Claims:WE CLAIM:

1. A retro reflection based forward scatter double pass opacity meter having retro-reflective dust sensor comprises of a probe head (3) containing a 45° mirror or Tx prism (1) and a 45° mirror or Rx prism (2) located on one side of the stack or probe (4), a transmitter (5) having a collimated light source (5’) which could be either LED or semiconductor diode laser and a receiver (6) being located on the same side of the probe and minimal optical alignment being required between the transmitter and the receiver, an air purge unit (15) being connected to both the transmitter and the receiver through air purge lines (14) for delivery of scavenge air for self initiated air purging while in operation, the opacity meter being provided with in-stack device calibration, a hollow shaft (7) running through the probe having the probe end (3) mounted on one end and a linear actuator (8) mounted on its other end, the linear actuator (8) adapted to move the probe head (3) to and fro to adjust the gap between the probe (4) and probe head (3) to make the sensing region programmable, the opacity meter being installed as a cantilever, the amount of light incident on the photo detector of the receiver (6) being directly proportional to the amount of light absorbed and/or scattered by the smoke/plume particles in the probe and being inversely proportional to the particle density.

2. The double pass opacity meter as claimed in claim 1, wherein a PCB unit (9) having a plurality of TX/RX PCBs is connected to the transmitter (5) by transmitter signal wire (11) and to receiver (6) by receiver signal wire (10), a control system (12) being connected to said linear actuator (8) through actuator signal wire (13) and also to the PCB unit (9), the connecting wires being colour coded for easy identification and installation.

3. The double pass opacity meter as claimed in claim 1, wherein the opacity meter device has on-line air purging mechanism and provision for incorporating a filter wheel for on-line multi point calibration.

4. The double pass opacity meter as claimed in claims 1 and 3, wherein said on-line calibration is normally self-initiated and automatic but can also be done manually at any time by the operator or remotely through a web based system.

5. The double pass opacimeter as claimed in Claim 1, wherein the reflecting mirrors or prisms are replaced by a Total Internal Reflecting (TIR) Dove Prism.

6. The double pass opacity meter as claimed in claim 1, wherein said retro-reflective dust sensor has provision for reflector alignment which is capable of being adjusted through a single threaded mechanism mounted on the other side of stack, the threading mechanism being located towards probe side (4) of the stack for easy optical alignment and having angular adjustment of the laser source for more accurate alignment, the opacimeter being provided with angular adjustment of the collimated laser/LED source (5’) through three positioning screws along the periphery for more accurate alignment.

7. The double pass opacity meter as claimed in claims 1 and 6, wherein said retro reflective dust sensor is provided with a linear actuator with dc motor for adjusting the measurement zone, whose stroke size is programmable.

8. The double pass opacity meter as claimed in claims 1 and 7, wherein the length of the measurement zone size is programmable by moving the probe head (3) connected to the linear actuator (8).

Dated this on 20th day of March, 2018

Subhajit Saha
Patent Agent (IN/PA-1937)
Agent for the applicant
, Description:FORM – 2

THE PATENTS ACT, 1970
(39 of 1970)

AND

THE PATENTS RULES, 2003

COMPLETE SPECIFICATION

(See Section 10; rule 13)

TITLE OF THE INVENTION

DOUBLE PASS OPACITY METER

Ace Gas Analysers Pvt. Ltd.
# 1, Biradri, Opp.Bank of Baroda,
M.G.Road, Ghatkopar (West), Mumbai,
Maharashtra, India - 400086

An Indian National

The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION

[001] The present invention relates in general to an apparatus for monitoring the pollution especially in an industrial space. The present invention in particular related to a double pass opacity meter, also referred as opacity meter in this description, which is based on optical absorption to measure particulate matter in gaseous smoke passing through a stack or chimney.

BACKGROUND OF THE INVENTION

[002] Many industrial processes normally emit gaseous smoke through a stack or chimney. Smoke and dust are collectively known as particulate matter (PM). One of the most obvious signs of PM emissions is a visible plume of smoke leaving the stack. Visible plumes caused by emission of particulate matter (PM) to the atmosphere from stationary sources raise public concern about the effects of PM on human health and atmospheric visibility. Hence, there is a regulation that the emission of smoke into the atmosphere should be continuously monitored by the regulatory authority.

[003] Over the years in the interest of human health and atmospheric visibility, standards have been developed and enforced by Environmental regulatory bodies in various countries to monitor and regulate particulate matter emissions from various sources. Industrial sources are one of the major entities being monitored by the Pollution Monitoring Authority. These particulate matter emissions can be monitored by various techniques, where optical opacity based measurement being one of the widely used methods. Opacity is the optical term for the property of stopping light from being transmitted and when it comes to exhaust gases it corresponds to how opaque they are. So when light passes through a gaseous mixture containing smoke and dust, called plume, some of the light is lost through scattering, absorption and reflection by the particles.

[004] Opacity based standards are established primarily because plumes can be more readily monitored by their optical properties rather than their mass-based properties. The opacity is measured in percentage, where 0 % means that all light is transmitted and 100 % corresponds to the case where no light at all is transmitted.

[005] Various kinds of opacimeters are known in the prior art. But these devices have numerous drawbacks in meter calibration. Some of the drawbacks include that the calibration can only be performed at a single point over the receiver response curve, the process needs to be stopped for performing the calibration of the opacity meter, and only off-line calibration is available. Further, the measurement zone is not adjustable;

[006] The calibration of the opacimeters in the prior art needs to be done on site after installation. The transmitter and receiver flanges are welded on the chimney at diametrically opposite sites. The transmitter and receiver alignment depends on the skill of the welder. Any misalignment after welding needs removal and rewelding. The optical alignment becomes more stringent for large diameter stack placing a constraint on installation, as the margin for error for angular misalignment is very tight.

[007] The present invention seeks to provide a novel and inventive opacity meter which overcomes the above drawbacks of the prior art.

OBJECTS OF THE INVENTION

[008] Accordingly, the primary object of the invention is to provide a double pass opacity meter which does not need the process to be stopped for calibration.

[009] Another object of the invention is to provide a double pass opacity meter which can calibrate the unit over the entire range by providing a mechanism for multipoint calibration with National Institute of Standards and Technology (NIST) traceable Optical Neutral Density (ND) filters mounted on a mechanized filter wheel operated by its control system.

[0010] Yet another object of the invention is to provide a double pass opacity meter where the gap between the probe-head and the probe is adjustable using a retractable mechanism within the probe to make the sensing zone variable.

[0011] How the foregoing objects are achieved will be clear from the following description. In this context it is clarified that the description provided is non-limiting and is only by way of explanation.

SUMMARY OF THE INVENTION

[0012] A double pass opacity meter consists of a transmitter and a receiver. An opacity probe head which comprises two retro reflecting mirrors or prisms or a dove prism that combines Tx and Rx into one unit are arranged to guide the light beam from the transmitter to the receiver. A hollow shaft is connected to an air purge unit for delivery of scavenge air to facilitate purging of mirrors and windows. The probe head and the hollow shaft are connected to a linear actuator which moves the probe head to and fro to adjust the gap between the probe head and probe according to the sensing region requirements. A control system drives the transmitter, linear actuator, purging unit and processes the receiver signals. The total linear dimension of double pass opacimeter is dictated by its stack diameter. Being a modular system, the length of probe is varied according to the stack diameter keeping the remaining components unchanged.

[0013] The double pass opacity meter has a provision for purging of air while the unit is sensing the opacity, called in-line purging. It also has a provision for incorporating mechanized filter wheel unit consisting of multiple optical neutral density filters to selectively attenuate the transmitter output during calibration, thus providing a multi-point calibration in the lab during testing and after installation in the field usage without disturbing the process in the stack.

[0014] The on-line calibration is normally self-initiated automatically but can also be done manually at any time by the operator, or remotely through a server for which provision is available in the system if needed. The linear actuator with dc motor is capable of adjusting the measurement zone, where the adjustment stroke size is programmable.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0015] The nature and scope of the present invention will be better understood from the accompanying drawings, which are by way of illustration of a preferred embodiment and not by way of any sort of limitation. In the accompanying drawings, figures 1 to 5 represent the prior art.

[0016] Figure 1 shows a graphic representation of the opacity measurement principle for Industrial stacks showing the mounting of transmitter on one side, receiver on the other side and a controller to control both.

[0017] Figure 2 shows a schematic representation of the principle of opacimetry where the light output I0 from a collimated source (on the right side) passes through the sample mixture in a single pass configuration and the resultant light output is collected by a receiver on the other side of the stack. This configuration is called single pass opacimetry.

[0018] Figure 3 shows a schematic representation of the method of purging the opacity meter according to the prior art to clean the optical windows where the transmitter and receiver are mounted on either side of the stack showing the provision for air purge.

[0019] Figure 4 shows a blown up photographic view of the opacity meter showing its major components and internal construction of transmitter and receiver for both single pass opacimetry and double pass opacimetry.

[0020] Figure 5 shows a diagrammatic representation of a filter cleaning system for a single pass opacity monitor which has a light source and light sensor on opposite sides of stack comprising a mechanism for offline calibration of the opacity monitor during non-functional period and a purging air mechanism showing the arrangement of purging valves including the cleaning of optics of the opacity meter windows through purging according to prior art.

[0021] Figure 6 shows a schematic diagram of the opacity meter with the configuration according to the present invention.

[0022] Figure 7 shows a mechanism for air purging process of the present invention.

[0023] Figure 8 shows the probe head with dove prism arrangement and reflecting prism/mirror arrangement.

[0024] Figure 9 shows a schematic diagram to explain the alignment mechanism of the hollow shaft and the linear actuator.

DETAILED DESCRIPTION OF THE INVENTION

[0025] Having described the main features of the invention above, a more detailed and non-limiting description of a preferred embodiment will be given in the following paragraphs with reference to the accompanying drawings.

[0026] In all the figures, like reference numerals represent like features. Further, the shape, size and number of the devices shown are by way of example only and it is within the scope of the present invention to change their shape, size and number without departing from the basic principle of the invention.

[0027] Further, when in the following it is referred to “top”, “bottom”, “upward”, “downward”, “above” or “below”, “right hand side”, “left hand side” and similar terms are strictly referring to an orientation with reference to the apparatus, where the base of the apparatus is horizontal and is at the bottom portion of the figures. The number of components shown is exemplary and not restrictive and it is within the scope of the invention to vary the shape and size of the apparatus as well as the number of its components, without departing from the principle of the present invention.

[0028] All through the specification including the claims, the technical terms and abbreviations are to be interpreted in the broadest sense of the respective terms, and include all similar items in the field known by other terms, as may be clear to persons skilled in art. Restriction or limitation if any referred to in the specification, is solely by way of example and understanding the present invention.

[0029] Referring to figures 1 and 2, the basic principle of the opacity meter is that generally collimated light is emitted from a light source, normally LED or Diode laser and a sensor comprising of a light detector placed some distance away registers the intensity of the light. If a sample, e.g. exhaust gas, with opacity more than 0 % is placed in between the light source and the sensor the measured light intensity will decrease. Through calibration, the measured intensity can be correlated to the opacity of the sample. When there is a perfect transparent matter, e.g. air, in between the opacity is 0 % and in the opposite case, where no light is transmitted, the opacity is 100 %.

[0030] The source and receiver are pre-calibrated in the laboratory. The calibration procedure consists of a no light signal and a full light signal. The slope is assumed to be linear. Based on the amount of received signal, the amount of absorption is estimated and correlated to the particulate monitor concentration. By introducing a filter wheel consisting of 4 Optically Neutral Density Filters (for ex., 10%, 25%, 50% and 75%), the calibration can be performed across different intensity levels in the lab.

[0031] As shown in Figure 3, the opacity meter uses scavenge air to protect the light source and the sensor from exhaust gas particles. The scavenge air is used for purging as shown in figure 7. In case of pressure/flow drop, the probe-head (4) is retracted by the linear actuator (8) so that the optical parts do not get contaminated by the smoke and dust particles.

[0032] Opacity meters measure the decrease in light intensity due to absorption and scattering as the beam crosses the stack according to Beer-Lambert’s Law. Referring to the figure 2, the basic operational principle of these instruments is that a collimated beam of light is directed through a smoke stream toward receiving optics. The receiving optics measure the decrease in light intensity, and the instrument electronics convert the signal to an instrument output. The amount of light lost depends on the number and the size of the particles, so we can use the loss as a measure of the PM concentration in the stack.

[0033] The opacity meter consists of either a continuous wave (CW) or a modulating light pulse transmitter having LED/Laser Diode light source, a photo sensitive detector based receiver unit having a focusing lens, required pipes, blower, air purge unit, shutters, I/O interface, mounting flanges, and controller and display unit. The transmitter unit consists of a solid state source (either LED or Laser Diode), collimating optics, optical windows housed in a unit with a suitable electronics processor. The receiver unit consists of a semiconductor photo detector with focusing lens and an optical window housed in a unit to prevent air leak during cleaning along with a suitable electronics processor. The opacity monitor incorporates a transmission meter and a number of other components which allow it to function reliably and to make it easy to install, calibrate and operate.

[0034] In this invention, a retro-reflection based forward scatter opacity meter using a solid state laser diode or a LED which has in-stack device calibration, minimal optical alignment requirement between transmitter and receiver, and self-initiated automatic air purging while the device is operation is described.

The principal components of the opacity meter are:

[0035] Transmitter and receiver: The transmitter and receiver are the heart of the system, containing the light source and detectors, user interface and main microprocessor.

[0036] Retroreflector: It is a passive reflector, also referred as the retro for brevity. A retroreflector differs from a mirror because it returns the light towards the source, so that the receiver is co-mounted with the transmitter reducing the installation complexity, need for field alignment while still allowing the ease of operation.

[0037] Hollow-shaft: The hollow shaft links the probe with the probe head. The transmitter and receiver are attached to the probe. The hollow shaft at the end of the transmitter/receiver contains a threaded screw which is rotated to align the retro reflecting prisms to the transmitter and receiver beams.

[0038] Purge Blower: Continuous air purge is used to protect the instrument’s delicate optical surfaces from the hot, corrosive stack gases.

[0039] Air Hose: This connects the purge blower to the transmitter and receiver.

[0040] Linear Actuator: Allows calibration without shutting off the process. Also acts as a safety mechanism and closes the probe to avoid contamination of optics, in case air purge fails. Linear actuator closes sensing zone thus allowing calibration and routine service without affecting the process.

[0041] Power Supply: The transmitter and receiver require suitable dc power, mains power input and convenient screw-terminal connections which avoid the need for a customer-provided junction box.

Performance steps:

[0042] Calibration: Temperature changes, voltage changes, accumulation of dirt on the optical windows, and alignment changes are the major sources of drift in smoke or dust density measuring installations; the calibration of such installations must be checked frequently. In the smoke-density monitoring instruments so far available, they cannot be calibrated during operation on the stack since, in a two-ended system in which light source and detector are on opposite sides of the smoke channel, it is necessary to stop the exhaust gas flow in order to obtain a calibration for zero percent opacity. An external device is often used to calibrate and re-calibrate the instrument periodically. Generally, the calibration is done when there is no particulate emission or in off-line condition.

[0043] The single-ended system of the present invention checks its zero and span calibration automatically at regular intervals without interrupting stack operation. The calibration checks in the present invention are done by retracting the probe head housing the retroreflectors (4) which are connected to the shaft, by activating the linear actuator. The zero calibration check indicates drift in the instrument due to contamination of the optics, variation in source power etc. Through a filter wheel containing Neutral density filters, it is possible to do a multipoint calibration check automatically. The calibration checks can also be activated manually at any time from the control room or remotely through an internet based application.

[0044] Purging: The optical windows and other surfaces can be damaged by the hot, corrosive, emissions from the stack. Such damage results in erroneous readings, resulting in inaccurate outputs. The windows need to be cleaned continuously by pumping external air to dislodge deposited particles. Thus, continuous air purge is used in the present invention to protect the instrument’s delicate optical surfaces from the hot, corrosive stack gases. An air hose connects the instrumentation grade compressed air to the optical transmitter and receiver.

[0045] Alignment between Transmitter and Receiver: The transmitted beam reaches the receiver through two prisms placed in the probe head, where, each prism reflects the beam by 90 degrees achieving 180 degree rotation. If the prisms are not properly aligned to the transmitted beam, then the signal is attenuated. Provision is given in the present invention to rotate the prisms about the optical axis to achieve alignment accuracy. Alternately, the two prisms can be replaced with a dove prism, best shown in figure 8, which simplifies the alignment requirement. The opacimeter is provided with angular adjustment of the collimated laser/LED source (5’) through three positioning screws along the periphery for more accurate alignment.

[0046] Now we refer to figure 6 which shows the configuration of the double opacity meter according to the present invention.

[0047] The opacity meter consists of a transmitter (5) having a collimated light source (5’), which could be either LED or semiconductor diode laser, a receiver (6) having a focusing lens with a photo detector, which could be either semiconductor or thermal detector, are placed on the same side of a stack. A corner reflector arrangement configuration or probe head (3) consisting of one or more mirrors or prisms is arranged within the probe head (3). The parallel light beam from the transmitter passes through the optical window through the stack containing the smoke plume and back to the optical window on the same side of the stack. The light is reflected by the two mirrors back to the incident side of stack through the optical windows and on to the receiver. The mirror can be replaced with a prism. Mirror material can be glass, plastic or metal. The receiver lens focuses the return beam on to a photo detector which converts the light beam into an electrical signal.

[0048] The double opacity meter according to the present invention comprises of a probe head (3) containing a 45° mirror or Tx prism (1) and a 45° mirror or Rx prism (2) located on one side of the stack or probe (4), and a transmitter (5) and receiver (6) located on the other side of the probe (4). An air purge unit (15) is connected to both the transmitter (5) and the receiver (6) through three air purge lines (14) for delivery of scavenges air. A hollow shaft (7), which runs from the linear actuator (8) on one end through the probe (4) and to the probe head (3) on its another end. The linear actuator (8) can move the probe head (3) to and fro to adjust the gap between the probe (4) and probe head (3) to make the sensing region programmable. The hallow shaft (7) is connected to an air purge unit (15) for delivery of scavenge air to facilitate purging of mirrors and windows. The probe head (3) is attached to the main unit through a hallow shaft (7). The said hallow shaft (7) consists of a screw mechanism to align the Tx prism (1) and the Rx prism (2) with the transmitter (5) and receiver (6) in the field. The probe head (3) and the hollow shaft (7) are connected to a linear actuator (8) which moves the probe head (3) to and fro to adjust the gap between the probe (4) and probe head (3) according to the stack diameter application conditions.

[0049] The collimated light from the transmitter (5) travels directly to the probe head (3). A PCB unit (9) having a plurality of TX/RX PCBs is connected to transmitter (5) by transmitter signal wire (11) and to receiver (6) by receiver signal wire (10). A control system (12) is connected to the linear actuator (8) through actuator signal wire (13) and also to the PCB unit (9). A control system (12) comprises human-machine interface unit (HMI) and an electronic controller. The connecting wires are colour coded for easy identification and installation. A control system drives the transmitter (5), liner actuator (8), air purging unit (15) and processed the receiver signals. The control system (12) is further configured to control and monitor the opacity meter from remote server.

[0050] According to the present invention, the disclosed double pass opacity meter is arranged as a cantilever and mounted on one side of the stack. The opacity meter consists of a retro-reflective probe-head. The amount of light incident on the photo detector of the receiver is inversely proportional to the amount of light absorbed and/or scattered by the smoke/plume particles in the probe and is inversely proportional to the particle density

[0051] The said retro reflectors are mounted on a hollow shaft (7) with a linear actuator (8) which is drawn out or in to clean and calibrate the system. This can be done while the stack process is still running. The unit also contains an air purge unit (15) to clean the optical windows.

[0052] Figure 7 describes the process of air purging in the present invention. The compressed air is pumped through a hollow tube located at the center of the main housing to clean the reflecting mirrors and the windows in the actuating mechanism. The transmitting and receiving sections in the main housing are purged through two different air valves.

[0053] Figure 8 shows a configuration of the opacimeter where the reflecting mirrors or prisms are replaced by a Total Internal Reflecting (TIR) Dove Prism.

[0054] The hollow tube used for air purging in the reflecting section is attached to a threaded mechanism. The two reflecting mirrors are rigidly attached. The transmitted and reflected beams are aligned to the transmitter (5) and receiver (6) by rotating the reflecting section to optimize the received power.

[0055] Thus the hollow shaft provides
• method of attaching a gap adjustable reflecting section
• means of purging the adjustable reflecting section
• means of aligning the transmitted beam and receiver beam
• method of in-line calibrating by closing the gap.
[0056] According to the figure 9 of the present invention, the hollow shaft (7) is being threaded (20) at the junction formed with the linear actuator (8) and is being welded (21) at the junction formed with the probe head (3). Once is the alignment is completed, the nut (19) is tightened onto the linear actuator (8), to ensure hollow shaft (7) and probe head (3) are to be in the locked state. Hence there is no chance of misalignment do the shock or vibration during the operation.

[0057] The advantages of retro reflector opacity are: due to the double pass of the light beam, the sensitivity is improved. The alignment between source and receiver modules is simplified as both the units are on the same side of the stack as this reduces issues due to diametrically opposite alignment not required.

[0058] In the present invention, the opacity meter is composed of modular units that can be replaced with equivalent devices. The device has on-line air purging mechanism and has provision for incorporating a filter wheel for on-line multi point calibration. The calibration can be carried out without disturbing the process. The purging is continuous. The pressure and flow of the scavenge air is continuously measured and in case it drops, the actuator is activated and the retro-reflectors are retracted, thus closing the sensing zone and protecting the optical elements from damage or contamination.

[0059] The retro-reflective probe head has provision for reflector alignment, which is adjusted through a single threaded mechanism mounted on the probe side giving advantage to make optical alignment easily. The double pass opacity meter is mounted from one side of the stack to avoid on-site alignment and installation from opposite sides of the stack. The measurement zone is dynamically adjusted and the probe size can be adjustable based on the sensing zone requirements.

[0060] The industrial emissions are monitored using optical techniques. An instack opacimeter quantifies plume opacity within the exhaust stack of the source and requires installation and maintenance of each opacimeter at each source.

[0061] Table 1 below provides a comparison between prior art and the present invention.

Factors/Parameters Prior art Present Invention
Parameter Calibration 1. Offline. The process needs to be stopped for the calibration.
2. Single point. One point calibration.
1. The process does not need to be stopped as the probe head closes and prevents the specimen dust from entering the system for calibration.
2. Multipoint calibration through Neutral Density Filters and filter wheel helps calibrate the unit over the entire range.
3. Remote calibration through web based system
Modular dust sensor features 1. In line air purging
2.Calibration when the process is stopped.
3.One side of stack installation only for scattering
4.Single point calibration
5.Fixed optical alignment in the lab 1. In-Line air purging mechanism
2. Calibration on-line without disturbing the process.
3. one side of stack installation configuration for scattering and opacity.
4. Provision for incorporating a filter wheel for multi-point calibration
5. Improved optical alignment between source and receiver
through a threaded mechanism.
Improved optical alignment between transmitter and receiver in an opacimeter Not provided Improved optical alignment between source and receiver through a threaded mechanism resulting in improved sensitivity. The laser source is also adjustable using a 3-screw mechanism to adjust angular alignment
Rotation adjustment of prisms in retro-reflective opacimeter Not provided The retro-reflective prisms alignment can be adjusted through a threaded mechanism provided for the air purge pipe.
Adjustable probe size Not provided Retro reflective probe head where the sensing zone is adjustable based on the stack diameter
Adjustable measurement zone Not provided Retro reflective dust sensor where the measurement zone is adjustable based on the stack diameter

[0062] The present invention has been described with reference to some drawings and a preferred embodiment purely for the sake of understanding and not by way of any limitation and the present invention includes all legitimate developments within the scope of what has been described herein before and claimed in the appended claims.