Automatic Fans

[sjamali@vpi-i.com]

I wonder if any of you can answer / guide me with a problem which we are facing at our site. We are seeing a strange delta between condensable and condensate temperature in ACC while our back pressure is controlled at a set-point.
The unit operates a dual speed 18 fanned ACC condensing for a 200 MW steam turbine [configuration is 3 rows x 6 fans] . Our ACC controls the back pressure and set the target back pressure and starts and stops the fans in a staggered fashion, so that At low load operation (20- 30% of ST load) we tend to get over cooling in the condenser.
Each row of ACC has 2 fans (6 in total for whole ACC) which are removing incondensable gases from the system. the other 4 fans in each row ( 12 in total) are cooling the condensate i.e. removing the latent heat to condense the water.
for efficient ACC operation condensable and incondensable temperatures should be close, so there is no overcooling on the system.(this is our quick check to sense check ACC performance)
The problem we are facing is that at whether at low load or high load operation as soon as the pressure setpoint rises the control system increases the number of fans which are connected to removal of incondensable, this increases the deviation between the condensate temp and incondensable temperature and it doesn’t fall back until the next 6 to 8 hours. Does this means that fans going in high speed is actually increasing air blanketing instead of removal of incondensable gases or are ineffective in removel of incondensable contant?
We get the same temperature deviation effect (incond temperature reduces dramatically) when the same high speed fans is reduced to low speed.
or does it mean that back pressure setpoint is not actively changing enough to compensate for the amount of condensate (t/hr) that is being condensed?

I wonder if any of you have experienced this problem and how have you overcome this issue?
Thanks
S Jamali

[sjamali@vpi-i.com]

The title should read: Automatic Fans and Condensable vs Incondensable temperature variation.

[huubhubregtse]

The control logic is based on the following two temperature differences.
1. is the temperature difference between the condensate leaving the ACC and the steam temperature at the turbine exhaust. If this temperture difference is below a certain limit the control system can increase the fan speeds, if possible. if the temperature difference is too big fans have to be reduced in speed or switched off as we are undercooling.

2. the second temperture difference is the temperature difference between the extraction of the incondensables and the steam temperture at the turbine exhaust. if this difference is too big the fans of the secondary cells have to be reduced in speed as the secondary bundles are cooling the steam too much.

[sjamali@vpi-i.com]

Thanks for your input @ HUUBHUBREGTSE;

1) Do you mean that we can use this as a Terminal temperature Difference?
We have individual street temperature (condensate) at each fan, and individual incondensable temperature for each of the 3 legs. I am analysing these readings to see if this is possible. We normally see a drop in incondensable temperature, while condensate temperature in each leg remains consistent. which make me think that the cooling effect is not in steam side but on incondensable side.

2) The difference of temperature b/w Turbine exhaust and temperature at Extraction Pump isn’t far off generally, but I have seen instances where the CEP temp is plunged, although we haven’t trended incondensable temperature vs turbine exhaust temp. We don’t see a reduction in actual back pressure while set-point for back pressure remains at its previous value.

What I understand from our DCS is that the Logic isn’t controlling the fans start/stop to temperature difference, but back pressure on the steam turbine, (not knowing whether this is the correct approach for us to follow but this is how the logic has been set-up) there is a cascade mode to control set-point for back pressure and fans are added in or out of service or when all fans are i/s then they are moved to high speed mode gradually.
What we have also find is that the system is not actively changing the back pressure set-point even if the load requirement on ST (& condensate amount in condenser) is increased from 30% to 80%.

To compensate for high cell temp deviation we have started to switch off those fans on the street where delta is high and start them again manually to control this anomaly. however this in not so popular among our teams.

Regards

[Author]

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