is likely to condense. The advantage of this type is that the temperature of the hold or cargo can be read on the deck or bridge without the necessity of entering the hold. The bulbs of these instruments should be so placed that they are protected against damage during cargo handling. Dew point may be measured either by wet and dry bulb thermometers or by a humidity indicator and dry bulb thermometer. Wet and dry bulb thermometers show the wet and dry bulb temperatures. By the use of a psychrometric chart (fig. 56) the dew point can be obtained. Wet and dry bulbs measure the air conditions very accurately, providing there is a brisk movement of air over the wet bulb. Where this air circulation is limited, it is necessary to use mechanical means, such as a hand-aspirator. By the use of a humidity indicator (instead of the wet and dry bulb thermometer) a direct reading of the relative humidity is obtained. However, before the dew point can be determined from the psychrometric chart the temperature (dry bulb) of the air must be known. For this reason a thermometer must be used in conjunction with the humidity indicator, and consequently these two instruments are often contained in one case. METHODS OF RECORDING DATA The method of recording the data obtained will depend upon the type of instrument used. Where recording instruments are used, a continuous record is made by the instrument itself. With indicating types of instruments, however, periodical readings and a form on which to record these readings are necessary. The headings shown in the Cargo Ventilation Log below are suggested for recording the data obtained by use of a long-distance thermometer in the hold and a wet and dry bulb thermometer or humidity indicator and dry bulb thermometer on deck or on the bridge. CARGO VENTILATION LOG [Condensation forms when dew point of entering air is higher than temperature of metal cargo Column No. 4.-Enter dry bulb and wet bulb readings when using psychrometer. Column No. 5.-Enter dry bulb and relative humidity readings when using humidity indicator. Column No. 6.-Plot dew point from No. 3 and either No. 4 or No. 5 on dew point (psychrometric) chart (fig. 56). Column No. 7.-At start and end of each voyage and at other times whenever necessary note the following: Nature and condition of cargo in each compartment; note free and hygroscopic moisture condition or content of cargo and dunnage material; presence and amount of sweat, if found; stowage of cargo affecting flow of air through holds; exact location of readings taken with reference to cargo, hatch, hull, etc. PROTECTION OF CARGOES AGAINST SWEAT OR MARINE-MOISTURE DAMAGE The methods that may be employed to eliminate or reduce cargo damage caused by sweat include: Conditioning of cargo prior to shipment; care in stowage and dunnaging; proper ventilation; and the use of a mechanical dehumidifying installation, such as the “cargocaire" system. Each of these preventive measures is discussed below. CONDITIONING OF CARGOES BEFORE SHIPMENT It has been shown that many products, chiefly agricultural, are likely to heat spontaneously and to deteriorate when they contain excessive moisture. Most of these goods, when sufficiently dry, could be shipped in a tightly sealed cargo space on long ocean voyages and would not need any ventilation or care whatever. The presence of hot or cold bulkheads or changes in the outside air would not cause a dangerous transfer of moisture under such conditions. Little, if any, moisture would tend to evaporate from the cargo and any excessive moisture in the air of the closed hold would be absorbed by the cargo. Therefore, no condensation could occur in cold weather on the cold metal of the ship. A logical procedure would be, therefore, to dry all goods likely to heat spontaneously below their safe moisture limits and then carry them to their destination without any protective measures whatever. Obviously such cargoes must not release any harmful odors, and also they must be prevented from taking on any moisture by absorption, rain, or otherwise, after they have been dried and before they are stowed in the ship's hold. The United States Department of Agriculture and several American steamship companies have made large-scale experiments along these lines with grain and other cargoes, proving that the above statements are correct. In view of this experience, many trades have recognized such procedure as one of the best (if not the only) solution of their troubles, and numerous drying plants for grain, seeds, nuts, and numerous other goods have been built and are being developed in all parts of the world. The canning industry has had a comparable experience. Experiments made in 1932 by the Forest Products Laboratory at Vancouver, British Columbia, indicated that the box shooks of which the wooden boxes were made should contain less than 17 percent of moisture in order to prevent the rusting of the tin cans during long ocean voyages. As a consequence, the British Columbia lumber and canning industries have been seasoning box shooks, accordingly, and this practice is spreading to other canning centers. S. J. Duly, of London, recently verified these findings. Obviously, the box shooks and filled boxes must not be allowed to absorb moisture before shipment or during the ocean voyage. The indications are, however, that practical and economic reasons will prevent a rapid growth of such sound practices. A grain drier that may be satisfactory and remunerative in the southern United States is likely to be much too expensive to operate in the interior of the Netherlands Indies or elsewhere. Therefore, it is safe to assume that many producing areas in oversea countries will have to continue taking chances on the spoilage of their goods, rather than install expensive drying equipment. Consequently, the ship owner (unless his vessels are equipped with dehumidifying installations) must watch very carefully to make sure that cargoes of the type under discussion are in a safe condition for carriage. It is not enough that the goods are "received in apparent good order and condition" at the ship. As an example, rice may look entirely sound at that time, but its moisture content may be so high that it is certain to deteriorate during a lengthy ocean voyage. The officer receiving the cargo should by all means insist that the moisture content of the dangerous cargo be determined. STOWAGE AND DUNNAGING Proper stowage and dunnaging are necessary if the full benefits of proper ventilation are to be secured in reducing or eliminating the formation of sweat. All cargo that requires treatment in the hold because of its condition or because of the presence of sources of moisture, heat, or cold, must be accessible to ventilation. It is, therefore, very important that the cargo be stowed and dunnaged in such a manner that air can freely enter the hold and intimately contact the cargo. It is equally necessary that the air can freely leave the hold after doing its work-a fact which is often overlooked in stowage. The stevedores must provide a continuous air space of at least 6 inches below the weather deck. Wet, hot, and cold cargo should be stowed with ample dunnage between tiers. The spaces thus created should have access, supply, and exhaust, to the main air streams through the hold by leaving space at two ends of the parcels. The stevedores should keep cargo from contacting exposed sections of the ship's structure by sweat battens, likewise from hot and cold bulkheads or decks. They should further protect it from condensation overhead by covering the cargo with tarpaulins or paper. VENTILATION Moisture has been shown to be the most important source of trouble in cargo and in air. The moisture in cargo can be controlled successfully by air of desirable condition. Ordinarily, on most ocean routes and in dry weather, the temperature and relative humidity of the outside air are satisfactory. An ample supply of such air should be distributed through the holds so that the areas of moisture, heat, and cold are well aired. The term "ample supply" means at least two complete changes of air per hour calculated for the empty cargo hold. For the ordinary cargo vessel, not fitted with the cargocaire system, the following recommendation for ventilating cargo spaces, by S. J. Duly, of the Department of Commercial Products, City of London College, in a paper read before the Royal Society of Arts on January 26, 1938, may be cited: Observe the dew point each watch on the bridge, and take the temperature of the cargo once a day. As soon as the dew point rises above the temperature of the cargo, the ventilators should be trimmed back to wind or should be covered and remain in that inactive condition until the dew point falls below the cargo temperature, when they are trimmed so as to ventilate the holds again. In certain trades the adoption of this practice several years ago has been accompanied by good outturns since. But it is not a generally understood principle at sea, nor is the practice based upon it generally adopted. It involves voluntarily closing the ship's ventilators for sometimes as long as a fortnight, and this proposal does not recommend itself to men whose experience has been that condensation is often the worst when ventilators have been compulsorily closed on account of bad weather. The observations of other students of marine-moisture damage tend to confirm Mr. Duly's findings. In general, it may be stated, therefore, that when the condition of the atmosphere is unsatisfactory, that is, either too wet or the relative humidity or dew point too high, it is best to keep the holds sealed. Cold goods, such as steel products and canned goods, loaded at a northern port should not be ventilated with warm, humid air. The present common practice of continuously ventilating steel and canned goods without regard to the dew point of the air and the temperature of the cargo should be discontinued. As long as the temperature of the cargo is below the dew point of the atmosphere, the admission of warm moist air into the hold must necessarily cause condensation on the cold metal cargo. After the cargo has become warmer than the atmospheric dew point, the admission of warm, moist air into the hold will not produce condensation. When a ship from the Tropics with its hold filled with warm, moist air enters cold water or cold weather suddenly, condensation from the warm air may form on the cold metal surfaces in the cargo holds. Such condensation may form on the beams and underside of the deck plates, even though the temperature of the cargo is above the dew point of the atmosphere. This condensation may be evaporated by ventilation without harm to the cargo, provided the ventilating air is not saturated with moisture and has a dew point lower than the temperature of the metal. On vessels proceeding from a cold to a warm region the temperature of the cargo should be raised above the dew point prevailing at the port of discharge to which the ship is traveling, so that the cargo will not be damaged by sweat when unloaded on the dock. This is accomplished by providing an ample supply of warm, low dew point air over the surface and through the body of a block of cold cargo. Atmospheric air, with a dew point lower than the temperature of the cargo, is frequently available for this purpose and can be admitted through the ship's ventilators; or such air may be provided by a "cargocaire" system, as explained below. If such air is not available, the hold should be kept sealed tightly to the outside air and natural transfer of heat through hull air and cargo will then slowly warm up the cargo, as the ship travels through hot climates and warm seas. In connection with this matter, it should be noted that cold decks and bulkheads near refrigerated spaces are undesirable, as they tend to keep the cargo next to them cool. Good insulation and dunnage will reduce this source of trouble. EXAMPLES OF IMPROPER VENTILATION Several examples of improper ventilation are cited below to further clarify the principles involved in preventing condensation. |