Pressure control in centrifugal evaporators

1. A method of operating a vacuum drying chamber having a vent valve, a non-return valve leading to a vacuum pump and a pressure control valve which communicates with the inlet to the pump, comprising the steps of:

(a) closing the vent valve;

(b) opening the pressure control valve, thereby allowing air at atmospheric pressure to enter through the pressure control valve and subjecting the non-return valve to atmospheric pressure on the pump side; and

(c) subjecting the non-return valve to a lower at least partial vacuum pressure in the chamber on the other side, such that the differential pressure keeps the non-return valve closed, thereby sealing the chamber from atmosphere while the open pressure control valve provides a ready supply of air at atmospheric pressure to the pump inlet which thus maintains a high flow rate therethrough, thereby to clear any solvent from the interior of the pump.

2. A method as claimed in claim 1 including the step of varying the chamber pressure by adjusting the amount by which the pressure control valve is opened.

3. A method of controlling the pressure within a chamber of a drying apparatus from which air and vapour is removed by a pump which is operated continuously during the drying process, wherein the chamber has a vent valve and a non-return valve is provided between it and the pump, and the pump has a pressure control value which communicates with an inlet thereof, comprising the steps of:

(a) opening both the vent valve and the pressure control valve so that air at atmospheric pressure is drawn by the pump directly from the pressure control valve, and via the chamber and the non-return valve from the vent valve;

(b) closing both vent and pressure control valves when the pressure in the chamber is to be reduced to allow the pump to remove air, gas and vapour from the chamber via the non-return valve;

(c) monitoring the dropping chamber pressure by a pressure transducer; and

(d) opening the pressure control valve after a required chamber pressure has been reached while the vent valve remains closed, whereby a high rate of airflow is maintained through the pump to clear the interior of the pump of any residual solvent.

4. Vacuum drying apparatus comprising a drying chamber, control means for controlling the pressure within the chamber, a pump arranged to remove air, gas and vapour from the chamber, a vent valve which when open admits air to the chamber from outside the apparatus, a pressure control valve which when open admits air directly to the pump inlet from outside the apparatus, and a non-return valve between the chamber and the pump inlet, wherein the control means is arranged to close both vent and pressure control valves when the pressure in the chamber is to be reduced by the removal of air, gas and vapour from the chamber by the pump through the non-return valve, and to at least partly open the pressure control valve while keeping the vent valve closed when a given partial vacuum pressure is achieved in the chamber, whereby a high rate of air flow from the pressure control valve is maintainable the pump by continuous operation thereof during the drying process to clear the interior of the pump of solvent, while the chamber pressure remains substantially constant.

5. Apparatus as claimed in claim 4 further comprising a pressure transducer means adapted to monitor the chamber pressure and to provide a signal to the control means.

6. Apparatus as claimed in claim 5 wherein the transducer signal is proportional to the chamber pressure and the control means compares the chamber pressure signal value with a programmed pressure value corresponding to the given chamber pressure to generate a signal to open the pressure control valve when the given pressure is reached.

7. Apparatus as claimed in claim 5 wherein the transducer means includes input means by which a given pressure can be entered and the transducer means delivers a signal to the control means when the given pressure is detected in the chamber, to open the pressure control valve.

8. Apparatus as claimed in claim 4 wherein the non-return valve is a flap valve.