CA1319021C - Co_ temperature control system for transport vehicles - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A cooling system for an insulated enclosure, comprising an evaporator containing CO2 in liquid and gaseous phases. A vent is coupled to the evaporator to release CO2 gas in order to reduce the pressure in the evaporator caused by the vaporization of CO2 therein. An electronic controller regulates the opening and closing operation of the vent to produce a generally cyclical pressure variation in the evaporator, having an amplitude selected in accordance with the heat absorption rate of the system, whereby regulating the CO2 discharge rate from the evaporator in accordance with the heat absorption rate. A ventilation system is also provided to produce an air current in the enclosure. The ventilation system includes ducts and a blower powered by the CO2 gas released from the evaporator.

Description

- I ~ 3 ~

'rlT~E- CO2'~EMPER~TURE ('ONTR~L SYSr~EIM E'OR
TRANSPORir VE,HICI.EC.
FI~L~ GE''f'HE, INVEN'.l'ION

S 'rhe present invention relates to the general field of temperature conl;ro:L anc1, more particularl.y, ~o a cr~loclenic cooliny system and to a method for cooliny an insulated enclosure, well suitecl for transport vehic.Le, such as straight body -trucks, trailer trucks, railroad cars or the like. The inventlon also extends to the comhinat.Lon of a eryogenic cooliny sys-tem and a heater unit to achieve a -temperature control under a wide ranye of environmental condi-tions.

BACKGROUN~ OE THE INV ~TION

A transport vehicle for perishable goods is, by definition, an insulated storage chamber where products are maintained at a predetermined -temperature duriny the transportation by means of a heatiny/cooling system. For a yood performanee, the heating/cooli.ny system must have the ability to quiekly absorh heat or eold which eould penetrate the chamber, as well as the heat yenerated ly certain type of products, in order to maintain the product in a s~tis~actory conclition.
Mechanical re~rigeration is the standard for refrigerated transport vehieles eventhouyh it has been widely reeognized that this tempera-ture control approach is qui-te complex and does not guarantee the basie requirements outlined above.
In an attempt to upyrade the dependability and the cooliny capability of refri.gerated transport vehieles and avoid dessieation problems as well as unadequate air eireulation assoeiated with mechanical refrigeration, eryoyenic cool:iny sys~;ems have been developed duriny the ,. q~

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~ 2 past recen~ y*ars, theoret:ically outperformincJ hy fclr mechanical refricJeratior! units.
Cryogenic coolinc~ has evolved aloncJ two di.fferent approaches to so:Lve -the problem of temperature cont:rol.
The i.nje(~tion approach is the s.implest one. It consists of spraying liquid cryogell, such as C02 kept uncler high pressure and low temperature, directly into the insu:Lated enclosure at atmospher:ic pressure. Immediately, dry snow (solid C0~) and C02 vapors a~ -110F are formecl. As the dry l.0 snow sublimates, it absorbs heat at the rate of ~46 Btu per pound of snow, or at 120 Btu per pouncl of liquid C02 injected.
The major cdra~back of the direct injection method resides in that it does nok guarantee a uniform temperature. The in~ected C02 can reduce the temperature of the enclosure in an un~ont.roll.ed manner and so rapidly to a point where damage to the stored product may occur, especially when the C02 comes in direct contact with a sensitive product. In acldition, the C02 rarefies the oxygen in the enclosure causing problems to human beings and to some breathing products. For all these reasons, the injection technique is mostly used for freezing where precise kemperature control is not really essential.
The other approach, usually referred to as "vaporization" consists in recuperatiny the latent heat obtained when liquid C02 converts to gas inside an evaporator. This method allows a more precise temperature control and does not affect the ox~yclen content of the atmosphere in the storage chamber.
Cooling systems, based on the vaporiza-tion approach, are definitely an improvement over cryogen injection units in terms of temperature control, however they are s-till far from being fully satisfactory in this respect.

OBJECTS AND STATEMENT OF THE INVENTION

An ohject of the present invention is an improved cryogenic cooling system of the vaporization type.
Another object of the invention is a vaporization type cryogenic cooling system and a method for cooling an insulated enclosure that allows to maintain a more precise and uniform temperature in the insulated enclosure, comparatively to conventional systems.
A further ob]ect of the invention is a cryogenic cooling system of the vaporization type with a ventilation system for the insulated enclosure, whose operation is regulated in accordance with the heat absorption rate of the cooling system.
Another object of the invention is a cryogenic cooling system of the vaporization type whose components are arranged to reduce the heat infiltration in the insulated enclosure.
Yet, a further object of the invention is a temperature control system with a cryogenic cooling unit and a heating device that can draw power from vaporized cryogen.
In accordance with one aspect of the invention, there is provided a cryogenic cooling system comprising an evaporator thermally coupled to the enclosure to be refrigerated, containing cryogen in li~uid and gaseous phases. The preferred cryogen is CO2 which is both inexpensive and readily available, however, any other suitable substance may be used.
The pressure of CO2 in the evaporator is adjusted by a vent whose operation is controlled by a system, preferably of the type that regulates the vent operation in accordance with data from various sensors such as a temperature senso~ in the insulated enclosure and a pressure sensor in the CO~ evaporator.

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As part of the liquid COl evaporates when it ahsorbs hea-t from ~he insulatecl enclosure, the ~otal pressure :Ln the evaporator increases up to a level where the cont:ro:L
system commands the vent to release some gaseous CO2 in order to reduce the pressure in the evaporator. The spreacl between the vent open:ing ancl closinc~l E~ressures determines the amoun~.of CO2 converting from llquid to gas, between two vent openings.
The amplitucle of -the pressure variation in the evaporator resulting from the vent operation is controlled in accordanca with the heat ahsorption rate of the evaporator for, in turn, obtaining a steadier discharye rate of vaporizecl CO2 from the evaporator.
A ventilation system comprising ducts and a yas operated fan is provided to obkain a more homogenous temperature in the insulated enclosure by creatiny a slight air current therein preventiny the formation of warm spots. The yas fan is powered by vaporized CO2 escaping the evaporator and as a result of the reyulated CO2 discharge, a steady fan operation is obtained.
In a preferred embodiment, the method to control the heat absorption rate by the evaporator consists of adjusting the average CO2 pressure therein. ~hus, when the system operates at a hiyh eapacity, the average CO2 pressure is relatively low and the spread between the vent opening and closiny pressures is large, and conversely, at minimum capacity the average CO2 pressure is relatively high and the spread between the vent opening and closincJ
pressures is small. The above translates into a large quantity of cryoyen gas generated at maximum capacity, d.riving the ven-tilation fan fast, and into a lesser quantity of gas at minimum capacity driving the fan slowly. By adjusting the spread between the vent opening and closiny pressures in accordance with the gas volume to be generated by the evaporator, a steadier ON-OFF

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cycling freque~cy vf the evaporator vent is obtained a:Llowing to ~ceep -the l)lower in a quasi-continllo~ls operation while the evaporator absorbs heat, regardless of the amount of ~as produced.
S By comparison, a system with a constant pressure differen~ial. between the ven-t openinc) ancl closing pressures throughout the entire average CO2 pressure range ln the eval~orator, would result into a considerable variatlon in the ON-OFF eyclin~ freqllency of the vent. At maximum capacity, the vent will cycle very quickly, continuously supplying gas to the ventilation blower.
However, at minimum capacity, the cycling frequency will he much lowe.r, supplying gas to the blower in bursts a-t long intervals, possibly causing the blower to cease :its operation between two consecutive bursts due to the lack of operating fluid.
For a more stable operation of the blower, it is preferred to store the exhausted CO2 in a temporary gas reserve .~ank before it is being supplied to the gas fan.
The location of the temporary gas reserve tank is not a crltical element for the satisfactory operation of the cooliny system. However, it has been found that an advantage may be gained by locating this tank so that it can intercept heat which o~herwise would have penetrated ~5 in the enclosure. This is particularly advantageous for refrigerated transport vehicles where the insulation of the enclosure must be limited due to space considerations, especially in the floor region which constitu~es a major area of heat infiltration.
It has been found that locating the temporary gas reserve tank underneath the floor reduces heat infiltration because the CO2 stored therein is cold and will absorb heat.
For added versatility, the cooling system is preferably coupled to a heating unit which allows to - 6 - ~3~9~2~

obtain an adeq~late temperature in the :insulclted enclosure whell the outsicle temperature :is below the freez:ing ~.)o.int.
The heatincJ unit is of conventional construc-tion, o:~ the diesel type :~or example, ~ouplecl to the ventila~ion system to supply hot air therein. For s:ituations where no e~ternal power so~lrce is ava:ilable antl when the heat requirement is limited, C02 gas may be .released from a C02 reserve tank to drive a generator supplying electric current ~o the diesel glow plugs, the gas also driving the blower of the ventilation system to obtain an aclequate warm air circulation through the same ducting .system used to convey cool air. Xf the liquid CO2 in the reserve tank is too cold to generate vapours, the li~uid C0~ is warmed up through a heat exchanger locatecl in the ventilation ducts.

Therefore, -She present invention comprises, in a general aspect, a cooliny system for an insulated enclosure, comprising:
- an evaporator thermally coupled to the enclosure, the evaporator containing cryogen in liquid and gaseous phases, heat from the enclosure transferred to the liquid to the gaseous phase whieh in~reases the pressure in -the evaporator;
- vent means coupled to the evaporator, the vent means being capable to assume an opened and closed condition in the opened condition the vent means releasing yaseous cryogen from the evaporator to reduce the pressure therein, in the closed condition the vent means preventing re]ease of cryogen from the evaporator;
- control means for commanding the vent means to assume the opened and closed conditions at respective and predetermined vent opening and closing pressures of cryogen in the evaporator, the control means constituting means for varyiny the pressure di.fferential defined 131902~

between the ven~ opening and closing pressures of cryogenin accordan-e with ~.he heat absorption rate of ~he cooli.ny system for regulating the rate of cryoyen discharge from the evaporcltor in accordallce with tlle heat absorption .rate;
- ventilation means Eor creatLncJ an air current in the enclosure, inclucling:
a) duct means for conveying air; and b) gas powered fan means in the duct means to propel air therein, the vent means being coupled to the fan means for supplying thereto opera~ing gaseous cryogen.

The invention also extends to a transport vehicle, comprising:
- a top wall;
- a bottom wall;
- side walls, these walls defining an insulated enclosure;
- door means on one of the walls for accessing the enclosure;
- a cooling system for the enclosure, including:
a) an evaporator mounted to the top wall, ~he evaporator containing cryogen in liquid and gaseous phasesr heat from the enclosure transferred to the evaporator causing cryogen therein to convert from the liquid to the gaseous phase which increases the pressure in the evaporator;
b) vent means coupled to the evaporator, the vent means being capable to assume an opened and closed condition, in the opened condition the vent means releasing gaseous cryogen from the evaporator to reduce the pressure therein, in the closed condition the vent means preventing release of cryogen from the evaporator;
c~ control means for commanding the vent means 5 to assume the opened and closed conditions, at respective 1~ 9~2~

and predeterminecl vent open~ncl and closincJ pressures O e cryogen in -the evaporator, the control means constit;u-ti.ng means for varying the pressure differential defined between the vent opening and closi.ny pressures of cryogen in accordallce with the heat absorption rate o:~ the cool:ing system for reyulatiny the rate of cryogen tlischarcJe ~rom the evaporator in accordance with the heat absorp-t:ion rate;
d) ~entilation means for creating an ai.r current in the enclosure, including:
i) duct means for conveying air; and ii) gas powered fan means in the duct means to propel air therein, the vent means he:ing coupled to the fan means for supplying thereto operating gaseous cryogen.
~he invention also comprehends a temperature control system for an insulated enclosure, comprising:
- a c!ooling system for an insulated enclosure, comprising:
- an evaporator thermally coupled ~o the enclosure, the evaporator containing cryogen in liquid and cJaseous phases, heat from the enclosure transferred to the evaporator causing cryoyen therein to convert from the licfuid to the gaseous phase which increases ~he pressure in the evapora~or;
- vent means coupled to the evaporator, the vent means being capable to assume an opened and closed condition, in the opened condition the ~lent means releasing gaseous cryogen from -the evaporator to reduce the pressure therein, in the closed condition the vent means preventing release of cryogen from the evaporator;
- control means for commanding the vent means to assume the opened and closed condi-tions at respective and predetermined vent opening and closing pressures of cryogen in the evaporator, the control means constitu~ing 1319~

means for vary:lng the pressure dlfferentia:L defi.ned between -the vent open:ing arlcl closing pressures of cryoyen ln accordallce wlth the heat absorption rate of the eool.lny system for recJu:Lat:ing the rate of cryogen discharge from the evaporator in accorclance wit,h the heat absorption ra-te;
- ventilation means for creatinq an alr cu:rrent, :in the enclosure, i.ncludlng:
a) duet means for conveying air; ancl 10b) yas powerefl fan means in the duct means to propel alr thereln, the vent means beiny eoupled to t,he fan means for supplying thereto operating gaseous c:ryogen.
- a heatiny unit coupl.ed to the duct means for supplying ~Jarm air in the enclosure.
15The inventlon further extends to a coolincl system for an insulated enclosure, comprlsing:
- an evaporator thermally coupled to the enclosure, the evaporator containing cryogen in li~uid and gaseous phases, heat from the enclosure transferred to the evaporator eausing eryogen therein to eonvert from the liquid to the gaseous phase which increases the pressure in the evaporator;
- vent means eoupled to the evaporator for releasing yaseous cryogen therefrom to reduce the pressure in the evaporator;
- control means for controlling the opening and elosing operation of the vent means to eause a generally eyelical pressure variatlon in ~he evaporator having an amp:Litude selected in aceordance with the desired heat absorption rate of the cooling system; and - ventilatlon means for creating an air curren-t in the enclosure, lncludlng:
a) duet means for eonveyiny air; anfl ~319~21 b) gas powered Ean means in the duct means to propel alr thereln, the vent ~einy couplecl to the f.ln means for s~lpplying tllereto operatlncJ gaseous cryogen The Lnvention furkher extends to a me-thod cor c~oo],lny an insulated enclosure, compLislncJ the steps oE
- placinc,l cr~oyen in liciuid and gaseous phases into an evaporator thermally couplecl to the enclosure~ the evapora~or comprising vent means to release gaseous cryogen -therefrom;
- openiny and closing said vent means to produce a generally cyclical pressure varlation in the evaporator having an amplitude selee~ecl in accordance with the heat absorption rate of the evaporatox;
- conveying gaseous cryogen from the vent means to a gas powered fan conveying air in duct means to create an air current in the enclosure.

BRIEF DESCRIPTION OF THE ~RAWINGS

- Figure 1 is a schematical view of the cooling/
heating system in accordance with this invention;
- Figure 2 is a top view of the evaporator, some elements heing omitted for clarity;
- Figure 3 is a vertical cross-sectional view of the evaporator;
- Fiyure 4 is a sectional view taken along lines 4-4 in Figure 3;
- Eigure 5 is an enlarged schematical view illustrating the connection between the evaporator and an aluminum skin enhancing heat transEer;
- Figures 6, 7 and 8 are schematical views of an insulated enclosure illustrating -the position oE the temporary gas reserve tanks and the C02 flow path therefrom;

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L, - E:igure 9 is a vertical sectiona:1 vi.ew of the insu:lated enc:losure showi.ng the gas -fans arrclncJement~ tlle heating unit and part o:E the ventilation clucts;
- Figure 10 is a schematical view OL the electronic S controller to regulate the operation of the cooling/heati.ng system;
- Ficlure lla is a c1iaclram of the CO2 pressure in the evaporator and in the reserve tank wi-th respect to time for a relatively low heat absorption rate;
- Figure llh is a diagram of the C02 vapour volume in the evaporator with respect to time for a relatively low heat absorption rate;
- Figure llc is a diagram of the CO2 pressure in the gas reserve tank with respect to time for a relatively low heat absorption rate;
- Figure lld is a diagram of the velocity of C02 escaping the gas reserve tank, with respect to time, for a relatively low heat absorption rate;
- Figure 12a is a diagram of the CO2 pressure in the evaporator and in the reserve tank with respect to time for a relatively high heat absorption rate;
- Figure 12b is a diagram of the CO2 vapour volume in ~he evaporator with respect to time or a relatively high heat absorption rate;
- Figure 12c is a diagram of the CO2 pressure in the gas reserve tank with respect to time for a relatively high heat abosorption rate; and - Figure 12d is a diagram of the velocity of CO2 escaping the gas reserve tank, with respect to time, for a relatively high heat absorption rate.

DESC'RIPTION OF A PREFERRED _BODIMENT

The cooling/heating system in accordance with the invention is particularly well adapted for use on ~31~21 - :L2 -transport vehicles such as straight body trucks, tru~k trailers or railroacl cars, that are designecl for holding peri.shable goods at a stable temperature.
Referring t.o Flgure 1, -the c-ooling sys~em comprises an evaporator ldentiEied by the reference numeral lO
mountecl in the insulated enc:Losure in orcler to absorb heat therefrom, for maintain:Lng the temperature of the enclosure at a desired level. The evaporator 10 is a reservoir containing C02 in yaseous and liquid phases at a predetermined pressure which determines the heat absorption capacity of the system.
The pressure in the evaporator is controlled by a vent 12 constituted by four valves VA, VB, ~C ancl VD
respectively, connected to the evaporator 10 through a conduit 14. A reducer 15 set at 95 PSI is mounted in the gas line 14 for reducing the pressure of the C02 gas coming from the evaporator 10 to an easier level to handle. A
safety vent 16 set at 110 PSI is also coupled to the gas line 14, downstream of the reducer 15 to discharge in the atmosphere the C02 gas should the pressure exceed the predetermined level. A safety vent 18 is also provided in the gas line 14, between the evaporator 10 and the reducer 15. The safety vent 18 is set at a pressure in the order of 450 PSI considered as a safe operating limit for the evaporator.
The Co2 gas leaving the valves VA to VD is stored lnto temporary gas reserve tanks 20 and 22 hefore being supplied to a gas motor driving a ventilation fan, as it will be explained in detail hereinafter. The temporary gas reserve tank 20 is fed exclusively by valves V~ and VB
through conduit. 17 and its pressure is malntained at a level below 60 PSI. The remaining vent valves, namely VC
and VD feed exclusively the gas reserve tank 22 through conduit 19.

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- .L3 ~ valve ~4 respons:Lve to the pressure :in the gas reserve tan]c 20 establishes a colnmunlcation between the tanks 2~ alld ~ when the pressl~re i.n the primary tank ~0 exceeds 60 PSI.
C2 cJaS iS discharged froln the tanks 20 arld ~2 :Into a gas line 26 through 30 PSX and 50 PSI reduce:rs 25 ancl ~7 respectively, to establish different pressure levels in the line 26 in accordance wlth -the flow rate of C02 gas from the evaporator 10. The gas line 26 conveys C02 gas to a gas motor 28 driving two fans 30 and 32. The gas motor 28 is in a continuous clriving relationship with the fan 30 whereas it is connected through an electric clutch 34 to the fan 32. When the gas motor 28 is supplied only from the reserve tank 20, the pressure in line ~6 is in the orcler of 30 PSI and only the fan 30 operates.
However, when the reserve tank 22 starts to discharge gas in the line 26, -the pressure therein rises to 50 PSI and the electric clutch 34 is engaged so that both fans are operated.
The fans 30 and 32 convey air into a ventilation system including ducts 36 with inle-t port 35 and an outlet port 37 both in the enclosure to be refrigerated. The purpose of this ventilation system is to provide an air clrculati.on in the enclosure so as to create a uniform temperature therein, to prevent the formation of warm spots and to push hot air toward the top where the evaporator is located. It should be appreciated that this ventilation system is different from prior art devices where an air stream is blown against a heat exchanger for assisting the evaporation of the cryogen therein. In the present case, the ventilation ser~es basically to homogenize the temperature.
The evaporator 10 is supplied wlth li~uid C0~ from an insulated reserve tank 33, through a condui-t 40 controlled ~y a valve 42. The transfer of liquid C02 from the reserve 1 3 ~

tank 38 -to the evapora-tor lO :Ls e~fectec1 by creatiny .a pressure d:Lffe3-er1t:ial between the reserve taQk 38 and t;he evaporator lO by opening valve 42 and vent 12 to lower the pressure ln the evaporator. It has been foul1d that this system is extremely simple and clepenclah:Le, however, the use of pumps or any other liq~1icl transfer ecluiplnent is clearly within the scope of this invention.
The pressure in the reserve tank 38 ls then rebuilt to its original value by transferring high pressure CO~gas from the evaporator lO to the reserve tank 38 throuyh a gas line 44 controlled by ~ valve 46 which closes when the desired pressure is reached in the reserve tan}c 38.
A safety vent 48, set at ~80 PSI, is provicled in the line 44 and a safety vent 39 set at 350 psi is provided in khe reserve tank 38 to burst in case of system failure.
The operation of the cooling system is controlled by a micro-processor based electronic controller 50 rece:iviny information from various sensors. More particularly, there is a pressure sensor 52 in the evaporator :L0, a pressure sensor 53 in the reserve tank 38, a temperature sensor 54 measuring the temperature in the insulated enclosure and a temperature sensor 56 for the outside temperature. Hiyh and low liquid levels switches 58 and 60 respeetively, are provided in the evaporator lO for providiny i~formation on the level of liquid COl therein.
Based on siynals provided by the sensors 52 to fiO, the controller 50 will regulate the operation of the cooling system in accorclance with the desired temperature in the enclosure. A more detailed description of the controller 50 will be provided hereinafter.
The controller 50 is powered by a yenerator 64, driven by the gas motor 2~, rechar~ing a battery 62 which provides electric power when the generator 64 is inoperative.

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A cli.esel heatinq unlt 300 is prQvided to genercl-te warm air in ~he ins,ulatecl enc~:losure when the ou~Lde tempe.rature is too low, as during the win-ter sedson. The heater unit 300, of a generally conventional corlstruction, 5i5 coupled to the venkilation clucts 36 by a conduit 302 through whicl- warm alr i~ suppllecl in the insu1atecl enclosure.
The heating un:it 300 has a dedicatecl ai.r intake conduit 303 from the insulated enclosure.
lOIt should be appreciatecl that the ducting system is such as to prevent hot air to be directly discharged from the heating unit 300 to the enclosure, by mixing the hot air challnelled by the condui.t 302 with cooler air drawn from the enclosure through the inlet port 35. This 15arrangement allows to raise the temperature of the enclosure steadily, preventing the formation of hot or cold spots therein.
When the heat re~uirement is minimal, such as when the temperature is close to the freezing point and when 20cooliny and heating periods may alternate, the electric power required to operate the heater 300 is extracted by vaporizing C02 from the reserve tank 38 to drive the generator 64 and the fan 30 for creating an air circulation.
25A heat exchan~er 304, mounted in the conduit 30~, is used tCl enhance the vaporization of li~uid C02. The heat exchanger 3Q4 conveys CO2 from the reserve tank 38 to the line l4 through a conduit 61, so as ~o maintain a minimum pressure of 95 PSI in llne 14 which feeds the gas motor 28 30driving the generator 64 and the fan 30. A valve 65 con-trol.s the fluid flow in the conduit 61.
However, when heat is required on a regular basis, the electric power requirecl to operate the heating unit 300 is normally supplied from the generator of ~he pulling ~319~2~

vehicle, continuously rechar~ing the battery 62 and actiVatincJ an e:lectric blo~1er (not-shown).
The concept of a cryogenic coolincJ~heatincJ system described ln relation to Figure 1, may he advantacJeously aclaptecl for a mecl~ rail truck or trailer used for the transport of ~resh beef carcases. Figures 2 to 9 illustrate the various components of the coolincJ system when installed in a trailer of this -type.
The evaporator 10, best illustrated in Figures 2, 3, 4 and 5, is installed at the top of the insulated chamber and comprises a plurality of extruded aluminum conduits 66, each conduit 66 being provided with two flancles 68 permitting attachment to the cross beams of the trailer roof, in d:irect contact with the aluminum ceiling of the trailer, and two flanges 70 in an inverted V-shape on which is mounted an aluminum skin 73 for a good therma:L
transfer between the conduits 66 and the insulated chamber.
The extruded aluminum conduits 66 are placed between the five meat rails 71 of the trailer. This arrangement creates six rows of two conduits 66 eachr leaving an empty space right in the center of the trailer ceiling.
The extremities of the conduits 66 are connected together by four manifolds 72, two in the center and one at each extremity of the trailer. The manifolds are shaped to clear the meat rails 71 in the trailer, as best illustrated in Figures 3 ancl 5.
Two steel tanks 74 containing liquid CO2 are mounted in the center of the trailer roof, between the two central manifolds 72 which are couplecl to the bottom of the tanks 74 by short conduits 76.
The manifolds 72 at the extremities of the trai:Ler ceiliny are connected to the top of each steel tank 74 by means of gas return conduits 78, best illustrated in Figures 2 and 4.

131~

A vertically ex-tendincJ receptable 80, connected to the top and to ~he bottom of the tanlcs 7~, is mourltecl therebetween and it is provicled with the liquid level switches 58 ancl 60 for identifyirlcl maximum and mlnimum levels of liquld C02 in the tanks 74.
As best i:llustrated in E'i.gure 5, the por-tion of the aluminum shee-t skin 73 associated Witil each conduit 66 comprises an inverted V-shaped central strip 92 fastened to the flanyes 70 by means of fasteners, such as bolts.
Strips 94 ex-tend between each extremity of the central strip 92 and the ceiling of ~he trailer. The strips 92 ancl 94 completely enclose the respective conduit 66 a:L1 along the ceiling of the trailer, between two meat rails 71. Drip pans 93 extend along and below the adiacent edges of the strips 92 and 94 to collect condensation water. It should be appreciated that the slanted walls of the strips 92 and 94 greatly assist in yulding the condensation droplets toward the drip pans 93.
The operation of the evaporator 10 is as follows.
When the trailer is moving on a flat road, the conduits 66 in the cargo spaGe and the manifolds 72 are filled by gravity with liquid C02 which is CominCJ down from the two steel tanks 74 inside the roof. These tanks 74 are supplied with fresh liquid C02 comi.ng from the storage tank 38, as described earlier whenever the level of the liquid C02 inside the tanks 74 has reached a preset minimum as determined by the low level liquid switch 60 The transfer of liquid C02 from the storage tank 38 to the tanks 74 stops as soon as the level of the liquid C02 inside the tanks 74 has reached a preset maximum detected hy the high level switch 58.
The gas return conduits 78 connecting the extremeties of the conduits 66 to the top of the tanks 74, are filled wi~h gaseous C0~, which is genera-ted by the evaporation of liquid C02 whenever heat is transferred to the evaporator ~3~2~
- l8 -10. The pressure in the evaporator can vary Erom 100 to 450 PSI, clependin~J on the required lleclt ahs0rpt:ioll of the system.
Whenever the trai~er is moving on an inc1lned roac1, part of -the licluid C'02 inside the conduits 66 will rise in the gas return concluits 78 whi~h are at the low encl of the trailer, while it slicles out of the conduits 66 at the opposite encl of the vehic:le. This movement of the licluid insicle the evaporator facilitates the evacuation of gaseous C02 from the conduits 66 toward the gas re-turn conduits 78 and the top of the steel tanks 74.
Since the two stee1 tanks 74 are in the exact center of the ceiling, ancl s.ince the volume of the conduits 66 has been calculated -to be equal to the volume of the gas return conduits 78r the level of the liquid C02 inside the vertical receptacle 80 holding the two level switches 58 and 60, is very stable regardless of the inclination of the roacl.
The C02 exhaust and ventilation system of the trailer is illustrated in Figures 6 to 9 of the drawings. The -trailer, identified generally hy the reference numeral 100, ~omprises a top wall 102, a bottom wall 104 ancl side walls 106 and 108, respectively. The trailer walls are provided with insulation material as it is customary in the art, to reduce heat infil~ration in the enclosure.
The temporary 4as reserve tanks are located be:Low the floor of the trailer 100 to limit hea-t infiltration therethrough. The tanks 20 and 22 are constituted by a series of rectangular shaped conduits extending the entire lenyth of the trailer and connected to the vent 12, located in the rear section of the ceiling, by the conduits 17 and 19 running around the opening of the back doors. This arrangement is particularly advantageous in limiting heat infiltration throuyh the floor and the doors ~3~2~

because the C02 gas stored in these tanks and condul~s is very colcl afte.r heing relaxed to 95 psi by the reducer lS.
The gas line 26 coupled to -the gas reserve tanks through the S0 PSI and 30 PSI reducers respectively, leads to the gas motor 23 operating at 3000 RPM maximulll, wi.th more or ].ess horse-power dependlncl of the gas sllpp:ly pressure. The gas motor 28 is continuously in driving relationship wi.th the blower 30 capable of deliveriny a maximum of 330 CFM at 3000 ~PM. The second blower 32, identical to the blower 30, is operated by the gas motor 28 through a belt transmission 100 and an electric clutch 34 made responsive to the pressure in the gas line 26 by a system of known construction, inclucling a pressure sensor to produce an engage/disengage control siynal.
The ductirlg system 36 comprises air intake sections 112 located at the top front of the refrigerated enclosure leading to the blowers 30 and 32, and outlet ducts 114 and 116, extending along the side walls 106 and 108 respectively, the entire length of the trailer close to the floor 104. The ducts 114 anc1 116 are provicled with a plurality of outlet ports leading in the reErigerated enclosure so that air drawn from the top front of the enclosure is distributed evenly near the floor thereof.
For an lncreased efficiency, the cold C02 gas ~5 exhausted by the blowers ~0 and 32 circulates through conduits 115 and 117 passiny in the ventilation ducting system 114 and 116, as schematically illustrated in Figure 8, to further cool the air circulating therein.
As gas C02 comes out of the ~alves VA or VA and VB, it progressively fills the tank 20; but, at the s~me time, it escapes by the 30 PSI reducer 25 in the gas line 26 to the gas motor 28 which activates blower 30. If ever the pressure inside tank 20 reaches 60 PSI, the excess pressure will go into tank 22. If the pressure in tank 22 is higher than in tank 20, then the air will escape in the - 20 -- 13~9021 yas line 2~ thro-lgll the 50 PSL reducer: 27, and will activate blowers 30 and 32, considering -that the electric clutch 3~ will be engaged. Evidentl~, if VC or VC and VD
are opened in acldition -to VA and VB~ tank 22 will fill very quickly and the two blowers will operate in priori~y.
The result is an air turbulence propc)rtionate to the heat absorption rate of the system.
Figure 10 illustrates a schematical diagram of the electronic controller 50. The controller is a micro-processor based circuit that receives data from ~arious sensors and outputs commancl signals to the various pneumatic valves of the cooling system to control the operation thereof. In addition, data such as temperature settings for the refrigerated enclosure, are entered through a standard keyboard/display unit.
The pressure sensor 52 mounted in the evaporator 10 has an operating range from O to 50~ PS~, and it is designed to perform accurate readings at relatively low temperatures. The pressure sensor commercialized under the trademark Omega, series 500, has been found satisfactory. The pressure sensor 53 mounted in the storage tank 38 is of identical construction.
The temperature sensor 54 measuring the temperature of the refrigerated enclosure is constituted by two sensor units 200 and 202 mounted at spaeecl locations in the refrigerated ènclosure to a].low for averaye temperature measurements. The sensor units 200 and 202 are basically current sensors which require local amplification and siynal processlng due to the weak signal procluced. In Figure 10, the local processor is illustrated by 204 which comprises an amplifier section and an analog circuit to average the signals from sensors 200 and 202. The processor 204 then supplies the output signal to the controller 50. The tempera~ure sensors, commercialized - 2.l -uncler the traclemark Omeyd, series AD590, have been foundsati..sfactory.
Tlle outsic1e temperature sensor 56 is sim:i.LaL^ in cons-truc-tioll to a sensor 54 except that it compr:ises a s:in~le SenSo:K unit 206, ldentical to the units 200 or 202, and a loca:L ampli-fier 208.
The liquid level indicators 58 and 60 are in the form of micro switehes coupled to the processor 50.
The data signals from the various sensors are processed by the controller 50 whieh outputs control signals on lines 210 to regulate the operation of the vent.
12 and the refilling of evaporator 10. The modifica-tion of the temperature setting in the refrigerated enclosure is perforrned through an input/output device 212. In a preferred embodiment, the input/output deviee 212 is an alphameric keyboard with a display. Howeverr other types of input/output deviees may also be used.
Electrie power is supplied to the eontroller 50 by the generator 64 through the battery 62 or the generator of the pulling vehiele.
Two pilot lamps 218 ancl 220, sueh as LE~ (light emitting~ diode) are coupled to the controller 50 to indieate its operating eondition. More partieularly, the LED 218 lndicates tha-t power is being supplied to the eontroller, whereas the indieator 220 indlcates a malfunetion thereof.
To turn off the controller 50, for maintainanee purposes for example, a switeh 222 is provided whieh, when open, turns off the eontroller 50 and consec.~uently, the entire eooling system.
The eontroller 50 eontains a memory in whieh is stored the state (opened or elosed) of eaeh control valve for different operating eonditions, in the form of a map.
In aeeordanee with a par~ieular operating condition determined fro~ data reeeived from the various sensors, ~3~ 2~

the prooessor accesses the pa~ticular memory slo~
correspondln~ to this operatiny conclition ancl outputC;
therefrom the state of each controllecl valve. This system is relatively simple and once properly programmecl, al:Lows to obtain consistant and trouble ~ree operation.
The ~ontrol:ler 50 is a:lso provided with a self-diagnostic ~ircuit which will indicate, through the I,~D
220, that a malfunction has occured and lmmediate attention is required.
The internal structure of the controller 50 is not an essential element to the invention considering that these types of controllers are relatively well lcnown and commercially available. Therefore, a detailed deseription of the system is not deemed to be necessary here.
lS The operation of the cooling system, in accordance with this invention, will now be described in conjunction with Figures 1, 11 and 12.
Assuminy that a certain quantity of liquid C'02 is present in the evaporator 10 and that a large amount of heat is penetrating in the enclosure or is being yeneratecl by the transported product, the cooling system will operate at maxlmum capacity to absorb the heat quickly and stop the temperature rising inside the enclosure. At this end, the controller 50 will regu].ate the operation of the vent 12 so that the vent closing and opening pressures will be relatively low and the spread between the vent opening and the vent closing pressure will be relatively large, allowing large amounts of C02 to vapori~e between two vent openings. More particularly, this will be achieved by opening valves VA t.hrough VD in order to allow a maximum flow of C02 gas from the evaporator.
Due to the successive vent opening and ven-t closing operations, the pressure in the evaporator will vary in a somewhat cyclical manner, as illustratecl in Figure 12a.
In this particular case, the actual evaporator pressure ~ 3 :L ~ 2 ~
~ 23 -setting co.rrespon-ling to the verlt c-losing pressure has been set at 16.0 bar; ancl the vent openinc.l pressure lla.c.
been set to 18.6 hars whereby the amplitude of -the pressure va~iation will be in the orcler of 2.6 bars.
Ihe C0~ gas leavincJ the vent 12 will f~ L both cJas reserve tanks 20 and 22 and will flow t~hrouclh the gas line 26 at a pressure of about 50 PSI. As a resu:L-t of the higher pressure in the gas line 26 the electric c:Lutch 34 will be engaged and -the c.Jas ~notor 28 will drive both blowers 30 and 32 in orfler to create a maximuln air curren-t in the refrigerated enclosure as a result of a high heat absorption of the system.
As the evaporator 10 absorbs heat from the enclosure liquid C02 converts to gas causing the level of liquid therein to progressively diminish. Figure 12b illustrates the variation of the vapour volume in the tanks 74 as a result of the evaporation/refilling cycle. ~hen the liquid level reaches the lower limit, the switch 60 is actuated indicating to the controller 50 that evaporator 10 must be refilled. The controller 50 will then open VA
and 42 for a period of time sufficiently long so that the pressure in the evaporator 10 drops below the pressure in the reserve tank 38 causing liquid C0~ to flow in the evaporator 10 as a result of the pressure differential created. When the liquicl level in the evaporator reaches the ma~imum causing switch 58 to close, the controller 50 will shut off the valves VA and 42 to interrupt the transfer of liquid C02.
Figures 12c and 12d illustrate the C02 pressure variation in the gas reserve tanks 20 and 22 and the velocity of the C02 gas leaving the tanks respectively for the above operating conditions. It will be appreciated that the C02velocity is constant thus driving the blowers 30 and 32 very regularly at a fast rate.

3 2 ~
- 2~ -When the heat penetrating :Lnside the enclo.sure isless, the heat absorption of the system has to he recluced by raising the ven-t opening and closing pressures, reclucing the spread between the vent opening ancl closing S pressure.s and reducing the number of vent valves allowecl to open. 'rhe heat absorption ra-te of the system i.s automati~ally further reduced with lesser hea-t penetration. At a very low heat absorption rate, only VA
will be allowed to operate. This condition is illustrated in Figure lla to lld. By comparison with Figures 12a to 12d, it will be noted that the pressure in the gas reserve tank 22 is more irregular resultiny in-to a jerlcy blower operation, which even stops for a short time period.
However, generally speaking, the blower remains in operation most of the time, thus allowing to obtain an adequate ventilation.
The liquid transfer from the reserve tanX 38 causes the pressure therein to drop slightly as illustrated in Figures lla and 12a. In order to repressurize the tank 38 for the nex-t refilling cycle, the valve 46 in the gas line 44 is opened to transfer high pressure C02 from the evaporator to the reserve tank 38 until it reaches the origlnal pressure in the reserve tank 38.
When the sensor 56 indicates a negative temperature outside and when the sensor 54 indicates a decrease of the temperature inside the enclosure compared to the set point, this is an 1ndication that there is a need for heat instead of cold. In this case, the controller 50 closes valve 42 ancl opens valve VA to provide gas pressure from the evaporator 10 to operate the glow and the electric motor of the diesel heater 300 with the 12V battery 62 which is recharged by the generator 64. Meanwhile the warm air generated by the diesel heater circulates inside the duct system 36 as it is forced by the blower 30 driven by the gas motor 28.

- 2~ - 13~ 2~

The air which is warmed up hy the di.esel heater 300 is aspiratecl from the enclosuxe by duc-t 302 before i.-t is mixed up witll air coming by duct 36, so the temperature of tlle warm air is modulated to the needs.
Located af-ter the exit of the diesel heatex 300 is a small heat excllanc~er 304 ab:le to vapori~e some liquicl C02 coming from the .reserve tank 38 when a valve 65 is opened to inject some gas C02 in the line 14 and to guarantee a minimum pressure oi 95 PSI inside the evaporator 10 and the associated piping.
As the evaporator never goes under 95 PSI there i.s no problem to refill the evaporator with fresh l.iquicl C'02 and reestablish the re~uired pressure inside the evaporator whenever there is need for cooling anew without running the risk oE generating dry snow.
Although the invention has been described with relation to a preferred embc,diment, it should be unclerstood that various changes and modifications obvious to one having the ordinary skill in the art may be made without departing from the scope of the invention. Eox example/ cryoyens other than C02 may be used such as nitrogen, oxygen, argon, hydxogen, helium, methane, freons, and carbon monoxyde. The scope of the invention is defined in the annexecl claims.

Claims (34)

1. A cooling system for an insulated enclosure, comprising:
- an evaporator thermally coupled to said enclosure, said evaporator containing cryogen in liquid and gaseous phases, heat from said enclosure transferred to said evaporator causing cryogen therein to convert from said liquid to said gaseous phase which increases the pressure in said evaporator;
- vent means coupled to said evaporator, said vent means being capable to assume opened and closed conditions, in said opened condition said vent means releasing gaseous cryogen from said evaporator to reduce the pressure therein, in said closed condition said vent means preventing release of cryogen from said evaporator;
- control means for commanding said vent means to assume said opened and closed conditions at respective and predetermined vent opening and closing pressures of cryogen in said evaporator, said control means constituting means for varying the pressure differential defined between said vent opening and closing pressures of cryogen in accordance with the heat absorption rate of said cooling system for regulating the rate of cryogen discharge from said evaporator in accordance with said heat absorption rate;
- ventilation means for creating an air current in said enclosure, including:
a) duct means for conveying air; and b) gas powered fan means in said duct means to propel air therein, said vent means being coupled to said fan means for supplying thereto operating gaseous cryogen.

2. A cooling system as defined in claim 1, wherein said control means constitutes means to increase said pressure differential with increasing heat absorption rate.

3. A cooling system as defined in claim 1, further comprising a pressure sensor in said evaporator, said control means being responsive to said pressure sensor.

4. A cooling system as defined in claim 3, further comprising a temperature sensor in said enclosure, said control means being responsive to said sensors.

5. A cooling system as defined in claim 1, wherein said evaporator comprises:
- a plurality of conduits generally horizontally extending and containing liquid cryogen;
- a feeding tank containing liquid cryogen above said conduit and being coupled thereto to cause liquid cryogen to flow from said feeding tank to said conduits by gravity; and - a gas return passage means connecting said conduits to a top portion of said feeding tank.

6. A cooling system as defined in claim 1, further comprising conduit means between said vent means and said fan means, said conduit means conveying vaporized cryogen driving said fan means.

7. A cooling system as defined in claim 6, wherein said conduit means includes a gas reserve tank for storage of vaporized cryogen.

8. A cooling system as defined in claim 7, wherein said conduit means includes two gas reserve tanks for receiving vaporized cryogen, passage means between said tanks, and valve means in said passage means for controlling exchange of vaporized cryogen between said tanks.

9. A cooling system as defined in claim 7, wherein said vent means comprises a plurality of valves individually controllable for regulating the flow rate of cryogen escaping from said evaporator.

10. A cooling system as defined in claim 9, wherein said conduit means includes a plurality of gas reserve tanks for receiving vaporized cryogen, each of said valves being associated with a given gas reserve tank through a predetermined fluid path, allowing to selectively feed said gas reserve tanks in accordance with the flow rate of cryogen escaping from said evaporator.

11. A cooling system as defined in claim 1, further comprising a pressure reducer in a fluid path between said evaporator and said vent means.

12. A cooling system as defined in claim 7, wherein said valve means is pressure responsive to establish a fluid path between said gas reserve tanks when the pressure in one of said tanks reaches a predetermined level.

13. A cooling system as defined in claim 1, wherein said fan means comprises:
- first blower means;
- a gas motor in driving relationship with said blower fan means;
- second blower means; and - a pressure responsive transmission means between said gas motor and said second blower means to establish a driving relationship therebetween when the pressure of vaporized cryogen supplied to said gas motor exceeds a predetermined level.

14. A cooling system as defined in claim 5, further comprising:
- a cryogen reserve tank;

- conduit means establishing a fluid path between said cryogen reserve tank and said feeding tank; and - valve means in said conduit means controlling the flow of cryogen therethrough.

15. A cooling system as defined in claim 14, further comprising a liquid level detection means coupled to said feeding tank for detecting the level of liquid cryogen therein, said valve means being responsive to said liquid level detection means.

16. A cooling system as defined in claim 15, wherein said liquid level detection means comprises maximum and minimum level switches mounted in said feeding tank, actuation of said minimum level switch causing said valve to open allowing cryogen to flow from said reserve tank to said feeing tank, upon actuation of said maximum level switch said valve means interrupting cryogen flow in said conduit means.

17. A cooling system as defined in claim 1, wherein said control means includes an electronic processing circuit, said cooling system further including a gas powered generator means coupled to said electronic processing circuit for supplying electric power thereto, said gas powered generator means being coupled to said vent means for receiving therefrom operating gaseous cryogen.

18. A transport vehicle, comprising:
- a top wall;
- a bottom wall;
- side walls, said walls defining an insulated enclosure;
- door means on one of said walls for accessing said enclosure;
- a cooling system for said enclosure, including:

a) an evaporator mounted to said top wall, said evaporator containing cryogen in liquid and gaseous phases, heat from said enclosure transferred to said evaporator using cryogen therein to convert from said liquid to said gaseous. phase which increases the pressure in said evaporator;
b) vent means coupled to said evaporator, said vent means being capable to assume opened and closed conditions, in said opened condition said vent means releasing gaseous cryogen from said evaporator to reduce the pressure therein, in said closed condition said vent means preventing release of cryogen from said evaporator;
c) control means for commanding said vent means to assume said opened and closed conditions, at respective and predetermined vent opening and closing pressures of cryogen in said evaporator, said control means constituting means for varying the pressure differential defined between said vent opening and closing pressures of cryogen in accordance with the heat absorption rate of said cooling system for regulating the rate of cryogen discharge from said evaporator in accordance with said heat absorption rate;
d) ventilation means for creating an air current in said enclosure, including:
i) duct means for conveying air; and ii) gas powered fan means in said duct means to propel air therein, said vent means being coupled to said fan means for supplying thereto operating gaseous cryogen.

19. A transport vehicle, as defined in claim 18, further comprising:
- conduit means between said vent means and said fan means, said conduit means conveying vaporized cryogen driving said fan means;
- a gas reserve tank in said conduit means for storage of vaporized cryogen; and - pressure reducer means between said evaporator and said gas reserve tank for reducing the pressure of cryogen in said gas reserve tank with respect to the pressure of cryogen in said evaporator.

20. A transport vehicle as defined in claim 19, wherein said gas reserve tank is mounted adjacent said bottom wall for absorbing heat passing therethrough,

21. A transport vehicle as defined in claim 20, wherein said gas reserve tank is shaped to extend along a substantial portion of the surface of said bottom wall.

22. A transport vehicle as defined in claim 21, comprising a conduit means between said vent means and said gas reserve tank, adjacent said door means.

23. A transport vehicle as defined in claim 19, further comprising exhaust conduit means to evacuate cryogen from said fan means, said exhaust conduit means extending along said duct means for absorbing heat therefrom.

24. A temperature control system for an insulated enclosure, comprising:
- a cooling system for an insulated enclosure, comprising:
- an evaporator thermally coupled to said enclosure, said evaporator containing cryogen in liquid and gaseous phases, heat from said enclosure transferred to said evaporator causing cryogen therein to convert from said liquid to said gaseous phase which increases the pressure in said evaporator;
- vent means coupled to said evaporator, said vent means being capable to assume opened and closed conditions, in said opened condition said vent means releasing gaseous cryogen from said evaporator to reduce the pressure therein, in said closed condition said vent means preventing release of cryogen from said evaporator;
- control means for commanding said vent means to assume said opened and closed conditions at respective and predetermined vent opening and closing pressures of cryogen in said evaporator, said control means constituting means for varying the pressure differential defined between said vent opening and closing pressures of cryogen in accordance with the heat absorption rate of said cooling system for regulating the rate of cryogen discharge from said evaporator in accordance with said heat absorption rate;
- ventilation means for creating an air current in said enclosure, including:
a) duct means for conveying air; and b) gas powered fan means in said duct means to propel air therein, said vent means being coupled to said fan means for supplying thereto operating gaseous cryogen.
- a heating unit coupled to said duct means for supplying warm air in said enclosure.

25. A temperature control system as defined in claim 24, further comprising:
- cryogen reserve tank for supplying said evaporator with liquid cryogen;
- a gas powered generator means coupled to said heating unit for supplying electric power thereto; and - conduit means between said cryogen reserve tank and said gas powered generator means for supplying thereto operating gaseous cryogen.

26. A temperature control system as defined in claim 25, further including a heat exchanger in said conduit means, thermally coupled to said heating unit, said heat exchanger enhancing vaporization of cryogen.

27. A cooling system as defined in claim 14, further comprising a gas return line between said evaporator and said cryogen reserve tank for transferring high pressure gaseous cryogen from said evaporator to said cryogen reserve tank, and valve means in said gas return line to control the fluid flow therein, whereby establishing a predetermined pressure level in said cryogen reserve tank.

28. A cooling system as defined in claim 14, wherein said cryogen reserve tank is insulated for limiting heat infiltration therein.

29. A temperature control system as defined in claim 25, wherein said cryogen reserve tank is insulated for limiting heat infiltration therein.

30. A cooling system as defined in claim 1, wherein said cryogen is CO2.

31. A cooling system for an insulated enclosure, comprising:
- an evaporator thermally coupled to said enclosure, said evaporator containing cryogen in liquid and gaseous phases, heat from said enclosure transferred to said evaporator causing cryogen therein to convert from said liquid to said gaseous phase which increases the pressure in said evaporator;
- vent means coupled to said evaporator for releasing gaseous cryogen therefrom to reduce the pressure in said evaporator;
- control means for controlling the opening and closing operation of said vent means to cause a generally cyclical pressure variation in said evaporator having an amplitude selected in accordance with the desired heat absorption rate of said cooling system; and - ventilation means for creating an air current in said enclosure, including:

a) duct means for conveying air; and b) gas powered fan means in said duct means to propel air therein, said vent being coupled to said fan means for supplying thereto operating gaseous cryogen.

32. A cooling system as defined in claim 31, wherein said vent means is of the variable flow rate type, said control means regulating the flow rate capacity of said vent means in accordance with the heat absorption rate of said cooling system.

33. A method for cooling an insulated enclosure, comprising the steps of:
- placing cryogen in liquid and gaseous phases into an evaporator thermally coupled to said enclosure, said evaporator comprising vent means to release gaseous cryogen therefrom;
- opening and closing said vent means to produce a generally cyclical pressure variation in said evaporator having an amplitude selected in accordance with the heat absorption rate of said evaporator;
- conveying gaseous cryogen from said vent means to a gas powered fan conveying air in duct means to create an air current in said enclosure.

34