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Heating of supply air and drive energy to fans and pumps predominate the energy use of HVAC systems in non-residential buildings in cold, dry climates. Hence heat recovery and the effect on fan energy is of interest whereas recovery of cooling and moisture is less of an issue. This study looks at dedicated outdoor air systems with a run-around loop heat recovery or an enthalpy wheel heat recovery. In nonresidential buildings heat recovery with full capacity is used only during a part of the year but it still causes a pressure drop in the air handling unit all year round. In this situation the most energy efficient system is not always the one with the highest temperature efficiency if one considers the penalty in fan energy. If the value of electricity, compared to the value of heat, is taken into account, the temperature efficiency might be even less important. In this case-study the enthalpy wheel with the lowest temperature efficiency was the most energy efficient choice on a system level. The same applies for the run-around loop systems, where the run-around loop with the lower temperature efficiency gave the better result.

In the case of run-around loop systems there are different strategies for capacity control of the liquid flow to obtain optimal operation. Common alternatives are constant flow with variable temperature by means of a shunt group and variable flow by means of two-way valve. A more energy efficient way is direct flow control of a variable speed pump. In direct flow control a much smaller pump power is used compared to valve control. Shunt groups, in particular, result in pump power that is constant and unaffected by the real demand. Calculations based on measurements on a run-around loop system with direct flow control during three months show that the pump power after optimizing the flow can be reduced by 80 %.

Citation: ASHRAE Conference Papers, 2010, vol. 116, pt. 2, Albuquerque, NM