BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a closed cycle dryer comprising a drum, an air blowing unit whereby the air is conveyed to the drum, a heating unit used to heat air that is blown into the drum, and a condensing unit placed upstream the heating unit for removing moisture. The invention relates also to a process for drying clothes in a closed cycle dryer. 
     2. Description of the Related Art 
     In the closed cycle dryers the air that receives moisture from the clothes is transferred to a condenser where moisture is removed, and then, after being heated, is supplied back to the drum. 
     Various solutions have been developed in order to improve the efficiency of the dryers using a closed cycle. For instance, a refrigeration cycle can be used in which the evaporator is used as a condenser for the drying cycle and the condenser of the refrigeration cycle is used as a heating unit. In another solution the condensing unit is provided with water nozzles which cool air and help to remove fluff. Of course these solutions, even if they increase the overall efficiency of the drying process, increase the complexity (and therefore the overall cost) of the dryer. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a dryer with an improved condensation efficiency thanks to a simple and not expensive modification of the air path in the machine. 
     Such object is reached thanks to the features listed in the appended claims. 
     One of the main features of the present invention is the use of a by-pass or short cut which creates a direct link between the outlet of the condensing unit and the outlet of the drum upstream the inlet of the blower. Where a filter is used for removing fluff downstream the drum, the outlet of the by-pass conduit may be placed downstream the filter housing. 
     According to the invention, the by-pass conduit enables a predetermined part of the process air which has already passed the condensing unit (and is saturated with moisture) to be mixed with the hot and humid air coming from the drum with tumbling clothes (which is not completely saturated with moisture). 
     According to an embodiment of the invention, the by-pass conduit can be a simple tube that links outlet of the blower downstream the condensing unit and the outlet of the drum or inlet of the blower. Both air flows (from the drum and from the by-pass conduit) will be mixed and will enter the condensing unit more saturated and pre-cooled than without the by-pass. 
     The unexpected main advantage deriving from the solution according to the present invention is that the energy needed for reaching the 100% humidity line (condensing line) in the Mollier diagram where condensation takes place is reduced if compared to a traditional drying cycle. 
     Less sensible heat (which would lead to energy losses) needs to be transferred to start condensation. The cooling power of the condensing unit is used more efficiently for the condensation (latent heat/phase change) itself. 
     Another advantage of the solution according to the present invention is that a part of the process air flow that passes through the by pass conduit reduces the overall resistance that the air blower has to overcome. This leads to higher air flow through the blower and so through the condensing unit as well. 
     Higher volume flow through the condensing unit, particularly in case a heat exchanger with plates is used, leads to better heat transfer and to higher condensation efficiency. Tests carried out by the applicant have shown an increased process air flow through blower and condensing unit of 10% and above. With the by-pass the overall energy consumption is reduced. According to the result of the above tests, the energy saving is higher than 0.01 kWh/kg dry laundry. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of a dryer according to the present invention will be clear from the following detailed description, with reference to the attached drawings in which: 
         FIG. 1  is a schematic view of a closed cycle dryer according to the invention; 
         FIG. 2  is a drying cycle according to prior art on a Mollier diagram; 
         FIG. 3  is similar to  FIG. 2  and shows the drying cycle according to the present invention; 
         FIG. 4  is a schematic view of a closed cycle dryer with a preferred air path according to the invention; and 
         FIG. 5  is a partial and more detailed view of a dryer according to  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the drawings, with 10 it is indicated a drum of a clothes dryer using a closed cycle. The drum  10  is fed in  10   a  with hot air heated by a heater  12  in which a heating power Q h  is transferred to air. The flow of air is driven by a blower  14 . Downstream the blower  14  and upstream the heater  12 , the dryer is provided with a condensing unit  16  for removing humidity (and heat) from the air flow. The cooling power of the condensing unit  16  is identified with reference Q c . The humid air is flowing in  10   b  from the drum  10  and passes through a filter  18  for removing fluff, before reaching the blower  14 . The condensing unit  16  could be placed upstream the blower  14  as well (solution not shown in the drawings). 
     According to the invention, the air circuit of the dryer is provided with a by-pass conduit  20  interposed between, on one side, a portion  22  of the circuit downstream the condensing unit  16  and the heater  12  and, on the other side, a portion  24  of the circuit downstream the filter  18  and upstream the blower  14 . 
     In the dryer according to prior art, i.e. without the by-pass conduit  20 , the drying process is shown in  FIG. 2 . Unsaturated air enters the condensing unit at point C of the Mollier diagram. To cool down the air to the condensing line (indicated with reference W in FIG.  2 —100% relative humidity) a certain cooling power is needed. Such cooling power is shown by the line identified with reference number  3  in  FIG. 2 . After cooling in the condensing unit, air needs to be heated up and this leads to further energy consumption. To cool the process air by 1° C. of to heat up such air by 1° C. a power of around 50 W is needed. 
     The process according to the invention, i.e. with the by-pass conduit  20 , is shown in  FIG. 3 . Line T of the diagram shows the situation inside the drum  10 , where energy is transferred from hot air to clothes and therefore to water contained therein for its evaporation (nearly constant enthalpy). In the drum  10  temperature of air from inlet  10   a  to outlet  10   b  is going down, and at the drum outlet  10   b  air is saturated at around 80% with water. Point M 1  shows the thermodynamic state of air before being mixed at portion  24  of the circuit. Such air M 1  is mixed with air coming out from the condensing unit  16  (point K in  FIG. 3 ). Such mixture changes the state of air along lines  4  and  4 ′ so that the final result of the mixture is air at point M 2  with a lower enthalpy than M 1 . It is therefore clear that for further cooling such air (line  3  in  FIG. 3 ), a lower amount of energy is needed for reaching line W if compared to prior art. This is due to the fact that the mixture of process air (M 2 ) is more saturated and pre-cooled when entering the condensing unit  16 . The power saving is comprised between 50 and 100 W. Line H of  FIG. 3  shows the heating phase in the heater  12 , where absolute humidity remains constant and where relative humidity at the outlet from the heating element  12  is below 15%. 
     Instead of mixing the two flows of air downstream the filter  18 , such mixing can be advantageously carried out in the filter housing (embodiment shown in dotted line in  FIG. 1 ), and this leads to a slightly reduced temperature in filter which increase the filtration efficiency. The filter  18  may also be placed downstream the portion  24  of the circuit where the by-pass conduit  20  flows in the main air circulation conduit (embodiment shown in dotted line, lower right part of  FIG. 1 ). 
       FIG. 4  shows a preferred embodiment for a dryer having a condensing unit  16  placed at the bottom of the dryer housing and in which the by-pass conduit  20  is defined by a shaped portion of the housing in which the blower  14  is installed, and particularly in which the by-pass is defined by an opening  22  in the housing of the condensing unit  16 . 
       FIG. 5  shows an enlarged structural detail of  FIG. 4 , where the same references used for  FIG. 1  have been used. Arrows A 1  shows the air coming from the drum. The hot and unsaturated flow A 1  is mixed with cold and saturated flow A 2  from the opening  22  in the condenser housing. The mixed flow A 3  (combination of flows A 1  and A 2 ) passes through the blower  14  and the condensing unit and it is split in a primary air flow A 4  to the heater and in the by-pass flow A 2  to the blower. The solution according to  FIG. 5  is particularly efficient since, in order to create a by-pass conduit, it is only necessary to open a part of the condensing unit housing close to the inlet of the blower. 
     Good results in terms of overall energy efficiency have been obtained with a total air flow through the blower comprised between 210 m 3 /h and 250 m 3 /h, preferably between 220 m 3 /h and 240 m 3 /h, with a fraction of the air flow diverted in the by-pass comprised between approximately 10% and 20%, preferably around 15%. 
     The following table shows a comparison between the air flows in a closed cycle dryer according to the prior art and according to the invention: 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Without by-pass 
                 With by-pass 
                   
               
               
                   
                 m 3 /h 
                 m 3 /h 
                 With by-pass 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Complete process 
                 210 
                 200 
                 −5% 
               
               
                 air loop 
               
               
                 Flow through by- 
                 0 
                 30 
                 15% 
               
               
                 pass 
               
               
                 Flow through 
                 210 
                 230 
                 110%  
               
               
                 condensing unit 
               
               
                 and blower
A closed cycle dryer comprises a drum, an air blowing unit wherein the air is conveyed to the drum, a heating unit used to heat the air that is blown into the drum, and a condensing unit upstream the heating unit for removing moisture. The dryer further comprises a by-pass between a first portion of the air circuit downstream the condensing unit and a second portion downstream the drum in order to increase the energy efficiency of the drying process.