The new HVAC technologies and approaches promise reductions both in first costs and long-term operating expenses, and herald improvements in indoor air quality.Do you want to learn more? Visit more info here
Low Temperature Air Distribution
A relatively new approach to HVAC system design – one that’s being employed increasingly often in new buildings and major renovations – lowers the temperature of the chilled air distributed through the facility. The chief advantages of this approach lie in reduced first costs and in longer-term savings realized through reduced “churn” costs.
When chilled air is distributed in the range of 46-48 F (as opposed to 55 F, the benchmark in conventional systems), less air is needed to cool interior spaces, so ductwork and piping can be significantly smaller than in conventional systems. That downsizing of air and water flow, in turn, means fans and pumps can be smaller. Fans operate continuously when spaces are occupied, but because less air is required to do the same job, energy costs also drop.
Air that arrives at an interior space at such a low temperature, however, cannot be released directly into the room. Dumping 46-48 F air into a room through a ceiling diffuser would likely cause some very real discomfort. But if the cold air is combined with room air before it enters the space, it can be brought to an acceptable temperature – warm enough that the occupants below won’t feel cold downdrafts.
This blending of chilled and room air can be accomplished by a fan-powered mixing box, set in the ceiling, that will produce a constant flow rate while varying the proportions of chilled and room air.
Experience shows that if the blended supply air is within the 50-52 F range, it will mix well enough with room air near the ceiling (typically about 75 F) that no discomfort will result.
Making sure that the air-change rates in interior spaces and the volume of fresh air being brought into a space conform with standards set by the American Society of Heating, Refrigeration and Air-conditioning Engineers (ASHRAE) can be an expensive proposition – especially in the summer, when warm, moist outside air must be cooled and dehumidified before being used for ventilation. One way to cut down on those refrigeration/dehumidification costs is through a demand-controlled ventilation system that modulates the quantity of outside air being brought into the building as occupancy and pollutant levels change.
The ability to know how much outside air should be brought into a space at any given time has been greatly enhanced by the refinement of low-cost electronic monitoring devices that accurately measure carbon dioxide levels. (Sensors that monitor other contaminants, such as volatile organic compounds, are also available, though their relatively high cost has so far limited their use.)
Demand-controlled ventilation can be incorporated into systems that mix fresh air with room air as well as into systems that have separate ducting for fresh air supply. Because installing an independent fresh air delivery system is fairly costly, however, the potential benefits of such a system must be carefully weighed against its substantial first costs.
As useful as they are – and as crucial as they’ve proved to be in improving indoor air quality in facilities – the fresh air standards set by ASHRAE are bedeviled by one basic problem. The standards necessarily assume that the air being brought into a building is clean. In reality, there can be an enormous variation in the quality of outdoor air, depending on factors such as time of day and a facility’s location. And it is difficult to predict how the general air quality at a location might change, for better or worse, over time.
Given this unpredictability, owners and developers of new facilities or those undergoing extensive renovations may wish to ask their consulting engineers to specify very high efficiency filters – as high as 85 percent efficiency – especially in some densely populated urban areas.
Owners may also be wise to instruct consultants to build extra space into designs for air-handling units so that additional filters – or higher-efficiency filters – can easily be added later if the quality of outdoor air declines or if, for example, the owner wants to attract tenants with very strict air quality requirements. If that space is not built in, the cost of installing additional filters later may be prohibitive.
The small additional cost of building-in that space at the start may be very easy to justify if the business environment changes in the future. For instance, if a commercial rental market should turn sharply competitive, the ability to ensure high indoor air quality might give an owner a much-needed edge in attracting prospective tenants.
Sizing Systems for Real-World Demand
For generations, good HVAC engineering practice has demanded that systems and components be sized large enough to accommodate peak use. For the most part, however, engineers have sized systems by textbook standards instead of real-world conditions. Because they have had little actual performance data from which to make their calculations, engineers have also tended to increase sizing even beyond what’s necessary to accommodate a theoretical peak load in order to to protect against under-capacity.
Sizing in this way creates systems that are far bigger than they need to be. Sizing systems to accommodate conditions that never occur is unnecessarily expensive for two reasons: First, there’s the added cost of buying equipment that’s bigger than a building is ever going to need; second, the part-load performance of most equipment (e.g., fans and pumps) is generally much less efficient than that equipment’s full-load performance.