WO2010128522A1 - Desiccant unit control system and method - Google Patents

Abstract

The present invention provides a method of controlling an active desiccant dehumidifier; the method comprising: a. modulating the airflow through a process sector to control the amount of dehumidification; b. modulating the airflow through a reactivation sector as a function of the modulation of the process airflow; and c. modulating the rotational speed of a desiccant wheel as a function of the modulation of the process airflow. The present invention also provides a system for the aforesaid method.

Description

DESICCANT UNIT CONTROL SYSTEM AND METHOD

The present invention relates generally to heating, ventilating, and air-conditioning

(HVAC) systems and methods, as also process drying system and methods, and more specifically to air conditioning or dehumidification or drying systems that incorporate a thermally activated desiccant wheel. The present invention also provides an improved method for conservation/reduction of energy consumed during use of such systems using desiccant wheels.

Desiccant wheels and energy recovery wheels are the two types of wheels used in HVAC, or for conditioning process air. Desiccant wheels are used to transfer moisture from one air stream to another. Desiccant wheels are of the two distinctive types: "active" desiccant wheels, and "passive" desiccant wheels.

"Active" desiccant wheels use an external heat source to heat one of the air streams, to reactivate/regenerate a portion of the wheel. "Active" desiccant wheels have been generally used for industrial applications requiring high moisture removal, but are being increasingly used in commercial HVAC applications. Examples of such active desiccant wheels and systems are disclosed in several patents e.g. patent no. 6,311.511, 5,551,245, 5,816,065.

"Passive" desiccant wheels do not use an external heat source and rely on the relative humidity difference between two or more airstreams to drive moisture transfer between the air steams. Examples of "passive" desiccant wheel system and use are disclosed in U.S.Pat. Nos. 6,237,354 and 6,199,388. As thermally activated desiccant wheel systems use substantial heat energy (steam, electric, gas etc.) to reactivate or regenerate the wheel, various methods have been adopted in the past, aimed to minimize the use of reactivation energy with various control methods and/or use of additional components. Methods such as heat recovery devices to transfer heat energy from process air to reactivation inlet air, or to transfer heat from outlet of reactivation air to inlet of reactivation air, have resulted in excessive "add on costs".

Dehumidification is a process of removing moisture from air. There are several known methods of dehumidifying air. However, the two commonly used methods use refrigeration and desiccants. In case of dehumidification using refrigeration, moisture is made to condense over a cooling coil, thereby removing moisture from an air stream passing over the cooling coil. In case of dehumidification using desiccants, the process employed is one of absorption or adsorption. In absorption, either liquid or solid desiccants are used, typically halide salts or solutions. For adsorption, solid desiccants like silica gel, activated alumna, molecular sieve, etc. are used.

Desiccant based dehumidifier systems can be either of the multiple tower, cyclic type, or of the continuously rotating type. The air to be dried is generally referred to as process air and the air used to regenerate the desiccant is referred as regeneration or reactivation air. Refrigeration based dehumidification systems are limited, in practice, in the moisture they can remove, because to achieve dewpoint humidities below freezing, frost builds up on the coiling coil making the system more complex, and often necessitating the need to provide reheat. Desiccant dehumidifier systems, on the other hand, work independently of the dew point of the air, and hence can achieve very low dew point humidities, necessary for many industrial applications. Known, common examples of use are pharmaceutical areas for production of drugs, and food processing areas, which require relative humidities or dew point humidities lower than those that can be technically or economically achieved through refrigeration alone.

Also hybrid systems using both refrigeration and desiccant units are commonly used and help to reduce energy usage and provide simple and reliable operation of the whole dehumidification system.

Compared to refrigeration type dehumidification units, desiccant dehumidifiers usually use more heat energy, mainly for regeneration or reactivation of the desiccant. Accordingly, over the years, several developments have taken place, in the desiccant equipment physical configuration and the control strategies for the capacity and energy control of the desiccant dehumidifier system, to miniπuze its energy use.

The desiccant dehumidifier units, for dehumidifying/ drying of air at atmospheric pressure, are generally today of the rotary type, wherein the desiccant is contained in a rotary bed (or wheel). The wheel moves on a continuous or intermittent basis, though, typically, two compartments (or sectors), one for process, and the other for regeneration. In the process sector, the air to be dehumidified (generally called the process air) is passed through the wheel and is dried by contact with the desiccant. In the regeneration sector, air is generally brought in from atmosphere, passed over a heat source, which elevates the temperature of the reactivation air, and is then passed through the remaining portion of the wheel, referred to as reactivation or regeneration sector, heating the wheel and driving out the water. Typically the process sector varies between 50 to 80% of the total bed/wheel area, though it could be more or less, the remainder being the reactivation sector.

Often, another sector is added between the process and regeneration sector, and is referred to as the purge sector. A third airstream (generally called the purge air) is passed through the purge sector and used as a portion of the regeneration air. The incorporation of the purge sector helps to recover some residual heat from the rotating wheel before it enters the process sector, thereby reducing the overall energy requirement for regeneration, as well as improving the overall moisture removed by the wheel.

In typical desiccant dehumidifier units, the process air flow rate and the reactivation flow rate are generally fixed and are set or adjusted with the help of manual or automatic dampers. In the design of a typical dehumidifier system for controlling the humidity in a given space, the airflow needed to control the space temperature may often be more than the dehumidified air quantity needed to control die space humidity. In such cases, a portion of die process air is typically bypassed around the dehumidifier unit, and is then combined with air exiting a dehumidifier unit, and then the combined air is cooled (or heated), and dien supplied to the controlled space.

As desiccant dehumidifier systems inherently use a significant amount of heat energy for regeneration, efforts have been made to find ways and means to reduce the amount of heat used by die system. One typical and well-known system and method used is to control the heated temperature of the regeneration air before it enters the reactivation sector of the wheel.

Another well-known method is to control the regeneration heat input amount by controlling the air temperature leaving the reactivation sector.

Depending upon the type and amount of relative humidity and dew point control, when the space or air condition is satisfied, the control strategy may employ the start/stop of the dehumidifier. Similarly, use may instead be made of automatic dampers to continuously vary the amount of air bypassing die dehumidifier unit to satisfy the operation and design needs.

The correlation of the process and reactivation sector area, the wheel rotating speed, the relative process and reactivation air flow rates and velocities through die two sectors, have in the recent decade been documented in Japan, India and USA resulting in robust mathematical modeling tools regularly used for the design, selection, and incorporation of a desiccant wheel, in a finite way, in a dehumidifier unit. Such tools are being used regularly to optimize a dehumidifying system at the design and build stage.

One such study and development of a mathematical model is detailed in a document "Modeling of Rotary Desiccant Wheels" by Harshe, Utikar, Ranade and Pahwa, in 2005.

In the case of rotating desiccant dehumidifier units, it has been known that equipment performance at the design and construction stage can be optimized by using such a mathematical modeling tool, to select a particular percentage as reactivation sector, as well as the process and reactivation flow rates, and also a given bed rotational speed. In such cases, under part loads and instantaneously changing moisture load, dehumidifier capacity control is achieved by using the traditional control strategies described above, some of which are well known and well documented, for example in the Bry Air design manual as well as the Munters design manual.

Widi traditional and known methods of dehumidifier capacity control, during the operation of such dehumidifier systems, reduction of the regeneration energy usage is limited. All of the above do not achieve the maximum energy reduction desirable, or to a large extent commensurate, with the changes in the instantaneous moisture load.

Several examples are provided below of prior arts practiced to reduce the regeneration energy and/or to regulate the desiccant wheel speed while optimizing dehumidifier capacity. U.S.Pat. No. 4,546,442 teaches a microcomputer-based programmable control system for fixed bed, multi-bed desiccant air dryers commonly used to dehumidify compressed air or other compressed gases. The control system is used to monitor the level of moisture in the desiccant and determine whether a regeneration cycle is required, and also to monitor the full depressurization and repressurization of the regeneration bed, and also to analyze and indicate valve malfunction. The application of the invention is limited to a compressed air system.

U.S.Pat. No. 4,729,774 teaches the profiling of air temperature in the regeneration sector to improve dehumidifier performance.

U.S.Pat. No. 4,926,618 teaches a desiccant unit having controllable reactivation air recirculating means and variable wheel speed means. The process air humidity is controlled by a master controller modulating wheel speed, reactivation air recirculation rate and reactivation heat input. Process and reactivation airflow rates through the wheel are fixed, and the reactivation air heater is controlled to maintain a constant reactivation air temperature leaving the wheel.

U.S.Pat. No. 5,148,374 teaches a system and method for real-time computer control of multi wheel sorbent mass energy transfer systems by optimization of calculated mass transfer ratios and measures of system effectiveness which are not subject to long system time constants.

The method relies on sensing at predetermined intervals a predetermined set of parameters selected from the group of wheel inlet temperature, and wheel outlet temperature, etc., to send a control signal to a predetermined one of a group of control means which includes controlling fluid flow temperature. The objective of the control method is to improve the response of the controlled device to a rapid change in load without causing unstable operation of the device and resultant fluctuations of the controlled variable.

U.S. Pat. No. 5,688,305 teaches an apparatus and method of regeneration of regeneration control for a desiccant dehumidification system in which the reactivation airflow is controlled to maintain a constant reactivation discharge air temperature and the reactivation air inlet temperature is controlled at a fixed value. The residence time of the desiccant in reactivation is also controlled in inverse proportion to the reactivation airflow. The object of this document is to reduce the over-generation of desiccant under part-load conditions, thus improving the operating efficiency of the desiccant dehumidifier. The application cited is for drying granular material in a bin or hopper using a dehumidified recirculated airstream, when the flow of granular material through the bin may occur in batches or at a variable rate. U.S.Pat. No. 6,199,388 Bl teaches a system and method for controlling the temperature and humidity level of a controlled space and is applied mainly to a combination of an enthalpy wheel, otherwise known as energy recovery wheel, a cooling coil, and a "passive" desiccant dehumidification wheel which does not employ any external thermal heat or energy input for reactivation. It further teaches a means for changing the performance of a "passive" desiccant wheel through change in rotational speed in response to the sensible and latent loads in the controlled space. Control of the desiccant wheel speed is discussed and the intent is to control the dehumidification capacity of the "passive" wheel rather than optimize the energy efficiency of the dehumidification process. It does not teach the use of process air face and bypass dampers to control the capacity of the dehumidification wheel. Both supply (process) and exhaust (reactivation) airflows are maintained at a constant value through all loading conditions.

U.S.Pat. No. 6,355,091 Bl teaches a unitary ventilation and dehumidification system for supplying outside ventilation air to a conditioned space. The unit includes a desiccant wheel which is rotated at a slow speed to accomplish more dehumidification, and at a fast speed to accomplish more heat recovery. Heat may be added to the space exhaust air upstream of the desiccant wheel to improve its dehumidification performance and to prevent frost formation during winter operation. Both supply and exhaust airflows are fixed, no bypass dampers are used, and rotor speed adjustment is for selection of operating mode and not efficiency improvement.

U.S.Pat.No. 6,767,390 B2 teaches a method to control the performance of a multi-bed, fixed bed desiccant dryer for compressed air and compressed gas applications and to optimize the regeneration and purge cycles to deliver the gas at the desired dew point. The intended field of application is compressed air for use in instruments.

U.S. Pat.No.7,017,356 B2 teaches about an HVAC system for cooling and dehumidifying comfort-conditioned spaces which includes a desiccant wheel in a passive dehumidification arrangement where the wheel's speed varies with airflow, and the wheel is operated for at least a set period during start up to prevent a surge of humid air into the conditioned space. This patent also teaches the use of a passive sensible recovery device and cooling coil to precondition the outside air before it mixes with the return air from the conditioned space.

U.S. Pat.No.7,101,414B teaches a method for reducing a sorbent concentration for a process fluid stream using a sorption bed system which includes material that is rotated through multiple zones, in addition to traditional process and regeneration zones, whereby one or two pairs of independent recirculated fluid streams, other than process and regeneration flow streams, are used to isolate process and regeneration flow streams from each other. The objective of the isolation may be to prevent cross-leakage of air between process and reactivation zones, permeation of sorbate through the sorption bed, or formation of condensation or frost on the sorption bed.

U.S.Pat. No.7,338,548 B2 teaches the use of an apparatus and a control method of conditioning humidity and temperature in a process air stream from a desiccant dehumidifier, where a portion of the process discharge air is used to preheat the regeneration air by use of an air-to-air heat exchanger. The field of use of the invention is in drying of structures and remediation of water damage.

US 7,389,646 B2 is a divisional application for previous work and is similar to 7,017,356 B2 by the same inventor. It also is intended for cooling and dehumidifying comfort-conditioned spaces and teaches an HVAC system which includes a passive desiccant wheel, wherein the wheel's speed varies with airflow, and relies on the wheel being energized for at least a set period, at start up, and employs a heat recovery system upstream of the wheel to enhance the system's ability to dehumidify air.

Most prior art control strategies have been only very partially successful in limiting and reducing the use of reactivation energy, not commensurate with the reduced moisture load at part-load conditions.

Also, during the use and application of the desiccant wheel and system, there is usually a considerable change in the instantaneous moisture loads, in the fresh air, if required to be treated, and the internal latent loads within the space where moisture is to be controlled, based on the changes of outdoor temperature and humidity, and product and occupancy loads.

A need therefore exists for a control method, along with necessary related components, that will substantially reduce the use of reactivation energy and that responds not only to changes in the dynamic/instantaneous moisture load but also simultaneously allows the optimization of energy use in the wheel, during these changes in moisture load. OBJECTS OF THE INVENTION

The general object and purpose of the invention is to substantially reduce the cumulative energy used in the ongoing operation of a thermally activated desiccant dehumidification system. The energy reduction is generally achieved by modulating the energy consumed by the desiccant unit in response to the instantaneous changes in moisture in the ambient air and/or the moisture load in the controlled space, and/or the moisture change of the process flow. Such instantaneous changes of moisture, and resultant moisture load, require the need to control the capacity of the dehumidification system.

With constantly varying and changing instantaneous moisture load, this dehumidification capacity control is mainly achieved by controlling the air flow through the process sector of the wheel; optimum/minimum energy use in the dehumidifier is achieved by proportionately controlling the air flow through the reactivation sector, and keeping constant the reactivation air temperature, while simultaneously and proportionately adjusting the rotation speed of the wheel, so that optimum energy efficiency is achieved.

While there are established methods for control of the capacity of the dehumidifier system, the present invention provides a novel method, achieving a substantial reduction in energy usage at part-load compared to the previous known methods.

The objects of the invention are achieved by a system and method to control dehumidification capacity comprising: a) controlling the airflow through the process sector of the rotor, and controlling a constant reactivation inlet temperature, and controlling the reactivation airflow as a function of the process airflow, and also controlling the rotor speed as a function of the process airflow, and the control functions are based on the ratio of instantaneous process airflow to design process airflow and the functions are all exponential functions with the exponents lying anywhere in the range of 0.5 to 2.0, and with the exponents for each controlled variable not necessarily being equal. b) controlling the airflow through the process sector of the rotor, and controlling a constant reactivation heat source temperature, for example, by use of steam at constant pressure as the reactivation heat source and use of a two position steam valve on the reactivation air heating coil, and by controlling the reactivation airflow as a function of the process airflow, and also controlling the rotor speed as a function of the process airflow, and the control functions are based on the ratio of instantaneous process airflow to design process airflow and the functions are all exponential functions with the exponent lying anywhere in the range of 0.5 to 2.0, and with the exponents for each controlled variable not necessarily being equal. c) controlling the airflow through the reactivation sector of the rotor while maintaining a constant airflow through the process sector and controlling a constant reactivation inlet temperature, and also controlling the rotor speed as a function of the reactivation airflow, and the control function is based on the ratio of instantaneous reactivation airflow to design process airflow and the function is an exponential function with the exponent lying anywhere in the range of 0.5 to 2.0. d) controlling the airflow through the reactivation sector of the rotor while maintaining a constant airflow through the process sector and controlling a constant reactivation heat source temperature, for example, by use of stream at constant pressure as the reactivation heat source and use of a two position steam valve on the reactivation air heating coil,, and also controlling the rotor speed as a function of the reactivation airflow, and the control function is based on the ratio of instantaneous reactivation airflow to design process airflow and the function is an exponential function with the exponent lying anywhere in the range of 0.5 to 2.0. e) controlling the airflow through the process sector of the rotor, and controlling a constant reactivation discharge temperature, and controlling the reactivation airflow as a function of the process airflow, and also controlling the rotor speed as a function of the process airflow, and the control functions are based on the ratio of instantaneous process airflow to design process airflow and the functions are all exponential functions with the exponents lying anywhere in the range of 0.5 to 2.0, and with the exponents for each controlled variable not necessarily being equal. f) controlling the airflow through the reactivation sector of the rotor while maintaining a constant airflow through the process sector and controlling a constant reactivation discharge temperature, and also controlling the rotor speed as a function of the reactivation airflow, and the control function is based on the ratio of instantaneous reactivation airflow to design process airflow and the function is an exponential function with the exponent lying anywhere in the range of 0.5 to 2.0.

Another object of the invention is to provide a system and method of controlling dehumidification capacity in accordance with the four control scenarios described above, and in addition incorporate a purge sector, disposed sequentially between the reactivation and process sector of the rotors with concurrent airflow through the process sector and purge sector, and control the purge airflow as a function of the reactivation airflow, the control function being based on the ratio of the instantaneous reactivation airflow and design reactivation airflow and being an exponential function with the exponent lying anywhere in the range of 0.5 to 2.0.

Another object of the invention is to provide a system and method to control dehumidification capacity in accordance with the four control scenarios described above, and in addition incorporate at lease one pair of purge sectors disposed between the process and reactivation sectors and each pair of sectors having means to re-circulate air through them, in accordance with US patent No. 7,101,414 B2, the improvement being to control the recirculation rate of the purge air as a function of the rotor speed, and the function being based on the ratio of instantaneous rotor speed to design rotor speed and the function being an exponential function with the exponent lying anywhere in the range of 0.5 to 2.0.

In the above embodiments there is a further object which is to provide a design feature for the basic cabinet and plenums to permit the reactivation sector size to be easily adjusted at the time of fabrication or after installation in the field to further optimize the design for any given application for the dehumidification system. The optimization is achieved by selecting the relative size of the process and reactivation sectors that permits the lowest reactivation energy use at design conditions and/or the lowest process discharge humidity.

One or more of the above objects of the invention are to provide a thermally activated dehumidification system employing an "active" desiccant rotor so that full advantage is taken of the dynamic behavior of the desiccant rotor under varying part load or process flow conditions.

BRIEF DESCRIPTION OF THE INVENTION

Accordingly the present invention provides an apparatus for dehumidifying air supplied to an enclosed space or process or drying bin, the apparatus comprising:

(a) a housing defining an interior space; (b) the interior space being separated by a separator into a supply portion for containing a supply air stream and a regeneration portion for containing a regeneration air stream, the supply portion being provided with an inlet for receiving supply air and an outlet for supplying air to the enclosed space, the regeneration portion being provided with an inlet for receiving regeneration air and an outlet for discharging regeneration air; (c) a rotatable desiccant wheel positioned such that a portion of the wheel extends into the supply portion and a portion of die wheel extends into the regeneration portion, the wheel being rotatable through the supply air stream and the regeneration air stream to dehumidify the supply air stream;

(d) a heat source to heat the regeneration air stream in order to regenerate the desiccant wheel as it rotates dirough the regeneration air stream; and

(f) at least one bypass damper between the inlet and the oudet of the supply portion for controlling the amount of supply air passing through the desiccant wheel by selectively bypassing the desiccant wheel.

In one embodiment, the apparatus can be a conventional HVAC unit or a hybrid air conditioning and dehumidifying apparatus.

In anodier embodiment, the regeneration portion is provided with a fan to move die regeneration air stream.

In anodier embodiment, a duct and control means is provided to permit die recirculation of a portion of die regeneration air. In a preferred embodiment, a damper and/or speed control means is provided to permit modulation of the airflow through the regeneration portion.

In anodier embodiment, die supply portion is provided with a fan to move the supply air stream; a cooling coil is positioned in the supply air stream; with the rotatable desiccant wheel being positioned downstream of the cooling coil. In another embodiment, a speed regulation mechanism is provided to vary the rotational speed of the desiccant wheel to control the amount of moisture removed from the supply air stream and/or minimize the amount of heat transferred to the supply air stream.

In a further embodiment, the heat source is a direct-fired gas burner. In a further embodiment, the heat source is electricity used in resistance heaters.

In a further embodiment, the heat source is a constant-temperature source such as steam or hot water.

In a further embodiment, the heat source is a source of recovered heat from a refrigeration condenser or recovered heat from another process. In a further embodiment, the heat source is a combination of two or more of the heat sources described above, used sequentially.

In a preferred embodiment, a heat modulating means is provided for the heat source to regulate the temperature of the regeneration air stream.

In another embodiment of the invention, a modulation means is provided for the bypass damper to regulate the amount of supply air passing through the desiccant wheel.

In another embodiment, the desiccant wheel is sized to handle a desired fraction of the air flow processed by the air conditioning system.

In another embodiment, means are provided to cool and/or heat the supply air after it passes through the dehumidifier and before it is delivered to the conditioned space. In another embodiment, the system includes a compartment housing a condenser, the apparatus being provided with a duct or opening connecting regeneration inlet air to the condenser housing compartment in order to enable pre-heating of regeneration inlet air by the condenser.

The invention also provides a method for controlling the temperature and humidity of a conditioned space or process or drying bin, the method comprising the steps of:

(a) providing an air conditioning system in communication with the conditioned space;

(b) providing an active desiccant wheel system defining an interior space; the interior space being separated by a separator into a supply portion for containing a supply air stream and a regeneration portion for containing a regeneration air stream, the supply portion being provided with an inlet for receiving supply air from the enclosed space or die air conditioning system and an outlet for supplying air to the air conditioning system or the enclosed space, the regeneration portion being provided with an inlet for receiving regeneration air and an outlet for discharging regeneration air; a rotatable desiccant wheel positioned such that a portion of the wheel extends into the supply portion and a portion of the wheel extends into the regeneration portion, the wheel being rotatable through the supply air stream and the regeneration air stream to dehumidify the supply ait stream; a heat source to heat the regeneration air stream in order to regenerate the desiccant wheel as it rotates through the regeneration air stream; and at least one bypass damper between the inlet and the outlet of the supply portion for controlling the amount of supply air passing through the desiccant wheel by selectively bypassing the desiccant wheel; (c) connecting the active desiccant wheel system to the air conditioning system;

(d) cooling and/or heating the supply air stream by passing it through the air conditioning system; and

(e) dehumidifying the supply airstream by passing it through the active desiccant wheel system while rotating the wheel through the supply air stream and the regeneration air stream to exchange moisture and/or heat between the air streams; and

(f) delivering air from the air conditioning system to the conditioned space.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other embodiments and advantages of the present invention will become more fully apparent from the following description and accompanying drawings where: FIG. l(a&b): is a schematic drawing of a typical thermally activated desiccant dehumidifier unit shown along with the regeneration blower and shows a typical/classic 25% regeneration sector.

FIG. 2 (a&b): is a schematic drawing of a typical thermally activated desiccant dehumidifier unit shown along with the regeneration blower and shows a typical/classic 25% regeneration sector, and also includes a purge sector.

FIG. 3 (a&b): is a schematic drawing of a typical thermally activated desiccant dehumidifier unit shown along with the regeneration blower and shows a typical/classic 25% regeneration sector, and also includes a pair of a purge sectors.

FIG. 4(a&b): is a schematic drawing of a typical thermally activated desiccant dehumidifier units shown along with the regeneration blower and shows a typical/ classic 25% regeneration sector and also includes additional two pairs of purge sectors.

FIG. 5(a& b): is a schematic diagram showing a typical prior art dehumidification system and method.

FIG. 6( a&b): is also a schematic showing a typical prior art product drying system and method.

FIG. 7(a&b): is also a schematic showing a typical prior art product drying system and method and also includes a purge sector..

FIG. 8(a, b, c d & e): are schematics showing an embodiment of the system and method of the present invention. FIG. 9: is a schematic showing an embodiment of the present invention as a flow chart for process drying/ dehumidification system.

FIG. 10(a&b): is a schematic and embodiment of the present invention as a diagram showing a product drying system and method. FIG. ll(a,b&c): is a graphical representation showing energy saving with the present invention in comparison with the prior art

FIG. 12: is a schematic showing an embodiment of the system and method of present invention and also includes several HVAC components that can be enabled /used or disabled/not used DETAILED DESCRIPTION OF THE INVENTION

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.

FIG. l(a) is a typical desiccant dehumidifier flow chart. As mentioned earlier, a typical rotating desiccant bed/wheel 1 has a process sector 2 and regeneration or reactivation sector 3.

The dehumidifier incorporating such a desiccant bed/wheel 1 would have a process flow 6, as well a regeneration flow 8. The regeneration flow is elevated in temperature by passing over a heat source 10 before entering the regeneration part of the bed 3. The regeneration air, exiting the reactivation sector 3 of the rotary bed is exhausted 9 with the help of the blower 5, referred to generally as the reactivation blower 5. The desiccant bed/wheel 1 is made to rotate through the reactivation and process compartments with the help of a bed drive arrangement 4.

FIG. l(b): shows a typical sector division of the wheel 1. The process sector 2, in a typical unit is 75% of the total bed area, as is shown as such, and can, in practice, generally vary from 50% to 80%, but can be designed to be even smaller or higher. The remaining area of the desiccant bed is shown as the reactivation sector 3, and can vary between 20% and 50% but can be designed to be even smaller or higher.

FIG. 2(a): shows the addition of another sector, referred to as the purge sector 11. The purge sector generally varies from 5 to 40% of the total bed area, the remainder being divided between process 2 and reactivation area 3. When the bed rotates from the reactivation sector 3 to the process sector 2, the bed is still hot. It is well known that the hot portion of the bed, particularly if it is of the silica gel type, will begin to perform (that is, remove moisture) when it has cooled down. Therefore, a certain portion of the bed is substantially inactive in performing the dehumidifying function while it is still hot. This segment or portion of the bed is often sectioned off and made into a purge sector 11. Air 12 is made to pass over this sector 11 , where the bed is hot, whereby the air 13 is preheated, before being made to pass through the reactivation sector 3, thereby both reducing the reactivation energy input needed, and also cooling that portion of the bed 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 because the bed is cooler when it enters the process sector. FIG. 2(b): shows the desiccant bed/wheel 1 from another angle, where various sectors are marked, and although shown in a typical way, these sector areas can vary, as explained above. FIG. 3(a): shows another flow chart of a rotary desiccant bed/wheel 1 system where a pair of sectors (11a ,12) has been added. In such a configuration, it is typical to continuously circulate a given amount of airflow through these sections, in a closed loop, with the help of a separate fan 15. The recirculated airflow acts as a buffer between the process and reactivation airstreams, capturing air leakage or moisture diffusion between the process and reactivation airstreams and thus improving the system performance. In some cases the recirculated airflow may also transfer heat between the sectors in the same manner as the purge sector shown in Fig. 2, further improving the system performance. It should be noted that the airflow in the recirculation loops described in all the figures may be in either direction, with the most advantageous direction depending on the specifics of a particular application. FIG. 3(b): shows the desiccant bed/wheel 1 from another angle, with various sectors marked, and although shown in a typical way, these sectors areas can vary, as clearly explained above.

FIG. 4(a),is a flow chart of a rotary desiccant bed/wheel 1 where more than one pair of purge sectors 11a, 12, 17,18 has been added. In such a configuration, it is typical to circulate a given amount of air 13, 19 through these sections, in a closed loop, by separate fans 15, 21.

FIG. 4(b): shows the desiccant bed/wheel 1 from another angle, with various sectors marked, and also shown in a typical way, these sectors can vary, as clearly explained above.

FIG. 5(a&b): shows a typical and traditional dehumidifier system for controlling a space 27. In this system, for example, the cooling needs for the space to be dehumidified, necessitate a certain quantity of overall supply air 26 to be taken over the cooling unit or coil 24, and the supplied to the controlled space. A greater airflow may be required to satisfy the space cooling needs than need be passed through the desiccant wheel to satisfy the space dehumidification needs. To accomplish this it is common practice to take a portion of the air through the dehumidifier and bypass 25 the balance to make up the total supply airflow passed through the cooling coil and delivered to the room. There is often a need to supply fresh air 31 to meet space ventilation/pressurization requirements. The fresh air is generally introduced at the inlet to the dehumidifier, combined with the air returning 28 from the controlled space. It may be advantageous to cool/heat the fresh air before combining it with the return air using air heating/cooling means 22 and 23 as shown in the figure. In this typical flow chart/schematic, use is made of dampers to the control flow of air. The fresh air flow is controlled with the help of the damper 35. The bypass damper 32 is used to control the flow that needs to bypass the desiccant dehumidifier unit. The overall supply air flow is controlled with the help of damper 33 positioned normally after the supply air flow. Each of these dampers may be adjusted manually, or automatically using actuators and appropriate controls.

The regeneration flow is also controlled with the help of a damper generally positioned after the reactivation fan 5. The regeneration heat input source 10 can be electric, steam, gas or oil burner, thermal fluid such as hot water, refrigeration condenser heat, recovered heat from another process, or any combination of these that can heat the reactivation air to the temperature required for the application. The reactivation heat energy input is regulated by a thermostat 30 which is generally positioned prior to the desiccant bed. This thermostat 36 may be located after the desiccant bed in the reactivation "out" section as shown in Fig. 5b. In some cases the alternate location results in reduced annual reactivation heat use, compared with the placement of the thermostatic control before the desiccant rotor. In both the above mentioned dehumidifier systems and reactivation heat input control methods, control strategies presently commonly used will sense the "satisfaction" of the relative humidity or moisture level of a given space, or process, or supply air, and stop the reactivation airflow, bed rotation and reactivation heat input when the humidity is satisfied, commonly referred to as "on-off" control.. In another known method, commonly used with fixed temperature heat sources such as steam or hot water, the reactivation airflow is modulated to regulate the dehumidification capacity of the unit.

FIG. 6 (a): shows a typical dehumidifier system used for drying applications. In this system, the dehumidified air 7 is heated through a heating source 22 as per the requirement of the material in the drying bin 37 . The return air 28 carrying moisture from the product is passed over a cooling coil 23 and passed through the desiccant wheel/bed 1 to adsorb the moisture.

The regeneration airflow 8 is provided by the reactivation blower 5. The heat source 10 is used to elevate the temperature based on the specific design of unit. The reactivation inlet temperature is controlled through thermostat.

FIG. 6 (b): shows the desiccant bed/wheel from another angle. The process sector 2, in a typical unit is 75% of the total bed area, as is shown as such, and can, in practice, generally vary from 50% to 80% but can be designed to be even smaller or higher. The remaining area of desiccant bed is shown as the reactivation sector 3, and can vary between 20% and 50% but can be designed to be even smaller or higher.

FIG. 7(a): Shows a typical dehumidifier system for drying application. This is similar to the system explained in Figure 6 (a&b), except a purge sector 11 has been added. This purge sector can vary from 5 to 40% of the total bed area. The object of using a purge sector has already been explained previously.

FIG. 7(b): shows desiccant bed/wheel 1 from another angle, where the various sectors are marked, and although shown in a typical way these sectors areas can vary, as explained above. FIG. 8(a): shows a typical space dehumidification system. In this 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. 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 air returning 28 from the design space 27. Depending on the application, it may be advantageous to heat or cool the fresh air before it mixes with the return air.

Air from outlet of the dehumidifier 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 4. Fig 8(b) is a schematic of typical space dehumidification system example. This is similar to the example in Fig.8( a ) except that a purge sector 11 has been provided in the desiccant bed/wheel . The purge sector can vary from 5 to 40% of the total bed area .The remainder being divided between process 2 and reactivation area 3 . Air 12 is made to pass over this sector 11, where the bed is hot, whereby the air 13 is preheated, before being made to pass through the reactivation sector 3, thereby both reducing the reactivation energy input needed, and also cooling that portion of the bed 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 because the bed is cooler when it enters the process sector.

Air from outlet of the dehumidifier 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 4. Fig 8 (c ) is a schematic of typical space dehumidification system example. This is similar to the example in Fig.8(a) except that a pair of purge sectors 11a, 12 has been provided in the desiccant bed/wheel. In such configuration, it is typical to circulate air in the sectors lla,12 in a closed loop using a separate fan 15. Heat from the wheel in section 12 following the reactivation sector, may be picked up, and passed on to "pre heat" the wheel in sector 11a following the process sector, with the help of airflow marked 13.

The mixed air 38 from the dehumidifier may be mixed with return air 28 and then passed through a cooling coil 24 for cooling the supply air 26 as required to cool the design space 27.

Reactivation inlet air 8 passes through a filter 42 and temperature of this air is elevated through a heat source 10 based on the specific design of the unit. This temperature is controlled and kept constant by a thermostat 30. To continuously vary reactivation airflow, a 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 4. Fig 8 (d) shows a schematic of a typical space dehumidification system example. This is similar to the example in Fig 8(c ) except that one more pair of purge sectors, 17, 18, has been added. In such a configuration, it is typical to circulate a given amount of air 13, 19 through these pairs 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 direction is most advantageous.

The mixed air 38 from the dehumidifier may be mixed with return air 28 and passed through a cooling coil 24 for cooling the supply air 26 to cool the space 27. The reactivation inlet air 8 passes through a filter 42 and the temperature of this air is elevated through a heat source 10 based on the specific design of unit. This temperature is controlled and kept constant by a thermostat 30. To continuously vary reactivation air flow, 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 4.

Fig 8(e) shows a schematic of a typical space dehumidification system. This is an example of 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 4000cfm. To satisfy the space cooling needs and moisture removal, 600cfm is taken as return air 28. The required fresh air 31(600cfm) is passed over cooling coil 23 and is mixed with return air 28. The face and bypass damper 40 controls the airflow through bypass/desiccant wheel. The return air 28 (2800cfm) 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 room temperature. FIG. 9 shows a flow chart for the process drying/dehumidification system. The ambient air 31 is passed through the cooling coil 23 to reduce the moisture load and is cooled. The bypass damper 32 modulates the airflow to be passed through the desiccant wheel and the remainder through the bypass. 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 FIG. 10(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 2 and reactivation 3 areas. The reactivation inlet temperature is controlled through thermostat 30 Fig.lO(b) shows the desiccant bed/wheel 1 from another angle where the various sectors are marked , and although shown in a typical way, the sector division can vary. .

FIG. 11 (a): compares the annual post cooling requirement when different control options are used.

Fig. 12: 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 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 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 CaI", 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, first a sample project was selected, with physical facts and assumptions, typical of the design of a dehumidification application. For this, 30% RH at 7OF was selected as the design condition. To get a better spectrum of the energy saving potential, a lower RH design of 15% at 70⁰F, also for the same pharmaceutical application, was selected. The city of Zebulon, NC. 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, NC, USA, and Mumbai in India.

Table -1.0

HOURLY BIN DATA

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.

Referring now to Fig. H(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.

Table 2

Ener consum tion data as er control o tion- 1

Table 5

Energy consumption summary as per control options - 1 ,2, 3 for the 30% RH

Table 6

Energy consumption summary as per control options - 1 ,2, 3 for the 15% RH

Design Example

While 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 using control option 1.

Accordingly, the resultant % of reduction in energy with the invention has been compared between all the three options, using control option 2 as the baseline, in Table 7, and using control option 1 as the baseline in Table 8. Referring now to Fig. ll(c), this graph shows the percentage savings in regeneration heat 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 Option 1, 2 and 3

Table 8

Ener Consum tion Anal sis

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 die 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 die spirit or scope of die invention. For instance, the numerous details set forth herein, for example, details relating to the configuration and operation of the presendy preferred embodiment of die active desiccant module and hybrid systems, are provided to facilitate die understanding of the invention and are not provided to limit the scope of die invention. Accordingly, die disclosure of embodiments of die invention is intended to be illustrative of the scope of the invention and is not intended to be limiting.

Claims

We claim:

1. A method of controlling an active desiccant dehumidifier; the method comprising: a. modulating the airflow through a process sector to control the amount of dehumidification; b. modulating the airflow through a reactivation sector as a function of the modulation of the process airflow; and c. modulating the rotational speed of a desiccant wheel as a function of the modulation of the process airflow.

2. A method of controlling an active desiccant dehumidifier the method comprising: a. modulating the airflow through a process sector to control the amount of dehumidification; b. modulating the airflow through a reactivation sector as a function of the modulation of the process airflow.

3. A method of controlling an active desiccant dehumidifier comprised of a housing containing at least: a desiccant wheel having a process sector with airflow means; a reactivation sector with airflow means; a means of rotating the desiccant wheel through the process and reactivation sectors; and reactivation air heating means; the control objective being to achieve improved operating efficiency at part-load conditions; the method comprising: a. modulating the airflow through a process sector to control the amount of dehumidification; b. modulating the airflow through a reactivation sector as a function of the modulation of the process airflow; and c. modulating the rotational speed of a desiccant wheel as a function of the modulation of the process airflow.

4. The method any as claimed in claims 1 to 3 wherein the modulation of process airflow comprises bypassing a portion of the process airflow around the desiccant wheel.

5. The method any as claimed in claims 1 to 3 wherein the modulation of process airflow comprises modulating a damper controlling the process airflow.

6. The method any as claimed in claims 1 to 3 wherein the modulation of process air comprises simultaneously controlling the airflow through the desiccant wheel and the airflow bypassing the desiccant wheel so the total airflow remains practically constant.

7. The method as claimed in claim 1 wherein the modulation of process airflow comprises varying the operating characteristics of the process airflow means.

8. The method as claimed in claim 1 wherein the minimum airflow through the process sector is limited to a predetermined value.

9. The method as claimed in any of claims 1 to 3 wherein the control function of the modulation of the reactivation airflow is a linear function.

10. The method as claimed in any of claims 1 to 3 wherein the control function of the modulation of the reactivation airflow is an exponential function with the exponent between 0.5 and 2.0.

11. The method as claimed in any of claims 1 to 3 wherein the control function of the modulation of the desiccant wheel rotational speed is a linear function.

12. The method as claimed in any of claims 1 to 3 wherein the control function of the modulation of the desiccant wheel rotational speed is an exponential function with the exponent between 0.5 and 2.0.

13. A method of controlling an active desiccant dehumidifier the method comprising: a. modulating the airflow through a reactivation sector while maintaining a constant airflow through a process sector to control the amount of dehumidification, and b. modulating the rotational speed of a desiccant wheel as a function of the modulation of the reactivation airflow.

14. A method of controlling an active desiccant dehumidifier comprised of a housing containing at least: a desiccant wheel having a process sector with airflow means; a reactivation sector with airflow means; a means of rotating the desiccant wheel through the process and reactivation sectors; and reactivation air heating means; the control objective being to achieve improved operating efficiency at part-load conditions; the method comprising: a. modulating the airflow through a reactivation sector while maintaining a constant airflow through a process sector to control the amount of dehumidification, and b. modulating the rotational speed of a desiccant wheel as a function of the modulation of the reactivation airflow.

15. The method as claimed in any of claims 1 to 3, claim 13 or claim 14 wherein the heated temperature of the air entering the reactivation sector is maintained at a fixed value.

16. The method as claimed in claim 15 wherein the heated temperature of the reactivation air is maintained at a fixed value by modulating the heat input to the reactivation air heating means.

17. The method as claimed in any of claims 1 to 3, claim 13 or claim 14 wherein the temperature of the reactivation air leaving the reactivation sector is maintained at a fixed value.

18. The method as claimed in claim 17 wherein the temperature of the air leaving the reactivation sector is controlled by modulating the heat input to the reactivation air heating means.

19. The method as claimed in any of claims 1 to 3, claim 13 or claim 14 wherein the reactivation air heat source is maintained at a fixed value and the temperature of the reactivation heated air is not controlled but allowed to vary, increasing with reduced airflow and decreasing with greater airflow.

20. The method as claimed in claim 19 wherein the reactivation heat source is activated whenever there is airflow through the reactivation sector.

21. The method as claimed in any of claims 1 to 3, claim 13 or claim 14 wherein the modulation of the reactivation airflow is achieved by modulating a damper in the reactivation airstream.

22. The method as claimed in any of claims 1 to 3, claim 13 or claim 14 wherein the modulation of the reactivation airflow is achieved by varying the operating characteristics of the reactivation airflow means.

23. The method as claimed in any of claims 1 to 3, claim 13 or claim 14 wherein die modulation of the reactivation airflow is achieved by bypassing a portion of the reactivation air around die desiccant wheel.

24. The mediod as claimed in any of claims 1 to 3, claim 13 or claim 14 wherein the minimum airflow through the reactivation sector is limited to a predetermined value.

25. The method as claimed in any of claims 1 to 3, claim 13 or claim 14 wherein the modulation of the rotational speed of the desiccant wheel is achieved by varying the operating characteristics of the desiccant wheel rotating means.

26. The method as claimed in any of claims 1 to 3, claim 13 or claim 14 wherein the effective rotational speed of the wheel is achieved by intermittently operating the desiccant wheel rotating means such that the percentage of time the rotating means operates is proportional to the control function desired.

27. The method as claimed in any of claims 1 to 3, claim 13 or claim 14 wherein the minimum rotational speed of the desiccant wheel is limited to a predetermined value.

28. The method as claimed in claim 13 or claim 14 wherein the control function of the modulation of the desiccant wheel rotational speed is a linear function of the reactivation airflow.

29. The method as claimed in claim 13 or claim 14 wherein the control function of the modulation of the desiccant wheel rotational speed is an exponential function of the reactivation airflow, with the exponent between 0.5 and 2.0

30. An active desiccant dehumidifier system comprised of a housing containing at least: a desiccant wheel having a process sector with airflow means; a reactivation sector with airflow means; a means of rotating the desiccant wheel dirough the process and reactivation sectors; reactivation air heating means; and a control system intended to improve the operating efficiency of die dehumidifier at part-load conditions; the control system logic comprising: a. modulating the airflow through a process sector to control die amount of dehumidification; b. modulating the airflow through a reactivation sector as a function of the modulation of the process airflow; and c. modulating the rotational speed of a desiccant wheel as a function of the modulation of the process airflow.

31. An active desiccant dehumidifier system comprised of a housing containing at least: a desiccant wheel having a process sector with airflow means; a reactivation sector with airflow means; a means of rotating the desiccant wheel through the process and reactivation sectors; reactivation air heating means; and a control system intended to improve the operating efficiency of the dehumidifier at part-load conditions; the control system logic comprising: a. modulating the airflow through a process sector to control the amount of dehumidification; b. modulating the airflow through a reactivation sector as a function of the modulation of the process airflow.

32. The system as claimed in claims 31 or 32 wherein the modulation of process airflow comprises bypassing a portion of the process airflow around the desiccant wheel.

33. The system as claimed in claims 31 or 32 wherein the modulation of process airflow comprises modulating a damper controlling the process airflow.

34. The system as claimed in claims 31 or 32 wherein the modulation of process air comprises simultaneously controlling the airflow through the desiccant wheel and the airflow bypassing the desiccant wheel so the total airflow remains practically constant.

35. The system as claimed in claims 31 or 32 wherein the modulation of process airflow comprises varying the operating characteristics of the process airflow means.

36. The system as claimed in claims 31 or 32 wherein the minimum airflow through the process sector is limited to a predetermined value.

37. An active desiccant dehumidifier system comprised of a housing containing at least: a desiccant wheel having a process sector with airflow means; a reactivation sector with airflow means; a means of rotating the desiccant wheel through the process and reactivation sectors; reactivation air heating means; and a control system intended to improve the operating efficiency of the dehumidifier at part-load conditions; the control system logic comprising: a. modulating the airflow through a reactivation sector to control the amount of dehumidification; and b. modulating the rotational speed of a desiccant wheel as a function of the modulation of the reactivation airflow.

38. The system as claimed in claim 30, claim 31 or claim 37 wherein the heated temperature of the air entering the reactivation sector is maintained at a fixed value.

39. The system as claimed in claim 38 wherein the heated temperature of the reactivation air is maintained at a fixed value by modulating the heat input to the reactivation air heating means.

40. The system as claimed in claim 30, claim 31 or claim 37 wherein the temperature of the reactivation air leaving the reactivation sector is maintained at a fixed value.

41. The system as claimed in claim 40 wherein the temperature of the air leaving the reactivation sector is controlled by modulating the heat input to the reactivation air heating means.

42. The system as claimed in claim 30, claim 31 or claim 37 wherein the reactivation air heat source is maintained at a fixed value and the temperature of the reactivation heated air is not controlled but allowed to vary, increasing with reduced airflow and decreasing with greater airflow.

43. The system as claimed in claim 43 wherein the reactivation heat source is activated whenever there is airflow through the reactivation sector.

44. The system as claimed in claim 30, claim 31 or claim 37 wherein the modulation of the reactivation airflow is achieved by modulating a damper in the reactivation airstream.

45. The system as claimed in claim 30, claim 31 or claim 37 wherein the modulation of the reactivation airflow is achieved by varying the operating characteristics of die reactivation airflow means.

46. The system as claimed in claim 30, claim 31 or claim 37 wherein the modulation of the reactivation airflow is achieved by bypassing a portion of die reactivation air around the desiccant wheel.

47. The system as claimed in claim 30, claim 31 or claim 37 wherein the minimum airflow through the reactivation sector is limited to a predetermined value.

48. The system as claimed in claim 30, claim 31 or claim 37 wherein the modulation of die rotational speed of the desiccant wheel is achieved by varying the operating characteristics of die desiccant wheel rotating means.

49. The system as claimed in claim 30, claim 31 or claim 37 wherein die effective rotational speed of the wheel is achieved by intermittendy operating the desiccant wheel rotating means such diat die percentage of time die rotating means operates is proportional to die control function desired.

50. The system as claimed in claim 30, claim 31 or claim 37 wherein the minimum rotational speed of die desiccant wheel is limited to a predetermined value.

51. The system as claimed in claim 37 wherein die control function of die modulation of die desiccant wheel rotational speed is a linear function of die reactivation airflow.

52.The system as claimed in claim 37 wherein die control function of the modulation of die desiccant wheel rotational speed is an exponential function of the reactivation airflow, widi die exponent between 0.5 and 2.0

53. The method as claimed in claims 1 to 3 and 16 wherein the dehumidifier also contains an intermediate "purge" sector between the reactivation and process sectors that pre-treats a portion of the reactivation air.

54. The method as claimed in claim 53 wherein the airflow passing through the purge sector is modulated in direct proportion to the reactivation airflow.

55.The method as claimed in claims 1 to 3 and 16 wherein the dehumidifier also contains one or more pairs of intermediate purge sectors disposed so they act as a buffer between the process and reactivation sectors and having means to circulate a closed flow of air through them.

56. The method as claimed in claim 55 wherein the airflow circulated through the purge sectors is modulated in direct proportion to the reactivation airflow.

57. The method as claimed in claim 55 wherein the airflow circulated through the purge sectors is modulated in direct proportion to the desiccant wheel rotational speed.