空气压缩机特性技术分析.ppt

* The variable speed control is steered by a pressure transmitter mounted at the compressor outlet.the transmitter develops a 4 to 20 mA signal which is proportionate to the pressure at the point. Depending on the strength of the current signal, the variable speed drive increases or decreases the supply frequency to the electric motor. This causes the motor to speed up or to slow down, there by controlling the capacity of the compressor in relation the air demand. Variable speed controls constitute the most efficient means of controlling air and gas compressors and save a great deal of energy for their users. * Let us now look at the controls of a dynamic machine. We consider two situations: the demand of air falls within the effective turn-down range of the machine the demand of air falls below the effective turn down range . * For con trolling the machine within the turn down range, two methods are employed. The inlet throttle valve is a butterfly valve, which completely opens when the machine is required to run at full capacity and partially closes when the demand reduces. The problem with this control is that local air velocities become very high,there by causing turbulence. This reduces the aerodynamic efficiency and wastes energy. The in the inlet guide vane control on the other hand, smoothly guides the air to change the velocity triangles,and impart a pre-whirl to the air, in order to achieve Capacity control.This method causes minimum turbulence and is therefore more efficient at part loads * This diagram shows the energy savings of an inlet guide vane system as against a throttle control at part loads.Savings of up to 9% can be realized, depending on the load levels An Inlet guide vane control involves higher initial cost, but this is quickly paid back with the savings,realized. * When the air requirement falls below the surge control point, there are two ways to control a centrifugal machine: The Modulated blow off control throttles