Handbook of air conditioning and refrigeration / Shan K

Fan-Coil Units
AIR CONDITIONING SYSTEMS: SPACE CONDITIONING SYSTEMS 28.5
at point ps. At ps, the dedicated ventilation system is combined with the space recirculating system
and forms the air system of the four-pipe or two-pipe fan-coil system. The outdoor air can be mixed
with the recirculating air in three ways:
● Outdoor air at point ps is supplied to the mixing plenum of the fan coil. It is then mixed with the
recirculating air, which is extracted by the fan in the fan-coil unit through the filter and forms the
mixture m. Air pressure at the mixing plenum is slightly less than the space pressure, so that space
air can be extracted into the plenum. The mixture then flows through the fan and coil in the fancoil
unit, cools and dehumidifies at the coil, and leaves the coil at point fc. Air that has been conditioned
in the fan coil is supplied to the conditioned space to offset the space load. State point fc
may or may not be on line pc-ps, but will be near point pf.
● Outdoor air at point ps is supplied to the conditioned space directly. In this case, space air is
filtered, and cooled and dehumidified (or heated), in the fan coil. The conditioned space air discharged
from the fan-coil unit at point fc is mixed with the outdoor air in the space. It is assumed
that the mixture at point m offsets the space load. In such an arrangement, the recirculating
airstream will recirculate along its own circuit.
● Outdoor air at point ps is supplied just before the supply outlet of the fan-coil unit. It mixes with
the conditioned air from the fan coil at point fc. The psychrometric cycle of this arrangement is
approximately the same as that of outdoor air supplied directly to the conditioned space.
Outdoor air supplied to the mixing plenum or before the supply outlet of the fan-coil unit provides
a balanced distribution of outdoor air in the conditioned space. On the other hand, outdoor air
supplied directly to the conditioned space has a shorter outdoor air supply duct, and the conditioned
air is better distributed when one of the fan-coil units is turned off during capacity control at partload
operation, as multiple fan-coil units are installed in a large room.
The temperature increase from fan power heat gain in the fan-coil unit is about 0.5°F (0.3°C) for
a permanent-slit capacitor fan motor and about 0.8°F (0.5°C) for a shaded-pole fan motor. The supply
duct in the ceiling plenum after the fan-coil unit is usually very short, so such a duct heat gain
or loss is negligible.
During winter heating mode operation, in extremely cold weather, outdoor air may be preheated
to point ph by the heating coil in the outdoor air AHU. After absorbing the supply fan
power to point pf and releasing the outdoor air supply duct heat loss to point ps in the dedicated
ventilation system, the outdoor air is mixed with recirculating air extracted by the fan coil at
point r, and forms a mixture m, as shown by line ps-m-r in Fig. 28.1b. The mixture is heated
in the fan coil to point cf by the fan power heat gain and then to point s by the heat released
from the coil in the space recirculating system, and the mixture is supplied to the conditioned
space.
Outdoor air can also be mixed with heated air from the fan-coil unit when outdoor air is supplied
directly to the conditioned space. In locations with a mild winter, only outdoor air is heated in
the outdoor air AHU.
A fan-coil unit, or a fan coil, is a terminal unit installed directly inside the conditioned space or in
the ceiling plenum just above the conditioned space. A fan-coil unit includes a small motor driven
centrifugal fan or two small centrifugal fans connected in parallel, a finned coil, a filter, an outer
casing, and controls. Sometimes, a cooling coil and a heating coil may be connected in series along
the airflow, as shown in Fig. 28.2.
A fan-coil unit can be a horizontal unit installed inside the ceiling plenum (Fig. 28.2a and b) or
a vertical unit mounted on the floor under the windowsill (Fig. 28.2c) or a stack unit installed vertically
along the two sides of the window. Vertical and stack units are usually used to offset the cold
draft on the inner surface of window glass or on the external wall during cold weather. Cold draft
often flows downward along the glass because it is heavier than the surrounding air. Warm air