10 replies to this topic

[Mithun_mk]

Hi everyone,

I am working on a sanitisation design of a tank where the tank will be filled with steam at 120 deg celsius. We need to condense the steam and remove it as condensate by

1) Spraying cold water at 10 deg celsius through spray ball of the tank
2) By using ambient air

Total volume of the tank is 275m3. Can anybody guide me in right approach to do the calculation for estimating the time required for the steam to condense?

[breizh]

Hi ,
Make sure you understand the risk associated with the condensation of steam using cold water .
Take a look at the attachment, in particular 2 .
Your tank must withstand vacuum and designed as a pressure vessel.

Hope this helps

Breizh

[Mithun_mk]

Yes...I do understand the issues of mass steam condensation with in the tank and the importance of a properly sized vent to allow sufficient air flow to the tank during the process.

[kkala]

Photos of post No 2 clearly indicate the risk of this activity. During past Operations in fertilizers, I had cought myself wondering about elementary things having escaped! Indeed, risks can be hidden in apparently simple activities.
Despite my lack of practical experience, following thoughts (subject to comments) on the specific topic could have some usefulness. Points are simplified to show principle; estimated figures need more precise recalculation.

1α. The tank will initially contain 275 m³ of steam (120 ^oC, 1 Atm), that is about 275 m³/(1.8 m³/kg) = 153 kg of steam, slightly superheated (saturation temperature 100 ^oC). Let us see the case of natural cooling by external ambient air only, of 20 ^oC assumed temperature.

1β. Supposing 7 m dia and 7.15 m height, tank area to ambient is about 200 m². With a heat transfer coefficient of U = 5 kcal/m²/h/^oC = 21 kJ/m²/h/^oC, heat losses (at the critical moment when condensation begins) are 21*200*(100-20)=336000 kJ/h. Latent heat=2257 kJ/kg and specific volume=1.67 m³/kg, so air coming into tank from breathers should be 336000/2257*1.67 = 249 m³/h.

1γ. Yet U can be higher (say five fold), especially in strong wind. U = 5 kcal/m²/h/^oC is just an indicative value, not precise enough for the case http://www.cheresour...lated-pipe/page, post No 13. All these indicate the risk of breathers inadequacy, even by natural cooling. Case may differ, if manhole and other openings could be left open.

2. In this sense, spraying water into the tank would multiply condensation as per para 1, and risk of vacuum would increase.

3. Ambient air introduced into the tank to displace steam seems not to cause vacuum (yet "by using ambient air" may mean something else). Suppose mixing 1 kg air (20 ^oC, 1.2 kg/m³) to 1 kg steam (saturated, 100 ^oC). Due to high latent heat of water, condensation will be minimum and loss in steam volume much less than the air introduced. Playing with the parameters of attached "mix.xls" seems to indicate this. The risk of overpressure can be checked by effective control of introduced air.
However the tank will be exposed to ambient air cooling at the same time, with risks as in para 1 above, possibly mitigated.

4. Assuming the above as right, risks could be limited if, for example: (α) tank gets insulated, (β) the manhole and any other opening is opened during cooling. (γ) introducing air at temperature higher than 100 ^oC, to prevent condensation and displace steam. There may be other measures too.

Besides there may be somewhere a procedure for such disinfection.

Note: "mix.xls" uses water97_v13.xla of IAPWS - IF97, http://www.cheresour...ater-and-steam. In case this is not installed, steam properties will not be automatically calculated.

Attached Files

•  mix.xls   23KB   115 downloads

Edited by Art Montemayor, 18 February 2012 - 12:54 PM.

[aliabdhs]

Hey , can any body share with me the exact procedure to understanding the main goal of use GCT (Gas cooling tower ) , in cement industry , The most they cooling the gases by spray water . Can i have an Excel sheet for this .

Best regards

[Mithun_mk]

I am planning to use water to speed up the cooling and bring in sterilised air to break the vacuum.. Problem is to evaluate the time of cooling.....There will be a filter to take care of the air flow(200m3/hr)...cold water will be sprayed at 5m3/hr and sterilised air will be passed into the tank at 930m3/hr to break the vacuum...Tank MAP is0.5 barg and MAV is 0.02barg....Tank is protected by a PVVV(+20/-15mbarg)....Now i like to make a comparison of 1) cooling time when using only air 2) Cooling time when using water and air

[kkala]

....
Note: "mix.xls" uses water97_v13.xla of IAPWS - IF97, http://www.cheresour...ater-and-steam. In case this is not installed, steam properties will not be automatically calculated.

Cheresources link as above is no longer active, can anybody advice where to find "water97_v13.xla" in cheresources?
At any case this useful excel add-in for steam properties can be downloaded from http://www.filestube...ater97-v13.html, or from other sites traced by googling "water97_v13.xla".

[Art Montemayor]

In my opinion, the proposal as outlined is a hazardous one.

The conitions that will exist at the moment of steam condensation by the cool water will be an instantaeious partial vacuum. The set pressure conditions for the PSVS are not enough, in my opinion. Whoever is process designing this proposal should bear in mind that the PSVS will not vent - nor will it allow the sufficient amount of venting (or air) when it starts to open. These type of relief valves do not exhibit their design capacity when they start to open, and consequently require either a much larger size or a higher setting for the pressure (or lower for the vacuum). Practice is to allow TWICE as much design pressure (the same for vacuum) as the operating set pressure in order to allow passage of the designed quantity of vapor through the PVSV. Check this out with experienced manufacturers.

The use of an air filter for the incoming, vacuum breaking air is just another problem because it introduces further resistance to flow into the tank and necessitates a larger PVSV. The vacuum set pressure is ridiculously too high. The pressure setting is too low. The range to be covered is too narrow. What is called for here is a vessel that is designed for FULL VACUUM. Of course, this means a pressure vessel - and more capital cost. Nevertheless that is what I would recommend.

[Mithun_mk]

Lead option now is to pass 1000 kg/hr of air (not water spray) into the tank to push the steam out and to cool the tank. My issue is estimating condensation that is happening when we pass air inside the steam space.
Heat capacitance of air is very low compared to steam and latent heat of steam is very high. So much more volume of air will be required to cool the steam and condense it than the volume of steam. Therefore while cold compressed air is introduced into the tank we will be pushing gases out of the tank than developing vacuum. But problem is to exactly tell how much steam condensation that is happening inside the tank.

Edited by mithunkmadhu, 20 February 2012 - 05:06 AM.

[Art Montemayor]

Mithunkmadhu

By your latest post, I am going to presume that you have abandoned your proposal to condense the steam with a cold water spray inside the vessel. Is that correct? If so, be honest and admit it directly so we can spare ourselves from wasting time on such a proposal. The original proposal simply was not thought out correctly and completely, considering the practical hazardous conditions it imposed.

Now, you propose to inject air – presumably atmospheric air that has a nominal humidity content. You propose to sweep (or purge) the steam out through a tank vent nozzle out to the atmosphere to get rid of it. Is that a fair and correct description?

If so, I attack your method of thinking out the application. You are failing to take into consideration the basic facts of what will occur under this proposal:

• If you use sufficient air (and I don’t know how you came up with the 1000 kg/m3 figure; this doesn’t make any sense. Do you mean 1,000 kg air/hr?), you will not condense any steam at all. Your problem is to furnish an outlet vent nozzle large enough to permit the flow of the exiting, humid air without reaching sonic flow at the nozzle and limiting your flow rare out.

• I have to assume that the atmospheric air you use will not be 100% saturated – as is the normal case. Therefore, this air will “take in” the steam as additional humidity and reach a more saturated state – but the mixture will remain in the VAPOR state. The steam will, therefore, not condense but will form a vapor mixture with the air, contributing its partial pressure.

Once again, as in your original proposal, you are not “seeing” the total, basic picture. You are focusing on the condensation of the steam, when in reality that doesn’t have to happen if you “engineer” the operation so that it doesn’t happen. The solution to evacuating the steam safely, in my opinion, has more to do with partial pressures and compressible flow than it has to do with condensation.

Of course, I have had to presume some of the basic data needed because you haven’t furnished it. If my assumed basic data is OK, then my above premise is also correct.

[Mithun_mk]

Thanks a lot for your time . I have started attacking the problem with cold spray option in mind intially(Considering fast cooling of tank). While using cold spray, we have huge condensation and the proposal was to put in clean air to compensate vacuum. Now we have ruled that option out.

I am sorry for the confusions. Our option now is cooling tank with clean air. I also made a mistake with the units. it is 1000 kg/hr. Now I need to get the tank certified for safety. For that, they asked me the exact condensation happening inside the tank while mixing cold air. We are putting in dry air from compressed air network after putting a PCV to reduce pressure to 1 bar before letting it inside the tank. We have a DN150 nozzle for PVVV(+20/-15mbarg). We have a DN200 filter housing with 3 nos of 20" catridge filters(micro filters) which allows 200m3/hr @ 15mbarg. But while doing cooling with air, we bypass the filter and pass the steam through an open vent DN65 to avoid chocking of filters due to steam condensate.