AIR CONDITIONING SYSTEMS
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All Air Systems
• An all-Air System is defined as a system providing complete
sensible and latent cooling capacity in the cold air supplied by
the system.
• No additional cooling is required
• Can be briefly classified and are discussed here in to basic
categories:
1. Single duct systems, or
2. Dual duct systems
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Cont ..
• The single duct systems can provide either cooling or heating
using the same duct, but not both heating and cooling
simultaneously. These systems can be further classified into:
1. Constant volume, single zone systems
2. Constant volume, multiple zone systems
3. Variable volume systems
• The dual duct systems can provide both cooling and heating
simultaneously. These systems can be further classified into:
1. Dual duct, constant volume systems
2. Dual duct variable volume systems
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• All-Air System can be adapted to all types of air conditioning
systems for comfort or process work.
• It is applied in off, schools, universities ,laboratories, hospitals,
stores, hotels and ships where they require individual control
of conditions.
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1. Single zone system: simplest form of the all-air system is a
single conditioner serving a single temperature zone.
-applied to small department stores, small individual
shops in shopping center, individual class rooms etc
A schematic of the single zone central unit is shown below:
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2. Multiple zone systems: for large building it is
usually not economical to provide a separate system
for each zone.
For such cases the basic control system concept is
expanded to meet the cooling and heating
requirements of the multi zone.
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Schematic diagram of terminal reheat zone
a. Constant-air Volume Systems
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b.
Variable-air Volume Systems: when air is the medium used to
balance the load, the designer may choose between varying the
supply air temperature constant volume) or varying the volume
(variable volume) as the space load changes.
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Variable air volume system (VAV) may be applied to interior or perimeter
zones with common or separate fan systems, common or separate air
temperature control, and with or without auxiliary heating devices.
The variable volume concept may apply to volume variation in the main
system total air stream and/or to the zones of control.
Variation of flow under control of a space thermostat may be
accomplished by positioning simple damper or a volume regulating
device in a duct, a pressure reducing device, or at the terminal diffuser or
grill.
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Cooling or Heating:
A single stream of cool air serves all the zones, and a thermostat in
each zone regulates a damper to control the flow rate of cool air into
the zone.
Application : in interior of an office building with no heating loads and
where only cooling loads prevail.
Diagram:
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Cont
ii.
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Dual Duct : conditions all the air in a central apparatus and
distributes it to the conditioned spaces through two parallel mains or
ducts. One duct carries cold air and the other warm air, thus
providing air sources for both heating and cooling at all times.
In each conditioned space or zone, a mixing value responsive to a
room thermostat mixes the warm and cold air in proper proportions
to satisfy the prevailing heat load of the space.
The dual duct system is very responsive to changes in load of the
zone and can simultaneously accommodate heating in some zones
and cooling in others
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Schematic diagram of dual duct system
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The system experiences a problem at very light cooling loads where the
air flow rate drops off so much that poor air distribution and/or
ventilation results.
The heating only VAV system the same structures that shown above,
but a heating coil is used to provide the source for constanttemperature warm air.
ii. Cooling –reheat VAV system:
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This system is the same as the above figure except that the branch
lines to each zone contains a reheat coil.
The control sequence is that as the cooling load drops off, the damper
progressively reduces the flow rate of air until about 25 to 30% ful
flow rate . At this point the air flow rate remains constant and the
reheat coil is activated.
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• The VAV reheat system overcomes a number of deficiencies of the
cooling only VAV system since it provides a means of obtaining
adequate air distribution and ventilation without paying the energy
penalty incurred in constant volume reheat applications.
• In the VAV dual duct system, the arrangement is similar to the
conventional dual duct system of the above fig. except for the flow
characteristics of the mixing boxes. Instead of providing a constant
flow rate of mixed air , the dampers are arranged so that the warm and
cool air flow rates drop appreciably before the other streams begins to
supply air.
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• The VAV reheat and dual duct system provide all the flexibilities
of the conventional reheat and dual duct systems in the sense that
a zone can be accommodated as it switches from heating to
cooling requirements and that some zones on a system can be
provided with heating at the same that the other zones need
cooling. There is some thermal bucking in both the VAV reheat
and dual-duct systems but the magnitude of heating and cooling
energy cancellation is modest because the low air flow rates at
which the thermal bucking occurs.
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AIR-AND -WATER SYSTEMS
• Accomplish space conditioning through the use of both air and
water source that are distributed to terminal units installed in
the habitable space.
• The air and water are cooled or heated in central mechanical
equipment rooms and distributed to the space in which comfort
conditions are being maintained.
• The most common types of terminal used with air and water
systems is the high pressure induction unit. The air supplied to
the induction unit is called primary air. It is distributed from
the central mechanical equipment room at high velocities
between 10 and 24m/sec. the water distributed to the induction
unit is called secondary water.
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• Air-and-Water systems are primarily applicable to multi zone
type exterior spaces of buildings where wide range of sensible
loads exists and where close control of humidity is not
required.
• Applications: hospitals, schools, apartment houses , research
laboratories
• Components: central air conditioning equipment, duct
distribution and water distribution systems and a room
terminal
• The air supply is generally constant volume and provides
outside clean air for ventilation
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• Category : Two-pipe, three-pipe , four-pipe systems. They are
basically similar in function and all incorporate both cooling
and heating capabilities for year round air conditioning.
• Their name is drive from the water distribution system:
two-pipe: the water distribution consists one supply and one
return pipe
three-pie: a cold water supply and warm water supply are
combined with a single common return pipe to make the water
distribution system
four-pipe : has cold water supply, cold water return, warm
water supply and warm water return
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ALL- WATER SYSTEMS
• Accomplish both sensible and latent space cooling by
circulating chilled water from ac central refrigeration system
through cooling coils in terminal units located in building
occupied spaces.
• Most common terminals: Fan coil units, unit ventilators, and
valence units
• The all-water system usually provides individual room control,
with no cross contamination of recalculated air from one space
to another
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Cont..
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• Advantage:
– Reduced need for building space to the elimination or reduction of
central fan room and duct space requirements
– The system provides all of the benefits of a central water chilling and
heating plant, while retaining the ability to positively shut off local
terminals in unused areas
• Disadvantage :
– Requires much more maintenance than central all-air system and work
must be done in the occupied areas
– Each unit requires a condensate pan and drain system that must be
cleaned and flushed periodically.
– It is difficult to clean coils, should this be required.
– Filters are low in efficiency, small, and requires frequent changing
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Duct Design and Fan Performance
• Commercial and residential and, industrial air duct system
design must consider
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2)
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Space air diffusion
Space availability
Noise level
Duct leakage
Duct heat gain and losses
Balancing
Fire and smoke control
Initial investment
System operating cost
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• Deficiency in duct design can result in systems that operate
incorrectly or are expensive to own and operate.
• Poor air distribution can cause discomfort, less of productivity
and even adverse health effects
• Lack of sound attenuators may permit objectionable noise
level.
• Poorly designed duct work can result in unbalanced system
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