THE ROLE OF THE

HVAC System Overview and Refrigerant Design 137
Reynolds number (Re): Ratio of inertial forces to viscous forces. Describes laminar
and turbulent flow. Re = υ sL/ν (where υ s is mean fluid velocity, L is length,
and ν is kinematic fluid viscosity).
Subcooling: The temperature of a refrigerant’s liquid below the boiling (condensing)
temperature at a specific pressure.
Superheat: The temperature of a refrigerant’s vapor above the boiling temperature
of its liquid at a specific pressure.
Thermal expansion valve (TXV): The component in the HVAC system that controls
the amount of refrigerant that goes into the evaporator.
Vapor compression cycle: A heating–cooling cycle that follows a refrigerant along
a temperature versus entropy diagram.
Zeotropic mixture: A mixture whose components have different mass fractions in
the liquid phase than in the vapor phase at equilibrium conditions.
9.9 need For alternate reFrIgerant systems
The European Commission has proposed legislation that will phase out any refrigerant
with a global warming potential greater that 150 by May of 2008 [14]. The commission
further states [14]:
Under the Kyoto Protocol, the EU has agreed to an 8% reduction in its greenhouse gas
emissions by 2008–2012, compared to the base year 1990. The reductions for each of
the EU-15 countries have been agreed under the so-called EU Burden sharing agreement,
which allows some countries to increase emissions, provided these are offset by
reductions in other Member States.
Emissions of the 6 greenhouse gases covered by the Protocol are weighted by their
global warming potentials (GWPs) and aggregated to give total emissions in CO 2
equivalents.
The Structural Indicator GHG emissions is [sic] defined as an index of greenhouse
gas (CO 2, CH 4, N 2O, HFC, PFC and SF 6) emissions normalized by the base quantity
in CO 2-equivalents (excluding land use changes and forestry). The base quantity is
defined by the GHG emissions in the base year. The base year for the non-fluorinated
gases (CO 2, CH 4 and N 2O) is 1990, and 1995 for the fluorinated gases (HFC, PFC and
SF 6).
In the face of these changes, the automobile industry must adapt its cooling strategies
to the new European requirements and possible future changes in the Americas.
With the belief that satisfying the imperatives will satisfy the customer, we also
address the market segment that is dedicated to the reduction of greenhouse gases
and bases its purchases on environmentally friendly vehicles.
9.10 reFrIgerant oIl mIXtures
Automotive refrigerant systems are not composed of refrigerant alone. For lubrication
purposes, an oil such as Mobil Artic EAL 68 is used. The significance of this is