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HVAC Cold Air Distribution

INTRODUCTION

HVAC Cold Air Distribution is an air-conditioning system type that yields greater cooling performance for a lower cost than conventional systems. Both equipment costs and energy costs are less for cold air systems. Humidity levels can be lower with cold air systems, resulting in improved air quality, comfort and productivity. The physical size of HVAC equipment is smaller, resulting in smaller mechanical rooms, less dead space above ceilings, and more room for occupants. Capital savings can pay for a thermal storage system, which will further reduce energy costs.

Cold air systems can do more for less. Nonetheless, HVAC designs based on cold air distribution are not as forgiving, so more attention to details is required in the design phase. This includes designing to prevent condensation on the cold surfaces of system components, and careful selection of ceiling diffusers to ensure adequate mixing of cold air with room air. Duct leakage is more of a concern than with conventional systems, and coil selection is based on parameters that are different from conventional designs. In short, cold air systems demand more from the designer and approach the limits of component capabilities. When properly applied, the benefits are lower capital costs, lower energy costs, and less humidity.

BACKGROUND

Cold air systems distribute air at a temperature of less than 50°F. This is lower than conventional designs, which distribute air at temperatures of 50-60°F, usually 55°F ±2°F. The conventional 55°F standard evolved from a need to prevent condensation on uninsulated duct components and diffusers, and from the desire to maximize coil temperatures in the days when chillers and compressors were far less efficient than current equipment. Over many years, it has become a standard design parameter upon which equipment selection specifications, streamlined design procedures, and rules-of-thumb are based.

The technology used by the HVAC industry is becoming more advanced. Three key developments are making cold air distribution the best option for many buildings: First, the efficiency of today’s best chillers and compressors is double those of 20 years ago. This means that reduced system temperatures no longer result in an overwhelming increase in energy use. It also means that fans and pumps are now a larger ‘pie slice’ of air-conditioning system energy use -- cold air systems use less energy because they require far less air and fluid flow than conventional systems. Second, the importance of air quality has become paramount. Larger quantities of outside air are being used. In humid climates this has resulted in excessive indoor humidity and fungal growth in many buildings. Cold air systems can be designed to control humidity without the expense of special equipment or the additional energy use of active dehumidification. Finally, computer advances have resulted in precise digital controls, and more accurate system design tools. These are needed because cold air systems are not as forgiving.

CASE STUDY: HEALTH SCIENCE CENTER

As an example, consider the application of Cold Air Distribution to a 72,000 square foot Health Science Center under construction on a Florida college campus. The HVAC system is based around a 225-ton rotary, water-cooled chiller (nominal 0.65 kW/Ton) and three central-station air-handling units. The building requires 14,650 CFM of outdoor air.

Capital Costs

Equipment costs for a carefully designed cold air system will be less for this application. Since the air is colder, more cooling can be provided per CFM (cubic feet per minute) so less airflow is needed. This means that air handlers, fans, ducts and chilled water pumps are smaller and less costly, and that they take up less space.

In theory, the colder the design air temperature, the less airflow is needed and the lower the capital costs, as shown in the chart. The coldest practical air temperature is about 38°F, with most cold air designs using 42-48°F. Lower temperatures are possible in special designs that deal with chilled water temperatures approaching the freezing point, the limitations of currently available coils designed for 55°F operation, and more critical duct leakage and condensation concerns. For the Health Science Center, a comparison was made between a conventional 55°F system and a 42°F cold air distribution system. As summarized in Table 1, the cold air distribution system provides an 11% savings on equipment costs. Costs for the air handling units, ductwork, pumps and piping are reduced by $70,945. Increased controls complexity, diffusers, and more detailed engineering added $17,144 for a net savings of $53,801.

Energy Costs

Electric use by the smaller air handler fans and chilled water pumps of the 42°F design will be less. This savings more than offsets the small increase in chiller electric use due to the colder water temperature. As shown in the chart below, minimum cost is achieved at around 42°F supply air temperature. . (The minimum cost for other buildings may be at a different temperature.) Compared with a 55°F system, annual fan electric costs are $15,000 less, and pump electric costs are reduced by $2,180. Chiller electric costs increase by just $3,750, giving a predicted net savings of $13,485 per year, a 24% savings compared with a 55°F system.

The 55°F design option included fan terminal units with reheat coils. Some amount of reheat would have been called for to keep humidity below 60% rh with the 55°F system. Since reheat is not a part of the cold air design, reheat savings could be as much as $6,000 per year additional. Total capital plus energy life-cycle (20-years) savings will be at least $340,000, or $4.70 per square foot with the cold air system. So not only is the cold air system less expensive to install, it also costs less to operate.

HUMIDITY CONTROL

Larger quantities of outside air are being used in HVAC system designs to improve air quality and to comply with ASHRAE Standard 62-1989. In humid climates, increases in outside airflow have resulted in excessive indoor humidity and fungal growth in many buildings. Cold air distribution systems can be designed to control humidity without the expense of special equipment or the additional energy use of active dehumidification

Cold air systems are better able to maintain indoor relative humidity below 60% rh, even with outside air flow rates approaching 70% of the supply air flow. With any system, space relative humidity increases as the fraction of outside air increases (as it can by design and/or during the normal operation of variable air volume (VAV) systems). Nevertheless, the humidity with cold air distribution is always less.

Relative humidity will probably exceed the ASHRAE recommended maximum of 60% rh when outside air fraction exceeds 20% with conventional 55°F systems. Furthermore, humidity usually increases at part load conditions, for example, in the morning and evening when the temperature is in the mid 70’s to low 80’s. Properly optimized cold air systems can keep humidity below 60% rh, even with large fractions of outside air at part load conditions, and in many cases without the use of costly reheat.

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