(1) Field of the Invention
The present invention relates to a process for cleaning containers, in particular containers such as casks or kegs. Particularly, the present invention relates to a process for cleaning such containers in which a cleaning solution is poured into a container, after which the container is rinsed with a suitable rinsing fluid such as water or the like. The present invention is further directed to an apparatus for cleaning containers in accordance with this process.
(2) State of the Prior Art
In the beverage industry, as in many other branches of industry, large fluid containers are used. In the beverage industry, such containers are casks or kegs that can have valves mounted thereto. These types of containers are normally returnable to the supplier after use by the customer for refilling at a bottling plant. Prior to a refilling operation, however, the containers have to be cleaned in order to remove any residue from the beverage or other liquid that was previously in the container and any other contaminants. The beverage industry, being a part of the food industry overall, has particularly high demands placed thereon for the degree of cleanliness of the containers used for beverages.
Cleaning of such containers is normally performed by first pre-rinsing the container in order to remove the coarsest residue. Thereafter, the container is rinsed with a cleaning solution having a high chemical content, or a similar cleaner, so that even stubborn contaminants are removed due to the high chemical content of the solution. Of course, before each container can be refilled and reused, the container has to be completely chemical free. As such, the containers are "post-rinsed" with a post-rinsing fluid, usually water. This post-rinsing operation is customarily carried out over a fixed period of time that is sufficient to ensure that the container will be completely chemical free. Certain precautionary steps, however, are also taken in order to account for situations where, for example, the chemical concentration of the cleaning solution or cleaning agent may be raised due to some malfunction in the cleaning operation. These precautionary steps involve having the post-rinsing take place over a considerably longer period of time than would normally be necessary to clean the container. While this of course results in an additional consumption of water, this is accepted for safety reasons.
Nor is it possible to check the post-rinsing fluid as it flows out of the container after rinsing to determine when the fluid becomes totally free of chemicals. If such a check could be made, the post-rinsing operation could be controlled, for example by means of a pH meter. Conventional solutions that are used as cleaning fluids have concentrations of 1.5 to 2.5%, and thus a pH value of 14. However, this pH value is obtained when the cleaning solutions are rinsed out of a container to such a degree that the post-rinsing fluid draining from the container shows only a residual concentration of about 0.4%. It is not until the concentration drops below this value that the pH value will fall rapidly. When the post-rinsing operation is completed, a pH value of 7 (neutral value) is reached. However, with the known methods of measuring, it is not possible to determine the pH value at the same time as the pH is dropping in the post-rinsing fluid draining out of the container. A conventional pH measuring cell will need a considerably longer period of time in order to fall from a pH value of 14 to a pH value of 7 than is actually required for the post-rinsing operation. Thus the reaction speed of conventional pH measuring cells is inadequate to control the post-rinsing operation during the cleaning operation of the containers.
German patent No. 3,424,711 uses at least one conductivity measuring probe in both an approach flow and a return flow of a cleaning agent being conveyed into and then away from an object being cleaned. A conductivity comparison element continuously processes paired readings of the probes and transmits a signal when the values of the paired readings reach a predetermined minimal difference. That is, when the conductivity measuring probe is located both in the approach flow at the entry to the working area of the cleaning facility and in a return flow at the exit from the working area, the change of the electrical conductivity of the cleaning agent as it passes through the work area can be determined. When the change falls below a pre-established minimal difference, the particular cleaning phase can be terminated and another working phase can be subsequently initiated.
However, in this patent the post-rinse phase requires the addition of a certain delay or run-out time. Furthermore, the present inventor has found that using electrical conductance as a parameter for stopping the introduction of a rinsing fluid is unsuitable. Employment of electrical conductance was one of the subject matters set forth in parent application 08/382,904. However, in subsequent tests, it has been found that when using hot water as a rinsing fluid in the last rinsing phase, the electrical conductance reaches the value of the hot water after six seconds, i.e. at a first measuring step. Thus using electrical conductance in practice has been found not to provide acceptable results.