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PostPosted: Feb 23, 2013 5:55 am 
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In stead of using tubes to condense the power cycle steam in, direct contact condensers use water spray to directly condense the steam in the same space. The water spray system is a closed loop system, yet sharing power cycle fluid (demineralized water), with air cooling or water cooling on the heat rejection side (using a more conventional heat exchanger). Apart from simplifying the condenser, a look at the heat transfer coefficient of condensing droplets (>60000 wmk!) shows that this approach is very promising.

http://www.thermopedia.com/content/654/

How come this is not more widely used in steam power plants? Is there a greater concern of contamination with salt or other environmental substances with this approach if seawater or poor quality evaporative water cooling is used?


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PostPosted: Feb 23, 2013 7:40 am 
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What does it get you?
All we've done is transfer the cooling problem to the spray water.
So now we have two heat exchangers instead of one.


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PostPosted: Feb 23, 2013 8:07 am 
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djw1 wrote:
What does it get you?
All we've done is transfer the cooling problem to the spray water.
So now we have two heat exchangers instead of one.


I'm not entirely sure, but it seems you can get a higher vacuum (lower pressure drop) with this approach, and also avoid fouling problems in the condenser (ie insensitivity to heat transfer degradation by fouling). You'd have a heat exchanger at the heat rejection side, but that's only a simple low temperature water-water heat exchanger (or water-air for a dry cooling unit).

Also, I've heard that for seawater plants, salt water ingress in the condenser is often a big problem. With a direct contact condenser, the heat reject HX could operate the primary cooling water at a higher pressure (using its own pump) than the seawater on the other side. Which would make seawater ingress virtually impossible.


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PostPosted: Feb 23, 2013 10:53 am 
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Seawater into the condensate is a real headache, at least on tankers.
Believe it or not the first line of defense is sawdust.
But as soon as the contamination overwhelms the water treatment plant,
you have to go in and start plugging tubes.
Marine condensors typically have 25% excess tubes
to allow this.

But we are talking absolutely massive heat exchangers.
I just cant see how paying for two of them to
avoid some of these problems is going to be economic.


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PostPosted: Feb 23, 2013 11:25 am 
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djw1 wrote:
Seawater into the condensate is a real headache, at least on tankers.
Believe it or not the first line of defense is sawdust.
But as soon as the contamination overwhelms the water treatment plant,
you have to go in and start plugging tubes.
Marine condensors typically have 25% excess tubes
to allow this.

But we are talking absolutely massive heat exchangers.
I just cant see how paying for two of them to
avoid some of these problems is going to be economic.


There's only one heat exchanger. It should be much smaller as it doesn't have low density steam in the tubes, but high density water. It's tubes or plates can be very thin because there's no big vacuum or pressure on them, reducing cost and improving heat transfer. These low temperature water-water heat exchangers are very cheap.

The actual condensing isn't done in a heat exchanger. It's done in a vacuum vessel with direct contact sprays. The working fluid and the condenser fluid mix, as they are both demineralized water. It's more of a mixing vessel. Perhaps the cost of this vessel will be considerable, I don't know. It's temperature and fouling requirements are very low but it is a vacuum vessel which could cost some.


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PostPosted: Feb 23, 2013 3:31 pm 
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What Tcold could you achieve this way versus the standard way?
You have one additional exchange step and each step implies some delta T.
But I would guess your two delta Ts might be less than the single one using standard methods.


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PostPosted: Feb 23, 2013 5:19 pm 
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Lars wrote:
What Tcold could you achieve this way versus the standard way?
You have one additional exchange step and each step implies some delta T.
But I would guess your two delta Ts might be less than the single one using standard methods.


The Sandia work assumed a 5 degree C drop in both the direct contact condenser and the heat rejection HX. The latter seems aggressive as it was air cooled and just 5 degrees drop is difficult with that. So they assume 25 degrees C air, cooling 35 C water to 30 C. The condensate pump sucks condensate at 35 degrees C. Fairly standard temperature.


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PostPosted: Feb 23, 2013 6:30 pm 
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Cyril R wrote:
Lars wrote:
What Tcold could you achieve this way versus the standard way?
You have one additional exchange step and each step implies some delta T.
But I would guess your two delta Ts might be less than the single one using standard methods.


The Sandia work assumed a 5 degree C drop in both the direct contact condenser and the heat rejection HX. The latter seems aggressive as it was air cooled and just 5 degrees drop is difficult with that. So they assume 25 degrees C air, cooling 35 C water to 30 C. The condensate pump sucks condensate at 35 degrees C. Fairly standard temperature.

So overall a 10 degree C drop (perhaps a bit optimistic at least for air cooled).

Now what is normal using the current methods?


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PostPosted: Feb 24, 2013 8:30 am 
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Lars wrote:
Cyril R wrote:
Lars wrote:
What Tcold could you achieve this way versus the standard way?
You have one additional exchange step and each step implies some delta T.
But I would guess your two delta Ts might be less than the single one using standard methods.


The Sandia work assumed a 5 degree C drop in both the direct contact condenser and the heat rejection HX. The latter seems aggressive as it was air cooled and just 5 degrees drop is difficult with that. So they assume 25 degrees C air, cooling 35 C water to 30 C. The condensate pump sucks condensate at 35 degrees C. Fairly standard temperature.

So overall a 10 degree C drop (perhaps a bit optimistic at least for air cooled).

Now what is normal using the current methods?


Surprisingly a comparable direct dry cooling system has about 2-5 degrees Celsius higher drop. The improved efficiency from cooling high density water with air appears to outweigh the temp drop in the direct contact condenser. Maybe this is not surprising, judging from the HTC of 60000 - 120000 wmk with direct contact droplet condensing - the highest value I've ever heard for any device. On the other had, perhaps the Sandia figure of 5 degree drop at the heat rejection side is too optimistic.

A direct contact condenser with indirect wet cooling or once through cooling version would be interesting to compare. Googling gave not much result.


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PostPosted: Feb 24, 2013 10:32 am 
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Cyril R wrote:

Surprisingly a comparable direct dry cooling system has about 2-5 degrees Celsius higher drop. The improved efficiency from cooling high density water with air appears to outweigh the temp drop in the direct contact condenser. Maybe this is not surprising, judging from the HTC of 60000 - 120000 wmk with direct contact droplet condensing - the highest value I've ever heard for any device. On the other had, perhaps the Sandia figure of 5 degree drop at the heat rejection side is too optimistic.

A direct contact condenser with indirect wet cooling or once through cooling version would be interesting to compare. Googling gave not much result.


Then perhaps it is a case of the proponents of a technology knowing the performance targets (in this case temperature drop) they need to hit to be successful so that becomes the estimated performance. (Kind of like our cheaper than coal). Still, it doesn't sound unreasonable so I hope they keep pursuing the idea.


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PostPosted: Feb 25, 2013 10:37 am 
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Building the sprayers could be quite a challenge. For a big power plant, you need to be moving ~2GW of waste heat. With only 5C of temperature change, that's 95 tonnes/sec of water to be sprayed, collected pumped through the final heat exchanger and returned to the sprayer.

If the whole loop pressure drop is 1 bar, including spraying and the hydrostatic head from the height of the spray vessel, that's 10 MW for the pump - ~1% of the electric output. How much do you gain in efficiency from the lower pressure drop on the back end of the turbine?


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PostPosted: Feb 25, 2013 11:10 am 
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Luke wrote:
Building the sprayers could be quite a challenge. For a big power plant, you need to be moving ~2GW of waste heat. With only 5C of temperature change, that's 95 tonnes/sec of water to be sprayed, collected pumped through the final heat exchanger and returned to the sprayer.

If the whole loop pressure drop is 1 bar, including spraying and the hydrostatic head from the height of the spray vessel, that's 10 MW for the pump - ~1% of the electric output. How much do you gain in efficiency from the lower pressure drop on the back end of the turbine?


Good point. A 2250 MWt station would produce 1 GWe electricity and 1.25 GWt of heat to reject. Because the spray capacity is large, likely multiple horizontal vessels will be used in series, and so there's little hydrostatic head. We might be able to achieve just half a bar pressure drop, depending also on the drop through the sprayers themselves. We might be down to only 4 MWe.

Regarding savings, indirect dry cooled systems save around 30 mbar compared to a direct dry cooled system, which gets you roughly +1% net output (eg from 43% to 44%). In terms of energy it seems worth it, real question is capital cost. I'm hoping that steel plate concrete vessels can be used to save vessel cost; one advantage with this concept is that the vacuum chamber is no longer your heat transfer path, so it needn't conduct heat well.


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PostPosted: Feb 26, 2013 4:56 am 
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You are talking about the Heller system, Yes? Spray condensers coupled to an air cooled tower where the warm condensate is circulated to and from.

I worry about the opportunity for contamination of the condensate, these systems are huge, the massive surface area exposed to clean condensate sounds like a challenge WRT leaks and transferring material from the coolers to the condenser to the polishers or the steam generator. They seem common enough so obviously they work.


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PostPosted: Feb 26, 2013 6:01 am 
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Lindsay wrote:
You are talking about the Heller system, Yes? Spray condensers coupled to an air cooled tower where the warm condensate is circulated to and from.

I worry about the opportunity for contamination of the condensate, these systems are huge, the massive surface area exposed to clean condensate sounds like a challenge WRT leaks and transferring material from the coolers to the condenser to the polishers or the steam generator. They seem common enough so obviously they work.


Yes the Heller cooling system is like that (but is proprietary and I didn't want to advertise).

With air cooling you don't have dirty water/salt water ingress into the condenser. Even with seawater cooling, the indirect system has the advantage of operating the condensate at higher pressure than the seawater, so leaks of salt into the condensate become virtually impossible.

The vacuum vessels could be coated with epoxy or the like. The heat reject cooler will be large in surface area, but if it's made of an appropriate stainless steel then would you really worry about it?


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PostPosted: Feb 26, 2013 12:03 pm 
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Ok, in this presentation there are lots of useful numbers on this direct cooling system (heller).

http://mydocs.epri.com/docs/AdvancedCoo ... Balogh.pdf

There are also 2 case studies, of a CCGT and a supercritical coal plant. The indirect dry cooling system performs much better than the direct air cooled condenser. Interestingly, there is a nuclear plant in Russia that has this Heller cooling system, the only dry cooled nuclear plant in the world.

One result is also worth mentioning: in a cold climate, the Heller fully dry cooled system does just as well as a full wet cooling tower! But without the cost of water and related auxilliaries (water supply, chemical conditioning, freeze protection). In a hot climate, the hybrid cooling system could step in if it gets too hot, with water spraying or deluging assist.


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