Calculate Drying Time of Nylon Parachutes

[smbarrett3]

Hello,

I'm designing the HVAC system for a parachute drying tower. What equation can I use to estimate drying time? I know the amount of water in the parachute, the temperature and CFM across each parachute, and I can approximate the exposed surface area. I've looked at various equations for water evaporation but none appear to take into account the convective air flow across the surface. One promising way would be to calculate the BTUs required to evaporate the water but I don't see how that could include the air flow. In my mind, higher air flow would result in quicker drying times. One formula I found involved the difference in vapor pressures between the water and the vapor pressure of the air but again, this seemed to be applicable to air at a standstill. Still another was a formula for evaporation of a pool of water which did include water temperature, air temperature, surface area of pool, and CFM. However, these resulted in unrealistically short drying times. They also assume the water is freely on the surface versus entrained in a fabric, so there may be some capillary action involved and there will be numerous folds in the parachutes since they are hung from the top of their canopies.

Anyways, I have virtually all of the data since I am designing the system but I do not know of a formula or empirical data regarding nylon (or other fabric) drying rates.

If it matters, our system is 100% outside air which will be heated to 120 F, ducted into the space at the bottom of the tower, and exhausted out of the top of the tower. I do know outside air conditions if humidity of the air plays a role.

I appreciate any ideas or advice.

Thanks!

 

[EdStainless]

The heat input is one part, the amount of air is the other.
You need to meet both criteria, and then use a multiplier of 3 or 4 on the time.

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P.E. Metallurgy, Plymouth Tube

 

[smbarrett3]

I agree that airflow is important but I don't see how I can apply velocity over the chute into any determination of drying time. Assuming energy is conserved, then it doesn't matter how I get the 179,000 BTUs into the parachute, so the only way airflow is important is ensuring that I size my heater for a LAT of 120 F at my selected flow rate (low enough velocity that the chutes don't billow) so that I'm outputting the BTU/hr that gives me my desired drying time (plus safety factor).

Still, I don't think this approach is sound because at ambient temperatures the chutes will still evaporate with no heat and no airflow. The chutes will evaporate faster with no heat and a high airflow rate. They'll evaporate faster still with heat and a high airflow rate.

For example, for a sheet hanging on a clothesline in the wind, there is energy from the ambient temperature of the air, plus the convective force of the wind blowing against the sheet. How would you calculate this dry time? Then take it a step further. What if you were in Arizona on a 110 F day with low humidity and a gentle breeze? That is what I'm trying to solve. If I can do that, I can apply it to my drying tower.

Part of me thinks this is a convective heat transfer question, but if the water in my chutes is the same temperature as the ambient air, there is no delta T, so I don't know how to model the evaporating water as heat transfer.

 

[IRstuff]

Your heat requirement seems to ignore the the sensible heat required to get the water from RT to your design temperature.

Air velocity changes the amount of heat that can be transferred; typically, the faster the flow, the more heat is transferred. However, since your air flow is never monoatomically thick, the air that's farthest from the parachute does nothing in the heat transfer process and conveys nothing away from the parachute. This is why there's a fudge factor of 3 or 4.

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[EdStainless]

OK, simplified example using numbers that I made up.
Let's say that given your inlet air conditions and heating to 120F that each lb of water removed will be enough to saturate 1,000 scf of air.
Now if you are trying to remove 800lbs of water total, that means that you need at least 800,000 scf of air, and more likely 2.5 million scf to allow for lack of mixing and such.
Now look at the energy balance, using the heat that it takes to evaporate water (phase change) and the heat input into the air. Heating with your heaters how much air will it take to deliver that much energy? Is it more or less than the number above? you need to work with the larger one.

In reality if you heat the air from 90F ambient to 120F, I would assume that the exit air after passing over the chutes is 110F.
You heat the air for two reasons, one to deliver energy to speed the process and also to supply unsaturated air to allow for transport by evaporation. If you supplied saturated air at 200F you would get no evaporation, there is no place for the water to go. Likewise you could supply very dry (-20F dew point) air at low temp (40F) and you would get drying, but it would be slower than using warm air.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[MintJulep]

DETERMINING DRYING TIME
The following three methods of finding drying time are listed in order of preference:

[ul]
[li]Conduct tests in a laboratory dryer simulating conditions for the commercial machine, or obtain performacne data using the commercial machine.[/li]
[li]If the specific material is not available, obtain drying data on similar material. This is subject to experience and judgment.[/li]
[li]Guess[/li]
[/ul]

 

[LittleInch]

I think a lot of the confusion and issue here is how you're thinking about this.

You're looking at it from a heat transfer / thermodynamics side where as what you're actually doing is drying something.

Part of the problem is how dry is "dry". Not easy to define in a technical sense.

As the water transfer is based in part of difference in humidty levels between the item 9 fabric and lines) and the air, then maximum water transfer is at the beginning, but then drops off as the RH / "dryness" of the fabric approaches the RH of the air.

Your statement " at ambient temperatures the chutes will still evaporate with no heat and no airflow." is not, IMHO, correct. If you simply strung the chutes up in the drying tower and then sealed it off you'd never get the chutes dry.
If you just add heat and no airflow then you would simply end up with a steam room / sauna
Just add airflow then chutes will slowly dry until they reach equilibrium with the RH of the air passing through them.

So if the drying time is not specified by the client, but you want to aim for a certain value (T hours) then I would go for the overall thermal quantity and then add a factor of 3. It may mean on some days it will take your time T - 1/3 or on a humid day T + 1/3. Is T that important?

Reserve some space for a de-humidifier and all you bases are covered.

Also can you explain "My initial thought was also to dehumidify the air (either subcooling or desiccant wheel) then heat it but that is a big waste of money." Why is it a big waste? If you include the waste heat from the process in the tower, you end up with warm dehumidified air. Where's the big waste in that?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[smbarrett3]

Drying time is not that important. I just need to have a basis for my design and some confidence that it will work. I don't want to be severely over or under-designing the system. So that's why I was looking for a calculation method to see if I was in the right ballpark.

Regarding dehumidification, I meant I'm doubtful the energy put into dehumidification (and capital cost) would justify the assumed improvement in drying times. If we don't have a reliable way to calculate drying time, I doubt we have a reliable way to calculate the effect of dehumidification, versus simply heating the outside air. And even if we took the 0.4% dehumidification data, the dehumidification improvement would only be applicable 1 day a year, and that's if they were even washing/drying chutes that day.

For an attempt to quantify dehumidification and reheat, versus simply heating, here goes. On the most humid day, the humidity ratio (lbs moisture per lb dry air) is 0.018 compared to the air moisture holding capacity of 120 F air of 0.08 lb moisture per lb of dry air. Cooling to 50 F to wring out some moisture and reheating would result in a humidity ratio of 0.008. So if drying performance is directly related to the delta between the humidity ratio of the air in the tower and the moisture holding capacity of air, then drying performance would be improved by max 16%. And again that's only on the most humid day of the year. The rest of the year it would have less effect and in the winter, no effect.

I will admit that if we were using DX cooling, there is potential of heat recovery from the hot gas, so that would help some of the "wastefulness" I was previously referencing. But I'm not sure what you mean by waste heat from the process in the tower. Do you mean a heat exchanger at the exhaust to pump hydronic heat back down the AHU? Blowing the air itself back down is impractical but there is probably an opportunity for heat recovery from exhaust. However, this would increase cost and system complexity.

I should clarify this is a 100% OA system. If the air was being recirculated, I would agree dehumidification is required because where else will the water go?

I would also like to point out that the air moisture holding capacity between 70 F and 120 F quadruples, which is why heating has such a dramatic effect on drying.

 

[LittleInch]

All I meant was that any energy you put into the de-humdification (heat, compressors, motors, fans etc) would all be inside the tower and hence contribute to the raising of temperature.

One issue with the 120F / 50C is the use of people every hour to shake out the canopies to ensure a good drying area given these things hang down in a straight manner.

Asking anyone to work for even a few minutes in that temperature is very uncomfortable and could be against regulations. If you've got 30- 40 canopies all strung up then that's not something which is going to take a minute only.

I these instances you are not going to get anywhere without some practical / existing system information. You should have a good start point based on the heat input required, but to get any more sophisticated you're going to need to benchmark this against existing drying systems, temperatures and air flow.

Or guess



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Compositepro]

People have dried their laundry on clothes lines for millennia. It really is not that complicated unless you are trying to predict drying time to great precision. The most important thing that must be done is to spread the fabric out so all the surfaces are directly exposed to airflow. If you do not do this there are no calculations that will tell you anything. So clip hanger lines to the center of the chute and to diametrically opposed points on the edges of the canopy so that the canopy hangs complete spread, without folds. Any airflow over the canopy using air that is less than 100%RH will eventually result in a 100% dry canopy. This means no liquid water, but there will be an equilibrium moisture content in the nylon polymer. Keep in mind that completely dry nylon is stiffer and has less elongation to break than nylon at 50%RH at room temp.

Clothes dried on a line tend to get "crunchy" due to minerals in the water that acts as a "starch". You may want to rinse the chute in distilled water after washing. If drying time is not critical then it would be best to not use heat but only ventilation. If the humidity outside is very high, a portable dehumidifier can handle the load of drying a chute, along with circulation fans.

 

[chicopee]

See if you can get help from Wright Patterson Air Force base as much of the research on physiology is done there. Drying parachutes may not be a physiological problem, however, you may be able to get help.

 

[chicopee]

Doing some search I came across this article: A7-09.36-PB-1.0-Patrick Airforce Base at

http://www.apricus.com/...Project-Patrick-Airforce-Base-Florida.pdf

 

[chicopee]

More search on drying time of parachutes yielded another articlearachute Drying | Bryair.com at

http://www.bryair.com/casedetails/brygram-6/parachute-dry...their

PDF article on dehumidifiers and dryers have several worksheets of calculations that should provide help in your quest.

 

[smbarrett3]

Chicopee, both of those yielded 404 errors.

 

[chicopee]

Yeah, I have the same problem, however under the general query "drying time of parachutes" you should be able to see the headings of these two articles.

 

[IRstuff]

http://www.apricus.com/upload/userf...ial-Project-Patrick-Airforce-Base-Florida.pdf

for the first one
The second link doesn't seem to go to anything showing worksheets, unless you're referring to:

http://www.bryair.com/download_pro....CCF115592B9Application Engineering Manual.pdf

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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