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MIXING VENTILATION CAN REDUCE ENERGY COSTS

TECHNICAL & PRACTICE

Increasing energy costs coupled with concern about carbon emissions has intensified interest in natural ventilation. Ventilation experts Shaun Fitzgerald and Andrew Woods study 'mixing' ventilation, which recent research from Cambridge University's BP Institute suggests can achieve better energy savings in buildings such as theatres and schools than conventional 'displacement' ventilation.

A number of UK theatres have recently been completed using displacement ventilation. Given the large ventilation requirements associated with the high density of occupants, displacement ventilation can provide an effective means of regulating temperature and air quality. Air is brought in at low level and is heated through a combination of heat loads, primarily from the audience and stage lighting, before rising through vents in the roof.

But displacement ventilation has inherent inefficiencies because it requires some heating and cooling capacity due to seasonal fluctuations in source air temperature. During cooler months the incoming air requires preheating to satisfy thermal comfort criteria. This increases the heat load and ventilation rate required to flush the heat from the theatre. In the summer, upward displacement ventilation can only operate if the theatre interior is warmer than the external temperature. This may lead to uncomfortably hot interior temperatures, necessitating a cooling system. The theatre interior then becomes cooler than the external temperature, resulting in downward natural ventilation through the stacks. Downflow may be undesirable if the cooling is provided by thermal mass at low level, since it can only function effectively with upflow ventilation. This can be achieved by low-wattage fans in the outflow stacks.

These principles of displacement ventilation have been used at Manchester's Contact Theatre and the Garrick Theatre in Lichfield. Both theatres include an underfloor plenum to buffer the temperature of inflowing air. To assess the actual operation of the ventilation in each theatre, we documented temperature fluctuations by locating thermocouples in the auditorium, the plenum, the ducts and the stacks, and recording temperatures at five-minute intervals over an extended period. At the Contact Theatre the internal temperature remained comfortable, but there was an inflow through one of the stacks during the cold evening and morning periods, while the other stacks continued to operate in an outflow mode. Our data was less conclusive at the Garrick, but it also appears to operate in a hybrid mixing-displacement ventilation mode - some high-level stacks provide inflowing air to complement air supplied at low level from the plenum.

Our research indicates that neither theatre is operating according to the displacement-ventilation principle upon which they were designed. Hybrid mixing ventilation has an important impact on both the ventilation rate and the thermal comfort of the theatres. With an inflow of cold air through a high-level vent, a descending plume of it develops within the space, becoming progressively more dilute as it approaches the occupied zone above the floor. A key issue is whether it is sufficiently dilute that occupants do not experience localised zones of cold air. Our data indicates that in tall spaces such as theatres, with interior heights of more than 5m, the ventilation plume will dilute sufficiently.

It is difficult to determine which flow pattern will develop at a given time, especially with multiple equally sized stacks. Fortunately, effective control of the stacks does not rely in detail on modelling the flow pattern, but often involves comparing the prevailing flow regime with the desired flow regime, and then establishing a procedure to evolve the flow to the desired regime.

To determine the prevailing flow regime, it is possible to measure temperature and CO 2 levels in the stacks and infer the flow pattern by comparing the data with the exterior and interior values. By installing small low-wattage fans in each stack, and using these in combination with the dampers, the flow can be adjusted. Once flow has equilibrated, the stack-driven flow becomes stable and fans may be turned off.

The potential financial savings of a mixing ventilation strategy can be estimated by examining the preheat requirements throughout the year. A graph (above) comparing the energy needed to preheat incoming air to 18ºC using displacement ventilation with the heating required to simply maintain the interior conditions at 22ºC using mixing ventilation in the winter shows that, once the external temperature falls to below 18ºC, the heating system is required for a displacement ventilation strategy, whereas the heating system is not required with a mixing ventilation strategy until the exterior temperature falls to around 12ºC.

Hybrid ventilation systems can also be used in buildings with conventional ceiling heights, such as schools, but when the vertical distance between the entry point to the stack and the occupied zone is less than 5m there is a risk of cold draughts in winter because the incoming fresh air is not able to mix sufficiently with the air in the room before reaching the occupants. This can be controlled by a new patent-pending e-stack natural ventilation system which uses a series of dampers, plus temperature and CO 2 sensors within the stack, to ensure that the appropriate ventilation rate is provided. The system has been installed in a number of schools. Data comparing the performance of a classroom using the e-stack natural ventilation system to the same classroom operating without the e-stack suggests that the CO 2 levels in the classroom are substantially lower when using the e-stack system.

Naturally ventilated theatres do not always operate in the simple upwards-displacement ventilation mode, even if they are designed to do so. Mixing ventilation, with counterflow in the stacks, can occur. The e-stack natural ventilation system has been developed to enable the energy-saving benefits of mixing ventilation to be applied in smaller buildings as well as in theatres.

Shaun Fitzgerald is managing director of e-stack and Andrew Woods is BP Professor at the BP Institute, University of Cambridge.

E-stack offers RIBA-approved CPD seminars and workshops on natural ventilation, low-energy building design and complying with Parts L and F.

Visit: www. e-stack. co. uk

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