Wrap-Around Heat Pipe for Enhanced Dehumidification and Energy Savings in a Multi-Story Condominium – Retrofit
Project Overview: Three, twenty story, luxury condominiums on the southeast coast of Florida. Each tower with its own rooftop 100% Dedicated Outside Air System (DOAS) Make-up Air Units (MUA). These units provide cool fresh air to the hallways and corridors of the buildings. The MUAs bring in 100% outside air, at approximately 17,000cfm. The air is cooled down to 55°F to remove the moisture from the air. Since the air conditioning load for the hallways is low (residents only pass through and the solar load is kept low through good architectural design), the air is then reheated to approximately 65°F to prevent the hallways from becoming too cool. This exiting systems reheat is provided by the direct expansion (DX) vapor compression system through a hot gas reheat coil. The roof top MUA units are AAON RN series units.
The Challenge: ACT was brought in to conduct an analysis on the MUAs to determine if ACT’s wrap-around heat pipe heat exchangers (WAHXs) could save operating costs by reducing the cooling load (tons of cooling savings) on the active cooling coil and supplying some of the reheat.
Local historical weather data was gathered and a BIN analysis was performed. The weather data was divided into dry-bulb temperature groups in 5°F intervals. ACT’s wrap- around heat pipe selection tool http://www.1-ACT.com/HVAC/WAHX was run at each of these temperature points to determine the tons of cooling savings and the reheat reduction. The wrap-around units utilized coils that were 3 rows deep at 12 fins per inch. The results were totaled for the year (sum of savings/hour multiplied by the number of hours per year at that condition). Those results, along with the local electricity rate and the conservatively estimated coefficient of performance (COP) of the AAON units, were used to determine the annual savings of approximately $7,000 per year. No savings credit was taken for the reheat because the units were utilizing compressor hot gas reheat. At this level of savings per year, the payback period for retrofitting ACT –WAHXs was on the order of three (3) years.
Project Details: The retrofitting process began by moving the DX evaporation coil inlet and outlet tubes (supply manifolds) and the hot gas reheat coil to make room for the additional WAHX coils. This work was done by a local commercial HVAC contractor hired by and paid by ACT as part of this job (it was a planned cost, not an additional unforeseen cost). ACT then installed the pre-cool and reheat coils into the units and cut openings in the wall of the AAON units for the WAHX connecting tubes to pass through. The connecting tubes were brazed and the heat pipes were evacuated and charged on site. A drain pan was added under the precool coil of the WAHX to collect any condensation that should occur on that coil during particularly humid times of the year. A new filter rack was also fabricated and installed in front of the pre-cool coil (another planned cost).
The entire job was managed by ACT with little disruption to the daily operation of the condominium. All three units were retrofit with WAHXs by ACT technicians. The job took approximately two weeks from start to finish with minimum downtime on the units.
The Condo Management Group also requested a means to verify the performance and savings. ACT provided an Energy Monitoring System (ACT-EMS) that calculates and records the daily, monthly and annual energy savings based on actual MUA operating conditions. This information can be viewed at the air handler or remotely via the web. The WAHX performance has met the original design goals and on warm, 90°F summer days, 10 degrees of pre-cooling is achieved by the ACT WAHX units.