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Desiccant embedded PPE suit to keep PPE dry and bearable

My idea is to embed PPE with desiccant packs to absorb the sweat. Desiccant readily absorbs liquid from air and can be worn underneath PPE to absorb a large amount of liquid depending upon the type of desiccant used. Given that the environment is not at too high of a temperature to risk the user of getting hyperthermia, a desiccant PPE suit should greatly extend the time the user can wear it.

Photo of jlocke8 .
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One of the problems of wearing PPE besides it being very hot to wear is the humidity.  As the user gets warmer they will sweat more.  Complaints arise from goggles fogging up and to having a shortage of socks because they are drenched in sweat and give a maximum of 45 minute wear time between changes.  http://www.npr.org/2014/10/07/354230895/ebola-protective-suits-are-in-short-supply

The thought is that a 90 degree dry heat can be tolerable where as a 90 degree humid day is very uncomfortable.  If the desiccant can keep PPE dry, this will extend the time the careworker can wear the suit.

Some desiccants are readily available and quite cheap.  Packs of these can be stored inside PPE during use then when user takes it off the desiccant can be recharged of its adsorption ability by heating off the adsorbed sweat --say over a fire ... or the packs can be thrown away and new packs be used.

Some possible dessicants could include zeolite, silica gel, or calcium chloride.
 

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Photo of Deborah Paterson
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Hi, thanks for posting this. Perhaps somethings to consider as you develop your idea - How much desiccant would be required for one suit do you think? How long would it extend the use by? How might we dispose of these in a non harmful and efficient way?

Feel free to post questions and needs from the community so people know how to add value :)

Photo of jlocke8 .
Team

Hi Deborah,

Thank you for guiding my thoughts and giving good feedback. I not sure how to post questions. Where can this be done at?

Photo of jlocke8 .
Team

I ran a couple quick calcs on your questions. I believe the numbers may put this idea as a secondary method instead of a primary solution.

An average person sweats a maximum of 1-2 liters per hour lets call it 1 liter/ hr for easy calculation. The absorption rate of the best desiccants may absorb at roughly 10 grams of water per hour for 100 grams of desiccant. The desiccant also maxes out absorption at a rough maximum of 30 grams of water per 100 grams of desiccant. This means for the absorption rate of the desiccant to keep up with the rate of sweating, there would need to be a minimum of 100 times 100 grams or 10kg of desiccant that the care worker would have to pack to handle a maximum rate of sweating. The desiccant would last roughly 3 hours before it would be totally saturated. 10kg is not exactly light weight when carried around for a few hours...

Of course this calculation is just estimation as temperature, relative humidity, desiccant types, and how well desiccant is in contact with the humid air are all variables that have effect of absorption rates and maximum absorption.

Photo of Rainer Winkler
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Water has a enthalpy of vaporization of 2260 kJ/kg, evaporating 1liter of water per hour causes a heat flow of 630 W. But a typical person produces only 100 W heat energy, so I suppose you do not need to evaporate so much, therefore you need less silicagel

Edited 2 November 2014: 300 W for a moderately active persob should be used, not 100 W (Rainer)

Photo of Rainer Winkler
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The problem with silica gel is, that the condensation energy of the water is released in the silica gel when the water is absorbed plus some more.

So the silica gel has to be cooled. And most of the energy for this cooling has to be drawn from the outside of the PPE, otherwise there would be no cooling for the person.

I have yet no good idea how to do this, so I prefer ice as cooling medium. Ice because it can probably absorb 50% more cooling power than phase change material.

Photo of Jason Holt
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These desiccant packs are for keeping huge shipping containers dry. They claim to be able to absorb their weight in water, and come in 1kg packages, which happens to be the weight of a liter of sweat. As I recall, they cost a few dollars each. Not sure how fast they can absorb that liter, though.
http://www.drypak.com/PDFs/Cargo_DryPak_2.pdf

Photo of Rainer Winkler
Team

Hi Jason, the packs probably contain calcium chloride. I did not yet take calcium chloride for a "fast cheap and easy" solution, because of harms if it accidentally comes into contact with the worker. Testing and improving a solution to prevent this costs probably too much time.
I think you can make the calcium chloride to absorb quickly. But then you have also to remove the heat that it releases.
With kind regards
Rainer

Photo of Jamie Beck Alexander
Team

Hi, thanks for the great idea. Have you gotten any clarity around what is contained inside the packs? Calcium chloride, or other desiccant? What information do you need in order to move this idea forward?

Photo of Jamie Beck Alexander
Team

A few other questions to consider:

1) What kind of information would need to be included in the packs that will help with training on how to use and safe protocol, especially in donning and doffing?

2) Distribution - how will these be distributed to remote communities that need them?

3) Manufacturing - who can manufacture these packs in a wearable version?

Photo of Jason Holt
Team

Here's a 4.4oz (125g) starch + calcium chloride desiccant pack that claims it can absorb 280% of its weight. They're <$3 each from mcmaster (awesome but expensive mechanical supply house). Also available with one side coated with adhesive: http://www.mcmaster.com/#2228k8/=ufuypk

McMaster is good about shipping worldwide, although I don't know if that'd apply to the affected countries.

I know nothing about chemistry, but Rainer does appear to be right about it being a skin irritant. The top two MSDS sheets for a google search on "calcium chloride" show pretty different assessments of its health hazards. Looks like they put it on roads to control ice, so that makes me think it can't be too toxic, but an expert should be consulted.

It shouldn't be coming into contact with humans unless the bag rips. Some of the PPE guidelines I've read have the worker wearing tyvek (permeable to gas but not liquid) under tychem (impermeable), so one option might be to wear it between the tyvek and tychem layers. The humid air ought to be able to penetrate the tyvek and get absorbed.

Rainer is also correct about the need to get rid of heat. This claims calcium chloride + water is exothermic with 795.8kJ/mol, which I think means there is indeed a significant amount of heat generated while absorbing the water (perhaps in addition to the heat being carried away from the body). So that's unfortunate and might be a showstopper, but I'm not an expert. Perhaps placing the pack on the inside of the tychem suit might be best for getting the heat outside the PPE. We'd certainly want to make sure it didn't get dangerously hot.

Another thing we'd want to be sure of is if there are any other reaction byproducts. CO2 perhaps? Releasing large amounts of gas could certainly be problematic inside PPE.

Photo of Jason Holt
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Posted on the Chemistry Stack Exchange site to see if any chemists want to chime in: http://chemistry.stackexchange.com/questions/19123/calcium-chloride-starch-desiccant-packs-inside-ppe-for-ebola-workers

Photo of Rainer Winkler
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Hi Jason, there are no byproducts if calcium chlorides absorb water vapor. The problem is the heat and how to remove it. But the same would be true for silica gel.
A problem is enough venting, the air has to come in contact with the adsorbant. After passing the adsorbant material the air is dryer and hotter. A sifnicant part of this heat has to be removed to have a cooling effect. To make that possible the ambient air temperature should be well below 34°C (or higher ?), otherwise the worker would probable be heated too much from the heat exchanger.

The challenge is not to cool, but to build a simple, not to heavy device that fits in the PPE and has a enough cooling power.

With kind regards, Rainer
PS: Thanks for posting on the site

Photo of Rainer Winkler
Team

If the air is blown through calcium cloride or silica gel it is warm thereafter. This warm air may be blown in a channel glued onto the inner side of the PPE. Maybe this is enough to have at least a simple heat exchanger.

Photo of Rainer Winkler
Team

Calcium chloride should be able to absorb more water. Silica absorbs less, but might be easier to handle. I would take both as an option.

Photo of Jason Holt
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Oh, I see, turn the PPE into a radiator by blowing air through channels next to the outer layer. Now we're talking batteries and motors, though, in which case we're probably better off exhausting the sweaty air and drawing in cool ambient air.

Photo of Jason Holt
Team

Silica gel absorbs a lot less from what I can tell; this claims silica absorbs only 25%-50% its weight in water, vs several hundred percent for calcium chloride: http://www.absortech.jp/eng/downloads/whitepaper-silica-eng.pdf

Photo of Rainer Winkler
Team

Hi Jason,

regarding your reply form 21:58, you are right, that is the problem I had. As far as I see, a quite strong fan is needed, together with a battery and a heat exchanger that works under a PPE not designed for this. And this with a final cooling power that is better than comparable techniques. Not simple.

My problem with exhausting the sweaty air and drawing in cool ambient air, is that you will have to filter it like the air the person breathes. But you need more air than for breathing. This leads again to bigger filter and more powerful fans.

I made some calculations for venting and I found values of 10 liters per second of air that are required depending on temperatures. This are 600 liter per minute. In scuba diving it is calculated that a diver breath about 20 liter per minute of air. So venting requires much more air.

I doubt that it will be accepted by the worker to vent with only moderately filtered air, but maybe I am wrong.

Silica gel absorbs less water then calcium chloride, you are right. But it is very easy to handle and not a problem when a capsule breaks and the silica gel comes into contact with the skin. So I would nonetheless analyze both material for possible usage.

In the moment I think of heat exchangers that are compatible with simple PPE. You connect something to the outside of the PPE. But only loosely coupled, so that it may not accidentally break the PPE. Then you cool through the PPE sheet or some other material something below. And this may cool a liquid cooling vest, or an adsorber or whatever is required.

With kind regards, Rainer

Photo of Rainer Winkler
Team

For calcium chloride I find an heat due to the adsorbtions of water that is about 7.5 times greater than the latent heat of vaporizations of water (http://en.wikipedia.org/wiki/Calcium_chloride, Standard enthalpy of
formation for the various hydrates of calcium chloride).

So using calcium chloride as desiccant appears problematic due to the high amount of heat that is to be removed.

Photo of jlocke8 .
Team

Hi I heard calcium chloride does give off energy when absorbing water but does silica gel really do that too? I wasn't sure that silica gel gives off heat when it absorbs can you point me to some more info about this? Thanks

Photo of Rainer Winkler
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Hi jlocke8,

you may for instance read this article " Experimental investigation of the silica gel–water adsorption isotherm characteristics", http://ulbuo.wdfiles.com/local--files/frigo:sources/Silicagel.pdf
The amount of water adsorbed is given by Henry´s Law. You find tables there for typical silca gel types. It depends on temperature and water vapor pressure. The air pressure is like very often in chemistry and physics not relevant.
The heat released while adsorbing is the latent heat of evaporation plus 30 - 100% for the heat of wetting.
The adsorbtion is a phyical process and goes fast.

It is not new that people try this and there may be better sources that explain what the problems are, when you do it in a PPE. The problem is, that to collect this information and for sharing it, people are required. My time is limited, many specialists are either not active here or already very busy.

To build a cooling in a PPE using silica gel, my impression in the moment is: a) you need a really strong fan, and this is a problem. b) After the water is adsorbed, you have to transfer the heat out of the PPE, which is a problem.

The best material I know in the moment when adsorption of water vapor is used, is silica gel, because it releases not too much heat during adsorption.

I doubt that we can build a machine using silca gel, that works under a PPE.

Maybe we manage to build a machine using silica gel, that works to cool tent like constructions that are in the moment evaluated for home/community care.

With kind regards
Rainer

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