Harvey,
The Perry formula given is essentially a statement of the skin temperature for the conditions specified.
First, let's us for the sake of this discussion say that the skin temperature of the HT surface we are taking about is always less than Tsat. Yes, the skin temp in some of the DSH zones of the Hx's I mentioned are above Tsat for parts of the zone, but not for all of it. But that is irrelevant.
Let's carefully look at what Perry is saying. First he addresses sensible heat transfer with the tube surface being above Tsat which we are not dealing with. Then he states that condensation will occur directly from the superheated vapor when the skin temperature is less than Tsat. He is right in saying this. BUT, HE DOES NOT STATE AT WHAT RATE THIS HEAT TRANSFER OCCURS.
I have done the calculations, and in English units, it is at a rate around a U value of 52 at reasonable velocities while the condensing U value for the same Hx after the steam is cooled to Tsat is in the 450 range.
These calculations were done (first manually and then with web tools) for a Roberts type rising film evaporator common to the sugar industry. The steam velocities were nowhere near the range of the type of Hx's I gave links to above, (Latexman, they are called 3 zone heaters, each having a DSH zone, a condensing zone and a condensate sub-cooling zone) which even with the velocities that they get to (to the point of causing hydrodynamic whip-vibration of the tubing) only attain a "U" value for the DSH zone in the range of 160.
The Roberts evaporator studied obviously couldn't have that much velocity because once the steam reached saturation and became wet, it would cut the copper tubed calandria to ribbons. That is the same reason the designers of the high pressure heaters linked to above are so concerned about maintaining a 5F dry wall margin over saturation as the steam exits the DSH zone, because they don't want the (steel and stainless steel) tubing cut to ribbons. I have personally witnessed many heaters where conditions changed and the DSH zone was destroyed by the presence of moisture in the DSH zone. Note for the record that the tcoolant in the DSH zone is below Tsat for the heater pressure and the tube skin temperature in the region of the steam exit from this zone is below tsat as well. But I digress.
Now to Perry/Latexman. They are both absolutely right. The superheated steam will (eventually) condense, but the 64,000 dollar question is; at what rate? Latexman hits the nail on the head when he uses the phraseology "sit there, cool and condense". Saturated steam, on the other hand, as it enters the heater and contacts a cold tube, condenses immediately. No "sitting there" involved.
The "sitting there" Latexman describes is the blanketing effect that gives the "air-bound" symptom that TBP mentioned in his 26May post. Oh yes, it is transferring heat while it is "sitting there" but at a much lower rate of heat transfer, sensible heat transfer rates (single phase as Perry puts it) instead of condensing (he might have said two phase for condensing since a change of state of the steam takes place).
Since the goal of most Hx designers is to minimize the cost of the Hx, minimizing its surface area is one of the most cost effective ways of doing that.
Since the U values for condensing, in the 400-600 range, are significantly higher than the best DSH zones that money can buy, in the 150-160 range in heaters where velocity is the limiting factor, it wouldn't take a smart designer long to figure out that adding more surface to account for the sensible heat transfer necessary to reduce the steam to saturation temperature isn't the thing to do.
The study where I got the numbers I quoted above was done to justify the addition of desuperheaters to the turbine exhaust steam headers supplying an evaporation station at a sugar mill. The calculations showed that easily 1/3 or more of the calandria was devoted strictly to desuperheating the incoming exhaust steam. It was easy to see this in the sight glasses on the body above the top tube sheet. The area of the top tubesheet nearest the steam inlet was virtually dead, very little of the percolation effect common to this type of rising film evaporator. A little, but not much.
The backside, however, once the steam was cooled to saturation, was very lively, with sugar juice jetting up higher than you could see through the sight glass. This was a newer evaporator with several good, clear sight glasses, and easy to make the observation.
After the desuperheaters were added, and the exhaust was brought down to Tsat, two things happened. The overall evaporation rate for the first stage evaporators (Pre's they are called in our part of the world) increased dramatically, and the visual observation in the sight glass showed that the lively boiling was uniform across the entire surface of the top tubesheet. Gone was the dead zone that was there during the previous grind.
Latexman,
has (or had) in their 'tools' section the capability to model heat exchanger performance, and/or calculate HTC's. I challenge you to go set up a couple of different scenarios. One being pure condensation where your inlet conditions are right at Tsat, and another where your inlet conditions are above Tsat keeping steam side pressure and Tcoolant constant.
I think if you do it right, you will see something different from what you are stating in this thread.
I have been there, done that, and I think I know the result you will find. BronYrAur, if you are still following this thread, I recommend the same for you. Mr Perry, I recommend the same for you too.
I have enjoyed this discussion. It has cleared out a lot of cobwebs.
rmw
