normally by steam injection from electrically heated units integralwithin the package. Air filtration is a requirement for such applications. Figure 14.25shows a typical cabinet type unit. Close control of conditions in the space together withthe need for quick analysis of component failure has led to the general use of microprocessorcontrols for these units.For computer room applications, a relatively small quantity of outside air is needed forventilation purposes and normally no extract system is provided, thereby allowing thesupply air to pressurise the space. The air-conditioning units would normally be duplicated,or in larger installations at least one redundant unit would be provided as a standbyin case of a unit failure or for use during routine maintenance operations.* Jackman, P. J., Displacement Ventilation. BSRIA Technical Memorandum 2/90.