Is it better to use a high-level thermal deaerator or a low-level thermal deaerator?

A lower position is sufficient; a higher position wastes energy.

Working principle of the rotary diaphragm deaerator (jet, suction, turbulence, heat transfer, mass transfer, water film skirt, rain-like flow, saturation). Condensate water and make-up water first enter the water chamber of the rotary diaphragm assembly in the deaerator head; under a certain pressure difference due to the water level, they are ejected at an angle from the small holes in the diaphragm tubes into the inner holes, forming jets. Since the inner holes are filled with rising heated steam, the water, as it moves in these jets, draws in a large amount of heated steam along with it； Intense mixing and heating occur over a very short distance and a small distance traveled, causing the water temperature to rise significantly. The rotating water continues to spin along the inner wall of the membrane tube, forming a rolling layer of water film. In this state of turbulence, heat and mass transfer are most efficient, and the water temperature reaches its saturated value. Oxygen is thus separated, as it cannot diffuse freely within the inner pores and can only be carried away to the atmosphere via the steam rising through the exhaust pipe. The feed water that has undergone rough deoxygenation in the film-forming section and the condensate introduced through the condensate pipes are mixed here for secondary distribution, falling in a uniform shower-like pattern onto the liquid-vapor mesh installed below, and only after further deep deoxygenation do they flow into the water tank. The oxygen content in the water in the tank is 0–7 цг/L at high pressure, and less than 15 цг/L at low pressure, which meets the regulatory standards for operation.      Since the rotary film deaerator keeps the water in a turbulent state during operation and possesses a sufficiently large heat exchange surface area, heat and mass transfer are more efficient, the steam discharge volume is low, and the deaeration effect is good. The excess capacity thus generated allows the deaerator to operate under overload conditions or to meet the operational standards even when water is added at a low temperature.

For high and low positions, one should mainly consider the NPSH of the boiler feed pump; I’m not an expert in this area, so I’m not very knowledgeable about it.