Avoiding air conditioning disappointments
Facilities managers eagerly awaiting improved environmental conditions following the completion of a refurbishment scheme may be disappointed with the air conditioning installation if all the facts have not been faced by the building services consultants.
Allen Williamson, a Director of FHP Consulting Engineers, explains:
"Not every building is suitable for a comprehensive air conditioning installation. While there are many options available for the introduction of mechanically generated comfort conditions, construction, age and space limitations within a building often severely limit the choice.
There are instances where the most basic of stand-alone units is the only alternative, and it is up to the consultants designing the services for the building to make sure the client is aware of the situation. Regrettably, some do not put their clients totally in the picture and attempt to incorporate a system that ultimately does not satisfactorily meet the requirement or expectations.
One of the biggest problems on most refurbishment schemes is the limited slab-to-slab height and the resultant shortage of space within the ceiling void once raised floors and suspended ceilings have been installed. Too often older buildings do not offer sufficient void depth to accommodate fan coil units or VAV units with their associated ductwork and pipework.
Space restriction may also he encountered within the risers and the addition of more - if this is a structural possibility - only leads to a reduction in floor space (and therefore, in the case of a speculative refurbishment, a reduction in income). And while we are considering the construction constraints, it has to be remembered that much of the heat rejection plant associated with the cooling equipment is mounted on the roof, and many buildings currently being considered for refurbishment have pitched roofs or limited flat roof area.
Designing the air conditioning for an owner occupier is always easier than for a speculative development, for the anticipated cooling loads are usually known and may therefore be addressed, with varying levels of comfort cooling being introduced to meet differing requirements. With speculative developments, sufficient flexibility needs to be built in to accommodate the needs of the incoming tenants.
Developers undertaking a speculative refurbishment are, naturally, seeking to obtain the highest rental levels possible, and these will normally be paid by financially biased institutions - the very organisations whose high tech offices require the highest levels of cooling. It is sometimes a possibility to overcome a slab to slab height restriction by installing a chilled ceiling, but these systems have a maximum output of 90W/sq m as opposed to the 200W/sq m from fan coils units. As a normal person imposes sensible load in the region of 9W/sq m of body heat, with lighting adding 12W/sq m and computing equipment adding as much as 133W/sq m for dealer areas, it is apparent that chilled ceilings alone are not able to cope with the requirements of dealing rooms and similar.
A method of utilising chilled ceilings in dealer rooms is by combining them with displacement ventilation which introduces slightly chilled air through floor grilles close to the workstations, but this again calls for a substantial floor to ceiling height measurement.
Another alternative comes in the form of chilled beams, either in passive form where air is naturally circulated over the cooling element, or in active form when air is forced over the chilled beam, but this again needs ductwork. But their cooling outputs are limited and therefore they are not appropriate for all projects.
Many smaller refurbishments unsuitable for any of these solutions may well benefit from a split system approach with one indoor stand-alone unit linked to an outer unit by refrigeration pipework. Some larger refurbishments may take a number of indoor units, again connected by refrigerated pipework to a single large outdoor unit, probably on the roof. Each of these larger outdoor units offers a maximum cooling load of approximately 30kW - and may be replicated as often as desired.
I would suggest, however, that in refurbishments where space (or cash) is a constraint or in listed buildings where aesthetic restrictions apply, close attention is paid to reducing internal and external heat gains. Solar control glazing should be considered, where appropriate, but remember this reduces light transmission and may lead to additional internal lighting requirements.
Reducing uncontrolled infiltration should be an objective in buildings to be comfort cooled, with the installation of mechanical fresh air ventilation at controlled, known rates appropriate to the building usage. Do as much as possible to reduce the reliance on comfort cooling by reducing heat gains internally and externally.
The answer, therefore, is to take the wider view of the refurbishment as far as air conditioning is concerned, ensure the installation will meet the required comfort conditions, and if the building has constraints, ensure the most appropriate system is selected, and its shortcomings are fully understood."
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