The new HVAC techn giso ologies and approaches promise reductions both in first costs and long-term operating expenses, and herald improvements in indoor air quality.
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.
When energy is cheap, those part-load inefficiencies may not seem to matter so much. But when energy prices go up, such inefficiencies could boost operating costs enormously.
It may be difficult to see the mistake that’s being made when an air conditioning system is unnecessarily oversized. If a facility contains a certain amount of heat-generating equipment, shouldn’t the cooling system be built big enough to handle the load that would occur if all that equipment were running at once?
The answer is no, for the simple reason that it’s extremely improbable that that would ever happen. For example, sizing chillers for a hotel should not be based on the sum of the peaks of all rooms, public spaces, and back-of-the-house areas. Calculations of cooling needs using such a sum might easily exceed 2.0 to 2.5 tons per room, including public areas. But the actual operating experience of hotels shows that total chiller sizing should be based on use in the range of 1.5 tons per room including public and back-of-house areas.
Obviously, each project must be reviewed individually. However, monitoring actual building use provides a sound basis for establishing diversity in lieu of design-day calculations and guesswork on diversity values. Building owners can be of great help to engineers if they keep actual records of equipment use, calculate performance data and make this information available to the design community. In some cases, leaving enough space to accommodate future additions of equipment is a good approach for avoiding initial oversizing.
For typical office buildings, lighting and equipment are the largest components of cooling loads. With advances in lighting design, typical open workspace can be easily reduced to 0.8 to 1.0 watts per square foot. Equipment loads, on the other hand, tend to be more difficult to estimate because of “diversity” – that is, the fact that patterns of actual use are constantly changing and that peak loads do not occur simultaneously throughout a facility.
Despite increases in office equipment use, however, office power on a gross-area basis does not usually exceed 2.0 watts per square foot. Nevertheless, hot and poorly ventilated spaces are common in many older buildings where new “high-tech” tenants are occupants.
Given this experience, convincing design engineers and owners not to oversize air-conditioning systems when designing for new construction is difficult. But systems that take advantage of diversity from space-to-space and floor-to-floor are usually effective at matching active loads without oversizing. Air-based systems that can vary temperature in sequence provide the most flexibility. Fan-powered mixing boxes that vary primary air while keeping ventilation of the space high for good air quality are efficient devices.
Moreover, this kind of system can be combined with low-temperature air to provide additional cooling capacity without a high-premium first cost. Variable air volume systems, which can deliver colder air if required but can be reset at a higher level when average conditions exist, are very efficient and allow for “built-in” capacity without much penalty on first cost.
Chillers that have from 10 to 20 percent additional capacity allow chiller temperatures to be reduced by 1 to 2 F. This is more cost effective for meeting high-load periods than adding a chiller to serve infrequent peaks.
Underfloor Air Distribution
One approach to HVAC system design that emphasizes the “air side” has been receiving a great deal of attention lately: low-pressure underfloor air distribution. In this approach, cool air, which is kept at a slight positive pressure, is distributed through access-floor plenums and delivered to occupied spaces through diffusers set directly in floor tiles, from which it rises toward ceiling return vents through a natural convective process, removing heat and contaminants from the space as it travels upward.
Underfloor air distribution offers multiple advantages over conventional supply-air delivery methods, including greatly enhanced individual control of comfort conditions, long-term reductions in energy consumption and improvements in IAQ. Still, the first costs associated with underfloor air, though falling, remain relatively high as compared with those of conventional systems, and an access floor system probably cannot be financially justified on the basis of air distribution alone. When underfloor air is coupled with access-floor power and voice/ data grids, however, the numbers become a lot easier to live with – especially in offices with high churn rates where the ease of reconfiguration that’s possible with plug-and-play systems can lead to mind-boggling reductions in the use of outside contractors.
There’s one trend that’s sure to make underfloor air distribution a viable alternative for a wider range of buildings in the near future: the progress being made in developing window glass that can regulate solar transmittance and heat rejection.
Today, the implementation of underfloor air systems is mostly limited to building interiors. In heavily glazed buildings, it has been necessary to supplement or replace underfloor air delivery at the perimeter with more conventional systems that are better able to handle the huge swings in temperature that can occur even over the course of a single day.
New glass technologies that immensely improve the efficiency of the building envelope will likely be on the market within the next decade. When that happens – perhaps as soon as five years – it will no longer be necessary to install a separate system to handle the heating/cooling needs of the perimeter, and the overall short- and long-term costs of underfloor air systems will drop precipitously.
Given all the new alternatives, facility executives can’t make decisions based on what’s been done in the past. Instead, the focus must be placed on applying those technologies that make the most sense in a given situation – and on simplifying and integrating systems. “Rules of thumb” provided a better road map when HVAC system goals and technology options were fewer and less complex than today.
Having a master plan, defining clear long-range goals, and making sure that systems are as simple and integrated as possible is the most sound advice that can be offered as HVAC technologies and approaches to system design continue to change.