4. COMPARACIÓN DE SISTEMAS
4.1 SISTEMA SUECO (ABOLICIONISTA)
Air systems generally provide a more rapid response than wet systems, so good control is essential for comfort con- ditions and energy efficient operation. DDCor BMScontrols are generally recommended as they provide more accurate control of temperature and volumes (see section 6). The control of ventilation and air conditioning is covered in CIBSE Guides B2(2)and H(35), and Good Practice Guide GIR 41(28). Table 7.8 provides guidance on the selection of controls for mechanical ventilation systems.
7.4.1
Plant start/stop
Air systems should generally be started and stopped in accordance with the guidance given in 5.5. Due to the faster response of air systems, optimum start control has a lesser effect but should still be used. Optimum stop is not normally used with ventilation systems due to the need to maintain minimum fresh air rates. Night purge systems pre-cool the building structure overnight to limit the Table 7.7 Humidification benchmarks for air conditioned offices
Parameter Benchmark for stated office type Air conditioned Air conditioned standard office prestige office (Type 3) (Type 4)
Good Typical Good Typical practice practice
Air handled 4 4 4 4
((litre·s–1)·m–2)
Specific fan power 2 3 2 3 (W/(litre·s–1)
Running hours (h/yr) 2750 3500 3000 3700 Energy use indicator 22 42 24 44 (EUI) ((kW·h)·m–2)/yr
Note: factors for converting treated floor area to nett and gross are given in Table 20.2
7-12 Part A: Designing the building
daytime peak cooling requirement and require operation at night.
Heater and cooler batteries, humidifiers, etc. should always be interlocked with the fan to ensure they are only able to operate when required. Terminal units such as fan coil units, fan assisted VAV boxes, etc. should all have effective time control. Demand based control systems to operate central plant only when required should be considered to minimise energy consumption.
7.4.2
Set points, dead-bands and
summated signals
Temperature and humidity set points must be selected for the minimum energy consumption consistent with com- fort conditions. Adequate dead-bands between heating and cooling are essential to minimise energy consumption and avoid simultaneous heating and cooling.
Where DDC is used with P+I control, the results of the heating and cooling calculations should be summated and a common signal produced to avoid simultaneous heating and cooling due to the integral action. The signal should then split into heating, free cooling (where appropriate) and mechanical cooling.
7.4.3
Sensor location
Sensors must be located in representative positions for the services being controlled. Where multiple terminal units are used in air conditioning systems, such as VAVsystems, one sensor per terminal unit is normally required.
7.4.4
Free cooling control
Free cooling can significantly reduce the running costs of air conditioning systems. Central air handling units Table 7.8Selection of controls for mechanical ventilation systems
Plant Control function
Ventilation systems start/stop up to 30 kW heating Time switch; resolution better than 15 minutes, spring reserve/battery back up Ventilation systems start/stop 30–100 kW heating Optimum start/stop recommended
Ventilation systems start/stop over 100 kW heating Optimum start/stop required by current Building Regulations Fresh air and exhaust dampers; tempered air systems Open/closed linked with time control of plant
Fresh air, re-circulation and exhaust dampers; Full re-circulation when unoccupied; minimum fresh air position when occupied; central air handling units; minimum fresh air minimum fresh air preferably controlled with respect to air quality
Fresh air, re-circulation and exhaust dampers; central Free cooling override of minimum fresh air from heating/cooling sequence; fresh air air handling units; free cooling temperature high-limit or preferably enthalpy control
Tempered air systems Heating/cooling sequence for constant supply temperature; possible reset supply condition according to ambient temperature or summated demand if appropriate Single zone air handling units Heating/cooling sequence from space or return air temperature; possible minimum
supply air temperature to prevent dumping of cold air; systems for large spaces (lecture theatres, etc.) reset supply air temperature from return air
Multi-zone air handling units; common cooling coil Re-heater batteries controlled with respect to space or return air temperature. Common cooling coil controlled from zone requiring most cooling; do not control cooling coil at constant off coil temperature, except in rare case of dew-point system
VAVair handling units: temperature Heating/cooling sequence for constant supply temperature; possible reset supply temperature according to ambient temperature or summated demand if appropriate
VAVair handling units; supply fan Speed or pitch control with respect to duct pressure 2/3of the way along supply duct VAVair handling units; supply fan, alternative methods Speed or pitch control with respect to point near supply fan with reset from summated
system demand, plus low limits; demand based control strategies based on VAVbox positions can be used but can be unduly influenced by poorly sized boxes
VAVair handling units; extract fan Control with respect to supply/extract differential volume
VAVboxes Control velocity between minimum and maximum with respect to space temperature Dual duct air handling units Control hot and cold decks for a constant temperature. Possible reset according to
outside temperature or summated demand
Dual duct mixing boxes Mix hot and cold duct supplies according to space temperature
VAVdual duct Fans as for VAV; temperature as for dual duct
Fan coil units Return air (or space) temperature control of heating & cooling in sequence Induction units Heating and cooling control in sequence from space or return air temperature;
segregation of heating and cooling hydraulic distribution essential for efficient operation Intermittently occupied areas Occupancy sensing controls with appropriate default values dependent upon system
response
Humidification Control from space or return relative humidity, supply modulating high limit
De-humidification Over-ride temperature control of cooling coil to cool air below dew-point; re-heat via re- heater batteries as per normal temperature control; supply temperature low limit may be required
Ventilation and air conditioning design 7-13
should have the supply, re-circulation and extract dampers controlled in parallel to provide fresh air as the first stage of cooling. When the system calls for cooling and outdoor enthalpy is below the setting on the enthalpy controller, the fresh air dampers modulate open. If the outdoor air enthalpy is above the enthalpy set point then the fresh air dampers remain closed. If the outside air can not satisfy cooling requirements of the conditioned space then mechanical cooling will be brought on in stages. Free cooling control is improved by enthalpy control which compares fresh air and return air enthalpy, controlling the fresh air to meet demand. Enthalpy control is essential where controlled humidification and/or de-humidification are provided by the AHU.
Free cooling is available with some packaged air con- ditioning units and should be used wherever possible. Free cooling is also available via cooling towers providing cooled water without chillers operating (see 8.1.3). Complex control strategies and additional filtration are necessary for effective operation, although significant savings are claimed.
7.4.5
Humidity control
Most comfort cooling applications do not require close control of relative humidity in the occupied space and do not require controlled de-humidification, see section 7.2.3.2.
Humidification may be required with air conditioning systems to provide a minimum relative humidity in winter. A relative humidity limit in the supply air is required to prevent saturation in the distribution ductwork.
Energy efficient humidifiers should be used, preferably with modulating control rather than control in stages or simple on/off control.
Where both humidification and de-humidification is essential, a wide dead-band between humidification and de-humidification set points is recommended to minimise energy consumption.
Humidistats or humidity sensors can suffer from drift causing inaccurate relative humidities and possibly higher energy consumption. Specifying a high quality sensor and regular calibration can help avoid this.
7.4.6
Multi-zone systems
Multi-zone systems with a common cooler battery must have the cooler battery effectively controlled with respect to the space requiring the greatest amount of cooling, as poor control can lead to excessive energy use. These systems must not be controlled at a constant supply temperature, except in the rare case of a dew-point system.
7.4.7
CO
2demand-controlled
ventilation
Where spaces have a large and unpredictable occupancy, there is potential to save energy by matching air supply to
ventilation demand. This is most commonly achieved by sensing CO2and can provide significant energy savings for both full fresh air and re-circulation systems. CO2 control is more expensive than conventional forms of control but can be set up more easily. However, it often has a rapid payback.
In recirculation systems, the minimum fresh air quantity is normally controlled via the supply, re-circulation and extract dampers in relation to air quality. This is partic- ularly effective for VAV systems where fixed minimum damper positions would provide varying fresh air content. The air volume can be controlled on full fresh air systems in relation to CO2 via variable speed drives or variable pitch axial fans. However, minimum ventilation rates may be required to provide adequate heating and cooling, air distribution, etc. Significant energy savings can still result even where turndowns in volume are limited (see GIR 41(28)).
BSRIA TN 12/94(36) suggests that significant energy savings may accrue from the use of CO2control. However, each application has to be judged on the ventilation requirements based on predicted occupancy profiles for the particular building and TN12/94 contains the means of assessing the viability of given schemes. In general, the energy benefits of a CO2 controlled ventilation system manifest themselves in buildings with spaces subject to variable occupancy.
7.4.8
VAV control
The control of VAVsystems is far more complex than other air conditioning systems. VAVsystems should normally only be used where the load is predominantly cooling throughout the year, such as a deep plan office building. Good space and duct sensor locations are essential for energy efficient operation(29).
Proportional space temperature control is preferable for stability, ease of commissioning and efficient operation. A dead-band between heating and cooling is necessary where reheat occurs.
Most modern VAVsystems use velocity reset VAVboxes. Primary air volume is reset between minimum and maximum settings in relation to space temperature. These are pressure dependent and the system is substantially self-balancing. Reheat is normally at minimum volume to prevent energy wastage. Fan-assisted VAVboxes generally have greater energy consumption, additional maintenance and higher capital costs.
Supply fans are normally controlled in relation to static pressure, two thirds of the way along the supply duct. Care must be taken to ensure the static pressure setting is not higher than necessary as this will waste energy and possibly cause increased noise at the terminal units at low loads. Extract fans are normally controlled in relation to differential volume. Difficulties can arise due to velocity sensor locations from which the volumes are calculated. A wide range of VAVcontrols is discussed in detail in CIBSE Guide H(35)and Good Practice Guide GIR 41(28).
7-14 Part A: Designing the building
References
1 Natural ventilation in non-domestic buildings CIBSE Applications Manual AM10 (London: Chartered Institution of Building Services Engineers) (1997)
2 Ventilation and air conditioning CIBSE Guide B2 (London: Chartered Institution of Building Services Engineers) (2001) 3 Energy efficient mechanical ventilation systems Good Practice
Guide GPG 257 (Action Energy) (1999) (www.action energy.org.uk)
4 The design team’s guide to environmentally smart buildings Good Practice Guide GPG287 (Action Energy) (2000) (www.action energy.org.uk)
5 A designers guide to the options for ventilation and cooling Good Practice Guide GPG 291 (Action Energy) (2001) (www.action energy.org.uk)
6 Environmental design CIBSE Guide A (London: Chartered Institution of Building Services Engineers) (1999)
7 HVAC Applications ASHRAE Handbook (Atlanta, GA: American Society of Heating, Refrigeration and Air Conditioning Engineers) (2003)
8 Avoiding or minimising the use of air-conditioning — A research report from the EnREI Programme General Information Report GIR 31 (Action Energy) (1995) www.actionenergy.org.uk
9 Natural ventilation in non-domestic buildings BRE Digest 399 (London: Construction Research Communications) (1994) 10 Best practice in the specification for offices (Reading: British
Council for Offices) (2000)
11 Testing buildings for air leakage CIBSE TM23 (London: Chartered Institution of Building Services Engineers) (2000) 12 Conservation of fuel and power The Building Regulations 2000
Approved Document L2 (London: The Stationery Office) (2001)
13 Technical standards for compliance with the Building Standards (Scotland) Regulations 1990 (as amended) (London: The Stationery Office) (2001)
14 Conservation of fuel and power The Building Regulations (Northern Ireland) 1994 Technical Booklet F (London: The Stationery Office) (1999)
15 Office service charges analysis(7th edn.) (London: Jones, Lang and Wootton) (1992)
16 Baker N V and Steemers K The LT Method 2.0. An energy design tool for non-domestic buildings (Cambridge: Cambridge Architectural Research)(1994)
17 Minimising pollution at air intakes CIBSE TM21 (London: Chartered Institution of Building Services Engineers) (1999) 18 Mixed mode buildings CIBSE Applications Manual AM13
(London: Chartered Institution of Building Services Engineers) (2000)
19 Brittain J R J Oversized air handling plant BSRIA GN 11/97 (Bracknell: Building Services Research and Information Association) (1997)
20 Energy efficiency in officesEnergy Consumption Guide ECG 19 (Action Energy) (2000) (www.actionenergy.org.uk)
21 Ductwork CIBSE Guide B3 (London: Chartered Institution of Building Services Engineers) (2002)
22 Energy efficiency in offices — 1 Bridewell StreetGood Practice Case Study GPCS 21 (Action Energy) (1991) (www.actionen- ergy.org.uk)
23 Selecting air conditioning systems. A guide for building clients and their advisers Good Practice Guide GPG 71 (Action Energy) (1993) (www.actionenergy.org.uk)
24 Cassar M Environment management: Guidelines for museums and galleries(London: Museums and Galleries Commission) (1994) 25 Efficient humidification in buildings BSRIA AG10/94.1
(Bracknell: Building Services Research and Information Association) (1995)
26 Bordass W (private communication)
27 Refrigeration and the environment — typical applications for air conditioningBSRIA TN 15/92 (Bracknell: Building Services Research and Information Association) (1992)
28 Variable flow control General Information Report GIR 41 (Action Energy) (1996) (www.actionenergy.org.uk)
29 New ways of cooling — information for building designersGeneral Information Leaflet GIL 85 (Action Energy) (2001) (www.actionenergy.org.uk)
30 Alamdari F and Eagles N Displacement ventilation and chilled ceilingsBSRIA TN2/96 (Bracknell: Building Services Research and Information Association) (1996)
31 Jones W P Air conditioning applications and design(2nd edn.) (London: Edward Arnold) (1997)
32 King G R and Smith M H VRF based air conditioning systems — performance, installation and operation notesBSRIA TN 10/97 (Bracknell: Building Services Research and Information Association) (1997)
33 Energy efficiency in offices. Understanding energy use in your office
Good Practice Guide GPG 33 (Action Energy) (1992) (www.actionenergy.org.uk)
34 Energy savings with electric motors and drives Good Practice Guide GPG 2 (Action Energy) (1998) (www.action energy.org.uk)
35 Building control systems CIBSE Guide H (London: Chartered Institution of Building Services Engineers) (2000)
36 Potter I N and Booth W B CO2controlled mechanical ventilation
systems BSRIA TN 12/94.1 (Bracknell: Building Services Research and Information Association) (1994)
Bibliography
HVAC systems and equipment ASHRAE Handbook (Atlanta, GA: American Society of Heating, Refrigeration and Air Conditioning Engineers) (2000)
Martin A J Control of natural ventilationBSRIA TN11/95 (Bracknell: Building Services Research and Information Association) (1995) Martin A J Night cooling strategiesBSRIA RR5/96 (Bracknell: Building Services Research and Information Association) (1996)
Energy demands and targets for heated and ventilated buildings CIBSE Building Energy Code 1 (London: Chartered Institution of Building Services Engineers) (1999)
Energy demands for air conditioned buildings CIBSE Building Energy Code 2 (London: Chartered Institution of Building Services Engineers) (1999)
Martin P L and Oughton D L Faber and Kell’s Heating and air- conditioning of buildings (London: Butterworth Heinemann) (1995) Steer J W and Doig R The specification of efficient ventilation systems in the decommissioning of redundant plant at Sellafield Proc. CIBSE Nat. Conf., Eastbourne 1–3 October 1995(vol. 2) 26–35 (London Chartered Institution of Building Services Engineers) (1995)
Green R H, Taylor M S and Fletcher P G The performance of a prototype, commercial building, balanced ventilation, heat pump Proc. CIBSE Nat. Conf., Eastbourne 1–3 October 1995(vol. 2) 36–43 (London: Chartered Institution of Building Services Engineers) (1995)
Riffat S B, Shao L and Shehata Mop fan for removal of air-borne pollutants Proc. CIBSE Nat. Conf., Eastbourne 1–3 October 1995(vol. 2)
Ventilation and air conditioning design 7-15
44–51 (London: Chartered Institution of Building Services Engineers) (1995)
Leaman A J, Cohen R R and Jackman P J Ventilation of office buildings: deciding the appropriate system Proc. CIBSE Nat. Conf., Brighton 2–3 October 1994(vol. 2) 90–101 (London: Chartered Institution of Building Services Engineers) (1994)
Channer G R A mixed mode ventilation system for an office tower which addresses the problems of infiltration, internal comfort and energy consumption Proc. CIBSE Nat. Conf., Brighton 2–3 October 1994(vol. 2) 109–120 (London: Chartered Institution of Building Services Engineers) (1994)
Butler D J G Chilled ceilings and beams — BRE research Proc. CIBSE Nat. Conf., Alexandra Palace 5–7 October 1997(vol. 1) 53–60 (London: Chartered Institution of Building Services Engineers) (1997)
Edwards M, Linden P and Walker RR Theory and practice — natural ventilation modelling Proc. CIBSE Nat. Conf., Brighton 2–3 October 1994
(vol. 2) 102–108 (London: Chartered Institution of Building Service Engineers) (1994)
W P Jones Air conditioning engineering (5th edn) (London: Edward Arnold) (2001)
Trott A R and Welch T C Refrigeration and air conditioning(London: Butterworth Heinemann) (1999)
Chilled ceilings and beams CIBSE Research Report RR5 (London: Chartered Institution of Building Services Engineers) (1998)
Air-to-air heat recoveryCIBSE Research Report RR2 (London: Chartered Institution of Building Services Engineers) (1995)
Fletcher J Pre-cooling in mechanically cooled buildingsBSRIA TN16/95 (Bracknell: Building Services Research and Information Association) (1995)
De Saulles T Free cooling systems — design and application guide BSRIA RR16/96 (Bracknell: Building Services Research and Information Association) (1996)
Jackman P J Air distribution in naturally ventilated officesBSRIA TN 4/99 (Bracknell: Building Services Research and Information Association) (1999)
Guide to air distribution technology for the internal environment (Heating, Ventilating and Air Conditioning Manufacturers Association) (2000)
A guide to energy efficient ventilation (Coventry: Air Infiltration and Ventilation Centre) (1996)
A practical guide to air leakage testingHVCA DW143 (London: Heating and Ventilating Contractors Association) (2000)
Kendrick C and Martin A Refurbishment of air conditioned buildings for natural ventilation BSRIA TN 8/98 (Bracknell: Building Services Research and Information Association) (1998)
REHVA guide to displacement ventilation in non-industrial premises
(Brussels: Federation of European Heating and Ventilating Associations (REHVA)) (2002)
Ventilation and cooling option appraisal — a clients guideGood Practice Guide GPG 290 (Action Energy) (2001) (www.actionenergy.org.uk)
HVAC strategies for well-insulated airtight buildings CIBSE TM29 (London: Chartered Institution of Building Services Engineers) (2001)
Air handling unitsHEVAC Guide To Good Practice (Marlow: HEVAC Association)
Fan application guide(Marlow: Fan Manufacturers Association/HEVAC Association)
Fan and ductwork installation guide (Marlow: Fan Manufacturers Association/HEVAC Association)
Butler D Pushing the limits of displacement ventilation for cooling
8-1
8.0
General
Most refrigeration plant is electrically driven and can add significantly to energy costs and CO2 emissions. Moreover, the need for cooling is increasing, due to the greater use of information technology and increasing comfort expectations.
Building Regulations Approved Document L2(7)* sets out a number of issues related to refrigeration plant, in partic- ular minimising the energy consumed in air conditioning and/or mechanically ventilated (AC/MV) systems, see
4.2.5.4. This includes the ‘carbon performance rating’ (CPR) for offices which sets out requirements for new and refurbished buildings as well as existing buildings where an AC/MVsystem is to be significantly altered or replaced. The CPRfor air conditioned offices is based on the total installed capacity (kW·m–2) of refrigeration plant and a series of factors representing plant management and control.
These requirement are summarised in the relevant parts of section 4 as they influence very early design decisions.