FIELD OF THE INVENTION

The present invention relates to heat exchangers. In particular, it relates to sludge/water heat exchangers for exchanging heat between countercurrent flowing water and sludge.

BACKGROUND OF THE INVENTION

Heat exchangers are used in many industries to remove heat from one fluid and transfer the heat to another fluid. A variety of heat exchanger designs are available, and each basic design has many possible configurations and materials of construction. The design chosen for a specific application depends on the conditions under which the heat exchanger must operate and the function it must perform. When the fluids passing through a heat exchanger are clean and not likely to form deposits on the heat transfer surfaces, any of the designs capable of handling the temperatures and pressures imposed by the application can be used. However, if fluids contain particulate matter or have a tendency to form deposits on the heat transfer surfaces, the available options become limited.

Almost any 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 in a digester kills such pathogens. The sludge may then be used as commercial fertilizer for farms instead of burying it in landfills.

Most wastewater systems involve batch processing of sludge. Primary and secondary treatments zones are employed as 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 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. 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 sludge passage resulting in repeated weekly or monthly maintenance. This also applies to other existing water/sludge heat exchangers, e.g. such as those offered by Laeckeby Water AB, Sweden. It is not uncharacteristic to devote a full day of labor to opening up existing heat exchangers and cleaning out the plugging debris.

GB1436685A discloses a heat exchanger for heating sludge comprising at least two horizontally aligned tube pairs having an outer tube and disposed inner tube. GB1436685A also shows return bends establishing a fluid flow connection between the inner tubes of one of the tube pairs with the inner tube of an adjacent tube pair, however these return bends are not configured as a compartment in which mixing of the contents from the different inner tubes takes place.

An object of the present invention is to provide a heat exchanger, which is capable of transferring heat between any two fluids. In particular, the heat exchanger of the present invention must work well for those applications where sludge is contaminated with solids or substances which are prone to accumulate on heat transfer surfaces and/or render the sludge highly viscous.

Another object of the present invention is to provide a heat exchanger, which is less expensive to build than prior art heat exchangers.

Still another object of the present invention is to provide a heat exchanger, which is easy to clean and maintain.

SUMMARY OF THE INVENTION

The foregoing objects are accomplished by the present invention. The heat exchanger of the present invention is comprised of: at least two, and preferably three, horizontally aligned tube pairs having an outer tube and at least three inner tubes, said inner tubes having a length of 75-95% of the length of the outer tube. Return bends establish a fluid flow connection between the inner tubes of one tube pair with the inner tubes of an adjacent tube pair, where each return bend is configured as a compartment having an external flanged end cap, which constitute an end wall of the outer tube, and an end plate provided with openings for receiving the ends of the inner tubes and restricting flow passage from the outer tube into the return bends, thereby providing a path for sludge flowing into the return bend from the inner tubes of one of said tube pairs and then entering the inner tubes of an adjacent tube pair. Also provided are flow channels between outer tubes of adjacent tube pairs thereby establishing a flow path for hot liquid in fluid flow connection between adjacent outer tubes.

In a preferred embodiment of the present invention three tube pairs are provided. Also preferred is that the flow channels between outer tubes of adjacent tube pairs are provided in a distance of 1-30 cm from the end caps. The flow channels are preferably provided alternately in the ends of adjacent tube pairs so that the flow passage through the outer tubes is shifting direction between adjacent tube pairs.

In a preferred embodiment the adjacent tube pairs are vertically stacked so as to establish a vertical flow in the return bends as well as in the flow channels between outer tubes. The externally flanged end cap is provided with means for accessing the return bend and the inner tubes, e.g. by screw ports, flanges, or ball valves.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying Figure 1 illustrates a complete embodiment of the invention in which the three tube pairs are used.

DETAILED DESCRIPTION OF THE INVENTION

The heat exchanger of the present invention allows for the exchange of heat between two fluids. Although the invention will be described on the basis of a complete system with three tube pairs, many of these elements may be provided as a pre-assembly before the final fabrication of the heat exchanger. Such a "modular" design helps reduce construction costs, especially for those applications where a large heat exchanger is required. Careful design and alignment of the inlet and outlet nozzles allows for multiple modules to be stacked together to create a heat exchanger optimized for flow velocity and total heat transfer area. Moreover, the modular design allows for easy disassembly to permit mechanical cleaning or replacement of fouled or damaged tubes.

Figure 1 depicts the heat exchanger with three tube pairs each having an outer tube (13) and three evenly disposed inner tubes (4). The inner tubes (4) open into the return bends (3) thereby establishing a fluid flow connection between the inner tubes (4) of the uppermost tube pair with the inner tubes (4) of the middle tube pair. The return bend (3) between the tube pairs has an external flanged end cap (10) thereby providing a path for sludge entering the return bend (3) from the middle tube pair and entering the inner tubes (4) of the uppermost tube pair. End plates (8) restrict flow passage from the outer tube (13) into the return bends, while the openings in the end plates receive the ends of the inner tubes (4) thereby allowing flow passage from the inner tubes to the return bends (3). The outer tubes (13) of the upper, middle and lower tube pair are in fluid flow connection thereby providing a flow path for hot liquid, which enters the system at reference numeral (1 1) and exits at reference numeral (12), while sludge enters at reference numeral (1) and exits at reference numeral (2). As elsewhere mentioned the inner tubes are shorter than the outer tubes and end in an end plate provided with wholes (or bores) to which the inner tubes are firmly attached and sealed; preferably by welding. The outer circumference of the end plate is tightly attached to the inner wall of the outer tube, preferably also established by welding.

Hence, the liquid (normally hot water) flowing in the outer tube cannot enter the return bend. The inner tube has a smaller diameter than the outer tube (such as 10-30% thereof), and both tubes share the same longitudinal axis. When the heat exchanger is in use, a viscous fluid (sludge) may flow through the inside of the inner tube and is then mixed in the return bend due to different velocities and gravities of the sludge moving in the three (or more) inner tubes.

Also shown are additional optional valves, such as reference numeral (6) for opening/closing outlet for e.g. sampling sludge for further analysis or processing. Further there is shown means (9) for accessing the return bends and inner tubes (4). Moreover, the fluid connections (14) between the outer tubes are shown.

Figure 1 also includes a cross-sectional view of a heat exchanger where three of the inner tubes (4) are visible in each of the three tube pairs. As mentioned above each end cap (8) has three openings therein for receiving the inner tubes (4). The size of the openings is about equal to the outer diameter of the inner tubes (4) such that the inner tubes (4) fit within the opening in the end cap (8) and are connected to it to form a leak- free connection. The number of openings is equal to the number of inner tubes in each tube pair.