FRUIT CARGOES

Evening Post, Volume CXIII, Issue 62, 15 March 1927, Page 15

FRUIT CARGOES

THE COOLING PROBLEM

OFFICIAL EXPERIMENTS

PRE-COOLING ADVISED

(From Our Own Corresponding)

LONDON, 26th January. Another interesting report on "Temperature Conditions in Refrigerated Holds Carrying Apples" has been issued by the Department of Scientific and Industrial Besearch. Observations and experiments have been made which disclose an unsuspected and strikingly large difference of temperature between one part of the hold and another, and this despite the fact that the procedure for securing uniformity of temperature adopted by engineering staffs of the vessels was in every case as good as existing knowledge permits it to be; These inequalities of temperature are due to the fact that the streams of cold air i;ake the lines of least resistance and apparently never distribute themselves uniformly among the boxes of fruit. The fruit itself liberates heat; therefore, in places where the air is more or less stagnant there may be relatively high temperatures. The problem of Securing equality of temperature through a space closely packed with obstacles such as boxes of fruit is one of great "difficulty. Ex-' periments on models,-carried out on the reoommenda.tiQn of thg board, have now been in progress' at the National Physical Laboratory for more than a year without securing as yet a solution which is practicable on board ship. The boara, therefore, desires to state explicitly that this report is not put forward so much in criticism of existing practice on board ship as to draw attention to a remarkable defect for which it may be hoped future research will find a remedy. EFFECT OF TEMPERATURE. Although direct evidence is at present wanting (says the report), it is clear from small-scale experiments that these differences of. temperature must be reflected in the condition of the fruit. For instance, apples carried at 45 degrees in the centre of a hold must ripen more rapidly than similar fruit which may be carried at 32 degrees near the grid" or the cold air trunks. The variability in the consignments of Australian fruit, so often commented upon by importers, may be due in a considerable measure to such circumstances. The systems with air circulation would naturally be expected to show better results than the simple grid system, with regard to rapidity of cooling and uniformity of temperature. Actually, however, this is not so, and the grid system holds its own with any of its rivals in these respects. Further, in holds with the grid system, circulation of the air by means of an auxiliary fan and trunk system is found to" have comparatively little effect on the temperatures. In fact, it appears to be almost impossible thus to drive air uniformly for any considerable distance through a closely-stowed mass of fruit cases. The air flow ia localised along certain paths, and the big inequalities of temperature still persist. A limited improvement in the temperature oondiI tions would doubtless be attainable by employing stronger air circulation of the present type, but the whole problem is not to be solved in so simple a manner. In addition to the forced movement of the air, there is movement due to temperature differences. Where the air is relatively cold it tends to fall, and where it is warm it tends to rise. Cold air introduced at the top of the hold will tend in this way to sink through the cargo to /the bottom, and if the movement is sufficiently unimpeded, a fairly uniform distribution of refrigoration can thus be achieved. This method is utilised at present when cooling is effected by means of brine grids on the roof of the chamber. The suggostion is made that all the systems would give better results if modified so as to supply cold air abovo the cargo, instead of from the sides as is usually the case at present. While it is certain that the maximum potontialitios of rapid cooling on board ship have not yet been realised m practice,.it is frankly conceded that tho task of cooling down a cargo of warm fruit is so formidable that really satisfactory results may never be obtainablo under the limitations imposod by considerations of space on board ship. Tho alternative is to cool the fruit bofore shipment. PRE-OOOLINa ADVISED. The ideal system would bo to cool tho fruit while it lies on tho wharf awaiting the arrival of tho overseas steamer; in this way delay would bo reduced to a minimum, extra handling avoided, and the risk of a subsequent warming up before shipment overcome. Tho difficulties in the way aro mainly economic and financial, and they would to a great extent disappear were the cost of this cooling, which apparently need not exceed a few pence per case, met by a proportionate reduction of the freight upon tooled fruit, justifiable in

view of the fact that the overseas steamer would thereby be relieved of tne most onerous part of its present task of refrigeration. The report gives detailed results of the experiments with the various systems of refrigeration. Summarising the results the report says: In each case it was found that a ™\ ona* le perioa of the vova Se (from 10 to 20 days) was occupied in cooling down the warm fruit to the temperature of carriage. , The temperature waß far from being uniform all over the hold, particularly while the fruit was being cooled down. In the lower holds the warmest and coolest parts of the cargo frequently differed in temperature by 15 degrees, and rajely less than 10 degrees. In tween decks the temperatures were more uniform. _ The systems of refrigeration which involved air circulation showed little superiority over the simple grid system with regard to uniformity of temperature, and none at all with regard to rapidity of cooling. The temperature distribution in holds employing forced horizontal draughts of cold air (battery system and screened grid system) indicated that in its passage across the hold the air stream descended, and passed mainly through the lower parts of the cargo, missing the upper central portions. The observation in holds cooled by grids emphasised the necessity of employing the hatch grids as well as the other roof sections, and showed the desirability of providing vertical air channels in the cargo. It is suggested that the systems with air circulation would give a more uniform temperature if modified so as to apply refrigeration at the top of the hold. Means for securing this end are tentatively put forward. In spite 1 of the improvements which may be effected in these ways, it appears that the task undertaken by the Steamer in cooling down a large cargo of warm fruit is so great, and the conditions attest so unfavourable to rapid refngeration, that really satisfactory results may only be obtainable if the fruit is chilled; before shipment, and so brought into line with other commodities shipped under refrigeration. 85, Fleet street.