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12 September 2011

Air Conditioning: Local vs Central Systems

PAUL HAY Capital Projects

Air-Conditioning, part 1 of 2

Author: Paul Hay


1.1     A major part of energy consumed in Jamaican buildings is used for cooling and fans.
Pie chart showing itemized energy end-use for a typical large office

Figure 1: Itemized energy end-use for a typical large office

1.2       Small one or two storey office buildings may be served by packaged air-cooled units installed on the roof slab.
            1.2.1   Equipment cost and installation is low; but
            1.2.2   This lacks flexibility.

1.3       Local systems are simple and require less space.
            1.3.1   There is no need for large mechanical rooms
            1.3.2   Ductwork is minimized; and
            1.3.3   Controls are simple

1.4       Roof locations are beneficial.
            1.4.1   The roof facilitates easy access to outdoor air for fresh-air supply; and
            1.4.2   Headroom is unlimited.

1.5       The system is sized to cope with the maximum design load of the building.

1.6       Direct Expansion (DX) systems are simple, economical and energy-efficient.
            1.6.1   Refrigerant passes through an expansion valve to the evaporator.
            1.6.2   Air is directly cooled at the evaporator; but
            1.6.3   Precise temperature control is difficult to achieve.

1.7       All-air single-zone systems transport air to conditioned spaces by one duct and has one room thermostat.

1.8       Direct refrigerant systems reduce the distribution tree by locating the evaporator adjacent to the conditioned space.

1.9       The Coefficient of Performance (COP) is one measure of A/C efficiency:

            COP   =          Cooling output to conditioned space       [1.1]
                                          Energy input to equipment

Table showing Coefficient of Performance [COP] for typical A/C units

Table 1: Coefficient of Performance [COP] for typical A/C units


2.1       Large commercial buildings have at least two independent thermal zones:
            2.1.1   The perimeter zone is predominantly affected by external heat-gains.
                          This zone is adjacent to the roof or up to 6 m in from external walls.
                          If the building is up to 12 m wide, this can be further divided into two sub-zones: each representing two different wall orientations;
                          If the building is wider, the zone can be divided into at least four sub-zones: each representing one wall orientation.
            2.1.2   The core is the zone located at the centre of the building at least 4.5 m in from external walls, which is predominantly affected by internal heat-gains.

2.2       Sub-zones can also be produced based on differences in the schedules of use.

2.3       The Jamaican Energy Efficiency Code gives specifications for zones as follows:
            2.3.1   Each zone shall have independent thermostat controls; and
            2.3.2   Zones not operating concurrently for more than 750 hours/year should have separate A/C systems.

2.4       Local systems are awkward in the thermal core.

2.5       DX systems are unsuitable for installations in the perimeter, because it requires multiple fan coil units.

2.6       Maintenance of units is difficult because of the distribution of the units.


3.1       Central Systems comprise one or more large mechanical spaces.
            3.1.1   The roof is a favoured location, however
            3.1.2   Equipment rooms occupy entire floors in very tall buildings.
                          There should be access to fresh air;
                          Ceilings should be high; and
                          Structure should be designed to support the weight.
            3.1.3   These spaces generate heat, moisture, vibration, noise and air motion.
            3.1.4   Multiple cooling units are recommended by code wherever loads exceed 500 kW.

3.2       Systems are sized to cope with the maximum design load of the zones served.

3.3       Central Systems are generally DX or chilled water systems.               
            3.3.1   Chilled water systems are marginally more efficient than DX systems.
            3.3.2   Chilled water systems cool water, instead of air, and passes it through heat exchangers to cool the air.
            3.3.3   They permit use of multiple coil-units and offer more precise temperature control.
            3.3.4   Chilled water systems can be either air-and-water or all-water systems.

3.4       Complex control systems are utilized.

3.5       Sizable distribution trees are used:
            3.5.1   Chilled water systems have smaller distribution trees.
                          All-water systems have the smallest distribution tree; but
                          Ventilation has to be dealt with separately.
                          Air can be exhausted locally; or
                          Exhaust can also be transported through return-air ducts.
            3.5.2   Vertical chases reduce the amount of space that can be rented and may limit the flexibility to partition the space.
            3.5.3   Horizontal runs influence the ceiling height.
            3.5.4   Maintenance is difficult in concealed distribution trees that are not properly detailed, but there are advantages.
                          There are less surfaces to clean;
                          Appearance of walls and ceilings are controlled; and
                          Noise is reduced.

3.6       Central DX cooling and local air-distribution takes advantage of benefits attributed to both the central and local systems.

3.7       Drawbacks of central systems are:
            3.7.1   Equipment failure can affect the entire building or multiple zones; and
            3.7.2   An entire system may have to be running to serve one zone or a section thereof.



4.1       Speculative office buildings have shifting tenancy.
            4.1.1   They typically require many independent zones.
            4.1.2   Air-and-water systems called induction-type air terminals may be used in the perimeter zones.
            4.1.3   All-water fan-coil units can also be used
            4.1.4   All-air systems called variable air-valume units can be used for the core.

4.2       Shopping Centres have transient occupancy.
            4.2.1   Air distribution is less critical because shoppers are moving about;
            4.2.2   Large shopping malls may use all-water fan-coil units in individual stores.

4.3       Hotels have variable occupancy.
            4.3.1   Central systems are usually desired in public areas alone, but
            4.3.2   Induction-type terminals or fan-coil units are used in individual rooms.

4.4       Apartment blocks have variable usage schedules and require individual control.
            4.4.1   They have as many zones as individual apartments.
            4.4.2   Apartment blocks over three-storeys high can use chilled-water systems with individual fan-coil units.

4.5       Hospitals need special temperature and humidity control.
            4.5.1   Operating Theatres do not use re-circulating air.
            4.5.2   All-air systems with adequate filtration may be used; but
            4.5.3   Induction terminals are often used in closed wards.


            Mechanical and Electrical Equipment for Buildings, 8th edition, Benjamin Stein, John S. Reynolds, John Wiley & Sons Inc., USA, 1992
Construction Materials & Processes, Don G. Watson, McGrawHill Book Co., USA, 1978;
            Ramsey/Sleeper Architectural Graphic Standards, A.I.A., Robert T. Packard (ed), John Wiley & Sons Inc., USA, 1981;
Architectural Handbook, Alfred M. Kemper, John Wiley & Sons Inc., USA, 1979
Jamaica National Building Code, Volume 2: Energy Efficiency Building Code, Requirements and Guidelines, 1994, Joseph J. Deringer (ed.), Jamaica Bureau of Standards, Jamaica, 1995.
Daylighting: Design and Analysis, Claude L. Robbins, Van Nostrand Reinhold, N.Y., 1986.


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