Minimum velocity to avoid freezing 5

[Shmulik]

Hi all,
I'm looking for a calculation procedure to find the minimum velocity required to avoid freezing of water flow over tube's surface in shell side of S&T HE.
Shell side: water inlet @ 10 ºC , water outlet 5 ºC
Tube side: liquid CO2 inlet @ -20 ºC , superheated CO2 outlet 0 ºC

Many thanks in advance.

 

[abeltio]

1st i would use a parallel flow heat exchanger... so the hottest water is in contact with the coldest CO2 and your differential is the maximum.
2nd the water is on the shell side so it is very difficult to accurately calculate a velocity... Donald Q. Kern in Process Heat Transfer outlines a procedure to calculate a "velocity" for the Shell side based on the wetted perimeter depending on the tube pattern... which is then used to calculate a Reynolds Nbr (Re) for the shell... in the shell side it is very easy to obtain very large Re's by modifying the pitch and arrangement of the HE tubes.
I would say that if the Re > 750,000 with fully developed turbulence then the water should not freeze.

To confirm, following the calculation of the heat transfer coefficient following DQK's method you can calculate the tube wall temperature... if the temperature of the wall is higher than 0.0 C then the water should not freeze... hopefully... maybe...
HTH
B"H


saludos.
a.

 

[Montemayor]

Shmulik:

Velocity is not a factor in whether water will freeze.

Temperature determines whether it will, or not, freeze.

What you are relating to, I believe, is the fact that if water can keep moving and find a "warmer" conducting surface, it will not freeze - or, if you flow sufficient flowrate quantity of water over a sub-freezing surface, it will not freeze. The latter is due to the rate of heat transfer not being large enough to remove sensible heat from the relative large quantity of available heat in the flowing water body. The former, you have to mechanically design and configure. The latter, you calculate.

Note carefully: if you have a sub-freezing source, there will always be an ice film being formed on the water surface, depending on how "deep" the low temperature is. And this film will have a tendency to accumulate - again, depending on the coldness of the conducting surface temperature and the water's flowrate. I know of no calculation procedure or magic solution for your dilemma. Sorry. The practical engineering answer is: don't do this.

Art Montemayor
Spring, TX

 

[Montemayor]

Shmulik

In my previous post I forgot to add that I've tried using both water and steam vaporizers on Liquid CO2 service - specifically for carbonated beverage bottler customers at their site. It was a disaster scene every time. We standardized on electric (Chromalox) heaters and never had any more vaporizer failures. This was over 40 years ago and I think the experience with most users is still the same. If you want another opinion, try TomCO2 at:

http://www.co2parts.com/home.htm

and tell them what you need or what you want to do. They have the experience and can guide you through the application. Hope this helps.

Art Montemayor
Spring, TX

 

[25362]

One may consider using a solute to depress water's freezing point. For example, adding ethylene glycol to water reduces its freezing point as follows: 10% mass by 3.4^oC, 20%, by 7.9^oC, 36%, by 20^oC.

 

[sterl]

If it really has to be water and CO2, the other design would be a vented tube heat exchanger with a third fluid in the interstitial space, brought to just above 0 deg. C at entry. Instrument air with depressed dewpoint is one possibility...A staganant eutectic might be the other.

Of course, 2-heat exchangers with anti-freeze as third fluid in a closed loop will do the same thing.

 

[Shmulik]

Hallow guys, thanks for your posts.
Montemayor / abeltio, you've exactly got my point. I'm aware of ice film created on tube surface; my concern is of ice accumulating at this area (and consequent tubes distortion). Do you have any experience with products of "Cryogenic Experts Inc." or "Wittemann" that are stated to provide such type of evaporators?

http://www.cexi.com/prd_energy_miser.php

http://www.wittemann.com/literature_web/Vaporizers/vaprorizer_water_literature.pdf

Anti freeze should not be added to the water according to the plant conditions requirements.

 

[25362]

Shmulik, could you use warmer than the 10-to-5 ^oC water ?

 

[Shmulik]

Hi 25362,
Water temp. that going to be used is significantly effected by ambient temp.
10-to-5 oC is the worst case (Normal temp. 14-to-9 oC).

 

[Montemayor]

Shmulik:

You've had 8 responses from guys who are trying to work out the problem with little basic data: you don't tell us
1) the LCO2 vaporized rate required;
2) the origin and availability of the water source;
3) the application (plant or customer site);
4) preference (or limited to) for water/steam instead of electric heater.

As I said, I've done this one - many times - and perhaps I didn't explain the background and depth of my experience in this particular application. I was, for 9 years, a project engineer, plant manager, and project manager for Liquid Carbonic when we were the world's largest CO2 producer. I handled international operations engineering and was assigned to special projects. I initiated and started the introduction of low pressure liquid CO2 production and distribution in to several countries. For example, I was the Plant manager for Liquid Carbonic del Peru during 1963-1966 when I operated 4 CO2 generating plants and 3 air separation units as well as Acetylene production. I designed and led the first fabrication of LP LCO2 production and distribution there and had to resolve the problem of vaporizing the LCO2 product at customer sites. Vapor was the only form that CO2 was consumed in. The LP LCO2 process and bulk distribution brought down the production/distribution costs radically - but all would be in vain if the customer couldn't consume the product. Prior to this, CO2 was produced and distributed in small steel cylinders as high pressure liquid. By going to LP LCO2, we revolutionized the way industry used the stuff. But we had to successfully vaporize the product at customer sites for their use.

The only installation that I was able to turn my back on was one where I installed a LCO2 automatic block valve on the LCO2 feed to a water-submerged coil that was fed continuous water with an overflow. The coil was a vertical helical arrangement in a round tank. The LCO2 was introduced through the bottom of the coil - as also was the water. The customer insisted on using water and in those days the electric elements would burn out frequently or worst, relay switches would fail and the vaporization would continue out of control until the tank relief operated. Today, Chromalox can be relied to produce reliable and safe elements. The installation of the automatic block valve and external coil + water was more expensive than the electric heater and the water turned out to not be reliable on supply.

Believe me, I personally know all the ins and outs, trade-offs and set-backs in this application. Been there, done that. I'm sharing this experience so you don't go through all the hassle and grief that we went through. I'm very familiar with the Witteman products and their processes. Hell, I installed and operated one of their plants in Trinidad, West Indies in 1967. It was the first By-product LCO2 plant in the Caribbean and I took the gaseous CO2 from the Grace Ammonia plant through a Plastic, 8" pipeline to the recovery plant and dry ice plant that I had installed at Point Lisas.

The reason I'm giving you all this background is that there are guys on this forum with similar experiences and track records. If you gave us all the basic data up front, a lot of confusion, guessing, conjecture, and wrong advice could be avoided. If given all the basic data I would resolve this problem as fast as I could write a response. We all want to help, but we need all the facts. The quality of the response is only as good as the quality of the data given.

If you're interested, I can send you an Excel Workbook I've prepared on Industrial Gas data and information that contains information on LCO2 vaporizers and LCO2 storage and customer tanks. I hope this helps and you resolve your problem.


Art Montemayor
Spring, TX

 

[Shmulik]

Hi Montemayor,
Thank you very much for your reply. Your willingness to help is greatly appreciated.

The problem we deal with is CO2 supply for carbonated beverage bottling facility with several bottling lines, and varying working regime.

The normal CO2 consumption is 3000 Kg/Hr, but we have to take into account peak loads of 4800 Kg/Hr and lowest load of 1200 Kg/Hr.
Liquid CO2 source is @ -20 ºC (19.25 Bar a).
Gaseous CO2 min. temp. supply requirement is @ 0 ºC.

Water source is 100 CU.M treated water storage tank. This is an external, not insulated tank, thus its temp. is effected by the ambient temp. The a.m. tank is foodstuff grad, so using of antifreeze is not permitted.
Preference of water heating is for energy saving considerations, and the experience of steam heating for this application is very troublous (and nearest steam source available is 200 MTR away).
Average water supply temp. is 14 ºC, lowest winter temp. is 10 ºC, max. temp. is 28 ºC.
Water supply circulation pump: 70 CU.M/Hr.

As I've mentioned before, my concern is of water freezing and ice accumulation on external tube surface. Another question is whether flow rate reduction is permitted when CO2 consumption is at its lower level? (In case CO2 consumption is stopped, water circulation remains, and CO2 space is drained off).

I hope now you can have the full information needed in order to handle this problem.

Excel Workbook you suggest for Industrial Gas data and information is most interesting to me for both this job and other jobs we have.
My address is: shmulik@boiler.co.il

Once again, thank you very very much for your assistance!

 

[Montemayor]

Shmulik:

As I stated, it doesn't take very long to resolve a problem when you have all (or most) of the basic data. You still haven't furnished all the basic data you possess, but I have made the necessary assumptions. I've sent you an Excel workbook where you will find your problem all worked out, using the cooling water you insist on. Look in the worksheet titled "LCO2 vaporization". You will find that you require a minimum of 285 gpm of the water you specify, so your pump should just barely be able to meet that requirement. However, you have various trade-offs (in other words, now for the bad news...):

1) Your 26,400 gallon tank only furnishes 1.5 hours of water supply;
2) You state that the max (design) CO2 vaporization rate is 4,800 kg/hr, but you don't state is this is a sustained and steady supply requirement. I mention this because yours has to be one (or the) of the world's largest bottling plant complexes! You must have a captive CO2 generation plant(s) on site working 100% of the time. I don't see how you could tolerate the tanker truck traffic to import LCO2 into your main storage tank at those consumption rates. I've been in many world-scale bottling plants - this one has to be the biggest.
3) What you intend to do with the "cooled" water that is used as the heat source is something you totally leave out. I assume you're going to heat it up back to the 50 oF. Otherwise, you're going to shut down your vaporizer afer 90 minutes of operation.
4) You must do something with your heating water, which you don't describe; you'll need another pump to take it some where else. I would not use a pressurized, enclosed, shell & tube unit. It is too expensive and not required.

Your vaporization duty of 1.3 MM btu/hr is going to have to be matched in capacity of some external heating source to raise the water temperature back to inlet specifications. I leave that up to you, assuming that you'll use a direct-fired gas heater. By the way, using propylene glycol solution is perfectly sanctioned and found to be OK by the USA's FDA. Propylene Glycol is not recommended for human consumption, but it is OK for human consumption. I have used it as a dry ice binder on direct cooling applications of sausage, pork, chicken, and other human foods with no legal problems and the sanction of the FDA because it is considered food-grade. Also, your statement implies that there is a possibility of the glycol getting into the vaporized CO2. This is not correct. The CO2 vapor exists at approximately 250 psig and the the glycol solution would be much lower. Any coil leak would allow CO2 to invade the solution cycle and not vice-versa. If you have to utilize a warm fluid vaporizer instead of an electric model (because of the size, for example), then I would use a propylene glycol solution in a closed circuit with a direct-fired heater. The size of the capacity required points towards this type of solution. Steam heating would be more expensive - especially in a bottling plant application.

Note the simplicity of the vaporizer controls in my detailed drawing in the workbook. Everything relies only on the successful operation of the automatic block valve. The control scheme is fail proof and is inherently automatic. The kicker is that you have to maintain the 285 gpm of constant heating water circulated 100% of the time.

I have no idea what you mean by "a.m. tank" or the phrase "CO2 space is drained off". I have a lot of questions about this applications and the constraints put on it, but it would draw out this thread endlessly. We don't have an identity of where this is or it's name, so I'll end this on this note.

Hope you're happy with the results.

Art Montemayor
Spring, TX