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ANSI/ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy, suggests that if the difference in temperature from ankle to head height is greater than 5.4°F, people will feel thermal discomfort. Although the supply air of a TDV system is not as cold as that of a standard mixed system, it still is 7°F to 10°F cooler than design temperature. This means certain areas of a space have a vertical temperature gradient greater than the recommended 5.4°F. Students should not be sitting in the areas shown in Figure 1.

Figure 2 shows the results of a CFD simulation in which a bookcase was placed 3½ ft from a terminal. The air was directed over the furniture, forcing it to flow farther into the room. This increased the chance that more students would feel a draft or discomfort because of a vertical temperature gradient. A new guideline instructed teachers to avoid placing objects of significant size, such as bookcases, within 4 ft of a diffuser.

RETURNING HIGH-LEVEL AIR

Returning high-level air can be more energy efficient because it often takes less energy to condition return air than outside air. The schools' design team performed a simulation to assess what CO₂ levels would be like in scenarios using 100-percent, 66-percent, or minimum outside air (figures 3, 4, and 5). The outside air was assumed to have CO₂ levels of 400 ppm. The American Society of Heating, Refrigerating and Air-Conditioning Engineers recommends that inside air not have CO₂ levels of 750 ppm above outside air. The simulation results showed that 100-percent outside air is excellent, 66-percent outside air is acceptable, and minimum outside air is unacceptable. Therefore, maximum CO₂ levels should be less than 1,050 ppm in school buildings.

USING HEATING WITH TDV

Heating can be a design challenge when utilizing TDV. In heating mode, supply air is much warmer than room air. Therefore, supply air rises and collects at a high level, where it is exhausted before it can provide heating to the occupied level.

In Southern California, however, little heating is required, usually only during the first hour or two of occupancy. That means it is possible to provide enough heating to an occupied classroom by supplying air that is only 2°F higher than the required room-air temperature, provided the space is preheated sufficiently. Preheating can be achieved by recirculating warm air through the space via a diffuser. One diffuser increases supply velocity, allowing adequate mixing, while two diffusers are needed when a room is occupied to keep air speed within comfortable levels. Figure 6 shows what happens when 70°F air is supplied to an occupied classroom via two diffusers. On a cold day, this continues for the first hour or two, after which cooling is required.

An alternative method can be used in cooler climates if the method shown in Figure 6 is insufficient to achieve occupant comfort. A separate diffuser significantly smaller than the one used in cooling mode is added to the bottom of the terminal. When the system is in heating mode, air is directed through the small diffuser at a much higher velocity than when the system is in cooling mode. This allows sufficient mixing before the warm air rises.

Based on these simulation results, AECOM consultants developed guidelines for use of TDV that could be used in Southern California school districts. AECOM also carried out full designs of TDV schemes in a few pilot schools. The simulation results demonstrated that TDV can provide comfortable classrooms at energy-consumption levels considerably lower than those of conventional air-conditioning systems. With guidelines in place, competitive bidding will be used to select contractors to install TDV in other schools.

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Information and images courtesy of Mentor Graphics (Mechanical Analysis Division) (formerly Flomerics).
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