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Multi-delivery refrigerate' hide design
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REFRIGERATED vehicle design and considerations on longdistance work have already been covered in previous issues of COMMERCIAL MOTOR (January 14 and April 8, 1966). This article now deals with design as applied to multi-delivery refrigerated vehicles. The major requirements apart from the means of cooling are the same for long-distance and multi-deliveries and these can be summarized as follows:— (I) The provision of a vessel (container or body) with a very high efficiency in terms of heat leakage and capable of maintaining that low level of heat transmission throughout its life (say eight, or even up to 10 years), and
(2) The need to transfer low-temperature goods from cold store to vehicle at cold-store temperature and, similarly, the need to transfer goods from vehicle to store at the receiving end without any material appreciation in product temperature.
Additionally, in the case of the multi-delivery vehicle, there is the heat load arising from the repeated door openings necessary in order to obtain access to goods at each delivery point.
Consider first of all the body. Current practice in regard to lowtemperature vessels of this kind is to insulate in either 4 in. or 5 in. of polyurethane or 5 in. or even 6 in. of polystyrene. Construction may be Orthodox—that is, framing in timber or metal or composite materials, with interior and exterior casings in metal or plastics sheeting. Alternatively, the construction may be all plastics. Also, the construction might be "body" or "container-in-body", where a lightly constructed boxvan body houses a separate or semi-separate insulated (refrigerated) container. At this point it might be advisable to discuss in more detail the advantages of the container-in-body construction.
Take the separate type of construction where the container is completely isolated from the housing body.
First of all there is the thermal advantage because the insulated section is relieved from the direct solar heat effects. The container is virtually being carried at all times in ambient conditions—in what might be termed "still air" ambient conditions.
Secondly, there is the protection from accidental damage and from any accelerated surface (casing) deterioration which might arise from direct contact with the elements. A degree of protection against accidental damage is valuable because any penetration of the outer casing of the insulated vessel, provided such a casing is the weather protection surface, means immediate repair, unless deterioration of the insulation, through ingress of moisture, is risked. Additionally, a separate container, inside a light boxvan body, is protected to quite a considerable extent from the "racking" strains arising from chassis deflection and deformation in running conditions.
Thirdly, there is the advantage of what might be termed "form". The outer casing—that is, the box-type housing body can be of any decorative or attractive shape or form, whilst the container can still remain as a basic "box", which is still the most economic, effective and efficient shape.
The container, being inside the body, is completely hidden, apart from, possibly, the rear end. Hence its external finish can be relatively rough; and fully effective, cheap means of completely vapour sealing the container casing panels can be employed.
Naturally one has to pay for these advantages but the extra cost involved is relatively little. In effect it amounts to the cost of one extra surface casing, although part of this will be offset because the extra casing does not require a high degree of finish.
Similarly, in regard to overall dimensions, the container-in-body will be perhaps 3 in. wider, 3 in. longer and If in. higher than a pure body of the same internal (stowage space) dimensions. This, however, does not seem to be a very serious handicap, particularly in a range of vehicles where construction up to maximum legal width is relatively rare.
In the case of the semi-separate type, the container is not freestanding inside the body, as in the separate form. The exterior wall of the container is attached to the inside of the (housing) body. pillars, and whilst the thermal characteristics of the separate type are very largely retained, the degree of protection both as regards external damage and chassis racking are diminished.
The dimensional increases, as opposed to pure body, stated at 3 in. wider, 3 in. longer and If in. higher for the separate type, become 2+ in. wider by 2+ in. longer by 1+ in. higher for the semiseparate version. Naturally there will be a slight increase in dead weight—for both separate or semi-separate constructions—over the pure body, amounting to approximately 2 cwt. for a body of internal dimensions 12 ft. by 6 ft. by 6 ft. high.
Typical heat leakages in an ambient temperature of 90°F (temperature difference, 90°F, that is 0°F internal 90°F external) for:— (a) orthodox construction refrigerated body, (b) orthodox construction container-in-body, and (c) all-plastics construction refrigerated body, where the internal dimensions are approximately 12 ft. long by 6 ft. wide by 6 ft. high and the insulation is taken
to be 5 in. polyurethane all round, would be:— (a) 1,700 Btu per hour, (b) 1,430 Btu per hour, (c) 1,250 Btu per hour.
These represent thermal efficiencies of approximately 69 per cent, 82 per cent and 94 per cent respectively.
The main points to note here are the very high efficiency of the all-plastics body and the high efficiency of a well-developed design of container-in-body.
This difference would, of course, tend to narrow in conditions of direct solar heat, because of the shielded conditions obtaining in the latter construction. The ideal would seem to be all-plastic container in a body.
Consider now the heat leakage arising from door openings—the door opening heat loss. This is very difficult to calculate because there are so many variables. There is, however, reasonable experimental evidence which tends to indicate that the average hourly heat loss over a period of eight hours on a summer's day, where the delivery working is at the rate of six per hour, will amount, in a vessel of the dimensions stated earlier, to approximately 1,400 Btu per hour. This presupposes relatively short "complete entry" periods and reasonable "air drop out loss" control arrangements.
The latter are of vital importance, and it is virtually impossible to deliver goods at a safe temperature in summer conditions without some means of controlling or otherwise considerably limiting the fall out of cold air from, and the entry of hot air to, the lowtemperature cabinet. Broadly, devices of this kind fall under one or other of the following headings:
Self-closing (refrigerator) doors
The operator opens the door and enters the cold zone, the door then closing to automatically behind him. The sequence is the same on exit, where, depending on the design, it might be necessary for the operator to secure the door in its locking catches before eeding to the next delivery point. Such arrangements can be
linked with, say, the vehicle starter switch the starter being erative until the refrigerator door is fully closed.
-flap door or doors iese are situated immediately inside the refrigerator door and )1. the centre swing type. They open inwards or outwards and ys return under their own spring tension to the closed position. nally flip-flap doors arc about 1 in. thick and double skinned.
can be insulated (approximately 1 in. thickness) between two casings, or the two casings may be translucent glassarced plastics with an air gap between. The operator opens the ;erator door, leaving it open during the delivery period, and .s the low-temperature cabinet via the flip-flap which closes matically behind him. The process is reversed on exit.
1Y C. A. REID, tains hese are situated immediately inside the refrigerator door and usually "strip" type and made of plastics (transparent or lucent) material. The operator opens the refrigerator door, h then remains open during the delivery period, and passes into mid zone through the strip curtain, which automatically falls its screening position behind him. The sequence is reversed xit.
Veil ere the cold air is retained in the vessel by means of a screen of owing over the aperture face. The appliance consists essentially high-efficiency fan, driven usually by a d.c. motor taking current the vehicle starter battery, or from an auxiliary battery. The tom the fan is taken to an angled type of exit duct usually ted at the top of the door and running the full aperture width. ; a curtain of air is directed downwards and slightly outward virtually "rolls back" the cold air as it seeks to drop out. The Alai features would appear to be the supply of sufficient air at equisite velocity plus precision in the setting of the efflux nozzle. his control or limitation of the "cold-air drop-out" is so importhat it is worth discussing in considerable detail. The vessel s, just as in the case of the long-distance refrigerated vehicle, now been developed to the state of all-round efficiency where t the economist might call the point of "diminishing returns" been reached. The vessel itself now has a very low fabric age and can maintain this low level throughout a very )nable life of, say, eight years.
is extremely difficult to see how the current fabric leakage possibly can be economically reduced: it is already less than the
• opening heat leakage. So logically one must strike at the crable point—the "Achilles heel", which is the door-opening loss. Returning to the means of control detailed above—that ap doors, curtains, and air veils—the first and the third are both ,ble of giving really good results, whilst the second can give 3nable results.
11, in normal conditions, are vulnerable. The flap doors and cur. can very readily be tied back by the operator, and in a similar the normally door-operated switch of the air veil can be ered inoperative. But the temptation to take the latter step, as s the operator is concerned, is not nearly so great as in the case le curtains or flap doors.
ere, in the case of the curtains, we have a fabric which is some3, wet on the outward facing surface, flapping around the ears of iperator, whilst in the case of the flap doors there is some small ical effort involved in opening them and in effecting an exit with rmful of goods. The choice then would seem to lie between a really well-designed type of flap door plus a conscientious operator (the former can be achieved at quite a low cost) and an air veil, which will cost, in broad terms, about four times as much as the flap doors, but will be much less liable to abuse. It is of interest to note that flap doors can be made foolproof by means of suitable interlocks.
So far my thoughts have been in connection with drop-out, control arrangements on vehicles where complete entry of the operator to the cold zone is always required, and it would seem that considerable heat leakage relief might be possible if total entry of the operator could be either eliminated or at least reduced.
The former presupposes the use of either of the following:— (a) Tunnel types of vehicles where the cold zone is divided into a number of tunnels or passages, each of which has a small access door. Thus, to obtain goods the operator does not enter the belly of the vehicle, but opens a small door, removes the required quantity of goods and then closes the door. The air drop out is limited and the driver does not carry any heat into the cold zone. The goods in the tunnel are on an inclined plane and always gravitate forward into the exit door region. There is, of course, a small premium to pay in that of necessity, the multiple doors increase the fabric leakage of the body or container, reducing the efficiency of the vessel. This increase, however, is only a relatively small proportion of the amount of heat saved through elimination of total entry.
(b) Well-type vehicles, where the goods are carried in wells or tanks and access to them is via the lid of the well or tank. This arrangement requires no further description; it is basic and quite sound and has been used on smaller applications for many years.
However, both (a) and (b) impose operational or other limitations. For example, a body of the tunnel type would cost a good deal more than a full-access body of the same internal dimensions. Additionally, the stowage space available for goods in a tunnel vehicle would be at least 20 per 'cent less than that available in a full-access type of the same internal dimensions.
Similarly, in the case of the well-type vehicle, the stowage space available would be at least 40 per cent less (perhaps 50 per cent) than on a full-entry type of the same overall dimensions.
Notwithstanding the above, further study of developments of this kind is, in my opinion, justified, particularly in regard to smalland medium-capacity vehicles operating in tropical or semi-tropical conditions, or where the maintenance of very low temperatures is absolutely imperative.
In the following section I will deal with methods of cooling, but first a final comment on the modern multi-delivery vehicle might be justified.
A correctly designed, cooled, maintained and operated multidelivery vehicle with adequate air-drop-out control is capable in maximum summer conditions (in the British Isles) of a temperature performance somewhat as follows:—
(X) Goods loaded on to vehicle with no surface (skin temperature of goods) in excess of 0°F.
(Y) Taking as a basis 60 deliveries in a 10-hour period on the road (1 hour lunch break) with the last delivery made 8+ hours after leaving the depot or cold store, the maximum surface temperature of any of the goods delivered will not exceed 5°F (27° of frost in the Fahrenheit scale). On this basis one could reasonably substitute a skin temperature of goods in statement X of —5°F (for the 0°F shown) and anticipate an end point in statement Y of 0°F (32° of frost Fahrenheit).
In general terms multi-delivery, or distribution, vehicles are "one day performers". They normally return to base each evening and are assumed to be on the road, for say, 12 hours out of each 24-hour day. Such an operating cycle lends itself to the storage system, and because this method of cooling is the most popular I will give it pride of place.
The storage plate contains a eutectic solution—usually a brine solution with certain additives—surrounding a cooling coil, the connections of which are coupled to the cooling source. The latter, in road transport applications, is normally an electrically powered compresser set, and the coupling is permanent. The refrigeration equipment (compresser set), permanently coupled to the storage plates, is carried on the vehicle, and overnight charging is via a flexible lead which is plugged into a bank-side power point.
Major disadvantages of the system are:—
Dead weight
Plant plus plates amounts (in the case of a body of say, 12 ft. by 6 ft. by 6 ft. internal dimensions, designed for 0°F holding) to about 15 cwt. This is rarely acceptable, but the position is not perhaps as bad as it appears. Normally delivery vehicles carrying bodies of this (or near) size are on so-called 4or 5-ton chassis, and if the effect of the extra dead weight of the storage-plate system over any alternative cooling system pushes one from a 4-tormer to a 5-tonner or from a 5-tonner to a 6-tonner, the financial penalty is by no means disastrous.
Need for regular desnowing and defrosting
Air-borne moisture separates and collects as "snow" or ice on the cold surface. This, of course, is fundamental and the heat ange between the cold source and the circulating fluid (the nal air in the body or container) is thereby impaired. In tice it is advisable to brush off the snow from the plates, so is possible, daily. Such brushing is normally a compromise use it is difficult to obtain access to the rear face—that is, race nearest the adjacent interior surface of the vessel. Howclearing the exposed face daily assists materially.
this connection it may be stated that from time to time the mounted storage plates have been carried on hinged mountThus, by slackening a couple of nuts, or otherwise removing fixing, the plates can swing away from the wall and the f can be brushed from the rear face. Such arrangements ippose some considerable flexibility, or capacity to extend he part of the pipe connections to the plates, and this would :ar to be difficult of achievement.
. the experience of the writer the premium one pays (liability :akage of refrigerant at the flexible points) is not worth the atful benefits which one might receive. After all, it is difficult gh in service to get the operator to brush off the exposed of the plate. It would be almost asking the impossible to ct a more complicated service.
efrosting is not such a serious handicap as one might imagine. r all, from the standpoint of elementary cleanliness the ior of the body or container must be cleaned out at very tar intervals. Fortnightly defrosting is generally considered adequate—although interior cleaning of the body could be on a weekly basis—so that if defrosting is carried out at ;ame time as interior cleaning no additional lay-off or service is really involved. Naturally, any water arising from the )st needs clearing from the inside of the body, adequately ped up and reasonably dried, just as in a domestic refrigerator. le great advantages of the storage-plate system are:—
ability
ie coupling of plant to plates is permanent, and provided the work is satisfactorily arranged and carried out there is very risk of circuit leakages. Also the modern electric-motor.11 compresser, whether fully sealed, semi-sealed or open, is )st reliable machine, silent and trouble free. The power cost tricity at say, 2d. a unit) is very low, and in general the :m is just about the cheapest of all to operate.
perature level
ie most popular (low temperature) eutectic is the so-called CF solution. Very few of the solutions behave like water, .e the whole of the latent heat is extracted at 32°F. One really rate eutectic solutions, performancewise, on the , of percentage of latent heat at a constant temperature. nme inferior solutions have a wide temperature band over h the latent is extracted (and conversely recovered). In any , using a —12°F eutectic to hold an environment temperaof 0°F, with door openings, requires a relatively large tee area because there is only 12°F of temperature difference .een the cooling source and the enveloping air. Some imement might be possible by forced-air circulation, although
are definitely difficulties in connection with the siting and :.ring of fans so that natural convection is assisted and not ded. The development of a really satisfactory —18°F :tic would also assist materially.
enerally, however, the advantages of cost, simplicity, reli.y and very reasonable temperature performance, would , to justify the storage-plate system at least for some time in remier position.
le remaining methods of cooling may be divided broadly into loss and currently powered systems.
the former we have the solidified or liquified gases (CO, nitroair), the first two of which lend themselves better perhaps to transportation duties rather than delivery working. Both ion-toxic, inert gases, but they do not support life. So to be le safe side it is necessary, if the CO, or nitrogen are used on a try basis—that is, the gas is released into the interior of the or container—for the refrigerator door to be left open whilst merator is inside. It would not, in my opinion, be sufficient st notices saying: "Leave the door open for one minute before entering—after which time you may safely go in and pull the door to behind you."
Modern safety precautions appear to reject the negative approach and say in effect: "Something positive has to be done." One can, of course, apply the cooling effect of the liquefied or solidified gases on a secondary basis (in a closed system where the "end point" gas is rejected to atmosphere). This, however, adds to the cost because some residual (sensible) heat is usually rejected with the waste gas.
The so-called "gas hazard" does not, of course, exist in the case of liquid air, although here there may be complications arising from oxygen enrichment where the liquid air is allowed to remain in the holding vessel (Dewar flask type) for fairly long periods.
In the main, however, the criticism is one of cost. As methods of cooling they have many advantages: they have a very low temperature (a wide temperature difference between the cold source and the cooled environment); they are quite positive (you put in X lb., and apart from the very remote possibility of a control failure you know your cooling is assured for Y hours). Liquefied gases, particularly nitrogen, are now extensively used in the United States, but this is not proof that we are wrong in not making greater use of them here. The picture could change if the cost per thousand Btu were substantially reduced; but this is difficult to see, particularly in view of distribution and storage costs.
Under "currently powered" systems we have all cooling arrangements where an orthodox refrigeration pump or compresser carried on the vehicle and driven from any power source whatever (separate i.e. or other reciprocating engine, turbine, storage batteries, power take-off from vehicle engine or transmission, and so on) currently supplies cooling or refrigeration to the vehicle. In the Majority of cases the cooling is direct—that is, the heat exchange takes place between the refrigerant, expanding direct into some form of gilled tube or similar evaporator, and the internal air of the body or container which, under forced draught, is caused to pass over the surface of the evaporator.
Such arrangements usually embody an alternative power source —for example, an electric motor for cooling the vehicle overnight or when standing at base. Additionally they are equipped with controls for limiting the temperature level of the internal, low-temperature cabinet air, means for defrosting the evaporator coils and a fan control, normally linked with the access door. In other arrangements the cooling is via storage plates, which is to this extent a secondary system.
Currently powered systems can give good results, but unless of excellent design tend to be less reliable than electrically powered storage systems. Also their operating costs (a petrol engine is usually employed as the prime mover) are higher. Good results at an economic cost, within a certain work output potential. can be obtained from systems which take compresser power from the vehicle engine via a hydraulic drive. An auxiliary electric motor drive must then be fitted for "at base" cooling. Such applications, however, are limited in that the vehicle engine must be running for more than some critical percentage of the "on road" time, and the mid-day shut down can satisfactorily be accommodated by the goods without assistance from the refrigeration plant. If one seeks to bridge the gap by the addition of storage plates then the main advantage, that of weight saving, is lost. You might just as well go in right away for a storage system.
Finally, a word about the future. This, or course, is purely a personal opinion.
Distribution vehicles deliver to outlets—to the shops where the last link in the chain (the customer) is reached. We hear a lot about night deliveries to shops—particularly those which are situated in the intensely populated areas, and for this purpose the current type of i.c.-engined vehicle would appear to be quite unsuitable because of noise. The only alternative available at present would seem to be the battery type of electric vehicle.
Current development then might well be directed toward a substantial reduction in dead weight, not only in regard to the body or container, but also the cooling plant. In the latter connection there would still seem to be considerable scope for the application of dry ice on the secondary basis, as its storage or carrying demands are not excessive. Also much more thought will require to be given to the subject of (considerable) reduction in the "air drop-out" or "air interchange" heat load and the possible elimination of operator entry to the cold zone.