25-09-2012, 12:31 PM
HEAT EXCHANGER DESIGN
HEAT EXCHANGER.pdf (Size: 1,010.59 KB / Downloads: 237)
INTRODUCTION
Objective
To design and evaluate an evaporator for a refrigeration unit capable of cooling air from 100 °F
to 50 °F while maintaining a flow velocity less than 500 ft/min. The overall capacity shall be one
ton (12,000 Btu/hr) and the maximum aspect ratio of the heat exchanger is 2:3. The working
fluids shall not incur “excessive” pressure drop. Estimated cost, dry weight, and efficiency also
guide the design of this heat exchanger.
The evaporator creates a cold reservoir by transferring heat from the air passing through / over it
to the refrigerant. This heat transfer into the refrigerant is called in Q and is known as the capacity
of the refrigeration unit. Refrigerant temperature through the evaporator is often considered
constant to simplify thermodynamic analyses.
Heat Exchangers
Several heat exchangers configurations exist, each lending itself to a different application. The
most common type for the purposes discussed in this study are crossflow exchangers that feature
a coil of copper tubing and an array of fins. Another type is the shell-and-tube heat exchanger,
which send flow through a baffled shell containing a number of parallel tubes of refrigerant.
Concept
We decided to model a shell-and-tube heat exchanger. This design is effective for its ability to
promote a high level of mixing (and in turn heat transfer) by moving the flow with baffles. The
configuration presented here uses counter-flow paired with annular fins, all encased in a compact
enclosure. For the sake of meshing, our design consolidates the bank of tubes within the shell
into a single, one-inch pipe running axially through the center.
The inlet and outlet locations were a topic of debate. One simple but less realistic approach is to
run air through the annulus axially. The physical realization of the design would present issues
with interference with the refrigerant tube, which would have to obstruct part of the inlet section
or run through the center of the fan (…a future consideration for the next phase of this study).
Instead, the design is based on flow sent into the shell through perpendicular ducts as the CAD
drawings demonstrate.
Geometry
The dimensions of the outer shell are four inches by twelve inches. This does not include a pair
of one inch offset rectangular ducts, which are themselves one and a half inches by two inches.
These serve as the inlet and outlet for airflow through the heat exchanger, and therefore must be
positioned diagonally across the volume. There are three annular fins and two annular rings that
obstruct flow and induce proper mixing. They are spaced axially two inches apart from each
other and the walls while also being a tenth of an inch thick. The outer diameter of the fins match
the inner diameter of the rings, which are both a one and a half inch radius. The pipe carrying
refrigerant runs axially parallel to the enclosure for the length of the enclosure. The total mass of
the aluminum needed for this heat exchanger is about 2.2 lbs, as determined using Solidworks.
COMPUTATIONAL FLUID DYNAMICS
Preliminary Concept Validation
Following a suggestion to perform a simple preliminary simulation to verify the potential for
adequate performance, a 2D-axisymmetric analysis was evaluated. A straightforward quad mesh
(52875 cells, 106665 faces, 53791 nodes) was generated and run using a k-e turbulence model.
The geometry is exactly that a cross-section of the three-dimensional model, except the boundary
conditions had to be altered since the device is necessarily a three-dimensional case. This
difference should not stray too far from realistic results since only the orientation of the inlet and
outlet have changed. As expected, the stream encounters a series of baffles that stirs up the flow,
a catalyst for improved heat transfer.
DISCUSSION
Taking the unconventional approach of modeling an experimental shell-and-tube heat exchanger
yielded surprisingly successful performance. Once the settings for the boundary conditions were
tweaked to correspond to realistic flow parameters based on hand calculations and common
sense, our heat exchanger was able to meets the requirements. Benefits of our design include
flexibility of implementation. The current perpendicular ducting for the inlet and outlet serves as
one of many options for the flow orientation. Sending the warm air into the shell axially may
require some alteration of geometry but will still likely demonstrate the effectiveness and
modularity of the design. One aspect of the design that sets it apart from typical evaporators is
the single, straight section of refrigerant pipe. Without the numerous bends observed with most
exchangers, the flow encounters fewer sources of head loss and therefore maintains its pressure.
The compact size makes for a light and relatively inexpensive device. For a cost estimate based
on materials and labor (at $11.00/hour), a one-off version of our design would cost about
$520.00. That is obviously not representative of the actual cost of this component for large-scale
production but would be an attractive price for an employer looking for an innovative and
thoroughly analyzed heat exchanger concept.
HEAT EXCHANGER.pdf (Size: 1,010.59 KB / Downloads: 237)
INTRODUCTION
Objective
To design and evaluate an evaporator for a refrigeration unit capable of cooling air from 100 °F
to 50 °F while maintaining a flow velocity less than 500 ft/min. The overall capacity shall be one
ton (12,000 Btu/hr) and the maximum aspect ratio of the heat exchanger is 2:3. The working
fluids shall not incur “excessive” pressure drop. Estimated cost, dry weight, and efficiency also
guide the design of this heat exchanger.
The evaporator creates a cold reservoir by transferring heat from the air passing through / over it
to the refrigerant. This heat transfer into the refrigerant is called in Q and is known as the capacity
of the refrigeration unit. Refrigerant temperature through the evaporator is often considered
constant to simplify thermodynamic analyses.
Heat Exchangers
Several heat exchangers configurations exist, each lending itself to a different application. The
most common type for the purposes discussed in this study are crossflow exchangers that feature
a coil of copper tubing and an array of fins. Another type is the shell-and-tube heat exchanger,
which send flow through a baffled shell containing a number of parallel tubes of refrigerant.
Concept
We decided to model a shell-and-tube heat exchanger. This design is effective for its ability to
promote a high level of mixing (and in turn heat transfer) by moving the flow with baffles. The
configuration presented here uses counter-flow paired with annular fins, all encased in a compact
enclosure. For the sake of meshing, our design consolidates the bank of tubes within the shell
into a single, one-inch pipe running axially through the center.
The inlet and outlet locations were a topic of debate. One simple but less realistic approach is to
run air through the annulus axially. The physical realization of the design would present issues
with interference with the refrigerant tube, which would have to obstruct part of the inlet section
or run through the center of the fan (…a future consideration for the next phase of this study).
Instead, the design is based on flow sent into the shell through perpendicular ducts as the CAD
drawings demonstrate.
Geometry
The dimensions of the outer shell are four inches by twelve inches. This does not include a pair
of one inch offset rectangular ducts, which are themselves one and a half inches by two inches.
These serve as the inlet and outlet for airflow through the heat exchanger, and therefore must be
positioned diagonally across the volume. There are three annular fins and two annular rings that
obstruct flow and induce proper mixing. They are spaced axially two inches apart from each
other and the walls while also being a tenth of an inch thick. The outer diameter of the fins match
the inner diameter of the rings, which are both a one and a half inch radius. The pipe carrying
refrigerant runs axially parallel to the enclosure for the length of the enclosure. The total mass of
the aluminum needed for this heat exchanger is about 2.2 lbs, as determined using Solidworks.
COMPUTATIONAL FLUID DYNAMICS
Preliminary Concept Validation
Following a suggestion to perform a simple preliminary simulation to verify the potential for
adequate performance, a 2D-axisymmetric analysis was evaluated. A straightforward quad mesh
(52875 cells, 106665 faces, 53791 nodes) was generated and run using a k-e turbulence model.
The geometry is exactly that a cross-section of the three-dimensional model, except the boundary
conditions had to be altered since the device is necessarily a three-dimensional case. This
difference should not stray too far from realistic results since only the orientation of the inlet and
outlet have changed. As expected, the stream encounters a series of baffles that stirs up the flow,
a catalyst for improved heat transfer.
DISCUSSION
Taking the unconventional approach of modeling an experimental shell-and-tube heat exchanger
yielded surprisingly successful performance. Once the settings for the boundary conditions were
tweaked to correspond to realistic flow parameters based on hand calculations and common
sense, our heat exchanger was able to meets the requirements. Benefits of our design include
flexibility of implementation. The current perpendicular ducting for the inlet and outlet serves as
one of many options for the flow orientation. Sending the warm air into the shell axially may
require some alteration of geometry but will still likely demonstrate the effectiveness and
modularity of the design. One aspect of the design that sets it apart from typical evaporators is
the single, straight section of refrigerant pipe. Without the numerous bends observed with most
exchangers, the flow encounters fewer sources of head loss and therefore maintains its pressure.
The compact size makes for a light and relatively inexpensive device. For a cost estimate based
on materials and labor (at $11.00/hour), a one-off version of our design would cost about
$520.00. That is obviously not representative of the actual cost of this component for large-scale
production but would be an attractive price for an employer looking for an innovative and
thoroughly analyzed heat exchanger concept.