Choosing an Air-Conditioning System
Each of the four general types of
air-conditioning systems has numerous variations, so choosing a system is not a
simple task. With experience, it becomes easier. However, a new client, a new
type of building or a very different climate can be a challenge.
We are now going to briefly outline the
range of factors that affect system choice and finish by introducing a process
that designers can use to help choose a system.
The factors, or parameters that
influence system choice can conveniently be divided into the following groups:
- Building design
- Location issues
- Utilities: availability and cost
- Indoor requirements and loads
- Client issues
Building
Design
The design of the building has a major
influence on system choice. For example, if there is very little space for
running ducts around the building, an all-air system may not fit in the
available space.
Location
Issues
The building location determines the
weather conditions that will affect the building and its occupants. For the
specific location we will need to consider factors like:
- site conditions
- peak summer cooling conditions
- summer humidity
- peak winter heating conditions
- wind speeds
- sunshine hours
- typical snow accumulation depths
The building location and, at times, the
client, will determine what national, local, and facility specific codes must
be followed. Typically, the designer must follow the local codes. These
include:
Building code that includes a section
on HVAC design including ventilation.
Fire code that specifies how the
system must be designed to minimize the start and spread of fire and smoke.
Energy code that mandates minimum
energy efficiencies for the building and components. We will be considering the
ASHRAE Standard 90.1 2004 Energy Standard for Buildings Except Low-Rise Residential
Buildings7.
In addition, some types of buildings, such
as medical facilities, are designed to consensus codes which may not be
required by local authorities but which may be mandated by the client. An
example is The American Institute of Architects Guidelines for Design
and Construction of Hospital and Health Care Facilities8, which has guidelines
that are extremely onerous in some climates.
Utilities: Availability and Cost
The choice of system can be heavily
influenced by available utilities and their costs to supply and use. So, if
chilled water is available from the adjacent building, it would probably be
cost advantageous to use it, rather than install new unitary refrigerant-based
units in the new building.
Then again, the cost of electricity may be very high
at peak periods, encouraging the design of an electrically-efficient system
with low peak-demand for electricity. The
issues around electrical pricing and usage have become very well publicized in
North America over recent years. The ASHRAE course Fundamentals of Electrical Systems
and Building Electrical Energy Use10 introduces this topic.
Indoor
Requirements and Loads
The location effects and indoor
requirements provide all the necessary information for load calculation for the
systems.
The thermal and moisture
loads –
Occupants’ requirements and heat output from lighting and equipment affect the
demands on the air-conditioning system.
Outside ventilation air – The occupants and other
polluting sources, such as cooking, will determine the requirements.
Zoning – The indoor arrangement
of spaces and uses will determine if, and how, the system is to be zoned.
Other indoor restrictions may be very
project, or even zone specific. For example, a sound recording studio requires
an extremely quiet system and negligible vibration.
The methods of calculating the heating
and cooling loads are fully explained, with examples, in the ASHRAE course Fundamentals of Heating
and Cooling Loads9.
Client
Issues
Buildings cost money to construct and to
use. Therefore, the designer has to consider the clients’ requirements both for
construction and for in-use costs. For example, the available construction
finances may dictate a very simple system.
Alternatively, the client may wish
to finance a very sophisticated, and more expensive system to achieve superior
performance, or to reduce in-use costs. In addition to cost structures, the
availability of maintenance staff must be considered.
A building at a very
remote site should have simple, reliable systems, unless very competent and
well-supported maintenance staff will be available. Clients’ approvals may be
gained, or lost, based on their own previous experience with other projects or
systems.
Therefore, it is important for the designer to find out, in advance,
if the client has existing preconceptions about potential systems.
System Choice
While all the above factors are
considered when choosing a system, the first step in making a choice is to
calculate the system loads and establish the number and size of the zones.
Understanding of the loads may eliminate some systems from consideration.
For
example:
- In warm climates where heating is not required only systems providing cooling need be considered.
- If there are significant variations in operating hours between zones, a system which cannot be shut down on a zone-by-zone basis may not be worth considering.
Typically, after some systems have been
eliminated for specific reasons, one needs to do a point-by-point comparison to
make a final choice. This is where the system-choice matrix is a very useful
tool.
Hope by this Article, now you can Choose best Air Conditioner.
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