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Heat pipes used in displacement ventilation

Ventilation is the introduction of outside (primary) air to a space while polluted internal air is extracted. Traditional methods of ventilation (mixing ventilation) involve the introduction of a mixture of treated outside air and recirculated indoor air at high level outside the occupied zone. This air will be cold at typically 12 to 15°C and supplied at relatively high velocity. Air needs to be cold so that when combined with the relatively high air volume it will offset the internal heat gain, and at relatively high velocity so that the air mixes with the indoor air effectively outside of the occupied zone. The supply air will typically leave the supply diffuser at 1 to 1.5m/s.

Conventional air conditioning systems can be either a) all air systems i.e. variable air volume, constant air volume or b) air-water systems using fan coil units. In all air systems space cooling is provided by supply air from the central AHU whereas an air-water system provides outside air from the central AHU and local cooling via the chilled water fed to the FCUs. The primary air can be mixed with room air at the FCU, or alternatively, fed via a separate supply. FCUs can provide both sensible and latent cooling to a space, whereas in all air systems all the latent cooling must take place at the central AHU. Displacement ventilation/chilled ceiling systems are a special case of an air-water air conditioning system where the space cooling equipment must do no latent cooling.

Displacement ventilation differs from mixing ventilation as follows: Air is introduced at low level directly into the occupied zone, at relatively warm temperatures, typically 18 to 19°C and at low velocity. Supply air volumes are low and only include treated outside air.

The theory behind displacement ventilation is:
Air around 3°C colder than the space is supplied at floor level into the occupied zone. Being colder than the space air it is more dense and displaces the space air above it. When the cool, dry displacement air contacts sources of heat and moisture it is warmed and becomes more humid, forming plumes of low velocity air around people and equipment, cooling them directly and transferring their heat and moisture upwards.

It is suggested that mixing ventilation is responsible for distributing contaminants associated with the room fabric and occupants whereas the plumes of warm air typical of displacement systems are unpolluted and result in contaminants collecting at ceiling level where they are extracted. Accordingly, displacement ventilation results in a healthier environment than mixing ventilation. Because air is introduced at floor level it must be at a comfortable temperature i.e. 18 to 19°C. It must also be supplied at low velocity not to create uncomfortable draughts. When the low supply air volume is combined with the relatively high supply temperature the amount of cooling that can be achieved using only displacement air is limited and typically accounts for only 25% of the total space cooling load.

Cooling provided by the ventilation air is referred to as primary cooling. A further source of cooling (secondary cooling) must be provided. When using displacement ventilation the low air velocities give rise to stratification, warm polluted air gathering at ceiling level. To counteract this, the displacement ventilation system always combines with a chilled ceiling and high level extract system.

Chilled Ceiling
A chilled ceiling can consist of chilled beams, chilled panels or a combination of both. Beams are normally found around the perimeter where solar gains are high, panels can be used in the interior of the zone where heat gains are low. Beams can be passive, relying on warm air to convect over the chilled fin surfaces, or active using low powered fans or induction units. As the air is cooled by the beams/ceiling it must not come into contact with surfaces below the dewpoint of the air. If it did then moisture would start to condense onto the ceiling and eventually drip into the room.

For the chilled ceiling to achieve the necessary cooling it must be fed with chilled water at a sufficiently low temperature, while still being above the dewpoint of the air which comes into contact with it. The water to the chilled ceiling is typically supplied at between 14 and 16°C. Assuming that the dewpoint of the displacement air increases by 1°C as it picks up moisture between the floor and ceiling, it is typically supplied at dewpoint of 2°C below the temperature of the chilled water feeding the ceiling.

Application of Heat Pipes
Displacement ventilation/chilled ceiling systems work against almost identical sets of conditions, with outside air volume being determined by size and usage of the space or level of occupancy. These conditions are:
Supply air volume: Typically 20 litres/s/person
Supply air temperature: 18°C
Supply air dewpoint: 12°C
Chilled beam water temperature: 14°C

To achieve the above supply conditions , the outside air must be overcooled to remove moisture, before being reheated to the comfortable supply temperature of 18°C. Conventionally this is achieved by oversizing the cooling coil to reduce the air to 12°C in a saturated state then reheating to 18°C using LPHW, steam or electric reheat.

This process is energy consumptive. By wrapping heat pipes around the cooling coil both total cooling and reheat requirements will be reduced or eliminated. ‘Wrap-around’ heat pipes are used, fitted around conventional cooling coils. Heat pipes are extremely effective heat conductors due to their internal construction. They absorb heat at their warmer end and transfer it to their cooler end with only negligible temperature differences along their length. Visually, heat pipes mimic the cooling coil consisting of bundles of heat pipe tubes expanded into continuous fins. The face size of the heat pipe will match that of the cooling coil.

The wrap-around heat pipe absorbs heat from air entering the cooling coil and transfers it to the leaving air . This process effectively precools the air prior to the cooling coil and reheats the air after leaving. Precooling and reheat effects are equal and eliminate or reduce the conventional costly overcooling and reheating. Typical design conditions for displacement ventilation heat pipe system are:-
Outside air: 29.0°C dry bulb/20.0°C wet bulb
Air off Precool: 23.0°C/18.0°C
Air off Cooling Coil: 12.0°C/11.8°C
Air off Reheat: 18.0°C/14.2°C
These conditions are plotted on a psychometric chart

Based on the above conditions, savings per cubic metre outside air accrued through adding heat pipes are:
Precool saving: 7.2kW
Reheat saving: 7.2kW
At design conditions the heat pipe provides all necessary reheat. Below this condition, supplementary reheat is needed - either a conventional reheat coil or a heat recovery heat pipe between supply and extract decks, utilising heat from the extract air.

Heat recovery heat pipes will typically be a straight heat pipe with its lower end in the warm extract air and its upper end in the cool supply air. This will then transfer waste heat from the extract air to the supply air to provide the necessary reheat free of charge. Figure 4 and 5 (see below) gives details of AHUs with and without the heat recovery device.

Concluding remarks
1. Displacement ventilation is more energy efficient than conventional systems as it involves treating relatively low volumes of only outside air.
2. Displacement ventilation directly cools people and equipment, the warm, moist plumes of air gathering at the ceiling.
3. Displacement ventilation systems are healthier than mixing ventilation systems as contaminants are not distributed in the space but concentrated at ceiling level were they are extracted.
4. Chilled ceilings are used to supplement the cooling available from the displacement air.
5. Humidity of the displacement air must be controlled to prevent the ceiling from dripping.
6. Outside air must be treated to remove moisture and provide a comfortable temperature for supply into the occupied zone. Heat pipes allow the most energy efficient means of conditioning the
outside air in this fashion.
7. Both ‘wrap-around’ heat pipes and heat recovery heat pipes can be utilised to give a complete solution irrespective of the external conditions and without the need for costly conventional reheat.
8. Heat pipes are external energy free, using only the refrigerant contained within the heat pipe circuits to achieve the necessary pre-cool and re-heat actions.
9. Heat pipes have no moving parts.

Email: spc@spcoils.co.uk