Question

1. Q1) What do you understand by terminal temperature different? Can it be negative? Explain. Q2) A 50% reaction turbine is supplied with steam at 60 bar, 600 °C. the condenser pressure is 0.07 bar. If the reheat factor is assumed to be 1.04 and the stage efficiency is constant throughout at 80%. Calculate the steam flow required for a diagram power of 25 MW. 2.

Answer 1

1. TTD is a measure of how close the outlet feedwater temperature is to the feedwater heater saturation temperature Generally it shows the heat transfer capability of the feed water heater.

TTD = Saturation temperature of bled steam – exit water temperature.

The value of TTD varies with the heater pressure.

1. TTD is positive and the optimum design TTD is 5 F. Too small TTD is good for the cycle efficiency, but would require a larger heater, which may not be justified economically. On the other hand too large value will decrease the cycle efficiency.
2. TTD can be negative at the case of high pressure feedwater heater, as the exit water may have a temperature higher than the saturation temperature of that steam, therefore TTD ranges from 0 to -5 0 F. The drain here is slightly subcooled.
3. The high pressure heater would almost receive steam that is still superheated (Energy above saturation temp/pressure) If the incoming steam has 15 degrees of superheat, and the outgoing feedwater absorbs all of that and is heated to 5 degrees above the saturation temperature of the supplied heating steam pressure, You wil have a negative 5 degree TTD. Hence TTD can be negative.