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 maximum possible heating 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 cooling stages 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 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/EP01/05277 of May 9, 2001, 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 heating or cooling power demanded at said evaporator for maintaining said defined temperature and minimize energy consumption, said controller operating said closed cooling circuit between a maximum possible heating power and a maximum possible cooling power in a sequence of different operational stages comprising a lowest operational stage and a sequence of at least two upper operational cooling stages, said controller operating said closed cooling circuit in each one of said upper operational cooling stages at a compressor speed related cooling capacity different from said compressor speed related cooling capacity in said other upper operational cooling stages and within said respective upper operational cooling 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 the various advantageous embodiments of the present invention it is not defined how it is possible to obtain heating power in addition to cooling power in the said closed cooling circuit.  
           [0009]    For instance it would be possible to provide specific heat transformers or heaters.  
           [0010]    One advantageous embodiment uses the closed cooling circuit to obtain heating power. In particular in said closed circuit heating power is obtained in addition to cooling power by providing an inoperable mode for said condenser in addition to an operable mode for said condenser. In said inoperable mode of said condenser the refrigerant heated by said compressor is not cooled so that the heated refrigerant arrives at the evaporator at high temperature and cannot evaporate anymore so that the evaporator finally is heated and consequently heating power can be obtained at the evaporator.  
           [0011]    It is particular of advantage if said inoperable mode of said condenser provides bypassing of said condenser.  
           [0012]    Since according to standard operation of the closed cooling circuit the refrigerant after said condenser passes through that expansion device it is particularly advantageous if said inoperable mode of said condenser provides bypassing of said expansion device.  
           [0013]    Such bypassing of said condenser can easily be realized by a bypass pipe and a valve which enables changes between the operable mode of said condenser combined with an inoperable bypass pipe and the inoperable mode of said condenser combined with an operable bypass pipe.  
           [0014]    This changes can be changes between the two aforementioned two alternatives by a switching valve all these changes can be continuous changes from one of the aforementioned modes to the other aforementioned mode.  
           [0015]    In accordance with the aforementioned embodiments of the present invention it is not defined how the controller operates the compressor in said lowest operational stage. It is particularly advantageous if in said lowest operational stage said controller operates said compressor in an uninterrupted mode at low speed and operates said closed cooling circuit by changing between the operable and inoperable mode of said condenser according to respective mode intervals adjusts said cooling or heating capacity by adjusting at least one of the parameters comprising speed of said compressor and duration of said mode intervals.  
           [0016]    The advantage of this embodiment of the present invention is that in the low operational stage it is allowed to run the compressor 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.  
           [0017]    It is particularly advantageous if the controller in said low operational stage maintains said speed of said compressor essentially constant and varies the duration of said mode intervals, for example the intervals within which the valve is switched on or off so that the cooling power is only controlled by controlling the valve interruption intervals.  
           [0018]    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 or heating power provided at that minimum speed a further reduction is performed by operating the mode intervals.  
           [0019]    With the various embodiments explained before the bypassing of the condenser and eventually the expansion device has not been explained in detail except that advantageously a bypass pipe and the valve are provided.  
           [0020]    A particular advantageous embodiment provides a bypass bypassing said condenser and that expansion device and a valve arranged in said bypass pipe. This is a very simple and advantageous embodiment and opening and closing the valve in the bypass pipe is sufficient for a bypassing said condenser and said expansion device because the design of the expansion device, in particular the pressure drop provided therein makes it not necessary to provide separate means for blocking the stream of refrigerant through said condenser and said expansion device.  
           [0021]    The valve provided in said bypass pipe can be a valve which has only two positions one closing said bypass pipe and one opening said bypass pipe.  
           [0022]    In such a case the bypass pipe can be closed and opened according to said operable or said inoperable mode of said condenser desired.  
           [0023]    However, it is also possible to provide a valve which enables continuous regulation of the flow through the bypass pipe so that the flow through the bypass pipe can be adjusted continuously and consequently the operation of the condenser can be continuously changed between the fully operable mode and the fully inoperable mode so that for example the condenser can be operated in a partially operable mode combined with a partially operable bypass line.  
           [0024]    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.  
           [0025]    A more advantageous manner of determining the cooling power demanded is to compare the temperature present within that transport volume and the requested temperature in said transport volume.  
           [0026]    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.  
           [0027]    Generally, the temperature within the transport volume can be detected anywhere therein.  
           [0028]    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.  
           [0029]    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.  
           [0030]    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.  
           [0031]    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.  
           [0032]    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 with the condenser being in the operable mode is constant within at least one of said upper operational stages.  
           [0033]    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 in said lowest operational stage with the condenser being in said operable mode to be the same as the compressor speed related cooling capacity in said one of said upper operational cooling stages covering the lowest range of cooling power of said sequence of upper operational cooling stages.  
           [0034]    If the controller has the possibility to switch from one upper operational cooling stage to another upper operational cooling stage such a switching is advantageously defined by a respective cooling power. To avoid at this respective cooling power a fast switching back and forth between one upper operational cooling stage and the other operational cooling 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 cooling stage to the next higher operational cooling stage is higher than the cooling power at which the controller switches from the higher operational stage to the next lower upper operational cooling stage.  
           [0035]    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.  
           [0036]    This is avoided if in the course of a transition from one of said upper operational cooling stages to another of said upper operational cooling 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.  
           [0037]    This means that even in the course of a transition from one operational cooling stage to the next operational cooling 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.  
           [0038]    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 compressor stages and in a second mode using all compressor stages of said compressor for compressing refrigerant. Such a design has the advantage that when operating the compressor at a reduced number of compressor 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 compressor stages.  
           [0039]    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.  
           [0040]    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.  
           [0041]    In an embodiment of particular advantage it is provided that said controller changes from an operational stage in which the compressor operates in said first mode to the operational 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.  
           [0042]    In another advantageous embodiment according to the present invention an economizer is provided in said closed cooling circuit.  
           [0043]    Such a economizer could be designed to be fully operable within the entire operational range of the cooling unit.  
           [0044]    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.  
           [0045]    For providing different compressor speed related cooling capacities it is of particular advantage if in at least one of said upper operational stages the closed cooling circuit is controlled to operate in an economizer off-mode or in an economizer on-mode.  
           [0046]    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 mode the energy consumption of the compressor is decreased with respect to the economizer off-mode.  
           [0047]    To avoid compressor overheating the controller can start using the economizer function. This ensures higher cooling capacity at same compressor speed and reduces compressor temperature by injecting refrigerant to the compressor via the economizer.  
           [0048]    One particular advantageous embodiment provides that said controller in said operational stage in which the closed cooling circuit can be operated in the economizer off-mode ir in the economizer on-mode switches from the economizer off-mode to the economizer on-mode if the temperature of the compressor exceeds a defined level.  
           [0049]    In connection with the preferred embodiment explained before the only operational stage defined in which heating power could be provided was the lowest operational stage.  
           [0050]    To obtain more heating power at the evaporator an advantageous embodiment provides an upper operational heating stage wherein said controller is operating said electric motor in an uninterrupted mode and adjusting said heating power provided by said closed cooling circuit by an essentially stepless speed control of said electric motor. Therefore, this upper operational heating stage provides the possibility to provide more heating power by a stepless speed control of said electric motor.  
           [0051]    It is in particular of advantage if said compressor speed related heating capacity of said closed cooling circuit with said condenser being in its inoperable mode is constant within said upper operational heating stage.  
           [0052]    To generate even more heating power a further advantageous embodiments provides a further operational heating stage in which the heating power of said cooling circuit is increased by a heating device. Such a heating device can be a heating device arranged for example in said bypass pipe but can also be a heating device arranged separately and closed to said evaporator where the heating power is needed.  
           [0053]    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.  
           [0054]    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  
       [0055]    [0055]FIG. 1 shows a sectional view along lines  1 - 1  in FIG. 2 of a container provided with one embodiment of the present invention;  
         [0056]    [0056]FIG. 2 shows a sectional view along lines  2 - 2  in FIG. 1;  
         [0057]    [0057]FIG. 3 shows a scheme of the various components of the cooling unit according to the present invention;  
         [0058]    [0058]FIG. 4 shows details of the compressor on an enlarged scale; and  
         [0059]    [0059]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  
       [0060]    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 .  
         [0061]    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 .  
         [0062]    Preferably, evaporator fan  20  and evaporator  30  are arranged at a front portion  32  of housing  14 .  
         [0063]    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 .  
         [0064]    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 .  
         [0065]    Evaporator  30 , condenser  50  and compressor  60  are part of a closed cooling circuit  70  shown in detail in FIG. 3.  
         [0066]    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 .  
         [0067]    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  in which check valve  73  is arranged. 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 .  
         [0068]    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 .  
         [0069]    Controller  90  is further connected to temperature sensor  98  provided in suction line  88  for detecting a suction temperature of compressor  60 .  
         [0070]    Controller  90  further controls the pressure within suction line  88  by pressure transducer  100 .  
         [0071]    In addition, condenser  50  is provided with temperature sensor  102  which is also connected to controller  90 .  
         [0072]    Controller  90  further controls the pressure within condenser  50  by pressure transducer  104 .  
         [0073]    Economizer  77  is cooled by condensed refrigerant branched off from pipe  76  by pipe  106  and fed to electronic 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 .  
         [0074]    Controller  90  further controls electronic thermo valve  108  by the temperature of compressor  60  detected by temperature sensor  114 .  
         [0075]    Controller  90  further controls frequency controller  122  which controls the speed of a motor  124  driving compressor  60 .  
         [0076]    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 .  
         [0077]    Controller  90  further controls evaporator fans  20  and condenser fan  54 .  
         [0078]    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 .  
         [0079]    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 .  
         [0080]    As shown in FIG. 4 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 .  
         [0081]    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 .  
         [0082]    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 .  
         [0083]    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.  
         [0084]    To defrost evaporator  30 , controller  90  is adapted to control heating of evaporator  30  within time intervals which can be determined. Heating 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.  
         [0085]    For heating of evaporator  30  a hot gas pipe  152  with a hot gas valve  154  provided therein is connecting inlet part  82  of evaporator  30  with discharge pipe  62  and for a bypass for condenser  50  and electronic thermo valve  80 . Hot gas valve  154  is controlled by controller  90 .  
         [0086]    In addition or alternatively evaporator  30  can be heated by heating elements  150  which can be controlled by controller  90 .  
         [0087]    Heating of evaporator  30  is performed in the same manner as when heating is demanded in normal operating mode.  
         [0088]    In addition, water cooled condenser  74  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 .  
         [0089]    If water cooled condenser  71  is activated by controller  90  condenser fan  54  is switched off.  
         [0090]    The cooling unit according to the present invention is operated as follows:  
         [0091]    Closed cooling unit  70  can be operated in various stages according to the cooling or heating power demanded at evaporator  30  for maintaining a defined temperature level within transport volume  16 .  
         [0092]    If a cooling or heating power between level A and a level-A is demanded at evaporator  30 , closed cooling circuit  70  will be operated in operational stage  0 .  
         [0093]    In stage  0  compressor  60  is operated in the first mode, e.g. with first stage  130  switched off. Further in stage  0  closed cooling circuit  70  provides the lowest possible compressor speed related cooling capacity which can be defined to be a first compressor speed mode.  
         [0094]    Further in stage  0  compressor  60  is running at a minimum speed level which is indicated by an (a).  
         [0095]    For controlling the cooling power hot gas valve  154  will be switched on and off by controller  90 , using pulse width modulation for operating said hot gas valve  154  wherein compressor  60  will run at minimum speed level (a).  
         [0096]    Even though hot gas valve  154  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.  
         [0097]    Changing the pulse width between “on” and “off” of hot gas valve  154  corresponds to “by-passing” or “not by-passing” of condensor  50  and electronic thermo valve  80  with the consequence that condensor  50  is in an inoperable mode or an operable mode.  
         [0098]    In the inoperable mode of condensor  50  cooling circuit  70  does not produce cooling power at evaporator  30  but produces heating power at evaporator  30 .  
         [0099]    Only in the operable mode of condenser  50  cooling circuit  70  produces cooling power.  
         [0100]    By switching between inoperable mode of condensor  50  and the operable mode of condenser  50  the cooling power can be varied depending on the relative duration of the operable mode interval in relation to the duration of the inoperable mode interval.  
         [0101]    If the durations of the mode intervals are equal with respect to net heating power and net cooling power generated the resulting cooling power is zero. If the duration of the mode interval of the operable mode exceeds the duration of the mode interval of the inoperable mode the cooling power of cooling circuit  70  is positive.  
         [0102]    If the duration of the mode interval of the inoperable mode exceeds the duration of the mode interval of the operable mode the cooling power of cooling circuit  70  is negative e.g. cooling circuit  70  has a heating power.  
         [0103]    Closed cooling circuit  70  can further be operated at operational stage  1  which extends between level (A) of the cooling power and level (B).  
         [0104]    In this stage closed cooling circuit  70  is still operated with the first compressor speed mode which is identical to the compressor speed mode in operational stage  0 .  
         [0105]    However, in operational stage  1  hot gas valve  154  is closed and the cooling power provided at evaporator  30  will be controlled by controlling the speed of compressor  60 .  
         [0106]    A transition between operational stage  0  and operational stage  1  can be easily achieved by terminating the pulse width modulated operation of hot gas valve  154  and keeping compressor  60  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 mode.  
         [0107]    Controller  90  is further adapted to operate closed cooling circuit  70  in operational stage  2  as indicated in FIG. 5.  
         [0108]    Operational stage  2  extends from a maximum cooling power corresponding to level (C) to a cooling power corresponding to level (D).  
         [0109]    In stage  2  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.  
         [0110]    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  2  closed cooling circuit  70  is operated with a second compressor speed related cooling capacity.  
         [0111]    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 (c) to the maximum possible speed in operational stage  2  which corresponds to level (d).  
         [0112]    A transition between operational stage  1  and operational stage  2  can be carried out only with a certain hysteresis for avoiding rapid switching back and forth of controller  90  between operational stage  1  and operational stage  2 .  
         [0113]    To obtain such a hysteresis, closed cooling circuit  70  will be operated in operational stage  1  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 (e) if only cooling power of level (B) is demanded.  
         [0114]    If, however, closed cooling circuit is operated in operational stage  2  and cooling power of level (B) is demanded at evaporator  30  closed cooling circuit  70  will remain at operational stage  2 . Even if the demanded cooling power is reduced closed cooling circuit  70  will remain in operational stage  2  until a level (D) of the cooling power which is below level (B).  
         [0115]    If the demanded cooling power is lowered to level (D) compressor  60  will be switched from its second mode used in operational stage  2  to its first mode used in operational stage  1 . 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 (c) at level (D) of the cooling power has to be increased up to level (f).  
         [0116]    Controller  90  can further operate electronic thermo valve  108  in closed cooling circuit  70 , which controls the flow of refrigerant to the economizer  77 . Electronic thermo valve  108  is activated if the temperature of compressor  60  exceeds a predetermined temperatur. Evaporated refrigerant will pass through economizer  70  and provide cooling of compressor  60 .  
         [0117]    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  2 , closed cooling circuit  70  will have a further compressor speed related cooling capacity which is the highest available compressor speed related cooling capacity.  
         [0118]    After opening of electronic thermo valve  108  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 (E) is required, controller  90  will reduce the speed of compressor  60  starting at speed level (g) according to the increasing effect of economizer  77  on the compressor speed related cooling capacity to speed level (h).  
         [0119]    If, however, during “economizer fade in” the cooling power demanded at evaporator  30  is between level (E) and level (F) 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.  
         [0120]    If during “economizer fade in” the cooling power demanded at evaporator  30  reaches level (F) 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 (F) of the cooling power will be achieved after a certain interval of time at a compressor speed at level (g) which corresponds to the cooling power at level (E) in operational stage  2 .  
         [0121]    Closed cooling circuit  70  can further be operated at operational heating stage- 1  which extends between level (-A) of the heating power and level (-B).  
         [0122]    In this stage closed cooling circuit  70  is operated with the first compressor speed mode which is identical to the compressor speed mode in operational stage  0 .  
         [0123]    However, in operational stage- 1  hot gas valve  154  is fully open and the heating power provided at evaporator  30  will be controlled by controlling the speed of compressor  60 .  
         [0124]    A transition between operational stage  0  and operational stage- 1  can be easily achieved by keeping the hot gas valve  154  open and keeping the compressor  60  running so that due to the first compressor speed mode heating power according to level (-A) is provided at evaporator  30 . If a higher heating power is required at evaporator  30  the speed of compressor  60  can be altered until value (i) which corresponds to level (-B) of the heating power when operating closed circuit  70  with the first compressor speed mode.  
         [0125]    Controller  90  is further adapted to operate closed cooling circuit  70  in operational stage- 2  as indicated in FIG. 5.  
         [0126]    Operational stage- 2  extends from a heating power corresponding to level (-B) to a heating power corresponding to level (-C).  
         [0127]    In this stage closed cooling circuit  70  is operated with the first compressor speed mode which is identical to the compressor speed mode in operational stage  0  and stage  1 .  
         [0128]    For controlling the heating power provided at evaporator  30  controller  90  runs the closed cooling circuit  70  corresponding to heating capacity (-B) and switching the electrical heaters  150  on and off using a pulse width modulating mode so as to obtain additional heating capacity.  
         [0129]    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:  
         [0130]    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.  
         [0131]    If the cooling power demanded at evaporator  30  is in the region between level (A) and level (-A) the cooling unit is operated in operational stage  0  in which compressor  60  runs at minimum speed at level (a) and the cooling power or heating power required is adjusted by controlling hot gas valve  154  to obtain appropriate duration of the mode intervals.  
         [0132]    If the cooling power required at evaporator  30  exceeds level (A) compressor  60  is operated in operational stage  1  and controller  90  will control the cooling capacity only by controlling the speed at which compressor  60  is operated.  
         [0133]    Closed cooling circuit  70  is maintained within operational stage  1  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  1  to operational stage  2 . 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  2  of closed cooling circuit  70  the cooling power can be controlled by controlling the speed of compressor  60 .  
         [0134]    The cooling requirement within cargo volume  16  can be detected in various ways.  
         [0135]    In a so-called chilled mode, in which the temperature within cargo volume  16  is above −10 20  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 .  
         [0136]    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.  
         [0137]    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 .  
         [0138]    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.