If I want to study the performance of plate heat exchanger,I should take my temperature recordings during the steady state operation.
How can I know that I reached the steady state conditions.
Can I do this by applying heat balance
Can I predict the waiting time to reach the steady state
The answer is rather simple. You do not need to perform any heat balance. If the inlet temperatures of the hot fluid and cold fluid are held steady, then steady state is indicated by the constancy of one of the outlet temperatures. If you are recording the temperatures and can see the temperature history on the computer screen, it is a simple matter to judge steadiness. The effect of noise on temperature of the fluid is so small that your thermometer cannot detect it. If you have serious disturbances, you can never get steady conditions. Eliminate those disturbances.
"Steady state" is a relative term. Unsteadiness can even be caused by, say voltage fluctuations, which may cause a change in flow rate. You might be able to set an arbitrary band (say 1% of the measured value about the mean), so that if the fluctuations are less than that, you may take it as 'nominally steady'. That should do for your purpose.
A.M. Alatyar,
I am sure that there is no phase change in your experiments.
Try to maintain a minimum of 5 to 10 degree C raise or drop between in let to out let of the fluids. Heat balance shall lead to the estimate Heat transfer coefficient.
With single phase heat exchange experiments, the waiting time to reach the steady state generally varies from 15 to 30 minutes.
Regards,
Dr.S.Ravindran].
Steady state is said to be reached when all thermodynamic properties(parameters) remain steady with time for one fixed state.
Temperature may be fluctuated due to noise or disturbances during steady state.
Predicting the time required to reach steady state may not be necessary. Simply run the system until the heat exchange rate no longer varies (at least not measurably) with time. Then steady state will have been attained (at least for all practical purposes).
The answer is rather simple. You do not need to perform any heat balance. If the inlet temperatures of the hot fluid and cold fluid are held steady, then steady state is indicated by the constancy of one of the outlet temperatures. If you are recording the temperatures and can see the temperature history on the computer screen, it is a simple matter to judge steadiness. The effect of noise on temperature of the fluid is so small that your thermometer cannot detect it. If you have serious disturbances, you can never get steady conditions. Eliminate those disturbances.
"Steady state" is a relative term. Unsteadiness can even be caused by, say voltage fluctuations, which may cause a change in flow rate. You might be able to set an arbitrary band (say 1% of the measured value about the mean), so that if the fluctuations are less than that, you may take it as 'nominally steady'. That should do for your purpose.
When would be the steady state achievement be quicker? Is it for laminar conditions or turbulent conditions?
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