Book - School of Science and Technology

Air supply for human emissions 341structure and the engineering systems. These buildings can, in certain circumstances,provide year round comfort, with good user control, usually at lower capital and maintenancecosts. This integrated approach to design of ventilation systems is incorporatedwithin the 2001 edition of the Guide Section 32.The CIBSE Applications Manual AM10: 1997 provides a comprehensive analysis of thesubject including, calculation methods and case studies and readers requiring more detailinformation are advised to refer to this document.It is appropriate to mention here so called `Mixed Mode' solutions that combinenatural ventilation with mechanical ventilation and/or cooling in the most effectivemanner to achieve satisfactory year round conditions whilst minimising energy use. TheCIBSE Application Manual AM 13: 2000 provides guidance on this subject.Air supply for human emissionsThe volume of air necessary to provide for human occupancy may be considered underthe following principal headings:. provision of oxygen for respiration. removal of products of exhalation. removal of body odour. removal of unwanted heat. removal of unwanted moisture. removal of contaminants.Respiration and exhalationAt rest, the normal adult inhales between 0.10 and 0.12 litre/s of air and of this only aboutsome 5% is absorbed as oxygen by the lungs. The exhaled breath contains between 3 and4% of carbon dioxide (CO 2 ) which amounts to about 0.004 litre/s.* The accepted level fora maximum concentration of CO 2 within an occupied space is 5000 parts per million, or0.5% by volume, for an exposure of 8 hours. The outside air requirement, at an equilibriumcondition, to restrict the level of concentration to the maximum level permitted,may be found by using the contaminant expression given on page 339: this indicates a verylowrate of 0.847 litre/s per person.Table 13.1 lists ventilation rates for concentrations other than the maximum and forother levels of activity but there are parallel aspects of human occupation of a space whichare more important criteria for the need for ventilation.As a result of these parallel effects, a room may feel fresh and pleasant with a CO 2content higher than that noted above and yet feel stuffy with a much lower content. Inconsequence, it has in the past been considered that it is an unsatisfactory datum forassessment of ventilation quality. However, the results of current research at BSRIA andelsewhere suggest that it might well be used as a means for ventilation control.* This is sometimes quoted as 0.004719 litre/s which is no more than an exact metric conversion of0:01 ft 3 /minute which, in turn, is an average between values of 0.513 and 0:684 ft 3 /hour. Since these lasttwo figures were derived from an estimate of the CO 2 content of the average exhalation from averagehuman lungs, 3±4% of 17 ft 3 /hour, the authors are inclined to believe that a somewhat rounder figure isto be preferred.