“Why are our boilers running all summer to keep the air cooled”, was the scream, and prompted by bemused calls from facilities managers, engineers at Fläkt Woods’ R&D centre in Jönköping, Sweden, have created a world first, an air handling unit with twin wheels. It uses approximately 50% less energy on the cooling coil without the requirement for a re-heater battery in the summer cycle.
Chilled beam systems offer exceptional comfort and indoor air quality (IAQ) however to avoid the issue of condensation on cold surfaces the dew point temperature of the air needs to be controlled at a level below that of the cold surface in question. The dew point temperature of outdoor air in the summer can often be well above the surface temperature of the coil in a chilled beam. This means that we must remove moisture before supplying it to the room. The traditional method is to use a cooling coil to condense out the water and then a re-heater to warm the air to a suitable supply-air temperature. This method demands a large cooling plant and is costly to run.
Explains Linden Shuttleworth, UK Systems Manager 'Our knowledge of twin wheel technology and active chilled beam systems”, engineers have created a more efficient method of providing dehumidification to the primary fresh air in order to avoid problems with condensation.
This technology is being utilised in London on one of the largest chilled beam projects in the Capital, where Fläkt Woods is supplying in excess of 2,300 chilled beams together with the AHU’s VAV boxes and Displacement terminals, on Peninsula Central office development, part of the Terry Farrell’s giant Greenwich Peninsula master plan.
Prior to installation, both M&E consultants, Hilson Moran and the M&E contractor, Skanska, visited the Flakt Woods facilities Centre at in Jönköping in Sweden, where they witnessed a successful purpose built mock-up of the system simulating both summer and winter conditions
The first two office buildings comprise two separate buildings with a central pedestrian route between them, and it is here that employees of Transport for London, who has pre-let the first building, will be able to experience the new system when they move in later this year. The second building comprising 115,000 sq ft will be ready for occupation at the end of 2009.
How the new system works
As the name implies, the system uses two thermal wheels (rotary heat exchangers).
In winter, the heat of the exhaust air is absorbed by an aluminium rotor, which then delivers the heat to the supply air; and vice versa, in summer. Rotary heat exchangers are used when the supply and exhaust air ducts converge at one point. Low velocities through the rotor can ensure optimum heat exchange efficiencies typically between 70 -85% and pressure loss of 60 Pa. What’s more, owing to the reheat wheel, a separate re-heater is no longer required. This is ideal for a chilled beam installation that avoids any moisture problems.
In winter operation, the benefits are substantial as well. Not only do you get high levels of heat recovery but also most of your humidification load is supplied by the hygroscopic wheel further reducing the energy demand.
Total recovery wheel
The total recovery wheel or ’hygroscopic’ wheel has a porous surface and can transfer moisture as well as heat. This means that if the humidity in the extract air is lower than the outdoor air then moisture will be removed and the air will be both cooled and dried at the same time. This has a significant effect on the cooling power required of the chiller and typically, the cooling power can be halved.
Twin wheel system
In the twin wheel system we make use of the total recovery wheel as mentioned above but we add a second thermal wheel to reheat the supply air to the required temperature. This reheating effect in the second wheel causes a cooling effect on the extract air, which improves the cooling recovery in the first wheel. This is illustrated in the diagram and example on the next page.
Complete with control package
The Fläkt Woods twin wheel system is best controlled by our stand-alone control unit, which ensures proper operation and needs a minimum of input from the main AHU controller.
The nominal efficiency of thermal wheels is typically between 70 and 85% depending on air velocity.
Click image above to enlarge
Example
Air flow rate 5 m3/s
Outdoor condition: 29°C dry bulb, 50% relative humidity Required supply air condition: 20°C with 8 g/kg absolute humidity
Exhaust air design condition: 23°C with 9 g/kg absolute humidity, the total cooling demand is 184.4 kW and the reheat demand is 56.5kW. With the twin wheel system the first rotary heat exchanger (green) recovers 100kW of cooling leaving just 84.4 kW for the cooling coil. The reheat demand is furnished by the second rotary heat exchanger (red) and the supply fan.
“In practice, with today’s emphasis on energy recovery, most systems will include some method of heat recovery. Nevertheless, this system shows a marked efficiency improvement in the summer cycle and is better in the winter cycle than most systems with some form of heat recovery”, concludes Linden Shuttleworth.
Press release [2009-09-15]
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