Actual rankine cycle - Work done by pump

In summary, the work done by the pump in an actual Rankine cycle can be neglected in most cases, unless a high level of accuracy is required. This is because the work done by the pump is significantly smaller compared to the other processes in the cycle. This is due to the fact that the pump is pumping liquid condensate, which requires less energy compared to compressing a gas or vapor. Therefore, neglecting the work done by the pump is usually only applicable for ideal Rankine cycles. The thermal efficiency of a Rankine cycle is typically calculated as the sum of work out and work in, divided by the heat input.
  • #1
Jameseyboy
25
1
Hi,

In what cases do I neglect the work done by the pump in an actual rankine cycle?

I am reading that h5-h4 is negligible but in what circumstances do I accept this.

Is this only for ideal rankine cycles?

As I understand thermal efficiency is (work out + work in)/Heat in

Thanks
 
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  • #2
Unless you are doing a very specific analysis which requires a good degree of accuracy, you can almost always neglect the work done by the pump. If you are unsure why, I'd recommend that you calculate the work term and compare it to the heat added and rejected by the boiler and condenser, respectively, and the work out from the turbine. You will see notice that the work done by the pump is very insignificant compared to the other processes. The reason the work done by the pump is so small is because it is pumping liquid condensate as opposed to a gas (or vapour) which would require a compressor and a much larger energy input.
 

1. What is the Rankine cycle?

The Rankine cycle is a thermodynamic cycle used in steam power plants to convert heat energy into mechanical work. It is based on the principles of the Carnot cycle and is a closed-loop cycle that involves the use of a working fluid to transfer heat and produce work.

2. How does the Rankine cycle work?

The Rankine cycle works by first heating the working fluid in a boiler to produce high-pressure steam. This steam then enters a turbine where it expands, producing mechanical work. The low-pressure steam is then condensed back into liquid form in a condenser, and the cycle repeats itself.

3. What is the purpose of a pump in the Rankine cycle?

The pump in the Rankine cycle is used to increase the pressure of the working fluid, usually water, before it enters the boiler. This allows for more efficient heat transfer and helps to maintain a constant pressure throughout the cycle.

4. How is work done by the pump calculated in the Rankine cycle?

The work done by the pump in the Rankine cycle is calculated by multiplying the mass flow rate of the working fluid by the difference in enthalpy between the inlet and outlet of the pump. This is represented by the equation W = m(h2-h1), where W is the work done, m is the mass flow rate, and h1 and h2 are the enthalpies at the pump inlet and outlet, respectively.

5. What factors affect the work done by the pump in the Rankine cycle?

The work done by the pump in the Rankine cycle is affected by several factors, including the type of pump used, the pressure difference across the pump, and the efficiency of the pump. Other factors such as the temperature and flow rate of the working fluid can also have an impact on the work done by the pump.

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