Abstract:
The invention relates a method for air conditioning of an IT environment which comprises hardware IT devices, computers, memories, telecommunication devices and peripheral devices, wherein a refrigeration supply system of the IT environment has a free cooler, refrigerators and an air circulation system which is operated by means of fans and contains heat exchangers for drawing off the heat generated by the hardware, wherein the method includes the following steps: determining a basic desired value for the feed air temperature of the IT environment on the basis of the existing hardware-configuration, measuring at least one parameter of the hardware which represents a disturbance variable which has a direct or indirect effect on the feed air temperature, and storing the measured value of the parameter, ascertaining a desired value for the feed air temperature from the measured parameter value of the hardware, and raising the target value for the feed air temperature until a critical parameter value is measured, then reducing the desired value for the feed air temperature until the stored value is achieved again, and optionally, if the stored value is not reached within a previously defined time period, resetting the desired value for the feed air temperature to the basic desired value.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Phase Application under 35 U.S.C. 371 of International Application No. PCT/DE2014/100008, filed on Jan. 15, 2014 and published in German as WO 2014/111087 A1 on Jul. 24, 2014. This application is based on and claims the benefit of priority from German Patent Application Nos. 10 2013 100 524.1, filed on Jan. 18, 2013 and 10 2013 111 053.3, filed on Oct. 7, 2013. The entire disclosures of all of the above applications are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    This section provides background information related to the present disclosure which is not necessarily prior art. 
       TECHNICAL FIELD 
       [0003]    The invention relates to a method for air conditioning of an IT environment which comprises, as hardware, IT devices, computers, memories, telecommunication devices and peripheral devices, for example, wherein in a refrigeration supply system, refrigerators and possibly free coolers are provided, as is an air circulation system which is operated by means of fans and contains heat exchangers for drawing off the heat generated by the hardware and other components of the computation system. 
         [0004]    The invention is also suitable for industrial applications and can be transferred to other application cases such as room air conditioning, for example. 
         [0005]    IT environment is understood to mean systems of any size that contain at least partially the above-mentioned hardware. An IT environment can be implemented, for example, as a switch cabinet, as an array of switch cabinets, or as a larger unit right up to a computing center. 
         [0006]    In general, air conditioning methods and air conditioners are used to maintain the temperature of the circulating air or of another cooling medium at a respective specified desired value. This is necessary if, without air conditioning activities, admissible limit temperature values would either be exceeded or not be reached. 
         [0007]    A large part of the current consumed in an IT environment thus is accounted for by the cooling, so that an optimally adapted system can contribute here to lowering the costs. In fact, 25% to 60% of the energy consumption is used for air conditioning, wherein the lower values can only be achieved with free cooling. Here, the largest energy consumer is the compressor in the air conditioner, which consequently should then be used only if the outside temperature is higher than the temperature in the IT environment. 
         [0008]    The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), in its handbook, a new edition of which is published every year, gathers developments and trends and gives recommendations for a climate-efficient design of IT environments that put free cooling in the foreground. Here, guidelines are also presented for the feed air temperatures for servers, which are considered noncritical and which can even be exceeded for short durations. A summary can be found in www.datacenterjournal.com/facilities/ashrae-guidelines-enable-years-round-free-cooling/. 
         [0009]    Further information on the guidelines can be found in Mark Hydeman “Implications of Current Thermal Guidelines for Data Center Energy Use,” ASHRAE Journal—August 2010, Aug. 30, 2010, pages 30-36, XP055113165, USA. 
         [0010]    Conventional air conditioning methods use the feed air temperatures for the servers as parameter for feedback control, wherein a constant desired value for the feed air temperature is used as the basis. As an example, the “LCP Inline” system is indicated, which is described in Handbuch [Manual] 33, Edition 2011/2012, pages 462, 463, of Rittal GmbH &amp; Co. KG. Here, by means of a continuous adaptation of the air volume flow based on the temperature difference between the feed air and the return air at the server, a constant server feed air temperature is ensured. In this way, the temperature change caused by various processes is used for the feedback control of the air conditioning. Since heating processes occur relatively slowly, an air conditioning feedback control of this design can also occur only slowly. 
         [0011]    U.S. Pat. No. 5,915,473 A discloses a method for air conditioning an IT environment, in which a basic desired value for a variable to be regulated, for example, the air temperature, is determined. A parameter is measured, namely the value for the humidity, which represents a disturbance variable which has a direct or indirect effect on the air temperature. 
         [0012]    GB 2,470,475 A also relates to the air conditioning of a room or computation center in which IT equipment is accommodated, wherein the air temperature is detected and this temperature is controlled on the basis of a predetermined temperature which minimizes the energy consumption of the air conditioning installation and of the IT equipment together. 
       SUMMARY OF THE INVENTION 
       [0013]    The problem of the present invention is to provide a method for air conditioning of an IT environment, an industrial environment or a room, which reacts rapidly to changes in the operating conditions and wherein, if present, the proportion of free cooling is kept as high as possible while complying with the ASHRAE recommendations for the feed air temperature for hardware or another heat generator. 
         [0014]    This can be achieved in particular if the limit temperature for free cooling can be set as high as possible. 
         [0015]    The method according to the invention comprises the following steps: 
         [0016]    determining a basic desired value for the feed air temperature on the basis of the existing hardware configuration; 
         [0017]    measuring at least one parameter that represents a disturbance variable which has a direct or indirect effect on the feed air temperature, and storing the measured value; 
         [0018]    ascertaining a desired value for the feed air temperature from the measured parameter value; 
         [0019]    raising the desired value for the feed air temperature until a critical parameter value is measured, then 
         [0020]    reducing the desired value for the feed air temperature until the stored value has been reached again; 
         [0021]    in the process 
         [0022]    if the stored value of the hardware is not reached within a previously defined time period, a resetting of the desired value for the feed air temperature to the basic desired value can occur. 
         [0023]    The invention now uses the cause of a temperature change for controlling the air conditioning. The cause of a temperature change, that is the disturbance variable, here is the supply or drawing off of energy, for example, in the form of electrical power dissipation, electrical power consumption of the hardware, heat irradiation from outside, full capacity utilization of heat-producing installations, passive heat transfer by temperature differences, volume flow of cooling media or heat-transporting substances and the like. 
         [0024]    According to a first aspect, the invention furthermore exploits the finding that the power consumption of the hardware essentially determines the raising of the desired temperature for the feed air, and thus the desired temperature for the feed flow of the heat exchanger is also raised, which leads to a higher limit temperature for the free cooling. In general, direct free cooling is possible if the outside temperature is less than 22° C.; indirect free cooling requires temperatures of 13° C. or lower. The limit temperatures can be raised if, as provided according to the invention, the electrical power consumption of the hardware is used as command variable for the feedback control. A critical parameter value is here defined by an increase—possibly subject to tolerances—of the electrical power consumption of the hardware. 
         [0025]    In the present invention, in order to satisfy the ASHRAE recommendations, a currently maximum possible desired value for the feed air temperature is determined, but, if an increase of the electrical power consumption of the hardware is measured, the desired value for the feed air temperature is then lowered again here until the stored value of the electrical power consumption of the hardware is reached again. 
         [0026]    Advantageously, the following steps are carried out: 
         [0027]    measuring the feed air temperature toward the hardware; 
         [0028]    measuring the return air temperature from the hardware; 
         [0029]    determining the desired value of the rotation speed for the fans of the air circulation system with the feed air temperature and the return air temperature as parameters; 
         [0030]    measuring the actual value of the rotation speed of the fans of the air circulation system; 
         [0031]    increasing or lowering the rotation speed in order to match the desired value and the actual value to one another, so that an updated actual value is obtained. 
         [0032]    By means of this measure, it is ensured that the maintaining of the desired value for the feed air temperature occurs as a matter of priority via the fans of the air circulation system, which should be operated in an energy efficient manner. 
         [0033]    Also advantageously, all the fans of the air circulation system are controlled in parallel with one signal. 
         [0034]    According to a variant of the method according to the invention, the desired value of the feed flow temperature of the heat exchangers is determined from the actual value of the feed air temperature. 
         [0035]    In particular, it is possible to provide that the feed flow temperature of the heat exchangers is used as setting variable for the rotation speed of the fans of the free cooler. 
         [0036]    According to an additional aspect of the invention, a verification is carried out at regular or irregular intervals in order to determine whether measured parameter values that correlate with currently acting disturbance variables are available. 
         [0037]    For this purpose, the evaluation unit verifies at regular or irregular intervals whether, in the input interfaces, measured data that provide information on the currently active disturbance variables are available. 
         [0038]    If no valid measured values are available, the output of the automatic controller is used as setting variable. This corresponds to a conventional temperature control without the influence of a higher-level evaluation unit. 
         [0039]    If sufficient measured values are available in order to conclude that, in the case of an appropriate setting of the setting variable, the actual temperature cannot exceed the predetermined desired value, a value obtained from the conditioning of the measured parameter values is used as setting variable. This corresponds to a direct control of the air conditioning activity by the evaluation unit without closed control loop. 
         [0040]    However, if measured values are available that are not sufficient to guarantee, by direct specification of the setting variable, that the desired value will not be exceeded, a correction value obtained from the conditioning of the measured values is applied to the output of the automatic controller. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0041]    The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
           [0042]    Below, the invention will be explained merely as an example in reference to the appended drawing. 
           [0043]      FIG. 1  shows the basic structure of a hydraulic system for the refrigeration supply of an IT environment; 
           [0044]      FIG. 2  shows the basic structure of an example of an IT environment; 
           [0045]      FIG. 3   a  shows a diagrammatic view of the hydraulic station of a refrigeration station; 
           [0046]      FIG. 3   b  shows a side view of a refrigeration station; 
           [0047]      FIG. 3   c  shows a view of the refrigeration station for illustrating the arrangement of the free coolers; 
           [0048]      FIG. 3   d  diagrammatically shows the encased refrigerators; 
           [0049]      FIG. 4  shows a diagrammatic representation of the parameter management in the present invention; 
           [0050]      FIG. 5  shows the corresponding process management in the optimized control; 
           [0051]      FIG. 6   a  shows a representation of the control loops for the determination of the fan rotation speed of the fans in the circulating-air cooling system as well as of the desired value of the feed air temperature; 
           [0052]      FIG. 6   b  shows a representation of the control loops by means of which it should be ensured that an optimal air conditioning feedback control occurs, and 
           [0053]      FIG. 7  shows a diagrammatic representation of the refrigeration supply according to the second aspect of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0054]    Example embodiments will now be described more fully with reference to the accompanying drawings. 
         [0055]      FIG. 1  shows the basic structure of a hydraulic system for the refrigeration supply of an IT environment. In such systems, in general, an n+1 redundancy is implemented. In the present case, two refrigerators  20  are provided which contain, for example, compressors, as well as a nonredundantly designed free cooler  30  downstream of which there is an EC fan  32 . The heat exchanger  31  of the free cooler  30  (represented with a broken line), implemented in the shape of a V, here offers a maximum heat exchange surface area relative to the available installation space. Two pumps  40 —whose rotation speed can be adjusted—, which again operate with n+1 redundancy, convey the cooling medium through the refrigerators  20  or the free cooler  30 . It is provided that the refrigerators  20  are bypassed hydraulically when the required refrigeration performance can be generated by the free cooler  30 . In the case of a hydraulic bypass, a bypass line is opened, wherein, for the fluid control, one uses motor-driven valves M that do not constrict the flow path, or to be precise a three-way valve  44 . In addition to a flow meter  42 , sensors T for the temperature are present. 
         [0056]    By means of such a system, the method according to claim  1  of the invention can be implemented. The system can here either be a component of the IT environment, as explained in reference to  FIG. 2 , or it can also be an independent module. In this way, for example, computation center environments that cannot be connected to a client-side refrigeration supply are supplied independently by use of a refrigeration station delivered ready for operation with refrigeration for cooling the hardware. An example of a refrigeration station is shown diagrammatically in  FIGS. 3   a  to  3   d.    
         [0057]      FIG. 2  shows the basic structure of an example of an IT environment with integrated refrigeration supply together with the components for the air conditioning. In a casing  10 , the hardware of the IT environment is accommodated, usually in racks  12  which are set up on a raised floor  14 . In terms of air conditioning technology, the space around the racks  12  is divided into a cold aisle  16  and a warm aisle  18 . Cold feed air, represented symbolically with the arrow A, enters the cold aisle  16  through a grid structure from the raised floor  14  and is conveyed through the racks  12 , where it absorbs the heat generated by the hardware and transports it into the warm aisle  18 . The return air, represented symbolically by the arrow B, enters the circulating-air cooling device  100  and is cooled there, wherein the cooling occurs by means of a heat exchanger  102 . The cooled air is conveyed by the fan  104  back into the casing  10 . In the present case, the value of the feed air temperature is acquired by two sensors T Z  which are arranged in the cold area, namely in the cold aisle  16  or in a feed air line to the raised floor  14 . The temperature of the return air is acquired correspondingly by a sensor T R  before the entry into the circulating-air cooling device  100 . Additional sensors T Z  and T R  can be provided in order to acquire a number of measurements for the temperature, which are then conditioned in accordance with certain specifications already discussed above. 
         [0058]    The refrigeration station as an independent module mounted, for example, in a steel frame construction, marked overall with  200 , is shown in  FIGS. 3   a  to  3   d . It is preferable to select as dimensions those of an ISO container, so that the refrigeration station can be transported with standard vehicles. 
         [0059]      FIG. 3   a  shows a hydraulic station H with redundant pumps  40 , control valves, for example,  44 , flow meters, etc., and a control unit  46  accommodated in a switch cabinet or two switch cabinets (not shown). Thus, it is possible to communicate with the feedback control of the computation center in such a manner that the feed flow temperature and water quantity in accordance with the operating conditions are automatically tracked within the computation center (or another IT environment). Here, the control unit  46  of the refrigeration station should be structured so that this regulation-side linking, in the context of the startup, needs to be activated only if a complete package consisting of an IT environment and a refrigeration station is used. In the case of an appropriate design of refrigerators and free cooler, the refrigeration station can thus be matched in terms of performance to the respective construction size of the computation center. 
         [0060]      FIG. 3   b  shows a side view of a refrigeration station, in which the already described hydraulic station H, a free cooler arrangement  30 , and refrigerators  20  are illustrated diagrammatically. 
         [0061]    The refrigeration station comprises two or more refrigerators  20 ; thus an n+1 redundancy is implemented. The refrigerators  20  are designed as air-cooled refrigerators, for example, and they have a geometry matched to the preferably more compact spatial situation in order to have the smallest possible footprint. This can be achieved, for example, in that the installation space for the refrigeration components is implemented height-wise in the form of a column, for example. For the optimization of the energy efficiency, the refrigerators  20  are provided with electronic expansion valves and, for this purpose, they can also contain rotation speed-controlled compressors. 
         [0062]    In order to achieve an energy-optimized free cooling, the refrigeration station contains, as shown in  FIG. 3   c , a free cooler  30  which, relative to the available installation space, offers a maximum area of the heat exchanger  31 , designed in the shape of a V. A fitting with EC fans (for example,  32  in  FIG. 1 ) is provided in order to adapt the free cooling performance at all times to the required cooling performance, even in transitional periods between seasons. 
         [0063]    As additional cold generators, refrigerators  20  are provided, which again are shown only diagrammatically in  FIG. 3   d . The components of the hydraulic station H are used for the purpose of conveying the cooling medium through the refrigerators  20  and in the end to the connected computation center. It is provided that the refrigerators  20  are hydraulically bypassed if the refrigeration performance that is necessary can be generated by the free cooler  30 . In the case of a hydraulic bypass, a bypass line is opened, and motor-driven valves that do not constrict the flow path are used for the media control. 
         [0064]    The line system is made of polypropylene and is thus absolutely corrosion resistant. Due to the wall thickness and the insulation effect of the plastic material used, it is possible, as a rule, to dispense with the use of a thermal insulation. Tests have shown that the line system can be operated with a media temperature of at least 10° C. without condensate forming on the line surfaces. 
         [0065]      FIG. 4  shows a diagrammatic representation of the parameter management according to the first aspect of the present invention. Before the beginning of the entire optimization process, an amount for the desired value of the feed air temperature is determined and stored  310 . According to the invention, the raising of the feed air temperature is made to be exclusively dependent on the electrical power consumption of the hardware. Therefore, said power consumption is measured continuously  312 , so that a detection upon an increase  314  of the electrical power consumption can occur. As long as no increase  314  is detected, the desired value of the feed air temperature is increased according to a linear function in accordance with a preferred embodiment of the invention. 
         [0066]    As soon as an increase in the electrical power consumption is detected  318 , the increase of the desired value of the feed air temperature is stopped  320 . A lowering  322  of the desired value of the feed air temperature to the value present at the time of the detection of the increase of the electrical power consumption occurs. At first it is assumed that this state represents the currently maximum possible feed air temperature. By means of a parametrizable offset, it is possible to ensure additionally that the switching threshold of the hardware is never reached. If, thereafter, the electrical power consumption falls  324 , the desired value of the feed air temperature is maintained  326  until the next optimization process. On the other hand, if the electrical power consumption does not decrease  328 , then this may be due to a higher load on the hardware, for example, as a result of reaching capacity utilization of the CPUs. In order to ensure that the higher power consumption is not caused by a still excessively high feed air temperature, a lowering  330  of the desired value of the feed air temperature to the value before the start of the entire optimization process, which has indeed been stored, is then carried out. If the electrical power consumption of the hardware increases outside of the optimization time window, that is to say when no change of the feed air temperature is thus active, then this indicates that possibly additional hardware equipment has been added or possibly that there has been a temporarily increased capacity utilization of the hardware. A detection of the increase in the electrical power consumption of the hardware outside of the optimization time window  332  does not lead to a change of the desired value of the feed air temperature. 
         [0067]    The described control loop according to the present invention becomes active within the context of a time program on a recurring basis in a permitted time period. The start times and the time period can be freely parametrized. 
         [0068]    In  FIG. 5 , the corresponding process management is indicated. At first, the parametrization  402  with desired values occurs, in particular with the basic desired value for the feed air temperature. The measured values for the electrical power consumption of the hardware are read in at predetermined intervals  404 . In accordance with the selected parametrization of the time function, the optimization process is then started  406 . In the process, as already described, the raising  408  of the desired value of the feed air temperature occurs until an increase of the electrical power consumption of the hardware is detected. The optimization process  406  then ensures that the desired value of the feed flow temperature is lowered  412 . If the decrease of the electrical power consumption then occurs, the optimization process  406  is terminated according to the time function and the time function is reset. After the passage of a parametrized waiting time, the optimization process  406  is started again. On the other hand, if the electrical power consumption does not decrease  416 , the desired value of the feed air temperature is decreased further  418 . If a decrease of the electrical power consumption is then detected  414 , the optimization process  406  is terminated according to time function c and the time function is subsequently reset. On the other hand, if the electrical power consumption continues not to decrease  420 , the optimization process  406  is terminated, and a query  422  addressed to the user occurs, in order to determine whether the hardware equipment has been changed since the last query time. If the hardware equipment was changed  424 , the time function of the optimization process  406  is reset, so that, in accordance with the parametrized start times, a new optimization can occur. However, if the hardware equipment has not been changed  426 , the time function is locked  428  and a disturbance message is output  430 . 
         [0069]      FIG. 6   a  diagrammatically illustrates how the desired value of the feed air temperature determined in the optimization process influences the further feedback control. The process course according to  FIG. 5  that has just been described in this context is summarized here by the control loop RK 7 . 
         [0070]    The purpose of the control loop RK 7  is to shift the desired value of the feed air temperature preferably to a higher value. In fact, by raising the desired value of the feed air temperature, the desired value of the feed flow temperature for the heat exchangers is also raised, which in turn increases the limit temperature for free cooling. Here, according to the invention, the measurement of the electrical power consumption of the hardware is used as command variable. The optimized desired value for the feed air temperature is used in further feedback control processes. 
         [0071]    According to the invention, the desired value of the feed air temperature is raised until the electrical power consumption of the hardware increases. When an increase is detected, a lowering of the desired value of the feed air temperature occurs until the electrical power consumption of the hardware again has assumed its initially detected value. According to the present invention, this state defines the currently maximum possible feed air temperature. 
         [0072]    If the electrical power consumption increases from this steady state on, then this is an indication of an addition of hardware or a temporarily increased capacity utilization of the hardware. A detection of the increase of the electrical power consumption of the hardware from the steady state leads to a resetting of the desired value of the feed air temperature to the parametrized and stored amount. 
         [0073]    In the course of the optimization, as far as the energy efficiency of the air conditioning is concerned, the feed air temperature is at first set preferably via the operation of fans associated with the racks to be cooled. For this purpose, as already explained, mean values of the feed air temperature T Z  and return air temperature T R  are determined. As is apparent from  FIG. 6   a , a temperature difference is calculated from the mean values of the feed air temperature and the return air temperature and compared to a desired value for the temperature difference. The control loop RK 6  then adapts the fan rotation speed of the fans. All the fans are controlled in parallel with one signal. Via the temperature-difference feedback control, the desired value of the feed air temperature can also be maintained in the case of variations of the power consumption of the hardware. 
         [0074]    If the higher capacity utilization of the hardware is only temporary, there will be a lowering of the rotation speed of the fans after the load peak has subsided. 
         [0075]    In the case of the invention, the feed air temperature is not increased or decreased depending on the rotation speed of the fan, since this in turn would influence the temperature-difference feedback control. A possible raising of the feed air temperature is made exclusively dependent on the electrical power consumption of the hardware. 
         [0076]    If, at the parametrized desired value for the feed air temperature, the load, for example, due to an addition of hardware, can no longer be dissipated solely by increasing the rotation speed of the fans, then this is noted automatically as described further below. 
         [0077]    The optimized, currently maximum possible desired value of the feed air temperature is used as command variable for the feedback control of additional components of the refrigeration supply system. This is explained in connection with  FIG. 6   b.    
         [0078]      FIG. 6   b  shows how the currently maximum possible desired value of the feed air temperature is used as desired value for the management of the control loop RK 4 . The control loop RK 4  for the feed flow temperature regulation of the heat exchangers of the circulating-air cooling system is implemented as a PID controller, for example. As starting signal, the variable, desired value of the feed flow temperature for the heat exchangers, is made available to the control loops RK 1  and RK 2 . The higher the currently maximum possible desired value of the feed air temperature is, the higher the desired value for the feed flow temperature of the heat exchangers will also be. 
         [0079]    With increasing outside temperature, the fans of the free cooler  30  ( FIG. 1 ) are adjusted up to 100% rotation speed. In the case of a decrease of the temperature difference in the cooling medium toward zero, which means that the feed flow is as warm as the return flow, and a fan rotation speed of 100%, the free cooler  30  is cut off. The purpose of the cutoff is to prevent a warming of the medium returning from the circulating-air cooling system during summer operation. 
         [0080]    The respective cutoff temperature, which corresponds to the outside air temperature at the time of the triggering of the cutoff, is stored and used for restarting the free cooler  30 . The free cooler  30  is restarted when the cutoff temperature is undershot by an amount ΔT which is freely parametrizable. In addition to this automatic startup, a manual influencing is possible, by resetting the cutoff temperature via a user interface or by manually parametrizing via a numerical input field. 
         [0081]    In case of falling outside temperature, the fans of the free cooler  30  are regulated to a minimum rotation speed. Below the minimum rotation speed, the free cooler  30  is locked in order to prevent undercooling of the medium. The minimum rotation speed is freely parametrizable as a percentage of 100. 
         [0082]    The free cooler  30  is restarted when the three-way control valve  40  is opened above a threshold value in the passage from the free cooler  30  and the feed flow temperature exceeds a threshold value of the temperature difference above the desired temperature. This threshold value is also freely parametrizable. 
         [0083]    The starting rotation speed of the fans of the free cooler  30  corresponds to the rotation speed at the time of locking. 
         [0084]    The control loop RK 2  is also implemented as a PID controller. The command variable is the feed flow temperature that is determined by the control loop RK 6 . 
         [0085]    In the case of falling outside temperature, the three-way valve  44  is regulated to 100% bypass. Since heat is generated continually in the computation center, a bypass opening to 100% is in practice never reached. In the case of increasing outside temperature, the three-way valve  44  is regulated to 100% passage. 
         [0086]    If a threshold value of the valve opening in the passage from the free cooler  30  is reached, the free cooler  30  is unblocked. After the unblocking of the free cooler, the three-way valve  44  in 100% position in the passage from the free cooler and is locked. The unlocking occurs when the free cooler  30  again has reached its parametrized measured rotation speed. 
         [0087]    In the control loop RK 3 , the continuous feedback control of the rotation speed of the pump for the flow of the cooling medium to the heat exchangers of the circulating-air cooling system occurs depending on the calculated desired value, and on the flow quantity measured by the flow meter  42 , the rotation number of the pump is changed continuously. The desired value of the conveyed quantity corresponds to the nominal media quantity of the heat exchangers and it is influenced by the control loop RK 5 . 
         [0088]    The purpose of the control loop RK 5  is to optimally exploit the possible conveyance performance of the pump. The increase of the media throughput leads to a performance increase of the heat exchanger of the free cooler  30  and of the heat exchangers of the circulating-air cooling system and thus to an increase of the free-cooling limit temperature. 
         [0089]    In the case of an increase of the rotation speed of the pump in the free cooler  30  above a parametrizable threshold value, for example, 95%, the rotation speed of the conveyance pump is continually raised until a threshold value of the electrical power consumption of the pump is reached. The power consumption decreases steeply from a certain value in accordance with an exponential function. The threshold value is therefore determined iteratively from the curve of the power consumption and is parametrized manually. 
         [0090]    The decrease of the fan rotation speed of the free cooler  30  in the case of falling outside temperature is detected and used to decrease the desired value of the conveyance quantity again until the nominal media quantity has been reached. In the case of the cutoff of the free cooler  30 , the threshold value of the conveyance quantity is again set to the nominal media quantity. 
         [0091]    The control loop RK 5  is also designed as a PID controller. Command variables are the measurement of the actual water quantity by the flow meter  42 , the measured value of the electrical power consumption of the pump as well as the rotation speed of the EC fan of the free cooler  30 . 
         [0092]    For the evaluation of the efficiency of the energy used in the computation centers, the power usage effectiveness PUE is used as nominal value. The PUE value sets the overall energy consumed in the computation center as a ratio with respect to the energy consumption of the hardware. When this ratio approaches the number 1, it is ensured that the computation center is working efficiently. With the invention, PUE values of 1.3 and less can be reached. 
         [0093]      FIG. 7  shows a diagrammatic representation of the air conditioning feedback control according to the second aspect of the invention. The following meanings are used in  FIG. 7 : 
         [0094]    Desired value: desired temperature value for air conditioning, which can be configured by the user 
         [0095]    Deviation: difference between desired value and current temperature 
         [0096]    Setting variable: specification of the performance of the air conditioning activity 
         [0097]    Disturbance variables: additional variables that have an effect on the temperature (for example, power dissipation, irradiation) 
         [0098]    Actual value: current temperature 
         [0099]    An evaluation unit  500  comprises physical input interfaces for recording sensor data for the determination of causative parameters that have an effect on the air conditioning demand, and can estimate, on the basis of recorded measured values, the air conditioning demand, so that a setting variable can be specified directly for the feedback control of the air conditioning activities, or additional influencing parameters can be made available for the determination of the setting variable. The evaluation unit thus allows a more demand-appropriate, rapid and precise feedback control of the air conditioning activities. As a result of the possibility of the more demand-appropriate feedback control, the energy efficiency of the air conditioning activities can also be improved. The evaluation unit can be implemented as an independent system or it can be integrated as a partial function in the control of an air conditioning device. 
         [0100]    The evaluation unit  500  verifies at regular or irregular intervals whether measurement data giving information on the currently acting disturbance variables are available at its input interfaces. 
         [0101]    If no valid measured values are available, the output of the automatic controller  520  is used as setting variable. This corresponds to a conventional temperature feedback control without influence of a higher-level evaluation unit and to the switch position S 1  of the switch  510  in  FIG. 6 . 
         [0102]    If sufficient measured values are available to conclude that, in the case of an appropriate setting of the setting variable, the current temperature cannot exceed the specified desired value, the value obtained from the conditioning of the measured value is used as setting variable. This corresponds to a direct control of the air conditioning activity by the evaluation unit  500  without closed control loop, that is to say switch position S 3  of the switch  510 . 
         [0103]    However, if measured values are available but they are not sufficient to guarantee, by direct specification of the setting variable, that the desired value is not exceeded, a correction value obtained from the conditioning of the measured values is applied to the output of the automatic controller  520 , which corresponds to the switch position S 2  of the switch  510 . 
         [0104]    For the conditioning the measured parameter values, the following are provided, among other measures: verification of the individual measured variables to determine their respective valid value range, determining the mean value of individual measured variables, reinforcing of individual measured variables, addition of individual measured values to form an output value, offset shifting of the output value, and delimitation of the output value to minimum and maximum values. 
         [0105]    The evaluation unit includes connection possibilities for several sensors, which can be used optionally by the user. Thus, it is possible for the user not to use the optimization possibilities, to use them partially for the optimization of feedback control, or to completely replace the feedback control. 
         [0106]    The decision as to whether it is the case that the available measured values are sufficient is made on the basis of rules that can be set by experts. The disturbance variables that have to be known by way of valid measured values in order to switch to a setting variable which is influenced by the evaluation unit are provided in the rules. The conditions that are used for the conditioning of the measured values can also be set by experts. 
         [0107]    The features of the invention disclosed in the above description, the drawing and the claims can be essential both individually and also in any combination for the implementation of the invention. 
         [0108]    The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the invention, and all such modifications are intended to be included within the scope of the invention.