Building service and building service energy modelling

The disadvantages of an All air systems are: Ducts installed in ceiling plenums require

additional duct clearance, sometimes reducing ceiling height and/or increasing

building height. In retrofits, these clearances may not be available; Larger floor plans

may be necessary to allow adequate space for vertical shafts (if required for air

distribution). In a retrofit application, shafts may be impractical; In commercial

buildings, air-handling equipment rooms represent non-rentable or non-revenue-

generating spaces; Accessibility to terminal devices, duct-balancing dampers, etc.,

requires close cooperation between architectural, mechanical, and structural

designers; Air balancing, particularly on large systems, can be cumbersome; and

Mechanical failure of a central air-handling component, such as a fan or a cooling-coil

control valve, affects all zones served by that unit.

All Air systems are classified in three categories: Single-duct systems, Dual-duct

systems and Multizone.

Single-duct systems contain the main heating and cooling coils in a series-flow air

path. A common duct distribution

system at a common air temperature

feeds all terminal apparatus. Single-duct

systems can serve single zone or

multizone. The simplest all-air system is

a supply unit serving a single zone.

The unit can be installed either in or

remote from the space it serves, and

may operate with or without distribution ductwork. Ideally, this system responds

completely to the space needs, and well-designed control systems maintain

temperature and humidity closely and efficiently. Single-zone systems often involve

short ductwork with low pressure drop and thus low fan energy, and can be shut down

when not required without affecting operation of adjacent areas, offering further

energy savings. A return or relief fan may be needed, depending on system capacity

and whether 100% outside air is used for cooling as part of an economizer cycle. Relief

fans can be eliminated if over pressurization can be relieved by other means. Single

duct multizone (zonal reheating) is actually a mixed air/water system since the

zone terminals with the reheating coils are feeded with an hydronic circuit. Multiple-

zone reheat is a modification of the single-zone system. It provides (1) zone or space

control for areas of unequal loading, (2) simultaneous heating or cooling of perimeter

areas with different exposures, and (3) close control for temperature, humidity, and

space pressure in process or comfort applications. As the word reheat implies, heat is

added as a secondary simultaneous process to either preconditioned (cooled,

humidified, etc.) primary air or recirculated room air. Relatively small lowpressure

systems place reheat coils in the ductwork at each zone. More complex designs

include high-pressure primary distribution ducts to reduce their size and cost, and

pressure reduction devices to maintain a constant volume for each reheat zone. The

system uses conditioned air from a central unit, generally at a fixed cold-air

temperature that is low enough to meet the maximum cooling load. Thus, all supply

air is always cooled the maximum amount, regardless of the current load. Heat is

added to the airstream in each zone to avoid overcooling that zone, for every zone

except the zone experiencing peak cooling demand. The result is very high energy

use, and therefore use of this system is restricted. However, the supply air

temperature from the unit can be varied, with proper control, to reduce the amount of

reheat required and associated energy consumption. Care must be taken to avoid high

internal humidity when the temperature of air leaving the cooling coil is allowed to rise

during cooling. In cold weather, when a reheat system heats a space with an exterior

exposure, the reheat coil must not only replace the heat lost from the space, but also

must offset the cooling of the supply air (enough cooling to meet the peak load for the

space), further increasing energy consumption. If a constant-volume system is

oversized, reheat cost becomes excessive. In commercial applications, use of a

constant-volume reheat system is generally discouraged in favor of variable-volume or

other systems. Constant-volume reheat systems may continue to be applied in

hospitals, laboratories, and other critical applications where variable airflow may be

detrimental to proper pressure relationships (e.g., for infection control).

Capacity can be controlled by varying the air temperature or volume in fact Single-

duct systems can be classified as constant volume system or variable volume system.

Constant Volume system: While maintaining constant airflow, single-duct constant

volume systems change the supply air temperature in response to the space load. A

VAV system controls temperature in a space by varying the quantity of supply air

rather than varying the supply air temperature. A VAV terminal unit at the zone varies

the quantity of supply air to the space. The supply air temperature is held relatively

constant. Although supply air temperature can be moderately reset depending on the

season, it must always be low enough to meet the cooling load in the most demanding

zone and to maintain appropriate humidity. VAV systems can be applied to interior or

perimeter zones, with common or separate fans, with common or separate air

temperature control, and with or without auxiliary heating devices. The greatest

energy saving associated with VAV occurs at the perimeter zones, where variations in

solar load and outside temperature allow the supply air quantity to be reduced.

Humidity control is a potential problem with VAV systems. If humidity is critical, as in

certain laboratories, process work, etc., constant-volume airflow may be required.

Dual-duct systems contain the main heating and cooling coils in parallel-flow or

series/parallel-flow air paths with either

(1) a separate cold- and warm-air duct

distribution system that blends air at the

terminal apparatus (dual-duct systems),

or (2) a separate supply air duct to each

zone with the supply air blended at the

main unit with mixing dampers

(multizone). In each conditioned zone, air

valve terminals mix warm and cold air in proper proportion to satisfy the space

temperature and pressure control. Dual-duct systems may be designed as constant

volume or variable air volume; a dual-duct, constant-volume system uses more energy

than a single-duct VAV system. As with other VAV systems, certain primary-air

configurations can cause high relative humidity in the space during the cooling

season. Constant Volume: Dual-duct, constant-volume systems using a single supply

fan were common through the mid-1980s, and were used frequently as an alternative

to constant-volume reheat systems. Today, dual-fan, dual-duct are preferred over the

former, based on energy performance. There are two types of dual-duct, single-fan

application: with reheat, and without. Variable Air Volume: Dual-duct VAV systems

blend cold and warm air in various volume combinations. These systems may include

single-duct VAV terminal units connected to the cold-air duct distribution system for

cooling only interior spaces and the cold duct may serve perimeter spaces in sync with

the hot duct. This saves reheat energy for the air for those cooling-only zones because

space temperature control is by varying volume, not supply air temperature, which

may save some fan energy to the extent that the airflow matches the load.

The Multizone system supplies several zones from a single, centrally located air-

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

through zone dampers at the air handler in response to zone thermostats. The mixed,

conditioned air is distributed throughout the building by single-zone ducts. The return

air is handled conventionally. The multizone system is similar to the dual-duct system

and has the same potential problem with high humidity levels. This system can

provide a smaller building with the advantages of a dual-duct system, and it uses

packaged equipment, which is less expensive. Packaged equipment is usually limited

to about 12 zones, although built-up systems can include as many zones as can be

physically incorporated in the layout. A multizone system is somewhat more energy-

efficient than a terminal reheat system because not all the air goes through the

cooling coil, which reduces the amount of reheat required. But a multizone system

uses essentially the same fan energy as terminal reheat because the airflow is

constant.

Air-water systems characteristic

The two mediums have partially different and complementary function. The function of

the primary air, treated in the AHU: are zone ventilation with external air; control

relative humidity; In same cases partial coverage of sensible thermal loads, while with

liquid medium (water) through

appropriate terminals – e.g. Fan coils

(inductors) the functions are: control zone

temperature and total or partial coverage

sensible thermal loads. Air-water systems

can be classified as Primary air and 1

hydraulic circuit (hot/cold) fan-coils or

Primary air and 2 hydraulic circuit (hot and cold) fan-coils.

Air systems components

Air outlet components

Room air distribution systems can be classified according to

their primary objective and the method used to accomplish

that objective. The objective of any air distribution system is

to condition and/or ventilate the space for occupants’ thermal

comfort, or to support processes within the space, or both.

Methods used to condition a space can be classified as one of

the following:

Mixed systems have little or no thermal stratification of air within the occupied

 and/or process space. Overhead air distribution is an example of this type of

system.

Full thermal stratification systems have little or no mixing of air within the

 occupied and/or process space. Thermal displacement ventilation is an example

of this type of system.

Partially mixed systems provide limited mixing of air within the occupied

 and/or process space. Most underfloor air distribution designs are examples of

this type of system.

Task/ambient air distribution and/or process control. Examples of

 task/ambient systems are personally controlled desk outlets and spot-

conditioning systems.

Emission systems (air distribution terminals) are grids, ceiling diffusers,

Displacement terminals, Induction diffusers feeded with primary air and chw, Fan coils

and Chilled beans

Centralised vs decentralised systems

Decentralised (es. split) have, compared to centralised systems the following

advantages: Lower investment cost, Easier to install (pre-insulated and flexible pipes

for refrigerant trasport), Flexible, often can operate as heat pump too and Easy to

control from the user. Decentralised systems disadvantages: Short operation life (less

than 10 years), Not allow proper IAQ control (scarse filtering, limited humidity control),

Total installed power is higher, Higher electricity consumption (almost no “free<