Ideal Regenerative Cycle And Its Process

 In order to increase the mean temperature of  heat addition (Tm1), attention was so far confined to increasing the amount of heat supplied at high temperatures, such as increasing superheat, using higher   pressure and temperature of  the steam, and using re-heat. The mean Temperature of heat addition can   also be increased by decreasing the amount of heat added at low temperatures. In a saturated steam   Rankine cycle, a considerable part of the total heat supplied is in the liquid phase when heating up water at a lower Temperature than T1, the maximum temperature of the cycle. For maximum efficiency, all heat should be supplied at T1, and feedwater should enter the boiler. This may be accomplished in what is known as an ideal regenerative cycle.

 

For any small step in the process of  heating the water,

Delta T (water) = - Delta T (steam)

Delta  S (water) = - Delta S (steam)

Q1 = h1 - h4' = T1 ( s1-s4') 

Q2 = h2' - h3' = T2 (s2' - s3)

Since 

s4' - s3 = s1 - s2'   or  s1 - s4' = s2' - s3 

n = 1 - (Q2 / Q1)  =  1 - (T2 / T1) 

The efficiency of  the ideal regenerative cycle is thus equal to the Carnot cycle efficiency.

Writing the steady flow energy equation for the turbine,

h1- Wt - h2' + h4 - h4' = 0 

Wt = (h1- h2') - (h4' - h4) .

The pump work remains the same as in the Rankine cycle, i.e, 

Wp = h4 -h3 

The net work output of the ideal regenerative cycle is thus less, and hence its steam rate will be more, although it is more efficient, when compared with the Rankine cycle. However, the cycle is not practicable for the following reasons : 

1. Reversible heat Transfer cannot be obtained in finite time.

2. Heat exchanger in the turbine is mechanically impracticable.

3. The moisture content of  the steam in the turbine will be high.