Topics: HVAC, Heating, ventilating, and air conditioning, Mechanical engineering Pages: 72 (3366 words) Published: May 18, 2014
Commercial Air-conditioning or
HVAC (Heating, Ventilation and Air


Commercial air-conditioning or HVAC (Heating,
Ventilation and Air Conditioning)
It refers to the mechanical systems which heat,
cool, filter or dehumidify air in a room or building.
HVAC systems control the ambient environment
(temperature, humidity, air flow, and air filtering)
in homes and commercial buildings and are
crucial in maintaining environmental conditions in
critical applications such as data centers, medical
rooms, computer server rooms, and other clean
room applications.

The most common air-cooling
systems are either direct expansion
(DX) type or the chilled water type.

In DX systems, the air is cooled with direct exchange of
heat with refrigerant passing through the tubes of the
finned cooling coil. All these systems are comprised of a
hermetically sealed or open compressor(s), evaporator
(cooling coil fabricated out of copper tubes and
aluminum fins), a supply air blower, filter, a condenser
and heat rejection propeller fan.

Unitary System
In a unitary system, the complete cooling
system is in one casing. Since all equipment is
prepackaged, the installation cost is usually
lower, and the performance quality is often
higher than field-erected systems. Window airconditioners, package units are typical examples of unitary DX systems

Split System
The second DX concept, where the evaporator is
separate from the condenser/compressor, is called a
split system. These are commonly found in residential
and small commercial installations with capacity
ranges varying 1 to 50 TR and suitable for an area of
100 – 10000 square feet. The new ductless systems
which can be conveniently mounted on the ceiling or
wall are in this family

In a chilled water system the air is cooled with chilled
water passing through the tubes of a finned coil
(cooling coil). The refrigerant is used to chill the water,
which is circulated throughout the building. When a
chilled water system is the refrigeration unit it is called
a ‘chiller’. These are usually pre-packaged by the
manufacturer with the evaporator and condenser
attached, so that only water pipes and controls must be
run in the field.

Centralized Ducted “All – Air” Systems
These are systems in which the primary movement of
heat around the building is via heated and cooled air.
These systems are the most common in large spaces
such as office buildings, common public areas, retail,
shopping, manufacturing areas, airports, hotel lobbies

Centralized Fluid Based Hydronic Systems
These are systems in which a fluid - typically
water but possibly refrigerant - is used to move
heat around the building. These systems are
fairly common in office rooms, hotel rooms,
schools, building perimeter control etc.

Decentralized Systems
These are systems in which heating and cooling is
conducted locally, with little or no bulk movement of
heat around the building. Individual unit ventilators
are dispersed in small rooms and around the
perimeter of a building. These systems are relatively
common in schools, small hotels, domestic
applications, residential homes and small offices.

In an ‘All-Air system’, the refrigerant or chilled water is used to cool and dehumidify the air in the air
handling unit (AHU). The cool air is then circulated
throughout the building thru the ductwork. Heating
can also be accomplished either by hot water or
electrical strip heaters. The centralization of these
systems allow for better management and system
operation. On the other hand, they also require a
mechanical room adjacent to the controlled space for
locating the AHU and large ductwork in building

1. Constant Volume Systems
2. Dual Duct Systems
3. Variable Volume...
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