Gas separation – With nonliquid cleaning means for separating media – Cohesive filter media cleaning
Reexamination Certificate
2000-02-14
2001-12-25
Hopkins, Robert A. (Department: 1724)
Gas separation
With nonliquid cleaning means for separating media
Cohesive filter media cleaning
Reexamination Certificate
active
06332902
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a filter unit for filtering gas, comprising at least one filter element having an outlet opening facing a clean-gas chamber, a filter cleaning nozzle, and a source of pressurized primary cleaning gas.
A filter unit of this type is known from DE patent 197 01 983 describing a nozzle of the slit ring jet type where the nozzle opening is an annular slot, which faces radially inwards towards the center of a flow pipe which passes centrally through a pressurized gas chamber and ends above the filter outlet opening. The control valve is pneumatically activated and built into the nozzle at the annular slot. The primary cleaning gas flows from the gas chamber through the annular slot and down along the inside of the flow pipe which acts as guide surface. The secondary gas is drawn into the flow pipe through its upper opening. The cleaning gas passes through a venturi mounted in the upper end area of the filter member.
A somewhat similar nozzle is described in U.S. Pat. No. 4,356,010 and in GB patent 1 455 281, but in the latter document the ring slit jet nozzle is mounted like a venturi in the top end of the filter element. The secondary gas is drawn centrally down through the upper end opening of the nozzle. GB patent 812,244 describes a simple nozzle shooting primary gas down through a venturi so that secondary gas is drawn down through the upper end opening of the venturi. GB patent 1,021,560 describes a nozzle pipe which is mounted in the bottom wall of a pressurized gas chamber and is closed at its upper end by a diaphragm valve. When the diaphragm valve is opened primary cleaning gas flows through the nozzle pipe and centrally down through a venturi at the upper end of the filter element. U.S. Pat. No. 3,729,903 describes a ring jet filter with an annular membrane valve mounted in the upper end of the filter element. A central pipe connects the filter element with the clean-air chamber. When the membrane valve is open secondary air is also drawn down through the upper end opening of the central pipe.
A feature common to all the prior art filter units mentioned above is that the secondary air is drawn in through the upper end opening of a pipe located centrally of the filter element.
BRIEF SUMMARY OF THE INVENTION
It is a purpose of the present invention to provide a very efficient filter cleaning nozzle of a simple design.
According to the present invention the filter cleaning nozzle is located in the clean-gas chamber at a distance from said outlet opening and is connected to the source of pressurized primary cleaning gas via a control valve; a guide body is located in the clean-gas chamber and has an outer surface; said nozzle has a nozzle opening for injection of primary cleaning gas onto a guide surface on the outer surface of the guide body; and the guide surface is open to secondary gas in the clean-gas chamber and directs said primary cleaning gas and entrained secondary gas towards the outlet opening of the filter.
When a burst of primary cleaning gas is shot out through the nozzle opening it flows along the outer side of the guide body and is in full and unrestricted contact with the secondary air in the clean-gas chamber. Compared to the prior art nozzles where the secondary gas has to be drawn in through the upper end opening of the nozzle, more secondary cleaning gas is entrained by the primary gas flowing on the outwards facing guide surface. As secondary gas is entrained the thickness of the air stream builds up and the diameter of the air stream increases. The larger diameter of the air stream facilitates further entrainment of secondary gas. In addition, there is no hindrance to the building up of a thicker air stream because it builds up outwards into the free air in the clean-gas chamber. These combined effects result in a nozzle design where the entrainment of secondary gas becomes more efficient the longer the air stream flows along the guide surface, and not less efficient as is the case when the guide surface is on the inside of a pipe and the air stream thickness builds up towards the center of the pipe.
In one embodiment the nozzle opening is an annular slot which is delimited by an annular rim of a feeding pipe for primary cleaning gas and the guide body positioned in front of the feeding pipe. This makes it possible to manufacture the guide body independently of the feeding pipe, because it simply needs to be mounted at a distance from the end of the feeding pipe corresponding to the desired width of the annular slot. The embodiment is furthermore suitable for retrofit mounting on existing filter units having feeding pipes.
In order to promote the gradual building up of the thickness of the air stream, the guide body portion facing towards the feeding pipe is preferably curved with its central area positioned closest to the feeding pipe. The curvature can vary along the guide body but for reasons of manufacturing simplicity the shape is preferably hemispherical. It is also possible to use a facetted shape or a shape composed of one or more conical portions.
In a preferred embodiment the guide body is integrated with the other part that delimits the nozzle opening. On the side facing away from the outlet opening of the filter the guide body extends into a housing, and an annular distribution chamber for primary cleaning gas in the housing ends at the nozzle opening, which is an annular slot delimited by an annular rim of the housing and the guide body. The two parts can then be manufactured as a unit having a very well defined and precise width of the annular slot, and the mounting work in the filter unit is facilitated. It is possible to make the annular chamber with a taper towards the annular slot, and preferably the included angle between an inner side wall of the distribution chamber and the outer side of the guide body is in the range of 2-45°. Due to the taper the chamber has a relatively large width in the end remote from the slot, which promotes efficient and even distribution of primary gas throughout the whole chamber, even when there is only one or a few inlet ports for primary gas to the chamber. At the opposite end of the chamber the width of the chamber approximately corresponds to the width of the slot, and this promotes more uniform gas flow conditions through the slot during the very short interval when primary air is shot out of the nozzle. If the included angle is less than 2° only an negligent effect is obtained, and angles larger than 45° will not result in any improvement of the distribution of the gas.
Preferably, the guide body is carried by the housing so that use of additional securing means is avoided, and even more preferably the guide body is permanently fixed to the housing, such as by welding the parts together. This reduces the risks of unintentionally loosening of a guide body during a filtration process or during inspection of the filter unit.
The efficiency of the nozzle in terms of capability for entrainment of secondary air is enhanced in an embodiment where the guide body from the nozzle opening has a first portion with increasing diameter in direction towards the outlet opening of the filter element. The increasing diameter allows on the one hand that the annular slot has an advantageously short length and thus a suitably large width for a certain nozzle opening area, and on the other hand that the flow of primary gas is spread out to have a larger diameter and consequently a large area in contact with the secondary gas.
In the most preferred embodiment the guide body has an end portion with decreasing diameter in direction towards the outlet opening of the filter element. The decreasing diameter reduces the contact area between the guide body and the gas flow in a gradual manner so that only small disturbances from eddy currents occur at the trailing end of the guide body.
In a further development the guide body is designed so that in between the first portion and the end portion the guide body has a second portion with substantially constant diameter.
Hansen Ove Emil
Simonsen Per
Hopkins Robert A.
Niro A/S
Sughrue & Mion, PLLC
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