Heat pack for survival in cold water

A heat pack for supplying thermal energy to the wearer on immersion in cold water comprises a capsule in which water can be admitted. The capsule contains a heat generating agent arranged in a tablet having such a shape that a reactive surface is created which allows the agent to react exothermally with the water at a controlled reaction rate.

BACKGROUND OF THE INVENTION 
The invention relates to a life preserving heat pack for supplying thermal 
energy to the wearer upon immersion in cold water. 
Sudden immersion in cold waters such as the North Sea poses a severe threat 
to human life. If the human body is immersed in cold water at near 
freezing temperature the cooling effect of the water may rapidly cause 
hypothermia and thus a drastically reduced blood circulation. 
DESCRIPTION OF THE PRIOR ART 
Contemporary approaches to this problem rely to a large extent on thermal 
insulation of the human body. In this respect "dry" immersion suits are 
known which prevent contact of water with the wearer's skin and garments 
designed on the "wet suit" principle, admitting some water to reach the 
skin but minimizing through-flow via the garment. It is also known in the 
art to provide for example diver suits and survival garments with an 
active heat source in order to counteract the cooling effect of the water. 
U.S. Pat. No. 3,884,216 discloses a heater system to supply heated fluid 
through a diver's suit using an electrochemical energy source which is 
shortcircuited to produce heat by the reaction of magnesium with seawater. 
U.S. Pat. No. 2,618,257 discloses an insulated life preserver garment 
comprising a shell in which a water activated heat generating means is 
arranged wherein the wearer may regulate the water supply to the heat 
generating means by opening or closing a valve, thereby regulating the 
amount of heat generated. West German Patent No. 2,941,116 discloses a 
survival garment containing at selected locations capsules with chemicals 
which react exothermally with each other and which may be brought in 
contact with each other by breaking a seal between the capsules. These 
previously known heat sources exhibit several disadvantages such as their 
bulky volume and difficulties encountered to activate the heat source and 
to subsequently regulate the amount of heat required. 
SUMMARY OF THE INVENTION 
The object of the invention is to provide a light-weight life preserving 
heat pack which can be combined with commonly used offshore clothing and 
which can be easily activated to generate heat in such a manner that, at 
least after the source has been activated, the heat output is regulated 
automatically without requiring manual control by the wearer. 
A heat pack according to the present invention comprises a capsule in which 
water can be admitted, the capsule containing a heat generating agent 
arranged in a tablet having such a shape that a reactive surface is 
created which allows the agent to react exothermally with the water at a 
controlled reaction rate. In an attractive embodiment of the invention the 
tablet is substantially cylindrical and comprises magnesium chloride as a 
heat generating agent. 
The heat pack according to the invention may be attached to a safety belt 
around the waist of the wearer, or be arranged in the wearer's clothing 
such as a body-warmer garment that can be worn under a rainproof suit.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The heat pack shown in FIG. 1 comprises a capsule 1 containing a heat 
generating agent arranged in a tablet 2. The tablet 2 has a substantially 
cylindrical shape and the heat generating agent consists of magnesium 
chloride (MgCl.sub.2). The magnesium chloride may be arranged in the 
tablet 2 either in the form of a solid block of anhydrous magnesium 
chloride or combined with a suitable binding agent. 
The capsule 1 comprises two interbonded impermeable parts 1A and 1B. The 
first capsule part 1A has the shape of a flat box having an opening 3 in 
one of its sides. The second capsule part 1B is arranged as a patch 
covering the opening 3 in the first capsule part 1A and bonded to the rims 
4 of the first capsule part 1A surrounding the opening 3 by means of a 
water soluble glue 5. 
The capsule 1 consists of two interbonded impermeable parts which allows 
storage of the tablet 2 in a dry environment during normal conditions but 
which allows quick entry of water via the opening 3 into the capsule 1 
upon immersion. After the water gets in contact with the tablet 2, 
exothermal hydration of the magnesium chloride at the surface of the 
tablet 2 starts. The rather small reactive surface-volume ratio of the 
tablet 2 enables the hydration to continue at a steady reaction rate so 
that a steady heat output during an hour or more may be achieved. For 
safety reasons it is essential that the hydration is tempered by creating 
a small reactive surface since otherwise a vigorous hydration of the 
magnesium chloride would take place resulting in a large heat output 
during a time interval of a few minutes only. 
In general, controlling the hydration of magnesium chloride by only 
regulating the amount of water entering the capsule should be avoided 
since in emergency situations the wearer may be unable to adequately 
control the water inflow. Since the wearer may also be unable to activate 
the heat pack at the moment of immersion, it is preferred to use a water 
soluble glue 5 to interbond the capsule parts 1A and 1B so that the heat 
pack is activated automatically upon immersion. Dissolving of the water 
soluble glues by rainwater can be avoided by arranging the capsule in a 
garment inside an impermeable sack, which is turned upside down under 
normal conditions so that dripping rainwater does not reach the water 
soluble glue but which fills up with water upon immersion. 
The heat pack shown in FIG. 2 comprises a capsule 11 consisting of two 
interbonded capsule parts 11A and 11B, which capsule 11 contains a 
magnesium chloride tablet 12 similar to the tablet described with 
reference to FIG. 1. In the heat pack of FIG. 2 however, the second 
capsule part 11B is bonded to the first capsule part 11A by means of a 
water soluble adhesive tape 14. The tape 14 consists of repulpable paper 
impregnated with glue means of a type known in the art. The large tape 
face area which is exposed to water upon immersion allows quick 
dissolution of the tape and thus quick activation of the heat pack. 
FIG. 3 illustrates a heat pack comprising a capsule 21 which is connected 
to a safety belt 20 that can be secured around the waist of the wearer. 
The capsule 21 comprises an open box-like first part 21A and a flat second 
part 21B which is bonded to the terminal rims 22 of the first capsule part 
21A by means of a water soluble glue 23. The first capsule part 21A 
consists of a rigid plastic shell which is provided at the outside thereof 
with a pair of molded-in belt loops 24 and at the inside thereof with a 
number of retaining ribs 25 for maintaining a magnesium chloride tablet 26 
in a fixed position inside the capsule 21. The tablet 26 is molded in the 
first capsule part 21A in such a manner that water introduced after 
opening the capsule cannot enter between the tablet 26 and the walls of 
the first capsule part 21A so that hydration can take place only at the 
top side 27 of the tablet 26, whereas the remaining tablet surface is 
shielded-off. In this matter a small reactive surface is created and thus 
a moderated heat-flux is generated over a long interval of time, at least 
until a substantial part of the tablet has been hydrated. Further 
moderation of the reaction rate may be accomplished by arranging a flow 
stabilizing cover mesh 28 over the tablet 26 which mesh may, after opening 
the capsule, reduce flow pattern variations of the water entering the 
capsule and simultaneously provide reaction control by concentration 
effects. 
As further illustrated in FIG. 3, the second capsule part 21B is provided 
with a pull tab 29 to enable manual activation of the heat pack 
immediately after immersion. 
FIGS. 4A and 4B illustrate another embodiment of the heat pack according to 
the invention. The heat pack shown in these figures comprises a capsule 41 
comprising two telescoping cup-shaped parts 41A and 41B. The cup-shaped 
capsule parts 41A and 41B are sealingly interconnected by means of a 
flexible roll-sock membrane 42 which allows the capsule parts 41A, 41B to 
telescope from a compressed position shown in FIG. 4A towards an extended 
position shown in FIG. 4B while maintaining the seal provided by the 
membrane 42. 
The second capsule part 41B is provided with a one-way flap valve 43 which 
allows water to be sucked into the capsule 41 in response to telescoping 
the capsule parts 41A, 41B towards the extended position thereof as shown 
by arrows 45 in FIG. 4B. Said ingress of water into the capsule 41 
activates hydration of one or more magnesium chloride tablets 44 arranged 
in the capsule 41 thereby generating heat. Since the one way flap valve 43 
does not allow escape of fluid from the capsule 41 a closed "hot bottle" 
is created and thus any contact of the products of said hydration with the 
wearer's skin is avoided. 
The heat pack shown in FIG. 4 may be secured to a safety belt 20 like the 
heat pack of FIG. 3 and be provided with a pulling wire 46 to telescope 
the capsule parts away from one another, but if desired the heat pack may 
also be loosely stored inside a pocket of a life preserving garment and be 
activated after retrieving the heat pack from the pocket whereafter it is 
inserted at a location inside the garment where heat is needed. 
Alternatively, the heat pack may be built-in inside a body warmer garment 
which can be worn as a vest under rainproof clothing commonly used 
offshore. 
If desired, the body warmer garment may further comprise flotation cells 
which fill up with gas automatically upon immersion as known in the art. 
The heat pack may be secured in such a manner to the garment that the 
telescoping capsule parts of the heat pack move automatically away from 
each other in response to inflation of said flotation cells, so that the 
heat pack is activated simultaneously with the flotation cells. 
To produce the amount of heat required, a life preserving garment may be 
provided with a single heat pack which heats the layer of water between 
the garment and the wearer but, alternatively, the garment may be provided 
with a plurality of heat packs. In particular the "closed" heat pack 
illustrated in FIGS. 4A and 4B which acts as a hot water bottle may be 
used for protecting sensitive areas such as the hands and neck/occipital 
regions of the head by direct heat convection. 
Reaction control is an important safety requirement under all circumstances 
and it has been found that the heat output/time profile of a heat pack can 
be successfully tailored to requirements by tabletting the heat generating 
agent to reduce the effective surface area where hydration takes place. 
For tablets where anhydrous magnesium chloride is used along or with a 
suitable binding agent, such as the heat generating agent, it has been 
found that stable tablets can be formed with moderate compaction pressures 
during tabletting. The tablets thus formed had a compacted density of 
around 2000 kg/m.sup.3 : about twice the "loose powder" density of a 
magnesium chloride powder. Experiments have shown that a cylindrical 
tablet with a diameter identical to the height of the tablet and having a 
volume of about 7.times.10.sup.-4 m.sup.3 containing 1.4 kg MgCl.sub.2 
with a typical density of about 2000 kg/m.sup.3 is able to provide a power 
output of 300 W for 2 hours and thus a total heat production of 2 MJ. 
Although the amount of heat required to avoid hypothermia of the human 
body depends on various factors such as body size, water temperature and 
the presence of further passive thermal insulating provided by clothing, 
it has been found that under practical circumstances a heat output of 200 
to 400 W is sufficient to counteract the cooling effect of water having a 
temperature between 0 and 10.degree. C. 
It is to be understood that the tablet or tablets used in the heat pack 
according to the invention may be cylindrical, cuboid, spherical or be of 
any other desired shape. 
Many other variations and modifications may be made in the apparatus 
described above by those having experience in this technology without 
departing from the concept of the present invention. Accordingly, it 
should be clearly understood that the apparatus depicted in the 
accompanying drawings and referred to in the foregoing description are 
illustrative only and are not intended as limitations on the scope of the 
invention.