Patent Application
20100212872
The present invention is directed to a heat exchanger for transferring heat energy from a heated liquid at a first temperature to sludge at a second temperature. The heat exchanger includes a conduit containing non-clogging motionless mixing elements to optimize heat transfer while preventing clogging of sludge within the apparatus.
Virtually every municipality has a waste water treatment system which is employed to remove nutrients such as nitrogen and phosphorus from waste water as well as to destroy pathogens and viruses which are found within waste sludge. Heating municipal sludge to 135° F. in a digester for 15 days or longer kills such pathogens. The sludge is then classified as class A sludge which may be used as commercial fertilizer for farms instead of burying it in landfills. Interestingly, the conversion of waste to class A sludge is mandatory in many jurisdictions such as the state of California; a standard which has prevailed in Europe for many years.
Most wastewater systems involve batch processing of sludge. Primary and secondary treatments zones ate employed as are are clarifiers and separators. It is common to have purified effluent discharge into streams or lakes while sludge drawn from a clarifier is oftentimes returned to the head of the activated sludge system and mixed with influent wastewater as a continuous process. As such, it is highly advantageous to have a mixer located within treatment zones and particularly within the heat exchanger to not only maximize the efficiency of the waste water treatment system, but also optimize the transfer of heat energy from a heating liquid, such as water, to the sludge and resulting digester.
Although there are various types of sludge, most can be characterized physically as including a high percentage of solids and stringy material. As such, there are basically two varieties of heat exchanger's which have been employed in this arena. The first involves a pipe with a hot water jacket. Such a configuration has the advantage of having an open six inch diameter or larger piping which eliminates plugging. However, such a heat exchanger assembly requires enormous floor space as it must be large due to the low heat transfer characteristics of the configuration. Multiple sections of jacketed piping must be used to achieve the requisite temperature increase. This results in higher installation costs than those involved in employing a spiral type of heat exchanger.
The spiral type of heat exchanger involves providing a spiraling passage for sludge and a spiraling passage for hot water. The two channels spiral in from the outside of the periphery of the system ending and exiting neat the central of the spiral. Such a configuration is relatively compact and thus results in space saving over the pipe/water jacket configuration discussed above. However, the spiral geometry characteristically results in periodic plugging of its narrow 1 inch×30 inch sludge passage resulting in repeated weekly or monthly maintenance. It is not uncharacteristic to devote a full day of labor to opening up the heat exchanger and cleaning out the plugging debris. Despite these limitations, due to its compact size, spiral type heat exchangers are used in over 95% of the waste water treatment plant installations now in service.
It is thus an object of the present invention to provide the non-plugging advantages of a jacketed pipe type heat exchanger while enjoying the relatively compact design of the spiral type heat exchanger while eliminating plugging and maximizing heat transfer in mixing of the sludge.
These and further objects will be more readily apparent when considering the following disclosure and dependent claims.
The present invention is directed to a heat exchanger for transferring heat energy from a heated liquid at a first temperature to sludge at a second temperature. The heat exchanger comprises a first conduit for receiving a stream of sludge, the first conduit having a length, substantially circular circumference, a longitudinal axis through said length and being opened at both ends thereof. The first conduit houses a plurality of mixing elements which are characterized as having no edges perpendicular to the longitudinal axis and which are sized and positioned within the conduit such that at any plane passing perpendicularly to the longitudinal axis, at least 75% of the circumference of the conduit is free of any mixing elements and, further, no mixing elements are in contact with one another resulting in an open region of travel for sludge passing through the first conduit along its longitudinal axis. A second conduit is also provided in the form of a sleeve having a longitudinal axis coincident with the longitudinal axis of the first conduit defining an annular space open at both ends for receiving and discharging heated liquid. As a preferred embodiment, a second set of mixing elements can be located in the annular space created between the first conduit and sleeve for enhancing heat transfer.
The heat exchanger of the present invention is shown, in its preferred configuration, in
The schematic pathway for the sludge and heating liquid is shown in
For the sake of simplicity, a single segment of conduit 30 has been selected for discussion. This conduit extends along spaced arrow 16 (
The sludge-carrying first conduit 30 of the present invention is shown in the shape of a cylinder provided with longitudinal axis 37. As shown in
The sludge carrying conduit 30 is provided with mixing elements 33, 34, 35 and 36 These elements are characterized as having no edges or surfaces perpendicular to longitudinal axis 37 and are sized so that no such elements are in contact with one another resulting in an open region of travel 96 for fluids passing through conduit 30 along its longitudinal axis. Ideally, each mixing element is seated within the sludge carrying conduit at an angle between approximately 30° to 45° to said longitudinal axis. Most importantly, however, the mixing elements are positioned within the conduit so that at least 75% of the conduit's circumference in any plane is free of any mixing element. Obviously, various mixing elements are provided with no points of contact so that there arc actually no “crotches” provided which would otherwise result in sludge hangup. In fact, it is the design objective of the present invention to enable debris having effective diameters of 75% or more of the conduit diameter to pass through the conduit without entrainment. Such a geometry is disclosed and claimed in applicant's U.S. Pat. No. 5,758,967. Mixers made according to the teachings of the '967 patent have been tested using flows of 2% to 4% sludge in pipe sizes for 2″ to 8″ in diameters. In no case did plugging occur over a period of more than six months.
As previously noted, it is the intent of the present invention to provide a device which is uniquely adaptable for the mixing of waste sludge and for enhancing heat transfer throughout a continuous stream of such material. Sludge is somewhat unique in its tendency to plug or clog mixing components for material within the sludge tends to migrate to and accumulate in low pressure or “dead spots” and long fibers will catch and build up in “crotches”. Both of these effects allow and encourage more material to accumulate until the sludge carrying conduit finally plugs By providing spacing 96 and, more importantly, by providing the placement of mixing elements whereby at least 75% of the conduit circumference in any plane is clear of any ancillary structure accomplishes the goals of the present invention. Even the most problematic components “slide” over the mixing elements without clogging under both laminar and turbulent flow conditions. Ideally, the mixing elements are provided as pairs such as 33/34 and 35/36. Each complimentary pair cause flowing material to rotate above the axis of the conduit in opposite directions. As further noted, the four mixing elements are each shown primarily as a circular segment configuration each of a height approximately D/10 and radius of D/2, wherein D is the diameter of the conduit. The various mixing segments or elements are set in a non-opposing fashion at the pipe wall so as to present to the fluid in any plane normal to the axis of conduit a non-symmetrical cross-section. This serves to break up the normal circular symmetry of flow and to substantially reduce the conduit length necessary to achieve effective mixing. As such, mixing is accomplished with minimal pressure drop and sludge hang up.
The primary performance limiting factor in both pipe-in-pipe and shell and tube heat exchangers is that associated with the internal film coefficient located at the internal surface of the tube well. It is well known that static mixing elements installed in the tube or tubes can improve overall heat transfer performance by a factor of three or more, but such mixing elements, as noted above, tend to plug. The next performance limiting factor is that associated with the external film coefficient and is usually much smaller than the internal film coefficient. The present invention allows mixing elements 53, 54, 55, and 56 to be installed on the outside of conduit 30 being installed in the annular space between conduit 30 and conduit 50 as shown in
