- At a star-delta start of electric motors it is often considered necessary to limit the compression work of the machine at the starting moment in order to reduce the starting torque of the electric motor.
- Usually, a solenoid valve is used in a by-pass arrangement which in the starting up phase short circuits the discharge side to the suction side of the compressor.
- At the same time, a non-return valve must be fitted in the discharge line to the condenser preventing the return flow of discharge gas to the compressor.
- When the electric motor has reached its max. number of revolutions per minute, a switch takes place from star to delta start.
- The solenoid valve is closed and the compressor now works under normal conditions.
Any opening in a pressure vessel is kept to a minimum and for a man entry an elliptical hole is lesser in size than the corresponding circular hole. More over it is prime concern to have a smoothed generous radius at the corners to eliminate stress concentration. Hence other geometrical shapes like rectangle and square are ruled out. To compensate for the loss of material in the shell due to opening, a doubler ring has to be provided around the opening. The thickness of the ring depends on the axis length along the dirrection in which the stresses are maximum and the thickness of the shell. It is important to align the minor axis along the length of the vessel, as the stress in this direction is maximum. Longitudinal stress: Pd/2t where P= pressure inside the vessel, d= diameter of the arc, t= thickness of the shell plating Circumferential stress: Pd/4t More over a considerable material and weight saving is achieved as minor is along the direction of maximum stress.