Steam Motors and Energy Conservation

Summary

In this laboratory, a steam motor and energy conversion test set was used to investigate the performance of a real steam plant cycle, and compare it to an ideal Rankine cycle, using specific steam consumption (SSC) and energy distribution analysis. A plot of SSC against motor power resembled an exponential with a negative exponent, as the mechanical losses became more insignificant in comparison to the power output. The presence of theses losses was also backed up by using a Willian’s line extrapolation, which implied roughly 0.18kW was used solely to overcome resistance. Using energy distribution analysis at an equilibrium point, the cooling flow out of the condenser (Q5) was found to be largest at 2kW, while the useful energy output (W1) was the smallest at 0.044kW. This experiment highlighted why a real steam plant cycle differed from an ideal case, due to various factors such as isentropic expansion and compression processes. In addition, the use of superheating and reheating in a Rankine cycle is also discussed as a means of reducing wear and improving efficiency, via minimising water droplet formation.

 

Key Data

The experimental readings from the test set.

 

The yellow dotted line is termed the Willian’s Line, and implies that there was around 0.18kW of mechanical losses from the data set obtained. If the fit of motor inlet pressure is extended to zero motor power as well, it can be seen that there is a non zero intercept.

 

Note how this plot roughly resembles an exponential (with a negative exponent) shape, with specific steam consumption tending towards a constant value of roughly 80kg/kWh as motor power increases. This is because, as shown by the Willian’s line, there are various mechanical losses involved in a steam engine, however as the motor output power becomes greater due to an increased condensate flow rate, these losses become more insignificant in comparison, hence less of a flow rate is required to increase power by the same amount, as the initial barriers to movement have already been overcome.

 

A comparison of the sizes of the individual energy flows in the system, where Q1 represents the heat input to the water and Q2, Q3 and Q4 represent the losses from the boiler, engine and condenser respectively.  Q5 denotes the cooling output from the condenser and W1 is the useful output of energy (see Figure 16). Ideally therefore W1, Q5 and Q4 would be maximised and Q2 and Q3 would be minimised.

 

Report
Due to the lab being repeated for other years, the full report has not been uploaded. It is however available on request, using the contact details available.