Sub cooling

Once you realize that the only way there is condensate sub-cooling if when the condensate passes thru an air pocket and gives up heat causing the sub-cooling you know that it is very important. Air locked in the ACC reduces the available area to condense steam and therefore can affect performance. After acceptance (completing a performance test) air in the ACC if the ambient temperature is above freezing cost the plant money because you are not condensing will the full surface of the ACC. Below freezing the air pockets if left unchecked can lead to ice plugs forming which can result in rupture of tubes.

Sub cooling also robs the boiler system of energy, because for every BTU that is removed from the condensate (more than needed for condensation) that water has to be heated back up again in the boiler resulting in a energy efficiency penalty. The boiler will make less steam if the boiler feed water is sub-cooled.

Because the ACC operate under saturated steam conditions, e.g. the condensate is always in contact with the steam from whence it came and therefore sub-cooling is of little concern for provided that the ACC is free from air ingress. For an ACC operating in automatic with a fixed backpressure setpoint sub-cooling will occur if air ingress is greater than the air extraction equipments’ capabilities. As the air collects and blinds the surface area, ACC backpressure will increase resulting in the control system in either speeding up the operating fans (VFD equipped) or starting additional fans. It is the increased air flow that will cause the condensate and the air take off NCG’s to become sub-cooled. Sub-cooling can also occur without major air ingress should the ACC be operated in manual mode supplying more air flow than is required. In this case the backpressure and saturation temperature will decrease.

Sub cooling mainly caused by the resistance of the exhaust pipe,steam condenser etc,normanly the sub cooling temperature is between 0.7~10 dgree at the full load of block jam conditions to TRL.not at the full load ,the sub-cooling is much bigger.for 660MW,
NO item content
1 output?MW? 660 660 660 660
2 sub cool??? 0 1 3 5
3 1???????t/h? 57.28 59.08 62.77 66.44
4 HR?kj/kwh? 7869 7869 7870 7872
5 consume of design coal ?t/h? 281.3 281.3 281.5 281.7
6 consume of design coal?t/d? 6752 6752 6757 6760
7 eff of the plant 36.65% 36.65% 36.62 36.60
PS, sorry, do not know how to paste the hate balance digram.

Sub cooling mainly caused by the resistance of the exhaust pipe,steam condenser etc,normanly the sub cooling temperature is between 0.7~10 dgree at the full load of block jam conditions to TRL.not at the full load ,the sub-cooling is much bigger.for 660MW,
NO item content
1 output(MW) 660 660 660 660
2 sub cool(?) 0 1 3 5
3 1??????(t/h) 57.28 59.08 62.77 66.44
4 HR(kj/kwh) 7869 7869 7870 7872
5 consume of design coal (t/h) 281.3 281.3 281.5 281.7
6 consume of design coal(t/d) 6752 6752 6757 6760
7 eff of the plant 36.65% 36.65% 36.62 36.60
PS, sorry, do not know how to paste the hate balance digram.

Two Issues I can share:

1. Thermal efficiency decreases (due to lower T than T_sat in condenser)
2. During sub-cooling incondensable gasses also tend to dissolve into the condensate, (and not escape through ejector) causing higher gas concentrations in the condensate which can lead to increased corrosion.

subcooling is a penalty in steam cycle efficiency and also increases the amount of oxygen that can be entrained in the condensate. It has one positive aspect in that it increases condensate pump NPSH, preventing cavitation in those pumps. Modern IR scanning detectors/ cameras can be configured to scan the ACC tube bundle to detect the degree of subcooling on each bundle, and perhaps this could be used to schedule fan speed to minimize subooling.

I have a question regarding air pockets. I have frequently seen cooled areas with IR camera on bundles. Sometimes I stopped the fan in order to know whether this is air leakega or air pocket. After seconds the fan is stopped, the cooled area disappear, but when the fan is re-started, this cooled area appear again!
Do you have any similar problem? Oxigen is not high in the cycle.

Air pockets occur in areas where there can be localized air ingress (cracked weld) and/or areas where steam flow through those particular tubes may be lower than adjacent tubes. On units operating in the South western USA these units can experience what is deemed as air logging in certain sections of the ACC. This is commonly the direct result of a unit reaching it’s maximum condensing load along with other environmental impacts, e.g. sun providing external localized heating, high winds starving fans, etc… The folks that operate these units and who routinely experience this phenomena have stated to me that they have learned to “Burp” the ACC in order to clear the air logged section(s). This is done by manipulating the fans in modules adjacent to the air logged zone. By stopping adjacent fans these areas become hotter and condensation decreases which allows for more steam to flow towards the air logged area due to differences with internal pressures between the hot and cold tube bundles. Normally the removal of the air logging occurs rather quickly but the fan configuration should remain for 10-15 minutes to ensure adequate air removal in that zone. If air returns rather rapidly once returned to normal fan configuration then one may want to investigate further as to localized air ingress. One more thing, A low dissolved O2 doesn’t necessarily equate to the absence of air ingress. A robust and properly functioning air removal system will ensure that dissolved O2 is removed from the condensate to design levels. Marty

• This reply was modified 7 years, 7 months ago by Martin Cyr.