Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CONTROL METHOD FOR A CRYOGENIC UNIT
CROSS REFERENCE TO CO-PENDING APPLICATIONS
The present invention is related to commonly assigned
U.S. Patent Application Serial No. 08/501,372, filed July 12,
1995, entitled AIR CONDITIONING AND REFRIGERATION UNITS
UTILIZING A CRYOGEN; and to commonly assigned U.S. Patent
Application Serial No. 08/560,919, filed November 20, 1995,
entitled APPARATUS AND METHOD FOR VAPORIZING A LIQUID CRYOGEN
AND SUPERHEATING THE RESULTING VAPOR, now U.S. Patent No.
5,598,709; both incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention generally relates to apparatus and
methods for temperature controlling a conditioned space and
more particularly relates to temperature controlling systems
which utilize a cryogen.
It has been known for some time to temperature condition
an enclosed space for the purpose of transporting temperature
sensitive materials, such as food stuffs. The most prevalent
current approach is to cool and/or heat a transportable
conditioned space (e. g. a refrigerated truck, trailer, or rail
car) with a mechanical, condensation/evaporation system
utilizing a fossil fuel powered compressor.
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Unfortunately, many such mechanical systems employ
refrigerants of the chlorofluorocarbon (CFC) family, because
of the desirable heat of vaporization and temperature/pressure
vaporization points. Certain studies have indicated that such
refrigerants may produce undue deterioration of the earth's
ozone layer. In response thereto, various laws and
regulations have been enacted to control the release of such
refrigerants to the atmosphere.
A relatively new and exciting alternative to mechanical
systems utilizing CFC refrigerants ,is a temperature
conditioning system based upon the controlled energy release
from a transportable store of cryogenic liquid. In the most
environmentally acceptable approaches, this involves the use
of a liquefied inert gas, such as nitrogen or carbon dioxide,
which may be simply and harmlessly exhausted into the
atmosphere at ambient temperature and pressure, after the
cooling potential in its cryogenic state has been utilized to
provide temperature conditioning of the controlled space.
Ideally, the entire cryogenic temperature control system
is powered to the greatest extent possible by the release of
the pressure stored by the cryogenic liquid with minimal or no
additional energy sources. This highly integrated design
promotes reliability, low cost of manufacture, and freedom
from acoustic and chemical pollution.
Control valves, for example, are preferably powered by
cryogenic energy rather than outside electrical or other
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energy sources. Similarly, attempts to provide mechanical
power from the cryogenic fluid have greatly enhanced through
the use of vapor powered motors. However, such conversions of
cryogenic energy to mechanical energy must be accomplished in
the most efficient means possible to prevent premature
depletion of the cryogenic liquid energy source. Whereas
great strides have been made concerning the design of the
individual components, efficiency of cryogenic liquid energy
usage is also a matter of system level design.
For example in prior art approaches, the vapor motor is
powered by the vapor retrieved from the low pressure end of
the evaporation coils. Whereas this is a particularly
efficient method for providing ventilation to the evaporation
coils during continuous operation, at system start-up there
may be substantial delay in the arrival of vapor to the vapor
motor thus encouraging clogging of the evaporation coils with
dry ice and uneven evaporation.
SUMMARY OF THE INVENTION
The present invention overcomes the disadvantages found
in the prior art by providing a methodology and a system which
both increase the degree to which a cryogenic temperature
conditioning system performs necessary functions utilizing
cryogenic energy and also increase the ef f iciency at which the
cryogenic energy is used.
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In the preferred mode of the present invention, the
energy stored within the cryogenic liquid is utilized in
performing three system functions in addition to the basic
heat absorption/release associated with temperature. The
first of these functions is the powering of virtually all
valves. In addition, a vapor powered ventilation blower motor
is prestarted and operated by the cryogenic fluid energy. The
third function is a compressed vapor take-off for powering
auxiliary tools which may be needed for maintenance of the
transport vehicle. ,
The efficiency of cryogenic energy usage is enhanced by
providing valve bleeder circuits for recycling excess
pressurized vapor through the vapor motor. Secondly,
efficiency is further enhanced through a separate vapor input
to the vapor motor directly from the storage tank. This
ensures that the vapor motor starts quickly and provides
ventilation to the evaporation coils immediately upon system
start-up, rather than delaying until vapor is produced at the
low pressure end of the evaporation coils. Elimination of
this delay ensures even evaporation at system start-up and
thus prevents evaporation coil clogging by uneven evaporation
of cryogenic liquid.
BRIEF DESCRIPTION OF THE DRAWING
The enclosed figure, being a schematic diagram, when
viewed in conjunction with the following detailed description,
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provides an enabling disclosure of the salient features of the
preferred embodiment of the present invention, without
limiting the scope of the claims appended thereto.
5 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The enclosed figure provides a schematic diagram of the
preferred mode of the present invention. Cryogenic tank
subsystem 10 contains an insulated storage vessel 12. In the
preferred mode, storage vessel 12 stores liquid carbon dioxide
at a temperature of about -50 degrees ,F. Therefore, the
overall efficiency of the- system will, be in large part
governed by the extent to which storage vessel 12 is
insulated.
During operation storage vessel 12 will contain a first
volume of liquid carbon dioxide 14 and a second volume of
carbon dioxide vapor 16. Of course, filling storage vessel 12
will increase first volume 14 and decrease second volume 16.
Similarly, operation of the system will decrease first volume
14 and increase second volume 16.
Storage vessel 12 has two vapor outputs and two liquid
outputs. A first vapor output 40 is suitable for powering
standard compressed air tools via regulator 38 and standard
compressed air tool fitting 40. In this manner, standard
compressed air tools may be used to maintain the transport
vehicle as required. The vapor output on vapor line 46 is
provided as an unregulated output of cryogenic tank subsystem
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10. Back pressure regulator 42 bleeds off vapor if the vapor
pressure in space 16 exceeds a designed limit. Typically,
this excess vapor is discharged to the atmosphere. In this
invention, line 44 feeds this excess vapor to the system
downstream from valves 56 and 58. This maintains the system
at a slight positive pressure when the refrigeration unit is
turned off. The positive pressure keeps out dirt and moisture
that can back feed into the system via the open end of muf f ler
76.
Back pressure regulator 90 maintains, the system pressure
above the triple point for carbon dioxide to prevent formation
of dry ice. Thermodynamic properties of COz are programmed
into the system microprocessor (not shown). Output from
pressure sensor 196 and temperature sensor 194 are compared
with the programmed data to determine how close the COZ fluid
is to the dry ice region. This also determines the degree to
which the C02 vapor is superheated. The microprocessor
responds accordingly by directing valve 54 to either open up
some more or close some so as to maintain a desirable level of
superheat of about 100°F. Although this is the preferred
method to determine the superheat condition of the C02 vapor
(you need both, the pressure and the temperature of the fluid
to determine the superheat), the system can perform
satisfactorily without the pressure sensor 196. The fluid
pressure in coils 62, 64 and line 74 are at substantially the
same pressure and this pressure can be determined by looking
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up the saturated pressure (from the programmed data) for the
corresponding saturated temperature valve output of
temperature sensor 192. The pressure value thus determined is
reasonably close to the actual pressure of the fluid as would
be determined by pressure sensor 196.
Main liquid output line 30 is directed through shut-off
valve 32, excess pressure relief valve 34, and out of
cryogenic tank subsystem 10 via liquid line 48. Line 18 is
heated through the insulated wall of storage vessel 12 and is
used as an internal pressure builder. ,Line 18 contains a
drain plug 20 for cleaning and maintenance of storage vessel
12. Line 18, via shut-off valve 50, pressure regulator 22,
pressure gauge 24, pressure relief valve 28 and shut-off valve
26 is used to maintain pressure within storage vessel 12 at
the desired level.
The cryogenic liquid supplied by main liquid line 48 is
filtered by filter 52 and flows through shut-off valve 54
before being applied to two-way valves 56 and 58 for selection
of cooling or heating mode. If heating mode is selected, the
cryogenic liquid is supplied by valve 56 to propane heater 60
for super heating as taught in the above referenced and
incorporated co-pending applications. If cooling mode is
selected, valves 58 and 66 route the cryogenic liquid through
evaporation coils 62 and 64 as also described in further
detail in the above referenced applications.
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Also in accordance with the above referenced commonly
assigned patent applications, line 74 directs vapor from the
low pressure end of evaporation coils 62 and 64 to power vapor
motor generator 68 before being released to the atmosphere via
muffler 76. However, as is discussed above, evaporation from
evaporation coils 62 and 64 tends to be uneven at system
start-up, because motor generator 68 has not yet received
sufficient vapor to begin rotation. Therefore, no ventilation
is present at evaporation coils 62 and 64 during system start-
up.
In the preferred embodiment of the present invention,
carbon dioxide vapor is directed via line 46 and shut-off
valve 70 to motor generator 68 via line 72 at system start-up
to provide immediate ventilation. This ensures even
evaporation and prevents clogging of evaporation coils 62 and
64 at system start-up.
As a further enhancement to efficiency, line 78 directs
vapor leakage from valve 66 to motor generator 68 as shown.
Having thus described the preferred embodiment of the
present invention in detail, those of skill in the art will
readily appreciate the construction and use of yet further
embodiments within the scope of the claims hereto attached.
