Background of the invention Water steam of high quality is required, for example in pharmaceutical and biotechnical production, for sterilization and other purposes. Especially pure steam can be defined as "water for injection"in the gas phase. In addition to being sterile and free of the impurities of the feed water, it is thus also pyrogen free.

In the production of pure water steam, it is essential that droplets of water and other non- gaseous impurities, as well as other gases than water steam are removed from the product steam flow. For this purpose, devices are normally fitted with various demisting baffles, cyclones and corresponding structures, which act to stop non-gaseous components. In a falling-film steam evaporator e. g. of the type described in US patent 3,875,017, precleaned and preheated feed water enters at the top of the device and evaporates traveling as a falling film down the heat transfer surfaces. The product flow thus arrives at the lowermost point of the device, turns 180° and then flows in a spiral pattern upwards, whereby impurities separate by centrifugal force to the periphery of the spiral path and depart, flowing down- wards as the pure product flow continues it upward motion. A small volume of water is formed in the bottom section of the device, the impurities thus collecting into that volume.

This amount of water, having a certain ratio to the amount of produced steam, exits as a reject stream. For retaining the quality of the produced steam, said ratio must be kept within certain limits.

By the same method, distillate may be produced by condensing the product steam to water.

Monitoring of product steam is a way of controlling the proper operation of the device, but such a measurement based on, for example, conductivity, signals a decreasing product steam quality too late. Continuous measurement of the reject water flow is very difficult

due to the irregularity of the flow produced by a valve or a moisture trap, and for this rea- son measurement of reject water has not been practiced earlier in the field of stills or steam generators.

Disclosure of the invention A method according to claim 1 has now been invented for measuring the amount of reject water from a steam generator or a still, in which two level sensors, provided in the reject water exit tube with a specified vertical distance relative to each other, control a valve ar- ranged below said sensors to effect opening of the valve as the water level reaches the up- per level sensor, and to effect closing of the valve as the water level reaches the lower level sensor.

A further object of the invention is a device according to claim 2, comprising a non- horizontal reject water exit tube or a branch or vessel connected thereto provided with a first level sensor to indicate a water level reaching said first level sensor, and at a lower level than said first level sensor a second level sensor to indicate a water level reaching said second level sensor, and at a lower level than said second level sensor a valve arranged to open by the signal from the first level sensor, and to close by the signal from the second level sensor.

Preferably, the level sensors are provided in a branch of the exit tube, the lower end of which branch communicates with the exit tube above the valve, while the upper end com- municates with the bottom section of the still or steam generator. In principle, the level sensors may be fitted in the exit tube proper, or in any side vessel communicating with the exit tube so as to provide a common water level. The type of level sensor may be swim- mer-switch, inductive, or any other type suitable for the conditions, as the person skilled in the art will contemplate.

According to the invention, the ambiguities relating to the measurement of reject water connected with the solutions according to the prior art are avoided. As the volume corre- sponding to the vertical distance of the level sensors is known, and the rate of opening and closing of the valve is recorded over time, the water flow per unit time can be determined.

As the flow rate of feed water is also known, the reject ratio of the still or steam generator is easily calculated by means of control electronics well known to the skilled person.

The system may include additional level sensors, for example mounted on a higher level than the aforementioned upper sensor. Such a sensor may provide, for example, a high level alarm indicating malfunction of the sensors below.

Brief description of the drawing Figure 1 shows a schematic view of a steam generator according to the invention; the gen- erator may constitute part of a still apparatus.

Description of an embodiment The invention is disclosed in greater detail below, with reference to the enclosed drawing.

Preheated feed water enters the still or steam generator (1) through line (2) and evaporates in the downflow channels (3), heat being transferred from the heating steam (4). In the bottom section of the device, the flow makes a 180° turn, and is freed of droplets and for- eign matter in the upflow channels (6). During operation, a minor volume of water en- riched in impurities is formed in said upflow channel and collects in the bottom section (5) of the device, and exits as a reject stream through line (7).

For measurement of the reject water flow, a branch (8) is provided in line (7), the upper part of which branch communicates with the bottom section of the still or steam generator.

As water accumulates in the tubing above closed valve (9) during operation of the still or steam generator, after some time the level reaches level sensor (10). Sensor (10) thereby transmits a signal to the control system, which opens valve (9). Water flows out until the level reaches sensor (11), the signal of which effects the closing of valve (9).

The volume of tubing (7,8) corresponding to the vertical distance of level sensors (10,11) is easily determined; as the control electronics monitors the rate of opening of valve (9), the flow rate of reject water is thus defined with sufficient accuracy.

When using the apparatus as a still, the product steam is condensed in condenser (12).