Steam turbines can be classified as either back-pressure or condensing.

Back-Pressure Steam Turbines

A back-pressure steam turbine reduces the pressure of the inlet steam to the steam turbine’s design back pressure. The exhaust steam from the turbine can then be used for process requirements e.g. heating tanks, comfort heating etc. Back pressure steam turbines are commonly used in industrial plants that require a large amount of process steam e.g. oil seed extraction plants and crude oil refineries. Note that back-pressure steam turbines are a type of non-condensing turbine.

Condensing Steam Turbines

Condensing steam turbines are used if the turbine is utilised solely for power generation i.e. it is connected to a generator and does not provide process steam to the plant. Unfortunately, the exhaust steam of condenser steam turbines must be condensed prior to being returned to the boiler (this is where condenser turbines obtain their name). This intentional cooling of the steam inherently leads to an efficiency reduction, due to the rejection of heat from the steam system.

Back-pressure turbines do not require a condenser as they exhaust steam at higher pressures; this steam is then used as process steam. Exhaust steam in a back-pressure turbine is not reduced to vacuum pressure, whereas a condenser turbine is. The total difference between the inlet and exhaust steam pressure of a turbine is what ultimately determines its overall efficiency. Therefore, a condenser steam turbine is more efficient than a back-pressure (non-condensing) steam turbine despite rejecting heat through a condenser. For power generation purposes, a typical condensing turbine may achieve operating efficiencies between 30-40%, compared to a non-condensing turbine which may achieve 15-35%. However, non-condensing turbines are cheaper.

Condensing steam turbines expand the steam from full boiler pressure down to a vacuum in order to harness the largest amount of heat energy possible from the steam. As we have already established, condensers have a negative effect on the system from an efficiency perspective, because we are rejecting heat. If the amount of waste steam discharged to the condenser can be reduced, this will result in efficiency gains. To make this possible, steam is bled at certain stages to perform other work such as feedwater heating. Whenever steam is bled, it is referred to as bleed steam, or extraction steam. Examples of bleed steam applications include condensate and boiler feed water heating (the steam is passed through a shell and tube heat exchanger). Bleed/extraction steam systems increase overall turbine efficiency because more heat from the steam is recovered rather than rejected.

In summary, both condenser and non-condensing turbine designs have advantages and disadvantages which have to be accounted for at the design stage in order to ensure the correct turbine design is suitable for its application. Selecting the correct turbine type is essential to obtaining a high overall plant efficiency.