A jet heater is not an ejector at all, even though it may resemble one externally. It is a mixing device for steam and water. Steam is condensed, giving up its latent heat of vaporization to heat water flowing through the unit. (Note: Other condensable vapors can also be used to heat up similar liquids. One example would be in a soybean oil plant, where cyclohexane vapors are used to heat up cyclohexane liquid.)
A jet heater consists of a water nozzle, a mixing tee with a knife edged orifice to ensure good contact between the water and steam and a mixing tube to provide residence time to assure that the steam is condensed. Sizing is based on water inlet and outlet velocities, steam inlet velocities and thermal duty.
A jet heater is not a self-regulating device and therefore the water and steam should be metered in through control valves. Care must also be taken that the outlet temperature is well below the boiling point of the water at the given outlet pressure. Otherwise cavitation and line hammer will occur.
A jet heater is not a pumping device. It provides no pressure increase across the unit. Generally, the steam must be a minimum of 5 psi higher that the water inlet pressure, and the outlet should be at a minimum of 5 psi lower that the water inlet pressure. For situations where the water needs to be heated with a pressure increase, a syphon should be used. For extremely large water and steam flowrates, a barometric condenser may also be used.