Steam turbine

In a condensing machine the turbine exhaust goes directly to a condensor. In a non condensing machine, the exhaust seam pressure is at a much higher pressure than a condensing machine and that steam is generally used somewhere else in the process.

Non condensing machines are often used as steam let down stations. We have steam at 800 psi, we need steam at 200 psi, so we run it through a turbine to get the lower pressure, but at the same time we run a fan, or drive a generator.

Condensing turbine.

Steam expands when it goes from high pressure to low pressure. Steam flows from the boiler at a higher pressure and leaves the turbine exhaust at a lower pressure. The lower pressure can still run a turbine because it can still expand into an even lower pressure area. The turbine has a condenser installed at the exhaust. It removes the heat and lowers the pressure. The condenser ‑turns the steam into water.

Condensing turbines are a combination of impulse and reaction turbines. They are quite complicated but produce more power than most other types of turbine. They are used to drive compressors and power generators. To get the maximum power out of a condensing turbine, the size of the turbine blades must increase towards the exhaust. Condensing turbines can have a problem with erosion the steam at low pressure. because of water droplets in large pressure drops are common in condensing turbines, the steam pressure is reduced in steps. Therefore, condensing turbines are usually multi‑stage. The exhaust pressure of a condensing turbine is below atmospheric pressure.

Non‑condensing turbines.

The exhaust pressure of a non‑condensing turbine is higher than that of a condensing turbine, and is above atmospheric pressure. When the steam is condensed it can be returned to the boiler for reheating. A non‑condensing turbine does not use all of the thermal energy present in the steam. Using the same amount of steam, a condensing turbine produces more mechanical work.

Condensers are to be found where the steam circuit is closed. An example might be marine propulsion equipment. The job of the condenser in this case is to create a low pressure, a partial vacuum, on the downstream end of the propulsion equipment, so as to get a better use of the energy to be found in the steam, and then return the condensate to the boiler for re-use.

Closed curcuits are handy where not a lot of boiler feedwater can be found. A marine boiler surrounded by seawater is a case in point.