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Thursday, 20 October 2016

CONCEPT OF REGENERATION IN RANKINE CYCLE

We were discussing “Rankine cycle” in our previous post, where we have seen the various components of Rankine cycle and its basic operations also. We have also discussed the “Rankine cycle with reheat”.

Today we will see here the basic concept of regeneration in Rankine cycle process with the help of this post. First we will see here the few basic concepts and terms and after that we will see the very important concept i.e. regeneration in Rankine cycle in this post.

 
First we will draw here a simple Rankine cycle and after that we will try to find here the method to increase the efficiency of a Rankine cycle. Let us see here the following figure, where a simple Rankine cycle is displayed.
As we can see here that heat energy will be added during the process 4 to 1 and heat energy will be rejected during the process 2 to 3. So let us see the efficiency of simple rankine cycle. Efficiency of a simple Rankine cycle will be calculated by following formula.

η= 1-[Temperature of heat rejection / Temperature of heat addition]

Temperature of heat rejection i.e. T2 will be constant because either heat energy will be rejected in to atmosphere or heat energy will be rejected at temperature lower than the atmospheric temperature but temperature of heat rejection will be constant throught out the heat rejection process.

We can also think to increase the efficiency of the rankine cycle by decreasing the temperature of heat rejection but we must note it here that temperature of heat rejection i.e. T2 could not be decreased below a specific level.

Let us see the temperature of heat addition, as we can see that heat energy will be added during the process 4-1 and therefore only partial heat energy will be added at constant temperature and rest heat energy will be added at varying temperature. So we will have one term known as mean temperature of heat addition i.e. Tm1.

Mean temperature of heat addition i.e. Tm1 will be termed as a constant temperature, located between T1 and T4, at which if same quantity of heat energy will be added then we will have same change in entropy as we were having changes in entropy during the process 4-1.

 
Therefore , Efficiency of a simple rankine cycle will be calculated by following formula.
η= 1-[T2 / Tm1]

Now in order to increase the efficiency of the Rankine cycle , we will have to increase the value of mean temperature of heat addition i.e. Tm1. We can increase the value of of mean temperature of heat addition i.e. Tm by increasing the maximum temperature of the Rankine cycle i.e. T1.

We must note it here that temperature of heat addition could be increased up to a limit only as it will be restricted by various practical parameters such as material properties of turbine blades. 

Turbine blade material will not work satisfactory if we increase the maximum temperature of rankine cycle i.e. T1 above a certain level and that level of temperature will be termed as maximum allowable temperature and we could not increase the temperature T1 of the Rankine cycle beyond this maximum allowable temperature.

We can also increase the boiler pressure in order to increase the efficiency of the Rankine cycle because range of temperature for heat energy addition will be increased.

Concept of regeneration in rankine cycle

So in order to increase the efficiency of the rankine cycle, we will require to increase the mean temperature of heat addition and it could be increased by increasing the amount of heat energy addition at higher temperature.

As we can see the above Rankine cycle, considerable quantity of heat energy will be added to the working fluid during its liquid phase or during sensible heating or during subcooled region. Only less part of heat energy addition will be added at maximum temperature i.e. at T1.

If we want to increase the efficiency of the cycle, we must be aware that all heat energy must be supplied at maximum temperature of the cycle i.e. at temperature T1 in this cycle and hence we will have to think the method by which we can permit the feed water to enter in to the boiler at state 5 so that all heat energy supplied by boiler to the working fluid will be carried out at maximum temperature of the rankine cycle i.e. T1 in this case.

 
So if we can use the heat energy of the high temperature steam, which is flowing through the turbine during the expansion process 1-2, to heat the feed water from 4 to 5 then all heat energy supplied by the boiler will be done at maximum temperature of the cycle.

Hence mean temperature of heat addition will be T1 itself because all heat energy supplied by the boiler to the working fluid will be completed at maximum temperature T1 during process 5 to 1.

Therefore, feed water leaving the feed pump will be circulated around the casing of the turbine in opposite direction of the direction of steam flow during expansion process 1-2 in the turbine.

The basic concept of regenration in rankine cycle is that we will have to make an arrangement to heat the feed water leaving the feed pump by the high temperature steam flowing through the turbine during the process 1-2.

So if feed water will reach to dry saturated liquid state, by receiving heat energy from the hot steam flowing through turbine, before entering to the boiler then all heat energy supplied by the boiler to the working fluid will be done at maximum temperature of the cycle and hence mean temperature of heat addition will be T1 and therefore in that situation we will have higher efficiency of the rankine cycle.

If feed water could not reach to dry saturated liquid state but also it is reaching at a point between 4 and 5, by receiving heat energy from the hot steam flowing through turbine, before entering to the boiler then all heat energy supplied by the boiler to the working fluid will not be done at maximum temperature of the cycle. 

But in that situation also we will have higher efficiency of the rankine cycle because we will have higher mean temperature of heat addition as compared to the simple rankine cycle without regeneration concept.

So this is the concept of regeneration in Rankine cycle, we will see another topic i.e. rankine cycle with regeneration

Do you have any suggestions? Please write in comment box.

Reference:

Engineering thermodynamics by P. K. Nag
Engineering thermodynamics by Prof S. K. Som
Image courtesy: Google

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