Washing process for an automatic dishwashing machine

Abstract: Washing process for an automatic dishwashing machine, including the steps of feeding a wash tub (5) of the machine with a predetermined water volume; adding a predetermined amount of detergent to said water volume; heating the so formed washing liquor; and subjecting the articles to be washed to the chemical and mechanical action of the washing liquor. After having reached a predetermined temperature (T1), the washing liquor is partially discharged and replaced with a corresponding quantity of water. The detergent is added in two steps, respectively before and after the partial discharge of washing liquor.

Description: The present invention relates to an improved washing process to be carried out in an automatic dishwashing machine. As is known, automatic dishwashing machines are provided with an electromechanical or electronic programme control device arranged to control the main operative devices of the machine so as to perform predetermined different programmes which can be selected by the user. A modern dishwashing machine, for example, is capable of performing operative processes, or programmes, consisting of one or more successive phases which may be selectively included in, or excluded from the programme. More particularly, an operative programme may include a prewashing phase with washing liquor at ambient temperature and/or a prewashing phase with heated washing liquor, an actual washing phase, a rinsing phase with water at ambient temperature and/or with heated water, a drying phase, etc. The operative programme consisting of the sole actual washing phase, so-called "washing process", is particularly important, also in the light of the current trends with regard to energy saving. As it is known, a washing process is carried out by supplying a water volume (usually at ambient temperature) into the wash tub of the machine; adding a metered amount of detergent (which is commonly a powder detergent including also surfactant substances) to the water volume; heating the so formed washing liquor; and subjecting the articles to be washed, which are exposed to the washing liquor, also to a mechanical action. In a dishwasher, in particular, the articles to be washed are sprayed with the washing liquor by means of special rotary spray arms, or the like. During the progress of the washing operation the detergent solubilizes the dirt (the organic fatty substances, for example) on the articles being washed and forms an emulsion thereof in the washing liquor which at least in part tends to deposit on the bottom of the wash tub. After a given time period, therefore, the detergent exerts its chemical action not only on the dirt which is still to be removed from the articles, but also on the dirt which has already been removed therefrom and suspended in the washing liquor or deposited on the bottom of the wash tub. More particularly, a part of the dirt released by the articles is deposited on the bottom water drain circuit of the dishwasher along with a corresponding amount of detergent which is then unused. In any case, after a certain time period from the beginning of the washing process the chemical action exerted by the detergent is partially wasted to attack an amount of dirt which has already been removed from the articles being washed. As already stated, in some known operative programmes the washing phase is preceded by a prewashing phase with heated washing liquor in order to obtain a more effective result. Mainly, the prewashing phase has the function of mechanically removing coarse dirt particles from the articles being washed, thereby promoting the effectiveness of the subsequent actual washing phase. To this aim, the prewashing phase terminates with a drain step in which the washing liquor is completely discharged, thereby discharging the dirt formerly removed; obviously, this reduces the above-mentioned problem of partially wasting the detergent used during the subsequent washing phase. However, the subsequent washing phase is carried out as described before, that is, by using a further water volume, to which a further amount of detergent is added and which has also to be completely heated up. Therefore, the whole washing process has an undesirably long duration and generally involves a remarkable consumption of water and detergent, as well as a corresponding energy consumption. Of course, this is in contrast with the current needs to save energy. From FR-A-2 128 575 it is also known a washing process for clothes washing machines including a washing phase in which the washing liquor is first heated up to a predetermined temperature and eventually discharged partially. After the washing phase, an additional washing phase is performed by adding a further amount of fresh water and a further amount of detergent to the remaining washing liquor, which is again heated up to said predetermined temperature. The additional washing phase replaces a traditional bleaching phase using corrosive bleaching agents, but undesirably involves prolongation of the whole washing process and substantial detergent and energy consumption. At any rate, no bleaching phase is commonly to be carried out in a dishwashing machine. It is the main scope of the present invention to provide a washing process for an automatic dishwashing machine by means of which a more effective remotion of dirt from the washed articles is obtained without the need of a substantial water, detergent and energy consumption. Another scope of the present invention is to provide a washing process of the aforementioned kind by means of which an effective remotion of dirt from the washed articles is obtained without the provision of a prewashing phase and substantially without lengthening the duration of the washing process itself. According to the invention, the scopes mentioned above are attained with a washing process for an automatic dishwashing machine embodying the features recited in the appended claims. The characteristics and advantages of the invention will be more apparent from the following description, given only by way of non-limiting example, with reference to the accompanying drawings, in which: Figure 1 diagrammatically shows an automatic dishwashing machine able to carry out the washing process according to the invention; Figures 2 and 3 are curves representing the changes in level and temperature, respectively, of the water fed into a dishwashing machine during a preferred embodiment of the washing process according to the invention. With reference to Figure 1, the dishwashing machine may be of a substantially common type and is not described in detail; for instance, it may be a dishwasher of the kind described in EP-A- 0 249 000 and is provided with a known detergent dispenser 7 having multiple compartments for selectively dispensing relevant quantities of detergent into a wash tub 5. The dishwasher is further provided with a programmer 4 capable of controlling the main operative devices of the machine to automatically perform one or more programmes, or processes, which can be selected by a user, particularly the washing process according to the present invention. The programmer 4 may be of the electromechanical type, or may be an electronic programmer comprising a Motorola 6804 or 6805 microprocessor, for example. In any case, it will be apparent to those skilled in the art that the programmer may readily be arranged so as to set the different parameters (e.g., the times and durations of operation of the various operative devices of the machine) determining the time-controlled progress of the process according to the invention. With reference also to Figures 2 and 3, the washing process according to the invention is substantially started at a time t0 by feeding the wash tub 5, via a supply circuit 6, with a certain volume (5-6 l, for example) of tap water at a given temperature T0 (see Figure 3). In a way known per se, during this step a first predetermined metered amount of detergent, which will be discussed later, is added to the water. Preferably, for instance as described in EP-A-0 118 719, the wash tub is filled with water by concurrently operating a circulating pump 8 capable of supplying at least a rotary spray arm 9 to spray the articles to be washed (not shown). The wash tub 5 is filled with water until a time t1, at which the water reaches a predetermined level L1 (see Figure 2). Substantially at this time the washing liquor so formed in the wash tub 5 begins to be be heated up, for instance by thermostatically controlled heating means, known per se and not shown. The washing liquor is heated until it reaches a first predetermined temperature T1, whilst the circulating pump 8 keeps being operated, preferably in a continuous way. The programmer 4 is arranged to actuate afterwards, during an interval t2-t3, a drain pump 10 capable of discharging the operative fluid collected on the bottom of the wash tub 5. The interval t2-t3 has a predetermined time duration (approximately 10 sec, for example), which depending on the general dimensioning of the machine is set so as to let a substantially small quantity of washing liquor, say about 1l, to be discharged, with the level of the washing liquor in the wash tub 5 accordingly decreasing to a value L2. Furthermore, the said temperature T1 preferably is set at such a value (at least 50°C, for example) as to allow dilution of the organic fatty substances forming part of the dirt on the articles to be washed. As a result, at time t2 a substantial amount of dirt is already released from the articles being washed and at least partially deposited on the bottom of the wash tub 5 along with a percentage of detergent which in practice cannot be used. The partial discharge occurring during step t2-t3, though it is negligible, enables the above amount of released dirt and unusable detergent to be substantially removed from the wash tub 5. The washing process can then progress with the heating means and circulating pump 8 in operation to spray the articles with washing liquor whose remaining percentage of detergent is used in practice to chemically attack only the dirt still to be removed from the articles being washed. In order to keep the circulating pump 8 under the best priming conditions, the water level in the wash tub 5 is preferably restored with fresh tap water after the aforementioned partial discharge, during a time period t3-t4. According to an aspect of the invention, such a level restoration, corresponding to the amount of washing liquor discharged during step t2-t3, is substantially negligible, say, not more than 1/5 of the water volume initially fed into the wash tub. Hence, the temperature of the washing liquor in the wash tub 5 decreases accordingly in a negligible way, down to a value T2. Therefore, the replacement of an amount of heated washing liquor with an equal amount of fresh water during time period t2-t4 advantageously does not involve a substantial loss of thermal energy. It is preferable, moreover, that at least a complementary metered amount of detergent be added to the washing liquor after step t2-t3 of partial discharge (e.g., during the water level restoration) in order to compensate for the percentage of detergent formerly discharged; as already stated, the complementary amount of detergent advantageously exerts its chemical action only on the remainder of the dirt. According to another aspect of the invention, the first metered amount and the complementary amount of detergent are such that their total quantity is substantially equal to the average amount of detergent which would commonly be added into the wash tub of the machine to carry out a usual washing process. Preferably, the first and the complementary amounts of detergent are substantially equal, each one corresponding to half the said common average amount. In case a powder detergent is used, for instance, a usual washing process is performed adding an average amount of 3g of detergent per liter of water supplied to the wash tub 5. By contrast, the washing process according to the invention provides addition of a first and a complementary amounts of detergent each one equal to 1.5g per liter of water. In a per se known manner, the washing process is performed after time t4 with operation of the circulating pump 8; in addition, the heating means are actuated until a time t5 at which the washing liquor reaches a predetermined temperature T3, which is higher than temperature T1 and has for instance a value of 65°C, approximately. The washing process is then terminated with a step t5-t6 in which only the circulating pump 8 is actuated, as well as a step t6-t7 in which the washing liquor is substantially completely discharged from the wash tub 5. Attention is directed to the fact that the washing liquor at the relatively higher temperature T3 promotes remotion of starchy substances from the articles being washed. As is known, starchy substances are particularly difficult to be removed, but according to the invention they are effectively attacked by substantially all the detergent contained in the washing liquor after step t2-t3. To summarize, the washing process according to the present invention advantageously involves differentiated remotion of dirt from the articles to be washed: fatty and dyeing substances are mainly removed at temperature T1 during step t1-t3, and starchy substances are removed at temperature T3 during step t5-t6. From the foregoing description it is apparent, and it was also experimentally found, that the washing process according to the invention is particularly effective to remove the dirt from the articles being washed, substantially as is a traditional washing process including, besides a washing phase, also a prewashing phase with heated washing liquor. On the other hand, the washing process according to the invention has a reduced duration, substantially equal to that of a usual single-phase washing process, involving also a water, detergent and energy consumption which is substantially equal with respect thereto. In addition, the remotion of a part of the dirt from the wash tub 5 through the partial discharge occurring during step t2-t3 reduces the total amount of dirt which could at least partially clog the recirculation filter (not shown) usually provided in a dishwasher. This results in a higher hydraulic pressure of the washing liquor jets issuing from the spray arms 9, thereby further improving the effectiveness of the washing process. The washing process according to the invention further provides an important advantage compared with a traditional washing process. In fact, the dilution of the washing liquor occurring during step t2-t4 reduces the final alkalinity of the washed articles, which thereby are more healthy in use. It was experimentally verified that, using the aforementioned exemplary parameters, the final alkalinity of the crockery subjected to the washing process according to the invention in an usual dishwasher is reduced by about 10%. Obviously, the washing process described above may undergo many modifications without departing from the scopes of the invention. For example, the quoted times, durations and amounts may be different, as the case may be.

Claims: Washing process for an automatic dishwashing machine, including the steps of feeding a wash tub (5) of the machine with a predetermined water volume; adding a predetermined metered amount of detergent to said water volume; heating the so formed washing liquor; subjecting the articles to be washed to the action of the washing liquor; discharging a predetermined partial quantity of the washing liquor from the wash tub (5) after the washing liquor has been heated up to a first predetermined temperature (T1); and eventually discharging substantially all of the washing liquor from the wash tub, characterized in that said partial quantity of washing liquor is equal to or lower than 1/5 of said predetermined water volume, the washing liquor in the wash tub (5) being further heated, after said partial quantity of washing liquor has been discharged, up to a second predetermined temperature (T3) which is higher than said first predetermined temperature (T1), this latter having a value of at least 50° C. Washing process according to claim 1, characterized in that said predetermined metered amount of detergent is added in at least two separate steps, respectively before and after said partial discharge of washing liquor. Washing process according to claim 2, characterized in that half the said predetermined metered amount of detergent is added during each one of said separate steps.

Inventors: MILOCCO CLAUDIO, MILOCCO, CLAUDIO
Applicants: ZANUSSI ELETTRODOMESTICI, ZANUSSI ELETTRODOMESTICI S.P.A.
Application year: 1991
Main classification: A47L15
IPC codes: A47L15