Tutorial - Underfloor Air-conditioning

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13/10/2010

Tutorial - Underfloor air-conditioning

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Tutorial - Underfloor airconditioning
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TUTORIAL - UNDERFLOOR AIR-CONDITIONING
Now that raised access floors are a standard item in most offices,
underfloor air-conditioning systems are becoming much more
popular. Here, we look at the types of system available.
CONTENTS
1. Definition
2. Comfort
3. Zonal displacement ventilation
4. Problems with such systems
5. A more flexible solution
6. Floor void supply - ceiling return
7. Floor void supply - floor void return
8. The egan initiative
9. The benefits
10. A disadvantage

1. DEFINITION
Air-conditioning is defined as the simultaneous control of temperature, humidity, air
movement and the quality of the air in the space.
(source: The Trane Company)

2. COMFORT
The Ideal Conditions
For optimum comfort one should have warm feet and a cool head without noticeable
draught. Air velocity in the workspace should not exceed 0.18-0.2 m/s and good air
movement should be maintained to avoid stuffiness and dilute pollutants such as ozone
and formaldehyde.
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The FM Dilemma
Air-conditioning systems have tended to be either central station systems in a variety of
configurations or smaller diversified plants such as fan coil systems. Facilities managers
have been experiencing high costs associated with the maintenance and reconfiguration of
systems above workstations and, on occasions, costly leaks of water have seriously
damaged computer networks. In the drive to achieve economy, systems have tended
towards higher volume/higher temperature differential solutions but this has led to
increased complaints of draughts and noise. Many aspects of office design are now
affected by legislation and operators have the task of adapting systems to meet changing
legislation and office layouts.

The options
Building services engineers have been charged with addressing these issues. As a result,
they have had to reconsider the options available, investigate new systems and use
computers for simulation of air movement patterns, stratification etc. It has been found
that air introduced from a low level offers improved conditions in the workspace while
offering additional benefits in cost and time.

Zonal displacement system - air circulation diagram

3. ZONAL DISPLACEMENT VENTILATION
Early developments in Sweden led to the system called displacement ventilation. But as it
is essentially a ventilation system with limited cooling (40-60 W/m 2 ), it does not fully
meet the definition of air-conditioning. Displacement systems, because of the large areas
they serve, often have a demand for heating and cooling within that large area. Additional
equipment in the form of underfloor fan coils or electric trench heating is often applied at
perimeters to reheat the cool air and introduce it into the space. However, because this
warm air is naturally buoyant it rises to a high level and is exhausted before completely
heating the space. The heating efficiency is impaired. Zones are usually large and therefore
can provide only an average humidity which often leads to complaints from users.

Swirl Grills in the floor
Air is introduced approximately 3°C below room temperature to avoid complaints of
draught, through "swirl" grilles or "perforated plate" outlets across the floor. This air picks
up heat from the occupants and machinery, along with solar and fabric gains, and rises.
Ceiling extract grilles remove this air, avoiding recirculation.
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4. PROBLEMS WITH SUCH SYSTEMS
Chilled Ceilings
Displacement ventilation systems are of limited use in spaces with high heat loads because
of the low cooling capacity available, so they are often teamed with a secondary system
of chilled ceilings. This offers a total cooling capacity in the order of 120-150 W/m 2 but
the cold ceiling may cause some "displaced" air to fall back into the room carrying with it
some of the contamination.

Ceiling Condensation
Where there is a high latent load causing high humidity, the possibility of condensation
occurring on cold surfaces of the metal ceiling needs to be checked. Controls can be
incorporated to raise the surface temperature in such conditions, but this in turn reduces
the cooling capacity often at the time it is most needed.

Central Station plants and ducting
Using large central station plants for the treatment of air supply requires floor voids of
approximately 600 mm deep for the supply ductwork, room heights in the order of 2.73.2 m and a ceiling void of about 600 mm to accommodate the exhaust air and chilled
ceiling equipment. Of course, the extra space needed to house the system means an
increase in the cost of construction. And although users find that such systems are quiet in
operation, they restrict flexibility, ceiling design and lighting design.

Lack of personal control
The environment created by displacement ventilation is reasonable but offers little or no
personal control over conditions. A large number of swirl grilles are needed and care
should be taken in their selection and location. This system has only limited application in
offices and other spaces of low height as the desired stratification cannot be achieved.

Energy Costs
Displacement ventilation offers a high level of free cooling in mid season. But in mid
winter, when the supply air requires tempering to about 18°C, it can be energy
demanding. Restricting the volume to reduce the energy requirement further limits the
cooling capacity.

Possible pollution levels
Mixing systems ensure good dilution whereas true displacement can have quite high levels
of pollution carried in the ascending air as previously demonstrated by cigarette smoke
trails in offices.

5. A MORE FLEXIBLE SOLUTION
Zonal system options
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Zonal systems are now receiving a lot of attention from developers and end users.
Generally there are two types: floor void supply - ceiling return (zonal displacement
systems); and floor void supply - floor void return (zonal mixing systems).
Both have their advocates, but in low height office space (2.35-2.6 m) the notion that high
level fume should be exhausted from a high level is somewhat flawed because the amount
of mixing that takes place in such space, caused by occupant movement, door swings,
etc, can render such design intention almost impossible.
Both solutions rely on similar central plant to that of fan coil systems, but have less onfloor engineering.

Reduced need for duct work
Compared with central displacement ventilation systems they have much smaller fresh air
ductwork and when well designed, lateral ductwork can be almost, if not completely,
eliminated, freeing up the floor for use by the tenant for data cabling and other systems.

Improved control and air quality
Both types of systems are served with chilled water operating at 6°C flow/11°C return to
achieve good levels of dehumidification in high occupation spaces. They can provide
closer control of humidity to smaller zones and can ensure individual zones maintain high
indoor air quality.

6. FLOOR VOID SUPPLY - CEILING RETURN
Zonal displacement systems
Zonal displacement systems use the floor void as a supply channel and the ceiling void or
office space as a return channel. Zones may be 100-250 m 2 . They usually work with
minimum fresh air rates offering savings in fan power and seasonal peak energy demand.
The supply air velocity under the floor may be low and designers should take care to
check temperature pick up. Simulation shows that lengths of paths under the floor should
be less than 15 m. Zone units are optimally positioned near the centre of the zone served.

Underfloor requirements
Such systems can make use of swirl grilles when the cooling load is small (60-80 W/m 2 )
and the supply temperature can be limited to 18°C, but with higher loads, swirl grilles
should be replaced by fan assisted terminals with vertical distribution. The vertical
distribution avoids draughts as warmer room air is entrained into the air stream instead of
cooler air being blown across the floor at a low level as in the case of swirl.

Good for high load areas
These systems are suitable for machine rooms where air can be introduced directly into
the base of the equipment at a low temperature. Such systems are used for internet hotels
and other high load areas where cooling loads can exceed 1200-1500 W/m 2 . Fresh air
is usually ducted to the top inlet of the zonal unit. This system has a more complicated
ventilation distribution and exhaust system than zonal mixing systems.
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System noise levels are in the region of 40 NR.

Zonal mixing system

7. FLOOR VOID SUPPLY - FLOOR VOID RETURN
Zonal mixing systems
Zonal mixing systems make use of the floor void as both supply and return channels.
Metal ducts are not used, but by the application of air segregation baffles usually fitted to
the raised floor substructure. Zones may range up to 300 m 2 . Filtration may be to EU7
standard which reduces or removes pollen and other allergens, and thus reduces hay
fever and other similar complaints.

Fan Assisted Terminals
The individual fan assisted terminals introduce the conditioned air into the space. Users
have freedom to adjust both the temperature set point and fan speed on the unit. The
space temperature is controlled by the flow rate of air introduced, which in turn is usually
controlled by a damper, operating under the dictates of onboard sensors. The need for
wall-mounted thermostats is eliminated.

Return Air
Air returns to the zonal unit via simple return air grilles positioned above the return air
plenum in the raised floor. Fresh air may be introduced into the return plenum or directly
to the unit and exhaust air may be extracted through toilets, kitchens and additionally
through exhaust air grilles at high level if required.

Flexibility with efficiency
In offices where cooling loads are high, the zonal mixing solution becomes more attractive
than the zonal displacement system as it can achieve cooling loads up to and in excess of
250 W/m 2 without undue draught, while offering the possibility for effective warm air
heating in perimeter zones in winter and cooling in summer.

8. THE EGAN INITIATIVE
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Reduction in floor heights
Office heights can be kept within "pension fund" limits of about 2.5-2.6 m and thus the
overall floor-to-floor height of the building can be in the region of 3.2- 3.6 m overall
compared with 5 m for conventional displacement systems. This offers dramatic savings in
cost and time of construction. Such solutions support the Egan initiative.

Efficient cooling, heating and ventilation
High levels of cooling can be achieved through the use of fan assisted terminals which
introduce about 500 m 3 of conditioned air/hour into the space vertically instead of
horizontally, thus permitting high induction and lower supply air temperatures (eg, 14°C)
to be applied. This allows about three times more cooling to be achieved for the same
volume of air distributed. Ventilation efficiency is high because of reduced shortcircuiting
compared with ceiling-based systems. These terminals, while offering about 1.3-1.5 kW
of sensible cooling and generally located every 12-15 m 2 , often offer personal control of
fan speed and temperature set point and leading solutions have fully intelligent links to the
central building management system.

Care in design
With low supply temperature conditions it is advisable to run the computer calculation for
condensation and possibly apply insulation and a vapour seal to the underside of the slab.
Maximum supply air to room temperature differentials should not usually exceed 10°C.
The velocity of the air should be 1 m/s to minimise temperature pick-up. Paths can be
extended under the floor to 30-35 m.
System noise levels are generally in the region of 32-35 NR.

9. THE BENEFITS
Floor voids need be no more than 300-350 mm high, offering a saving in building
height. In new-build projects, this overall reduction in the height of the building (1015%) can amount to a 5-7% saving in construction cost
(source: Swedish National Pension Fund)
Construction time can be cut by 15-25%
(source: BAA Lynton)
Zonal mixing systems do not require ceiling voids and thus can be used where head
room is limited.
The low-pressure plenum (usually 5-15 Pa) reduces fan power and noise.
Computer simulations indicate energy savings in the order of 25% compared with
VAV systems
(source: Building Simulation)
Computer simulations indicate night-time cooling can achieve energy savings of
about 9% and lower carbon dioxide emissions
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(source: J Roger Preston)
Multi-tenanted buildings or large single occupant buildings can benefit from
diversified operational hours.
Humidity control can be installed on a zone-by-zone basis offering further energy
saving.
Floor voids are easily accessed for cleaning whereas ductwork is difficult to access
and expensive to clean.
The extent of diversification reduces the impact of equipment failure, maintenance
and reconfiguration.
The systems are quick and easy to design.
Relocation of equipment is fast, offering savings in the cost of change - £100170/m 2 compared with ceiling systems
(source: Baker Wilkins and Smith)
Capital allowances/depreciation can be granted when the floor void is used as a
ventilated plenum.
High levels of user satisfaction have been recorded in many countries
(source: Swedish Clinic for Occupational Medicine)

10. A DISADVANTAGE
Zonal units may take up 0.5% of floor space
served, but riser sizes are usually much smaller than
VAV and central displacement systems and floorby-floor plant rooms can be reduced or even
eliminated. Ceiling-based fan coils have no need for
floor-based equipment but require increased
building height and more costly relocation and
maintenance.
Air handling unit

This article was produced originally as a contribution to CIBSE Continuing Professional
Development (CPD) and published in Building Services Journal.

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Technologies Ltd
The Center
201-203 London Road
East Grinstead
West Sussex RH19 1HA
Tel: 44 (0)1342 310400
Fax: 44 (0)1342 310401
Mobile: 44 (0) 7785-280908

E/Mail: [email protected]
Homepage: www.FlexibleSpace.com/

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