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Performance
Temperatures and cooling capacities are presented in Table 1 for three DOAS delivering 1,000 cfm outdoor air. The first row, "48˚F Coil," is a benchmark cooling coil presented for comparison. The second row, "DX RaR," is the DX runaround system. The bottom row is the heat-pipe chilled-water system. All three have been selected to meet the design leaving dew point of 48.6˚F for the classroom example; therefore, all three provide essentially equal latent-cooling capacity. Because a cooling coil alone overcools, it requires 8,472 Btuh of reheat. The DX RaR and heat-pipe systems deliver supply air at the desired 56.6˚F temperature and 48.6˚F dew point and do not require additional reheat energy.

Table 2 compares energy usage of the three 1,000-cfm DOAS systems. Because the cooling coil alone provides excess cooling capacity, it consumes more electrical energy and requires heat energy to cancel out the excess sensible capacity. The benchmark energy efficiency before new reheat is 10.1 Btuh per watt, and total efficiency, including reheat energy, is 6.5 Btuh per watt. The DX RaR and heat-pipe systems deliver just the right amount of sensible and latent cooling to meet the space load, which maximizes energy efficiency at 11.6 and 11.8 Btuh per watt respectively (15-percent and 17-percent improvement in energy efficiency compared with recovered reheat).

Conclusion
Engineers can achieve equal or better performance—without the complexities and cost of a reheat coil and controls—using DOAS equipped with heat-pipe coils and/or using DX DOAS based on refrigerant runaround and coil bypass. Because of their inherent simplicity, they have a lower first cost than comparable units relying on enthalpy wheels and/or hot-gas or liquid reheat coils and controls. Little or no excess cooling capacity and avoidance of reheat translates directly into significant energy savings relative to a cooling coil alone. DOAS like these are similar to the mixed-air units that engineers are comfortable specifying and hopefully will encourage engineers to sustain the long-predicted paradigm shift to DOAS.

References

1. Mumma, S., & Shank, K. (2001). Selecting the supply air conditions for a dedicated outdoor air system working in parallel with distributed sensible cooling terminal equipment. ASHRAE Transactions, 107, 562-571.
2. Brooke, T. (2010, January). Wrap around heat pipes in DOAS units. Paper presented at the ASHRAE Winter Conference, Orlando, FL.

Did you find this article useful? Send comments and suggestions to Associate Editor Megan Spencer at megan.spencer@penton.com.

A member of HPAC Engineering's Editorial Advisory Board, Michael West, PhD, PE, is the principal building-systems scientist for Advantek Consulting Inc. He is responsible for the development and testing of new HVAC technologies, as well as the engineering of energy-efficiency and indoor-environmental-quality projects throughout the United States and the Caribbean. A recipient of the Association of Energy Engineers' (AEE’s) International Project of the Year award, he has extensive experience applying renewable sources of energy. He is an active member of AEE and the American Society of Heating, Refrigerating and Air-Conditioning Engineers. For patent information on the technologies referenced in this article, contact the author via e-mail at mwest@advantekinc.com.

Resources
For additional information on this topic, check out:

Bradley, B., & Stanke, D. Dedicated ventilation systems. (n.d.) Retrived from www.trane.com/commercial/uploads/pdf/673/enews_30_03_090601.pdf

Brooke, T. (2007). Neutral air units with heat pipes in chilled water systems. Gainesville, FL: Heat Pipe Technology Inc.

Mumma, S. DOAS supply air conditions. (2008). Retrived from doas.psu.edu/IAQ_DOAS_SA_Cond_Sp_08.pdf

Murphy, J. (2006, July). Smart dedicated outdoor air systems. ASHRAE Journal, pp. 30-37.

Desert Aire. (2005). 100% outdoor air dehumidification methods. Milwaukee, WI: Desert Aire.