15-6 THERMAL PROTECTION SYSTEM

THERMAL PROTECTION SYSTEM

All saturation diver life-support systems include diver thermal protection consisting of a hot water suit and a breathing gas heater. The thermal protection is designed to minimize the diver’s heat loss caused by helium’s high thermal conductivity. Helium conducts heat away from the body rapidly and causes a significant heat loss via the diver’s breathing gas. The diver’s metabolic rate may not be great enough to compensate for the heat loss when breathing cold gas, resulting in a drop in body temperature and increasing the chance of hypothermia.

Diver Heating

Because of the high thermal conductivity of helium and depths attained, most conventional diving suits (i.e., wet suits/dry suits) provide inadequate insulation in a helium environment. As a result, thermal protection garments for helium-oxygen saturation diving must employ active heating. The most successful thermal protection currently used is the non-return valve (NRV) hot water suit using circulating hot water as the heat source. The typical NRV hot water suit is constructed from closed-cell, pre-crushed neoprene with an outer layer of tough canvas-type nylon. The interior is lined with a softer nylon with perforated hot water hoses along the limbs, chest, and backbone. Divers are required to wear Polartec Diveskins or Neoprene liners under their NRV suits. The liners or Diveskins offer almost no protection from cold water. The liners or Diveskins keep the divers from getting burned by hot water discharge from the NRV suit and minimize chafing of skin.

The effectiveness of the hot water suit in keeping the divers warm is dependent upon maintaining an adequate flow of water at the proper temperature. A 4-gallon per minute (gpm) (3 gpm to the suit and 1 gpm to the breathing gas heater) hot water flow rate with the suit inlet temperature adjusted to diver’s comfort generally provides adequate protection. During normal operation, hot water is distributed through the NRV hot water suit and is then discharged to the sea through the NRV. If there is a diver heating system failure, the diver shuts the NRV and opens the bypass valve, trapping sufficient hot water in the suit to allow him to return to the PTC. To prevent burn injury to the diver, the water temperature at the suit inlet should not exceed 110°F. Hot water thermal protection systems should be designed to provide individual control of water temperature and rate of flow supplied to each diver. All divers normally use umbilicals of similar length.

Inspired Gas Heating

The thermal protection system includes a breathing-gas heater to warm the gas to a temperature sufficient to minimize respiratory heat loss. A typical breathing-gas heater is a hot water heat exchanger that can raise the breathing-gas temperature by 30–50°F. Breathing cold helium-oxygen at deep saturation diving depths can cause incapacitating nasal and trachea-bronchial secretions, breathing difficulties, chest pain, headache, and severe shivering. These symptoms may begin within minutes of starting the dive excursion. The thermal protection system includes a breathing-gas heater to warm the gas to a temperature sufficient to minimize respiratory heat loss. A typical breathing-gas heater is a hot water heat exchanger that can raise the breathing-gas temperature by 30–50°F. Breathing cold helium-oxygen at deep saturation diving depths can cause incapacitating nasal and trachea-bronchial secretions, breathing difficulties, chest pain, headache, and severe shivering. These symptoms may begin within minutes of starting the dive excursion.

Breathing apparently comfortable but low-temperature helium-oxygen at deep depths can rapidly lower body temperature through respiratory heat loss, even though the skin is kept warm by the hot water suit. The diver usually remains unaware of respiratory heat loss, has no symptoms, and will not begin to shiver until his core temperature has fallen. Metabolic heat production may not compensate for continuing respiratory heat loss. Table 15-1 contains guidelines for the minimum allowable temperatures for helium-oxygen breathing gas. These limits are based on a 4-hour excursion with a maximum core body temperature drop of 1.8°F (1.0°C) in a diver wearing a properly fitted and functioning NRV or hot water suit.

TABLE 15-1

TABLE 15-1 Guidelines for Minimum Inspired HeO2 Temperatures for Saturation Depths Between 350 and 1,500 fsw.*

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