SOME ENGINEERING AND OPERATIONALASPECTS OF CONTROLLED ATMOSPHERE

STORAGEby

S. W. Porritt

Research Station, Summerland, British Columbia

INTRODUCTION

Controlled atmosphere storage(CA) also known as modified atmosphere storage or gas storage refers tothe technique in which the compositionof the storage atmosphere is regulatedor controlled. The value of the pro-cedeure was first demonstrated inEngland in 1927, was introduced commercially in New York State in 1940and has increased spectacularly in theUnited States and Canada in the lastfew years. The Okanagan Valley hasCA storage capacity of 522,000 loosebushels with a possible expansion to1,000,000 bushels by conversion ofother existing facilities.

CA storages in British Columbia areoperated at 30-35°F with atmospheresof 2-5% carbon dioxide and 2-3%oxygen depending on variety. Successful operation of CA storage is effectivein maintaining good quality and condition of well selected fruit for a con

siderably longer period than regularcold storage. A successful operation,however, depends upon good management and properly designed plant.

BUILDING CONSTRUCTION

Structural Material

A CA storage is essentially a coldstorage with addition of a gas-proofmembrane. Because the gas seal mustbe accessible for inspection and repairit is located on the inside surface of

the rooms. Hence it becomes an almostperfect vapour barrier which traps anymoisture penetrating the exteriorvapour seal. The hazard of moisturecondensation is minimized to a greatextent and long structural life ensuredby building CA storages of concreteslab or block and insulating exteriorwalls with moisture resistant material

such as expanded polystyrene. Insidepartitions less subject to high moisturegradients may be constructed of woodand insulated with bat-type insulation.Long experince in New York State (8)has proved that loose fill inorganic insulation in the ceiling without externalvapour seal is consistently satisfactorywhen a vented attic space is provided.

Structural failures have occurred inframe buildings used for CA storageparticularly old converted buildings

with inadequate vapor barrier. Somewooden frame buildings planned andbuilt specifically for CA storage, however, are giving satisfaction. Buildingsof this type using plywood for bothstructural material and gas barrier havebeen operating five to six years withno evidence of deterioration.

It has been shown (5) that whereground water is lower than 10-12 feetbelow the floor level the cost of insulation in the floor is not justified. However, insulation extending from the

k

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^-Mir.i

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Figure I. Structural details of insulation and gas sealat floor-wall juncture.

walls below grade two to three feetdown the foundation wall (figure 1)should not be omitted.

Gas Seal

Many materials are sufficiently impervious to oxygen and carbon dioxideto constitute a satisfactory gas seal forCA storage. They include numerousplastic films such as polyethylene;mylar; vinyls; sprayed, brushed orplastered coatings; metal foils; plywoodand sheet metals. The limiting factor inmost instances is lack of a satisfactorymethod of application or failure afterone or two years of operation. Mosteasily applied and durable is sheetaluminum or 28 gauge galvanized iron.This is nailed at 3-inch intervals totreated wooden furring pieces set inthe insulation and secured to the walland ceiling structure. Joints are lappedabout 2 inches and nails and joints arecaulked with a non-hardening compound. A good grade fir plywood withjoints sealed has proven sufficientlygas-proof in a number of storages inthe United States and Ontario duringseveral years. The life expectancy ofplywood, however, has yet to be determined. Five inches of well laid dense

CANADIAN AGRICULTURAL ENGINEERING. FEB. 1966

concrete on a properly compacted fillforms a satisfactory gas seal for thefloor. To avoid fractures, floors shouldbe poured in sections of about 175-200square feet with joints cleaned andfilled with non-setting mastic. Thefloor seal at the floor-wall juncture(figure 1) can be made effectively byprovision of a perimeter channel about\V2 inches wide and 2-3 inches deepinto which wall metal is extended andwhich is finally filled with non-settingmastic. This type of floor constructionpermits minor deflections under loadwithout cracking or rupture of the gasseal. Metal clad doors are made gasproof by sealing with carefully appliedsponge rubber gaskets. Provision mustbe made for gas-tight entry of all refrigeration and water piping and electrical conduit. Water drain lines mustbe trapped and electrical conduitsealed internally. Before a room is putinto operation it must be proven gastight. In a simple method of estimatinggas tightness, air is pumped into thesealed CA room to 1-inch static waterpressure with room temperature atequilibrium with outside temperatureand cooling system off. According toSmock (6) a 10,000 bushel room issufficiently gas proof if some positivepressure persists for 30 minutes ormore. In the larger CA rooms in theOkanagan positive pressure for aminimum of one hour is correlatedwith good room performance and hasbecome the accepted standard.

Room Shape and Size

Because of economy, and to facilitate rapid oxygen reduction free spacein the room and surface to volumeratio should be kept to a practicalminimum. A cube would constitute theideal shape of a CA room and for thisreason high stacking is desirable. InCA storages in the Okanagan Valleyfruit is stacked 25 feet high inrooms which are approximately 42 feetsquare with 28 foot ceilings. Spaceabove the fruit is required for air movement from blower units mounted onthe ceiling along one side of the room.Dimensions of the rooms should be amodulus of the container dimensionswith wall clearance and allowance foiair movement between stack rows. Forexample, 43 inch x 48 inch bins are

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stacked tightly in rows 48 inches wideparallel to the air movement with 8inches clearance at the walls and 5inches between rows. Twelve to 18inches clearance is provided at thewall under the blower units to permitunobstructed access of return air tothe coils. Doors likewise should besized and located so as to facilitatefinal loading.

REFRIGERATION

A frank appraisal of CA performance admits that the rapid coolingand good temperature control obtained in CA rooms accounts for a significant part of the beneficial effects of thismethod of storage. Design of the refrigeration system for CA must incorporate features which guarantee rapidcooling, high humidity, and accuratetemperature control with minimumfluctuations and spacial variations.Satisfactory performance may beachieved with freon or ammonia usingceiling units or floor mounted evaporators with air ducts. In general, ceilingmounted evaporators with blade typefans have design features which areadvantageous. These include large coilsurface and high air volume with expenditure of about lA the horsepowerthat is normally characteristic of largefloor units with duct systems.

Cooling Capacity and Temperature

Prompt cooling of fruit destined forlong storage is of utmost importance indelaying ripening and preventing certain physiological disorders. Calculation of the heat load should be basedon average peak intake of fruit andexpected prevailing temperature at thetime of harvest. In addition to sensibleheat of fruit and containers, respirationheat must also be anticipated (figure2). It is sometimes not recognized that

TEMPERATURE OF

Fiaure 2. Heat load incurred bv a daily loadina rateof 40 bins of apples at any temperature between 40°and 80°F. and cooled to 30°F. in 7 days. Net contentof bins (43" x 48" x 24") is equivalent to 1,000loose bu. boxes. If boxes are substituted for binsthe sensible heat load due to container must be

increased by a factor of 2.

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rapid cooling actually requires lessrefrigeration than slow cooling. This isso because heat of respiration is reduced by as much as 2A for each 18°Freduction in temperature. Rapid cooling, however, can be achieved withadequate coil and compressor capacityonly if air volume and distribution arealso adequate. Theoretical values of1,000 to 1,100 cfm per ton of coilcapacity used in the past in many fruitstorages has seldom proved adequatein practice. Chronic problems of localized warm areas, frozen fruit, extendedcooling time and low humidities characterize many of these installations. Inthese storages even when fully loadedwith fruit properly stacked, the magnitude of free space is such that muchlarger volumes of air would have to becirculated in order to ensure positivemovement in all areas. Evaporatorsrated on a 10°F.T.D. and capable ofdelivering 2000 cfm per ton capacityof coil can be expected to do an efficient job of cooling.

HumidityThe turgid crisp texture of fruit is

lost with dehydration. Shrivelling occurs with 4 to 5% water loss fromapples. Greatest rate of water lossoccurs when fruit is warm and thevapor pressure deficit is correspondingly high. Weight loss during coolingperiod may amount to as much as 1-\V2% (4). When a stabilized temperature is reached rate of moisture lossvaries inversely with the relative humidity of the storage atmosphere (9).Therefore, to avoid excessive waterloss during the long period fruit is keptin CA storage relative humidities of90-95% are required. Once a storageis sealed only the cooling coils andfruit containers remove moisture. Itmight be assumed on theoreticalgrounds that a dry apple box inequilibrium with air at 70°F and 20-30%rh would contain sufficient mois

ture to satisfy equilibrium requirements at 30°F 90% rh. In practice,however, the caintainer increases inweight considerably. Apple boxes havebeen shown to increase in weight by 8to 9% under these conditions (table1). This means that apples in CAstorage suffer a further 0.8% weightloss to the 25-bushel bins in whichthey are stored. During a long storageperiod, however, the greatest potentialfor moisture loss is by condensation orfreezing on the cooling coils. Controlof this factor by proper design andoperation constitutes the most satisfactory method of regulating water lossand humidity in fruit storages. Highhumidity is provided by a system inwhich coils have sufficient heat transfer when operating with small temperature difference between refrigerantand storage air. For example, (figure3) in a 35°F storage, 95% rh isobtained when return air is not cooled

Figure 3. Relationship of relative humidity to discharge and return air temperatures. The qraph wasdeveloped from values found in Table II. Determination of thermodynamic properties of moist air,

ASHRAE Guide and Data Book, 1961.

more than 1.3 %F when passingthrough the coil. Coils which are selected for the cooling load on the basisof a 10°F temperature difference andequipped with suction pressure controlwill provide a satisfactory humidityduring the holding period. In actualpractice during five months at 35°F,90-93% rh weight of Mcintosh applesdecreased on an average only 2.45%(table 2).

TABLE I. CHANGE IN WEIGHT OF APPLE BOXESRELATIVE HUMIDITY

WITH CHANGE, IN

% R.H.

Wt. gm.

Temp. F. A B

70

70

30

21

65

87

2650

2856

2864

2483

2700

2707

TABLE II. WEIGHT LOSS OF McINTOSH APPLES IN CA STORAGE

Avg. net wet. of fruit/bin(12 or more bins) % loss

Lot No. Before Storage After 5 mo. storage of weight

1 804.6 786.3 2.27

2 800.6 781.8 2.34

3 890.5 770.5 2.53

4 802.2 780.8 2.66

Average 799.5 779.8 2.45

CANADIAN AGRICULTURAL ENGINEERING, FEB. 1966

EQUIPMENT LAYOUT

Equipment in CA rooms filled withfruit is not readily accessible and anexternal source of oxygen must besupplied to workmen in atmospherewith less than 16% oxygen. For thisreason and to reduce risks of refrigerant leaks, only a minimum of pipingand equipment is included inside theCA rooms. This is limited to the coilsand supply and return headers forrefrigerant and water where water defrost is used. Main suction and liquidlines, suction traps, solenoids, floatcontrols, expansion valves, etc. shouldbe kept outside the CA rooms. Watersupply and drain lines must be properly graded to avoid blockage by freezing. Water drains must be trapped toform a gas seal and the trap must beprotected from freezing.

To protect the gas seal in CA roomsfrom excessive pressure fluctuationsthat may be caused by severe barometric changes or by rapid carbondioxide absorption during oxygen reduction, all rooms should be equippedwith pressure relief valves. In itssimplest form this may be a 2-inchdiameter U tube with means of adjusting water level so that venting occurswhen pressure difference between theinside and outside atmosphere exceeds1-1 '/2 inches of water. An ethyleneGlycol solution is used to avoid freezing.

ATMOSPHERIC REGULATION

With some exceptions which are notdiscussed here, the respiration processof the fruit is relied upon to reduce theoxygen content of the CA room to thedesired level. At this point, to preventoxygen dropping below the criticalminimum of 2-2 Vi%, the room isvented to the outside through a smallport provided for the purpose. Undernormal metabolic conditions carbor:

dioxide is produced by the fruit inquantities equivalent to the oxygenutilized. Respiration studies at Sum-merland have shown that Mcintoshapples at 70°F produce 8-10 cubicfeet of C02 per ton in 24 hours butonly 11/2-2 cubic feet at 36°F. Smock(6)concluded that respiration rateunder CA conditions (5% C02 plus3% 02) was !/3 that in air. Within2-3 days of sealing the room carbondioxide will have reached the desiredmaximum concentration and must becontrolled at this level. Scrubbing, theprocess of removing excess C02 fromthe storage atmosphere, may be accomplished in a variety of ways. Themost common method in the past wasto circulate storage air through a cascading solution of sodium hydroxideor sometimes a slurry of lime. Recently

Mann (3) in England used monoethan-olamine to absorb C02 from the CAatmosphere and in a more sophisticatedprocedure molecular sieves of thezeolite types have been found effective(2). Both of the latter materials canbe regenerated by heating and may becycled repeatedly. As a result of observations and experiments in NewYork (7) a system was developedusing water to remove C02 from thestorage atmosphere. One of the simplest and cheapest methods of scrubbing,however, was devised by Eaves inNova Scotia (I) who used bags ofagricultural lime, Ca(OH)2 in a chamber connected to the storage by pipesfitted with valves to control rate ofC02 diffusion.

In the Okanagan Valley of BritishColumbia lime scrubbing has beenadapted for use in large CA roomsholding 385 tons of fruit. In these installations, lime chambers adjoiningthe CA rooms and connected by twolouvered openings, hold 200 - 50 poundbags of lime (figure 4). Lime is

Figure A. A view of a lime scrubber showing one ofsix pallet loads of lime and the fan used to circulate

storage atmosphere through the scrubber.

handled conveniently on pallets makingit possible to load a scrubber or recharge it rapidly. A 500 cfm fan operated automatically by a time switchcirculates storage atmosphere throughthe lime room. When the fan is notoperating louvers close to prevent excess scrubbing. With storage roomblower fans and scrubbing fans off,the door to the scrubber room can beopened and the lime replaced withnegligible effect on the gas concentration in the storage. Mann (4) has calculated that 1 pound of lime is capableof absorbing 4.4 cubic feet of C02.However, rate of absorption decreases

CANADIAN AGRICULTURAL ENGINEERING, FEB. 1966

as increasing amounts of lime reactwith C02. Consequently, scrubbersholding a small percent of the season'slime requirements are inefficient. Onthe other hand, the gross absorptioncapacity of a large quantity of lime asused in the Okanagan is sufficient tocontrol C02 levels during the periodof declining efficiency. This has beenapparent in the past season's operationin which 0.54 pounds of lime perbushel (32 pounds) was required during the six month storage of Deliciousapples using 10,000 pounds of lime inthe scrubber. Under comparable conditions, however, where the scrubberswere charged with 6700 pounds oflime, 0.76 pounds per bushel were required and Eaves reported about 1pound per bushel was used in a scrubby charged with 5800 pounds of lime.Undoubtedly, water or lime scrubbingat present are the cheapest methods ofC02 removal. Construction cost ofwater scrubbers is generally more thanfor lime but operating costs are small.Reduction of oxygen concentration isnot as rapid with water scrubbers andmaintenance of 2% or less C02 andlow 02 is more difficult than withlime. Average cost of lime per bushel(32 pounds) for storage of 440,000bushels of apples during a six monthperiod in the 1964-65 season wasabout one cent per bushel includingcost of handling.

INSTRUMENTS AND CONTROLS

Room Temperature Control

A reliable thermostat to controlroom temperature with narrow differential is essential. Fluctuations inair temperature which may producesmall or insignificant changes in fruittemperature are acceptable in air storage but should be minimized in CA because corresponding atmospheric pressure changes in the room increase thedifficulty of maintaining low oxygenconcentration. The use of a properlyadjusted pressure regulator tends todampen fluctuations that would otherwise occur in discharge air temperature.

A modulating back pressure control,ideal for CA storage, automaticallyregulates suction pressure to suit theheat load thus ensuring even dischargetemperature and high humidity duringthe holding period. Evaporator coilsshould have a positive method of defrost scheduled automatically by a timeswitch. To avoid undue increase in atmosphere pressure in the room thedefrost period should be the minimumrequired to clear the coils and the fandelay should be set with sufficient time

continued on page 39

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. . . IRRIGATION SYSTEMS

continued from page 14

for system design under different soilsand crops; each individual irrigationsystem will be designed with capacitiesto meet the requirements of meteorological events having different probabilities of occurrence. It is suggestedthat a more rational approach to designmay be accomplished by using a peakconsumptive use value of a given probability. Several examples are presentedshowing the reduction in capital costsof a sprinkler irrigation system whenusing the proposed design criteria inplace of the conventional procedures.

LITERATURE CITED

1. Gumbel, E. J. Statistical Theoryof Extreme Values and somePractical Applications. NationalBureau of Standards. AppliedMathematics. Series 33, Washington. 1954.

2. Jensen, M. C. and King, L. DesignCapacity for Irrigation Systems.Agr. Engr. 43:522-525, 1962.

3. Kerr, H. A. The Development ofan Irrigation Budget. UnpublishedM.Sc. Thesis. University ofSaskatchewan Library, Saskatoon,1963.

4. Nicholaichuk, W. Frequency ofOccurrence of EvaporationAmounts. Unpublished M.Sc.Thesis. University of Saskatchewan Library, Saskatoon, 1964.

. . . CONTROLLED STORAGE

continued front page 21

to permit coils to be cooled to roomtemperature by refrigerant beforeblowers start.

Gas Analysis

It is accepted practice to analyzestorage atmospheres daily and regulatescrubbing time or venting if necessary.Faithful attention to this routine task is

Figure 5. A convenient arrangement of gas samplingand analysis equipment in the office of the CAstorage, in addition to gas analyses, temperaturereadings in one or more locations and static pressurecan be determined in any of the rooms from this

central point.

essential and much time is saved byproviding for gas analysis at a centralpoint such as office or engine room(figure 5). Polyethylene tubing, XAinch, makes satisfactory gas samplinglines which can be connected to a

manifold fitted with stopcocks andequipped with a small diaphragm pumpto draw the sample. Oxygen and C02can be analyzed simultaneously byusing simple orsat type equipment forC02 and a Beckman D2 instrumentfor 02. The convenience and economyof time inherent in this arrangementjustifies the cost where six or moreanalyses are made daily.

Thermometers

An accurately calibrated recordingthermometer to measure return airtemperature is an invaluable means ofassessing plant operation as well asproviding a permanent record. For additional verification, an indicating thermometer should be located in the storage so as to be visible from one of theglass inspection ports.

While not essential, remote temperature indicating equipment may be usedto advantage particularly in a largestorage, in monitoring air and fruittemperatures in various parts of theroom. Thermister type equipment withnarrow range and matched, calibratedprobes is inexpensive and has provenhighly satisfactory in practice. The instrument with multi-point switch canbe located centrally with gas analysisequipment for convenient determination of temperature at the time gasanalysis is made.

Static Pressure Gage

One more instrument which providesuseful information to the CA operatoris a static pressure gage which indicates+ or — static pressure in fractions ofan inch to ± one inch to two inchesof water pressure. Various commercialinstruments are available but a simplymade "U" tube manometer suffices.The pressure gage connected one to aroom or one common instrument connected to the gas manifold helps theoperator to account for changes inoxygen levels in the storages and permits assessment and adjustment ofpressure fluctuations due to refrigeration cycling and defrosting.

REFERENCES

1. Eaves, C. A. 1959. A Dry Scrubber for CA Apple Storages. Transaction Am. Soc. Agr. Eng. 2(1),127-128.

2. Johansson, J. 1963. Use of Molecular Sieve as a C02 Adsorbent

CANADIAN AGRICULTURAL ENGINEERING FEB. 1966

in Experimental Controlled Atmosphere chambers. Proc. Am.Soc. Hort. Sci. 83, 172-174.

3. Mann, G. 1958. The Use ofEthanolamine in Scrubbers for

Fruit Stores. Mod. Refrig. 61(727): 990,992-994.

4. Mann, G. 1960. Construction ofCold Stores for Fresh Fruit and

Vegetables. 2. Building and Engineering Requirements. DittonLab. Memoir 10, 5-20.

5. Sainsbury, G. F. Heat LeakageThrough Floors, Walls andCeilings of Apple Storages. U.S.Dept. Agr. Market Res. Rep. 315.

6. Smock, R. M. 1958. ControlledAtmosphere Storage of Apples.Cornell Univ. Exp. Sta. Bui. 759.

7. Smock, R. M. 1959. Water Removal of Carbon Dioxide from

Controlled Atmosphere Storages.Proc. N.Y. State Hort. Soc.

8. Smock, R. M. Private Communication.

9. Wells, A. 1962. Effects of Storage Temperature and Humidityon Loss of Weight by Fruit. U.S.Dept. Agr. Market. Res. Rep.539.

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