Energy Recovery: Rotary Heat Exchanger. Rotary Heat Exchanger (Enthalpy Wheel) Schematics

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1 Energy Recovery: Rotary Heat Exchanger Adding a rotary heat exchanger to both air handling units would decrease the energy consumption of each resulting in lower operating costs. The proceeding in depth analysis describes how heat exchangers work and would be incorporated into the HVAC system of the von Liebig Center for Science. Rotary Heat Exchanger (Enthalpy Wheel) Schematics Rotary heat exchangers revolve in a plane perpendicular to the airflow and work off of exhausted air on the principle of sensible and latent energy transfer. Harnessing this wasted energy by means of an aluminum wheel and desiccant material helps cool and heat the flow of air into the building. The Science Center currently supplies its spaces with outdoor air as well as return air from various spaces. Many of the spaces are labs in which air must be exhausted and cannot be directly put back into the system. The proposed heat exchanger will work off of this exhausted air. The diagrams below help depict how the process works. 1

2 The system recovers both sensible and latent energy from the exhaust air. Sensible energy is captured on the aluminum wheel and as the wheel turns, it transfers this energy to the outside air stream coming into the building. The latent energy is captured in a similar manner. The desiccant absorbs moisture from a stream with high vapor pressure and desorbs it to the lower vapor pressure stream. 2

3 Psychometrics The psychometric chart below represents the conditions in which the air will be cooled or heated. Cooling Heating 3

4 Sizing There are two main air handler units. Both of these units exhaust a different CFM and will use different heat exchanger models. AHU-1 35,375 CFM AHU-2 26,825 CFM Rotor is constructed of corrugated aluminum media bonded with a synthesized, Non-migrating silica gel desiccant permanently bonded to the matrix. 4

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6 Cooling/Dehumidifying Analysis Design conditions were based off of 75F DB/62.5F WB and 64.9Gr/lb. Weather data was taken from Bin Maker Plus using Pittsburgh as the nearest data. Hours of operation were assumed to be 7am-6pm. For cooling, the heat exchanger will only work if the outside WB temperature is greater than the inside WB temperature. HOURS DB (ºF) W (Gr/lb) DB (ºF) W (Gr/lb) WB (F) COOLING/DEHUMIDIFICATION AHU 1 (BTU/h) (BTU/h) (TON) (TON) (TON HRS) (TON HRS) (kwh) (kwh) COOLING $1, HOURS DB (ºF) W (Gr/lb) W DB (ºF) (Gr/lb) WB (F) COOLING/DEHUMIDIFICATION AHU 2 (BTU/h) (BTU/h) (TON) (TON) (TON HRS) (TON HRS) (kwh) (kwh) COOLING $1, By using a heat exchanger, the latent and sensible savings for AHU-1 is 141 tons. This means the chiller can be reduced by this amount. AHU-2 had a 134 ton savings. I would like to thank Adam Warriner for helping me set up the previous charts. 6

7 Heating/Humidifying Analysis Design conditions were based off of 75F DB/62.5F WB and 64.9Gr/lb. Weather data was taken from Bin Maker Plus using Pittsburgh as the nearest data. HOURS DB (ºF) W (Gr/lb) W DB (ºF) (Gr/lb) HEATING/HUMIDIFYING AHU 1 (BTU/h) (GRAINS) (TON) (lbs) (TON HRS) (BTU/h) (kwh) (kwh) HEATING $3, HOURS DB (ºF) W (Gr/lb) W DB (ºF) (Gr/lb) HEATING/HUMIDIFYING AHU 2 (BTU/h) (GRAINS) (TON) (lbs) (TON HRS) (BTU/h) (kwh) (kwh) HEATING $2, By using a heat exchanger, sensible savings for AHU-1 is 1,546,538.4 BTU/h AHU-2 had a 1,172,746.1 BTU/h savings. Total Cost Savings $ $ $ $ $