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The size of the tank must be sufficient to prevent the low-pressure receiver from becoming over pressurized. The size of the receiver depends upon both the flow of condensate and the input steam pressure. Because the unit is considered a pressure vessel, it must be constructed in accordance with ASME code and is supplied with a pressure relief valve. The tank is also elevated 27" above the floor level.
As the pressurized high temperature returns enter the receiver, it hits the inlet cascade baffle. The purpose of the baffle is to slow the flow of the incoming condensate minimizing water turbulence inside the tank. The condensate flows out the sides of the baffle and drops into the receiver. A portion of the high-temperature condensate also flashes into steam. This flashing occurs when high-temperature condensate that is under pressure is discharged into the lower pressure receiver.
The thermostatic vent trap serves a dual function. At startup, the low pressure inside the receiver forces air and non-condensable gases, that might have accumulated within the receiver, out through the thermostatic vent trap.
During operation, the thermostatic vent trap functions as a mechanical trap. When steam reaches the trap, the thermostatic air vent closes in response to higher temperature -- keeping the steam inside the unit. However, as the temperature of the steam in the piping between the check valve and the thermostatic vent trap drops below the saturation point, the steam condenses. A small amount of condensate accumulates in the piping above the trap. At a certain point, the trap opens an orifice and discharges the condensate into the drain piping. This process is shown on the animation of the HT unit.
The returning condensate causes the water level in the receiver to rise, in turn causing the float to rise.
The proportioning controller controls the level of water in the tank by matching the amount of water discharged to the amount of water input to the tank. If the controller detects an increase in the tank's water level, it sends a signal to widen the opening of the proportioning motor valve (PMV) allowing an increased amount of water to be discharged.
The widened proportioning motor valve (PMV) causes the water level in the receiver to fall.
If the controller detects a decrease in the tank's water level, it sends a signal to tighten the opening of the proportioning motor valve (PMV) thereby reducing the amount of water discharged.