Air Conditioning and Refrigeration Center. College of Engineering. University of Illinois at Urbana-Champaign.
Abstract
Hydrocarbon refrigerants (HC's) are one alternative to hydrofluorocarbons (HFC???s) since they have zero
ozone depletion potential and negligible global warming potential. However, due to their flammable nature, the
amount of refrigerant used in systems is regulated for safety reasons.
This report presents simulation results for a 3-ton R290 (propane) air-conditioning system, and identifies
the optimum heat-exchanger geometries that would minimize system charge while trying to retain the same system
efficiency. An existing R410A microchannel system simulation served as the base case, and then the geometries
were optimized for the R290 system, and the results were compared to the base case. The model was then analyzed
for the off-design conditions, and the conclusions presented.
The optimal condenser geometry tended to have smaller port diameter and core depth with thicker webs
between the ports. Also, the fins tended to be taller, thinner and more densely packed. Similar results were noted for
the evaporator geometry. The optimal design reduced the combined heat exchanger charge by more than a factor of
5. The system efficiency was reduced by 3% in the process, but the loss could be recovered because the pressure
drop was low enough to permit increasing the air-flow rates.
The off-design behavior of the R290 microchannel system is very different from a traditional R410A
round-tube plate-fin system. Typically with the increase in ambient temperature, charge from the evaporator and the
liquid line moves to the condenser. In the R290 system, because of the oil/refrigerant solubility characteristics,
charge from the compressor sump also moves to the condenser.
In the microchannel systems, the heat exchangers account for only 20% of the system charge as opposed to
70% in the tube fin systems. At higher ambient temperatures, the additional charge flowing from the other
components, provides the condenser with the additional ~7% charge it needs at hot ambient conditions. However,
due to the small internal volume of the heat exchangers in microchannel systems, an additional 60% charge flows
into the condenser, resulting in high values of subcooling, thus reducing system efficiency. One solution to this
problem would be to install a receiver at the outlet from the condenser, to retain high levels of efficiency across a
wide range of operating conditions.Air Conditioning and Refrigeration Project 14
