the present invention will now be described with reference to accompanying drawings which are illustrative of certain embodiments of the invention . variations and modifications are possible without departing from the spirit and scope of the invention . fig9 ( a ) is a schematic illustrating an embodiment of a space dehumidification system . there is an internal bypass 39 interlinked with the process airflow 6 through a face and bypass damper 40 . based on the humidity measured in the design space 27 , and with instantaneous and changing loads , the face and bypass damper 40 modulates the amount of airflow passing through the wheel , while bypassing the rest of the airflow . as and when there is a need to supply fresh air 31 for the space design need , it is generally introduced at the inlet of the dehumidifier , and combined with the returning air 28 from the design space 27 . depending on the application , it may be advantageous to heat or cool the fresh air 31 before it mixes with the return air 28 . dehumidifier outlet air 38 may be mixed with return air 28 before passing through a cooling coil 24 and filters 44 , 45 and delivered as supply air 26 to the design space 27 . reactivation airflow 8 passes through a heat source 10 which elevates the air temperature based on the specific design of the unit . the thermostat 30 controls the temperature as per the set point . to control the reactivation air flow , the reactivation blower 5 is continuously variable in speed , with a suitable design for the purpose . to get optimum performance , the rotor speed is also varied through a continuously variable speed bed drive arrangement 106 . the term “ rotational driver ”, or “ driver ” or similar may be used in this application to indicate any type of driving system , for example electric , hydraulic , pneumatic , or other . fig9 ( b ) is a schematic illustrating a second embodiment 120 of a space dehumidification system . this embodiment has a purge sector 11 at the wheel 1 . the purge sector 11 can vary from 5 % to 40 % of the total bed area , the remainder being divided between process area 2 and reactivation area 3 . air 12 is passed through purge sector 11 , where the wheel 1 is hot ; it produces preheated air 13 that subsequently passes through the reactivation sector 3 , thereby reducing the reactivation energy input needed . air 12 also cools that portion of the wheel before entering the process zone 2 , whereby the dehumidification performance through the process sector 2 is improved . in addition , less heat is imparted to the process air 7 because the bed is cooler when it enters the process sector 2 . fig9 ( c ) is a schematic illustrating a space dehumidification system 140 . a pair of purge sectors 11 a , 12 is at the wheel 1 . the air in the sectors 11 a , 12 is circulated in a closed loop using an air mover 15 . heat from the wheel in section 12 following the reactivation sector may be picked up by airflow 13 and passed on to preheat the wheel in purge sector 11 a following the process sector . thus the wheel 1 will require less regeneration energy . fig9 ( d ) is a schematic illustrating an embodiment 150 space dehumidification system . this is similar to the example in fig9 ( c ) except that one more pair of purge sectors 17 , 18 , has been added . a given amount of air 13 , 19 is circulated through these pairs of sectors in two separate closed loops with separate fans 15 , 21 . as stated previously , the airflow in each of the closed loops may be in either direction depending on which is most advantageous . fig9 ( e ) shows a schematic example of a space dehumidification system . this example is a pharmaceutical production area , for which design conditions of 15 % and 30 % rh at 75 ° f . have been selected for the room 27 . the total supply air quantity 26 calculated in this example is 4000 cfm . to satisfy the space cooling needs and moisture removal , 600 cfm is taken as return air 28 . the required fresh air 31 ( 600 cfm ) is passed over cooling coil 23 and mixed with return air 28 . the face and bypass damper 40 controls the airflow through bypass 39 and desiccant wheel 1 . the return air 28 ( 2800 cfm ) is mixed with the process out air 7 to provide the desired supply air flow 26 . the total air is then passed through cooling coil 24 to provide the desired temperature for room 27 . fig1 is a flow chart of an embodiment of the present invention for a process drying / dehumidification system . ambient air 31 is passed through the cooling coil 23 to reduce the moisture load and is cooled . bypass damper 32 modulates the airflow to be passed through desiccant wheel 1 and the remainder through bypass 39 . the mixed air 38 ( process out 7 and bypass air 39 ) is passed over heating 24 / cooling 22 sources and is tempered depending upon the requirement of the supply air 26 . the regeneration flow 8 is also controlled with the help of the damper 34 generally positioned after the regeneration blower 5 . the regeneration heat input 10 can be electric , steam , gas burner or from a variety of heat sources that can elevate the temperature based on the specific design of the unit . this temperature is controlled by thermostat 30 fig1 ( a ) shows a product drying system and method . in this system , based on the conditions required in the drying bin 37 , the mixed air ( process out 7 and the bypass air 39 ) 38 is passed over a process heat input 22 to provide the necessary drying temperature . the return air 28 is cooled through a cooling coil 23 and blown through the process sector 2 and the purge sector 11 of the rotor . the face and bypass damper 40 is used to control the flow that needs to bypass the dehumidifier . air exiting the purge sector is recycled and mixed with the return air upstream of the cooling coil . this enables the dehumidifier to deliver drier air . the purge sector generally varies from 5 to 40 % of the total area , the remainder being divided between process sector 2 and reactivation sector 3 . the reactivation inlet temperature is controlled through thermostat 30 . fig1 ( b ) shows the desiccant bed / wheel 1 from another angle where the various sectors are identified . although shown in a particular way , the sector division may vary . fig1 is a flow schematic showing various hvac element options . each element may be included or not included based on the performance requirements of the application . the overall supply air quantity to be passed through cooling coil 59 / heating source 60 / humidifier 57 is based on the requirement of the space to be conditioned . the return air 28 may pass through a cooling coil 54 or heating coil 53 to give a desired condition for mixing with the fresh air 31 . the fresh air 31 may pass through heat recovery unit 50 , if the required temperature needs to be increased and heating is required via the heat source 22 . fresh air may be cooled , if advantageous , using the cooling coil 23 . the mixed air passes through heating source 55 and cooling source 56 , based on the requirement , and then passes through face and bypass damper 40 . this controls the flow that needs to pass through the desiccant wheel and be dehumidified . the exhaust air passes through heat recovery unit 52 to the outside through blower 23 . the regeneration air passes through heat recovery unit 49 and then goes through heating source 10 to elevate the temperature as per the specific design of the unit . the reactivation airflow going out of the reactivation sector 3 passes through heat recovery sector 48 and through regeneration blower 5 . the use of heat recovery unit 52 reduces the load . the thermostat 30 controls the temperature of reactivation inlet after the heat source , or alternatively may be located and control the reactivation air temperature leaving the desiccant wheel . as explained earlier , the invention relates to a method and a system for the capacity control of the desiccant dehumidifier , which has an active desiccant wheel . as there are instantaneously changing moisture loads , there is a need to control the capacity of the dehumidifying unit and system . while there are several currently known and practiced control methods for reducing the reactivation usage , this invention provides a novel method of substantially further reducing the reactivation energy compared to earlier known methods . in the present invention , the fundamental approach is to continuously provide a means to continuously vary the amount of air that will bypass the desiccant wheel , out of the total process flow . this reduction in process flow through the desiccant unit generally tracks the change in instantaneous moisture loads . when the process flow through the desiccant wheel is reduced , there is no longer a necessity to retain full regeneration flow through the reactivation sector of the wheel . where the regeneration flow is correspondingly reduced in some defined correlation , a considerable reduction is achieved in regeneration energy usage . in this invention , through a control function , the regeneration flow rate can be made to continuously reduce or increase based on the continuously varying process flow rate through the process sector . with the changes in technology , it is today economical and commonplace to use variable speed drives , based on several known methods , which now allow continuous varying of reactivation air flow . similarly , it is also a basis of the invention to use such technology for continuous speed variance of the rotational speed of the wheel , also through a correlating control function . in this , the development of the control function , use is made of the knowledge of the mathematical modeling tool “ dri cal ”, or any other similar tool e . g . “ procal ”, both of which are similar tools , currently , in use worldwide for the selection of a desiccant unit / wheel geometry and flows . while developing this invention of continuously controlling the process variables of the dehumidifier , the energy usage was compared with several known and practiced control methods . to develop the invention , a sample project was selected , with physical facts and assumptions , typical of the design of a dehumidification application . for this , 30 % rh at 70 f was selected as the design condition . to get a better spectrum of the energy saving potential , a lower rh design of 15 % at 70 ° f ., also for the same pharmaceutical application , was selected . the city of zebulon , n . c . was selected for weather conditions typical of the southeastern u . s . however , to demonstrate the effect of more humid climates , the city of mumbai , india was selected as being typical . a flow chart was made and prepared of the sample project / design . with the given hourly weather data available and used today for providing a more detailed load profile of the project design , ambient weather bins were created in increments of 10 grains / lb . air with mean coincident dry bulb temperature and frequency of occurrence in hours / year . this allowed the calculation of several “ bins ” of the instantaneous loads , to enable simple simulation , to estimate the total energy usage with each control method . table 1 below shows the hourly bin data that was created for both the cities , zebulon , n . c ., usa , and mumbai in india . with this method , the reactivation energy usage analysis is more defined compared to applying the design data on the basis of two or three design points , for all the three control methods considered and defined below . ( a ) control option 1 — fixed reactivation airflow , fixed reactivation inlet temperature , fixed rotor speed , variable process flow ; ( b ) control option 2 — fixed reactivation airflow , fixed reactivation discharge temperature , fixed rotor speed , variable process flow . ( this is , for the purposes of the invention , considered as a baseline control option ); ( c ) control option 3 — fixed reactivation inlet temperature , variable reactivation airflow , variable rotor speed , variable process flow through the wheel with the balance bypassing the wheel . based on the hourly bin data , and the aforementioned three control methods / options , option 3 being based on the current invention , the energy used in therms / year for all three options was charted and compared . the comparison is given below in tables 2 , 3 , 4 , 5 and 6 . the amount of energy used in the after cooler is also tabulated in tables 5 and 6 , which clearly show that , in addition to the reduction in regeneration energy usage , there is a considerable overall reduction in cooling energy usage as well . fig1 ( a ): compares the annual post cooling requirement when different control options are used . referring now to fig1 ( b ), this graph shows the comparison of reactivation heat consumption ( in therms / year ) for control options 1 , 2 and 3 . the case study is for 15 % and 30 % rh conditions considered for zebulon and mumbai . it is observed that in case of control option 2 ( baseline control option ), in zebulon for the 15 % rh design the consumption of reactivation heat is 11071 therms / year . if control option 1 is selected , this rises to 13059 therms / year . however , if control option 3 is selected , the consumption comes down considerably to 5747 therms / year . tables 2 , 3 and 4 provide complete data for the energy consumed as per control options 1 , 2 and 3 for 15 % and 30 % design rh in mumbai and zebulon . table 5 is a summary of energy consumed in control option 1 , 2 and 3 for the 30 % rh design and table 6 is a summary of energy consumption per control option 1 , 2 and 3 for 15 % rh design . initially energy usage analysis for the invention , per control option 3 , was benchmarked against the baseline of control option 2 . it was further considered useful to complete the analysis using another commonly and currently used method of dehumidifier capacity control , control option 1 . accordingly , the resultant % reduction in energy with the invention has been compared between all three options . control option 2 is the baseline in table 7 . control option 1 is the baseline in table 8 . referring now to fig1 ( c ), this graph shows the percentage savings in regeneration heat when using different control options . as shown , by using control option 3 , the percentage saving can be as high as 47 %. however if the control option 1 is selected as another baseline , there is a further increase in the percentage saving . this would then be a comparison between control option 1 , and 3 . table 7 provides a detailed energy consumption comparison between control options 1 , 2 and 3 from the foregoing it is evident that this invention presents a novel system and method for dehumidifier capacity control , providing a significant energy saving compared to known arts and methods the system of the invention also incorporates several other advantages such as the design of the basic cabinet and plenums so reactivation sector size can be selected from the range of 12 % to 45 % of the total desiccant rotor face area and set during fabrication with no modification to the cabinet design . in addition , if desired , the design of the basic cabinet and plenums is such that reactivation sector size can be manually field adjusted anywhere in the range of 66 % to 150 % of its original design value using hand tools , to adapt to modified performance requirements . when the system is used with a purge sector with concurrent air flow , the basic cabinet and plenums design enables a purge sector size in the range of 2 % to 25 % of the rotor face area to be added without major modification of the design . although the invention has been described with reference to specific embodiments , it will be understood by those skilled in the art that various changes may be made without departing from the spirit or scope of the invention . for instance , the numerous details set forth herein , for example , details relating to the configuration and operation of the presently preferred embodiment of the active desiccant module and hybrid systems , are provided to facilitate the understanding of the invention and are not provided to limit the scope of the invention . accordingly , the disclosure of embodiments of the invention is intended to be illustrative of the scope of the invention and is not intended to be limiting .