Colds to Rages Past

8/6/2019 Colds to Rages Past

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Paper for National Workshop on ColdChain Management and Rural WarehousingBombay, 22nd December 2004.

COLD STORAGESPast, Present and FutureBy

Krishna V.Jog, Head, Technology Cell,Kirloskar Pneumatic Co.Ltd, Pune-411013, India.

E-mail ID: [email protected] / [email protected]

Introduction

Cold storages are used for preserving foodstuff and other perishables for a long time

and also for delaying the process of ripening by CA or MA methods. Various types ofcold storage structures, types of insulating materials or insulated panels, variety ofcooling units various types of compressors, condensers etc. and many refrigerants havebeen used for over 50 years in India.

A lot of wonder refrigerantshalocarbons (popularly known as Freons) have been usedand then it was established that they are responsible for skin cancer and hence bannedprogressively. New research is required for widely using natural refrigerants like NH3and CO2 or hydrocarbons, which do not have ODP or GWP. Efficient systems must bedeveloped which contain minimum charge in the system and do not create anyenvironmental problems and leaving the globe a safer place for generations to come.

The paper discusses various types of equipment and refrigerants and gives guidelinesfor using natural refrigerant NH3 in DX compact system with water cooled or air cooledapplications.

Cold Storages

Earlier cold storages were mostly for Potato with very simple insulating materials likerice husk, which were very low in first cost. Nobody was bothered for low k values orprefab panels, which would cost more, as the first cost was the only consideration.Power was not very costly and was available relatively plenty as the total population as

well as the cold storages was not too high as is common today.

Indias population has exceeded 1.2 billion (120 crores) and it is estimated that by 2040it will cross 1.5 billions surpassing China. China is the No.1 most populated country inthe world. They have taken very strict measures to restrict the growth in theirpopulation. We are a country with various religions and we do not have a code ofconduct applicable uniformly same for all the religions. Due to the explosive growth inpopulation, all our infrastructure is under severe strain and the common man is not ableto get the benefit of such vast advancements in Science and Technology.

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There has been a tremendous growth in the number of cold storages all over India, notonly for potatoes but also for other products such as apples, vegetables, grapes,oranges, meat, fish, dairy and other sea foods. In spite of this, we do not have propercold storages for onions. Even storages are not available (let alone cold storages) andhence when the bumper crops are harvested, a lot of onions get damaged due to

unseasonal rains or hail- storms and then we have to import them at more than 6 timesthe standard price. India is the largest producer of onions in the world, but we are notable to get this product in a very good condition year-round at a reasonable price.

Spurt in Infrastructue

We see a tremendous increase in a lot of ways in the last 50 years after independencesuch as railway network, 4/6 lane express and super-express ways connecting theMetro cities of India. GQ project is building 11 km / day against the speed of 11.12 km /year before 1997. With such an increase in rail, road and waterways, it has becomepossible for farmers to get farm produce quickly to the centers where cold storages are

available. The time from harvesting to cold storage processing centers is very critical indetermining the life and quality of the product. This has been cut down tremendously.

The increase in electrical power to run the machinery of cold storages has not been asdramatic as the other infrastructure. Electrical energy should be available which isreliable and uninterrupted. If it goes on and off, then the food stuff kept in the coldstorages will undergo cooling / freezing and thawing in a cyclic fashion and the storedproduce will become useless for human consumption and will have to be thrown anddiscarded. Proper running of cold storages requires proper supply of electrical energywithout fluctuations or without interruptions and we also need back -up by generators asstand by in case of power failure. Otherwise, the product stored in the cold storage will

get fully damaged.

Multiple Products

Some cold storages can maintain different temperatures and R.H. conditions requiredfor different products. These are very critical, for example, Mangoes and Onions. Indiais the largest producer of both these in the world, but our export of these is verynegligible because we do not have proper cold storages facilities.

Mangoes

Mango is the national fruit of India. We produce more than 12 million tones of morethan one thousand varieties of mangoes in various states. The highest rated of them isthe king of Mangoes Alphanso from Ratnagiri in Maharashtra but not widely advertisedor known in export market. Our share of export of mangoes is less than 0.6% of theglobal trade. There is a growing realization that Alphansos are of high quality and tasteand with proper quick transportation we can increase our exports. Mangoes aloneaccount for more than 50% of world productiona fact not known widely.

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Onions

India is the largest producer of onions in the world. Onion is an important vegetablecrop. It is available in plenty during the season and is very cheap and within reach ofpoorest of the poor. It is mainly grown in the states of Maharashtra, Bihar, Karnataka,

Gujarat, Andhra Pradesh, Uttar Pradesh, Orissa and Madhya Pradesh.

Storage Requirement

The process consists of three steps.Step 1: Dry: Consists of removing surface moisture on outside crackling. 30 to 35C, 65% RH and air flow 60 m3 / tonne.hr in 2 to 3 days.Step2: Cure: Internal sap moisture: 25 C to 27C and 75% RH, 4 to 5 weeks andStep 3: Cool: Bring down to store to temperature. 0 to 1C, 75% RH and air flow 40M3/tonnes.hr. 1 to 8 months. When onions are to be taken out of the cold storage forconsumption, it should be warmed up for 2 to 3 days above dew point prior to

distribution.

If the above procedure is not followed, we will get outside layers blackened and alsoundue shrinkage and dryness will result.

The size of the Onion crop in a year depends upon weather conditions. It has been inthe range of 40 to 60 lakh tons about 5 years ago and is now expected to be about 70lakh tons due to increased productivity. In some years of excessive rains / scanty rains

/ delayed rains the crop gets affected and results in low production or sometimes evenafter good production, because of lack of storage facilities it gets spoilt and then pricesshoot up beyond the reach of the common consumers. The 1997-98 year showed an

unprecedented hike and onions had to be imported from various countries at a very highprice. Hail storm and sudden drop in soil temperature affected the root activity andbulb formation and hence acute shortage of the product.

Traditionally, onions are stored by conventional methods in various parts of the country.These include hanging bunches along with top, storage in various types of godowns.Some of these are provided with ventilation from side and bottom, so that heatgenerated is not accumulated in the godown. The losses in weight due to shrinkagevaries from 10% to 35% due to drying, 10 to 12% by decay and 8 to 12% by sproutingdepending upon RH and temperature during the rainy season. The total loss variesfrom 30% to as high as 56% depending on the type of storage construction used.

Refrigerated cold storages for onions are almost non-existent, but efforts in thatdirection are being seriously now given.

The BARC (Bhabha Atomic Research Centre) Bombay has developed techniques ofirradiation of Onions and Potatoes by which the storage life can be extendedcomparatively at higher temperatures even without refrigeration. This has however notyet been used on a commercial scale.

The above two have been discussed as typical examples where we need to focus ourattention.



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Refrigerants and Equipment

We will now look into various aspects of equipment used for refrigeration plants as wellas the refrigerants.

The cold storages attached to various Five Star Hotels where the product is to be storedfor about a week or ten days, a small condensing unit with product cooler is commonlyused. These units used to have R-12 in older units. Now R-12 being a CFC can nomore be used, mostly they are substituted with R-22 units. Some of the imported unitsuse HFCs such as R-134a, R-404A or R-407C.

R-404A and R-407C are zeotropic blends and are comparable to R-22 as substitutes.The relative performance of these refrigerants is as shown in Table 1, Annexure -1.

HFC.jpg

Some manufacturers use R-410A, although it gives higher capacity, it has higherpressure also. All the components and safety valves etc. have to be designed for quitehigh values particularly for air cooled applications. All these refrigerants like R-404A, R-407C, R-410A etc. are to be imported, as they are not manufactured in India. The priceof these is also very high.

The larger cold storages of upto 5000 Tons capacity are generally with NH3 asrefrigerant. Ammonia is a natural refrigerant having O ODP and O GWP. If leaked,returns back to soil as a fertilizer since it mixes with water in any proportion.

Almost all large and very large (above 5000 Tons) cold storages use NH3. They useeither gravity feed or liquid pumping system. For very large capacities, where longrefrigerant piping is involved, it is more efficient to use liquid pumping. Ammonia hasbeen used as a refrigerant right from very early days of refrigeration and will continue tobe used till the end of the world. It has very high latent heat of evaporation and is veryeconomical and has very low power consumption (bhp / TR)

Compressors

For majority of sizes except the very large ones, reciprocating open type compressorsare favoured. They are low cost, efficient, easy to maintain and are work-hogs. Theywork day and night, 24 Hrs. a day without any problem if basic preventive maintenanceis taken care of.

The modern open recips have 1 to 16 cylinders in various designs such as, inline, Vshape, W shape, etc. The capacities vary from a fractional TR to as high as 400 TR ina single machine. The speeds can vary from about 250 rpm to 3600 rpm or more.They are available for various refrigerants like halocarbons, CFCs and Ammonia. Thecompressors for Ammonia application cannot use copper, brass, bronze materials in



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construction. They can only use Steel / SS or Aluminium. Ammonia reacts with non-ferrous materials. The recips are used in single stage or two stage versions. Thesingle stage units are generally designed for compression ratios of about 8 to 9.5.Some heavy-duty industrial units are designed for compression ratios of upto 12, butthey suffer from lower volumetric efficiencies.

The single stage machines used for Ammonia can go down to about -10C to -15Cevaporating temperatures with +40C condensing temperatures. The same compressorcan go down to a bout -25C to -30C evaporating temperatures with +40C condensingfor typical R-22 or equivalent HFC refrigerant.

For temperatures lower than those given above say in the range of -40C and below,two stage internally compounded compressors are available. They can go down to -60C evaporating with +40C condensing temperature. These are normally used forwater cooled condenser applications. For air cooled applications, there are other typesof open recips which can withstand high condensing temperatures of +60C or above

but these are restricted to the lowest evaporating temperatures of no lower than -5C.For transport applications of refrigeration and / or air conditioning lightweight, highspeed relatively lower capacities machines are available.

Efficiency of Refrigeration Systems

This is identified in three different ways 1) EER 2) COP 3) kW / TR or bhp / TR. Foropen recips, it is the bhp / TR most commonly used. For open compressors, there isnothing like input kW as there are no electrical terminals fitted to the compressor motor,as is the case with hermetic or semi hermetic compressors. In case of hermetics (which will also include semi-hermetics) there is no way to measure brake horse power

or brake kW since the compressor shaft is inside the common shell for compressor andmotor. The motor efficiency thus comes into picture for such hermetics. If highefficiency motors (97% to 98%) are used, a lower IkW figure would be obtained andhigher IkW would result for lower efficiencies. Typically for small compressors andsmall motors, motor efficiency can be as lower as 60% or so. For open compressors,brake Horse Power is measurable and, therefore, for open systems only bhp / TR canbe defined.

The efficiency of effectiveness of a refrigeration system depends on several factors.The ratings published must give the datum or the basis for such data. Many-a-times,these factors are not given and then a true and fair comparison of various systems

cannot be made. Some of the main factors on which the capacity and powerconsumption depend are: 1) Speed 2) Degree of sub-cooling 3) degree of superheating4) Pressure drops allowed in suction and discharge.

Energy Efficiency at Design Conditions

ARI (Air conditioning and Refrigeration Institute, USA) has developed a rating systemand given basic parameters at which these can be compared. Most of the time theseARI set of parameters are just non-existent in India, but it is often used as a commonbasis for comparison. The parameters like cooling capacity TR, Power consumption,



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kW or bhp, COP etc. can be compared if they are brought on the same basis and if acomparison is made apple to apple. In actual practice, the plant never works at designconditions and the performance has to be judged at off-peak or off-design conditions.ARI provides this by comparing the integrated Part Load Values (ILPV) for variousoffers. The ILPV uses weighted national weather averages, weighted averages of

building operation and a blending of the kW / TR at four rating points: 100%, 75% , 50%and 25%.

Efficiency of Recips Vs Screws

The screw compressors are excellent rotary machines, which are also positivedisplacement machines. They are of two designsmono screw or dual screw. Thescrews are basically capable of high compression ratios in a single stage even upto 20to 25. They can also be used for multistage operations. Screws are fully balanced

rotary machines with high precision required for manufacturing of screws. Thelubricating oil used is plenty and they require special oil tank separator.

Actual screw compressor is a very small piece of equipment but with motor, controlpanel, OTS (Oil Tank Separator ) etc. it looks very bulky.

It can be seen that at moderate temperature of 35F evaporating and 115Fcondensing, screw consumes 1.2 BHP / TR whereas, a typical open recip. Compressorconsumes less than 1 BHP / TR. These figures are for R-22 refrigerant and forAmmonia, the recips consume even lesser than that of R-22. Thus in many instances,the power consumption of screws at full load is higher than that of Recips. The

difference in power consumption at lower temperature around -30C /-35C evaporatingwith +40 C condensing and for Ammonia is much more and Recips become clearly thewinner. Screws consume much more power apart from oil management systemcomplications.

The same story is true for part load operation as well. A typical ILPV comparison atARI conditions is shown in figure 1 for Screws and Recip chillers as referred in thereference 1 at the end of this paper.



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Figure 1

Another comparison of part load performance of Piston (Recips) compressor and screwcompressor is reproduced here with courtesy of P.C.Koelets book on industrialRefrigeration ( Ref.No.2) Figure 2.



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Figure 2

Another independent source ref. Cold and Chilled Storage Technology Edited byC.V.J.Dellino published by Blackie and Sons Ltd. Glasgow have made the comparisonof COP at part load conditions as shown in fig.3 for reciprocating, screw and centrifugal.

Fig.3. Part Load Compressor Performance



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The above figure shows a lower COP both for centrifugal and screw compressorwhereas that for recips it is substantially higher. The above comparison is forcompressor only.

If the effect of other ancillaries in the system is considered the Part Load Curve varies

as shown in Fig.4.

Fig. 4. Part Load System Performance

The compressor performance and hence power cost, is only a part of the totalownership cost of a refrigeration system. The minimum cost can only be achieved bygood system design and by maintaining the actual logs at the design level conditionsand by taking all corrective actions such as, cleaning of condenser tubes / cooling towercircuit, cleaning of choked filters, choked nozzles etc. maintaining proper refrigerantlevels in heat exchangers, degree of superheat, degree of sub-cooling, pressure drops

etc. within limits.

Most of the systems thus continue to use the most trusted open reciprocatingcompressors on NH3 as refrigerant.

Faster Pull Down

Low temperature applications such as, plate freezers, Blast freezers, Frozen storagesetc. essentially require compressors to be selected at low evaporating temperatures at-30C to -40C or -45C or lower. For these conditions, either booster systems or two



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stage compound compressors can be selected. In India most selections use internallycompounded 2 stage compressors like KC-51, KC-42, KC-72, KC-63 or KC-102 or KC-93 etc. All these represent two stage models. The first digit represents the Low stagecylinders and the second digit shows the High stage cylinders. Thus a model KC-51and KC-42 is a 6 cylinder model which would become KC-6 if it were a single stage

compressor.

When we start the system from ambient pull-down conditions, the compressor will workas a high stage compressor only. When the system achieves temperature of around -15C, it can be operated as a two-stage system. Various steps of loading are given inthe compressor manual. The two stage KC-51 compressor will operate as KC-1 withonly one cylinder till the temperature comes down. If we select KC-42 and start thecompressor it can be made to work as KC-2 compressor by taking care of oversizingthe condenser and motor kW suitably. This will give rise to a faster pull down.Depending upon the requirements of initial cooling i.e. the extent of room size, load,brine tank reservoir size etc. and how often this takes place, it may be worthwhile to

design the system right at the beginning for a faster pull down. It will be difficult orsometimes impossible to incorporate these changes in the field for achieving faster pulldowns once the systems are charged with the refrigerant and commissioned. It isnecessary to visualize these right at the design stage so that we get quick cooling of aproduct in stipulated time periods. The quality of the product to be frozen depends onthe speed at which this takes place and the time it requires so that the product is quicklyfrozen and can be kept for a period of 8 months or longer in a frozen storage roomwithout affecting its overall quality and texture.

Other Equipment

There has been considerable progress in cooling units as well. In old days, bunkercoils were commonly used because of low cost and space was never at a premium.Floor mounted air-cooling units with flooded gravity fed NH3 or R-22 on DX or with brinespray became common. They had centrifugal fans and could supply chilled air throughducting to various floors as well as larger distances in bigger halls. They occupied floorspace. Nowadays, ceiling suspended units either blow-thru or suckthru have becomequite common. A lot of units use carbon steel OD tubes and also have the option ofstainless steel tubes with various fin configurations suitable for various temperatures.Low temperature units have less fins per inch than the high temperature units becauseof frost formation at low temperatures.

Condensers

In older units, atmospheric condensers of bare pipes with water trickling through holes,without any fans were quite common. These have now been replaced by Shell andTube or Evaporative condensers. Evaporative Condensers are installed in open spacesuch as terraces and they combine the condenser and cooling tower function and giverise to lower condensing, as they utilize evaporative cooling and hence lower runningcosts due to lower kW / TR.



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Some installations may be with Plate Heat Exchangers (PHEs). They are compact andcan achieve lower temperature leaving differentials. The first cost is however muchhigher than the conventional shell and tube.

Challenges for Future

We must concentrate more on the use of natural refrigerants like NH3 and CO2. Manycountries are taking CO2 seriously. CO2 has higher pressures and proper equipment towithstand high pressures must be designed.

We must develop even smaller units with reliable open type NH3 compressors and alsolook for semi-hermetic recips or screws for NH3 duty. Some European manufacturersare offering these for NH3 duty with motor windings of Aluminium. We should developunits with minimum charge in the system. We need to find out the maximum chargeallowable for units to be installed in residential / commercial places from relatedauthorities in our country. This is possible if we switch over to DX ammonia instead of

flooded systems. For DX we can use OD or 3/8 OD Carbon steel or Aluminiumtubes and also for condenser. If we can make small capacity 3 TR to 5 TR (10 TR to 15TR) self contained DX ammonia systems with low charge then by checking thelimitations on charge in the system, with the concerned authorities, they can be useduniversally not only in Industrial units but also in other residential and commercialapplications. We should even develop smaller systems with DX-ammonia air-cooledunits similar to R-22 air-cooled ones.

Industry must come up with low charge DX ammonia systems, which can be used, evenfor small capacity cold storages. For large capacity ones, NH3 is already the mostfavoured refrigerant with gravity feed or pump recirculation system. We must focus our

attention on NH3 more than trying other zeotropes, which are not manufactured in ourcountry, are costlier and have higher temperature glides.

References

1) Air conditioning and Refrigeration Journal, Vol.7, No.1 Jan-Mar 2004 Issue,published by ISHRAE, Bombay Chapter Advantages of Recip Compressors byK.V.Jog.

2) Industrial RefrigerationPrinciples Design and Applications by P.C.Koelet with T.B.Gray 1992, published by Marcel Dekkar Inc. New York.

3) Cold and Chilled Storage Technology edited by Clive V.J. Dellino, Published by

Blackie and Sons Ltd. Glasgow, U.K.4) Cold Storage Industry in India (Keynote presentation at Refrigerated Warehouse

and Transport Association of Australia Ltd. (RWTA) conference, Melbourne,Australia in October,2004) by K.V.Jog.

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Colds to Rages Past