Impact of Elevated Ambient Temperatures on Capacity and Energy Input to a Vapor Compression System: Literature Review. Letter Report.
Impact of Elevated Ambient Temperatures on Capacity and
Energy Input to a Vapor Compression System: Literature
Review. Letter Report.
Motta, S. F. Y.; Domanski, P. A.
Letter Report for ARTI 21-CR; Research Project
605-50010/605-50015; 9 p. 2000.
refrigerants; air conditioning; alternative
refrigerants; coefficient of performance; refrigeration;
coefficient of performance
Operation of a system at elevated ambient temperatures
inherently results in a lower Coefficient of Performance
(COP). This conclusion comes directly from examining the
Carnot cycle. The COP relation, indicates that the COP
decreases when the condenser temperature increases at a
constant evaporation temperature. This theoretical
indication derived from the reversible cycle is valid
for all refrigerants. For refrigerants operating in the
vapor compression cycle, the COP degradation is greater
than that for the Carnot cycle and varies among fluids.
The two most influential fundamental thermodynamic
properties affecting refrigerant performance in the
vapor compression cycle are refrigerant's critical
temperature and molar heat capacity. For a given
application, a fluid with a lower critical temperature
will tend to have a higher volumetric capacity and a
lower coefficient of performance (COP). The difference
between COPs is related to different levels of
irreversibility because of the superheated vapor horn
and the throttling process. The levels of
irreversibility vary with operating temperatures because
the slopes of the saturated liquid and vapor lines
change, particularly when approaching the critical
point. Refrigerants with a low critical temperature have
a high pressure, a low drop of saturation temperature
for a given pressure drop, and a low
condenser-to-evaporator pressure ratio. These properties
offer some advantages, which can be exploited in a real
system for the betterment of its performance. Some
researchers reported that a low pressure ratio promotes
an improved compressor isentropic efficiency. The low
drop of refrigerant saturation temperature for a given
pressure drop allows designing heat exchangers with a
high refrigerant mass flux, which promotes an improved
refrigerant-side heat transfer coefficient.