Transcript central air-handling units

IAED 204
Construction and Materials II
Spring 2015
HVAC SYSTEMS
HVAC SYSTEMS
(HVAC) system integrates mechanical equipment into one
complex system that is designed to provide thermal comfort and
air quality throughout a building.
The temperature,
humidity,
purity,
distribution, and motion of air
within interior building spaces are all controlled simultaneously by
an HVAC system.
These systems use air, water, or both to distribute heating and
cooling energy.
Systems include furnaces that supply hot air and boilers that heat
water or produce steam. Some systems include electric heaters
that use electrical resistance to convert electricity to heat.
HVAC SYSTEMS
HVAC systems can have CENTRALIZED or LOCAL EQUIPMENT.
LOCAL EQUIPMENT:
Advantages:
Because climate is such a strong factor in small buildings and
heating and cooling needs may vary room by room, localized
equipment rather than a centralized system may be the better
choice.
• A local system can respond more rapidly to individual room
needs and scheduling differences.
• No large central equipment spaces are needed, as equipment
can be distributed throughout the building or over the roof in the
case of low-rise buildings.
HVAC SYSTEMS
LOCAL EQUIPMENT:
Advantages:
• Distribution trees are shorter, and breakdowns are localized.
• Control systems are greatly simplified.
• Energy is conserved when heating and cooling is used only as
needed.
Disadvantages:
• The noise from many machines all over the building can be
objectionable.
• The presence of maintenance workers within occupied areas can
be disruptive.
• Multiple pieces of equipment mean many filters to clean and
change to assure air quality.
• There is less opportunity to use waste energy than with a
centralized system.
HVAC SYSTEMS
CENTRALIZED EQUIPMENT:
Advantages:
• The equipment has its own space, and does not take up room
within occupied spaces.
• Maintenance need do not interrupt room activities.
• Energy recovery from boilers and chillers is possible in a central
equipment room.
• Centralized mechanical spaces concentrate the noise and heat
for easier control.
• Air intakes can be high above street pollution, and regular
maintenance of centralized air filtering equipment results in
longer equipment life.
HVAC SYSTEMS
CENTRALIZED EQUIPMENT:
Design and Some Disadvantages:
• Mechanical rooms need to be centrally located near the area
served, with direct access to the outside for fresh air and for
installation and removal of equipment.
• Achieving both a central location and outside access is
often difficult.
• Rooms for heating, cooling, and air handling equipment need
ceilings around 3.7 meters high.
• Distribution trees of centralized systems are larger and controls
are more complex than for localized systems.
• Breakdown of a single piece of equipment may affect the entire
building.
• Energy is wasted when the entire system is activated to serve
one zone.
HVAC SYSTEMS
INTERIOR DESIGN:
OFFICES:
• Uniform ceiling heights,
• lighting placement,
• and HVAC grille locations
increase flexibility in office arrangements and extend
the building’s useful lifespan.
HVAC SYSTEMS
INTERIOR DESIGN:
OFFICES:
Four basic types of office space can be interchanged
within a flexible overall plan. These include
• Enclosed offices;
• bullpen offices with repeated, identical
workstations with desk-height dividers;
• uniform open plan offices with higher partitions;
• and free-form open plan offices with partitions of
varying heights.
HVAC SYSTEMS
INTERIOR DESIGN:
OFFICES:
The design of the air-circulation and ventilation system
interacts with the layout of furniture.
Even furniture like filing cabinets and acoustic screens
less than 1.5 meters high can impede air circulation,
especially if they extend to the floor.
Some sources recommend an open space of at least 25
to 51 mm at the base for furniture pieces.
If walls or full-height partitions enclose spaces, each
enclosed space should have one supply vent and one
return or exhaust vent.
TYPES OF HVAC SYSTEMS
TYPES OF HVAC SYSTEMS
Direct Refrigerant Systems are heating and cooling
systems that respond directly to the needs of
individual zones.
The other three, all-air, air and water, and all-water
systems, produce heating and cooling in a central
location, far from some zones. Air-handling
equipment for these last three systems is either
central or located on each floor.
HVAC SYSTEMS
Design Issues:
• Exposing the mechanical system within the space
permits easy access for maintenance, repair, and
alteration.
• Exposed systems may add visual interest. Office
buildings sometimes expose the HVAC system in
corridors and service areas, but conceal it in
offices.
Centre Georges Pompidou in Paris,
designed by Piano +
Rogers, Architects.
Centre Georges Pompidou in Paris,
designed by Piano +
Rogers, Architects.
HVAC SYSTEMS
Design Issues:
• The location of centralized HVAC equipment in a large
building has implications for the building’s space use
and function.
• The heat, noise, moisture, air motion, and vibration
from equipment may annoy adjacent floors or
neighboring buildings.
• Mechanical floors can be used to separate floors of
apartments from floors of offices, isolating daytimeuse spaces from housing used more heavily at night.
• Very large buildings often require several intermediate
mechanical floors.
HVAC SYSTEMS
Design Issues:
• Basement locations offer noise isolation, utility
access, and support for heavy machinery.
• Rooftops provide access to air for rejecting excess
heat, plus unlimited headroom. However, top floor
spaces often bring top rental fees, which are lost
when these spaces are devoted to mechanical
equipment.
TYPES OF HVAC SYSTEMS
HVAC SYSTEMS: HVAC
EQUIPMENT
UNITARY AIR-HANDLING UNIT
Unitary HVAC systems combine heating, cooling, filtration, humidity
management, and air-handling functions in a single, unified package.
• Unitary air-handling units are installed on the roof directly above
the space to be air-conditioned, or mounted on a concrete pad
along the exterior building wall. Rooftop units may be placed at
intervals along long buildings.
• Unitary air-handling units are self-contained, and are used where
the utility services of individual tenants are metered separately.
• Packaged systems with vertical shafts that connect to horizontal
branch ducts can serve buildings up to four or five stories high.
• They have a life of about 10 years and require little maintenance.
HVAC SYSTEMS: HVAC
EQUIPMENT
UNITARY AIR-HANDLING UNIT
HVAC SYSTEMS: HVAC
EQUIPMENT
COMPUTER ROOM UNITS
Computer room units use a highly reliable airhandling unit with extremely precise temperature,
humidity, and dust controls for sensitive electronic
equipment, all located in the space served.
HVAC SYSTEMS: HVAC
EQUIPMENT
CENTRAL AIR-HANDLING UNITS
Central air-handling unit systems are found in large
buildings with multiple zones of at least 450 square
meters and in tall multistory buildings.
Hospitals with stringent air quality controls use central
air-handling systems exclusively.
• Large central air-handling unit systems require routine
daily checking and regularly scheduled maintenance.
• They are built on site, and take longer to install than
prefabricated units, but may be more energy efficient.
• Central air-handling systems have a 20- to 30-year life
expectancy.
CENTRAL AIR-HANDLING UNITS
1. In air-handling unit systems for large
buildings, air is passed over the airhandling unit’s heat exchanger coil,
which contains steam or hot water
pipes.
2. Heating is transferred to the air, then to
ducts and into the spaces where it will
be used.
3. The air distributed from the central airhandling unit ends up in the building’s
individual occupied spaces.
Small terminal units may be mounted
directly below a window or in openings in
the exterior wall of each space served.
Window mounted units typically are used to
retrofit existing buildings.
HVAC SYSTEMS: HVAC
EQUIPMENT
HVAC systems are made up of a number of separate
pieces of equipment:
1. Pre-heaters warm air that is below 0°C to a
temperature slightly above freezing before it is sent on
for other processing.
2. Blowers (fans) supply air at a moderate pressure, to
create the forced drafts that operate the HVAC system.
3. Humidifiers maintain or increase the amount of water
vapor in the air.
4. A chilled water plant powered by electricity, steam, or
gas delivers chilled water to the air-handling equipment
for cooling, and pumps condenser water to the cooling
tower to dispose of excess heat.
HVAC SYSTEMS: HVAC
EQUIPMENT
5. The boiler generates hot water or steam for heating,
and requires fuel and air supplies. Hot water (hydronic)
heating is more common than steam heat today.
6. The furnace generates hot air for heating and requires
fuel and air supplies.
7. The fan room contains air-handling equipment in large
buildings. Sometimes individual fan rooms are located
on each floor or in each zone.
8. Chimneys exhaust gases from the burning fuel.
9. Cooling towers extract heat from the water that has
been used for cooling by blowing air over the water to
create water vapor and liberate the latent heat. They are
typically located on the roof.
HVAC SYSTEMS: HVAC
EQUIPMENT
10. Special control equipment in HVAC systems for
large buildings maintains comfort through central
monitoring and coordination of equipment and
building conditions. Controls increase fuel economy
by running equipment in the most efficient way, and
act as safety devices to limit or override mechanical
equipment in emergencies.
HVAC SYSTEMS: Distribution
Trees
Centralized HVAC systems distribute heating and cooling
through systems called distribution trees.
Distribution trees take up a lot of space, both horizontally
and vertically.
They need to be coordinated with the lighting, ceiling
design, and other interior design elements.
Like trees in nature, distribution trees have roots—the
machines heating or cooling the air or water.
The trunk of the tree is the main duct or pipe from the
mechanical equipment to the zone served.
The tree’s branches are the many smaller ducts or pipes
leading to individual spaces.
HVAC SYSTEMS: Distribution
Trees
An HVAC system can distribute heating and cooling by means of
air,
water,
or both.
• All-air systems have the largest trees, followed by air and water systems.
Air systems use ducts, which are bulky and have a significant visual
impact.
• All-water systems with local control of fresh air have the smallest
distribution trees. Water systems use pipes, which take up less space, and
are easier to integrate with the building’s structural columns.
All-air systems provide the best comfort of these three systems. The air is
heated or cooled,
humidity controlled, and filtered,
and fresh outdoor air is added, all under controlled conditions.
Within each zone, supply registers and return grilles ensure a stream of
conditioned air to all areas.
HVAC SYSTEMS: ALL-WATER
DISTRIBUTION SYSTEMS
All-water systems are a simpler alternative for heating and cooling
than all-air systems. The distribution trees are slim.
Water systems provide temperature control only. Air quality is
provided by windows or infiltration, or by a separate fresh air supply
system.
Water for the HVAC system is heated in a boiler to
between 71°C and 121°C.
Cold water is cooled in a chiller to 4°C to 10°C.
Once the water is heated or chilled, it is then piped to a fan-coil unit
(FCU) or radiant panel for both heating and cooling.
An FCU contains an air filter and a fan for drawing a mixture of room
air and outdoor air over coils of heated or chilled water, then blowing
it back into the space.
HVAC SYSTEMS: ALL-WATER
SYSTEMS
HVAC SYSTEMS: AIR-WATER
DISTRIBUTION SYSTEMS
Air-water systems provide superior comfort control. The
water distribution tree does most of the heating or
cooling.
A small centrally conditioned airstream filters, controls
humidity, and adds fresh air.
Air is not recirculated in air-water systems, making this a
good choice for hospitals and other facilities where
contamination may be a problem.
Exhaust air is returned via return air ducts, making energy
recovery possible, or is exhausted locally.