ASHRAE Journal - March 2012 - 58

energy use for this pod of classrooms.) This difference is primarily due to the fact that Chicago and Minneapolis experience more hours when outdoor enthalpy is low, which is when proper wheel control is important. For this example, 82% of this difference in cooling energy use in Atlanta is due to operating the wheel when it should be off (when hOA < hEA), and 18% of this difference is due to operating the wheel at full heating capacity when capacity should have been modulated. For Chicago, 64% is due to operating the wheel when it should be off, and 36% is due to operating the wheel at full capacity, rather than modulated.

Correct
510 cfm 69.0°F 56.0 gr/lb 510 cfm 57.7°F 51.0% 25.3 B/lb 38.0 lb/min 50.3°F

0 mbh

0 Tons

0 mbh

630 cfm

Dry Bulb: HumRat: Airflow: Wet Bulb: RelHum: Enthalpy: Massflw: Dew Point:

46.8°F 45.5 gr/lb 630 cfm 45.7°F 92.4% 18.3 B/lb 46.9 lb/min 44.7°F

46.8°F 45.5 gr/lb 630 cfm 45.7°F 92.4% 18.3 B/lb 46.9 lb/min 44.7°F

50.0°F 47.0 gr/lb 630 cfm 47.6°F 84.5% 19.3 B/lb 46.9 lb/min 45.6°F

50.0°F 47.0 gr/lb 630 cfm 47.6°F 84.5% 19.3 B/lb 46.9 lb/min 45.6°F

50.0°F 47.0 gr/lb 630 cfm 47.6°F 84.5% 19.3 B/lb 46.9 lb/min 45.6°F

Incorrect
510 cfm 69.0°F 61.6 gr/lb 510 cfm 59.0°F 56.0% 26.2 B/lb 38.0 lb/min 52.9°F

Summary

DOAS Cooling Energy Use, Percent of No TEW Control

Achieving maximum energy savings and minimizing the payback period for exhaust-air energy recovery depends on proper control of the energy-recovery device. On the hottest and coldest days of the year, energy savings can be sig0.78 Tons 0 mbh 0 mbh 630 cfm nificant. But, during less severe outdoor conditions, improper operation of the 46.8°F Dry Bulb: 46.8°F 60.1°F 50.0°F 50.0°F 45.5 gr/lb HumRat: 45.5 gr/lb 55.2 gr/lb 49.4 gr/lb 49.4 gr/lb energy-recovery device may actually in630 cfm Airflow: 630 cfm 630 cfm 630 cfm 630 cfm 45.7°F Wet Bulb: 45.7°F 54.0°F 48.3°F 48.3°F crease system energy use. 92.4% RelHum: 92.4% 68.6% 88.8% 88.8% 18.3 B/lb Enthalpy: 18.3 B/lb 23.0 B/lb 19.6 B/lb 19.6 B/lb Although this article focused on con46.9 lb/min Massflw: 46.9 lb/min 46.9 lb/min 46.9 lb/min 46.9 lb/min 44.7°F Dew Point: 44.7°F 49.9°F 46.6°F 46.6°F trol of a single, total energy wheel in a dedicated OA system, understand that Figure 10: Modeling modulating capacity control on a cool day. Correct (top): When it is the control strategy may differ depend- cool outside, wheel capacity should be modulated to avoid overheating, and it is. Wheel ing on the equipment configuration, ap- recovers only enough heat to warm the OA to 50°F (10°C) (winter setpoint). Incorrect (botplication, and climate.1,2,4 tom): When it is cool outside, wheel capacity should be modulated to avoid over-heating, Finally, energy simulation software but instead the wheel is operating at full capacity. The result is recovery of too much heat, must be capable of modeling proper warming the air above 50°F (10°C) (winter setpoint), which may require re-cooling. control of the energy-recovery device to accurately predict the energy-saving benefits. Since the deciHouston Atlanta Chicago Minneapolis sion to invest in exhaust-air energy recovery is often based on 100 financial return, underestimating energy savings may result in underuse of this technology.
90

References
1. Mumma, S. 2001. “Dedicated outdoor air-dual wheel system control requirements.” ASHRAE Transactions 107(1). 2. Murphy, J., B. Bradley. 2009. Air-to-Air Energy Recovery in HVAC Systems. La Crosse, Wis.: Trane. 3. GTI. 2011. BinMaker® Pro, Version 3.0. Des Plaines, Ill.: Gas Technology Institute. 4. Mumma, S. 2005. “Tempering cold outdoor air.” ASHRAE IAQ Applications, Summer 2005. 5. Mumma, S. 2007. “DOAS and desiccants.” Engineered Systems 24(8):37 – 49. 6. Trane. 2011. TRACE® 700, Version 6.2.6.5. La Crosse, Wis.: Trane. 58 ASHRAE Journal

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60 No TEW Control TEW Control

Figure 11: Annual DOAS cooling energy impact of proper total-energy wheel control.
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ASHRAE Journal - March 2012

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