Design, Measurements and Performance Predictions for a Micromolded Refrigerant Expansion Device.
Design, Measurements and Performance Predictions for a
Micromolded Refrigerant Expansion Device.
Yashar, D. A.
NISTIR 7327; 137 p. April 2006.
refrigerants; heating; ventilation; air conditioning;
microfabrication; refrigerant expansion
The design, fabrication and testing of a thermopneumatic
microfabricated valve for controlling refrigerant mass
flow rate during expansion is presented in this report.
This device was fabricated through nickel electroplating
within very thick SU8 molds, thereby realizing expansion
devices useful for small vapor compression systems
through an inexpensive modified Ultraviolet-LIGA (German
acronym for lithography, electrodeposition and molding)
process. This work begins with process development to
create meso-scale nickel pieces through electroplating
into SU8 molds. Two test benches were constructed and
used to benchmark the performance of a prototype with
compressed air and R134a. Three dimensional numerical
simulations were performed on the flowfield within the
device to predict its ability to control compressed air.
A code was also developed to predict the device's
temporal response and relationship between actuation
level and power input. The prototype was able to reduce
the mass flow rate of the compressed air by 22% at the
conditions used in the analysis. The performance was
then demonstrated in a 1.5 kW to 2 kW R134a vapor
compression system to characterize both steady state and
transient response. Steady state data showed that the
mass flow rate of refrigerant could be controlled using
the valve. The level of refrigerant subcooling defined
the magnitude of the response. Steady state data taken
at 750 kPa inlet pressure shows the mass flow rate was
reduced by 4.2% at 1 oC subcooling and up to 10.8% at 5
oC subcooling for a given level of actuation. Transient
system response was characterized using cyclic actuation
of the device. The change in capacity was approximately
5%, at the conditions used during these tests.