Note: Descriptions are shown in the official language in which they were submitted.
Subcritical Carbon Dioxide Dehumidifier
Field of the Invention
[0001] The present invention relates to dehumidifiers, in particular, to
dehumidifiers that use
carbon dioxide as a refrigerant.
Background
[0002] The use of carbon dioxide as a refrigerant is well known,
however, the fact that the
critical point of carbon dioxide is relatively low, particularly compared to
many other common
refrigerants, creates challenges for its successful application. Carbon
dioxide has a critical point
of 31.1 C and 1,071 psia, above which it behaves as a supercritical fluid.
Normal ambient
temperatures can exceed the critical temperature of carbon dioxide, resulting
in high operating
pressures within a dehumidifier system.
[0003] As a result, dehumidifiers using carbon dioxide as a refrigerant
have hereto been
designed and built to withstand pressures of up to 1,300 psia. This increases
the cost of such units
to the point where they are commercially uncompetitive with dehumidifiers that
use other
refrigerants, such as R-404a or R-407c. However, carbon dioxide has a number
of desirable
characteristics as a refrigerant. For example, many common refrigerants, such
as R-407c, are
hazardous if inhaled, whereas carbon dioxide is non-toxic. Carbon dioxide is
also a by-product of
many industrial processes and, as a result, is readily available at a low
cost. Further, refrigerants
such as hydrofluorocarbons (HFCs) can have a serious environmental impact,
through their high
global warming potential (GWP).
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[0004] In applications where ambient temperature in the operating
environment generally
remains below the critical temperature for carbon dioxide, such as in a hockey
arena, dehumidifiers
that use carbon dioxide as a refrigerant may be designed and built having
lower operating
pressures. However, during a power outage or when the seasonal ambient
temperatures are above
.. the critical temperature and pressure within the dehumidifier system can
increase above the critical
point, the excess pressure created by the supercritical carbon dioxide must be
released by
discharging a portion of the refrigerant. The system will then require a new
charge of refrigerant
before it can return to normal operation, which can cause delays and be costly
for large
dehumidifiers, such as those used in hockey arenas.
Summary of the Invention
[0005] A subcritical carbon dioxide dehumidifier, according to the
present invention, has a
compressor, a condenser, a receiver, an expansion valve, an evaporator, and an
expansion tank and
uses carbon dioxide as a refrigerant.
Brief Description of the Drawings
[0006] In order that the invention may be more clearly understood, a
preferred embodiment
thereof will now be described in detail by way of example, with reference to
the accompanying
drawings, in which:
[0007] Figure I is a schematic view of a subcritical carbon dioxide
dehumidifier, according
to the present invention.
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[0008] Figure 2 is graph showing the compressor operating envelope for
the subcritical carbon
dioxide dehumidifier.
Description of the Invention
[0009] A subcritical carbon dioxide dehumidifier, according to the
present invention, uses
.. carbon dioxide as the refrigerant and maintains the system at a subcritical
temperature throughout
the refrigeration cycle. The dehumidifier is described herein with reference
to the example
application of a hockey arena, but may be used in any application where the
ambient temperature
is generally below the critical temperature of carbon dioxide. The
dehumidifier has an expansion
tank to accommodate an increase in pressure resulting from the carbon dioxide
refrigerant
temperature increasing above its critical point, such as during a power outage
or seasonally. This
permits the system to avoid discharging refrigerant to relieve excess pressure
and thereby retain a
full refrigerant charge when the ambient temperature is in the supercritical
range of carbon dioxide.
[0010] As shown schematically in Figure I, the carbon dioxide
dehumidifier, referred to
herein as the dehumidifier, has a compressor 1, a condenser 2, a receiver 3,
an expansion valve 4,
an evaporator 5, and an expansion tank 6. The operation of the compressor 1,
condenser 2, receiver
3, expansion valve 4, and evaporator 5 in the basic closed circuit
refrigeration cycle are well known
and, accordingly, are described briefly herein. These parts are located within
a housing (not
shown), having an air inlet and outlet, which is generally located on the roof
of the facility and the
air is ducted to and from the dehumidifier. The dehumidifier may also be
located directly in the
space it is conditioning, such as on a mezzanine of a hockey arena, and the
air inlet receives air
from the space at one location and the air outlet blows air back into the same
space at a different
location.
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[0011] The basic closed circuit refrigeration cycle of the dehumidifier
operates as follows.
Low-pressure low-temperature carbon dioxide gas is drawn into the compressor 1
and is
compressed and supplied to the condenser 2 as a high-pressure high-temperature
gas. In the
condenser 2, the high-pressure high-temperature carbon dioxide gas transfers
heat to the air
passing over the condenser 2 and condenses to high-pressure high-temperature
liquid carbon
dioxide. The high-pressure high-temperature liquid carbon dioxide then enters
the receiver 3,
where it is stored until needed. The high-pressure high-temperature liquid
carbon dioxide then
flows through the expansion valve 4, which restricts the flow of liquid and
lowers the pressure of
the liquid. The low-pressure low-temperature liquid carbon dioxide then enters
the evaporator 5,
typically as an aerosolized mixture of gas and liquid, where it absorbs heat
from the air. This
causes the liquid carbon dioxide to evaporate as it absorbs heat in the
evaporator 5, to result in a
low-temperature low-pressure gas that is supplied to the compressor 1 to
continue the refrigeration
cycle.
[0012] The above refrigeration cycle is carried out in the main line 7
of the dehumidifier,
which is defined by the compressor 1, the condenser 2, the receiver 3, the
expansion valve 4, the
evaporator 5, and the connections therebetween. The main line 7 of the
dehumidifier has a high-
pressure side 9, between the compressor 1 and the expansion valve 4, and a low-
pressure side 10,
between the expansion valve 4 and the compressor 1.
[0013] As shown in Figure 1, the expansion tank 6 is on a secondary line
8, which is connected
to the main line 7, and together with the main line 7 forms a closed
refrigeration circuit. Preferably,
the expansion tank 6 connects to the main line 7 in two places, a first
connection 11 on the high-
pressure side 9, between the evaporator 5 and the compressor 1, and a second
connection 12 on
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the low-pressure side 10, between the condenser 2 and the receiver 3. This
permits the first
connection 11 to operate as a drain line from the expansion tank 6 to the
compressor suction line.
[0014] A control valve 13 on the first connection 11 prevents the flow
of high-pressure liquid
carbon dioxide into the expansion tank 6 under normal operating conditions.
Preferably, the
control valve 13 is a normally open solenoid valve, so that if the control
valve 13 is not energized,
for any reason, the solenoid valve opens and allows the high pressure gas to
expand into the
expansion tank 6. The control valve 13 is opened when pressure in the high-
pressure side 9
increases beyond a safety threshold value, to permit high-pressure carbon
dioxide to flow into the
expansion tank 6 and thereby relieve pressure from the main line 7.
[0015] Preferably, the main line 7 is provided with one or more pressure
relief valves 14,
which operate as a failsafe to release pressure in the event the control valve
13 of expansion tank
6 fails and pressure builds up in the main line 7. The expansion tank 6 may
also have a pressure
relief valve 14, in case the pressure in the expansion tank 6 exceeds
acceptable levels. Because
carbon dioxide is used as the refrigerant, there is minimal safety hazard in
venting excess
refrigerant through the pressure relief valves 14. Nonetheless, preferably,
the pressure relief valves
14 discharge excess refrigerant to an outside area.
[0016] A filter 15 may be included on the main line 7 between the
receiver 3 and the expansion
valve 4 to maintain the clean and moisture free operation of the dehumidifier.
A view port 16 may
also be provided on the main line 7 between the receiver 3 and the expansion
valve 4 to permit
observation of the liquid column passing between the receiver 3 and the
expansion valve 4.
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[0017] A control system monitors temperatures and pressures throughout
the dehumidifier
and controls the operation of the dehumidifier. As shown in Figure 1, the
control system monitors
the temperature and pressure in the main line 7 between the compressor 1 and
the condenser 2 and
between the receiver 3 and the expansion valve 4. The control system may also
monitor the
humidity and temperature of the space and control the operation of the
dehumidifier based on set
humidity and/or temperature thresholds.
[0018] In a typical hockey arena, the ambient temperature is roughly 6
C. Accordingly, the
air passing over the condenser 2 cools the high-pressure carbon dioxide vapour
into a high-pressure
liquid and at the same time increases the temperature of the air to
approximately 10 C. The air
then passes through the evaporator 5, where the temperature is below the dew
point of the air,
thereby dehumidifying the air and cooling it to approximately 7 C. The net
increase in temperature
of the air passing through the dehumidifier is desirable for applications such
as hockey arenas,
because it reduces the amount of heating otherwise required for the space.
[0019] At normal operating conditions, the high side pressure is between
450 psi and 650 psi,
while the low side pressure is between 300 psi and 400 psi. High side pressure
refers to the
pressure of the carbon dioxide in the high-pressure side 9 of the main line 7.
Similarly, low side
pressure refers to the pressure of the carbon dioxide in the low-pressure side
10 of the main line 7.
[0020] The present invention has been described and illustrated with
reference to an
exemplary embodiment, however, it will be understood by those skilled in the
art that various
changed may be made and equivalents may be substituted for elements thereof
without departing
from the scope of the invention as set out herein. Therefore, it is intended
that the invention not
be limited to the particular embodiments disclosed herein.
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