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The use of thermal wheels has come of age with the increased demand for energy efficient air handling systems. One area which has provided significant energy savings is two-wheel systems using desiccant and sensible wheels with a regenerative heater and adiabatic cooling.

These have been effective in air handling units providing fresh air to active chilled beams in lecture room applications. The main drawback with the chilled beam system is the requirement for the air in the occupied spaces to be maintained at a comparatively low relative humidity, which for a standard air handling unit, can only be achieved by a dew-point type temperature control to limit the moisture supplied to the space. In lecture rooms this is exacerbated by the high density of occupation within the rooms.

Dew-point temperature control can be achieved using a total energy wheel and sensible wheel with a cooling coil. This reduces cooling load and cooling coil size, and eliminates the need for regeneration heaters during summer cycle.

Two-wheel systems
The two-wheel system with regenerative heater and adiabatic cooling works by evaporation of water within the air returning from the building, which in turn cools a thermal wheel which cools the supply air stream.

Cooling performance is enhanced by the desiccant wheel which dehumidifies the incoming fresh air. Heat is then required to regenerate the desiccant wheel. This system is especially useful if waste heat or heat from chp is available, and no other refrigeration is required on site.

However, for most applications, waste heat is not available and refrigeration is required, ie for chilled beams or fan coil units. RMJM has contributed to Flakt Woods development of a simplified two-wheel system with a cooling coil which does not require a regeneration heater or adiabatic humidifiers. The energy components required are: a total energy thermal wheel, cooling coil and a sensible-only thermal wheel.

This system, when incorporated with an active chilled beam system, allows dew-point control of the supply air, using up to 50% less energy on the coils, compared to a conventional ahu, without the requirement for a reheater battery during the summer cycle.

The example shown in figure 2. however, does include a heater battery and a frost coil for winter conditions and heat-up times.

Operation
The basic operation of the system is as follows (figure 2):

• Summer: E-G outdoor air is initially cooled by the total heat wheel; G-H it is further cooled and dehumidified, using the cooling coil, to the required dew-point temperature; H-C the supply air is then reheated to the required supply condition using the sensible heat wheel which extracts heat from the return air.
  
• Winter: E-F outdoor air heated by frost coil to 2°C; F-G outdoor air heated and humidified by the total heat wheel; G-C supply air further heated by the sensible heat wheel; C-J heated to design condition by the heater battery.

With an occupancy level well below the design, acceptable humidity levels will be maintained, removing the need for humidifiers. Of course, if close control of the winter low level humidity were required, a humidifier may be necessary.

Terminal heating/cooling
A typical system for a densely occupied space, such as a university lecture room, would be fan coil units providing the heating and cooling to the space with fresh air being ducted to the back of the fcus within the false ceiling. This system has the advantage that the fresh air can be provided to the space without consideration of the humidity levels of the supply air, since the fan coils will remove excess moisture and run the condensate to drain.

RMJM has used chilled beams for university lecture rooms, which have reduced the maintenance requirement within the ceiling voids (or even removed the requirement for a ceiling void).

As previously stated, a system utilising chilled beams needs to ensure that the humidity levels in the space are kept to a maximum level of 53% saturated to prevent condensation forming on the beam. This means that the fresh air ahu needs to be controlled on its dew point (unless it's the evaporative cooling/regen system).

With the development of this two-wheel system with cooling, it will also be possible to utilise the advantages of the chilled beam system, and provide fresh air controlled on its dew point without the inherent energy inefficiencies of traditional air handling units with dew point control.

The simplicity of this system in its controls and reduced plant items serves to limit capital costs as well as operating costs.

It must be noted that although the fan coil system doesn't require the fresh air system to keep a close control on its humidity, the energy used by the fan coil units to remove moisture from the room recirculated air will offset, to a certain degree, the additional energy used by the ahu to maintain humidity levels in chilled beam systems. Hence chilled beam systems are not as energy inefficient as may be initially thought.

RMJM has designed a system for the Manchester Metropolitan University IT laboratories which has incorporated the use of active chilled beams in conjunction with two-wheel air handling units providing fresh air via variable volume units to the beams. This will enable the rooms, when unoccupied, or sparsely occupied, to revert to minimum fresh air volumes, reducing energy usage to a minimum.

A typical IT laboratory
The example of a typical IT laboratory illustrates the method used to calculate the psychrometrics of this system and illustrate the energy savings to be realised.

First, look at the psychrometric chart with the example of the chilled beam. The base conditions are: primary air from the ahu at 18°C db, 13·3°C wb; room air condition of 24°C, 50% sat.

From the chilled beam manufacturer's data, we know that with a primary flow rate of 20 litres/s/m, the total sensible output of the chilled beam is 814 W/linear metre.

Knowing the temperature and volume of the primary air, we know what contribution it makes to this 814 W. It is 6°C x 1·01 kJ/kgK x 0·02 m³/s 4 0·835 m³/kg = 0·145 kW, (145 W) therefore the secondary contribution to the cooling effect of the beam is 814-145 = 669 W.

If we assume that the induced room air, after the cooling coil, is cooled to 20°C, we can calculate its notional volume flow rate, which allows us to calculate the mixed air condition and plot the result. This is possible, because to achieve the stated output, the volume will increase/decrease depending on temperature, giving the same result.

From this the mixed temperature will be notionally 19·76°C (see figure 3 for the psychrometry of the chilled beam.) From this point, plot the room ratio to the room condition and it can be seen that it stabilises at 50% as assumed. The conditions shown are: condition A – room condition; condition B – induced air cooled by the chilled beam; condition C – primary air from the ahu; condition D – notional mixed air supplied to the room.

This may only be achieved by the use of dew-point control on the fresh air supply to ensure that the moisture content does not result in too high a value for the space's percentage saturation.

Figures 4 to 7 show the psychrometric cycles of the two-wheel ahu and a conventional unit without heat recovery. The psychrometric examples shown for the two-whe el ahu are based on tests carried out at the Flakt-Woods factory in Sweden. The heating effect of the fan motors and the mixing effect of the purge volumes on the thermal wheels has not been shown on these charts.

The psychrometric chart, figure 3, shows the two-wheel air handling unit cycle operating under dew point control at summer design conditions. The conditions shown are: condition E – outside air design condition; condition G – supply air after total energy wheel; condition H – supply air after cooling coil; condition C – supply condition from the ahu after the sensible wheel.

For design conditions the energy requirement for the coils are as follows: cooling coil: 18 kJ/kg; heating coil: 0 kJ/kg.

For an air handling unit, delivering 5 m³/s, assuming a typical bypass volume on the thermal wheel of 10%, the volume through the cooling coil is 5·5 m³/s resulting in a cooling load of 120 kW.

The psychrometric chart (figure 5), shows an air handling unit cycle operating under dew point control at summer design conditions with a conventional air handling unit. The conditions shown are: condition C – supply condition from the ahu after the heater battery; condition E – outside air design condition; condition G – supply air after cooling coil.

For 5 m³/s the loads would be as follows: cooling coil – 28 kJ/kg; heating coil: 7 kJ/kg. Again, for a 5 m³/s air handling unit, this results In loads of 171 kW cooling and 42 kW heating. The energy savings for this system at summer design conditions are: heating, 42-0 = 42 kW; cooling, 171- 120 = 51 kW.

Winter cycle
The base conditions for the winter design conditions are: room condition – 21°C, 50% sat. Primary air temperature from ahu 21°C, 40% sat. The psychrometric chart (figure 6) shows the two wheel air handling unit operating at winter design conditions.

Assuming a frost coil off-temperature of 2°C, with the twin wheel system the heat input required at the heater battery is 2 kJ/kg. The resulting heating load for a 5 m³/s ahu is 12 kW. The twin wheel system results in a higher percentage saturation within the space because of the total wheel, giving superior comfort.

The psychrometric chart (figure 7) shows a conventional air handling unit operating at winter design conditions.

Again, assuming the off coil temperature from the frost coil is 2°C, the heat required at the heater battery is 17 kJ/kg. The resulting heat load for a 5 m³/s ahu is 102 kW giving an increased load of 90 kW. The saving would be greater if the conventional unit were to include humidification to improve comfort within the space.

There are certain considerations to bear in mind when selecting the twin wheel system:

• Purge volumes: both of the wheels will have a bypass volume from supply to extract (or vice versa) depending on position of fans and pressures within the system. This will need to be considered when calculating the energy usage of the coils and fans.
  
• Controls: the controls would be set up as follows. Summer: the total energy wheel would be set to run at constant speed to reclaim maximum energy from the vitiated air. The cooling coil would be set to give a constant off-coil temperature of say 10°C. The sensible wheel would be set to vary the speed to maintain a supply temperature of 18°C. Winter: the total energy wheel would run at constant speed to reclaim maximum energy from the vitiated air. The sensible wheel would vary its speed to maintain the supply temperature of 21°C. The heater battery is used to top-up when there is a shortfall from the wheels.
  
• Air handling unit selection: it is relatively difficult to select and ascertain the performance of the air handling unit components directly from the present catalogue data, since the performance of the two wheels are interdependent and also rely on the purge volumes through the wheels being calculated accurately. Flakt Woods has developed a calculation method to enable performance of the twin wheel units to be predicted, along with their energy usage at all conditions to enable payback periods to be calculated.

Conclusions
The twin wheel system shows an efficiency improvement in the summer cycle and is better in the winter cycle than most systems with other forms of heat recovery. The two-wheel system should also be considered for other applications such as:

• Fan coil systems – the dehumidification of the fresh air to the fan coils will result in reduced moisture drop-out on the fan coils, hence reduced energy costs.
  
• Displacement ventilation systems – full fresh air systems without adiabatic cooling reduces the maintenance associated with humidification systems.

When a few of these systems have been installed and proved in operation, it would be surprising if it were not to become one of the standard systems.