HVAC Air Time
Both novel and conventional technologies are being put to effective use in air movement strategies. The newest innovations are in controls and monitoring, as well as airflow modeling in the design phase. CSE: What are some of the newest approaches to moving air for energy-efficient heating and cooling? ROWLAND: There are several alternatives currently in use that, while not new, are different ...
Both novel and conventional technologies are being put to effective use in air movement strategies. The newest innovations are in controls and monitoring, as well as airflow modeling in the design phase.
CSE: What are some of the newest approaches to moving air for energy-efficient heating and cooling?
ROWLAND : There are several alternatives currently in use that, while not new, are different from the conventional office HVAC systems. Low temperature, primary air systems provide air at considerably lower temperatures. The low temperature air is then blended with return air to be supplied to the spaces at a higher, more comfortable temperature.
Another alternative is the task-based system. These provide heating or cooling at the point of use—right at the desk—putting conditioning right where it’s needed and tending to give the occupant a feeling of control.
Also, underfloor air distribution (UFAD) is being applied in some office facilities, particularly where task-based systems are used, supplying air through the furniture system.
KALLSTROM : It’s true that UFAD is gaining popularity, due to benefits such as occupant comfort, indoor air quality (IAQ) and reduced energy costs. It gives occupants individual control over the airflow in their workspaces, which has greatly reduced HVAC-related complaints. UFAD’s floor-to-ceiling air movement improves IAQ by removing contaminants at the ceiling level before they have a chance to mix with the supply air. Also, less ductwork is required because the space under the floor is used as a plenum to distribute the supply air. In climates where humidity is not a concern, 63
KNAPP/RAINEY : In addition to UFAD, some of the most interesting systems now coming into use are cooling beams, high-induction diffusers and de-coupled outside-air systems.
Where there are high internal loads, cooling beam systems have advantages over conventional air. These systems use the sensible cooling effect of the cooling beams to offset a large portion of the high internal loads, thereby allowing the supply-air system to be downsized to handle only the minimum ventilation air and cooling requirements. This reduces air handler equipment and ductwork sizes, and consequently, costs and operating energy.
High-induction diffusers can improve ventilation effectiveness and reduce dumping. De-coupled outside-air systems improve IAQ by ensuring that minimum ventilation air rates are maintained, regardless of space load fluctuations.
BENSON : We are seeing an increased use of variable-frequency drives with direct-drive Arrangement-4 type fans. This concept allows the fan to perform at its peak point of operation, regardless of the amount of air the building requires. It’s a more efficient choice than outlet dampers or inlet guide vanes. With a VFD, the fan’s operation can be integrated with the rest of the HVAC system, allowing for maximum comfort with minimum energy consumption. As a side benefit, the fan is quite compact and requires very little maintenance.
CSE: Did the anthrax scares of recent years effect any significant changes in air movement designs?
BENSON : We observed three changes. The first was a strong interest of system designers and specialty equipment manufacturers to develop equipment that would filter or destroy contaminants, such as anthrax, introduced into the building’s HVAC system. However, the level of interest in such systems seems to have moderated over the past year, in part due to the high cost of retrofitting HVAC systems with this equipment.
The second change was a greater awareness of the location and accessibility of building air intakes, and the third was a trend toward smaller, more localized zones with the objective of limiting the spread of any contaminant released in the system.
KNAPP/RAINEY : Something else to consider is evaluating air distribution zoning and pressure control within buildings, regarding building entrances and material handling areas, in order to assure possible contaminants released in these areas will not be circulated throughout the entire building. In some cases, this can mean using dedicated air-handling systems or once-through ventilation systems, where previously, conventional recirculating systems may have been used. Local-spot exhaust systems can also be used to capture possible contaminants at their source, such as mailroom tables. Spaces that could possibly be exposed to contamination should be maintained under negative air pressure relative to the adjoining spaces. Areas that could be prone to contamination and need to be re-evaluated regarding air distribution include entry lobbies, medical waiting rooms, mail rooms, loading areas and similar spaces.
CSE: What are some common errors that result in poor and inefficient air circulation? And how can these mistakes be corrected?
ROWLAND : The biggest efficiency problems come from unnecessary pressure drops requiring additional energy to move the same amount of air for cooling or heating. The biggest culprits causing this are non-standard fittings, undersized ductwork, crushed flexible ducts and poor inlet and outlet conditions. The inlet/outlet issue can be particularly bad at fan inlets where turbulent conditions can reduce the fan’s capability.
Another issue is duct leakage. A duct system that leaks 10% or more severely penalizes the energy efficiency of the system by requiring the fan to move—and heat and cool—more air than is needed to meet the required load.
Finally, we see problems with zoning where spaces with dissimilar load profiles are grouped together in a single thermostatic control zone. Adjacent areas with dissimilar loads wind up being either over- or under-cooled or heated.
KALLSTROM : The most common error we see, by far, is the underestimation of system effect losses. System effects are caused by anything placed in close proximity before or after the fan that affects cataloged performance. The most common causes of an underperforming fan or system, as Mr. Rowland mentioned, are improper inlet and outlet conditions, non-uniform inlet flow or swirl at the inlet.
Designing the system as closely as possible to the way the fan was tested should eliminate these common causes of system effect loss. By allowing for an unobstructed area before and after the fan, laminar airflow is achieved into and out of the fan, maintaining cataloged performance. Proper design of inlet elbows and inlet boxes will also greatly reduce system effect losses.
If system effects are not accounted for in the design, they can be corrected after the fact—but at a cost—either by increasing the fan speed or changing the ductwork. The most common fix is to increase the fan speed to deliver the designed airflow. However, with increased fan speed, the horsepower requirements will also be increased, which results in higher operating costs. These costs can become significant, as every 10% increase in fan speed causes a 33% increase in horsepower requirements.
For example, increasing the speed of a fan with a 20-hp motor by 10% will result in an increase of $3,296 in annual operating costs for a fan that is operating 261 days per year and 16 hours per day at $0.15 per kwh.
But, of course, increasing fan speed also increases the potential for noise problems. Even though this is the quickest fix, the long-term impact of running at higher than expected operating costs must be weighed against the one-time cost of correcting the ductwork.
KNAPP/RAINEY : A common problem directly affecting air distribution is the fact that the HVAC engineer is often hesitant to request from the architect additional ceiling plenum space required to properly distribute the air. More and more frequently, architects are striving for high open ceiling spaces uncluttered by diffusers. However, this can have a serious impact on the HVAC design, if the engineer does not identify a potential problem soon enough in the design. In addition, diffuser locations cannot always be located where the engineer would like them to be, resulting in uneven temperatures, drafts and unsatisfied occupants.
CSE: What kind of improvements and technological advances do you foresee in the near future for fans, air-handling units, diffusers, dampers and controls?
KALLSTROM : Monitoring and variable-speed operation of these products continue to become more sophisticated, as well as more economical. With these advancements, I foresee the integration of controls into a broader range of products direct from the manufacturer. Such improvements will allow for ventilation products to be more responsive to changing conditions while optimizing energy efficiency and IAQ.
ROWLAND : I expect most of the improvements to be on the software and control end, but another area of advancement is the concept of airflow modeling during design. With this powerful software tool, airflow and distribution alternatives can easily be compared to determine the best solution for a particular application.
KNAPP/RAINEY : There will probably not be drastic, revolutionary technical improvements made to these devices and equipment. The focus will remain on what it has been over the past decade or so, which is evolutionary, continual improvement in quality, flexibility, efficiencies, sound reduction, IAQ and control system capabilities.
For the complete M/E Roundtable discussion, visit www.csemag.com .
Gary Benson , Vice President of Marketing, New York Blower, Willowbrook, Ill.
Dave Kallstrom , Sales/Marketing Manager, Fan/Vent, Greenheck, Schofield, Wis.
Doug Knapp , P.E., Vice President/HVAC, and Teresa Rainey , P.E., Senior Mechanical Engineer, Vanderweil Engineers, Alexandria, Va.
Charles Rowland , P.E., Director, Mechanical Services, Kling, Philadelphia
The control system is crucial to today’s air-handling systems, as variable-frequency drives, reset schedules, carbon dioxide sensors, ASHRAE 90.1 and a multitude of other considerations make today’s systems more complex than ever. These increasingly complex requirements often challenge commissioning teams to verify that the building systems are installed correctly and are operating according to design.
Thoughtfully designed control systems can simplify verification of these system complexities by displaying critical parameters on a building automation system monitor, a laptop computer or even a personal digital assistant or other hand-held device. Direct-digital control systems with carefully crafted sequences of operation permit the commissioning team to simulate a host of building operating conditions and confirm proper response of the controls and mechanical systems.
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