Miniaturization of Heat and Mass Transfer Devices

Miniaturization of Heat and Mass Transfer Devices
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Background

Absorption space-conditioning systems are environmentally friendly; because no CFCs or HCFCs are used and it could be powered by either of waste heat, steam or gas. Because it can be operated by using natural gas as the main energy input and use less electrical energy per unit of cooling than vapor compression systems, there is the ability to shave the summer peak electricity demand and to fill the valley for natural gas demand. Currently absorption systems are used for large industrial and commercial applications. The use of absorption systems in residential applications has been hindered by the lack of inexpensive, compact refrigerant-absorbent heat and mass exchangers.

Motivation

Understandings of the heat and mass transfer phenomenon in the absorber are required, since the absorber is the one of key components to improve the system performance. It is required to develop miniaturized heat and mass exchangers for use in residential sized absorption cooling systems.

Past Work

A miniaturization technology for absorption heat pump components has been developed and patented. A lattice of microchannels is used to accomplish heat and mass transfer between ammonia-water and coupling fluid streams. In an absorber application, dilute solution flowing due to gravity over the microchannel array absorbs ammonia vapor flowing up due to buoyancy. The heat of absorption is removed by coolant flowing through the microchannels. In a desorption application, hot fluid flowing through the tubes desorbs ammonia vapor from a concentrated solution. Solution, vapor and coolant are all in a counterflow arrangement, facilitating heat and mass transfer across the applicable temperature and concentration gradients. Excellent mixing and species redistribution is achieved due to the flow over the microchannel array. Tube-side thermal resistances are extremely low due to the small hydraulic diameter. An absorber for a 10.55 kW (3 ton) residential heat pump was fabricated using a bank of 1.575 mm OD x 140 mm long tubes, resulting in an extremely compact 178 mm x 178 mm by 0.508 m tall component. Experiments covering a wide range of solution and coolant flow rates and vapor fractions demonstrate that high transfer rates are achieved with no surface treatment and with low solution and coolant pressure drops.

An absorber with optical access was also fabricated and tested to understand the flow mechanisms and prompted improvements to the solution distribution mechanism. The original absorber with an improved distributor was then demonstrated as a desorber achieving heat duties of 17.5 kW. Scaled up prototype absorbers with visual access were subsequently fabricated and tested, and transferred up to 21 kW with overall heat transfer coefficients ranging from 540 to 1160 W/m²-K and solution side heat transfer coefficients ranging from 800 to 2900 W/m²-K. A different compact shell-and-tube heat exchanger, also using small diameter tubes, was experimentally investigated as an absorber, and was shown to be capable of heat duties of up to 19.8 kW. Together, these studies show that these compact, modular and versatile microchannel geometries, capable of mass production, are suitable for all components in an absorption heat pump, and could enable the increased use of this environmentally benign technology in the small capacity heat pump market. Similar components derived from this technology could also readily be implemented in energy intensive industries such as chemical processing, paper and pulp processing, biorefineries, and other applications.

Future Work

Micro-sized channel effects on the absorber will be compared with the absorber consisted of the conventional tube bundles (3/8in).

Detailed analysis on the heat and mass transfer for the binary mixture in the absorber is required.

The absorber is required to be tested in realistic operating conditions within entire absorption system.

It is required to build the residential size absorption system and to test the stability and operating performance.

Sponsors

ARTI, NSF, ASHRAE, IEC

Papers

1. Determan, M.D. and S. Garimella. Ammonia-Water Desorption Heat and Mass Transfer in Microchannel Devices. International Sorption Heat Pump Conference. 2005.

2. Meacham, J.M. and S. Garimella, Ammonia-Water Absorption Heat and Mass Transfer in Microchannel Absorbers with Visual Confirmation. ASHRAE Transactions, 2004. 110(1): p. 525-532.

3. Determan, M.D., S. Garimella, and S. Lee. Experimental Demonstration of a Microchannel Desorber for Ammonia-Water Heat Pumps. Seventeenth National Heat and Mass Transfer Conference and Sixth ISHMT/ASME Heat and Mass Transfer Conference. 2004. Kalpakkam, India.

4. Meacham, J.M. and S. Garimella, Modeling of Local Measured Heat and Mass Transfer Variations in a Microchannel Ammonia-Water Absorber. ASHRAE Transactions, 2003. 109(1): p. 412-422.

5. Meacham, J.M. and S. Garimella. Experimental Demonstration of a Prototype Microchannel Absorber for Space-Conditioning Systems. International Sorption Heat Pump Conference. 2002. Shanghai, China.

6. Meacham, J.M. and S. Garimella. Miniaturized Shell-and-Tube Heat and Mass Exchangers for Absorption Heat Pumps. 12th International Heat Transfer Conference. 2002. Grenoble, France.

7. Garimella, S., Microchannel Components for Absorption Space-conditioning Systems. ASHRAE Transactions, 2000. 106(1): p. 453-462.

8. Garimella, S. Miniaturized Heat and Mass Transfer Technology for Absorption Heat Pumps. Proceedings of the International Sorption Heat Pump Conference. 1999. Munich, Germany.

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