|
Terminal Systems ,Fan coils,Induction Units
IN-ROOM TERMINAL SYSTEMS condition spaces by distri buting air
and water sources to terminal units installed in habitable spaces
throughout a building. In some systems the air is distributed
to the space directly, not through the terminal unit. The air
and water are cooled or heated in central equipment rooms. The
air supplied is called primary or ventilation air; the water supplied
is called
secondary water. Sometimes a separate electric heating coil is
included in lieu of a hot water coil. This chapter describes induction
units and fan-coil units used in in-room terminal unit systems.
In-room terminal unit systems are applied primarily to exterior
spaces of buildings with high sensible loads and where close control
of humidity is not required. In some cases they may be applied
to interior zones. They work well in office buildings, hospitals,
hotels, schools, apartment buildings, and research laboratories.
In most climates, these systems are installed in exterior building
spaces and are designed to provide (1) all required space heating
and cooling, (2) outside air for ventilation, and (3) simultaneous
heating and cooling in different parts of the building during
intermediate seasons.
FAN-COIL UNITS
Fan-coil unit systems include cooling as well as heating, normally
move air by forced convection through the conditioned space,
filter the circulating air, and introduce outside ventilation
air. Fan coil units with chilled water coils, heating coils, blowers,
replaceable air filters, drain pans for condensate, etc., are
designed for these purposes. These units are available in various
configurations to fit
under window sills, above furred ceilings, in vertical pilasters
built into walls, etc. These units must be properly controlled
by thermostats for heating and cooling temperature control, by
humidistats for humidity control, by blower control or other means
for regulating air quantity, and they must have a method for adding
ventilation air. into the building.
In-Room Terminal Systems
Basic elements of fan-coil units are a finned-tube coil, filter,
and fan section (Figure 2). The fan recirculates air continuously
from the space through the coil, which contains either hot or
chilled water. The unit may contain an additional electric resistance,
steam,
or hot water heating coil. The electric heater is often sized
for fall and spring to avoid changeover problems in two-pipe systems.
A cleanable or replaceable 35% efficiency filter, located upstream
of the fan, prevents clogging of the coil with dirt or lint entrained
in the recirculated air. It also protects the motor and fan, and
reduces the level of airborne contaminants in the conditioned
space. The fan-coil unit is equipped with an insulated drain pan.
The fan and motor assembly is arranged for quick removal for servicing.
Most manufacturers furnish units with cooling performance certified
as meeting Air-Conditioning and Refrigeration Institute (ARI)
standards. The prototypes of the units have been tested and labeled
by Underwriters’ Laboratories (UL), or Engineering Testing
Laboratories (ETL), as required by some codes.
Fan-coil units, with a dampered opening for connection to apertures
in the outside wall, are available. These units are not suitable
for commercial buildings because wind pressure allows no control
over the amount of outside air that is admitted. Also, freeze
protection may be required in cold climates. However they are
often used in residential construction because of simplicity of
operation, low first cost, and the operable windows can unbalance
a duct ventilation air system. Fan-coil units for the domestic
market are generally available in nominal sizes of 200, 300, 400,
600,
800, and 1200 cfm, often with multispeed, high-efficiency fan
motors. Where units do not have individual outside air intakes,
means must be provided to introduce retreated outside air through
a duct system that engages each room or space.
Types and Location
Fan-coil units are available in many configurations. Low vertical
units are available for use under windows with low sills; however,
in some cases, the low silhouette is achieved by compromising
such features as filter area, motor serviceability, and cabinet
style.
Floor-to-ceiling, chase-enclosed units are available in which
the water and condensate drain risers are part of the factory-furnished
unit. These units are used extensively in hotels and other residential
buildings. The supply and return air must be isolated from each
other to prevent air and sound interchange between rooms.
Vertical models or chase enclosed models located at the perimeter
give better results in climates or buildings with high heating
requirements. Heating is enhanced by under-window or exterior
wall locations. Vertical units can be operated as convectors with
the fans turned off during night setback.
Horizontal overhead units may be fitted with ductwork on the
discharge to supply several outlets. A single unit may serve several
rooms (e.g., in an apartment house where individual room control
is not essential and a common air return is feasible). Units must
have larger fan motors designed to handle the higher pressure
drops of ductwork.
Horizontal models conserve floor space and usually cost less,
but when located in furred ceilings, they create problems such
as condensate collection and disposal, mixing of return air from
other rooms, leakage of pans causing damage to ceilings, difficulty
of access for filter and component removal, and air quality concerns.
When outside air is introduced from a central ventilation system
it may be connected directly to the inlet plenums of horizontal
units or introduced directly into the space. If introduced directly,
provisions should be made to ensure that this air is pretreated
and held at a temperature equal to the room temperature so as
not to cause
occupant discomfort when the unit is off. One way to prevent air
leakage is to provide a spring-loaded motorized damper that closes
off the ventilation air whenever the unit’s fan is off.
Coil selection must be based on the temperature of the entering
mixture of primary
and recirculated air, and the air leaving the coil must satisfy
the room sensible and latent cooling and heating requirements.
,
|
