can be applied. Where they are used, it is important that they have controls to automatically reset temperatures at the central air handling unit to provide minimum heating and cooling to satisfy the hottest/coolest zone. Dual duct VAVsystems are available and offer significantly improved energy efficiency.
7.3.1.3 Variable air volume
In general, energy efficiency is improved by moving to variable air volume (VAV) systems to minimise transport energy. However, VAV systems should be matched to suitable applications. They are primarily suited to applica- tions with a year round cooling load such as deep plan office buildings and can also have other limitations to their application such as ceiling height due to box throw etc. The VAVboxes for each zone should be individually controlled, making it necessary for appropriate locations for temperature sensors to be available. VAVsystems can provide flexibility for future fit out and partition changes, provided the locations of supply and extract grilles and sensors are carefully considered.
VAV systems reduce the airflow rate in relation to the demand, rather than change the supply air temperature. The air handling unit (AHU) supplies cooled air (normally at 10–18 °C but this can be at 6 °C from ice storage) to VAV boxes which vary the volume with respect to space temperature. The supply fan is normally controlled with respect to static pressure and it responds to demand by varying volume. The location of the static pressure sensor is critical in achieving energy savings(28). The extract fan volume is controlled with respect to the supply fan volume. The AHUfans achieve variable volume by variable speed drive (VSD), variable pitch or the less efficient inlet guide vane control. VAVboxes with reheat and fan assisted boxes are available but care must be taken to ensure that
these are only used where necessary to avoid wasting energy. The advantages and disadvantages of VAVsystems are shown in Table 7.6.
7.3.2
Partially centralised air/water
systems
Systems in which the air does not transport all the cooling and heating may offer energy savings due to reduced flow rates from the plant room to the spaces served. To satisfy zones that have variable requirements, both heating and cooling pumped water circuits are needed i.e. four pipe circuits. Three pipe systems with a common return should be avoided at all costs as cooling and heating energy will be wasted when the return water is mixed.
Tempered fresh air systems limit the humidification and de-humidification capacity. However, this is normally adequate for most applications and discourages attempts at unnecessary close control of humidity, which is very wasteful of energy.
Savings due to reduced air flows must be balanced against the restricted free cooling from fresh air, the additional energy used due to higher pressures and local fan energy, and the energy required for heating and chilled water distribution pumps.
The AHUshould be sized for minimum fresh air duty only, to reduce transport energy. The minimum fresh air volume can be controlled with respect to air quality via VSDs to minimise the amount of heating and cooling of fresh air. Turndown may be limited, dependent upon application. Varying fresh air volume is unlikely to be satisfactory with induction systems due to the minimum pressure required at the induction terminal units. Heat recovery via thermal wheels or run around coils should be considered at the AHU.
Where the building is likely to require cooling in one area and heating in another, or where common cooling coils are used with multiple re-heater batteries, heat recovery from the chillers to supply the terminal unit heating coils, or re-heat batteries, should be considered. This will require a low temperature (warm water) heating circuit. Top-up heat can be provided by condensing boilers, which will always operate in the condensing mode.
Table 7.6 Advantages and disadvantages of variable air volume systems Advantages Disadvantages
Often the most efficient form of Complex in comparison with other air-conditioning types of air conditioning system Highly flexible for initial/future Integration of controls from fit-out requirements concept stage essential
Opportunity to reduce AHUand Special diffusers may be required
duct size compared to multizone for good room air distribution at system if diversity allowed on low loads
cooling load
VAVsystems, using DDCcontrols VAVsystems with conventional with communications, will gen- controls, or additional VAVbox erally require less maintenance coils/fans, could require more in occupied areas than multizone maintenance than multizone systems with reheat coils systems
Additional sound attenuation may be required for maximum velocity
7-8 Part A: Designing the building
All partially centralised systems should have local controls with communication to provide demand-based control of main plant.
7.3.2.1 Centralised air with reheat
These systems (commonly known as multi-zone) require the full volume of air necessary for heating and cooling loads to be transported to and from the AHU. The AHU should incorporate a controlled re-circulation system via dampers (or other means of heat recovery) and free cooling (enthalpy) control to minimise energy use. Most systems incorporate a common cooling coil, which must be controlled with respect to the zone requiring the greatest amount of cooling. Additional re-heating for the other zones is therefore required, necessitating simul- taneous heating and cooling of the same air. This increases energy consumption compared with correctly applied multiple single zone systems.
Systems with minimum air treatment at the AHUand local heating and cooling coils are likely to be far more efficient than systems with a common cooling coil at the AHU, although capital costs will be increased.
Dew-point systems provide saturated air at the cooling coil at all times to provide very stable humidity conditions when air is reheated to the desired space temperature. However, these systems are only necessary for special applications, such as laboratories, and should normally be avoided since they can be very inefficient. Systems with the cooling coil controlled in relation to the zone requiring greatest cooling are much more efficient and should normally be used.
7.3.2.2 Induction
In order to produce the air jet velocity needed to induce room air through the casing and over water coils, induction systems need to operate at higher pressures than those of low velocity systems. Also, having high pressure sufficient to drive room circulation in the furthest zone means that the extra pressure needs to be removed from other branches in an induction system. Typically, this is achieved by careful sizing of the ductwork, together with dampers for the final trimming. Restrictions such as jet nozzles and dampers for flow regulation inherently absorb fan power and thus it is important to consider how much energy is used compared with other means of creating room air circulation.
Two coil induction units should be used where both heating and cooling are required at the terminal. Systems with one coil terminal units require much higher air volumes from the AHUto provide the heating, increasing transport energy. Even where the common heating coil is effectively controlled, simultaneous heating and cooling is inevitable. This can be reduced by effective zoning of the AHUs serving areas with different load profiles such as facades with different orientations. Single-coil systems should have the supply temperature reset in a similar manner to multi-zone systems; they also require a sophisticated control system. Where this is not possible, such as in existing systems, the supply temperature should be scheduled in relation to ambient temperature.
Simultaneous heating and cooling is a common energy problem found in induction systems and often results in a poor user control with associated comfort problems. This can be avoided by good zone selection and control.
7.3.2.3 Fan coils
Fan coils place the motive power for distributing heat and cooling within or close to the zone being served. This means that fan power is not used to move this air down ducts from the central plant. However, additional power is required at fan coil units to circulate the room air. The cross-flow and tangential fans used in fan coil units typically have an efficiency of around 40% — around half that of the most efficient AHUfans.
The energy consumption of these alternative approaches should be assessed. Fan units with free cooling are suitable for some applications (on outside walls of low-rise buildings) and can provide additional economy of operation.
Simultaneous heating and cooling is a common energy problem that can be found in fan coil systems and often results in a poor user control with associated comfort problems. In particular, where there is perimeter heating fighting fan coils providing cooling. This can be avoided by good zone selection and control with interlocks to prevent conflicts with perimeter heating.
7.3.2.4 Chilled beams and ceilings
Conventional cooling methods such as fan coils and VAV systems provide cooling almost entirely by convective heat transfer. Chilled beams/ceilings can provide up to 60% of the cooling by radiation and therefore cool the objects within the space as well as the space itself.
Operating at a higher chilled water temperature than conventional systems offers improved energy efficiency and the possibility of using passive cooling instead of mechanical cooling(29). Chilled beams/ceilings are often used with upward or positive displacement distribution systems which are inherently more efficient than conventional air conditioning. This system also requires less fan power due to the lower air flow requirements. The BCO’s Best practice in the specification for offices(10)indicates annual CO2emissions of 60–90 kg·m–2for displacement ventilation systems as opposed to 80–140 kg·m–2 for conventional air conditioning. The temperature control provided by any separate fresh air system is critical as simultaneous heating and cooling may occur and reduce the overall efficiency of operation.
7.3.2.5 Displacement ventilation
Displacement ventilation provides buoyancy-driven flow rather than less energy efficient conventional forced methods. ‘Fresh’ air is introduced gently at a low level and at a temperature just slightly below the room ambient, with a view to providing a local cooled environment around the people and heat sources, eliminating the need to temper the entire space, e.g. a theatre. Any local heat sources in the lower part of the room create convection currents that also contribute to the general upward movement. The warmed air and contaminants collect
Ventilation and air conditioning design 7-9
below the ceiling and are then exhausted at high level. The temperature of the stratified air near the ceiling can be allowed to rise above comfort conditions allowing greater temperature differential between supply and extract, increasing cooling capacity (typically 40 W·m–2). The higher supply temperature also achieves energy savings by improving the CoPof central cooling plant. Displacement systems operate with lower volume airflow (typically 4 ACH) than traditional methods, thus saving energy. However, supply air has to be warmer than for mixing ventilation, thereby increasing heating requirements, although cooling may be more important in these situations. There may also be more opportunity to utilise free cooling. Further information about displace- ment ventilation can be found in section 4.2.5 of CIBSE Guide B2(2), General Information Leaflet GIL 85(29)and BSRIA TN 2/96(30).
7.3.2.6 Unitary heat pumps
For buildings where there is a significant spatial variation in load, especially if there is a need for simultaneous heating and cooling in different zones, the use of localised reversible heat pumps connected to a circulating water system can be advantageous. This provides the oppor- tunity to transfer energy around the building, i.e. by carrying heat from a cooled zone into a zone requiring heat. This effectively recovers heat or coldness from different parts of the building. Controls should be linked where more than one unit is used in a common zone, to avoid individual heat pumps simultaneously heating and cooling the same space.
A boiler and cooling tower are normally connected to the water system to top up or reject heat as necessary. This system also has reduced transport losses by circulating low temperature water as the heat source/sink. Also, because there are several items of plant around the building, areas with several heat pumps will not lose the service com- pletely during individual unit breakdowns. Conversely, maintenance is costly and can cause considerable disturbance in occupied areas.
The efficiency of the local heat pumps is reduced compared with other types of air conditioning due to the common circuit being used as both a heating and cooling source. This reduction in efficiency can be minimised by scheduling the circuit temperature in relation to outside temperature so that it increases when cold and vice versa. Typically, the cooling CoPfor smaller individual units is 2.4 to 2.6(31), although it is important to consider the energy consumption by the other components in the system, such as the boiler, cooling tower and pumps for the low temperature water circuit. A tempered fresh air system will also be needed for minimum fresh air requirements.