'*ASHRAE
SRCH
IJHVAC
4-4-ENGL
I1998
m
0759b50
0540197
809
m
HVAC&R RESEARCH
332
FIN
HEIGHT.
L
///
A'RFLoW
REFRIGERANT
FLOW
Figure
2.
Single
Finned
Tube
Heat
Exchanger
to both heat and mass transfer from the air-water vapor mixture. This may be done by expressing
temperatures for both the air-water vapor mixture and the refrigerant in terms
of
an effective
air-water vapor enthalpy, and multiplying the thermal resistance terms by appropriate values of
effective air-water vapor specific heat (Braun
et
al.
1989). This approach assumes that the air
side of the heat exchanger
is
fully wetted, and is
one
that has been used for cooling coil design
for both air-water vapor to refrigerant and air-water vapor
to
water (Mirth 1993).
Dry
air is
assumed for
the
air side analysis of the condensedgas cooler.
The evaporation process
of
both fluids was modeled by considering
a
series
of
individual
finite tube lengths, where the refrigerant experiences
a
vapor quality increase of
5%
(0.05)
over
each finite tube length. Transport properties required for evaluation
of
pressure drops and heat
transfer coefficients were evaluated for each finite tube length using the arithmetic mean of the
vapor qualities of the inlet and outlet vapor quality.
The heat rejection process of the carbon dioxide was also modeled by considering a series of
individual finite tube lengths, where the carbon dioxide experiences
a
temperature reduction of
5°C
over each finite tube length. Transport properties required
for
heat transfer coefficient eval-
uation were evaluated at the average of the inlet and outlet temperatures for each tube length.
The same approach was used to model the heat rejection
of
R-22
prior to condensation.
For
the
cooling of superheated R-22, a
3°C
temperature reduction was considered for each finite ele-
ment. For the condensation process, the vapor quality
of
R-22 was reduced by
5%
(0.05)
over
each finite tube length. In the same manner as the evaporation process, transport properties
required for evaluation of pressure drops and heat transfer coefficients during condensation were
evaluated at the average of
the
inlet and outlet vapor quality for each finite tube length.
In conjunction with a constant outside surface heat transfer coefficient corresponding
to
each
fin length, a balance can be made where the heat transfer rate based on refrigerant mass flow rate
and enthalpy change matches that of internal convection, conduction and external convection
based on fluid flow velocity, and surface area. R-22 two-phase property values and the transport
property values
of
carbon dioxide were obtained from the 1997
ASHRAE
Handbook-Fundamen-
tals.
The transport properties of single phase R-22 were taken from EES (Engineering Equation
Copyright ASHRAE
Provided by IHS under license with ASHRAE
Licensee=Naval Aviation Depot/5918326100
Not for Resale, 01/19/2008 02:11:22 MST
No reproduction or networking permitted without license from IHS
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