Abstract:
A process and apparatus for disinfection of spaces using a disinfecting liquid droplet spray atomization are described, in which an electric fan  107  is used to dispense the atomization from an atomization chamber  104  via venture outlets  106  into the space to be disinfected. The operation of the electric fan  107  is modulated in a substantially random sequence to vary the rate of dispensing into the space in order to improve the dispersal of the atomized droplets in the space and hence the efficacy of disinfection.

Description:
BACKGROUND 
       [0001]    The present invention relates to a method and apparatus for disinfecting spaces such hospital rooms and the like. 
         [0002]    The problem of micro-organism contamination of, for example, hospital bedrooms and the like has gown into an acute one in recent years and one particular method of disinfecting such spaces and any equipment located therein is the use of disinfecting sprays (atomisations) of liquid droplets such as hydrogen peroxide. These systems are often known as hydrogen peroxide fogging systems and such hydrogen peroxide fogging systems can involve vaporization of a low concentration hydrogen peroxide solution using a high frequency ultrasound (typically 1.6-2 MHz) droplet generator in an atomization chamber partially filled with solution. WO-A-2009-138430 discloses such a system and method designed to provide improved efficacy of such methods and which involves the steps of: (a) determining a first value of the relative humidity of the air in the space; (b) atomizing a disinfecting liquid in the space until a predetermined second value of the relative humidity of the air is reached in the space, and; (c) maintaining the relative humidity of the air for a predetermined time at the second value by means of atomizing the disinfecting liquid, wherein the method further comprises of decreasing the relative humidity of the air in the space prior to and/or during atomizing of the disinfecting liquid. 
         [0003]    In such processes, under the influence of the ultrasonic droplet generator and through the mechanism of forced cavitation, small sized droplets (circa 1 μm diameter) are produced and ejected from the solution surface. Airflow from an electric fan then carries these droplets as a mist from the chamber via a venturi into the space to be treated. A combination of the fan&#39;s airflow and natural convection currents in the room carry these droplets until they contact surfaces where they are deposited and operate to destroy any organisms present. Given appropriate generating capacity for the space to be disinfected, if sufficient vaporised hydrogen peroxide is expelled from the system, statistically a good fog density and hence hydrogen peroxide distribution will be achieved in the space. 
         [0004]    This traditional process is controlled via a closed loop system, with relative humidity (RH) being the controlling variable as described in WO-A-2009-138430. Tests have shown that in a closed environment (which is required for safety), RH increase is proportional to hydrogen peroxide concentration increase. 
         [0005]    One of the primary factors affecting efficacy of the process is the profile of the airflow within the space being treated. Traditionally, with a regular electric fan operating in a closed space with no external ventilation (as required for safety), a reasonably static air distribution pattern is achieved which, depending on the layout of the room and its contents, can lead to areas of poor fogging coverage and hence droplet deposition, a phenomenon known as spatial differentiation and this in turn can lead to incomplete disinfection. 
       SUMMARY OF THE INVENTION 
       [0006]    According to the present invention, a process for disinfection of spaces using a disinfecting liquid droplet spray (atomisation), in which a fan is used to dispense the atomisation into the space to be disinfected, can be improved by modulating the operation of the electric fan in a substantially random sequence. 
         [0007]    Apparatus for carrying out the method comprises a hydrogen peroxide fogging device having an electric fan for dispensing a fog of hydrogen peroxide droplets into a space to be disinfected, and a control system for modulating the operation of the electric fan in a substantially random sequence. 
         [0008]    Preferably, apparatus for carrying out the process includes a venturi injector into which atomized droplets are introduced as a secondary airflow and the electric fan is arranged to provide a primary flow of entraining air through the venturi into which the atomized droplets are entrained for dispensing into the space to be disinfected. 
         [0009]    This is preferably achieved by controlling the ‘duty cycle’, i.e. the ON/OFF cycle of operation of the fan in accordance with a pseudo-random sequence. This may have the effect of adjusting the fan speed, depending on the time periods being used. Alternatively, the fan speed may be varied directly by adjusting the electrical power supplied to it in accordance with a desired pseudo random characteristic. 
         [0010]    During the fog generation phase of treatment, the microcontroller controls the fan speed to create the required the turbulent airflow. 
         [0011]    The random sequence may, as in the current embodiment, be provided by a software pseudo random number generator running on the control system hardware. This is used to switch the electric fan on and off using a pulse-width modulation scheme determined by the pseudo random number generator, the pulse width being determined, on each successive cycle of on/off times, by the random number generator. Using a pulse width modulation scheme with a relatively short (say 1-3 second) period for both ‘ON’ and ‘OFF’ parts of the cycle, causes the fan&#39;s speed-up and speed-down ramps to overlap, which gives rise to variable air speed. 
         [0012]    This higher fog density in conjunction with optimisation by the control system of the attained humidity level to match the ambient temperature of the space, gives rise to micro-condensation on surfaces in the space being treated. This has been shown to increase efficacy of the decontamination process. 
         [0013]    While pulse width modulation is preferred, other methods of achieving the randomized airflow profile may be used. 
         [0014]    Randomization of the airflow has the effect of improving dissipation by introducing a more dynamic airflow, which greatly reduces the number of poor coverage areas by creating a more turbulent and random airflow within the space. Also, the atomization droplet density being expelled from the machine is being constantly changed (whilst maintaining the sub 1 μm droplet size required for mobility). This further assists with creating a dynamic, turbulent airflow. 
         [0015]    Preferably, additionally, the attained RH is based upon the air temperature of the space being treated, and the desired RH target is linked to the ambient temperature of the space being treated. 
         [0016]    Once the mist generation phase is complete, it is required to allow a period of time for deactivation of the hydrogen peroxide where it breaks back down into water and oxygen. During this time, the fan is controlled to operate at normal, i.e. full, single speed, operation, to maximise airflow and to minimise deactivation time. 
         [0017]    The combination of fan modulation and venturi, gives the advantage that with a constant atomisation/fog generation rate, during the low fan speed/pwm off periods a higher fog density builds in the venturi, and is expelled as the fan speed re-increases. Given the random nature of the fan speed, the fog density is constantly changing, which also assists with generation of a turbulent airflow profile. 
     
    
     
       BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES 
         [0018]      FIG. 1  is a perspective view of apparatus according to the invention, from the front; 
           [0019]      FIG. 2  is a block diagram illustrating flow of hydrogen peroxide within the apparatus and the corresponding components; 
           [0020]      FIG. 3  is a block diagram illustrating the main components of the atomisation, delivery and control components of the apparatus; 
           [0021]      FIG. 4  is diagrammatic view illustrating the arrangement of the atomization unit of the apparatus; and 
           [0022]      FIG. 5  is a flowchart of the disinfecting process. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    One example of apparatus according to the invention will now be described with reference to the accompanying figures described above. 
         [0024]    The apparatus has a portable fogging unit  1  mounted on a trolley  10 , a portable relative humidity and temperature sensor unit  2  on a stand  21 , and a remote control and indication unit (or process monitor)  3 , likewise on a stand  31 . In use, the portable fogging unit  1  and the humidity sensor unit  2  are disposed within a space to be disinfected and the remote control and indication unit  3  is disposed outside the space which is sealed from the surrounding area for safety. The control and indication unit  3  includes a hydrogen peroxide monitor  33  arranged, on detecting hydrogen peroxide gas outside of the space under treatment, to provide a signal to the fogging unit  1  to halt operation of the apparatus. 
         [0025]    The fogging unit  1  includes a storage reservoir  101  for liquid hydrogen peroxide and which is supplied from a hydrogen peroxide cartridge  102 . A pump  103  is provided to supply hydrogen peroxide from the storage reservoir to an atomization chamber  104 , in the base of which is provided an ultrasonic atomization unit  105 . This operates in use, to generate high frequency ultrasound (typically at 1.6-2 MHz) to cause cavitation in the liquid hydrogen peroxide in the atomization chamber and hence creates a fog of atomized droplets above the level of the liquid in the atomization chamber. To create a hydrogen peroxide ‘fog’ within the space to be disinfected, a pair of venturi injection outlets  106  are provided, the primary (or entraining) airflow through which is provided by a fan  107  disposed within a housing  108  surrounding the atomization chamber  104 . Atomized droplets entrained in the primary airflow are emitted into the atmosphere in the space to be disinfected and, in use, alight on surfaces within the space, causing those surfaces to be disinfected. The atomized droplets preferably have a diameter of around 1 μm. Overflow lines  117 , 118  from the cartridge  102  and atomization chamber  104  respectively back into the storage reservoir  101  are provided to prevent overflow/spillage in case of control system failure. 
         [0026]    The fogging unit carries a power supply  114  housed in a casing  115  and supplying power for driving the various electrical components including an oscillator  116  which in turn drives the atomization unit  105 . The fogging unit  1  also includes a user interface in the form of a control panel and display  120  which provides instructions and information on the status of the operation to the operator during the setup and post-decontamination phases. It also displays error and warning messages. As it is not accessible during the process (being inside the space under decontamination), information is provided via a data cable (not shown) to the process monitor  3  which thus gives status information to the operator from outside the space during the disinfection process. 
         [0027]    Having now described the main components, the operation of the apparatus will now be described. 
         [0028]    Before the treatment begins, the space to be treated is prepared by sealing doors, ventilation openings etc. and by placing the portable fogging unit  1  and the relative humidity and temperature sensor unit  2  in the space, the relative humidity and temperature sensor unit being located remote from the fogging unit  1 . The process monitor  3  (remote control and indication unit) is placed outside the space to be treated. 
         [0029]    At the start of the process a control switch (not shown) on the fogging unit  1  is operated and, via a signal to a microcontroller  109  which in turn activates a control valve  110 , hydrogen peroxide liquid is admitted to the storage reservoir  101  from the supply cartridge  102 . The control valve  110  allows the liquid hydrogen peroxide to enter the reservoir  101 , with an electronic level sensor  111  transmitting a signal reporting reservoir level to the microcontroller  109 . Multiple supply cartridges may be used before the desired level is reached in the storage reservoir  101  and the control valve  110  is closed once sufficient cartridges have been drained into the system to fill the storage reservoir. 
         [0030]    Once the reservoir  101  is filled to the desired level as determined by the sensor  111 , the control system microcontroller  109  commands the pump  103  to fill the atomization chamber  104  to a predetermined level (in this example 53-57 mm). The level in the atomization chamber  104  is communicated to the control system microcontroller  109  via a level sensor  112 . Once the correct level is attained, the pump  103  is stopped and the system enters the standby state. Personnel are removed from the space and it is finally sealed. 
         [0031]    The decontamination cycle is then started by a key operated switch on the process monitor  3 . Indicators on the process monitor show the current status of the system during use. The decontamination cycle starts by reading the initial relative humidity value of the space from sensor in the RH and temperature sensor unit  2  which includes a transmitter  22  connected to the microcontroller  109 , and by operating the atomization unit  105  in the atomization chamber  104 . At the same time the duty cycle of the fan  107  is pseudo-randomly varied by pulse width modulating the electrical supply to the fan. 
         [0032]    Fog generated in the atomization chamber  104  is expelled into the space under treatment by the action of the fan  107  and venturi outlets  106 . 
         [0033]    The optimum relative humidity is determined based on current temperature measured from RH sensor unit and other programmable parameters as described below in relation to the flowchart of  FIG. 5 . When this RH level is attained, the atomization unit  105  in the atomization chamber  104  is controlled in a closed loop system including the microcontroller  109  to hold this level accurately for the duration of the active phase of the process (90 minutes in the current example). The atomization unit preferably includes multiple transducers disposed over the bottom of the atomization chamber, but all driven by the same input signal. The fluid level in the atomization chamber  104  is also held at 53-57 mm by a closed loop control system between the pump  103 , level sensor  112  and the central processor  109 . 
         [0034]    The process works by logging the starting humidity and running the atomization unit  105  until a predetermined RH difference above the ambient is reached (typically this is the relative humidity at start plus 25%). The humidity is then maintained at this level for a predetermined time period set into the control system (typically 60-90 minutes). The control system microcontroller also logs the ambient temperature. Once the desired RH level is reached and rather than just holding this RH level static, changes in ambient temperature can be monitored to change the setpoint of the atomization RH control system. The process is illustrated in the flowchart of  FIG. 5  which is self-explanatory. 
         [0035]    Since heating and ventilation to the space under treatment is blocked/disabled during a disinfection process, the temperature generally falls over the course of a disinfection cycle. This can be improved by heating the space prior to decontamination, and removing the source of heat just prior to starting decontamination. In practice, the dew point may be approximated by: T dew =T ambient −(100−RH/5), which, although not dimensionally correct, has been found, as a rule of thumb approximation to be generally accurate to within a degree or so, as long as the RH is over 50% (which it is at the point in the process where the approximation is used). This is sufficiently accurate to fulfil the requirements of the control system. It is desirable to achieve micro-condensation on surfaces where possible to improve efficacy (this occurs significantly at the dew point), the control system microcontroller  109  monitors the rate of change of ambient temperature via the RH and temperature sensor unit  2 , and attempts to attain an RH value which gives rise to the dew point being achieved during the process as the temperature falls. For example, if the RH (after the initial period of increase) is 70%, the change in temperature over a 5 minute period has been −0.5° C., the current ambient temperature is 20° C., and there is 45 minutes of process time remaining, the dew point will be approximately 14 degrees. With a temp loss of 0.1° C./minute one would need 60 minutes to reach the dew point, but only 45 minutes remain. The control system calculates the required RH to achieve the dew point within the remaining time at the current rate of change of temperature. In this case, the temperature would drop by only 4.5 degrees (assuming a fixed rate of change), so the RH would need to be increased to 77.5% to achieve dew point during the process. Of course, rate of change of temperature during the process is not constant and will reduce as thermal equilibrium is approached. The control system is adaptive and constantly changing based upon the above relationship. It may not be possible to achieve dew point for every disinfection operation (if there is little or no ambient temperature change during the process), and in this case, the control system will timeout on attempting to achieve dew point and hold a ceiling RH. 
         [0036]    When the active phase is completed, the atomization unit  105  is turned off, and the fan modulation ceases, with the fan  107  then being driven to provide a full 120 m 3 /hr for a further deactivation period (45 minutes in the current example), while the hydrogen peroxide breaks down. Whilst this deactivation period is current, the control system microcontroller  109  instructs control valve  113  to open, draining any remaining fluid from the atomization chamber  104  back into the storage reservoir  101  for subsequent re-use. 
         [0037]    After this deactivation period, signals on the process monitor  3  indicate to the operator that it is safe to return to the space. During the set up and post process phases, the control panel and display  120  provides instructions to the operator, and also displays and warning/error messages. 
         [0038]    In the current example, tests have shown a random fan operating time between 1 and 3 seconds to be optimal, with pauses of similar length (also randomly determined by the pulse width modulator). This range allowing a 750 ml volume (chamber capacity above liquid level) to be fog filled between fan cycles, gives an increased fog density as compared to running the atomizer with a fan running permanently at the full 120 m 3 /hr rate.