Actual Vapor Cycle Process And Its Working Principle

The processes of an actual cycle differ from those ideal cycle. In actual cycle conditions might be as indicated. The thermal efficiency of cycle is

Nth = Wnet / Q1

where the work and heat quantities are the measured values for the actual cycle, which are different from the corresponding quantities of the ideal cycle.

T-S diagram

PIPING LOSSES :

Pressure drop due to friction and heat loss to the surroundings are the most important piping losses. States 1 ' and 1 represent states of the steam leaving the boiler and entering the turbine respectively.

1' - 1" represents the frictional losses, and 1" - 1 shows the constant pressure heat loss due to the surroundings. Both the pressure drop and heat transfer reduce the energy or availability of the steam entering the turbine.

A similar loss is the pressure drop in the boiler and also in the pipeline from the pump to the boiler. Due to this pressure drop, the water entering the boiler must be pumped to a much higher pressure than the desired steam pressure leaving the boiler, and this requires additional pump work.

TURBINE LOSSES :

The losses in the turbine are those associated with frictional effects and heat loss to the surroundings. The steady flow energy equation for the turbine

h1 = h2 + Wt + Q loss

Wt = h1 - h2 - Qloss

For the reversible adiabatic expansion, the path will be 1 - 2 s. For an ordinary real turbine the heat loss is small, and Wt is h1 - h2, with Q2 equal to zero. Since actual turbine work is less than reversible ideal work output, h2 is greater than h2s. How ever, if there is heat loss to the surroundings, h2 will decrease, accompanied by a decrease in entropy

Nt = Wt / (h1 - h2s) = (h1-h2) / ( h1 - h2s)

where Wt is the actual turbine work, and (h1 - h2s) is the isentropic enthalpy drop in the turbine (i.e , ideal output)

PUMP LOSSES :

The losses in the pump are similar to those of the turbine, and are primarily due to the irreversibilities associated with fluid friction. Heat transfer is usually negligible.

The pump efficiency is defined as,

'Np = (h4s - h3) / Wp = (h4s - h3) / (h4 -h3)

CONDENSER LOSSES :

The Losses in the condenser are usually small. These include the loss of pressure and the cooling of condensate below the saturation temperature.

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