As you review the technologies, be sure to keep in mind the requirements of your particular application. The pros and cons of each technology will help you select the personal cooling system that best meets your needs.

Umbilical systems.

Fluid-chilled systems:

Fluid reservoir holds ice to cool circulating fluid. Variable-speed pump circulates fluid and controls rate of flow. Tube garment carries cool fluid to body and draws away heat toward reservoir.

The fluid in the reservoir is chilled by ice to 33º to 34º F, then circulated by the pump through tubing passages in the vest-like garment. The chilled fluid will rise 6º to 7º F while moving from the pump to the garment. As the fluid passes over the skin, the body transfers heat toward the cooler fluid, which then carries the heat back to the reservoir. As the fluid re-enters the reservoir, it is chilled back down by the ice and the circulation process begins again.

The circulation pump is operated by either batteries or an AC adapter. (Batteries are used when mobility is required.) The battery life is typically 4 to 5 hours between recharges. This can create a problem when long-duration cooling is needed, as a change of batteries will be required at least once during the day.

The temperature of the fluid at the body can be controlled somewhat by changing the speed of the pump motor. This regulates the rate of flow, which, in turn, determines the amount of heat drawn from the body.

Fluid-chilled systems are efficient and work well. But mobility is limited because the reservoir and pump are separate from the garment. Some systems operate with a hip pack containing an umbilically attached reservoir and pump. These systems allow greater mobility, but add weight to the body and have limited space for ice, which limits the length of operation.

Another concern with fluid-chilled systems is that they can promote the formation of condensation due to their cool operating temperature range. This can cause some efficiency loss and dampness to the body.

PROS:

• Garments are available in all sizes and fit most areas of the body.
• Can be worn against the skin and are generally close fitting; uniforms easily fit over garments.
• Long-duration cooling.

CONS:

• Limited mobility due to umbilically connected circulating reservoir and pump.
• Formation of condensation can cause efficiency loss and dampness.
• More expensive than other technologies.
• Requires electricity or batteries to operate.

Pre-chilled- or forced-air systems:

Vapor compressor pre-chills air and pumps toward vest. Umbilical carries chilled air to garment from fixed compressor location. Body garment forces chilled air against body and carries heat toward atmosphere via convection.

Pre-chilled- or forced-air systems consist of a torso garment, a compressor, and an umbilical.

The compressor forces pre-chilled air through the umbilical and into a bladder in the garment. The air is then forced against the body through a series of orifices in the inner surfaces of the garment. As the cooler air passes near the surface of the skin, it convectively draws heat away from the body and into the atmosphere.

These systems are lightweight and provide efficient cooling, but mobility is restricted by the length of the umbilical. They also employ many moving parts in the compressor and require regular maintenance.

PROS:

• Garments are available in all sizes and fit most areas of the body.
• Provides a comfortable cooling temperature range.
• Lightweight garment construction.
• Long-duration cooling.

CONS:

• Mobility is limited and encumbered by the umbilical, which must be attached to a fixed compressor location.
• More expensive than most technologies.
• Requires electricity to operate.
• Moving parts require regular maintenance.

Passive systems.

Ice or gel pack vests.

Carrier holds ice packs against body. Insulation minimizes absorption of ambient heat by ice packs. Ice packs solidify at 32º F and absorb body heat until saturated.

Ice or gel pack vests consist of a torso garment containing pockets, surrounding the chest cavity, that hold ice packs. Body heat, carried to the surface of the skin by the circulatory system, is absorbed by the ice packs.

The garment fully loaded with packs is heavier than an umbilical-type garment, but is completely unattached to any external devices, making it much more portable. Typical weight is 9 pounds for a 100º F product and 11 pounds for a 125º to 130º F product. Cooling duration is approximately 1 to 1-3/4 hours between recharges based on average workload and individual metabolic rate. The packs recharge in five hours in a freezer and can be recharged literally thousands of times. When considering whether an ice or gel pack vest is the right choice, first determine how easily the wearer will be able to change packs on the job. After all, once the packs lose their cooling charge, they do nothing but add weight to the wearer.

A concern with this technology is that the ice packs condense, since their temperature is below the typical dew point. The condensation generates heat, which is then absorbed by the pack, reducing the duration before another recharge is needed. Also, the condensation is absorbed by clothing, causing discomfort and adding weight, which creates a greater load on the body.

Ice technology provides a reduction of body core temperature when used for short periods. However, with prolonged exposure (several hours of continuous use) the core temperature can actually begin to rise. This is due to vasoconstriction that occurs in the blood vessels carrying core heat to the surface of the skin. The vasoconstriction is caused by the 32º F temperature of the packs. The continued cold exposure of the packs to the skin fools the brain into thinking it's cold outside. The body then attempts to retain heat when, in reality, it should be giving up heat.

Since workload continues to generate even more heat, the core temperature rises. This condition can cause faintness and dizziness. If full workload continues, there is a serious risk of heat stroke.

In addition to vasoconstriction and the resultant physical problems, extended cold exposure to the skin can cause harm to skin tissue and the development of flu-like symptoms.

PROS:

• Inexpensive.
• Portable; no umbilical device needed.
• Rechargeable.

CONS:

• Fools body into thinking it's cold, which could be potentially dangerous (Vasoconstriction).
• Packs condense; uncomfortable to wear.
• Undergarments required to avoid direct contact with skin; defeats purpose of cooling.
• Bulkier to wear than umbilical systems.
• Requires freezer to chill.
• Limited-duration cooling.

CoolPack Technology.

Carrier holds CoolPack against body. Wicking fabric carries perspiration away from body toward outside of garment. Insulation minimizes absorption of ambient heat by CoolPacks. CoolPacks solidify at 59º F and absorb body heat until saturated.

Phase change material vests consist of a torso garment with chest pockets that hold CoolPacks packs, similar to ice and gel pack vests. Body heat, carried to the surface of the skin by the circulatory system, is absorbed by the packs, which operate at 59º F.

The garment fully loaded with packs is heavier than an umbilical-type garment, but is completely unattached to any external devices, making it much more portable. Typical weight is 3 pounds for a 100º F product and 9 pounds for a 125º to 130º F product. Cooling duration is approximately to 2.5 hours between recharges based on average workload and individual metabolic rate. The packs recharge in 20 minutes in ice water or a freezer, and can be recharged indefinitely.

Because the temperature range of the packs (59º F) is well above the typical dew point, the packs will not condense and will remain dry against the body.

The temperature of phase change material packs is within the comfort range of the body, so the garment can be worn for extended periods of time without risk of vasoconstriction or skin tissue damage.

Phase change material packs can not absorb as much heat as ice packs, but this limitation is offset by a lower loss of cooling potential to the ambient air temperature (because the differential between the ambient and the 59º F packs is less than the differential between the ambient and 32º F ice).

PROS:

• Comfortable temperature against the skin; no undergarment required.
• Inexpensive.
• Portable; no umbilical device needed.
• Packs won't condense; no irritating moisture against the skin.
• Wicking action removes perspiration.
• Easy to recharge in ice water; no freezer needed.

CONS:

• Bulkier to wear than umbilical systems.
• Limited-duration cooling.

Evaporative cooling.

Carrier holds water absorption crystals against body. Crystals absorb water when immersed, then evaporate the water to atmosphere to create cooling.

Evaporative technology consists of a garment and a water absorption material. The garment is extremely lightweight and the technology is inexpensive.

To use the garment, simply soak it in water and put it on. The crystals in the cloth swell up and contain water held closely against the body. The process simulates the body's evaporative cooling system as it evaporates the water held in the garment to the atmosphere. The phase change from water fluid to water vapor creates a tremendous cooling energy. There is some efficiency loss over natural perspiration evaporation because the water is not in actual conductive contact with the skin, but is actually cooling air between the absorption crystals and the skin.

The concept is simple, but has several drawbacks. Most obvious is that evaporative technology works well only in warm, dry air. When the humidity is high and the air already saturated with water vapor, the technology cannot work. There is a hybrid version of this technology, which suggests placing the water-saturated crystals in a freezer to solidify. The frozen crystals provide some absorption cooling in high humidity, but only for a very short duration, since the total amount of retained water is minimal.

Another drawback is that the garment is always damp, which can cause skin irritation, bacterial growth, mold, and odor.

PROS:

• Most inexpensive.
• Extremely lightweight.
• Portable; no umbilical device needed.
• Longer-duration cooling.

CONS:

• Requires the movement of warm, dry air across them to be effective; completely ineffective under any type of protective garment.
• Won't work in high humidity.
• Tends to be damp against the body; can cause skin irritation, bacterial growth, mold, and odor.

Environment cooling.

Spot-cooled area restricts worker mobility. Workers outside cooled area derive no cooling benefit.

There are many methods of conditioning and ventilating the air around workers. The purpose of this guide is not broad enough to include all methods, but a brief overview of benefits and concerns is presented.

If workers operate in a closed, contained space, it's possible to provide spot environment cooling. There are a number of methods available, and results will vary based upon the ambient temperature, humidity, and mobility of workers.

The simplest method is to provide good ventilation and a flow of fresh outside air. This can be an effective method against lower temperatures of 80º to 90º F, as the air movement will induce evaporation of perspiration. When the temperature rises above 90º F, however, this method is no longer effective, as the differential between circulating air temperature and skin temperature is too small, so body heat can no longer be drawn away.

Another method is to use spot air pre-chillers. This is effective for cooling a small group of workers in higher heat, but is an expensive approach and limits workers mobility to the cooled area. Also, as workers constantly move back and forth between hot and cold areas (e.g., to get parts or to perform other temporary work), they can develop flu-like symptoms over extended periods of exposure.

In addition to the initial expense of these types of environment cooling systems, keep in mind that there will be an ongoing maintenance cost, plus the utility cost to run them.

PROS:

• Allows good worker mobility, but only within cooled area.
• Temperature is easily controlled.
• Can be turned off when not required.

CONS:

• Most expensive.
• Not effective for outdoor situations.
• Not effective for large groups of workers.