Traditional AHU Designed for Dehumidification Duty. Air is subcooled to pull moisture from the air, then reheated using a new energy source to raise the supply air temperature and lower the supply air relative humidity.
Typical system design includes small cooling and reheat coils, high CHW flow rates, low CHW temperature differential and high AHU air pressure drops. 45°F CHW enters the cooling coil (5) at 70 GPM and leaves the cooling coil at 55°F. A new source of 140°F water enters the reheat coil (6) at 4 GPM and leaves the reheat coil at 87°F. The unit requires 479,319 BTU’s per hour to cool, dehumidify and reheat 10,000 CFM of air at the design conditions in this example
High Efficiency Dehumidification System (HEDS) AHU (53% Peak Day BTUH Savings) Similar to the traditional approach, except that 100% of the reheat energy is provided by the low quality heat captured in the cooling process, and the cooling load delivered to the chiller plant is reduced by exactly the amount of energy that was reclaimed in the reheat process.
Typical HEDS design incorporates very large cooling and cooling recovery coils, low CHW flow rates, high CHW temperature differential and low AHU air pressure drops. 45°F CHW enters the cooling coil (5) at 27 GPM and leaves the cooling coil at 70°F. This 70°F water then enters the CRC coil (6) at 27 GPM and leaves the CRC coil at 62°F while heating the air to 65°F. The HEDS unit requires 226,187 BTU’s per hour to cool, dehumidify and reheat 10,000 CFM of air at the same conditions, a BTUH savings of 53% and a CHW flow reduction of 62% in this example.
The figure above highlights the reduction in cooling load possible from a constant air volume HEDS system in a kitchen application during a high load day in the fall. The blue line represents the baseline cooling load of the unit if a typical dehumidification system with reheat had been used. The orange line represents the actual cooling load that was sent back to the chiller plant. The area between the two lines is the amount of the cooling load reduction, and also the amount of new reheat energy that is saved for supply air relative humidity and temperature control. HEDS works by capturing the low quality “waste heat” off of the large cooling coil and using that wasted energy to reheat the supply air instead of using another reheat source such as hot water or electricity, the figure shows that the cooling load is able to be reduced by more than 25%. The recovered energy completely eliminates the need for additional reheat energy for RH control and space comfort.
The figure above highlights the cooling load savings from HEDS compared to typical reheat AHU designs. During this period, savings from HEDS range from 25% to 35%, significantly reducing the energy use of the chilled water system while increasing available capacity.
ESPC's and UESC's financial benefits
HEDS is the only "low hanging fruit" that is left when it comes to energy efficiency projects. By combining relatively short payback period HEDS projects with larger, more capital intensive projects, the size of ESPC and UESC projects can be increased significantly.