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HVAC Fundamentals: Refrigeration
ized theoretical values for R-134a and should be less
than the corresponding theoretical values of
Equations 2.1 and 2.2, which assume both an “ideal”
refrigerant and reversible processes. Thus, the values
computed using Equations 2.1 and 2.2 should be greater
than those found using Equation 2.3c and 2.4c for the
same evaporating and condensing temperatures.
Operating Characteristics of
Real Vapor Compression Cycles
A familiarity with this process and the values generated
in the preceding discussion is valuable even though
the process is idealized. Limits are set and the system
designer can determine the relative importance of operating
conditions and component selection that will have
the greatest impact on total system optimization. However,
there are a number of constraints that limit the
capacity and efficiency of actual vapor compression
cycles. Several are listed here:
• Refrigerant enters the compressor as a superheated
vapor to protect it from damage by liquid refrigerant
(it is essentially incompressible and may
remove oil from bearing surfaces). Therefore, the
inlet gas density will be slightly lower compared to
saturated vapor, thereby reducing mass flow rate
and compressor capacity.
• There are pressure drops (across ports, valves, and
mufflers), friction in the compressor and fluid, and
heat generated by the compressor drive. Thus, the
pressure difference from inlet to outlet will be
greater than ideal, and entropy will be generated
since the processes are irreversible.
• There is pressure drop and internal heat generation
through the condenser with a corresponding change
in saturation temperature as the pressure changes.
• Condensers are heat exchangers of finite size and
will operate at temperatures elevated above the idealized
condensing temperatures.
• The refrigerant must be subcooled several degrees
below the saturation temperature to ensure the
refrigerant is 100% liquid since the performance of
many expansion devices is compromised by the
presence of vapor.
• Heat transfer and entropy generation are encountered
when refrigerant passes through real expansion
devices.
• Evaporators are heat exchangers of finite size and
will operate at temperatures below the idealized
evaporation temperatures.
• The evaporation temperature in many cases must be
suppressed to low values to meet the needs of the
application (ice making, dehumidification, etc.).
• Many alternative refrigerants are zeotropic mixtures
that do not behave as a pure substance. Thus, condensation
and evaporation saturation temperatures
change (glide) with phase concentration. This typically
results in lower system efficiency and/or
larger heat exchanger size.
Figure 2.3 is a p-h diagram that demonstrates the
impact of a number of these constraints.
Figure 2.3 P-h process diagram of a real vapor compression cycle (ASHRAE 2005).
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