Super heated steam to blame?
Super heated steam to blame?
(OP)
We have a HX at our plant that has both 600# and 160# steam attached. We have been running on 600# steam and I am currently looking at the economic implications of switching to 160# steam. My problem is that when looking at historical data, the required flowrate for 160# steam is 33% higher than when 600# steam is used. I am not sure what is going on. I would think that as 160# steam has a higher heat of vaporization, less steam would be required. I just wondered if anyone had some ideas on why this is the case.
Thanks
Thanks





RE: Super heated steam to blame?
RE: Super heated steam to blame?
RE: Super heated steam to blame?
rmw
RE: Super heated steam to blame?
Joe Lambert
http://www.control-associates.com/
RE: Super heated steam to blame?
The 600# steam provides slightly more energy than the 160# steam when condensed to a fixed condensate temperature. What is the shell pressure for each case?
If you are using more 600# steam, perhaps you are providing more heat to the heated side than with the 160# steam.
Orifice plates are unreliable until proven otherwise. Measure the condensate instead.
RE: Super heated steam to blame?
If the delta temp is sufficient, even superheated steam quickly condenses on the cold tube. The delta T should however be based on saturated steam temperature, and not the superheated inlet temperature.
RE: Super heated steam to blame?
Good luck,
Latexman
RE: Super heated steam to blame?
So while the delta t difference of superheated steam gives a higher driving force or heat tranfer rate than the delta t difference of its equivalent saturated steam at condensing temperature, and yes there is some heat transfer rate at the sensible heat transfer from superheat to saturation, it is the latent heat transfer as the steam condenses that give the high heat transfer rates-shall we say the bang for your buck.
But it is what is going on around the tubes while this sensible heat transfer is taking place that is what is important. When saturated steam gives up its latent heat and the good heat transfer happens, the volume change from a pound of steam to less than a teacup of water (I am not going to get steam tables out here to be more specific) that draws more steam into the heater creating steam flow and velocity around the tubes resulting in higher Reynolds numbers.
So sure, superheated steam has a higher driving force for a given amount of sensible heat transfer, but the time required that the steam has to sit there in contact with the tube surface to transfer that heat is enormous compared to steam entering at saturation and collapsing to condensate immediately upon touching a cool tube, bringing some more steam right in to do the same microseconds later.
Superheated steam blankets the tubes and slows down the flow velocities in the tube bank to a point that the reynolds numbers become so low that to repeat a line that I do remember from my heat transfer text, 'for Reynolds numbers below 2000, it is debatable whether or not heat transfer even occurs at all' or words to that effect.
rmw
RE: Super heated steam to blame?
To bmemmott, historical data may refer to different heat load conditions. You should check all the factors, including the process side, not just the recorded steam consumption.
RE: Super heated steam to blame?
The heat capacity of the condensate can be 38% more but that need not get tanslated into 33% more steam flow.
The extra heat content (considering a back pressure of say 160psi or below) is about 135btu/lb. The latent heat of steam at 160#a is about 850 btu/lb. So, the steam flow should only increase by 135/850 = 16%
Further, the difference in enthalpies of steam at 600# and 160# is 8.33btu/lb, the steam flowrate should not vary much if the outlet condition is same in both cases.
Apart from 25362's suggestion, I would also check for the flow compensation with both temperature and pressure if 600# steam is in super heated condition.
RE: Super heated steam to blame?
Good luck,
Latexman
RE: Super heated steam to blame?
My point was that the temp driving force for superheated steam is NOT more than for saturated steam.
There is not really a "desuperheating zone" for superheated steam. The superheated steam in contact with the cold tube immediately condenses on it. Superheated steam then condenses into the saturated condensate around the tube. Thus, the tube always sees saturated condensate around it, even for superheated steam, and the temp difference should reflect it.
RE: Super heated steam to blame?
Heat exchangers that are designed to handle superheated steam such as power plant feed-water heaters have desuperheating sections where there is no moisture present and, actually where moisture isn't wanted because it is very detrimental. In order to get any meaningful sensible heat transfer at all, (or said differently to get the type of Reynolds numbers that produce good sensible heat transfer) the velocities in a DSH zone are very high and if condensation does begin within the DSH, it will cut the tubes to ribbons. So normally DSH zones are designed to have a 5-10 deg F outlet temperature above saturation to insure a dry exit as the steam exits the DSH and enters the condensing zone of the heater. The heat transfer rates even at those Reynolds numbers (velocities) are still very low overall, in the 50-150 B/hr/ft^2/F compared to the condensing zone which starts at 500 and can be higher.
Many believe that superheated steam condenses, but sorry, I've tested heaters both ways; with and without SH and I have seen the difference.
rmw
RE: Super heated steam to blame?
Why does a cold glass of ice tea sweat? Same thing. Local temperatures condense vapors too.
Good luck,
Latexman
RE: Super heated steam to blame?
rmw
RE: Super heated steam to blame?
I believe CJKruger is speaking about condensers that are fed superheated steam. If the tube is less than the saturation temperature, condensate will form on the tube and superheated steam will transfer heat into the condensate and condense into it. The key is the local tube temperature. Condensers that have tube temperatures that are everywhere lower than saturation temperature do not have desuperheater zones. I suspect these are handling steam with much less superheat than what you are talking about.
I believe you are speaking of desuperheaters whose tubes everywhere are above the saturation temperature. They are not meant to condense steam, or they will quickly be destroyed by erosion corrosion.
It's apples and oranges, right?
Good luck,
Latexman
RE: Super heated steam to blame?
Heat exchangers that transfer heat by condensing steam work (that is to say make their duty) by condensing steam, which collapses the steam vapor volume which draws more steam to be condensed into the condenser to be condensed which condenses and collapses the steam.......etc, etc and now we have a performing heat exchanger.
While a glass of tea will condense moisture out of the atmosphere and a cold tube will in some finite amount of time condense steam from superheated steam around it. However, the rate at which that happens is drastically different from the action of condensating heat transfer.
Remember for a point of discussion, if superheated steam encounters saturated condensate on the surface of a tube, in order to transfer heat to that condensate, and in so doing desuperheat itself, it has to boil away some of the condensate, which then has to be recondensed by the tube bundle adding load to the heater.
The heaters I mentioned are condensers that operate at high pressure and that have special zones to desuperheat the steam so that it can do what is described in the first paragraph above. It is not that they won't condense that steam without desuperheating it first rather that the rate at which they do the condensing would be so slow that the heater would have to be tens of dozens of sizes larger.
So, a heat exchanger of any type that is designed to transfer heat by condensing steam and reaping the benefit of latent heat transfer is greatly hampered by the presence of superheat-any superheat, obviously the more there is the worse it is. The slow heat transfer rates of the sensible heat transfer (as compared to the heat transfer rates of the condensation) has the effect of blanketing the tube bundles, slowing down the velocities through the tube bundle and overall choking the heater.
But, yes, don't misunderstand that I am saying that a cold tube wont eventually condense the steam around it. I just can't afford to build a HX big enough to operate that way.
It is apples and oranges and that is why I asked the OP about his steam conditions. If he is trying to get his HX to do a certain duty with saturated steam at one of the two pressures given and superheated steam at the other, no matter which one it is, then he is trying to compare apples and oranges. That is what I am trying to draw out of him/her.
rmw
RE: Super heated steam to blame?
With superheated steam the dT is greater, however the U is MUCH smaller, hence the lower heat exchange rate with superheat vs. saturated steam.
A gross estimate of the 'U' for superheat I have seen is 16 while that for saturated is 205.
Then there is the BTU's available from the phase change.
Joe Lambert
http://www.control-associates.com/
RE: Super heated steam to blame?
The best description I found on the internet related to this issue, is Section 3.3.4 from:
http://www.wlv.com/products/databook/ch3_3.pdf
They conclude with: "It is both simpler and more conservative to assume that condensation will occur directly from the superheated vapor, using the saturated temperature and a condensing heat transfer coefficient in the rate equation..."
RE: Super heated steam to blame?
Until the superheat is gone, the HX will behave as if it is airbound.
RE: Super heated steam to blame?
It was a clear and unambiguous visual observation of what that I had known theorhetically for years. I'll never believe otherwise.
rmw
RE: Super heated steam to blame?
Good luck,
Latexman
RE: Super heated steam to blame?
Did I mention that once the DSH is started the overall performance of the heater took off like a rocket?
Good Point Latexman. I think it makes my case.
rmw
RE: Super heated steam to blame?
Good luck,
Latexman