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Microclimate Cooling in a PPE

Research on Microclimate Cooling in protective clothing is not a new topic. Yet we are not able to provide cooling to the health care workers. In this research contribution I try to give some information on where the problems are.

Photo of Rainer Winkler
21 12

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Version 6: 3 December 2014 - Changed description on the picture for "Evaporation Heat Flow Tychem".
Version 5: 2 December 2014 - New discussion of evaporative cooling in the description of figure with total heat flow.
Version 4: 23 November 2014
Version 3: 31 October 0:00 New details on heat balance and possible cooling strategies.

Data on heat flow in Tychem QC

I now add a document that describes the heat flow in a Tychem QC in more detail. The main heat flow at high temperatures und not to high humidity is due to evaporation cooling.
 

Generated Head

Typical Heat gerneration in resting adults range from 70 to 110 W. During walking 280-350 W and at maximal excertion more than 1000 W.[1]

Actual Situation

Health Care Worker in the actual epidemic use PPE but without cooling. This causes heat stress and limit working time. The CDC describes detailed what has to be done, to prevent Heat-related Illnesses.[2]

The worker starts to sweat, but as the humidity is trapped inside the PPE only a small quantity of the sweat can evaporate. The moist will then condense at cooler parts, for instance the inside of the PPE. When 1 kg of water evaporates 2260kJ are required. When this water condenses on a surface, the same quantity is released as heat. So the condensing water heats the outside of the PPE. Another part of the energy is transfered by convection of by radiation. But the flow of thermal energy from the person to the outside is too small, so the working time is limited.

Most of the sweat cannot evaporate and is flowing to the bottom of the clothing. This sweat is useless for cooling but causes loss of water and salt.

The heat capacity of a human body is about 3.49 kJ / ( kg K).[3] Assumed a person with 80 kg weight is able to work 1 hour in a PPE until the body temperature raised by 1 °C (= 1K), this would mean, that a power of 75 W was heating the person. This is a crude estimation of how much heat is to be removed. If his temperature rises bei 2 °C in 30 minutes, 300W where heating. By this calculation, the power that a colling system has to absorb is estimated. This assumes, that the cooling system does not cause extra isolation.

Climate

Temperatur in Monrovia is betwen 23 °C Min and 30 °C Max, the dew point averages between 22 °C and 25°C. The humidity ranges between 65 and 95 %.[4] 

Heat Balance and clothing

An article by George Havenith gives an introduction on human heat balance and the effects of clothing when protective clothing is weared.[5]. The original article that developed the Heat index gives detailed numbers to perform at least a basic calculation of heat and water vapor balance inside a PPE.[6] A good overview over cloothing and heat balance is given in the Handbook of clothing.[7]

Climate in the PPE

Hot but still below 30°C

I do not have empirical data from worker in PPE in West Africa. In the moment I use the following model. This will probably change as soon as I have more informations.
Thermal power to be dissipated (excluding breathing): 300 W
Outside temperature 26 °C
Inside temperature 34 °C (The normal skin temperature)
The PPE has a material density of 0.237 mm (Tychem C), with assumed heat conductivity 0.03 W / (m K). With an area of 2 m^2 and a temperature difference of 1.3 K the heat current is 330 W.

On the surface heat energy is transfered by conduction and by radiation.
P (conduction) = alpha A (Tppe - Tair).
alpha is between 3 and 20 W / (m^2 K) for natural convection, and may rise to 100 with venting.
I assume alpha = 10 W / (m^2 K), with a tempature difference of 7 K (33°C -26°C) this gives P (conduction) = 140 W.

The energy flow by radiation is given by
P (radiation) = A sigma epsilon (T^4 - T0^4), whith the area A, the Stefan–Boltzmann constant sigma, the emissivity epsilon (assumed to 1), the Temperature of the PPE T and the temperature of the surrounding T0. 
With T0 = 26 °C, T = 33°C, this give P(radiation) = 88 W.
Total energy transfered is 228 W, given the uncertainty, this is not far from 300W.

This model is not really correct, because the temperature flow inside the PPE is not taken in account.

This calculation leads two the following theses:
a) If a person walks and is outside of buildings, so that wind speed is not neglible, the energy loss due to conduction might also be sufficient at 26°C.
b) The air inside the PPE is mostly at rest, the main barrier is probably this mass of not moving air.
c) As humidity is trapped inside the PPE and the inside temperature of the PPE is with 34°C quite high, no effective sweeting is possible.
 

Temperature rises to 30°C and more

During the day and inside a tent which is heated by the sun, the temperature may rise above 30°C. The tent tissue itself may be even hotter if it is in direct sunlight. In that case the worker would be additionally heated by thermal radiation from the hot tent.

In that case cooling by conduction and radiation will be low. With rising temperature the PPE may even be heated from the outside.In that case it might be better to reduce the head conductivity of the PPE, so that the worker is only heated by himself and not also from the environment. In that case active cooling with a cooling power of 300W or more is the only method to allow longer working times.

Models in detail

I used [6] to make more detailed assessments of the heating situation inside the PPE. Even though a simple table calculation is sufficient for this, the details are tricky, so my result should be regarded as preliminary.

Worker in a PPE with thin or minimal clothing

All calculations are done for a heat generation of 180 W/m^2 of the worker, with a skin surface of 1.78 m^2, this corresponds to 320 W. This is used by [6] and is the heat produced by a walking person.
I assume a clothing thickness of 0.3 cm, a air thickness of 0.1cm (small, because the worker moves). Then the worker would be in thermal equilibrium if the outside temperature is about 0°C.

If the worker usess only minimal clothing under the PPE , equilibrium is a arrived at 7 °C. The highes heat barriers are: a) the air space between skin and inside of the PPE, with 5"C temperature difference required and b) the temperature difference between outside of PPE and air, where I come to 14°C temperature difference. The latter appears quite high, so it should be checked again.

As long as the temperature is not too high, to cool effectively: a) minimize clothing under PPE, b) vent the inner air space of the PPE to increase thermal conductivity due to convection and c) Vent the outside of the PPE, to increase thermal conductivity there and d) prevent hot surfaces is the vicinity to prevent heating due to radiation, or even allow radiative cooling.

Worker in a PPE in a hot environment

If the environment of the worker is not cool, but rather hot (tent heated by the sun) and the temperature rises to nearto 37°C the heat flow changes the direction. In that case the worker is internally heated by two sources: a) internal metabolic heat and b) heat from outside.

When the outside temperature is 45 °C, the heat flow towards the worker is about 45 W, in addition to 320 W due to internal heating.

This quantity appears not so high, so it makes probably not much sense, to isolate the worker with thicker closing from the hot environment in this situation.

Challenge of cooling

It is not difficult to cool a person inside a PPE a bit. The problem is to cool sufficiently to absorb a flow of thermal energy of a few hundred Watt. One kg of Ice for instance absorbs 330kJ when it melts. If a working person produces 300 W of heat and all this heat could be absorbed by the ice block, it would melt in 18 minutes. Using liquid cooling it would be possible to use this to produce cool water to cool the body. This could be done with a garment filled with tubes that cover much of the skin. If all ice is molten, this garment would isolate the body so the worker will start to heat faster. A big problem is then the reliability and complexity of such a machine. It appears that no existing microclimate cooling systems promised to give a real advantage to be broadly used in the epidemic in West Africa.

Cooling by venting from outside

It was proposed to cool a PPE by venting from outside. I will try to assess the amount of air that is required for venting. By venting energy is removed either due to the heating of the air and also due to the increased humidity. Supposed the PPE is vented with 14 liter per second, the temperature is raised by 3 °C and the relative humidity is raised by 10 % at 30 °C air temperature. With the isobaric heat capacity of air (1005 J / (kg K) ), density of air 1.2 kg/m^3, latent heat of evaporation 2260 kJ/kg the transfered power due to heating is about 50 W, due to evaporation (if the worker is sweating) 95W, this gives a cooling power of 145W. To press 14 liter per second through a filter into a PPE and again out of the PPE is technically probably not simple.

Cooling in the PPE

A cooling mechanism that cools the inner air of the PPE is problematic. As soon as the inside of the PPE hull is cooled, thermal energy will flow from the outside of the PPE into the inside. The thin foil of the PPE is not a significant barrier to heat. Whith each 1.4 °C the inner side of the PPE becomes cooler, an additional 320 W would flow into the PPE. This additional heat flow would required an even stronger cooling system.

The low heat resistance of the PPE foil there forces all cooling system inside the PPE to use one of two options: a) Cool the skin directly and provide a thermal barrier to the outside (This causes problem if the cooling stops, as this barrierr than works as an isolating layer that prevents heat transfer) or b) Condense water to reduce the humidity in the PPE. In that case the worker would be cooled if due to venting or other means, he is able to sweat effectively.

Cooling due to sweating

I it is possible to reduce the humidity by condensing, the flow of thermal energy would not only occur due to temperature differences. Evaporating air on the skin takes thermal energy from the body. This energy will then be released in the absorbing system. 
Such tools require:
a) Proper venting inside the PPE
b) Proper removal of excessive heat of adsorption

Cooling using phase change material

This may be ice or some other material. This material should cool the skin directly, this is problematic with ice, as it is too cold to be placed directly on the skin.
An insulating layer on the outside is required to prevent "heating" the environment.
This technique extends working times in cases where cooling from the outside is neglible. A thermal insulation is not a problem, or in cases of even higher outside temperatured advantageous.
This is the only fast and cheap method I know at the time I write this, that can be used in an existing PPE originally not designed for cooling.
 

References

[1] A. D. Flouris, S. S. Cheung. Design and Control Optimization of microclimate Liquid Cooling Systems Underneath Protective Clothing. Annals of Biomedical Engineering March 2006, Volume 34, Issue 3, pp 359-372
[2]Interim Guidance for Healthcare Workers Providing Care in West African Countries Affected by the Ebola Outbreak: Limiting Heat Burden While Wearing Personal Protective Equipment (PPE). (http://www.cdc.gov/vhf/ebola/hcp/limiting-heat-burden.html)
[3] Gustavo Zubieta-Galleja, Poul-Erik Paulev. New Human Physiology.(http://www.zuniv.net/physiology/book/chapter21.html)
[4]https://weatherspark.com/averages/29194/Monrovia-Margibi-Liberia
[5]George Havenith. Heat balance when wearing protective clothing. Ann Occup Hyg (1999) 43 (5):289-296. (http://annhyg.oxfordjournals.org/content/43/5/289.short)
[6]
R. G. Steadman, 1979: The Assessment of Sultriness. Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science. J. Appl. Meteor., 18, 861–873.
doi: http://dx.doi.org/10.1175/1520-0450(1979)018<0861:TAOSPI>2.0.CO;2(http://journals.ametsoc.org/doi/abs/10.1175/1520-0450%281979%29018%3C0861%3ATAOSPI%3E2.0.CO%3B2)
[7]Ralph F. Goldman, Berhard Kampmann 2007 (editors), Handbook on cloothing, http://www.environmental-ergonomics.org/Handbook%20on%20Clothing%20-%202nd%20Ed.pdf
[8]T.B.Cavarello et al., "Apparent evaporative resistance at critical conditions for five clothing ensembles," Eur J Appl Pyhsiol, vol. 104, pp. 361-367, 2008.



 

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Photo of Shane

Excellent research Rainer! Perhaps you might be interested to contribute your insights to help develop this idea on improving PPE cooling measures?

https://openideo.com/challenge/fighting-ebola/research/microclimate-cooling-in-a-ppe

Photo of Rainer

Hi Shane,
the link you gave is probably wrong. Which contribution did you mean? With kind regards
Rainer

Photo of Shane

Sorry for the mis-link! Here is an idea contribution on improving PPE cooling measures: https://openideo.com/challenge/fighting-ebola/ideas/active-or-passive-cooling-built-into-ppe-clothing-or-worn-underneath

You would be interested in checking out this like minded idea as well: https://openideo.com/challenge/fighting-ebola/ideas/wear-phase-change-cooling-vests-under-ppe-refreeze-in-ice-water-or-refrigerator

Photo of Shane

Rainer, here is another contribution that can use your insights! https://openideo.com/challenge/fighting-ebola/research/adding-ventilation-channels-to-ebola-protective-suits

Photo of Rainer

Hi Shane, thanks for the links I will check them. There are now so many ideas, that it is really important to point to them. Thank you very much! Rainer

Photo of Rebecca

Dear Rainer & Shane, do you think some passive cooling techniques could easily lower the temperature enough in the tents of the field hospitals so less is demanded on the PPE heat-reducing capabilities ?
https://openideo.com/challenge/fighting-ebola/ideas/help-cool-the-ppe-wearers-using-traditional-building-cooling-methods

in the article below is a video with view of a field hospital (for tent layout/passive cooling ideas)

http://blog.usaid.gov/2014/10/ive-never-had-a-job-like-this-life-inside-an-ebola-treatment-unit/

Photo of Rainer

Hi Rebecca, I think you are right. There should be passive techniques possible. Cooling the health care worker in the PPE is a huge effort. And each degree you cool the tents or buildings makes cooling of the PPE easier. I apologize for not having understood the importance of your idea.
With kind regards,
Rainer

Photo of Jason

Rainer, I just noticed your post. I independently did some of the same calculations and came up with almost identical results, which is great: https://openideo.com/challenge/fighting-ebola/research/ppe-cooling-calculating-how-much-heat-needs-to-be-removed

Photo of Rainer

Hi Jason, thanks. There are so many posts that finding becomes a big problem. Also, Iam always happy if someone checks my calculations, :-) Rainer

Photo of Justin

Structure cooling would be hugely effective and would no doubt help the PPE wearers tremendously. Building cooling requires a substantial amount of power (even with energy efficiency technology in place) to be effective. So long as this power could be provided, this would definitely be valuable.

Photo of Rainer

Hi Justin,
what do you mean by structure cooling?

Photo of Justin

Rainer - sorry to be unclear, I was referring to changing the ambient temperature in a structure/building. It usually takes a great deal of energy to do so. But yes, every degree would be valuable.

R/
Justin

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