GB1591888A - Method of drawing gas from a region - Google Patents

Description

(54) A METHOD OF DRAWING GAS FROM A REGION (71) We, AAF-LlMxTED, a British Company of Bassington Industrial Estate, Cramlington, Northumberland, NE23 8AF., do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a method of drawing gas from a region where the level of demand differs widely at various times.

It is often necessary, particularly in cyclic processes, to provide draught at hood sets or spaces at widely differing levels corresponding to the demands of the process. The conventional method is to install a fan or fans capable of satisfying the maximum demand and to regulate the draught by simple means such as a damper or fan speed control.

However, the characteristics of many fans employed in such applications are such that peak efficiency is usually only obtainable at one of the required draughting levels and it therefore follows that this method of draught control can be very wasteful of power. The cost of power so wasted becomes increasingly significant: (a) the higher the maximum exhaust rate; (b) the higher the turn-down ratio; (c) the greater the proportion of total fan running time spent in mechanically inefficent operation.

Power costs would be considerably reduced, relative to conventional draughting methods, if peak fan efficiency were obtainable at all the required draughting levels.

According to the present invention, this can be achieved by installing fans in separate flow paths with each fan selected for maximum efficiency at a specific exhaust rate and selecting one or a group of such paths to obtain the desired draughting level at a particular time. Thus, the invention provides a method of drawing gas from a region at two or more diffferent flow rates at different times along at least two flow paths, each of which has a fan therein operable at a peak efficiency speed, the method comprising drawing gas from the region for a first time period at a first flow rate along at least one of said flow paths, the other path or paths being closed; and drawing gas from the region for a second time period at a second, different flow rate along a flow path or group of flow paths different from that or those of the first time period, the other path or paths being closed, wherein in each time period the or each fan in the open path or paths operates at its peak efficiency speed, and wherein the dimensions of each flow path are matched to the fan operable therein to optimize gas flow therethrough.

The flow paths may include a common duct, extending from the region, to a point from which they diverge, or they might be entirely separately connected to the region.

In processes particularly suitable for the application of the invention, the maximum exhaust rate is normally required for short periods relative to those in which a lower rate or rates would be used. In other words, one of the time periods will normally be considerably greater. It is then, sometimes convenient if the maximum exhaust rate is achieved with a plurality of flow paths open, which plurality includes that or those open at the or a lower rate.

By using a plurality of flow paths, and associated fans, two particular advantages are obtained. Firstly, the dimensions of the duct defining the paths may be chosen for maximum economy and secondly, each duct may be designed whereby the flow therethrough is always within a range which inhibits settlement of particulate matter entrained in the gases carried by the duct, and minimizes the possibility of abrasion by such matter.

Three or more paths permits more than two alternative exhaust rates, by allowing flow through one, two or three ducts at any one time. As noted above, different exhaust rates do not have to be achieved through quite independent paths, but the number of fans will normally be at least equal to the number or draughting levels required. For example, with three ducts leading from the region to be draughted, a first fan, with capacity x, may be selectively coupled to a first duct, thus prodtlcing a gas velocity zv in that duct. Another fan, with capacity 3x, may be coupled with the first duct. through a damper, and with the second and third ducts, to be operative when the first fan is shut down and the inlet thereto closed.

whereby two alternative draughting rates are selectively available, one through the first fan and first duct and the other through the second fan and all three ducts. The three ducts may be similarly designed such that. at each driughting level the gas velocity zX is substantially the same along teach. The use of independent ducts is however advantage olis where the gascs to be removed contain substances having different ch;iracteristics.

For example, at a high flow rate (greater suction at the region to be draughted), solids can be drawn into the duct or ducts which have greater abrasive qualities or sx which have a greater tendency to settle out of the moving gas.

The invention will now be described by way of example and with reference to the accompanying drawings wherein: Figure 1 is a diagrammatic representation of a draughting system of the prior art; Figure 2 is a diagrammatic representation of a first embodiment of the invention; and Figure 3 is a diagrammatic representation of a second embodiment of the invention.

In Figure 1 region 2 defined by hood 4 is draughted through duct system 6 and fan 8.

The fan 8 is driven by motor 10 through a gearbox or other variable speed device 12.

Cjas flow through the duct system 6 is controlled by damper 14.

In use, the fan is driven at a first speed to provide a first draughting level and the speed is changed through gearbox 12 to provide a different level. Alternatively, or additionally, the draughting level may be controlled by the damper 14. In any case, this system requires the fan to be operating at less than maximum efficiency at at least one of the draughting levels and. as the higher level is normally required for shorter periods and the fan is normally more efficient at higher speeds, the fan will be running at a lower efficiency for the majority of the time the system is in use.

In the embodiment of the invention illustrated in Figure 2, the duct system 6 includes two ducts 1 6 and 18 which lead from a common duct 20 respectively to two fans 22 and 24. Each duct 16 and 18 is controlled by a damper 26 and 28 respectively.

The fan 22 is driven by a motor 30 and the fan 24 by a motor 32, but no gearbox is required as will be seen.

In use, a first draughting level is provided by the fan 22 drawing gas through ducts 16 and 20. Damper 28 closes duct 18 in this mode. To provide a second draughting level damper is opened, fan 24 being operative to dra gas through ducts 18 and 20. Damper 26 may be closed if desired, although the selective use of fan 22 and duct 1 6 affords a third alternative draughting level. As each fan always runs at a given speed, no gearboxes are required.

The second embodiment of the invention, illustrated in Figure 3, utilizes a duct system including two separate ducts 16' and 18' which both lead directly from the region 2 to be draughted to a fan 22', 24'. A damper 26', 28' is located in each. The fan 22' is driven by motor 30' and fan 24' by motor 32'. This draughting system is operated in the same way as that of Figure 2 but has the advantage that each duct may be designed for use with its associated fan whereby abrasion and settlement can be substantially avoided by predetermining the gas velocity therein.

It will be appreciated that the invention may be extended to provide three or more substantially predetermined draughting levels using various numbers off ducts and fans.

To demonstrate the degree of power saving that can be achieved by utilizing the present invention, an analysis is set forth in the following table. The analysis compares the single fan system (A) of Figure 1 with the two fan system (B) of Figure 2 or Figure 3.

It assumes two desired exhaust rates; 30,000 and 5,000 m3 of gas per minute with corresponding fan pressures of 1,400 and 1 50 mm w.g. and gas temperatures at the fan of 1300C and 50"C respectively. The analysis is for a system operable for 8,000 hours per year, 200 of which are to be at the higher exhaust rate. Identical fans would be used for the higher rate but while one fan is turned down for the lower rate in System A, a different fan, operating at higher efficiency, is used in System B.

SYSTEM A SYSTEM B power consumption higher rate (mew) 9.15 9 15 lower rate 0.5 0.165 Annual power consumption for System A would be 200 hrs x 9.1 5 MW plus 7800 hrs x 0.5 which totals 5730 MW Hrs. The power consumption for System B would be 200 hrs x 9.15 MW plus 7800 x 0.165 MW totalling 3117 MW Hrs; a saving of 2613 MW Hrs.

Running costs can also be reduced as maintenance of the fans in System B will be minimized as each will only be operated at a single speed.

WHAT WE CLAIM IS:- 1. A method of drawing gas from a region at two or more different flow rates at different times along at least two flow paths, each of which has a fan therein operable at a peak efficiency speed, the method comprising drawing gas from the region for a first time period at a first flow rate along at least

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. first duct, thus prodtlcing a gas velocity zv in that duct. Another fan, with capacity 3x, may be coupled with the first duct. through a damper, and with the second and third ducts, to be operative when the first fan is shut down and the inlet thereto closed. whereby two alternative draughting rates are selectively available, one through the first fan and first duct and the other through the second fan and all three ducts. The three ducts may be similarly designed such that. at each driughting level the gas velocity zX is substantially the same along teach. The use of independent ducts is however advantage olis where the gascs to be removed contain substances having different ch;iracteristics. For example, at a high flow rate (greater suction at the region to be draughted), solids can be drawn into the duct or ducts which have greater abrasive qualities or sx which have a greater tendency to settle out of the moving gas. The invention will now be described by way of example and with reference to the accompanying drawings wherein: Figure 1 is a diagrammatic representation of a draughting system of the prior art; Figure 2 is a diagrammatic representation of a first embodiment of the invention; and Figure 3 is a diagrammatic representation of a second embodiment of the invention. In Figure 1 region 2 defined by hood 4 is draughted through duct system 6 and fan 8. The fan 8 is driven by motor 10 through a gearbox or other variable speed device 12. Cjas flow through the duct system 6 is controlled by damper 14. In use, the fan is driven at a first speed to provide a first draughting level and the speed is changed through gearbox 12 to provide a different level. Alternatively, or additionally, the draughting level may be controlled by the damper 14. In any case, this system requires the fan to be operating at less than maximum efficiency at at least one of the draughting levels and. as the higher level is normally required for shorter periods and the fan is normally more efficient at higher speeds, the fan will be running at a lower efficiency for the majority of the time the system is in use. In the embodiment of the invention illustrated in Figure 2, the duct system 6 includes two ducts 1 6 and 18 which lead from a common duct 20 respectively to two fans 22 and 24. Each duct 16 and 18 is controlled by a damper 26 and 28 respectively. The fan 22 is driven by a motor 30 and the fan 24 by a motor 32, but no gearbox is required as will be seen. In use, a first draughting level is provided by the fan 22 drawing gas through ducts 16 and 20. Damper 28 closes duct 18 in this mode. To provide a second draughting level damper is opened, fan 24 being operative to dra gas through ducts 18 and 20. Damper 26 may be closed if desired, although the selective use of fan 22 and duct 1 6 affords a third alternative draughting level. As each fan always runs at a given speed, no gearboxes are required. The second embodiment of the invention, illustrated in Figure 3, utilizes a duct system including two separate ducts 16' and 18' which both lead directly from the region 2 to be draughted to a fan 22', 24'. A damper 26', 28' is located in each. The fan 22' is driven by motor 30' and fan 24' by motor 32'. This draughting system is operated in the same way as that of Figure 2 but has the advantage that each duct may be designed for use with its associated fan whereby abrasion and settlement can be substantially avoided by predetermining the gas velocity therein. It will be appreciated that the invention may be extended to provide three or more substantially predetermined draughting levels using various numbers off ducts and fans. To demonstrate the degree of power saving that can be achieved by utilizing the present invention, an analysis is set forth in the following table. The analysis compares the single fan system (A) of Figure 1 with the two fan system (B) of Figure 2 or Figure 3. It assumes two desired exhaust rates; 30,000 and 5,000 m3 of gas per minute with corresponding fan pressures of 1,400 and 1 50 mm w.g. and gas temperatures at the fan of 1300C and 50"C respectively. The analysis is for a system operable for 8,000 hours per year, 200 of which are to be at the higher exhaust rate. Identical fans would be used for the higher rate but while one fan is turned down for the lower rate in System A, a different fan, operating at higher efficiency, is used in System B. SYSTEM A SYSTEM B power consumption higher rate (mew) 9.15 9 15 lower rate 0.5 0.165 Annual power consumption for System A would be 200 hrs x 9.1 5 MW plus 7800 hrs x 0.5 which totals 5730 MW Hrs. The power consumption for System B would be 200 hrs x 9.15 MW plus 7800 x 0.165 MW totalling 3117 MW Hrs; a saving of 2613 MW Hrs. Running costs can also be reduced as maintenance of the fans in System B will be minimized as each will only be operated at a single speed. WHAT WE CLAIM IS:-

1. A method of drawing gas from a region at two or more different flow rates at different times along at least two flow paths, each of which has a fan therein operable at a peak efficiency speed, the method comprising drawing gas from the region for a first time period at a first flow rate along at least

one of said flow paths, the other path or paths being closed; and drawing gas from the region for a second time period at a second, different flow rate along a flow path or group of flow paths different from that or those of the first time period, the other path or paths being closed, wherein in each time period the or each fan in the open path or paths operates at its peak efficiency speed, and wherein the dimensions of each flow path are matched to the fan operable therein to optimize gas flow therethrough.

2. A method according to Claim 1 wherein a plurality of flow paths are open in the second time period, which plurality of paths includes the path or paths open in the first time period.

3. A method according to Claim 1 wherein the flow path or paths which are open in the first time period is or are closed in the second time period.

4. A method according to any preceding Claim wherein each flow path includes a common duct leading from the region to a point at which the flow paths diverge to their respective fans.

5. A method according to any preceding Claim wherein the flow paths and the fans therein are matched such that in either time period, the flow rate through an open flow path is substantially the same.

6. A method drawing gas from a region at two or more different flow rates at different times substantially as described herein with reference to and as illustrated by Figure 2 or Figure 3 of the accompanying drawing.