Note: Descriptions are shown in the official language in which they were submitted.
CA 02429857 2003-05-22
WO 02/42695 PCT/NO01/00454
Refrigerating or heat pump syste~a
with heat rejection at supercritical pressure.
Field of the invention
The present invention relates to refrigerating or heat pump systems, in
particular to
refrigerating systems for retail and/or storage cabinets for cooling or
freezing of food or
beverages, or heat pumps for building heating, in both cases using carbon
dioxide as the
refrigerant.
Description of prior art
Refrigerating systems for cooling or freezing cabinets usually have a
refrigerant that
operates in a vapour compression cycle with evaporation and condensation. The
refrigerant is chosen so that its critical temperature is well below the
required heat
rejection (condensing) temperature. In order to achieve effective condensation
in air-
cooled systems, a relatively high air flow rate is required, with large space
requirements
for the condenser and the air flow system. A fan is needed in most systems to
circulate air
over the condenser. One problem with this solution is the relatively large
power
requirement for the fan, and the additional space requirements for the fan and
its air flow
system. The forced air flow and the fan and its motor may also result in noise
problems,
and the installation of a fan gives added cost and complexity to the system.
Residential and light commercial heat pumps that supply heat to the indoor air
usually
have an indoor unit with forced air circulation over the condenser. Again, an
air
circulation fan or blower is needed, giving additional power consumption and
noise.
Furthermore, the thermal comfort may be compromised due to draft from large
air flow
rates and/or high-velocity air currents with temperature only slightly above
room
temperature. Due to the large air flow requirements, the indoor unit design
needs a large
volume, which reduces the options for attractive product design
Refrigerants in present refrigerating or heat pump systems are either
fluorocarbon-based
chemicals that are undesirable due to ozone-depleting properties and/or their
contribution
CA 02429857 2003-05-22
WO 02/42695 PCT/NO01/00454
2
to man-made climate change, or they are flammable hydrocarbon-based fluids
that are
questioned due to safety concerns.
In a trans critical system, heat is rejected by reducing the temperature of
the super
critically pressurized refrigerant, and not by condensation at constant
temperature as in
conventional systems. As the supercritical-pressure refrigerant flows through
the heat
exchanger, it gives off heat and its temperature is reduced (gliding
temperature). Ideally,
the refrigerant temperature will approach the air inlet temperature, with
counter current
refrigerant and air flow.
In a situation with gliding temperature heat rejection from the refrigerant,
the air flow rate
may be reduced and the air outlet temperature increased compared to the
situation in a
condenser. In a condenser, the air outlet temperature necessarily has to be
below the
condensing temperature. In a trans critical system, the high air temperature
and reduced
air flow rate will be beneficial for natural convection air flow over the heat
exchanger, it
will reduce noise, and will also be advantageous with respect to thermal
comfort in heat
pump applications.
Summary of the invention
In view of the above problems and shortcomings it is therefor an object of the
present
invention to provide a refrigerating system that uses a safe and
environmentally friendly
refrigerant in a system with a compact natural air circulation heat rejection
system
without fan power requirements, or with only minor fan power in high load
situations.
To achieve these objects, the present invention describes a system using the
nonflammable, nontoxic and environmentally friendly fluid carbon dioxide (C02)
as the
refrigerant.
The invention is characterized in that the refrigerant rejects heat at a
supercritical pressure
with gliding temperature through a heat rejecting heat exchanger which is
cooled by
CA 02429857 2003-05-22
WO 02/42695 PCT/NO01/00454
3
natural upwards circulation/convection of air as defined in the attached
independent
claim 1.
Preferred embodiments of the invention are further defined in the dependent
claims 2- 8.
By taking advantage of the special thermodynamic properties of COa and by
properly
designing the system, heat rejection may, as stated above, take place with
natural
convection flow of the air, with greatly reduced air flow rate and without the
need for a
special air circulation fan.
The invention will be further described in the following by way of example and
with
reference to the drawings where:
Fig. 1 shows a trans critical vapor compression system including a compressor,
an
air-cooled heat rejecting unit with natural air circulation, an expansion
device and
an evaporator connected in a closed circuit..
Fig. 2 shows a cross-sectional view of a heat rejecting unit with natural air
circulation
including an air flow conduit and a heat rejecting heat exchanger based on
round
tubes in an in line layout accoxding to the invention.
Fig. 3 shows a cross-sectional view of a heat rejecting unit with natural air
circulation
including an air flow conduit and a heat rejecting heat exchanger based on
round
tubes in a staggered layout according to a second embodiment of the
invention..
Fig. 4 shows a side view of a heat rejecting unit with natural air circulation
having an air
flow conduit and a heat rejecting heat exchanger based on folded tubes
according
to a third embodiment of the invention.
CA 02429857 2003-05-22
WO 02/42695 PCT/NO01/00454
4
Fig. 5 shows a cross-sectional view of a heat rejecting unit with an air flow
conduit and a
heat rejecting heat exchanger formed into a spiral geometry according to a
fourth
embodiment of the invention,.
Fig. 6 shows a heat rejecting unit where the tubes are attached to a plate to
increase the
air-side heat transfer surface according to a fifth embodiment of the
invention.
Fig. 7 shows a fully counter-flow heat rejecting unit with natural air
circulation using
Multi Port Extruded (MPE) heat exchanger with plate fin extended surface on
one
or both side of the said heat exchanger.
Fig. 8 shows example of the embodiment according to claim 5 used in a
refrigerator or
similar devices.
Detailed description of the preferred embodiments.
With reference to Figures 1 to 6, the embodiments of the invention will be
explained in
detail in the following text.
Fig.1 shows an example of a vapor compression system including a compressor
20, air-
cooled heat rejecting unit 21, expansion device 22 and evaporator 23. The
components
are connected in a closed circuit that operates in a trans critical vapor
compression cycle,
i.e. with super critical high-side pressure. The heat rejecting heat exchanger
21 is cooled
by natural upwards circulation/convection of air.
Fig. 2 shows a cross-sectional view of a heat rejecting unit with an air flow
conduit or
outer air flow shell or jacket 11 and heat exchanger tubes 10. The tubes are
arranged in
line above one each another within the shell 11. Air enters at the inlet i in
the lower end
of the system, and exits at the outlet o at the top. Air circulation is
achieved by natural
CA 02429857 2003-05-22
WO 02/42695 PCT/NO01/00454
convection when the air is heated by the heat exchanger tubes. High-
temperature
refrigerant from the compressor enters through the heat exchanger refrigerant
inlet 12 and
flows through the heat exchanger while rejecting heat to the air whereby an
efficient
chimney effect is achieved. The cooled refrigerant exits from the heat
exchanger through
the outlet 13. In order to further increase the air flow rate, an extra
vertical length of
conduit l la may be added above the heat exchanger, to increase the chimney
effect. The
"chimney" or stack may also be built with a converging and diverging nozzle
cross
section, in order to improve air flow.
As shown by the cross-sectional view in Fig. 3, the heat transfer tubes 10 may
also be
arranged in a staggered fashion inside the flow conduit, to increase the
surface and
improve the heat transfer.
Fig. 4 shows a side view of a natural air circulation heat rejecting unit with
air flow
conduit 11 and a heat exchanger based on folded tubes 10. In order to maximize
air
circulation and heat exchanger efficiency, the refrigerant should flow in a
generally
counter current direction to the air. With refrigerant inlet at the top 12 and
outlet at the
bottom, 13 as indicated in the figure the desired relationship between the two
different air
and refrigerant flows is achieved.
Another possible embodiment is shown in Fig. 5, where the air flow conduit 11
has a
circular cross-section, and the heat transfer tube 10 is formed into a spiral
inside the air
flow conduit 11. In order to optimize the cross-section of the air conduit 11
with respect
to air flow, an annulus containing the heat transfer tube may be established
by inserting an
inner circular tube into the conduit, the inserted tube being closed at the
ends.
As shown by Fig. 6, the heat transfer tube may form an integral part of the
shell in a plate
or a conduit 11, i.e. being built into the conduit or shell, in order to
increase the heat
transfer surface facing the air flow. If necessary, thermal conduction along
the height of
the conduit can be reduced or eliminated by having slots, splits or louvers 14
in the plate.
CA 02429857 2003-05-22
WO 02/42695 PCT/NO01/00454
6
The shell plate or conduit may have a flat surface, or the surface may consist
of vertical
fins or open or closed duct-like structures that improve natural-convection
air flow.
The invention as defined in the attached claims is not limited to the examples
as shown in
the figures and explained above, thus in all the above embodiments, one or
several walls
of the conduit or shell may be applied as heat transfer surface as well.
Further, even
though the heat transfer tube is shown with a circular cross section in the
diagrams, any
tube geometry may be used, including flat tubes, oval tubes, mufti port tubes
and more
complex geometry. Still further, the refrigerant tube may also be integrated
into the air
flow conduit material, giving an integral heat rejecting and air conduit unit
which can also
enhance heat transfer by radiation. Several enhancements and exterior surface
extensions
are also possible for the heat transfer tube, including wires, fins, studs
etc. An example is
shown in Fig 7 using Mufti Port Extruded (MPE) heat exchanger with plate fin
extended
surface where high temperature refrigerant enters at the top and leaves from
the bottom
after being cooled by natural upwards circulation/convection of air in fully
counter-flow
heat exchange process which is ideal in such cases.
Fig 8 shows example of the embodiment according to claim 5 used in a
refrigerator or
similar devices. The heat exchanger 10 is placed in the bottom compartment,
with the air
flow conduit l la behind the refrigerator, extending the air flow shell or
jacket 11 in order
to enhance the natural air flow/circulation.
