Modern microchannel tube technology – widely used in the automotive industry for heat exchangers – offers an excellent opportunity to rethink the use of ammonia as a refrigerant, according to scientists at the University of Illinois.
In a recent feasibility study, UI researchers used a commercially available microchannel heat exchanger to create a 15-kilowatt refrigeration system with the smallest ammonia charge in the world. A similar technique could be used to significantly reduce the amount of ammonia in large chillers.
Ammonia is widely accepted as the most efficient and environmentally friendly refrigerant. But its unpleasant odour and mild toxicity have limited its use to industrial plants away from heavily populated areas.
To reduce risk and expand applications in urban areas, the amount of ammonia required to charge refrigeration and air conditioning systems must be substantially reduced.
‘New designs in microchannel heat exchangers allow much smaller refrigerant charges to be used than in conventional heat exchangers,’ said Pega Hrnjak, a UI professor of mechanical and industrial engineering and co-director of the university’s Air Conditioning and Refrigeration Centre. ‘Charges in these systems could be hundreds of times smaller than in conventional systems.’
To explore the feasibility of using air-cooled condensers with microchannel tubes and ammonia as the refrigerant, Hrnjak and graduate research assistant Andrew Litch constructed an experimental chiller facility.
The laboratory facility consisted of a re-circulating calorimetric wind tunnel, an ammonia chiller test bed and a pumped glycol heat-recovery loop.
The researchers then evaluated two similarly sized aluminium condensers: one with a parallel microchannel tube arrangement and the other with a single serpentine macrochannel tube.
Measurements of heat transfer, pressure drop and charge were taken over a variety of operating conditions.
‘The microchannel system significantly outperformed the conventional system,’ Hrnjak said. ‘The amount of refrigerant was reduced several times, while significantly increasing the heat transfer capability.’
Using microchannel tubes, Hrnjak and Litch are said to have successfully reduced the refrigerant charge to 2.5 ounces of ammonia per ton of evaporator capacity, which is considerably lower than the 12.5 ounces per ton used in current air-cooled ammonia chillers.
Further charge reduction would be possible through better design of the condenser headers and optimisation of the heat exchanger as a whole, said Hrnjak.
‘This is not only an excellent way to reduce refrigerant charge, but also to drastically reduce the condenser size,’ Hrnjak said. ‘Chillers could be reduced to nearly one-third their present size.’