A Superportable Immersion Cooler for Vaccination Work in Remote Areas
The Refrigeration Problem
Vaccination offers enormous benefits to tribal peoples in many areas. This is especially true among South American Indians, where, for example, mortality rates of 50% or more have resulted from measles epidemics. Vaccination can provide protection from measles and other diseases. However, vaccination work in remote areas such as the Amazon Basin is difficult - the vaccine must be kept cold in hot, dense forests without electricity or other energy sources.
Portable refrigeration equipment transported by aircraft, boat, or jeep is a partial solution. Some small refrigerators are commercially available. The Jordan/Fogel company of Philadelphia produces a small refrigerator (6 kg.) with a lightweight thermoelectric immersion cooler and insulating jug. These operate on a 12-volt auto battery and cost only $150-$200. There are also heavier but nevertheless portable refrigerators which burn propane or kerosene as energy sources.
Portable refrigerators can be very useful where resources exist to pay for mechanized transport. However, because of their weight and other requirements - lead batteries, generators, fuel - they are inconvenient for extended use in areas accessible only by foot. For this reason, in Brazil, for instance, Indians are very seldom vaccinated unless they are readily accessible by airplane, boat or jeep. Many die in epidemics long before roads or airstrips are available.
Superportable Insulated Storage
To facilitate vaccination in areas without mechanized transport a Superportable system has been devised. It is light enough to be easily carried in a backpack, with enough fuel for 2-3 weeks. It can be constructed from cheap, readily available materials.
This system depends on excellent insulation so that only a low-capacity refrigeration unit is needed. The best insulation commonly available is a thermos bottle. One dose of most vaccines is only 0.5 ml.; the amount needed for 200 recipients has a volume of 100 ml., or less than 4 oz. This volume may be doubled because of the glass vials, but it will still take up only about 25% of a quart or liter vacuum bottle. The vials of vaccine sit inside the vacuum bottle in enough cold water to fill the vacuum bottle 85%-90% full. (The empty 10%-15% prevents overflow and minimizes contact with the less well-insulated neck and cap.) This water keeps the vials from jostling, and serves as a cold temperature reservoir and a medium for heat transfer.
A quart (.95 l.) glass-lined, narrow-mouth vacuum bottle (Thermos brand) was tested under heat conditions similar to those of a day-night cycle in the tropical forest. Over a 24-hour period 850 ml. of water rose from 2°C to 7°C, a gain of 5°C. This means that about 4,250 calories of heat enter the vacuum bottle each 24 hours under those conditions. (One calorie raises one ml. of water 1.0°C.) That is only about the amount of heat required to melt two one-ounce ice cubes.
In the system being described the thermos with water and vaccine is opened once a day, preferably in the cool of morning, and the immersion cooler is used to extract the approximately 4,250 calories which have accumulated. This drops the temperature back down about 5°C. The suggested optimal temperature range for storing most vaccines is 2°-8°C (35.6°-46.4°F).
Vials can be tied together with string or rubber bands for easy retrieval from narrow-mouth bottles. Widemouth vacuum bottles are more convenient but let more heat enter. Stainless steel vacuum bottles are unbreakable, but they allow almost twice as much heat to enter as do good quality glass-lined bottles. The usual styrofoam cooler lets in about 15-20 times as much heat as a glass-lined thermos bottle.
Superportable Immersion Cooler - Basic Principles
The immersion cooler used for the daily cooling of the thermos' contents operates on the same principle as a kitchen refrigerator. When a liquid becomes a gas it absorbs energy. The amount of energy absorbed for the particular substance is called its heat of vaporization. In the immersion cooler liquified propane is released from a tank through a coil of copper tubing. As it turns to gas it absorbs heat from the copper tubing, which in turn absorbs it from the water in the vacuum bottle. The propane gas escapes from the tail of the coil; it is not burned or recycled. So the immersion cooler must be used outdoors away from open flames.
The heat of vaporization of propane at room temperature is 83 calories per gram. The immersion cooler is about 75% efficient: each gram of liquid propane expended extracts roughly 60 calories from the contents of the vacuum bottle. This is the efficiency when the temperature of the contents of the thermos ranges between 3°-8°C. If the temperature range is kept lower, say 0°-5°C, then the efficiency is somewhat lower (about 55 cal./g.). Also efficiency drops somewhat near the end of a tank of propane when the rate of flow is lower. Even assuming an average figure of 55 cal./g., we could extract the required daily 4,250 calories with less than 80 g. (2.8 oz.) of liquid. In practice it is probably wise to add a safety margin and figure 100 g. (3.5 oz.) of propane per day to keep a thermos of water and vaccine cold.
Construction of the Immersion Cooler
The construction of the immersion cooler is simple. Most of it is purchased. All that is necessary is to saw off the shaft of a common propane torch, construct the coil, and cement the coil into the shaft. Of course larger propane tanks could be used. Cooking and lighting attachments could be used to recycle the gas.
In the model illustrated, about 35 mm. of the shaft was cut off. The coil was made by wrapping about 7.5 ft. (2.5 m.) of 1/8 in. (3.175 mm.) outside diameter copper tubing around a 3/4 in. (19 mm.)tube, which was later removed. The resulting coil fits into the one-inch (25.4 mm.) mouth of a thermos. Before coiling the tubing around the 3/4 in. tube, the exhaust tail was tucked through the tube. After the tube is removed the exhaust tail remains inside the coil so as not to add more width. Avoid making sharp bends or kinks in the tubing.
The tip of the coil, where the propane enters, must be constricted to form a very narrow opening. If this is done the propane cannot expand into a gas until it is in the coil. Otherwise it will expand immediately, chilling the entire valve and lowering efficiency. The hole in the tip should be between .004 and .025 in. (0.1 and .64 mm.). A household pin is .025 in. In diameter, an extra-fine needle is .015 in., and a fine wire can be .005 in. or less. To form the narrow aperture in the tip of the coil, insert an extrafine needle into the tip, squeezing it closed with pliers. Use the pliers to extract the needle, leaving a hole of the proper size.
To attach the coil to the shaft, first cement a plastic disk of the proper size about 3 mm. below the tip of the coil. When the coil is inserted into the shaft and cement poured in around it the disk prevents the cement from dripping out or clogging the tip. Epoxy cement is excellent because of its strength and low thermal conductivity. Wrap insulation around the shaft and the neck and tail of the coil to keep heat from entering and to block ice formation. Several layers of waterproof tape will work for this purpose.
The total cost of the immersion cooler (with one tank) is less than $15.
Operation of the System
When in use, the mouth of the tank must always face downward. Insert the coil of the assembled immersion cooler into the thermos. Open the valve wide - dribbling the propane out allows it to expand before entering the coil. The coil tip apertures of .025 and .015 in. best results are obtained by releasing the propane in short bursts of 3-8 seconds, waiting 10 seconds or more between bursts for heat to enter the coil from the water. With an aperture of .005 in. bursts of 10-20 seconds were used, but ice tends to form on the top of the coil and must melt before withdrawing the coil. The exhaust gas should be cool, but not cold. If cold, wait longer between bursts. If the valve fitting becomes heavily frosted, reduce the duration of bursts or narrow the aperture more.
Check the contents of the thermos occasionally with a thermometer. When the desired temperature is reached, withdraw the immersion cooler. If ice forms inside the thermos and won't melt after 10 minutes, 0°C has been reached. Use a fine wire to unplug the aperture if it clogs.
Roughly 55% of the weight of a full propane tank is the weight of the container; 45% of the weight is the propane itself. A standard small tank of 14 oz. (400 g.) of propane weighs about 1.9 lb. (0.9 kb.) and will provide 4 (or more) days of refrigeration (assuming we start with a thermos and contents already at 8°C). A 5 kg. tank would provide over 20 days of refrigeration. A large 20 kg. tank would provide over 80 days of refrigeration. This last size is not superportable, but is very suitable for a base camp where vaccine is kept in quantity. Smaller tanks have the advantage of being discardable when empty, reducing weight.
Butane is superior to propane in the system described above, although it is less widely available. It extracts 5% more calories per gram and has relatively lightweight, thin-walled containers because it liquifies at a low pressure.
The immersion cooler can be used as an ice maker by placing it in a small, snug-fitting container of water. However, efficiency is lower and the ice is difficult to remove, especially with the exhaust tail inside the coil. Perhaps a different shape, a flat, elongated spiral instead of a coil, would be better for making ice.
The exhaust propane from the cooler can be used for heating or lighting, but in that case a tip aperture of .004 in. or less is needed and no small apertures can be allowed downstream from the coil.
The superportable system can also bring out refrigerated biological specimens from remote areas, e.g., sputum or serum samples. The quality of vaccination work should improve with a better refrigeration system since such work can be carried out less hurriedly.
Aside from the refrigeration problem, more organization is needed in areas such as South America. For example, locating and preparing the target population for vaccination can be done by someone in advance of the medical personnel, reducing the amount of time they must spend. More reliable means of recording who was vaccinated are needed. In some areas, smallpox vaccinations are given at the same time as other vaccinations so that the tell-tale scar will signify which people received vaccinations.
The choice of vaccines and the scheduling of their use needs to be adapted to the specific needs and conditions of tribal peoples. For instance, South American Indians are known to have enormous rates of deadly bronchopneumonia complications of respiratory diseases (36% in a measles epidemic among the Yanomami, 20% in an influenza epidemic among the Kreen-Akorore), yet the pneumococcal pneumonia vaccine is still not deployed in this population, the very one which would benefit most from it.
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