|Title:||Detection of Evaporator Frost
|Author(s):||Aviles, E.A.; Miller, N.R.; Newell, T.A.
|Abstract:||Frost growth is a major concern in many air conditioning and refrigeration systems. Frost buildup causes an
increase in the airside pressure drop and thermal resistance of the evaporator, yielding a substantial decrease in
operational efficiency. To date, methods for reliable detection of frost have not been available. Approaches in
which “point” sensing of frost is used for detection have difficulties when the growth pattern of the frost changes
due to manufacturing variability, installation variations, and differences in operational conditions. Defrost cycles
rely almost exclusively on timers set by the manufacturer based on previous tests. In this project, two different
methods were investigated as possible frost detection techniques. Small accelerometers were used to record and
compare acceleration power spectrum signals from the evaporator structure as frost adhered to the evaporator
surface. The white noise vibration from the expansion valve, used as an excitation source for the accelerometers, did
not propagate through the evaporator tubes because the liquid of the two-phase flow refrigerant damped the
vibrations. It appears that the method is not useful as a means of frost detection for home refrigerators although it
may have utility for other types of refrigeration systems. In the case of home refrigerators, other sources of
vibrational excitation of the evaporator should be explored.
The electro-mechanical impedance method is being currently studied as a structural health monitoring
technique. In this project, the method was tested as a mean of monitoring frost formation on the evaporator. The
impedance signal obtained using small piezoceramic wafers showed significant changes as frost was formed on a
test structure (an isolated evaporator fin). These results suggest that the electro-mechanical impedance method could
be used as a frost detection technique and further investigation should be performed.
|Publisher:||Air Conditioning and Refrigeration Center. College of Engineering. University of Illinois at Urbana-Champaign.
|Series/Report:||Air Conditioning and Refrigeration Center TR-215
|Sponsor:||Air Conditioning and Refrigeration Project 143
|Date Available in IDEALS:||2009-06-12