Handbook of air conditioning and refrigeration / Shan K

Induction Systems
annoying. A receiver may receive a sound pressure level between 35 and 40 dBA in a conditioned
space produced by a typical fan coil of 400 cfm (189 L/s).
● Fan coils and water-source heat pumps are scattered terminals. Both have moving parts, filters,
and condensate pans and require site maintenance directly above, near, or within the conditioned
space, which causes difficulties, inconvenience, and possible leaks of water and condensate.
In addition, because only low-efficiency or coarse filters are used in the fan coils and water-source
heat pumps, more cleaning work is required for the coils, ducts, and air distribution components.
● The volume flow rate of outdoor ventilation air supplied to the conditioned space is not sufficient
to pressurize the floor immediately above or below the fire floor, to prevent smoke contamination
in case of a building fire.
In an induction system, recirculating air extracted through the coil is due to a negative pressure
formed adjacent to the high-velocity supply nozzle. An induction system sees a considerably
greater energy use than fan-coil systems and is not recommended in new and retrofit projects.
28.2 FAN-COIL SYSTEMS AND FAN-COIL UNITS
System Description
Operating Characteristics
AIR CONDITIONING SYSTEMS: SPACE CONDITIONING SYSTEMS 28.3
A four-pipe or a two-pipe fan-coil system includes boilers and chillers in the central plant, water
system supplying chilled or hot water to the fan coils, a space recirculating system using fan coils
to condition the space recirculating air, and a dedicated outdoor ventilation air system using an outdoor
air AHU to condition the outdoor air, as shown in Fig. 28.1. Both the space recirculating and
dedicated outdoor air systems have ducts, diffusers, inlets, controls, and accessories.
Outdoor air is often cooled and dehumidified, heated, or sometimes even humidified in a separate
outdoor air or makeup AHU. The outdoor air is then transported to the fan coils where outdoor
air is mixed with the recirculated air and conditioned. If the conditioned outdoor air is supplied directly
to the conditioned space via supply ducts in the dedicated ventilation system, then it is mixed
with the conditioned recirculating air from the fan coil of the space recirculating system in the
space.
Outdoor (primary) air mixing with recirculating air in the outdoor air AHU is not economical
because recirculating air must be transported back to the outdoor air AHU and conditioned there instead
of in the fan coil in or near the conditioned space. Normally, there are no return fans or return
ducts in four-pipe or two-pipe fan-coil systems.
Chilled water or hot water is supplied to the fan coil through a two-pipe or four-pipe water system.
In locations with a moderate winter, or where electric rates are favorable in winter, an electric
heating coil in each fan-coil unit is sometimes used for winter heating. This greatly simplifies the
operation of fan-coil systems.
Four-pipe and two-pipe fan-coil systems and water-source heat pump systems are the air conditioning
systems that use dedicated ventilation and space recirculating air systems.
During summer cooling mode operation, outdoor air at point o in Fig. 28.1 is cooled and dehumidified,
and filtered in the outdoor air AHU. The outdoor air leaves the cooling coil at point pc, flows
through the supply fan outlet at point pf in the dedicated ventilation system, and is transported by
the supply duct to the fan coil in the space recirculating systems or directly to the conditioned space