©2012 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 54, no. 3, March 2012.
By John Murphy, Member ASHRAE
About the Author
John Murphy is an applications engineer with Trane, a business of Ingersoll Rand in La Crosse, Wis.
With increased focus on reducing energy consumption in buildings, the use of exhaust-air energy recovery in HVAC systems is becoming more common, somewhat prompted by the requirements of ASHRAE Standards 90.1 and 189.1. Proper control of the energy-recovery device is critical to realize the expected energy savings. Although there are several other types of energy-recovery devices, and exhaust-air energy recovery is also applied in mixed-air systems, the focus of this article is on proper control of a total energy wheel (or enthalpy wheel) when applied in a dedicated outdoor air system (DOAS).
Exhaust-Air Energy Recovery
Exhaust-air energy recovery refers to the transfer of energy between the outdoor and exhaust airstreams. For the configuration discussed in this article, a total energy wheel is arranged to precondition the entering outdoor air (OA) by exchanging sensible heat and water vapor (latent heat) with the exhaust air (EA) stream (Figure 1).
During the cooling season, when it is hot and humid outside, the total energy wheel pre-cools and “pre-dries” (pre-dehumidifies) the outdoor air by transferring both sensible heat and water vapor to the cooler, drier exhaust airstream. During the heating season, when it is cold and dry outside, this same device pre-heats and pre-humidifies the outdoor air by removing both sensible heat and water vapor from the exhaust air and transferring it to the entering outdoor airstream.
Importance of Proper Wheel Control
Many proponents explain the benefits of exhaust-air energy recovery by focusing on the hottest and coldest days of the year. The more extreme the outdoor conditions, the greater the energy savings and the more that cooling and heating plants can be downsized.
However, during less-severe outdoor conditions, improper operation of the energy-recovery device can actually increase overall system energy use. Therefore, proper control of the device is critical for maximizing the energy-saving potential, while avoiding (or minimizing) energy waste.
Proper control of the energy-recovery device depends on whether sensible or total energy recovery is used and what type of HVAC system it is applied to. As mentioned previously, this article focuses on control of a total energy wheel when applied in a dedicated outdoor air system (DOAS) (as depicted in Figure 1).
Turn the wheel off to avoid transferring unwanted heat to the entering OA. For a total energy wheel, when the enthalpy of the outdoor air drops below the enthalpy of the exhaust air (hOA < hEA), the wheel provides no cooling energy recovery benefit. In fact, unless it is turned off, the wheel will actually increase the load on the cooling coil by increasing the dry-bulb temperature (by transferring sensible heat) and/or increasing the humidity ratio (by transferring water vapor) of the outdoor airstream.
At the example conditions depicted in Figure 2, the enthalpy of the outdoor air (hOA = 24.3 Btu/lb [56.5 kJ/kg]) is less than the enthalpy of the exhaust air (hEA = 28.2 Btu/lb [65.6 kJ/kg]). If the total energy wheel continues to operate at this condition, it increases the enthalpy of the air leaving the wheel (OA′) to 27.0 Btu/lb (62.8 kJ/kg), which increases the load on the cooling coil (Figure 2).
However, if the wheel is turned off when hOA < hEA, the enthalpy of the air entering the cooling coil is lower, avoiding this increase in coil load. For a 10,000 cfm (4700 L/s) dedicated OA unit, operating the wheel at this condition increases the cooling coil load from 14 tons (50 kW) with the wheel off, to 25 tons (88 kW) with the wheel on … an 80% increase!
In this configuration, consider adding bypass dampers (Figure 1) on one or both sides of the wheel. Opening these dampers when the wheel is turned off can reduce the airside pressure drop and minimize fan energy use. Note: In this operating mode, the wheel is typically cycled on for one or two minutes each hour to help keep it clean.
Citation: ASHRAE Journal, vol. 54, no. 3, March 2012
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