The present invention relates to an automatic weighing gravimetric feeder.
The purpose of a raw coal feeder in a coal pulverizing circuit of a modern steam generator is to feed the raw coal from the bunker to the pulverizers in controlled and varied quantities, corresponding to the steam generators load demand. Earlier to introduction of gravimetric feeders, BHEL had been supplying rotary volumetric feeders. The volumetric feeder feeds coal by volume, and cannot take into the variation in bulk density of raw coal in the feeding section at various feeder speeds. In order to take care of the above variations, gravimetric feeder - which continuously weighs the coal and feeds it by weight and not by volume - is ideal. The conventional gravimetric feeder is offered with a center distance of 2134 mm. However for retrofit of the rotary volumetric feeder with a gravimetric feeder, the required center distance is 533mm only. To meet the requirements a short centered gravimetric feeder development work was taken up by the applicant.

Background of the invention
For a conventional gravimetric feeder, a minimum center distance of 2134mm (7 feet) is required between feeder inlet and feeder discharge for effective weighing of coal fed. However in most power plants, Rotary volumetric feeders are equipped wherein the center distance available is only 533mm. While customers are eager to modernize their power plants to reap the benefit of gravimetric feeder, the mismatch of center distance hitherto was a constraint. In order to meet customers requirement of modernizing their plants by way of retrofitting Fuel Systems group of the applicant designed and developed 24" inlet 533mm center distance gravimetric feeder.
Description of the invention
The electronic weighing gravimetric feeder uses two precision strain gauge type load cells (06 Fig-03) for weighing. These load cells measure the weight of the coal acting on the weighing span. The weigh span is defined by two fixed rollers (05 Fig-03), precisely located in the feeder body and a third roller (07 Fig-03) is located midway on the span. Two load cells, one on each side are suspended on the center weigh roller. As coal passes over the weigh span the weigh roller transmits force to the load cells, which is

directly proportional to the load acting between the fixed, weigh span rollers. The load cells convert this force into an electrical signal (millivolt). In terms of the weight of coal passed per unit of belt length (kg/m). A high-resolution optical encoder measures belt speed (in case of solid state control) or tacho generator (in case of micro control) and produces a signal (frequency representing belt length traveled per unit of time (m/sec). These signals are multiplied to obtain instantaneous rate of coal fed as given below.
Weight (kg/m) x Speed (m/sec) = Rate (kg/sec)
The Short center gravimetric feeder consist of both mechanical and electrical and electronic components, housed in a remote power cabinet and feeder mounted integral cabinet.
An automatic weighing gravimetric feeder comprising a pair of strain gauge load cells for weighing, a pair of fixed roller span forming a weighing , a third roller , two fixed rollers being suitably located in a feeder body and said third roller being positioned mid way on said span, said load cells being suspended , one on either side of a center weight roller, wherein said short center gravimetric feeder comprises both mechanical and electrical /electronic components housed in a remote power cabinet and feeder mounted integral cabinet.

In the aforesaid automatic weigh gravimetric feeder, said load cells are provided with means to convert weight span force in to an electrical signal in term of weight of coal passed per unit of belt length (kg /m). Further, such gravimetric feeder is attached to a high resolution optical encoder measuring belt speed or tacho-generator (Mounted in drive 01 Fig-04) and a signal means to represent belt length traveled per unit of time (m /sec).
In the aforesaid feeder of the present invention short-center gravimetric feeder comprises both mechanical and electrical / electronic components housed in a remote power cabinet and feeder mounted integral cabinet.
The components of the short gravimetric feeder can be divided in to the following sub groups:
* Body.
* Feeder Belt and drive assembly.
* Conveyor belt 8B drive assy.
* Weighing assembly.
* Cleanout Conveyor drive assembly.
* Miscellaneous components.
* Feeder mounted electrical & electronics components.
* Control cabinets (an integral feeder mounted control cabinet and remotely located power cabinet).

* Interconnecting Cabling - Power & control.
The Present invention is described is hereinbelow with reference to the accompanying drawing, Wherein,
Fig. 1 shows the gravimetric feeder of the present invention.
Fig. 2 shows the flow of coal inside short centered gravimetric feeder.
Fig. 3 illustrates the section of 24 " short centered gravimetric feeder.
Fig. 4 illustrates the end view of 24 " short centered gravimetric feeder.
Operation and construction
1) The feeder belt (03) carries the coal from the inlet collar (01) to the weighing assembly and discharges the weighed coal to the conveyor belt (04) assembly at a distance of 2134 mm from inlet. The drive, take up and tensioning arrangements (07) for the belt are same as the conventional gravimetric feeder. The feeder belt (03) runs at variable speed to match the load requirement of the boiler (0.2 to 2.2m/min). (All references in FIG-02)
2) The discharge skirt (02) is provided to guide the coal from feeder belt (03) to conveyor belt (04). The skirt can be replaced easily whenever it is worn out. (All references in FIG-02)

3) The coal discharged by feeder belt (03) is carried back to the discharge assembly, which is at 533 mm from feeder inlet. The conveyor belt (04) is same as that of feeder belt in construction but for the length of the belt. The center distance of the pulleys of conveyor (04) is 1450mm. The drive,(fig 04,item 03) take up arrangements are similar to the feeder belt arrangements. The conveyor (05) belt runs at constant speed (14m/min (All references in FIG-02).
4) The clean out conveyor collects the spilt over coal (06) from the floor of
feeder and discharges it. The center distance between the COC
sprockets is 1390mm. (All references in FIG-02)
5) The feeder shell is not the conventional cylindrical shell. It consists of
a flat surface in between the curved portions of the shell.
6) An end door (04,05) is provided for mounting and maintenance of conveyor belt and clean out conveyor chain. The tensioning arrangement (06) for conveyor belt is provided at this door end. (All references in FIG-04)
7) Two access doors (07) with bulls eye cleaning is provided on either side for easy access to the conveyor belt. (All references in FIG-03)
8) Belt scrapper arrangement (04)has been provided for the conveyor belt assembly. (All references in FIG-03)

9) The overall dimensions are as follows
a. Overall height - 1595mm
b. Overall length - 3613mm
c. Overall width - 1800mm
d. Total weight - 4405kg
Salient features of short - centered gravimetric feeders
The short - centered gravimetric feeder according to the present invention has minimum retrofit work in existing thermal power plants where the CE - design rotary volumetric feeders are equipped. The feeder is designed to incorporate all the state of art technology that is available in our conventional gravimetric feeder.
a) The short- centered gravimetric feeder can withstand an explosion pressure of even up to 100 psi against the NFPA requirement of 50 psi and thus ensuring personal safety.
b) Both solid - state control as well as microprocessor control is possible.
c) Variation in speed can be obtained either by eddy current clutch control or variable frequency drive control.
d) The down spout (bunker to feeder ) and feed pipe (feeder to mill ) can remain vertical to ensure free smooth coal flow,

e) It is designed to accommodate a variety of CE designed rotary volumetric feeder like 50 # (suction), 50 # (pressurized), 67 # (suction), 100 # (pressurized) volumetric feeders.
The start up and trip sequence
The following are the sequence of operation for dual belt gravimetric feeder of the present invention.
Start - up
The conveyor motor (return belt) is started first along with clean out conveyor motor followed by feeder motor with a time delay. The time delay required is fine tuned during trial operation.
STOP
The feeder motor (weighing belt) is stopped first followed by conveyor is motor with a time delay. The time delay required is fine tuned during final operation.
TRIP
All motors are tripped simultaneously, in the event both duterual and external trip condition.

WE CLAIM:
1. An automatic weighing gravimetric feeder comprising a pair of strain gauge load cells for weighing (06) , a pair of fixed roller span (05) forming a weighing , a third roller , two fixed rollers being suitably located in a feeder body and said third roller ( 07) being positioned mid way on said span, said load cells (06) being suspended , one on either side of a center weight roller, wherein said short center gravimetric feeder comprises both mechanical and electrical /electronic components housed in a remote power cabinet and feeder mounted integral cabinet.
2. An automatic weighing gravimetric feeder as claimed in claim 1 wherein said load cells (06) are provided with means to convert weigh span force in to an electrical signal in term of coal passed per unit of belt length (Kg/m ).
3. An automatic weighing gravimetric feeder as claimed in claim 1 or 2 wherein said gravimetric feed is attached to a high resolution optical encoder measuring belt speed or tachogenerator and a signal means to represent belt length traveled per unit of time (m / sec).
4. An automatic weighing gravimetric feeder as claimed in any of the preceding claims wherein said short center gravimetric feeder comprises mechanical and electrical / electronic components housed in a remote power cabinet and feeder mounted integral cabinet.
5. An automatic weighing gravimetric feeder essentially comprising
I. A body ;
II. A feeder belt and drive assembly ;
III. A conveyor belt and drive assembly ;
IV. A weighing assembly ;
V. A cleanout conveyor drive assembly ;
VI. A feeder mounted electrical and electronic components ;
VII. A control cabinet; and
VIII. An interconnecting cable.
6. An automatic weighing gravimetric feeder substantially as herein
described and as illustrated in the drawing accompanying the Complete
Specification.