Design and performance of insulated and refrigerated ISO intermodal containers

Design and performance of insulated and refrigerated ISO intermodal containers*

R. D. Heap Shipowners Refrigerated Cargo Research Association, 140 Newmarket Road,

Cambridge CB5 8HE, UK Received 18 April 1988; revised 5 October 1988

The various types of intermodal containers available which can be used for temperature-controlled transportation of perishables are described. Details are given of typical performance of modern equipment, including thermal performance, air movement, and temperature control. Likely rates of temperature change and ranges of temperature are indicated for various operating conditions. Likely future developments are considered. (Keywords: refrigerated containers; design; thermal performance)

Concept ion et performance des conteneurs intermodaux ISO isothermes et frigorifiques

On d~crit les divers types de conteneurs disponibles pouvant &tre utilis~s pour le transport d temperature dirig~e des denr~es p~rissables. On donne des d~.tails sur la performance type des materiels modernes, y compris la performance thermique, le d~.placement d'air et la r~gulation de la temperature. On indique les taux probables de variation de temp~.rature et les domaines de temperature pour diverses conditions de fonctionnement. On examine l'~volution future probable. (Mots clrs: conteneurs frigorifiques; conception; performance thermique)

ISO intermodal containers are widely used for the international transportation of goods. In insulated or refrigerated forms they are suitable for maintaining close control of transport temperatures for perishable commodities. The containers are mostly either 20 or 40 ft in length, so that the larger size holds approximately twice the volume of the smaller, and the total capacity of groups of containers is often quoted overall in 'teu', that is 'twenty-foot equivalent units'. At the last published census 1, there were nearly 5 x 10 6 teu o f lSO containers in use, of which approximately 160000 were refrigerated and 80 000 were insulated.

Refrigerated containers incorporate refrigeration and air handling equipment which is generally electrically driven. They are typically designed to maintain the temperature of perishable goods at set levels from - 18°C or below to + 25°C in ambient temperatures from - 10°C to +38°C or above. The general layout of one such container is given in Figure 1.

Insulated containers incorporate thermal insulation to a similar extent to refrigerated containers, but do not incorporate refrigeration equipment or fans. Instead, porthole apertures at one end of the container are connected to the air inlet and outlet of the cargo space and, in use, either a clip-on refrigeration machine or an external air circulation system is attached to the container. Provision is made for closing the air apertures when they are not being used. The general arrangement of

* Material in this paper includes information previously presented at IIR meetings in Orlando, USA (1985), Bristol, UK (1986), and Vienna, Austria (1987) ~ 8

systems using these containers has been described by Scrine 2 .

The cargoes carried in these containers fall into two categories, generally described as 'frozen' and 'chilled'. Frozen cargoes include frozen meat, butter, vegetables, dairy products, fish, and the whole range of pre-prepared frozen foodstuffs. These require temperatures of - 18°C or lower if quality is to be maintained but precise temperature control is not necessary. Chilled cargoes include fresh meat, fruits, vegetables, and horticultural produce, all of which have a limited storage life which is very dependent on temperature 3. The approximate effects of temperature on storage life for a selection of goods are shown in Figure 2. There is a commercial need to maintain storage life during transport, thus precise control at the lowest temperature the goods can tolerate is necessary. Furthermore, because of operational

Evaporator fan ~ I I Evaporator c o i l ~

Condenser f a n ~ ~_~ Condenser coil ~ Compressor

'T-bar' floor

Figure 1 Figure 1

\ I

- A

Schematic diagram of refrigerated container arrangement Schbma de l'agencement d'un conteneur frigorifique

O140-7007/89/0301374)9503.00 © 1989 Butterworth & Co (Publishers) Ltd and IIR Rev. Int. Froid 1 989 Vol 1 2 Mai 137

ISO intermodal containers." R. D. Heap

80

A

60

O

4o x o Q.

o 2o J~ I-

0 5 10 15

Temperature (°C)

Effects of temperature on storage life. 1, Apple (Granny Figure 2 Smith), carrot; 2, Grape (Flame Tokay); 3, peach, brussels sprout; 4, apricot, asparagus; 5, sprouting broccoli; 6, watercress Figure 2 Influence de la temperature sur la dur~e de conservation. 1, Pommes (Granny Smith), carottes; 2, raisins (Flame Tokay); 3, pdches, choux de Bruxelles; 4, abricots, asperges; 5, pousses de brocoli; 6, cresson

constraints, all refrigerated containers must be suitable for the carriage of both frozen and chilled goods on different occasions.

The wide range of ambient and internal conditions, combined with the requirements of strength and durability for marine transportation, make the modern refrigerated container a much more sophisticated piece of equipment than may at first appear to be the case.

Thermal performance

Insulated and refrigerated containers incorporate about 70mm of insulation in their walls. Generally this is polyurethane foam and almost always the foam incorporates a low conductivity halocarbon gas within closed cells to provide good performance. Such foams deteriorate with age, partly due to loss of halocarbon and partly due to moisture ingress.

The heat leakage of a new 20 ft refrigerated container should be 24 W K - ~ or less, and that ofa 40ft unit should be 35 W K - ~. A new insulated 20ft unit could have a heat leakage of 22 W K-~ or less (all heat leakage figures quoted here are measured at 10°C mean insulation temperature).

Aging of container insulation has been studied over many years 4. A good insulated container will typically increase in heat leakage from 22 to 26 W K-~ after six years, and to 3 0 W K -~ after 12 years. An 'average' container might be 10% worse than this (i.e. 33 W K - 1 at 12 years). The overall range of rates of deterioration is from 3 to 5 ~o per year on a linear basis.

Some insulated containers designed solely for the carriage of fruit have thinner insulation, with a heat leak of around 35 W K-~ when new for a 20ft unit. Rates of

aging are a similar percentage per year to those for the more heavily insulated containers.

Refrigerated container performance

The performance of refrigerated containers is basically related to three separate parameters. These are air movement (necessary for heat transfer), overall refrigeration capacity, and controllability. These will be considered in turn below.

Air movement and air freshening

Air movement is provided by fans which circulate air through the refrigeration machinery and around the cargo. Generally, these fans operate continuously.

The rate of air circulation depends on the resistance of the cargo. Total flow is measured for a range of added pressure drops using an auxiliary fan and duct system. The performance of three different containers is shown in Figure 3; details of the containers are given in Table 1. There is an increase in flow with 60 Hz rather than 50 Hz electricity supply of about 20%, with an associated increase in fan power of over 50~o.

The uniformity of airflow across the width of the container is measured by checking velocity in each channel of the T-bar floor. There is no generally accepted standard of uniformity but a variation of _+ 20-30~o about the mean velocity is typical, with the high values being deliberately concentrated at or towards the sides of the container.

For containers used to carry fruit a system of vents is installed which replaces a small proportion of the recirculated air with fresh air. This prevents the build-up of carbon dioxide or ethylene within the container. General fruit cargoes in 20ft containers need up to ~-8 m3h 1, but at least 10 times this amount may be needed for some flower bulbs which are particularly susceptible to the effects of ethylene.

4 %

3 A

× 2 B

~ 1 c

o 100 200

Added pressure drop (Pa)

Figure 3 Air flow characteristics of three refrigerated containers Figure 3 Caract~ristiques de l'~coulement d'air de trois conteneurs frigorifiques

Table 1 Air circulation rates with 50 Hz supply (see Figure 3) Tableau 1 Taux de circulation d'air

Container A B C Length fit) 40 20 20 Refrigeration (hp) 7.5 7.5 5 Air circulation rate at no added pressure 78 115 92 (changes per hour) Corresponding fan power (kW) 1.26 1.18 0.70

1 3 8 Int. J. Refrig. 1989 Vol 12 May

1

°i • . - - ~ I 1 I I 4

- 3 0 - 2 0 - 1 0 0 10

Internal temperature (°C)

Figure 4 Refrigeration capacity of a 40ft unit in 38°C ambient temperature. 1, Gross capacity; 2, nett capacity; 3, 5 0 W K - I ; 4, 3 5 W K -I Figure 4 Puissance frigorifique d'un conteneur de 40 pieds d une temperature ambiante de 38°C. 1, Puissance brute; 2, puissance nette; 3, 5 0 W K - t ; 4 , 3 5 W K 1

s j

50 - /s/j~SS S s O.

s / / s E /X • < 30 -

I I - 3 0 - 2 0 - 1 0

Minimum internal temperature (°C)

Figure 5 Refrigeration capacity and minimum achievable internal temperature fora 5hp20 f t uni t . - × -, As tested, 24WK 1;_ _ 3 0 W K -1 Figure 5 Puissance frigorifique et temperature int~rieure minimale oouvant dtre obtenue dans un conteneur de 20 pieds de 5 ch. , D'aprds l" essai, 24 W K-1; - - , 30 W K - l

Refrigeration capacity Container refrigeration systems usually operate with refrigerant R12, and have reciprocating compressors mostly in the 5-8 hp range, although some units exist with two separate smaller compressors. Refrigeration capacity is the measure of cooling power available at a particular running condition. Gross capacity is the total cooling done by the refrigeration system; nett capacity is that available to the container after removing the heat generated by evaporator fans and motors. The nature of the vapour compression refrigeration cycle is such that: capacity reduces as the temperature difference between inside and outside the container increases; capacity also reduces as internal temperature falls.

The performance of a typical 40ft unit in a fixed ambient temperature of 38°C is shown in Figure 4. The dashed lines show the cooling needed to take away the container heat leakage. This particular unit can cool down to -24°C in a 38°C ambient temperature with a 35 W K- 1 heat leakage, and has five times the necessary capacity at 0°C internal temperature. With a heat leakage of 50WK -1 it can only cool to -21°C.

The variation of minimum achievable internal temperature with ambient temperature is shown in Figure 5, for a 5 hp 20 ft unit with heat leakage of either 24 or 30WK -1. With this unit every 2°C rise in ambient

ISO intermodal containers: R. D. Heap

temperature gives (approximately) a 1 °C rise in minimum achievable internal temperature.

The most usual reference point for refrigeration capacity is the ISO condition of 38°C ambient and -18°C internal temperature. Some operators require evidence of satisfactory operation in higher ambient temperatures of 50°C or 55°C if operation in the Middle East is anticipated.

When running on a 60Hz supply, gross capacity increases. However, the simultaneous increase in fan motor power usually means little change in nett capacity.

Temperature control Methods of temperature control include simple on/off compressor switching (nowadays frequently restricted to use on frozen cargoes), cylinder unloading, hot gas bypass, and suction throttling, the latter often in combination with an evaporator bypass to maintain refrigerant flow. For chilled cargoes accurate temperature control is essential as the requirement is to keep the produce as cool as possible without causing damage.

There are three measures of temperature control (Figure 6):

• the unit must maintain the correct steady temperature at the control sensor, which for chilled cargoes should be in the air delivered to the cargo. The average temperature over an hour or so should be held to within +0.1°C, with short term variations as measured by rapid response sensors of less than + 0.5°C, and these tolerances can be held in the better modern units throughout the operating range of ambient temperatures, using electronic three-term controllers;

• the temperature spread across the width of the container should be small, typically within a range of +0.3°C. This is more difficult to achieve at low refrigeration loads when the evaporator may be only partially used for refrigeration; and

• however steady and uniform the control, there will be a temperature range through the cargo due to heat ingress through the container walls, augmented in the case of respiring produce by cargo generated heat. This is discussed below.

The combination of these aspects of control with the wide range of operating conditions and the need for

Figure 6 Schematic diagram illustrating temperature variations Figure 6 Schema illustrant les variations de temperature

Rev. Int. Froid 1989 Vol 12 Mai 139

G

-1

Q .

E I - -

E ==

" r "

E I-

-SIa - 1 0 ~

-15

-20

-25 0 20 40 60

Time (h)

40 b

30

20

10

/SO intermodal containers. R. D. Heap

0 20 40 60 Time (h)

Figure 7 Rise of maximum temperature in frozen herrings without refrigeration at various ambient temperatures: V, 35°C; ~ , 30°C; ©, 20°C; +, 10°C: x, 0°C. (a) As measured; (b) as percentage temperature rise Figure 7 Elbvation de la temperature maximale dans des harengs congeles, sans production de froid, it diverses temperatures ambiantes: • , 35°C: A , 30°C; O, 20°C: +, IO°C; x , O°C. (a) Mesuree; (b) en proportion de l¥1evation de temperature

reliability makes refrigerated transportation one of the most demanding applications of refrigeration technology. Present day standards of control are very high but are obtained at the expense of high energy costs. Every unit has to have the capacity to cope with frozen cargo in high ambients, and also the controllability to hold chilled cargo in low ambient temperatures when the refrigeration requirement may be a very small proportion of the capacity available. For example, power draw at zero nett refrigeration requirement can be as high as 6.5 kW; under these conditions power factors can be as low as 0.5. Various methods of reducing refrigeration capacity under these conditions are being introduced by manufacturers, including partial compressor unloading and controlled suction throttling.

Rates of temperature change in containers

Rates of cargo temperature change in containers are important under two distinct sets of circumstances. First, in door-to-door container operations there will be short periods when refrigeration cannot be provided, and it may be necessary to know how long these periods can be before critical temperatures are reached. Second, control set-point temperature may be altered, or cargo may be loaded at the wrong temperature, so the rate of temperature change under refrigeration or heating may be of interest.

To obtain accurate data on these changes, close monitoring under highly controlled conditions is necessary. This requires the use of experimental cargoes in test chambers. Because of this, there is relatively little data available and even this is specific to particular

cargoes in particular types of equipment. The results presented below cover a range of cargoes and situations including those most likely to be of interest to container users.

EfJect of switching off refi'igeration In a given insulated or refrigerated container, the rate of cargo temperature change for a particular cargo will depend primarily on the difference in the temperature between the cargo and the outside air. This is seen in Fiyure 7a, which shows temperature change with time for a container of cartonned frozen herrings in a range of ambient temperatures, in each case stabilized at -25c 'C prior to switching off refrigeration.

Experiments to obtain such data are very time consuming. Fortunately, the effects of temperature difference can be simply accounted for if, instead of plotting temperature against time, the temperature rise as a percentage of the ultimate rise in unlimited time is plotted. This has been done in Figure 7b for the data of Figure 7a, and all the results are seen to lie on a single curve with a maximum deviation of 2'~.0. This enables experimental results to be applied easily to any combination of cargo and ambient temperature provided that the cargo temperature is not so far different that the produce has different thermal properties (e.g., frozen cf. chilled produce). For most produce, such differences are negligible as long as temperatures are either all below - 5 ° C or all above 0°C, but latent heat effects between these temperatures can be significant.

Rates of temperature change vary with position in the stow. An example is shown in Figure 8, for a cargo of frozen butter. Over three days, the corners have risen by over 20~o, but the centre has only risen by a little over 2 '~;,. In SRCRA trials, about 14 temperature sensors are distributed through the cargo in a pattern designed to study the extremes of the cargo rather than the true average. 'Mean' temperatures reported here are the mean of these measured values, and are likely to represent more rapid changes than occur in the bulk of the cargo.

The results of several series of actual temperature measurements in cargoes under controlled conditions are shown in Table 2. Here, all the results refer to cargoes in insulated containers of 25 + 3 W K - ~ heat leakage with the exception of the 'fruit' container, which had thinner wall insulation and a heat leak of 45 W K-1 . 'Fastest ' refers to that temperature which was changing most rapidly out of those measured. Cargoes were generally at an initial temperature around - 18°C, except for cheese which was at +4°C.

- 2 • r- 20

~- 0 I 1 2 3

Time (days)

Figure 8 Butter cargo temperature rise off refrigeration. 1, Top front, left; 2, bottom door end, left; 3, mean; 4, centre Figure 8 EMvation de la temperature d'une caroaison de beurre sans production de froid. 1, En haut, it l'avant, it gauche; 2, en has it l'extr~mit~ de la porte, it 9auche; 3, moyenne; 4, au centre

140 Int. J. Refrig. 1989 Vol 12 May

Table 2 Temperature rise times for containerized cargoes off refrigeration Tableau 2 Temps dYldvation de la temperature pour des cargaisons en conteneurs sans production de froid

10~o 15% 25~

Butter in insulated container: mean 35 70 fastest 25 51 -

Butter in 'fruit' container: mean 17 - fastest 8.5 17

Butter, defrosting external refrigeration unit: fastest 4.5 Butter with portholes open: fastest 1.5 3 8.5 Cheese, with portholes open: fastest 4.5 7 Lamb in insulated container after liquid nitrogen treatment: mean 19 30 55 Frozen fish in insulated container: mean 28 57 138

maximum 6 11 27.5

~ S e t - p o i n t =]-" Set-point 0°C = - 2.5oC 4

v

2

0

- 1

. . . . . .

" ' . . . _ . . . . . . . . . . . . . . . . . • ~

I I 1 - 8 0 -- 40 0 40 80 120

Time from change of set-point (h)

Figure 9 Temperature changes in cartonned apples on reduction of set-point temperature in 15°C ambient. , Maximum pulp; • , air return; - - , mean pulp; . . . . , air delivery probe; . . . . minimum pulp; . . . . . , minimum air delivery Figure 9 Variations de la temperature dans des potatoes en cartons lors de I'abaissement de la temperature de consigne dans une ambiance de 15°C. - - - , M a x i m u m dans la pulpe; - . - , reprise d'air; - - - , moyenne de la pulpe ; . . . . . . , [t la sonde; - - - - - - , minimum de la pu lpe : . . . . , minimum de l'air soufflb

The results show a clear difference in performance between the insulated and 'fruit' containers, as would be expected. Rates of change during defrosting or with portholes open are rapid, but normally these conditions are only of short duration. Lamb (frozen carcases) responded more rapidly than butter, having both a lower mass in the container and greater scope for convective air movement. These results cannot be applied directly to other cargoes, but they do illustrate both the order of magnitude of rates of change and the influence of some of the important factors.

Change of set-point temperature It is not common to change set-point temperature whilst carrying cargo, although for some varieties of plum and for some in transit quaratine treatments it may be necessary. Data is, therefore, scarce but is useful in providing guidelines for cooling and heating ability and in indicating the likely effects of rare occurrences of operator errors.

Because, as shown above, results expressed in terms of percentage change may be applied to a wide range of temperatures, it is possible to carry out accurate


measurements for small changes in set-point and extrapolate the results to possible larger changes in practice. Some results are shown in Figure 9, in which a refrigerated container of cartonned apples was first stabilized at a set-point of 2.5°C in a 15°C ambient, and then the set-point was reduced to 0°C. Air temperatures and minimum pulp temperatures were close to their final values within 24 h, but both maximum and mean pulp temperatures took three days. The particular container used had a high air circulation rate (110 changes per hour) and a large amount of spare refrigeration capacity. In this case, as the controlled air delivery temperature was rapidly achieved, the overall rate of cooling was limited by heat transfer from the fruit, which is controlled by the type of packaging and the rate of air circulation.

Times for 50~o and 87.5~o temperature change are shown in Table 3, for the cartonned apples, and also for both cooling and heating of frozen butter. Butter cooling is slower than apple cooling because of the greater resistance to heat transfer through the individual items of cargo (25 kg butter blocks cf. single apples), also because of the greater heat removal requirement, and because of the smaller excess refrigeration capacity at lower temperatures. Comparison with the butter warming data shows the latter factor to be very important. The butter tests were conducted in an insulated container in 27°C ambient temperature, using an external clip-on refrigeration unit with electric heating.

It is worth emphasizing here that container refrigeration systems are designed to control temperature by heating or cooling as necessary. Unlike many cold stores, increasing set-point temperature does not just switch off refrigeration - air circulation fans continue to run and heating is provided. It would be more accurate to refer to 'controlled temperature containers' rather than 'refrigerated containers'. Heating is necessary for sub- tropical fruits carried in northern winter conditions, for example.

Rates of temperature change are very dependent on commodity, packaging, air flow and storage equipment. Results have been presented for a range of situations with half cooling times from under 1 h to over one week. Apart from the value of specific information given, these results allow limits and reasonable estimates to be given for temperature changes under a wide variety of conditions.

It is particularly notable that rates of temperature

Table 3 Cargo temperature change on change of refrigeration set- point temperature Tableau 3 Variation de temperature de la cargaison lors de la variation de la temperature de consigne

Time for change (h)

50Yo 87.5%

Apples, cooling (2.5 to 0°C) maximum 21 64 mean 13 53 minimum 5 22

Butter, cooling ( - 14 to - 20°C) maximum 26 77 mean 29 65 minimum 24 59

Butter, warming ( - 2 0 to - 14°C) maximum 7 21 mean 9 34 minimum 18 48



-1 .5 N ' " -2 .0

-2 .3 ~ - - -1 .8 ---.- - - 1 . 1 -

Figure 10 Temperature (°C) in a simulated lamb cargo in an ambient temperature of 15°C Figure i0 Temp&atures (°C) dans une cargaison simulke d'agneau d une temp&ature ambiante de I5°C

change in the absence of refrigeration are highly dependent on the insulation value of the container, and that cooling rates with refrigeration depend particularly on packaging of produce. Containers are not primarily designed to cool produce, and if they are used for this then cooling times of several days are to be expected, in contrast with the few hours required in a pre-cooling system.

Likely temperature distributions in containerized cargoes

However good the container and however well cooled, packed, and stowed the cargo may be, there is of necessity a temperature gradient within the container, dependent on outside conditions. Such gradients are known and understood by container operators, and as part of its research programme SRCRA has collected data on them over the last 20 years.

With developments in temperature monitoring equipment, commodity purchasers are now taking an increased interest in temperature in transit. Greater awareness of what is a normal and reasonable temperature distribution is needed, and the theoretical and practical importance of various factors needs to be understood.

A particular need for a narrow temperature range arises for cold treatment sterilization of fruits in transit, for example, to prevent importation of fruit fly from Mediterranean countries to the USA. Such treatment requires maintenance of a temperature close to the lowest tolerable temperature for the fruit, for a specified period.

As an example of temperature distribution, in one particular experiment a simulated cargo of meat in cartons was held in an insulated container with a clip-on refrigeration unit in a test chamber at an ambient temperature of 15°C. Temperatures recorded under steady conditions at some points in the cargo are shown in Figure 10.

In this experiment, the refrigeration unit was set to control delivery air at - 2.5°C. Air entered the container at - 2.3°C, and the coldest cargo was at - 2.0°C. There was a vertical gradient of 0.6°C at the front end and 1.0°C at the rear of the container, and a front-to-back gradient of around 1.0°C.

The refrigeration unit circulated 60 air changes per hour and the container heat leakage was 2 8 W K - Using the calculation procedure outlined below, the expected difference between delivery and return air temperature was 0.8°C, as was the measured value. The estimated cargo temperature range is twice this value, i.e. 1.6°C, which is not very different from the measured range of 1.9°C.

The stowage in this experiment included dunnage battens as were used in lamb shipments around 1980. A comparion with a block stow (no battens) showed a slight improvement in temperature uniformity when the battens were absent. A tight stow with all the cool air forced uniformly through the spaces between the cartons gives the best temperature distribution. If a container is not quite filled at the door end, this can be assisted by blocking the remaining space with empty cartons, polystyrene insulation or any other convenient material.

Adequate pre-cooling is necessary to ensure maximum life. It takes several days for meat only a few degrees above 0°C to be cooled to carriage temperature, and this corresponds to a loss of storage time at the carriage temperature of over a week.

Reasons for temperature variations

The refrigerated air supply to the cargo space in a container picks up heat through the container walls, and in some cases from the cargo. Areas where heat leakage is greatest or air movement is least will pick up more heat than the average position, so the warmer cargo will be warmer than air leaving the container. The temperature of the air delivered to the cargo is the only factor which can be actively controlled.

Efject of ambient temperature

The higher the difference between ambient temperature and cargo temperature, the greater will be the heat flow through the container walls. Taking a heat leak figure of 25 W K - ~, the heat gain is 1500 W for a cargo at - 20°C in a 40°C ambient, 500 W for a cargo at 0°C in + 20°C. If the container is static and in sunshine, the effective ambient temperature may be higher than the shade air temperature.

Effect of container age and condition

Although the effect of aging of polyurethane foam insulation will increase heat leakage by 3-5 ~o for every year since manufacture, larger increases are possible if containers are damaged or inadequately maintained and the insulation becomes wet. The importance of prompt and effective repair of damage is well understood by experienced container operators and problems in practice are rare.

Air circulation rate

All other factors being equal, the range of temperature in a cargo under refrigeration will be inversely proportional to the air circulation rate, once steady conditions have become established. Thus, for a narrow range of temperatures a higher circulation rate is required. Higher rates mean more fan power, thus a compromise is necessary.

In a ship's hold which is maintained relatively cool, a circulation rate through insulated containers of around 30 to 40 changes of the empty volume per hour is sufficient, but for free-standing refrigerated containers at least 60 changes are necessary. Some modern refrigeration units have higher circulation rates and these are able to maintain chilled cargo within a narrow range even in very high ambient temperatures.


Table 4 Results of dunnage trial in a stow of weighted meat cartons. (Ambient temperature controlled at 30°C, air circulation at 60 changes per hour) Tableau 4 R~sultats de l'essai de montants transversaux dans une pile de cartons de viande pond~r~s

Vertical Vertical No dunnage dunnage dunnage every alternate (block row rows stow)

Mean cargo temperature (°C) - 0.3 - 0.2 - 0.2 Calculated air temperature range (°C) 1.5 1.5 1.5 Measured cargo temperature range (°C) 3.5 4.0 3.4

Air flow pattern Both the design of the container and the methods of packing and stowage affect the pattern of air flow. The more uniform the pattern, the more uniform will be the temperature distribution. Generally speaking, the standards of stowage in temperature-controlled containers are high, short-circuiting of air flows is avoided, and temperature uniformity is good.

There are various opinions regarding the benefits of dunnage to break up the stow and encourage air flow. In a trial at SRCRA, the effects of two different dunnage patterns were compared under steady conditions with a block stow using weighted meat cartons, with results shown in Table 4. The dunnage used comprised 37 × 16 mm wooden vertical strips, and it was clear that this dunnage did nothing to reduce temperature gradients. A tight stow of undunnaged cartons gives a uniform distribution of air in the spaces between the cartons, and in the case of ventilated fruit cartons, helps to force air around the fruit.

Refrigeration control system The very short term temperature changes due to modulating or on/off refrigeration control are generally smoothed out by the thermal capacity of the cargo and have little effect, as long as air is circulated continuously. Over longer times, overall temperature ranges can be increased if the control system is not fully compensated for changes in ambient temperature and consequent refrigeration load. Modern electronic controllers are fully satisfactory in this respect, but some older electromechanical systems need careful supervision to obtain the best results.

At low refrigeration loads, it is possible to have a range of temperature across the evaporator of a refrigerated container, which will result in a non-uniform supply air temperature. This is something for purchasers of equipment to look for in equipment acceptance trials; it can be a problem if the temperature control sensor is not appropriately positioned.

Loading temperature As has been stated previously, container systems are not designed to cool down produce, but to hold temperatures at a steady level. If non-respiring produce is loaded above the carriage temperature, there will be an increase in the temperature range for several days whilst the cargo cools, with a corresponding loss of storage life.


In the case of respiring produce giving out metabolic heat, the combination of this heat, the fact that heat production rises rapidly as temperature rises, and the method of packing and stowage can lead to a situation where the cargo can never reach carriage temperature if it is loaded more than a few degrees too warm.

Prediction of temperature ranges

If the heat input to a given flow of air is known, the temperature rise can be calculated from the flow rate, specific heat capacity and density. Applying this to a typical 20 ft refrigerated container, the temperature rise, DT (°C), is given by

DT= 0.11H(TA - TD)/C

where: H is container heat leak (W K-1); TA is ambient temperature (°C); TD is air delivery temperature (°C); C is air circulation rate, changes per hour (ach). This equation needs appropriate modification if there is a respiring cargo giving additional metabolic heat.

Results collated from 21 separate trials on cargoes of apples, cheese, butter and lamb have been analysed. Calculated values of DT have been compared with measured delivery to return air temperature rise, with results shown in Figure 11. These results represent values of ( T A - TD) from 12.5 to 59, values of H from 24.3 to 44.8 and values of C from 45 to 110. Considering the wide

%. ~2

LU

a

I [ I 1 2 3

Delivery/return difference (°C)

b X

§2 x X~x~~X 0 I I I I

1 2 3 4 5

Cargo temperature range (°C)

Figure l l Estimated and measured temperature ranges in SRCRA trials for: O , apples; + , cheese; x , butter; O , lamb Figure 11 Temperatures estim~es et mesur~es lors d'essais de la SRCRA pour: O, potatoes; +, fromage; x , beurre; 0 , agneau



Table 5 Predicted overall temperature range for non-respiring cargo ( C} Tableau 5 Domaine general de temperatures pr~,vu pour des car qaisons m, respirant pas

Ambient temperature cargo temperature

20C 30C 40 ('

Air flow (ach) 40 60 90 40 60 9(I 40 ~l) 9(~ Heat leak 2 2 W K ~ 2.4 1.6 1.1 3.6 2.4 1.6 4.8 3.2 2.1 Heat leak 26 W K i 2.9 1.9 1.3 4.3 2.9 1.9 5.7 3.8 2.5 Heat leak 30 W K l 3.3 2.2 1.5 4.9 3.3 2.2 6.6 4.4 2.9

range of conditions, agreement between measurement and calculation is reasonably good.

For the same trials, maximum cargo temperature range was also measured. As is seen in Figure 11, this range is approximately twice the value of DT. This is due to the effects of increased heating at edges and corners of the stow and to non-uniformities in air flow.

Such measurements on real cargoes are necessary to obtain these results. Mathematical analysis of temperature distributions, even for relatively simple cargoes such as frozen blocks of butter in cartons, is unable to assist, as the cargo as a whole behaves somewhere between a single large cuboid and a single carton. Determining all the heat transfer factors necessary for a full mathematical model would be far more difficult than obtaining results experimentally.

Further results from tests on empty refrigerated containers at a number of ambient and internal temperatures have shown DT to be consistently predictable to within +0.5°C, as long as air delivery temperature is an average over a representative number of locations.

Suppose that some cargo was to be carried within a total range of 1.5°C in a container of heat leakage 25 W K - 1, with negligible metabolic heat. Assuming the range will be twice DT, application of the equation above shows that an air circulation rate of 37 ach would be sufficient if ambient temperature is within 10°C of cargo temperature, but a 30°C difference requires 110 ach. This means that the type of container necessary for particular carriage requirements can be defined, or alternatively for a given container the allowable range of ship's hold temperature can be stated.

As the factors which affect temperature range are straightforward, they allow prediction of the difference between air delivery and air return temperature in a container. Experimental results show actual maximum cargo temperature ranges to have about twice this value. Using these facts, overall temperature ranges can be predicted in containerized cargoes. Table 5 presents data for heat leak rates corresponding to new, average and old 20 ft insulated containers for three different air flow rates. These predicted ranges may be used as guidelines for both container operators and cargo shippers as to what is reasonable to expect in practice.

Future developments

Modern refrigerated containers are able to provide adequate refrigeration capacity and good temperature

control. The next stage of development, now in progress, aims to reduce energy use and maintenance costs whilst maintaining performance.

Microprocessor-based control and monitoring systems are appearing and these could reduce maintenance and repair costs by automated checking procedures and by better equipment condition monitoring. They also have the potential by using alternative defrost and fan control procedures to reduce energy use. The most promising energy saving measures, which could be used in conjunction with the microprocessor controllers, are compressor speed control and electric expansion valves. The former, using a.c. invertor frequency and voltage control, offers the energy use of on/off control with the controllability of hot gas bypass, but at present is relatively expensive and bulky. The electric expansion valve is available in smaller sizes but has yet to be demonstrated in a suitable form for transport use.

More complex speed control strategies (especially for evaporator fans), more efficient fan motors, and improved heat exchangers are likely to appear. The use of a single/double stage rotary compressor combination for chilled/frozen use is a possibility to be investigated. Refrigerant mixtures are unlikely to find favour in transport refrigeration due to difficulties of in-field maintenance.

The obvious technical solution of separately designed distinct units for frozen and for chilled cargoes would give both capital and running cost savings. However, the commercial problems of container positioning make this solution unacceptable except in very specialized applications.

Clip-on refrigeration units are likely to incorporate the control features of the latest refrigerated containers, and are likely to operate with higher air circulation rates to reduce cargo temperature gradients. To achieve this, more efficient fan systems will be needed to minimize the additional heat to be extracted by the refrigeration system.

Further development of machinery to provide control of atmospheric composition in containers is likely, as has been summarized by Champion 9. Such machinery incorporates means for measuring oxygen level and in some cases also carbon dioxide level. The container must be well sealed and either some means of scrubbing carbon dioxide or a supply of low oxygen atmosphere must be available. The latter may be provided either from bottled gas or from a membrane air separation system. Catalytic scrubbing of ethylene may be incorporated as part of or as an alternative to atmosphere control. The likely


commercial scope for these systems is limited to high value, limited life respiring produce, as there must be considerable additional costs involved in supplying relatively complex additional equipment for a limited range of goods to be carried.

In the foreseeable future, the effects of limitations on use of chlorinated fluorocarbons will affect design of transport refrigeration equipment and may also affect the availability of thermal insulation materials. It is too soon to predict the scale or rate of changes this will produce.

Conclusion

The technical performance of refrigerated and insulated freight containers has been under continuous development for two decades now, and modern equipment must represent one of the most flexible and best controlled applications of mass produced refrigeration equipment that exists. Nevertheless, the laws of physics dictate that time is necessary to obtain temperature change, and temperature changes require temperature gradients, so completely uniform temperatures can never be achieved.

Experimental results presented in this paper show what sort of performance can be expected from these containers in practical situations. The effects of the ongoing development of equipment are demonstrated for the benefit of ever-changing world-wide trading patterns.


Acknowledgements

The results reported in this paper are a summary of part of the work of all the staff of SRCRA over many years, and the role of every one in making this paper possible is gladly acknowledged. Some of the experimental work was partly funded by the U K Department of Trade and Industry, which is also acknowledged.

References

1 World Container Census Containerisation International, London, UK (September 1986)

2 Serine, G. R. Marine Refrigeration and Container Ships Proc Inst Mech Eng (1981) 195(7) 37-52

3 The transport of perishable foodstuffs SRCRA, Cambridge, UK (19851

4 Scrine, G. R. Further aspects of insulation deterioration Proc 16th Int Cong on Refrigeration Paris, France (1983) 557 564

5 Heap, R. D. Temperature distributions in frozen and chilled commodities in insulated and refrigerated ISO containers IIFqlR Commission D3 Orlando, USA (1985) 201-206

6 Heap, R. D. Rates of temperature change in refrigerated and in ventilated commodities in ISO containers IIF-IIR Commission D3 Orlando, USA (1985) 207-213

7 Heap, R. D. Container transport of chilled meat IIF-IIR Commission C2 Bristol, UK (1986) 377 381

8 Heap, R. D. Refrigerated freight containers: performance and potential for energy saving Proc 17th Int Cong on Refrigeration Vienna, Austria (1987) 325-330

9 Champion, V. Atmosphere control - an air of the future Cargo Systems International (November 1986) 11 28-33


Documents

Design and performance of insulated and refrigerated ISO intermodal containers