Abstract:
To provide a transportable cooling unit for maintaining a transport volume at a defined temperature, comprising a closed cooling circuit and a controller sensing a temperature present within said transport volume and controlling said cooling circuit so as to provide the cooling power demanded at said evaporator for maintaining said defined temperature and minimizing energy consumption, said controller operates said closed cooling circuit between a minimum possible cooling power and a maximum possible cooling power in a sequence of different operational stages, said controller further operates said closed cooling circuit in each one of at least two upper operational stages at a compressor speed related cooling capacity different said other upper operational stages and within said respective upper operational stages said controller operates a compressor in an uninterrupted mode and adjusts said cooling power stepless speed control of said compressor.

Description:
[0001]    The present disclosure relates to the subject matter disclosed in international application No. PCT/EP00/10994 of Nov. 8, 2000, which is incorporated herein by reference in its entirety and for all purposes. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The invention relates to a transportable cooling unit for maintaining a transport volume at the defined temperature, comprising a closed cooling circuit serially including a multi-stage compressor, a condenser, an expansion device and an evaporator arranged in said transport volume as well as a speed-controlled electric motor driving said compressor.  
           [0003]    Such transportable cooling units are, for example, disclosed in the article of R. D. Heap “Refrigerated containers in . . . ”.  
           [0004]    The object of the present invention is to provide a transportable unit cooling which provides minimized energy consumption in combination with optimized temperature stability within the transport volume irrespective of the environment.  
         SUMMARY OF THE INVENTION  
         [0005]    This object is achieved by a transportable cooling unit for maintaining a transport volume at a defined temperature, comprising a closed cooling circuit serially including a multi-stage compressor, a condenser, an expansion device and an evaporator arranged in said transport volume, a speed-controlled electric motor driving said compressor and a controller sensing a temperature present within said transport volume and controlling said electric motor so as to provide the cooling power demanded at said evaporator for maintaining said defined temperature and minimize energy consumption, said controller operating said closed cooling circuit between a minimum possible cooling power and a maximum possible cooling power in a sequence of different operational stages comprising a lowest operational stage (stage  1 ) and a sequence of at least two upper operational stages (stage  2  to  4 ), said controller operating said closed cooling circuit in each one of said upper operational stages (stage  2  to  4 ) at a compressor speed related cooling capacity different from said compressor speed related cooling capacity in said other upper operational stages and within said respective upper operational stages in an uninterrupted mode and adjusting said cooling power provided by said closed cooling circuit by an essentially stepless speed control of said electric motor.  
           [0006]    The advantage of the present invention is to be seen in the fact that due to the sequence of different operational stages the compressor can be run within a reasonable speed range which is advantageous for an optimized compressor design and optimized compressor energy consumption but within the speed range different levels of cooling power can be achieved by using different operational stages of the closed cooling circuit, which makes it possible to minimize energy consumption of the entire system.  
           [0007]    According to the present invention control of the speed of the electric motor could be achieved by various means. It turned out to be advantageous for the speed controllable electric motor to be a frequency controlled AC-motor, because in such a frequency controlled AC-motor the energy consumption can be reduced in accordance with the speed of the controllable electric motor.  
           [0008]    In accordance with the aforementioned definitions of the present invention it is not defined how the controller operates the compressor in said low operational stage. It is particularly advantageous if in said lowest operational stage said controller operates said compressor in an interrupted mode at low speed and adjusts said necessary cooling capacity by adjusting at least one of the parameters comprising interruption interval and speed.  
           [0009]    The advantage of this embodiment of the present invention is that in the low operational stage it is allowed to switch the compressor on and off so as to be able to control low cooling power of the closed cooling circuit and to maintain the energy consumption dependent on the demanded cooling capacity but to maintain a certain level of speed if the compressor is operable for maintaining a reasonable level of compressor efficiency.  
           [0010]    It is particularly advantageous if the controller in said low operational stage maintains said speed of said compressor essentially constant and varies the interruption interval, e.g. the intervals within which the compressor is switched on or off so that the cooling power is only controlled by controlling the interruption intervals.  
           [0011]    In such an embodiment it is of particular advantage if in said lowest operational mode said speed of said compressor is in the dimension of the minimum possible speed for the compressor. This means that the compressor is run at the lowest allowable speed for proper operation and that if only cooling power is needed which is lower than the cooling power provided at that minimum speed a further reduction is performed by interrupting the compressor.  
           [0012]    In connection with the aforementioned explanations of various embodiments of the present invention it has not been defined how the controller determines the cooling power demanded.  
           [0013]    One manner of determining the cooling power demanded would be to only detect the temperature present within said transport volume and to reduce the speed of the compressor to the lowest possible level.  
           [0014]    A more advantageous manner of determining the cooling power demanded is to compare the temperature present within that transport volume and the temperature of ambient air.  
           [0015]    With respect to the temperature detection within the transport volume the location of detection has not been defined in connection with the explanation of the aforementioned embodiments.  
           [0016]    Generally, the temperature within the transport volume can be detected anywhere therein.  
           [0017]    For obtaining a fast response of the temperature detection it is advantageous if the controller senses the temperature in a stream of air circulating within said transport volume because in such a case the controller obtains the proper temperature values with a short response time.  
           [0018]    In addition, it is advantageous to sense the temperature within said transport volume close to said evaporator because in this case the cooling power demanded can be determined more precise.  
           [0019]    In general, the controller could start in the uppermost operational stage or in the lowermost operational stage and follow the sequence of operational stages until the desired temperature is obtained.  
           [0020]    To be able to respond precisely to temperature changes it is of advantage if the controller selects the currently necessary operational stage in accordance with the cooling power demanded.  
           [0021]    In accordance with the present invention, as discussed above, it would be possible to have a varying compressor speed related cooling capacity of said closed cooling circuit within at least one of said upper operational stages, however, for designing an easily controllable system it is of advantage if said compressor speed related cooling capacity of said closed cooling circuit is constant within each of said upper operational stages.  
           [0022]    With respect to the lowest operational stage the compressor speed related cooling capacity could vary too. However, it is also of advantage if said compressor speed related cooling capacity of said closed cooling circuit is constant within said lowest operational stage.  
           [0023]    With respect to a cost effective design of the inventive cooling unit it turned out to be advantageous for said compressor speed related cooling capacity of said closed cooling circuit to be the same as the compressor speed related cooling capacity in said one of said upper operational stages covering the lowest range of cooling power of said sequence of upper operational stages.  
           [0024]    If the controller has the possibility to switch from one upper operational stage to another operational stage such a switching is advantageously defined by a respective cooling power. To avoid at this respective cooling power a fast witching back and forth between one upper operational stage and the other operational stage it is advantageous if the controller switches from one upper operational stage to another upper operational stage with a hysteresis with respect to the level of cooling power, which means that the cooling power at which the controller switches from one upper operational stage to the next higher operational stage is higher than the cooling power at which the controller switches from the higher operational stage to the next lower operational stage.  
           [0025]    In the course of such a switching from one operational stage to the next operational stage the cooling power provided by the closed cooling circuit could come out of control.  
           [0026]    This is avoided if in the course of a transition from one of said upper operational stages to another of said upper operational stages said controller maintains full control of the cooling power provided by said closed cooling circuit by adjusting the speed of said compressor in accordance with a change of the compressor speed related cooling capacity.  
           [0027]    This means that even in the course of a transition from one operational stage to the next operational stage, which has the consequence that the corresponding compressor speed related cooling capacity changes, precise control of the cooling power provided is still maintained due to the fact that the controller even in the course of such a transition is still able to adjust the cooling power by adjusting the speed of the compressor.  
           [0028]    An advantageous embodiment of the present invention provides a compressor designed as a multi-stage compressor which is operable in a first mode using a reduced number of stages and in a second mode using all stages of said compressor for compressing refrigerant. Such a design has the advantage that when operating the compressor at a reduced number of stages the compressor speed related cooling capacity can be reduced and in addition the energy consumption is reduced due to the lower amount of energy which is needed for operating such a multi-stage compressor in a reduced number of stages.  
           [0029]    It is of particular advantage if such a multi-stage compressor is controllable by said controller of said closed cooling circuit so as to operate in said first mode or said second mode.  
           [0030]    It is of particular advantage according to the present invention if in one of said upper operational stages said compressor operates in said first mode and in another of said upper operational stages said compressor operates in said second mode because then different operational stages can be defined by operating the compressor in different modes, e.g. a first and a second mode, and the controller can be used to switch the compressor between said first mode and said second mode.  
           [0031]    In an embodiment of particular advantage it is provided that said controller changes from a stage in which the compressor operates in said first mode to the stage in which the compressor operates in said second mode at a defined level of cooling power which is higher then the defined level of cooling power at which the controller switches from the operational stage in which the compressor operates in said second mode to the operational stage in which the compressor operates in said first mode. Such a hysteresis used for changing between two operational stages is advantageous insofar as it prevents the controller at a certain level of cooling power from switching back and forth between the operational stages and therefore providing an unstable controlling characteristic which in particular has the consequence that the tolerances with respect to the defined temperature within the transport volume increase.  
           [0032]    In another advantageous embodiment according to the present invention an economizer is provided in said closed cooling circuit.  
           [0033]    Such a economizer could be designed to be fully operable within the entire operational range of the cooling unit.  
           [0034]    However, it is of particular advantage if said economizer can be switched by said controller between an economizer off-mode and an economizer on-mode.  
           [0035]    For providing different compressor speed related cooling capacities it is of particular advantage if in one of said upper operational stages the closed cooling circuit is controlled to operate in an economizer off-mode and in another of said upper operational stages said closed cooling circuit is controlled to operate in an economizer on-mode. Such an embodiment of the present invention has the advantage that within the same range of speed of the compressor, different compressor speed related cooling capacities can be obtained and these different compressor speed related cooling capacities also result in a different energy consumption by the compressor, because in the economizer on-mode the energy consumption of the compressor is increased with respect to the economizer off-mode.  
           [0036]    To avoid an unstable behaviour of the cooling unit and, therefore, to avoid increased temperature tolerances due to unstable conditions an advantageous embodiment provides that said controller switches from the operational stage in which the closed cooling circuit is in the economizer off-mode to the operational stage in which the closed cooling circuit is in the economizer on-mode at a defined level of cooling power which is higher than the defined level of cooling power at which the controller switches from the operational stage in which the closed cooling circuit is in the economizer on-mode to the operational stage in which the closed cooling circuit is in the economizer off-mode.  
           [0037]    The aforementioned object is further achieved by a refrigerated container comprising a thermally insulated housing enclosing a transport volume to be cooled, a cooling unit for cooling air circulating in said transport cooling volume, wherein said cooling unit is designed according to the features of the various embodiments as explained before.  
           [0038]    Further advantages of the present invention are the subject matter of the detailed description of one embodiment of the present invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]    [0039]FIG. 1 shows a sectional view along lines  1 - 1  in FIG. 2 of a container provided with one embodiment of the present invention;  
         [0040]    [0040]FIG. 2 shows a sectional view along lines  2 - 2  in FIG. 1;  
         [0041]    [0041]FIG. 3 shows a scheme of the various components of the cooling unit according to the present invention;  
         [0042]    [0042]FIG. 4 shows details of the compressor on an enlarged scale; and  
         [0043]    [0043]FIG. 5 shows a schematic representation of the relationship between cooling capacity and speed of the compressor in various stages of operation. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0044]    A refrigerated container  10  designed for the transport of perishable cargos  12 , such as, for example, frozen fish, chilled meat, fruit or chocolate or flower bulbs comprises a thermally insulated container housing  14  enclosing a transport volume  16  which is cooled by cooling unit  18 .  
         [0045]    Within the insulated housing air is circulated by an evaporator fan  20  receiving a return air stream  22  extending along a cover  24  of housing  14  and blowing this return air stream  22  through an evaporator  30  so that the stream of air is cooled and thereafter blown towards a bottom  26  of housing  14  as a supply air stream  23  and extending along bottom  26  between T-bars  28  of a T-bar floor of housing  14 .  
         [0046]    Preferably, evaporator fan  20  and evaporator  30  are arranged at a front portion  32  of housing  14 .  
         [0047]    Preferably, the evaporator  30  extends over a major portion of the width of front portion  32  between side walls  34  and  36  of housing  14 .  
         [0048]    Preferably, a front wall  38  of front portion  32  extends downwardly from cover  24  along a front end  40  of housing  14  and below evaporator  30  a portion  42  of front wall  38  steps back from front end  40  to form a space  44  between front end  40  and portion  42  of front wall  38  which is separated from transport volume  16  by portion  42  and in which a condenser  50  and a compressor  60  are arranged. Space  44  can be penetrated by a stream  52  of ambient air extending through condenser  50  and around compressor  60  for cooling of condenser  50  and compressor  60 , said stream  52  of ambient air being blown through space  44  by a condenser fan  54 .  
         [0049]    Evaporator  30 , condenser  50  and compressor  60  are part of a closed cooling circuit  70  shown in detail in FIG. 3.  
         [0050]    As can be seen in FIG. 3 compressor  60  receiving evaporated refrigerant compresses this refrigerant and discharges it into discharge pipe  62  extending between compressor discharge port  64  and an inlet port  66  of condenser  50 .  
         [0051]    The refrigerant after having passed through condenser  50  leaves through an outlet port  68  and is fed to a water cooled condenser  71  by pipe  72 . After having passed through a water cooled condenser  71  condensed refrigerant passes a drying filter  74  arranged in pipe  76  guiding condensed refrigerant to economizer  77 . After having passed through economizer  77  condensed refrigerant is passed via feed pipe  78  to electronic thermo valve  80  which is the expansion device and from electronic thermo valve  80  to an inlet port  82  of evaporator  30  and after being evaporated within evaporator  30  to outlet port  84  which is connected to a compressor inlet  86  by suction pipe  88 .  
         [0052]    Closed cooling circuit  70  is controlled by a controller  90  which is connected to a temperature sensor  92  detecting the temperature of return air stream  22  before entering into evaporator  30 . Controller  90  is further connected to temperature sensor  94  detecting the temperature within evaporator  30  and further connected to temperature sensors  96  provided for detecting the temperature of supply air stream  23  coming from evaporator  30  and being guided back into transport volume  16  for cooling cargo  12 .  
         [0053]    Controller  90  is further connected to temperature sensor  98  provided in suction line  88  for detecting a suction temperature of compressor  60 . In addition, suction line  88  is further provided with a low pressure transducer  100 .  
         [0054]    In addition, discharge pipe  62  is provided with temperature sensor  102  which is also connected to controller  90 .  
         [0055]    Controller  90  further controls the pressure within water cooled condenser  71  by pressure transducer  104 .  
         [0056]    Economizer  77  is cooled by condensed refrigerant branched off from pipe  76  by pipe  106  and fed to thermo valve  108  controlling the amount of refrigerant flowing to economizer  77 . After having passed economizer  77  the amount of refrigerant is guided to an intermediate pressure inlet  110  of compressor  60  by pipe  112 .  
         [0057]    Thermo valve  108  is controlled by the temperature of compressor  60  detected by temperature sensor  114  and the pressure within pipe  112  detected via capillary tube  116  extending from pipe  112  to thermo valve  108 .  
         [0058]    Economizer  77  can be switched on or off by solenoid valve  120  arranged within tube  106  and being controlled by controller  90 .  
         [0059]    Controller  90  further controls frequency controller  122  which controls the speed of a motor  124  driving compressor  60 .  
         [0060]    Controller  90  is further connected to cargo temperature sensors  126  for detecting the temperature of the cargo and to ambient temperature sensor  128  for detecting the temperature of the ambient air used for a cooling condenser  50 .  
         [0061]    Controller  90  further controls evaporator fans  20  and condenser fan  54 .  
         [0062]    As shown in FIG. 4 compressor  60  is a two stage compressor having, for example, two cylinders forming a first, low pressure stage  130  and two cylinders forming a second high pressure stage  132 .  
         [0063]    First stage  130  can be switched off by a solenoid valve  134  being able to close a low pressure inlet  136  of first stage  130  which is connected to compressor inlet  86 .  
         [0064]    An intermediate pressure inlet  138  of second stage  132  and an intermediate pressure outlet  140  of first stage  130  are internally connected by an internal duct  142  arranged within compressor  60  and this internal duct  142  is connected to suction pipe  88  via check valve  144  for enabling a connection of suction line  88  and intermediate pressure inlet  138  of second stage  132 .  
         [0065]    As long as solenoid valve  134  keeps low pressure inlet  136  open, first stage  130  generates an intermediate pressure within intermediate pressure duct  142  which is above the pressure at low pressure inlet  136  and consequently the pressure within suction line  88 . In this case, check valve  144  closes so that all refrigerant from suction pipe  88  enters low pressure inlet  136  of first stage  130 .  
         [0066]    If, however, solenoid valve  134  closes low pressure inlet  136  the pressure within intermediate pressure duct  142  will decrease and check valve  144  will open to allow refrigerant from suction pipe  88  to directly enter into intermediate pressure duct  142  so as to be guided to intermediate pressure inlet  138  of second stage  132  which in any case compresses refrigerant and discharges compressed refrigerant through high pressure outlet  146  which is connected to compressor discharge  64 .  
         [0067]    Therefore, compressor  60  can be operated in a first mode, in which solenoid valve  134  is closed and only second stage  132  is operative or in a second mode in which both stages  130  and  132  are operative.  
         [0068]    To defrost evaporator  30 , controller  90  is adapted to control heating elements  150  within time intervals which can be determined. Heating elements  150  after being switched on can be switched off when a preset temperature at temperature sensor  94  is detected, because then it can be assumed that evaporator  30  is completely defrosted.  
         [0069]    In addition, water cooled condenser can be activated or deactivated by controller  90 . When water cooled condenser  71  is not activated air cooled condenser  50  is cooled by condenser fan  54  which can be operated at various speeds. The actual speed of condenser fan  54  is controlled in accordance with the actual pressure detected by high pressure transducer  104 .  
         [0070]    If water cooled condenser  71  is activated by controller  90  condenser fan  54  is switched off.  
         [0071]    In addition, discharge temperature sensor  102  is used to detect the discharge temperature of the refrigerant discharge by compressor  60  and controller  90  will reduce the speed of compressor  60  in case the temperature of the discharge refrigerant exceeds a certain level.  
         [0072]    The cooling unit according to the present invention is operated as follows:  
         [0073]    Closed cooling unit  70  can be operated in various stages according to the cooling power demanded at evaporator  30  for maintaining a defined temperature within transport volume  16 .  
         [0074]    If a cooling power between  0  and a level A is demanded at evaporator  30 , closed cooling circuit  70  will be operated in operational stage  1 .  
         [0075]    In stage  1  compressor  60  is operated in the first mode, e.g. with first stage  130  switched off. Further in stage  1  economizer  77  is inoperable so that closed cooling circuit  70  provides the lowest possible compressor speed related cooling capacity which can be defined to be a first compressor speed related cooling capacity.  
         [0076]    Further in stage  1  compressor  60  is running at a minimum speed level which is indicated by an (a).  
         [0077]    For controlling the cooling power compressor  60  will be switched on and off by controller  90 , wherein compressor  60  if switched on will run at minimum speed level (a) and after evaporator  30  has provided sufficient cooling power to supply air stream  23  compressor  60  will be switched off.  
         [0078]    Even though compressor  60  is switched on and off after certain time intervals the precision of the temperature control within cargo volume  16  is still high due to the sufficiently high thermal inertia of the entire system and due to the low cooling power required.  
         [0079]    Closed cooling circuit  70  can further be operated at operational stage  2  which extends between level (A) of the cooling power and level (B).  
         [0080]    In this stage closed cooling circuit  70  is still operated with the first compressor speed related cooling capacity which is identical to the compressor speed related cooling capacity in operational stage  1 .  
         [0081]    However, in operational stage  2  compressor  60  is running uninterruptedly and the cooling power provided at evaporator  30  will be controlled by controlling the speed of compressor  60 .  
         [0082]    A transition between operational stage  1  and operational stage  2  can be easily achieved by terminating the temporary interruptions in operation of compressor  60  and keeping compressor  60  continuously running so that due to the first compressor speed related cooling capacity cooling power according to level (A) is provided at evaporator  30 . If a higher cooling power is required at evaporator  30  the speed of compressor  60  can be altered until value (b) which corresponds to level (B) of the cooling power when operating closed cooling circuit  70  with the first compressor speed related cooling capacity.  
         [0083]    Controller  90  is further adapted to operate closed cooling circuit  70  in operational stage  3  as indicated in FIG. 5.  
         [0084]    Operational stage  3  extends from a cooling power corresponding to level (H) to a cooling power corresponding to level (C).  
         [0085]    In stage  3  compressor  60  is operated in its second mode in which its first stage  130  and its second stage  132  are operable so that compressor  60  operates as a two stage compressor. In operational stage  3  economizer  77  is still inoperable.  
         [0086]    Due to the fact that compressor  60  is now operating in its second mode, e.g. as a two stage compressor, the compressor speed related cooling capacity of closed cooling circuit  70  is higher than when compressor  60  is only operated with its first mode so that in operational stage  3  closed cooling circuit  70  is operated with a second compressor speed related cooling capacity.  
         [0087]    For controlling the cooling power provided at evaporator  30  controller  90  controls the speed of compressor  60  between its minimum speed which corresponds to level (g) to the maximum possible speed in operational stage  3  which corresponds to level (d).  
         [0088]    A transition between operational stage  2  and operational stage  3  can be carried out only with a certain hysteresis for avoiding rapid switching back and forth of controller  90  between operational stage  2  and operational stage  3 .  
         [0089]    To obtain such a hysteresis, closed cooling circuit  70  will be operated in operational stage  2  until level (B) of the cooling power and when level (B) is achieved compressor  60  will be switched from its first mode to its second mode and consequently closed cooling circuit  70  will be operated with the second compressor speed related cooling capacity so that the speed of compressor  60  has to be reduced from level (b) to level (c) if only cooling power of level (B) is demanded.  
         [0090]    If, however, closed cooling circuit is operated in operational stage  3  and cooling power of level (B) is demanded at evaporator  30  closed cooling circuit  70  will remain at operational stage  3 . Even if the demanded cooling power is reduced closed cooling circuit  70  will remain in operational stage  3  until a level (H) of the cooling power which is below level (B).  
         [0091]    If the demanded cooling power is lowered to level (H) compressor  60  will be switched from its second mode used in operational stage  3  to its first mode used in operational stage  2 . Since the first compressor speed related cooling capacity is lower than the second compressor speed related cooling capacity the speed level of compressor  60  which is (a) at level (H) of the cooling power has to be increased up to level (h).  
         [0092]    Controller  90  can further operate closed cooling circuit  70  in operational stage  4 . In operational stage  4  compressor  60  is operated in its second mode, e.g. as a two stage compressor, and further in operational stage  4  economizer  77  is operable.  
         [0093]    Due to the fact that economizer  77  is able to further increase the compressor speed related cooling capacity of closed cooling circuit  70  in operational stage  4 , closed cooling circuit  70  will have a third compressor speed related cooling capacity which is the highest available compressor speed related cooling capacity.  
         [0094]    A transition between operational stage  3  and operational stage  4  is also possible with some kind of hysteresis.  
         [0095]    If cooling circuit  70  is operated in operational stage  3  and has the second compressor speed related cooling capacity the maximum possible cooling power is defined by level (C) and obtained at a speed level (d). At this point controller  90  switches on evaporator  77  by actuating solenoid valve  120  to open pipe  106 .  
         [0096]    After opening of solenoid valve  120  a so-called “economizer fade in” takes place, which means that economizer  77  starts to affect the compressor speed related cooling capacity and the “economizer fade in” is terminated if economizer  77  is fully operable. During this “economizer fade in” controller  90  will adapt the speed of compressor  60  in response to the cooling power provided at evaporator  30  and in response to cooling power demanded. If, for example, a cooling power at a level corresponding to level (C) is required, controller  90  will reduce the speed of compressor  60  according to the increasing effect of economizer  77  on the compressor speed related cooling capacity.  
         [0097]    If, however, during “economizer fade in” the cooling power demanded at evaporator  30  is between level (C) and level (D) controller  90  will reduce the speed of compressor  60  to a lesser extent so that at the end of the “economizer fade in” closed cooling circuit  70  will provide the respective cooling power.  
         [0098]    If during “economizer fade in” the cooling power demanded at evaporator  30  reaches level (D) the speed of compressor  60  will not increase but due to the increasing effect of economizer  77  on the compressor speed related cooling capacity level (D) of the cooling power will be achieved after a certain interval of time at a compressor speed at level (d) which corresponds to the cooling power at level (C) in operational stage  3 .  
         [0099]    If closed cooling circuit  70  is in operational stage  4  and the cooling power demanded at evaporator  30  decreases closed cooling circuit  70  is maintained in stage  4  even if the level of cooling power decreases below level (C) as long as a level (F) is reached which is below level (C).  
         [0100]    After level F of the cooling power has been reached economizer  77  will be switched off so that a so-called “economizer fade out” takes place due to the fact that economizer  77  does not suddenly affect the compressor speed related cooling capacity.  
         [0101]    Consequently, controller  90  will adjust the speed of compressor  60  in response to the change in the compressor speed related cooling capacity due to the “economizer fade out” until the second compressor related cooling capacity is reached so that closed cooling circuit  70  has returned to operational stage  3 .  
         [0102]    If the demanded cooling power corresponds approximately to level (F) controller  90  will increase the speed of compressor  60  in accordance with the degree of “economizer fade out”.  
         [0103]    If, however, during the “economizer fade out” the demanded cooling power decreases to level (G) controller  90  will maintain the speed at level (f) so that the cooling power of closed cooling circuit decreases in accordance with the “economizer fade out”.  
         [0104]    In operational stage  4  closed cooling circuit  70  can be operated between level (F) up to the highest possible cooling power which corresponds to level (E). As an example for the purpose of illustration a start-up of a transportable cooling unit according to the present invention will be performed by controller  90  as follows:  
         [0105]    As shown in FIG. 5 if the cooling unit is switched on compressor  60  starts running at minimum speed as indicated at level (a) in FIG. 5. In addition, evaporator fans  20  start running.  
         [0106]    If the cooling power demanded at evaporator  30  is in the region between zero and level (A) the cooling unit is operated in operational stage  1  in which compressor  60  runs at minimum speed at level (a) and will be interrupted after the desired temperature level at evaporator  30  is obtained. Even though compressor  60  is switched off temporarily the precision of the temperature control within cargo volume  16  is still high because the entire system has a sufficient inertia due to the low cooling capacity required.  
         [0107]    If the cooling power required at evaporator  30  exceeds level (A) compressor  60  is operated in operational stage  2  and controller  90  will control the cooling capacity is only by controlling the speed at which compressor  60  is operated.  
         [0108]    Closed cooling circuit  70  is maintained within operational stage  2  until a cooling power at level (B) or higher is required. If a cooling power at level B or higher is demanded controller  90  switches closed cooling circuit  70  from operational stage  2  to operational stage  3 . In the second mode the cooling capacity of closed cooling circuit  70  is increased and for this reason the speed at which compressor  60  is driven has to be decreased. This enables a higher cooling capacity to be obtained at even lower speed of compressor  60  so that even higher cooling capacity can be obtained if the speed of compressor  60  is increased again. In operational stage  3  of closed cooling circuit  70  the cooling power can be controlled by controlling the speed of compressor  60 .  
         [0109]    When cooling power level (C) or higher is demanded controller  90  switches closed cooling circuit  70  from operational stage  3  to operational stage  4 .  
         [0110]    In operational stage  4  controller  90  controls the cooling power by uninterruptedly varying the speed of compressor  60 .  
         [0111]    The cooling requirement within cargo volume  16  can be detected in various ways.  
         [0112]    In a so-called chilled mode, in which the temperature within cargo volume  16  is above −10° Celsius controller  90  is operated in the chilled mode program and in the chilled mode program controller  90  detects the temperature within cargo volume  16  by means of the supply air sensors  96  which detect the temperature within supply air stream  23 .  
         [0113]    In the chilled mode program the evaporator fans  20  are also operated at maximum speed for obtaining very small deviations from the desired temperature level. These deviations are in the range of +/−0,25° Celsius.  
         [0114]    In another case, a so-called frozen mode, the temperature within the cargo volume  16  is below −10° Celsius and in this case controller  90  is in the frozen mode program, in which the temperature within cargo volume  16  is detected by temperature sensor  92  detecting the temperature within return air stream  22  before reaching evaporator  30 .  
         [0115]    In this case, evaporator fan  20  is operated at a speed below its highest speed, a so-called low speed level because the tolerances from the desired temperature can be higher. In case of the frozen mode the tolerances can be of about +/−1° Celsius.