ABSTRACT
The system pressure of a flow is determined by the feed-in rate of a gas and the pumping-out rate of a vacuum system. The monomer flow-in rate is determined by the opening of an orifice (e.g., a metering valve) and the differential pressure applied across the orifice. The pumping-out rate, however, is determined by the overall pumping-out capability of a pump system. The latter is determined by the capacity of the pump, the size and length of the vacuum line that connects the pump and reactor, and the type of cold trap (or absence of it) used in the system. As will be shown, the cold trap, particularly a liquid N2 trap, acts as an excellent pump for many organic compounds. The system pressure of a flow can be controlled to a predetermined value by employing a throttle valve of which the opening is regulated by the pressure of the system within a certain range of flow rates. In order to establish a meaningful flow system LCVD operation, it is mandatory to have a pressure-controlling throttle valve in the pumping system.
