This study is called the Manikin Evaluation of Personal Cooling Systems for Soldiers conducted at the Institute for Environmental Research at Kansas State University by Elizabeth A. McCullough, Ph.D.
The testing is designed to evaluate the Texas Cool Vest light cooling vest and heavy cooling vest products for military use. A complete pdf version of this study is available for download (link to study evaluating cool vest effectiveness).
Introduction to Overall Project
There is an immediate need for personal cooling systems (PCS) that can mitigate heat stress
for Soldiers deployed in the Middle East – particularly during the summer months when the high air
temperatures and radiant load from the sun can cause the body to retain heat. These environmental
conditions, combined with the use of heavy protective clothing and carrying a load of supplies and
equipment, can put a thermal strain on Soldiers – especially when they are working and their
metabolic heat production increases.
In extremely hot environments and/or at high activity levels, the only way the body can lose
excess heat is by the evaporation of sweat from the body surface. The rate of evaporative cooling is
dependent upon the vapor pressure gradient between the skin surface and the environment and the
rate of air movement around the body and between clothing layers. Unfortunately, protective clothing
such as body armor and helmets can inhibit the evaporation of sweat. In addition, the weight,
rigidness, and design of protective garments may increase the energy cost associated with wearing
them during activity. Consequently, Soldiers operating in hot environments often experience heat
stress symptoms that affect performance on extended operations.
To overcome these limitations, the Army has been searching for new technological advances
in personal cooling systems (PCS) that have been developed by manufacturers and evaluating their
effectiveness for military use.
Purpose
The purpose of this study was to measure the heat removal rate and cooling duration of
selected PCS using a sweating manikin in an environmental chamber. The manikin test provides a
repeatable, unbiased, quantitative method for comparing the cooling effectiveness of a variety of
different PCS. Data comparing the base ensemble with the PCS in cooling mode and turned off (or
used up) was collected.
Personal Cooling Systems and Clothing
Each PCS was evaluated as part of a military base ensemble which consisted of Army combat
garments and the Soldier Plate Carrier System (i.e., body armor). The ensemble weighed 15.054 kg
(33 lbs. 3 oz.) and is shown in Figure 1. The base ensemble consisted of
- ACH Advanced Combat Helmet (with cover, suspension system, and pads)
- Hanes Premium boxer briefs (75% cotton 25% polyester knit, fitted style)
- Gold Toe Ultra Tec crew socks (cushioned, antimicrobial, 79% cotton, 14% nylon, 6%
polyester, 1% spandex) - ACS Army Combat Shirt – Fire Resistant (knit portion on torso replaces T-shirt) shirt tucked
into pants - ACP Army Combat Pants – Fire Resistant (use the drawstrings at the bottom of the trousers to
blouse around boots; when bloused, the trousers should not extend below the third eyelet from
the top of the boot) - Belt
- External Knee Protectors
- ACG Army Combat Gloves (worn under sleeve cuffs)
- MCB Mountain Combat Boots
- SPCS Soldier Plate Carrier System (lighter body armor with plates in place)
- ESAPI Enhanced Small Arms Protective Inserts (front and rear hard armor plates)
- ESBI Enhanced Side Ballistic Inserts (small side hard armor plates)
The base ensemble had an intrinsic clothing insulation value of 1 clo (0.155 m2.°C/W)
and an intrinsic evaporative resistance of 26.2 m2.Pa/W.
The cooling effectiveness of the personal cooling systems was measured according to ASTM
F 2371, Standard Test Method for Measuring the Heat Removal Rate of Personal Cooling Systems
Using a Sweating Heated Manikin (ASTM, 2010). In addition, each PCS was weighed so that its
watt-to-weight ratio (W/lb.) could be determined and compared.
Apparatus
The insulation values and evaporative resistance values for the clothing systems were
measured using an electrically-heated manikin in thermal equilibrium with its surroundings. The
manikin at Kansas State University – STAN – consists of a shell formed to simulate the physical
shape and size of a typical man (i.e., 1.80 m² surface area, 177.2 cm height). The manikin consists of
20 independently heated thermal zones (see Figure 2), with an additional fluid heater inside the
manikin. All thermal zones are fit with heaters to simulate metabolic heat output rates and distributed
wire sensors for measuring temperature. A chart describing the body segments (zones) and their
surface areas is shown in Table 1.
The power cables, measurement cables, fluid supply tubes, and fluid return tubes connect to
his face. A photograph of the manikin in his sweating suit is shown in Figure 3. The entire system is
computer operated. The ThermDAC control software is a 32-bit Windows based program that
provides control capabilities, data recording, and real-time numerical and graphical displays of
section temperatures.
Manikin Procedures for Sweating Tests
The environmental conditions for the isothermal sweating manikin tests were controlled as
follows:
- ambient air temperature, 35ºC (95ºF)
- air velocity, 0.3 m/s
- relative humidity, 40%
- manikin surface temperature, 35ºC (35ºF)
The manikin was covered with a knitted “skin” and sprayed with distilled water to simulate
skin saturated with sweat (i.e., 100% skin wettedness). Then the flow rates to the manikin were
adjusted so that enough water was distributed through his pores to keep the skin saturated. Two air
temperature sensors and one relative humidity sensor were hung in back of the manikin at waist level
about 2 ft. from the manikin. The air velocity was measured periodically using an anemometer.
Baseline test. First a baseline test was conducted on the ensemble with the PCS turned off. In
the case of phase change materials, a “used” component of the PCS was tested (e.g., cartridge of
water at 35°C instead of ice, etc.). To conduct a baseline test, the manikin was dressed in the PCS
ensemble, and all closures were secured. The manikin was hanging from his metal stand by a hook in
his head. His feet did not touch the floor because excess water runs out of small holes in his shoes
during a test and pools in a tray beneath him. As soon as steady-state conditions had been reached, a
30 minute test was run and the power level was recorded.
PCS test. Next the “heat difference” program was opened on the manikin’s computer. This
program quantifies the cooling rate of the PCS by subtracting the average power level during the
baseline test from the power used to keep the manikin’s skin temperature at 35°C when the PCS is
turned on. When the program was ready, the PCS was turned on (or in the case of phase change
materials, a new component was added to the PCS), and the experiment was started immediately.
Data were collected for 2 hours.
Three replications of the baseline tests with the PCS turned off, followed by the heat
difference test with the PCS turned on, were conducted for each type of PCS.
Results
The standard defines the cooling rate as the time average of the power input to the manikin
from the time the PCS was activated and data collection was started until the effective power (power
to the manikin minus the baseline power level) decreased to 50 W – for a maximum test of 2 hours.
However, some of the PCS we tested never reached 50 W to begin with, so we ran each test for 2
hours. We calculated the cooling rate two ways: 1) the time the system was drawing 50 W or more of
power, as the standard specified, and 2) the average cooling rate over 2 hours – even though this is
somewhat meaningless if a system did not cool for very long.
For this part of the study, we tested two versions of the phase change technology cooling
system from Texas Cool Vest: Version A – the lightweight vest holding the standard size packs and
Version B – the modified lightweight vest holding the heavy packs. Photographs of the systems and
a graph of their average cooling performance during the tests are shown in Figures 4 – 7. Cooling data
are given in Table 2. The system is described in detail in Appendix A.
The standard packs performed well averaging 57 W for the two hour test; however the
cooling level dropped below 50 W after 79 minutes had passed. The heavy packs provided greater
cooling power averaging 88 W for the two hour test. Although more cooling was generated with the
heavy packs, the cooling watts per weight ratio was lower since they weighed more. The horizontal
3mounted packs in the back of the modified light vest were difficult to fit on the manikin when frozen
because of their length and inability to bend and match the curve of the torso.
For both weights of PCM packs, the cooling effectiveness of the phase change material
decreased as the test progressed. To maintain the cooling effectiveness on a Soldier, the packs would
have to be replaced every 1 to 2 hours. This would require extra frozen packs and the facilities to
freeze and store these units. Since the solidification temperature of the PCM is relatively high at
65 ºF, the Texas Cool Vest PCS will require less time and power to re-solidify than ice-based types,
and will be easier to transport in the field in insulated vehicles. In addition, the “freezing”
temperature of the packs would not be as likely to cause vasoconstriction on the body as compared to
water/ice based systems.
Type of Technology: Phase Change Materials
Short Technology Description: Phase Change Material (PCM) packaged in flexible plastic packs.
{n-Hexadecane; cetane. CAS No.: 544-76-3 CH3(CH2)14CH3} solidifies at 64.8ºF ( 18.22ºC) liquid
at 70ºF ( 21.11ºC)
Physical Description: Quick dawning upper torso vest style garment. Velcro adjustable waist and
shoulders with zipper front closure, each containing 4 phase change chemical packs held in internal
pockets. PCM packs are insulated from the ambient air and local environment by “Thinsulate ™”
insulation layer built into the outer shell of the garment. Unique adjustment system allows for custom
and individual fitting with body weights from 140 to 275 pounds (65 to 125.0 kg). Light vest
contains four vertical pockets for light, and standard phase change packs. Modified light vest
contains two front vertical pockets for heavy phase change packs on the front of the vest and two
horizontal pockets for heavy packs along the back of the vest.
Sizes Available: Fully adjustable one size fits all. 145 to 275 pounds.
System Weight: (for all available systems)
Light vest
1340 g of PCM material
Standard vest
1600 g of PCM material
Heavy duty vest
3400 g of PCM material
Power Requirements:
total system weight 3.8 lbs
total system weight 4.8 lbs
total system weight 8.7 lbs
1.72 kg
2.18 kg
3.95 kg
None
Estimated Runtime (per Charge/Fill): 1.5 – 3.0 hrs after 20 minute immersion in ice water. Runtime
is dependent on heat generated by the physical activity and body mass of the individual using the
garment. Vest cool packs will recharge in 20 minutes, (three individuals and four packs with a chilled
water container can maintain the all systems indefinitely). Every 20-30 minutes a different
13individual replaces his cool packs with a recharged set in the ice water cooler. This allows for each
vest to be changed out and recharged every 90 minutes. This has worked well in the Arabian Gulf for
the US NAVY on small craft.
Supplies Required: A cooling location below 60ºF (15.5ºC). Submergence in ice or cold water works
the best, allowing for maximum heat transfer in recharging the PCM cool packs. Refrigerator, room,
or vehicle air conditioner will work but charging time is based on temperature difference between the
PCM material and the air flowing over the pack.
