A distillation unit (10) employs a rotary heat exchanger (32) forming a multiplicity of evaporation chambers (56) into which a liquid to be purified is sprayed for evaporation. Spray arms (58) spray at a steady rate into all of the evaporation chambers (56) simultaneously but not at a rate that is adequate to maintain the wetting required for efficient transfer of heat to the liquid. A scanning sprayer (140) supplements this steady spray with spray from nozzles (142 and 144) into only a few of the evaporation chambers at a time, visiting all of them cyclically. The overall rate of spray from the two sources thus combined to spray the chamber cyclically maintains proper wetting even though on average it is lower than the rate that would be required of a constant-rate spray into all of the evaporation chambers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to distillation. It has particular, but not exclusive, application to using rotary heat exchangers to purify water by distillation.

2. Background Information

One of the most effective techniques for purifying water is to distill it. In distillation, the water to be purified is heated to the point at which it evaporates, and the resultant vapor is then condensed. Since the vapor leaves almost all impurities behind in the input, feed water, the condensate that results is typically of a purity much higher in most respects than the output of most competing purification technologies.

One of the distillation approaches to which the invention to be described below may be applied employs a rotary heat exchanger. Water to be purified is introduced to one, evaporation set of heat-exchange surfaces, from which the liquid absorbs heat and evaporates. The resultant water vapor is then typically compressed and brought into contact with another, condensation set of heat-exchange surfaces that are in thermal communication with the set of evaporation heat-exchange surfaces. Since the water vapor on the condensation side is under greater vapor pressure than the water on the evaporation side, vapor that condenses on the condensation side will be hotter than the evaporating liquid on the evaporation side, and its heat of evaporization will therefore flow to the evaporation side: the system reclaims the heat of evaporization used to remove the relatively pure vapor from the contaminated liquid. To minimize the insulating effects to which a condensation film on the condensation surfaces would tend to contribute, a rotary heat exchanger's heat-exchange surfaces rotate rapidly, so the condensate experiences high centrifugal force and is therefore removed rapidly from the condensation surfaces.

This removal of liquid from the condensation-side heat-exchange surfaces is important, because a significant drawback of using distillation for water purification is the energy cost that it exacts. That cost tends to be greater when the temperature difference between the rotary heat exchanger's evaporation and condensation sides is relatively great. On the other end, a low temperature difference tends to result in a lower rate of heat exchange, and this then necessitates a greater heat-exchange area for a given volume rate of distillation. Such an additional heat-exchange-surface area exacts its own cost penalties not only in initial equipment cost but also in the power needed to operate the unit. The reason why rapid condensate removal tends to ameliorate the energy-cost problem is that reduction of the condensate film's insulating effects tends to increase the heat-exchange rate for a given temperature difference.

The rotary heat exchanger's centrifugal force also tends to reduce the water-film thickness on the evaporation side and thereby further benefit heat-exchange efficiency. Of course, introducing liquid to the evaporation side at too great a rate will compromise the centrifugal force's beneficial effect on heat transfer, so evaporator efficiency is best served by keeping the rate of feed-water introduction relatively low. Unfortunately, too low a rate of feed-water introduction is counterproductive; it allows surface tension to defeat proper surface wetting and thus heat transfer to the liquid.

SUMMARY OF THE INVENTION

But I have recognized that heat-exchanger efficiency can be improved by employing a technique that keeps the evaporator surfaces substantially wetted but uses an average rate of liquid feed substantially lower than the steady-state rate required to maintain proper wetting. In accordance with my invention, the rate at which the evaporator-side heat-exchange surfaces are irrigated so varies as repeatedly to reach a peak irrigation rate that is at least twice its average rate. Preferably, that average rate is less than half the steady-state rate required to maintain proper wetting, while the peak rate preferably exceeds that steady-state rate. Even though the average rate is low, the repeated increases to such a peak rate can prevent those surfaces from dewetting. The result is a significantly greater heat-exchange rate, and less power consumption, than in a similar system employing the minimum steady-state rate required to maintain wetting.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

FIG. 1is an exterior isometric view of a distillation unit in which the present invention's heat-exchanger-irrigation approach can be employed. In general, the distillation unit10includes a feed inlet12through which the unit draws a feed liquid to be purified, typically water containing some contamination. The unit10purifies the water, producing a pure condensate at a condensate outlet14. The volume rate of condensate produced by the unit10will in most cases be only slightly less than that of the feed liquid entering inlet12, nearly all the remainder being a small stream of concentrated impurities discharged through a concentrate outlet16. The unit also may include a safety-drain outlet18. The illustrated unit is powered by electricity, and it may be remotely controlled or monitored. For this reason, electrical cables20are also provided. In the illustrated embodiment, the distillation unit10is intended for high-efficiency use, so it includes an insulating housing22. But the present invention's teachings are applicable to a wide range of heat-exchanger applications, not all of which would typically employ such a housing.

FIG. 2is a simplified cross-sectional view of the distillation unit. It depicts the housing22as having a single-layer wall24. In single-layer arrangements, the wall is preferably made of low-thermal-conductivity material. Alternatively, it may be a double-layer structure in which the layers are separated by insulating space.

The present invention is an advantageous way to supply feed liquid to the unit's heat exchanger32. While the present invention's teachings can be employed to feed a wide variety of heat exchangers, the drawings illustrate a particular type of rotary heat exchanger for the sake of concreteness. As will be explained in more detail directly, the illustrated embodiment's rotary heat exchanger is essentially a group of stacked plates, one plate34of which will be described in more detail in connection with subsequent drawings. That heat exchanger32is part of an assembly that rotates during operation and includes a generally cylindrical shell36driven by a motor38. The rotating assembly's shell36is disposed inside a stationary vapor-chamber housing40on which is mounted a gear housing42that additionally supports the motor38. The vapor-chamber housing40in turn rests in a support omitted from the drawing for the sake of simplicity.

As FIG.3's exemplary heat-exchanger plate34illustrates, each plate is largely annular; it may have an outer diameter of, say, 8.0 inches and an inner diameter of 3.35 inches. Each plate is provided with a number of passage openings46.FIG. 4, which is a cross section taken at line4-4ofFIG. 3, shows that the passage openings are formed with annular lips48that in alternating plates protrude upward and downward so that, as will explained in more detail presently, they mate to form passages between the heat exchanger's condensation chambers.

To form alternating condensation and evaporation chambers, the heat-exchanger plates are provided with annular flanges50at their radially inward edges and annular flanges52at their radially outward edges. Like the passage lips48, these flanges50and52protrude from their respective plates, but in directions opposite those in which the passage lips48protrude.FIG. 5, which depicts the radially inward part of the heat exchanger on the left and the radial outward part on the right, shows that successive plates thereby form enclosed condensation chambers54interspersed with open evaporation chambers56. A recently tested prototype of the heat exchanger employs108such plate pairs.

As will be explained in more detail below, a sprayer in the form of a stationary spray arm58located centrally of the spinning heat-exchanger plates sprays water to be purified onto the plate surfaces that define the evaporation chambers56. (The use of the term spray is not intended to imply that the water is necessarily or preferably applied in droplets, although some embodiments may so apply the liquid.) That liquid absorbs heat from those surfaces, and some of it evaporates. FIG.2's compressor60draws the resultant vapor inward.

FIG. 6depicts compressor60in more detail. The compressor spins with the rotary heat exchanger and includes a (spinning) compressor cylinder62within which a mechanism not shown causes two pistons64and66to reciprocate out of phase with each other. As a piston rises, its respective piston ring68or70forms a seal between the piston and the compressor cylinder62's inner surface so that the piston draws vapor from the heat exchanger's central region. As a piston travels downward, on the other hand, its respective piston ring tends to lift off the piston surface and thereby break the seal between the cylinder wall and the pistons.

When their respective pistons are traveling downward, annular piston-ring stops72and74, which respective struts76and77secure to respective pistons64and66, drag respective piston rings68and70downward after the seal has been broken. The piston rings and stops thus leave clearances for vapor flow past the pistons as they move downward, so a downward-moving piston does not urge the vapor back downward as effectively as an upward-moving piston draws it upward. Additionally, the pistons reciprocate so out of phase with each other that there is always one piston moving upward, and thereby effectively drawing the vapor upward, while the other is returning downward.

As will be explained in more detail below, the vapor thus driven upward by the pistons64and66cannot pass upward beyond the compressor's cylinder head78, but slots80formed in the compressor wall's upper lip provide paths by which the vapor thus drawn from the heat exchanger's central region can be driven down through an annular passage82formed between the compressor cylinder62's outer surface and the rotating-assembly shell36. This passage leads to openings83in an annular cover plate84sealed by O-rings85aand85bbetween the compressor cylinder62and the rotating-assembly shell36. The openings83register with the openings46(FIG. 3) that form the passages between the condensation chambers.

In short, the compressor cylinder62, the cylinder head78, and the rotating-assembly shell36cooperate to form a guide that directs vapor along a vapor path from FIG.5's evaporation chambers56to its condensation chambers54. And the compressor compresses the vapor that follows this path, so the vapor pressure in the condensation chambers54is higher than that in the evaporation chambers56, from which the compressor draws the vapor. The boiling point in the condensation chambers therefore is also higher than in the evaporation chambers. So the heat of vaporization freed in the condensation chambers diffuses to the (lower-temperature) evaporation chambers56.

In the illustrated embodiment, the rotating assembly rotates at a relatively high rate of, say, 700 to 1000 rpm. The resultant centrifugal force causes the now-purified condensate to collect in the outer ends of the condensation chambers, between which it can flow through the passages that the heat-exchanger-plate openings46form. AsFIG. 7shows, the condensate therefore flows out through the openings83in the top of the heat exchanger and travels along the channel82by which the compressed vapor flowed into the heat exchanger.

Like the compressed vapor, the condensate can flow through the openings80in the compressor wall's lip. But the condensate can also flow past the cylinder head78because of a clearance86between that cylinder head78and the rotating-assembly shell, whereas the condensate's presence in that clearance prevents the compressed vapor from similarly flowing past the cylinder head. An O-ring88seals between the rotating-assembly shell36and a rotating annular channel-forming member90secured to the cylinder head78, but spaced-apart bosses92formed in the cylinder head78provide clearance between the cylinder head and the channel member so that the condensate, urged by the pressure difference that the compressor imposes, can flow inward and into channel member90's interior.

Like the cylinder head78to which it is secured, the channel-forming member90spins with the rotary heat exchanger to cause the purified condensate that it contains to collect under the influence of centrifugal force in the channel's radially outward extremity. The spinning condensate's kinetic energy drives it into a stationary scoop tube94, from which it flows to FIG.1's condensate outlet14by way of a route that will be described in due course.

While the scoop tube94is thus removing the liquid condensate that has formed in the condensation chambers, centrifugal force drives the unevaporated feed liquid from the evaporation chambers to form an annular layer on the part of the rotating-assembly wall36below plate84: that wall thus forms a liquid-collecting sump. Another scoop tube, which will be described below, removes this unevaporated liquid for recirculation through the rotary heat exchanger.

Before we deal with the manner in which the recirculation occurs, we summarize the overall fluid circuit by reference toFIG. 8. A pump100draws feed liquid from the feed inlet12and drives it to the cold-water inlets102C—INand104C—INof respective counterflow-heat-exchanger modules102and104. Those modules guide the feedwater along respective feed-water paths to respective cold-water outlets102C—OUTand104C—OUT. In flowing along those paths, the feedwater is in thermal communication with counterflows that enter those heat exchangers at hot-water inlets102H—INand104H—INand leave through hot-water outlets102H—OUTand104H—OUT, as will be explained in more detail below, so it is heated. (The terms hot and cold here respectively refer to the fluid flows from which and to which heat is intended to flow in the counterflow heat exchangers. They are not intended to refer to absolute temperatures; the liquid leaving a given counterflow heat exchanger's “cold”-water outlet, for instance, will ordinarily be hotter than the liquid leaving its “hot”-water outlet.)

For reasons that will be set forth below, counterflow-heat-exchanger module104receives a minor fraction of the feed-water flow driven by the pump100. Its volume flow rate is therefore relatively low, and the temperature increase of which it is capable in a single pass is relatively high as a consequence. For modularity purposes, counterflow-heat-exchanger module102in the illustrated embodiment is essentially identical to counterflow-heat-exchanger module104, but it receives a much higher volume flow rate, and the temperature increase that it can impart is correspondingly low. So the cold-water flow through counterflow-heat-exchanger module102also flows serially through further modules106,108, and110to achieve a temperature increase approximately equal to module104's.

The series-connected modules' output from outlet110C—OUTis fed to a degasser112, as is the single heat exchanger104's output from outlet104C—OUT. For the sake of simplicity,FIG. 2omits the degasser, but the degasser would typically enclose the motor38to absorb heat from it. The degasser thus further heats the liquid. Together with the heat imparted by the counterflow heat exchangers, this heat may be enough to raise the feed-liquid temperature to the level required for optimum evaporator/condenser action when steady-state operation is reached. From a cold start, though, a supplemental heat source such as a heating coil (not shown) would in most cases contribute to the needed heat. The residence time in the degasser is long enough to remove most dissolved gasses and volatiles from the stream. The thus-degassed liquid then flows to a filter assembly114, where its flow through a filter body116results in particulate removal.

The resultant filtered liquid flows from the filter body116to an annular exit chamber118, from which it issues in streams directed to two destinations. Most of that liquid flows by way of tube119to a nozzle120. AsFIG. 9shows, nozzle120delivers the filtered feed liquid to the rotating-assembly shell36's inner surface, where it joins the liquid layer formed by the liquid that has flowed through the evaporation chambers without evaporating. Only a minor fraction of the liquid that flows into the evaporation chambers evaporates in those chambers in one pass, so most of it contributes to the rotating layer, whereas the feed nozzle120delivers only enough liquid to that layer to replenish the fluid that has escaped by evaporation.

Stationary scoop tubes122and124scoop liquid from this rotating layer. The scooped liquid's kinetic energy drives it along those tubes, whichFIG. 10shows in plan view andFIGS. 11 and 12show in cross-sectional views respectively taken at lines11-11and12-12ofFIG. 10. To minimize the kinetic energy's dissipation, each scoop tube bends gradually to a predominantly radial direction. Also, each scoop tube is relatively narrow at its entrance but widens gradually to convert some of the liquid's dynamic head into static head. Those tubes guide the thus scooped liquid into an interior chamber126(FIG. 11) of a transfer-valve assembly128. Ordinarily, a transfer-valve member130is oriented asFIG. 12shows. In this orientation it permits flow from the interior chamber126through entry ports132into spray arms58but prevents flow through a port134into a conduit136that leads to an upper entrance of FIG.8's filter assembly114. The static head drives the liquid up the spray arms.FIG. 13, which is cross-sectional view taken at line13-13ofFIG. 12, shows that each of the spray arms58forms a longitudinal slit138. These slits act as nozzles from which the (largely recirculated) liquid sprays into the evaporation chambers56depicted inFIG. 5.

In short, the liquid-collecting inner surface of the rotating-assembly shell36, the scoop tubes122and124, the transfer-valve assembly128, and the spray arms58form a guide that directs unevaporated liquid along a recirculation path that returns it to the evaporation chambers56. And, since FIG.8's nozzle120supplements the recirculating liquid with feed liquid, this guide cooperates with the main pump100, the counterflow heat exchangers102,104,106,108, and110, the degasser112, the filter assembly114, and the tubes that run between them as well as tube118and nozzle120to form a further guide. This further guide directs feed liquid along a make-up path from the feed inlet12to the evaporation chambers56.

Now, so long as its evaporator-chamber surfaces stay wetted, heat-transfer efficiency in the rotary heat exchanger is greatest when the water film on these surfaces is thinnest. The flow volume through the spray arms58should therefore be so controlled as to leave that film as thin as possible. In the illustrated embodiment, the flow rate through those spray arms is chosen to be just high enough to keep the surfaces from drying completely between periodic wetting sprays from a scanner140best seen inFIG. 9. The scanner includes two scanner nozzles142and144that provide a supplemental spray at two discrete (but changing) heights within the rotary heat exchanger.

The nozzles' heights change because a drive rod146reciprocates, in a manner that will presently be described in more detail, to raise and lower a yoke148from which the scanner140extends. Control of the scanner feed is best seen inFIG. 14, which is a cross-sectional view, with parts removed, of the vapor-chamber housing40's lower interior.FIG. 14depicts the valve member130in the closed state, but when the valve member130is in its opposite, open state, it permits flow not only into the spray tubes' ports132but also into a path through a separate feed conduit150by way of an internal passage not shown into a vertically extending tube152. A telescoping conduit154that slides in tube152conducts the flow, as best seen inFIG. 9, through the yoke148and into the scanner140. So these elements guide liquid along a further branch of the recirculation and make-up paths.

As the reciprocating rod146drives the yoke148and thereby the scanner140up and down, successive evaporation chambers momentarily receive a supplemental liquid spray. This spray is enough to wet the evaporator surfaces if they have become dry, or at least to prevent them from drying as they would if they were sprayed only through the spray arms58. The flow rate experienced by each of the evaporation chambers is therefore cyclical. The steady flow from the spray arms can be low enough not to keep the surfaces wetted by itself. Indeed, the cyclical spray can keep the surfaces wetted even if the average flow rate that results when the supplemental scanner spray is taken into account would not be great enough to keep the surface wetted if it were applied steadily.

Under testing conditions that I have employed, for example, the irrigation rate required to keep the plates wetted is about 4.0 gal./hr./plate if the irrigation rate is kept constant. But I have been able to keep the heat-transfer surfaces wetted when the spray arms together sprayed 216 gal./hr. on 216 plates, or only 1.0 gal/hr./plate. True, this spray was supplemented by the spray from the scanner. But the scanner nozzles together contributed only 30 gal./hr. Since the scanner nozzles together overlap two evaporation chambers in my prototype so as to spray an average of four plates at a time, this meant that the scanner sprayed each plate for about 4/216=1.9% of the time at about 30 gal./hr. ÷4 plates=7.5 gal./hr./plate. Although the resultant peak irrigation rate was therefore 8.5 gal./hr./plate, which exceeds the constant rate required to keep the plates wetted, the average irrigation rate was only 1.14 gal./hr./plate, or only 28% of that constant rate of 4.0 gal./hr./plate. Such a low rate contributes to heat-exchanger efficiency, because it permits the average film thickness to be made less without drying than would be possible with only a steady spray. While it is not necessary to use these particular irrigation rates, most embodiments of the present invention will employ average rates no more than half the constant rate required for wetting, while the peak rate will exceed that constant rate.

The manner in which the scanner140's reciprocation is provided is not critical to the present invention; those skilled in the art will recognize many ways in which to cause reciprocation. But the way in which the illustrated embodiment provides the reciprocation is beneficial because it takes advantage of the mechanisms used to refresh the rotary-heat-exchanger fluid and to back flush the filter. To understand those mechanisms, it helps to refer toFIG. 14.

FIG. 14shows that the transfer-valve assembly128is provided on a vapor-chamber base160sealingly secured to the vapor-chamber housing40's lower annular lip162. Together that lip and the vapor-chamber base can be thought of as forming a secondary, stationary sump that catches any spillage from the main, rotating sump. The heating coil mentioned above for use on startup may be located in that sump and raise the system to temperature by heating sump liquid whose resultant vapor carries the heat to the remainder of the system.

Among the several features that the vapor-chamber base160forms is a vertical transfer-pump port164, through which the drive rod146extends. That rod extends into a transfer pump166thatFIG. 14omits butFIG. 15illustrates in cross section. The transfer pump166includes an upper cylinder half168that forms a cylindrical lip169, which mates with the transfer-pump port164ofFIG. 14. It also forms a flange170by which a bolt172secures it to a corresponding flange174formed on a lower cylinder half176.FIG. 15also depicts a mounting post178, which is one of two that are secured to FIG.14's vapor-chamber base160and support the transfer pump116by means of flanges, such as flange180, formed on the upper cylinder half168.

A piston182is movably disposed inside the transfer-pump cylinder that halves168and176form, and a spring184biases the piston182into the position thatFIG. 15depicts. As that drawing illustrates, the drive rod146is so secured to the piston182as to be driven by it as the piston reciprocates in response to spring184and fluid flows that will now be described by reference toFIG. 8.

It will be recalled that the filter assembly114's output is divided between two flows. In addition to the liquid-make-up flow through tube119to the feed nozzle120, there is a second, smaller flow through another tube186. This tube leads to a channel, not shown inFIG. 14, that communicates with an upper section188, whichFIG. 14does show, of the transfer-pump port164. During most of its operating cycle, the piston182shown inFIG. 15moves slowly downward in response to the force of its bias spring184and thereby draws liquid from FIG.8's tube186through port164into the portion of the transfer pump's interior above the piston182. As will be seen, this portion serves as a refresh-liquid reservoir, and the components that guide feed liquid from FIG.8's feed inlet12through the filter assembly114cooperate with tube186and port164to form a guide that directs feed liquid along a feed-liquid-storage path into that reservoir.

As will also be seen, the pump's lower portion serves as a concentrate reservoir. While the piston is drawing liquid into the refresh-liquid reservoir, it is expelling liquid from the concentrate reservoir through an output port190formed, asFIG. 15shows, by the lower cylinder half176. The lower cylinder half further forms a manifold192. One outlet194of that manifold leads to the filter assembly114, whichFIG. 15omits butFIG. 16depicts in cross section.FIG. 16shows that the filter assembly includes a check valve196that prevents flow into the filter assembly from manifold outlet194. AsFIG. 15shows, the flow leaving the transfer pump from its lower outlet190must therefore flow through the other manifold outlet198.

FIG. 8shows that a tube200receives that transfer-pump output. A flow restricter202in that tube limits its flow and thus the rate at which the transfer-pump piston can descend. By thus limiting the transfer-pump piston182's rate of descent, flow restricter202also limits how much of the filter assembly114's output flows through tube186into the transfer pump166's upper side, with the result that the transfer pump receives only a small fraction of the filter output and thus of the output from the input pump100. A flow divider comprising a flow junction203and another flow restricter204so controls the proportion of pump100's output that feeds counterflow-heat-exchanger module104's cold side that this cold-side flow approximates the hot-side flow that flow restricter202permits: main pump100's output is divided in the same proportion as the transfer pump166's output is. As was mentioned above, the resultant relatively low flow rate into module104is what enables the entire heat transfer to occur in a single module104, whereas the higher flow rate through modules102,106,108, and110necessitates, their series combination.

Because of the flow restricter202, FIG.15's transfer-pump piston182moves downward under spring force at a relatively leisurely rate, taking, say, five minutes to proceed from the top to the bottom of the transfer-pump cylinder. As the piston descends, it draws the drive rod146downward with it, thereby causing FIG.9's scanner nozzles142and144to scan respective halves of the rotary heat exchanger's set of evaporation chambers. At the same time, it slides an actuator sleeve206provided by yoke148along an actuator rod208.

AsFIG. 17shows, a spring mount210is rigidly secured to the actuator rod208and so mounts a valve-actuating spring212that the spring's tip fits in the crotch214of a valve crank216. The spring engages the crank in an over-center configuration that ordinarily keeps that actuator rod208in the illustrated relatively elevated position. The valve crank216is pivotably mounted in the transfer-valve assembly and secured to FIG.12's transfer-valve member130to control its state.

When the valve crank216is in its normal, upper position depicted inFIG. 17, the transfer-valve member130is in the lower position, depicted inFIG. 12, in which it directs liquid from the scoop tubes122and124(FIG. 10) to flow into the spray arms58and scanner140but not into the filter inlet port134. As FIG.9's yoke148continues its descent, though, its actuator sleeve206eventually begins to bear against a buffer spring218that rests on the spring mount210's upper end. The resultant force on the mount and thus on the actuator rod208overcomes the restraining force of FIG.17's valve-actuating spring212, causing the valve crank216to snap to its lower position. It thereby operates FIG.12's valve member130from its position illustrated inFIG. 12to itsFIG. 18position, in which it redirects the scoop-tube flow from the spray arms58to the conduit136that feeds the filter assembly's upper inlet220(FIG. 16).

Now, whereas fluid ordinarily flows through the filter at only the relatively low rate required to compensate for evaporation, the flow directed by this transfer-valve actuation into the filter is the entire recirculation flow; that is, it includes all of the liquid that has flowed through FIG.5's evaporation chambers56without evaporating. Since only a relatively small proportion of the liquid that is fed to the evaporation chambers actually evaporates in any given pass, the recirculation flow is many times the feed flow, typically twenty times.

The pressure that this high flow causes within the filter assembly opens the filter assembly's check valve196(FIG. 16) and thereby permits the recirculation flow to back through the outlet194of FIG.15's transfer-pump-output manifold192and, because of the resistance offered by flow restricter202(FIG. 8), back through the transfer pump's outlet190to the concentrate reservoir. With the transfer valve in this state, that is, the scoop tubes122and124(FIG. 10), the transfer-valve assembly128, and the filter assembly114(FIG. 16) form a guide that directs concentrate from the liquid-collecting inner surface of the rotating-assembly shell36(FIG. 9) along a concentrate-storage path to the transfer pump's concentrate reservoir.

That redirected flow flushes the filter so as to reduce its impurities load and thus the maintenance frequency it would otherwise require. It also drives the transfer-pump piston182(FIG. 15) rapidly upward. The piston in turn rapidly drives the feed liquid that had slowly accumulated in the transfer pump's upper, refresh-reservoir portion out through the vapor-chamber base's port164(FIG. 14) along a refresh path. AsFIG. 14shows, that is, it flows into ports132by way of a check valve224provided to prevent recirculation flow from entering the refresh reservoir. With that flow now redirected to the transfer pump's lower side, i.e., to the concentrate reservoir, the resultant rapid flow through the check valve224and ports132enters the spray arms58and scanner140, replacing the temporarily redirected recirculation flow. All this happens in a very short fraction of the recirculation cycle. In most embodiments, the duration of this refresh cycle will be only on the order of about a second, in contrast to the recirculation cycle, which will preferably be at least fifty times as long, typically lasting somewhere in the range of two to ten minutes.

The effect of thus redirecting the feed and recirculation flows is to replace the rotary heat exchanger's liquid inventory with feed liquid that has not recirculated. As was explained previously, the rotary heat exchanger continuously removes vapor from the evaporation side, leaving impurities behind and sending the vapor to the condensation side. So impurities tend to concentrate in the recirculation flow. Such impurities may tend to deposit themselves on the heat-exchange surfaces. Although the periodic surface flushing that the scanner nozzles perform greatly reduces this tendency, it is still desirable to limit the impurities concentration. One could reduce impurities in a continuous fashion, continuously bleeding off some of the recirculation flow as concentrate exhaust. But the illustrated embodiment periodically replaces essentially the entire liquid inventory on the rotary heat exchanger's evaporation side. This results in an evaporator-side concentration that can average little more than half the exhaust concentration. So less water needs to be wasted, because the exhaust concentration can be higher for a given level of tolerated concentration in the system's evaporator side.

As the transfer-pump piston rises rapidly, it slides FIG.9's actuator sleeve206upward rapidly, too. Eventually, the sleeve begins to compress a further buffer spring226against a stop230that the actuator rod208provides at its upper end. At some point, the resultant upward force on the actuator rod208overcomes the restraining force that FIG.17's valve-actuating spring212exerts on it through the spring mount210, and the actuator rod rises to flip the valve crank216back to its upper position and thus return the transfer valve130to its normal position, in which the recirculation flow from FIG.9's scoop tubes122and124is again directed to the spray arms and scanner. So the unit returns to its normal regime, in which the transfer pump slowly expels concentrate from its concentrate reservoir and draws feed liquid through the feed-liquid storage path to its refresh-liquid reservoir. AsFIG. 8shows, tube200, counterflow-heat-exchanger module104, and a further tube232guide the concentrate thus expelled along a concentrate-discharge path from manifold outlet198to the concentrate outlet16.

To achieve approximately the same peak concentration in different installations despite differences in those installations' feed-liquid impurity levels, different refresh-cycle frequencies may be used in different installations. And, since the typical feed-liquid impurity level at a given installation may not always be known before the unit is installed-or at least until rather late in the distiller's assembly process-some embodiments may be designed to make that frequency adjustable.

For example, some embodiments may make the piston travel adjustable by, for instance, making the position of a component such as FIG.9's stop230adjustable. In the illustrated embodiment, though, that travel also controls scanner travel, and any travel adjustability would instead be used to obtain proper scanner coverage. So one may instead affect frequency by adjusting the force of FIG.15's transfer-pump spring184. This could be done by, for instance, making the piston182's position on the drive rod146adjustable. Refresh-frequency adjustability could also be provided by making the flow resistance of FIG.8's flow restricter202adjustable.

In any case, flow restricter204, which balances the two counterflow-heat-exchanger flows to match the relative rate of concentrate discharge, would typically also be made adjustable if the refresh-cycle frequency is. The flow restricters could take the form of adjustable bleed valves, for instance.

Having now described the distillation unit's rotary heat exchanger, we will describe one of its counterflow-heat-exchanger modules. Before doing so, though, we return toFIG. 8to complete the discussion of the fluid circuit in which those modules reside. The flow of purified liquid that issues from FIG.7's condensate scoop tube94is directed to FIG.8's accumulator236, which the drawings do not otherwise show. The accumulator236receives condensate in a resiliently expandable chamber. The accumulator's output feeds heat-exchanger module110's hot-water inlet110H—INto provide the hot-side flow through the serial combination of heat exchangers110,108,106, and102. A condensate pump238drives this flow. After being cooled by flow through the serial heat-exchanger-module combination, the cooled condensate issues from module102's “hot”-water outlet102H—OUTand flows through a pressure-maintenance valve240and the concentrate outlet16. Valve240keeps the pressure in the hot sides of counterflow heat exchangers102,106,108, and110higher than in their cold sides so that any leakage results in flow from the pure-water side to the dirty-water side and not vice versa.

The main pump100's drive is controlled in response to a pressure sensor242, which monitors the rotary heat exchanger's evaporator-side pressure at some convenient point, such as the transfer valve's interior chamber. Finally, to accommodate various leakages, tubes to the drain outlet18may be provided from elements such as the pump, pressure-maintenance valve, and sump.

It can be seen from the description so far that the counterflow-heat-exchanger modules102,104,106, and108act as a temperature-transition section. The rotary-heat-exchanger part of the fluid circuit is a distiller by itself, but one that relies on a high-temperature input and produces high-temperature outputs. The counterflow-heat-exchanger modules make the transition between those high temperatures and the relatively low temperatures at the feed inlet and condensate and concentrate outlets. The counterflow-heat-exchanger modules in essence form two heat exchangers, which respectively transfer heat from the condensate and concentrate to the feed liquid. We now turn to one example of the simple type of counterflow-heat-exchanger module that this arrangement permits.

FIG. 19, which is an isometric view of counterflow heat exchanger102with parts removed, shows tubes that provide its cold-water inlets102C—INand102C—OUT. It also shows the hot-water outlet102H—OUTbut not the hot-water inlet, which is hidden.FIG. 20is a cross section taken through the cold-water inlet102C—INand the hot-water outlet102H—OUT. That drawing shows that heat exchanger102includes a generally U-shaped channel member250, which provides an opening252that communicates with the heat exchanger's “hot”-side outlet. Similar openings254in a cover258and gasket260(both of whichFIG. 19omits) provide the cold-water inlet102C—IN. A folded stainless-steel heat-transfer sheet262provides the heat-exchange surfaces that divide the cold-water side from the hot-water side, and elongated clips264secure the folded sheet's flanges266, channel-member flanges268, cover258, and cover gasket260.

AsFIG. 19shows, spacer combs270are provided at spaced-apart locations along the heat exchanger's length. One spacer comb270's teeth272are visible inFIG. 20, and it can be seen that the teeth help to maintain proper bend locations in the folded heat-transfer sheet262. Similar teeth274of a similar spacer comb at the opposite side of the heat-transfer sheet262also serve to space its bends.

FIG. 19shows the upper surfaces of diverter gaskets278, which extend between the upper spacer combs270and serve to restrict the cold-water flow to regions close to the folded heat-transfer sheet262's upper surface.FIG. 19also shows that the module includes end plates280and281. These end plates cooperate with the channel member250, the cover258, and the cover gasket260to form a closed chamber divided by the sheet262. Additionally, the leftmost diverter gasket278cooperates with the end plate280and the cover258and cover gasket260to form a plenum282(FIG. 20) by which cold water that has entered through port102C—INis distributed among the heat-exchange-surface sheet262's several folds.

End plate280similarly cooperates with another diverter gasket284(FIG. 20) to form a similar plenum286by which water on the hot-water side that has flowed longitudinally along the heat-exchange surfaces issues from the heat exchanger102by way of its hot-water outlet102H—OUT. Incoming hot-side water and outgoing cold-side water flow through similar plenums at the other end.

It can be appreciated from the foregoing description that the present invention's teachings can significantly increase an evaporator-and-condenser unit's operating efficiency. It thus constitutes a significant advance in the art.