By Michael Pieterse, Michael Carl and Martin Stangl
Architects and engineers are often asked to shoehorn high-performance buildings into tight locations in dense urban environments. On such projects, it’s tempting to squeeze mechanicals and service equipment onto rooftops or in other confined, out-of-the-way spaces. But, like people, such equipment can suffer from overcrowding and bad ventilation. Finely tuned equipment for high-performance buildings can be especially sensitive. The impacts of heat and humidity from equipment exhaust plumes are becoming a significant issue as available footprints decrease and demands for large cooling loads and emergency power increase. So, finding space for mechanicals isn’t the end of the design challenge; it’s just the beginning.
A true solution involves a careful review of how mechanical units will interact based on placement and airflow, followed by an assessment of risks and mitigations. The tradeoffs get complicated very quickly, especially when densely configured rooftop mechanicals are sited in a densely built context. We’ll walk through some typical issues and some ways to think more broadly about designing for challenging equipment in challenging spaces.
Problematic equipment can include backup generators, evaporative cooling towers and air-cooled chillers. A primary issue is contamination at building fresh air intakes, whether by particulates, humidity or thermal load. The impacts of heat and humidity from equipment exhaust plumes is quickly becoming a significant issue as available footprints decrease and demands for large cooling loads and emergency power increase.
When exhausts are placed near obstructions or in areas with limited airflow, the exhaust air can also be drawn into the inlets of the same or neighboring pieces of equipment. The result can be reduced performance or, in extreme circumstances, shutdown. For mission-critical equipment, this risk can be unacceptable. The fix could be as simple as a shift in equipment location, use of porous screen walls to improve wind penetration into roof-wells, or greater separation of intake and exhaust points.
A particular example of contamination by exhaust air is seen in evaporative cooling towers, which can cause several problems in a cramped setting. First, exhaust air may be recirculated back into the tower’s own inlets. The issue here is the high wet-bulb temperature and humidity of the re-entrained exhaust, which diminishes the tower’s capacity to add additional moisture to the air. A second problem is that during colder weather, the humid exhaust may condense in the air, becoming a visible plume and causing condensation or ice formation on building façades or other architectural elements, as shown in the photos. These issues can be avoided by using plume abatement equipment to reduce the moisture content in the exhaust.
But impacts to equipment aren’t the only issue. When exhaust equipment is placed in cramped spaces, it’s usually also close to building air inlets, open windows, and so on—a situation that can cause building air quality issues (pollutant/chemical levels, odors).
The design community can benefit by being aware of these potential issues and how to fix them. Proactively identifying any issues and addressing them at the design stage can not only save headaches down the road but also save money.
The design of mechanicals starts with deciding where the equipment will fit and whether the structure hold its weight. Too often, designers stop there. To achieve a truly high-performance design, several more questions need to be asked:
- Is the equipment suited to its placement, especially given how this particular equipment “breathes”?
- If problems are possible, what is the risk they will occur?
- How does the risk align with the “risk appetite” of the client?
- If the risk is unacceptable, what mitigation is possible?
- Is the added cost of mitigation justified?
Mitigation can be costly if a problem isn’t detected until a building is operational. On the other hand, the added cost of mitigation equipment may not be justified if the risks are low.
Understanding the risks and, if necessary, choosing the right mitigation strategy requires sophisticated tools. Analyses can include a combination of numerical, wind tunnel and/or computational fluid dynamics (CFD) modeling, conducted by an experienced practitioner. The best tool will depend on the location and type of equipment of concern, the temperatures of exhausts and the local geometry, and similar factors. Analyses should consider at least five years of local meteorological conditions combined with modeling outputs of local sources of heat and humidity (psychrometrics). The goal of such analyses is to quantify the number of hours per year when, for example,
- Cooling tower inlet wet-bulb temperature is above the design condition or within various ranges
- Cooling tower operation may lead to visible plumes of varying lengths or to condensation or icing on surfaces
- Air-cooled chiller, generator, or building fresh air inlet temperatures are above design temperature.
If the risks are too high, then potential mitigation strategies can also be assessed by using the same methods. The images here illustrate the temperature rise associated with air-cooled chillers external to a building, as predicted using CFD. The use of stack extensions separates the exhausts from intakes, allowing more mixing and dilution with fresh air, thus moderating the air temperature at neighboring units.
The demand for high-performance buildings will continue to push architects and engineers to get creative in their placement of service equipment. Understanding the risks associated with the placement of that equipment can give designers confidence as well as peace of mind about advanced designs.