System Air Conditioning 2

7/29/2019 System Air Conditioning 2

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4.0 Air Conditioning System

Air conditioning for people is the control of temperature, humidity, air movement and

air cleanliness, heat radiation sometimes, normally with mechanical means, to achieve

human thermal comfort. Air conditioning systems can be categorized according to the

means by which the controllable cooling is accomplished in the conditioned space. They

are further segregated to accomplish specific purposes by special equipment

arrangement. In selecting a suitable air conditioning system for a particular application,

consideration should also vent to the following:

System constraints : Cooling load, Zoning requirements, Heating and ventilation

Architectural Constraints : Size and appearance of terminal devices, acceptablenoise level, Space available to house equipment and its location relative to the

conditioned space, acceptability of components obtruding into the conditioned space

Financial Constraints : Capital cost, Operating cost, Maintenance cost

There are four basic system categories of air conditioning, first is central chilled

water air conditioning systems used all air systems. Second, central chilled water air

conditioning systems used all air and water systems. Third, central chilled water airconditioning systems used all water systems, including cooling towers. Fourth is,directexpansion systems such as direct expansion of refrigerant, without the chilled water

cooling medium.

4.1 Central Chilled Water Air Conditioning used all Air System

4.1.1 Single zone

The all-air single-zone air conditioning system is the basic central system which can

supply a constant air volume or a variable air volume at low, medium or high pressure.

Normally, the equipment is located outside the conditioned space but can also beinstalled within the conditioned are if conditions permit. Typical applications include:

Space with uniform loads

Small spaces requiring precision control

Multiple systems for large areas


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Figure 1 : Single Zone system

4.1.2 Reheat

The reheat system is a modification of the single-zone system. It provides:-

Zone or space control for areas of unequal loading.

Heating or cooling of perimeter areas with different exposures.

Close control for process or comfort applications. In the reheat system, heat is

added as a secondary process to either preconditioned primary air or recirculated

room air. The heating medium can be hot water, steam or electricity.

The advantage for this type is, it can closely controls space conditions, but this

advantages for this type system, it is very expensive to operate.

Figure 2 : Reheat system


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4.1.3 Variable Air Volume

The variable air volume system compensates for varying cooling loads by regulating the

volume of cooling air supplied through a single duct.

a. Simple Variable Air Volume (VAV)

A simple VAV system typically cools only and has no requirement for simultaneous

heating and cooling in various zones.

Figure 3 : Simple Variable Air Volume (VAV)

b. Variable Air Volume Reheat

It integrates heating at or near the terminal units. It is applied to systems requiring

full heating and cooling flexibility in interior and exterior zones. Heating is turned on

when the air flow reaches a predetermined minimum.

Figure 4 : Variable Air Volume Reheat


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Advantages

When combined with a perimeter heating system, it offers inexpensive temperature

control for multiple zoning and a high degree of simultaneous heating-cooling

flexibility.

Capital cost is lower since diversities of loads from lights, occupancy, solar and

equipment of as much as 30% are permitted.

Virtually self-balancing.

It is easy and inexpensive to subdivide into new zones and to handle increased loads

with new tenancy or usage if load does not exceed the original design simultaneous

peak.

No zoning is required in central equipment.

Lower operating cost because Fans run long hours at reduced volume

Refrigeration, heating and pumping matches diversity of loads

Unoccupied areas may be fully cut-off

Reduced noise level when the system is running at off-peak loads.

Allows simultaneous heating and cooling without seasonal changeover.

4.1.4 Dual Duct

The dual-duct system employs two air ducts to supply cold air and warm air to a mixing

terminal unit which proportions the cold and warm air in response to a thermostat

located in the conditioned space. The system is well suited to provide temperature

control for individual spaces or zones.

Figure 5 : Dual Duct System


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Advantages (in addition to those common to all air systems)

Systems with terminal volume regulation are self-balancing.

Zoning of central equipment is not required.

Instant temperature response is achieved because of simultaneous availability of

cold and warm air at each terminal unit.

No seasonal changeover is necessary.

Disadvantages

Initial cost is usually higher than other VAV systems.

Does not operate as economically as other VAV systems.

4.1.5 Multi-zone

The multi-zone system applies to a relatively small number of zones served by a single,

central air-handling unit. Different zone requirements are met by mixing cold and warm

air through zone dampers at the central air handler in response to zone thermostats.

Figure 6 : Multi-zone System

Advantages

Easy to balance.

Air transmission and distribution is simplified.


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4.2 Central chilled water air conditioning systems used air and water systems

4.2.1 Induction

The inducting system is designed for use in perimeter rooms of multi - storey, multi-room

building that may have reversing sensible heat characteristics. It is especially adapted

to handle the loads of skyscrapers with minimum space requirements for mechanical

equipment. In the induction system, ducted primary air is fed into a small plenum

chamber where its pressure is reduced by means of a suitable damper to the level

required at the nozzles. The plenum is acoustically treated to attenuate part of the noise

generated in the duct system and in the unit. The primary air is then delivered through

nozzles as high velocity jets which induce secondary air from the room and over the

secondary coil. Induction units are usually installed at a perimeter wall under a window.

Some hotel rooms are providing with induction coils.

Figure 7 : Induction System

The induction system employs air ducts to convey treated air with higher pressure levels

and of the right adjustable quantities to various cooling/heating coil units. These coil

units are built in with induction nozzles such that when high pressure air goes through


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them, air room the room is inducted across the fin surface of the water-circulated coils.

This inducted air stream is either cooled or heated after passing through the coil, and

then mixed with the air coming out of the nozzle. The right quantity of high pressure air is

adjusted automatically in response to a thermostat located in the conditioned space. The

system is well suited to provide temperature control for individual spaces or zones.

Advantages

Individual room temperature control.

Separate sources of heating and cooling for each space available as needed to

satisfy a wide range of load variations.

Low distribution system space required as a result of reducing the air supply by

use of secondary water for cooling and high velocity air design.

Reduced size of central air handling equipment.

Dehumidification & filtration performed in a central plant room remote from

conditioned space.

Outdoor air supply is positive. Minimal maintenance required for individual

induction units which have no moving parts.

Air duct dimensions are smaller than VAV systems or CAV systems

Zoning of central equipment is not required.

No fan comes together with the coil, making the conditioned space quiet.

Disadvantages

Limited to perimeter space.

The primary air supply is usually constant with no provision for shutoff.

Not applicable to spaces with high exhaust requirement.

Higher energy consumption due to increased power required by the primary

pressure drop in the terminal units.

Controls tend to be more complex than for all-air systems.

A low chilled water temperature is needed to control space humidity adequately.

Seasonal changeover is necessary.

Initial cost is usually higher than fan coil systems.


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4.2.2 Fan Coil

The fan-coil system is similar to the inducting system, with the induction unit replaced by

the fan-coil unit. The basic elements of the fan-coil units are a finned-tube coil and a fan

section. The fan section recirculates air continuously from within the perimeter space

through the coil which is supplied with either hot or chilled water. Auxiliary air may be

delivered to the conditioned space for dehumidification and ventilation purposes.

Figure 8 : Fan Coil system

Advantages

System can be operated with the primary air turned off.

The air velocity is fairly constant regardless of the primary air quantity.

Primary air can either connect directly to fan-coil unit or supply the room

separately.

4.2.3 Two-pipe

In two-pipe systems for induction coil, fan-coil or radiant panel systems, the water

distribution circuit consists of one supply and one return pipe. The secondary water is

cold in summer and intermediate seasons and warm in winter. The primary air quantity

is fixed and the primary air temperature is varied in reverse proportion to outside

temperature to provide the necessary amount of heating during summer and

intermediate seasons. During winter cycle operation, the primary air is preheated and

supplied at about 10C to provide a source of cooling.


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Figure 9 : Two-pipe system

Advantages Usually less expensive to install than four pipe systems.

Disadvantages

Less capable of handling widely varying loads or providing widely varying choice of

room temperature than four-pipe systems.

Cumbersome to change over.

More costly to operate than four-pipe systems.

4.3 Central chilled water air conditioning systems used to all water systems, including

cooling towers.

4.3.1 Fan-coil units

Use when heating and cooling loads that proves to be too great for induction units can

be dealt with by separate fan and coil units fitted into the false ceiling of each room or

buildings module. Better air filtration can be achieved than with the induction unit. A

removable access hatch below the unit is required to facilitate motor and filter

maintenance.

Care is taken to match the fan-generated noise to the required acoustic environment. As

with the other systems, the extracted air can be taken through ventilated luminaires to

remove the lighting heat output at source and avoid overheating the room. The supply

and extract ducts only carry the fresh air. All recirculation is confined to the room


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Figure 10 : Fan coil unit installation in false ceiling and vertical unit for fan coil unit

system

4.3.2 Water cooling tower

A water cooling tower cools the water entering it from 35 deg. C to 30 deg. C nominally.

The warmer water is sprayed inside the cooling tower amidst the stream of an upward

air flow produced by the fan at the top of the tower. The air stream going out carrieswater particles. These water particles should not be taken into buildings, to avoid

Legionnaire disease to occur. Condenser water pumps move condenser water through

this circuit. Water in this circuit has to be treated. There is water loss to atmosphere in

using cooling towers.


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4.4 Direct expansion Systems

4.4.1 Window air conditioner

A window unit is an encased assembly designed primarily for mounting in a window,

through a wall, or as a console. These units are designed for comfort cooling and to

provide delivery of conditioned air to a room either without ducts or with very short ducts.

They include a prime source of refrigeration, dehumidification, means for circulating and

cleaning air, and may also include means for ventilating, and/or exhausting and heating.


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Figure 11 : Direct Expansion System for window air conditioner

In a window air conditioner, the indoor unit and outdoor unit of the split system is put into

one single unit. The refrigerant compressor now is part of the machine locating at the

window area. Since this compressor gives out most noise, among other components, the

window unit will make the room acoustically inferior to other air conditioning systems.

Fresh air exchange for the room can be provided by:-

a. setting the ventilator switch of the window air conditioner to open position.

b. installing a ventilating extract fan in the room to extract room air to outside

caution- not to oversize the fan.

c. naturally leaking of air in and out of the room.

4.4.2 Unitary and Rooftop Air Conditioners.

These are commonly air-cooled units. The units are the floor are standing type designed

for installation outdoors or on the roof. A supply air duct and a return air duct are to be

connected to the cooling unit.Be used for general air conditioning of stores, residences,schools, offices, etc. Particularly suitable for single flat building with extensive floor

areas. A remote controller should be installed on an easily accessible wall, incorporating

a temperature selection switch & thermostat.


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Figure 12 : Direct Expansion System for Unitary and Rooftop Air Conditioners.

4.4.3 Split type and package air conditioning systems

A package air conditioning systems are used in factory assembled (floor mounting)

package, placed indoor, containing direct expansion coil, controls, fan and compressor,

with the condenser remotely placed outdoor. A package air unit is a self-contained air-

conditioning unit comprising a hermetically sealed refrigeration compressor, a refrigerant

evaporator coil to cool room air, a hot-water or electric resistance heater battery, a filter,

a water or air cooled refrigerant condenser and automatic controls. Packaged units caneither be completely self-contained, needing only a supply of electricity, or piped to

central heating and condenser cooling-water plant. Small units are fitted into an external

wall and have a change-over valve to reverse the refrigerant flow direction. This enables

the unit to cool the internal air in summer and the external air in winter.

Heat rejected from the condenser is used to heat the internal environment in winter. In

this mode of operation it is called a heat pump. A separate ventilation system may be

needed. Compressor and fan noise levels are compared with the acceptable background

acoustic environment. Maintenance requirement are filter cleaning, bearing lubrication

and replacement of the compressor when it becomes too noisy or breaks down.

Split system unit have a separate condenser install outside the building. Two refrigerant

pipes of small diameter connect the internal and external equipment boxes. This allows

greater flexibility in siting noise-producing compressor. Ducted models provide


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conditioning and ventilation and are often sited on flat roofs. Figure 5 shows a typical

through-the-wall installation. Split system used at same place with package air system.

Figure 13 : Packaged air-conditioning unit.

4.4.4 Heat pumps

The term heat pump, as applied to a year-round air conditioning system, commonly

denotes a system in which refrigeration equipment is used in such a manner that heat is

taken from a heat source and given up to the conditioned space when heating service is

wanted, and is removed from the space and discharged to a heat sink when cooling and

dehumidification are desired.

Heat pumps for air conditioning service may be classified according to

a. type of heat source and sink.

b. Heating and cooling distribution fluid.

c. Type of thermodynamic cycle.

d. Type of building structure.

e. Size and configuration.


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4.4.4.1 Air-to-Air Heat Pumps

The air-to-air heat pump is the most common type of heat pumps. It is particularly

suitable for factory-built unitary heat pumps, and has been widely used for residential

and commercial application. Air is used as the heat source and heat sink. Extended

surface, forced convection heat transfer coils are normally employed to transfer the heat

between the air and the refrigerant. When selecting or designing an air-source heat pup,

two factors in particular must be taken into consideration:

a. the variation in temperature experienced in a given locality.

b. the formation of frost.

4.4.4.2 Water-source Heat Pumps

The water-source heat pump uses water and air as the heat source or heat sink

depending on the mode of operation. When cooling, water is used as the heat sink, and

the heat pump operates as a water-cooled air conditioner. When heating, water is used

as the heat source and the equipment operates as a water chiller. The water-source

heat pump is suitable for many types of multi-room buildings, including office buildings,

hotels, schools, apartment buildings, manufacturing facilities and hospitals.

Advantages

Affords opportunity for energy conservation by recovering heat from interior

zones and/or waste heat and by storing excess heat from daytime cooling for

night time heating.

No wall openings required.

Longer expected life than air-cooled heat pumps.

Lower noise level because condenser fans are eliminated.

Energy for the heat pumps can be metered directly to each tenant.

Total life cycle cost frequently compares favorably to central systems when

considering relative installed cost, operating costs, and system life.

Disadvantages

Space required for boiler, heat exchanger, pumps and heat rejecter.

Higher initial cost than for most other multiple-packaged unit systems.

Reduced air flow can cause the heat pump to cycle cutout. Good filter

maintenance is imperative.

System Air Conditioning 2

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