Session: 12-04-01: Heat and Mass Transfer in the Natural and Built Environments

Paper Number: 170573

Thermal Hydraulic Performance Analysis of Different Heat Exchanger Geometries for Moisture Control in Buildings 

<Motivation> Moisture management in buildings is critical to ensure occupants’ thermal comfort and prevent sick building syndrome. According to ASHRAE handbook, optimal indoor humidity should be maintained around 30%–60%. Due to the increasing demand for thermal comfort in built environments, by 2050 the energy consumption for latent load (humidity) management is projected surpass the energy consumption for sensible load (temperature) management. To accommodate this additional latent load management demand, retrofitting of existing HVAC systems is necessary. In a conventional air-conditioning system air is dehumidified below the dew point. Thus, dehumidified air needs to be reheated, leading to large amount of energy waste. In contrast, in a desiccant-based system outdoor air is dehumidified by adsorbing water vapor into a porous medium at a temperature much higher than the air dewpoint. Thus, desiccant system eliminates the need for below-dewpoint cooling and reheating, leading to energy savings. Hence, advancing desiccant technology for energy-efficient moisture management in the buildings is crucial to ensure the national energy security.

<Contribution towards advancing science> A major challenge that remains unsolved in current desiccant technology is to obtain isothermal dehumidification to suppress the supply air temperature rise by the release of enthalpy of water vapor adsorption. Usually, a metal heat exchanger is used as substrate on which porous adsorbents are coated using polymer binders, also known as desiccant coated heat exchanger. The most used heat exchanger type is a fin-tube heat exchanger, in which fins are coated with porous adsorbents. Humid outdoor air flows over the fins and chilled water flows through the tubes to remove the enthalpy of water vapor adsorption. However, in a fin-tube assembly, heat exchange from hot porous desiccants under adsorption to chilled water is limited as the heat transfer surface area with the air side is larger. Therefore, other designs of heat exchangers having large heat transfer surface area with the water side should be investigated. To minimize this gap, this study evaluates the thermal hydraulic performance of several un-coated heat exchanger types including fin-tube, metal foam, microchannel and a newly proposed multilayer spiral-coil heat exchanger intended to use for moisture management in buildings.

<Methodology, Results and Conclusion> A thermal hydraulic test loop is built at Oak Ridge National Laboratory, TN, USA to experimentally assess the heat transfer and pressure drop characteristics of different heat exchangers as mentioned above prior to coating desiccants on them. The test loop consists of a wind tunnel, a heater, a chiller, a fan, several sensors for humidity, temperature and pressure measurement, and mass flow meters to measure volumetric flowrates of air and water. A Campbell scientific data logger is used to record the measured data from the sensors. The test loop is placed inside an environmental chamber to supply humid air at desired temperatures and humidity levels. The thermal hydraulic performance evaluation of the prototype heat exchangers is currently ongoing. The initial results reveal that heat transfer area with the water side is the rate limiting step to obtain isothermal dehumidification by enhancing heat exchange between chilled water and hot desiccant.  More details on the heat transfer and pressure drop at different chilled water flowrates, chilled water and humid air inlet temperatures will be reported later upon completion of the experimental runs.

Presenting Author: Kashif Nawaz Oak Ridge National Laboratory

Presenting Author Biography: Dr. Kashif Nawaz is a distinguished leader in the field of energy conversion science and technology. He earned his BSc in Mechanical Engineering from Ghulam Ishaq Khan Institute of Technology, Pakistan, in 2007, followed by an MSc and PhD in Mechanical Engineering from the University of Illinois at Urbana-Champaign (UIUC) in 2010 and 2013, respectively.

Upon completing his PhD, Dr. Nawaz joined the Heat Transfer Center of Excellence at Johnson Controls (JCI), where he served as a Senior Heat Transfer Engineer until 2016. Concurrently, he held an adjunct faculty position in the Department of Mechanical and Aerospace Engineering at the University of Oklahoma. In 2016, Dr. Nawaz transitioned to Oak Ridge National Laboratory (ORNL) as a senior scientist in the Building Equipment Research Group, while also holding a joint appointment at the Bredesen Center, University of Tennessee, Knoxville. He currently serves as the sub-program manager for the cross-sector technologies area within the Industrial Efficiency and Decarbonization Office (IEDO).

Dr. Nawaz has an extensive publication record, having authored or co-authored over 250 peer-reviewed publications, including journal articles, conference papers, and technical reports. His innovative work is reflected in more than 20 invention disclosures and seven patents in heat exchanger design and energy conversion processes.

Recognized by his peers, Dr. Nawaz is a Fellow of both the American Society of Mechanical Engineers (ASME) and the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE). His accolades include ASHRAE's Exceptional Service Award (2021) and Distinguished Service Award (2018), the JCI Excellence in Innovation Award (2016), and the UIUC Alumni Teaching Fellow Award (2011, 2012). Additionally, he is a four-time recipient of the prestigious R&D100 Awards (2021, 2022, twice in 2024).

Dr. Nawaz is also a respected reviewer for leading academic journals such as International Journal of Heat and Mass Transfer, International Journal of Refrigeration, Energy, Applied Energy, Applied Thermal Engineering, Carbon, Applied Physics Letters, and Langmuir.

Authors:

Jubair Shamim Oak Ridge National Laboratory
Easwaran Krishnan Oak Ridge National Laboratory
Kai Li Oak Ridge National Laboratory
M Muneeshwaran Oak Ridge National Laboratory
Kashif Nawaz Oak Ridge National Laboratory