Building Services Engineering 5th Edition Handbook

132 Ventilation and air conditioning
of the building in varying the area of glazing, walling and roofing and the thermal transmittance
of each component. Life-cycle costing of each alternative thermal design will reveal the total
cost of constructing and using the building for 50 years, with reasonable assumptions on price
changes each year and the cost of refitting the building every 20 years because of improvements
in technology.
Selection of the indoor design air set points for temperature and relative humidity will be
determined by legislative and comfort standards. Short-term variances within an allowable range
can minimize the use of energy at peak times. It may be possible to avoid the installation of
mechanical cooling systems where the users of the building agree to accept regular, short-term
divergence from the standard design conditions. The provision of ceiling-mounted or portable
fans with comfort breaks and refrigerated drinks dispensers might allow a building to avoid the
installation of mechanical cooling, depending upon the number and frequency of divergences
from the normal standards. Allowing the indoor air temperature to rise to 25 ◦ C d.b. in summer
before switching on the air-conditioning chiller during the afternoon, and whether the building
is occupied, the time is before 3.30 p.m. and the outdoor air temperature is above 25 ◦ C d.b.;
all these conditions can be accessed through the computer-based building management system,
if one is provided. Automatic control programming can be set to minimize the use of electrical
energy by the mechanical cooling system as a deliberate strategy by the designers and operators
of the building. Dynamic thermal analysis software is used to model buildings and their services
systems to assess the indoor air conditions that are expected to occur.
The provision of outdoor air ventilation is a legislative requirement based on the activities of
and the numbers of people within the enclosure. The minimum quantity of outdoor air must
be maintained for each person to comply with the standards. Outdoor air ventilation does more
than provide air for breathing; it also flushes the building with outdoor air to remove heat and
to control the concentrations of odours and atmospheric pollutants that are produced within the
building. Toilet exhaust air flows often partly match the inflow rate of outdoor air in commercial
buildings. Any balance of flow between the exhaust quantity and inflow of outdoor air may
be allowed to leak through doors, permanent ventilators and other openings in the structure,
such as gaps around window frames. The inclusion of carbon dioxide sensors in the return or
exhaust air ducts allows the building management system to minimize the opening of the outside
air intake motorized dampers and consequently reduce the heating or cooling load of the airconditioning
plant and save energy. Carbon dioxide in air that is removed from the occupied
space is a direct assessment of the number of occupants and their activity level, allowing energy
use to match the instantaneous load on the building. Other means of assessing room occupancy
are available from infrared or ultrasonic sensors. If the designers of the building are able to know
the divergence in the patterns of occupancy of rooms or spaces, then decisions can sometimes
be taken to diversify the provision of lighting, heating and cooling zones to minimize the use of
mechanical and electrical plant and systems, with benefits in the initial plant capacity and in the
use of energy in the long term.
Where incoming outdoor unconditioned air can be preconditioned, an air-to-air flat-plate heat
exchanger may be used. This is a compact box around the size of a suitcase having counter flow
passages for incoming and outgoing air separated by aluminium foil plates. Incoming winter air
is warmed by the room air being exhausted; in summer, room air at the correct temperature cools
the outdoor ventilation air stream.
Natural ventilation can be used in atria and industrial buildings where the stack effect of height
within the building is used to create air movement. Low-level outside air intakes and roof-level air
extractors or openable ventilation units can be mechanically controlled to match the heating and
cooling load on the building to the flow of air through the spaces. The avoidance of draughts