ASHRAE Journal - March 2012 - 57

times these devices are not purchased Correct because the predicted energy savings does 510 cfm not result in an acceptable financial return. One reason may be that capabilities 70.4°F 50.8 gr/lb of energy simulation software to mod510 cfm 57.0°F el this strategy differ significantly. It is 44.2% 24.8 B/lb important to verify that control of the 38.0 lb/min 47.7°F energy-recovery device is being modeled properly to accurately predict the energy-saving benefits. 0 mbh 0 mbh 0.33 Tons Figures 9 and 10 depict the simulat630 cfm ed operation of a total energy wheel 55.8°F Dry Bulb: 55.8°F 55.8°F 50.0°F 50.0°F in a dedicated OA system that serves 34.3 gr/lb HumRat: 34.3 gr/lb 34.3 gr/lb 34.2 gr/lb 34.2 gr/lb 630 cfm Airflow: 630 cfm 630 cfm 630 cfm 630 cfm a single elementary school class46.6°F Wet Bulb: 46.6°F 46.6°F 43.9°F 43.9°F 50.2% RelHum: 50.2% 50.2% 61.8% 61.8% room, according to one commonly 18.7 B/lb Enthalpy: 18.7 B/lb 18.7 B/lb 17.3 B/lb 17.3 B/lb 46.9 lb/min Massflw: 46.9 lb/min 46.9 lb/min 46.9 lb/min 46.9 lb/min 6 used energy simulation program. 37.6°F Dew Point: 37.6°F 37.6°F 37.5°F 37.5°F This particular program is capable of modeling proper wheel control, but Incorrect the user has the ability to change the 510 cfm default settings to model improper 70.4°F 59.7 gr/lb wheel control. 510 cfm 59.1°F Figure 9 displays the simulation re51.9% 26.2 B/lb sults on a mild day, when outdoor con38.0 lb/min 52.1°F ditions are 56°F (13°C) dry bulb and 38°F (3°C) dew point. At this hour, the enthalpy of the outdoor air is 18.7 Btu/lb (43.5 kJ/kg), which is less than the en0.96 Tons 0 mbh 0 mbh 630 cfm thalpy of the exhaust airstream. The top half of Figure 9 depicts 55.8°F Dry Bulb: 55.8°F 65.2°F 50.0°F 50.0°F 34.3 gr/lb HumRat: 34.3 gr/lb 49.4 gr/lb 46.9 gr/lb 46.9 gr/lb 630 cfm Airflow: 630 cfm system operation where the wheel is 630 cfm 630 cfm 630 cfm 46.6°F Wet Bulb: 46.6°F 54.7°F 47.6°F 47.6°F 50.2% RelHum: 50.2% 51.5% 84.5% 84.5% turned off to avoid increasing the en18.7 B/lb Enthalpy: 18.7 B/lb 23.3 B/lb 19.3 B/lb 19.3 B/lb 46.9 lb/min Massflw: 46.9 lb/min 46.9 lb/min 46.9 lb/min 46.9 lb/min thalpy of the entering outdoor air. The 37.6°F Dew Point: 37.6°F 47.0°F 45.6°F 45.6°F bottom half of this figure depicts operation where the wheel is not controlled Figure 9: Modeling on/off total energy wheel control on a mild day. Correct (top): properly, and remains turned on. If the When hOA < hEA the wheel should be off, and it is off. Incorrect (bottom): When wheel continues to operate, it increas- hOA < hEA the wheel should be off, but it is on. This increases enthalpy of the air es the enthalpy of the air leaving the entering the cooling coil, which increases the coil load. wheel—to 23.3 Btu/lb (54.2 kJ/kg) at this example condition. This increases the load on the cool- ly, this may or may not require re-cooling to the desired 50°F ing coil. (10°C) setpoint. Figure 10 displays the simulation results on a cooler day, The annual energy impact of proper total energy wheel when outdoor conditions are 47°F (8°C) dry bulb and 45°F control for a specific building depends on climate, build(7°C) dew point. When it is cool outside, the wheel could op- ing use, and system design. Figure 11 depicts the impact erate to preheat the entering outdoor air, but wheel capacity of proper wheel control on cooling energy used by the may need to be modulated to avoid over-heating or over-hu- dedicated OA equipment serving a pod of K-12 classmidifying the air. rooms. For this example analysis, the dedicated OA sysThe top half of the Figure 10 system depicts operation with tem was modeled to deliver the conditioned OA at 50°F modulated wheel capacity. The exhaust-side bypass dampers (10°C) dry bulb year-round (12 months of occupancy). modulate so the wheel recovers only enough heat to warm the Proper control of the total energy wheel—turning off the OA to the desired leaving-air temperature, which is a 50°F wheel when hOA < hEA and modulating an exhaust-side by(10°C) winter setpoint for this example. pass damper to prevent over-heating—reduced the cooling The bottom half of this figure depicts operation without energy used by the dedicated OA equipment by 5% to 10% wheel capacity modulation. If the wheel continues to operate at in the hotter climates of Houston and Atlanta, and by 15% full heating capacity, it over-heats the air leaving the wheel—to to 20% in the milder climates of Chicago and Minneapolis. 60°F (16°C) at this example condition. As discussed previous- (This equates to about 2% to 3% reduction in the overall
March 2012 ASHRAE Journal 57

ASHRAE Journal - March 2012

Table of Contents for the Digital Edition of ASHRAE Journal - March 2012