Saturday, 10 December 2016

EFFECT OF INTERCOOLING ON BRAYTON CYCLE

EFFECT OF INTERCOOLING ON BRAYTON CYCLE

We were discussing Brayton cycle, an ideal cycle for gas turbine engine in our recent post. We have also seen the effect of regeneration on Brayton cycle efficiency in our previous post.

Today we will see here the effect of intercooling on Brayton cycle with the help of this post.

Brayton cycle with intercooling

 
As we have already discussed during study of open cycle gas turbine engine and also during study of closed cycle gas turbine engine that resultant work from a gas turbine engine will be the difference between the work output from the turbine due to the expansion of working fluid and work energy required by the compressor for compressing the working fluid.

Net work from the gas turbine engine = Work output from the turbine – Work input to the compressor
Wnet = WT-WC

We can conclude from above equation that net work from the gas turbine engine could be increased by decreasing the compressor input work or by increasing the turbine work or both. Hence, efficiency of Brayton cycle could also be increased in this way by increasing the net work from the cycle.

Work energy required for compressing the gas between two specific pressure levels could be reduced by carrying out the process of compression in stages and cooling the gas between two successive stages. This method of increasing the net work from the gas turbine engine is termed as multi-stage compression with intercooling.

Let us see the following figure, we will have basic arrangements of various components, PV diagram and TS diagram here to show the effect of intercooling on Brayton cycle


Process 1-2: Adiabatic compression of the working fluid

Process 2-3: Working fluid is cooled to the initial temperature in a heat exchanger which is termed as intercooler.  

Process 3-4: Further adiabatic compression of working fluid to the specified pressure level

Process 4-5: Heat energy addition to the working fluid at constant pressure in heating chamber

Process 5-6: Adiabatic expansion of the working fluid through turbine or also termed as power stroke

Process 6-1: Rejection of heat energy at constant pressure in cooling chamber

Process 1-2’: Adiabatic compression of the working fluid from one specified pressure level to another specified pressure level and we have not considered the effect of intercooling here.

Therefore, cycle 1-2-3-4-5-6-1 indicates the gas turbine cycle with intercooling having two stage compression and cycle 1-2’-5-6-1 indicates the gas turbine cycle without intercooling i.e. ideal closed gas turbine cycle or ideal closed Brayton cycle.

 
Suppose a compressor has to compress the working fluid from pressure P1 to pressure P2, there will be three methods for compressing the working fluid from pressure P1 to pressure P2.
Isentropic compression
Isothermal compression
Multi-stage compression with intercooling

If we plot these three processes on PV diagram to know the work energy required for compressing the working fluid from pressure P1 to pressure P2 for each process, we will come to know that work will be required maximum during the process of isentropic compression and work will be required minimum during the process of isothermal compression.
PV diagram for three processes for compressing gas from P1 to P2
Whereas during the process of multi-stage compression with intercooling, work will be required less than as for isentropic compression process and will be more than as for isothermal compression process.

As we know that when compressor will compress the working fluid, temperature of working fluid will be increased and hence isothermal compression process will not be possible and therefore work input for compressor for compressing the working fluid between two specified pressure levels could be decreased by adopting the multi stage compression with intercooling.

 
We have shown above two stage compression of working fluid with one intercooler between two compressors. There might be more stages of compression process but usually more than two intercooling will not be preferred because it will be quite complex.

We have shown above in PV diagram one intermediate pressure PX, compressor input work will be dependent over the value of intermediate pressure PX. Compressor input work will be minimum if it will follow the following equation.
Do you have any suggestions? Please write in comment box.


We will see another topic i.e. Brayton cycle with reheating in our next post in the category of thermal engineering.

Reference:

Engineering thermodynamics by P. K. Nag

Engineering thermodynamics by Prof S. K. Som

Image courtesy: Google

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