Flow reduction is maximized when airflow is measured at the cold-deck and total-flow positions. Using 100-percent outdoor air allows VAV systems to control individual-space airflows over a large range of outdoor-air ventilation rates. Occupancy sensors can be used to reduce minimum ventilation rates to zero when spaces are unoccupied, reducing the control of the cold deck to shut off VAV. Total-airflow sensors can be used to monitor and trend actual air-delivery rates to individual spaces to document compliance with Standard 62 requirements. Local heat typically is supplied and controlled locally via radiant floors, ceilings, or baseboard radiation.

APPLICATION

The RGDD System should not be employed in facilities smaller than 20,000 sq ft. It specifically supports ventilation-dominant variable-volume applications in which outdoor-air ventilation rates exceed 21 percent of peak system design airflow. Able to support almost any mix of occupancies, the RGDD System can be designed to deliver airflow of up to 140,000 cfm per unit. It is applicable in laboratories, hospitals/clinics, casinos, convention centers, educational facilities, and other high-density-occupancy structures. Viable in hot/humid and arid environments, the system is appropriate in multifunction applications in which multiple individual HVAC systems can be replaced by a single system and air quality is or has been a problem.

CAUTIONS

Physically, the RGDD System is straightforward. By efficiently processing ventilation and eliminating systemic inefficiencies, it can significantly reduce primary-plant heating-capacity requirements. Cooling capacity also can be reduced significantly. For example, in the RGDD System prototype installation, the educational facility's required boiler-plant capacity was reduced by 75 percent; actual primary-cooling-plant capacity was reduced by a factor of 8-to-1.

Psychrometrically, the RGDD System is different from other types of HVAC systems. With six sets of design conditions, the RGDD System's airflows are only partially based on cooling loads. Psychrometric calculations must be prepared for heating, cooling, and intermediate operating conditions. Most elements in the RGDD System serve multiple functions that vary depending on ambient conditions and thermal loads. Its processes are selected, configured, and designed to work synergistically by converting energy back and forth between latent and sensible forms and exchanging energy between airflow streams.

The system creates and relies on the interaction of its various components, permitting it to amplify energy availability for recovery. However, this interaction makes the system difficult to design and commission. Because the RGDD System requires commissioning, engineers and commissioning agents will need training in its application.

The system can be used for thermal storage. Because smaller chillers consume less energy and reduce electricity demand, the implications for primary-cooling-plant size and energy and cost savings are significant.

A designer also must keep in mind that the RGDD System strategy is new, and contractors and temperature-control specialists probably will not have had previous experience with it. Because the system employs 100-percent outdoor air, proper testing and balancing, system-control setup, and commissioning are critical to a successful installation. A system designer must have command of the system's engineering and control issues and be prepared to provide substantial field supervision and troubleshooting services to help ensure a successful installation. The system's design is unusual enough that a first-time designer probably will not be able to provide a successful application without proper instruction and guidance. Designers and commissioning agents must be trained and licensed, and projects must be registered for the protection of the system, those involved in its installation, and public health and safety.

REFERENCES

1. Lentz, M.S. (2009, January). Regenerative Dual Duct: A case study. HPAC Engineering, pp. 20-26.

2. Carrier, W.H. (1937). The contact mixture analogy applied to heat transfer with mixtures of air and water vapor. Transactions, 59, 49-53.

For past HPAC Engineering feature articles, visit www.hpac.com.

The president of Lentz Engineering Associates Inc. (www.lentzengineering.com) and a member of HPAC Engineering's Editorial Advisory Board, Mark S. Lentz, PE, is nationally recognized for having successfully developed, tested, and proven several advanced HVAC-system strategies designed to exceed the performance requirements of ANSI/ASHRAE/IESNA Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, while meeting or exceeding the requirements of ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality. He is the recipient of numerous national engineering awards, including an American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Energy Award, and an ASHRAE Distinguished Service Award.

Evaporative Cooling and the Risk of Legionnaires' Disease

Evaporation is the “greenest” of all HVAC processes. It has the ability to alter psychrometric state points without expending new energy assets. It is 100-percent efficient and has zero carbon footprint. When properly applied, evaporative processes have the potential to save more energy and better enhance air quality than other green solutions, such as geothermal, solar, and wind power. Evaporation alone can close the thermodynamic loop on latent energy.

However, the use of evaporative cooling often elicits concern for Legionnaires' disease. Legionnaires' disease requires operating temperatures between 85°F and 125°F to flourish. Evaporative-cooling systems never approach these temperatures and, therefore, Legionnaire's disease has never been traced to this kind of equipment. Additionally, research has demonstrated that corrugated evaporative media not only do not aerosolize bio-aerosols into the air downstream of direct evaporative equipment, they actually remove particulate from air. An 8-in.-deep bed of media will remove particulate and many gaseous contaminants from an air stream, as well as approximately 70 percent of bioaerosols when it is irrigated with water. Therefore, it is not surprising to find microbes in sump water.

The real concern is the formation of bio-slimes, which can cause “swamp-cooler odor” when they die. The trick is to prevent them from setting up. ASHRAE Guideline 12, Minimizing the Risk of Legionellosis Associated With Building Water Systems, recommends the use of low-level photochemical ozone generators. These devices, in combination with appropriate purge and dump cycles, can control the level of microbes in a sump and prevent the scale formation in the media.