ACR Project Consultants, Pune
A post graduate in refrigeration and air-conditioning from University of Roorkee with 35 years experience, specialising in process air-conditioning, cold stores and freezing plants.
Refrigerated storage which includes cold storage and frozen food storage is the best known method of preservation of food to retain its value and flavour. Preservation of perishable food is a matter of vital importance and the loss due to the lack of proper storage facilities is very high.
Preservation of food by proper storage assumes great importance in a country like India where food shortage is a perennial problem. Several kinds of fruits and vegetables are cultivated seasonally depending on rainfall land other climatic conditions. These have to be preserved both at producing centres as well as consuming centres, and by employing correct storage practices it is possible to store them for periods ranging from two weeks to over eight months. See Table 1
Temperature-wise there are two of cold stores used for different applications:
These are generally designed for storing a variety of products at 0 to 8 deg C. Commodities include various types of fresh fruits and vegetables, dry fruits, spices, pulses, milk products etc. Most cold stores in U.P. Bihar, Punjab, West Bengal and M.P are designed for a single commodity storage such as potatoes. Units in Maharashtra and parts of Gujarat and the Southern states are of multipurpose type designed for storage of various commodities. The occupancy levels in the multi commodity stores are much better than those used for single commodity storage. However, the chambers in multi commodity stores have to be designed for maintaining different temperatures and sometimes relative humidities to suit different products.
Construction practices vary largely, in different parts of the country depending on the type of usage. The bulk commodity stores have chambers of large sizes whereas the multi-purpose units have many chambers of smaller sizes to suit the customer needs. These medium temperature cold stores are generally constructed with 2 to 6 floors. The capacity range is from 500 M.T. to 10,000 M.T. or more. However, in case of cold stores attached to pre-cooling plants the capacities are much smaller and the units are designed with single floor construction.
There are a number of commodities which have to be processed and frozen for preservation over long periods of time. These include green peas, corn, okra (bhendi), mixed vegetables, mango pulp and tomato puree. The other items are ice cream, butter, fish and meat products. Frozen food stores are designed for a temperature of - 20±2 deg C for most foods. But for items like ice cream, lower temperatures in the range of -25 to -30 deg. C are specified.
Frozen food stores are, normally, a part of a food processing and freezing complex. However, they are also set up as a part of multi-purpose cold store or as independent units to offer facilities for storage of products already frozen at the food freezing plants
The conventional construction includes brick walls with RCC frame and a roof a with RCC slab. In U.P. Bihar and M.P. the roofs are truss type with G.I. / A.C. Sheet covering. Internal floors are constructed with RCC columns and beams or with steel frame work with wooden batten flooring as a normal practice. In some of the recent units RCC battens and steel grating have also been used. In case of cold stores for pre-cooling plants and frozen foods, the stores are generally of single floor construction without any columns and beams in the chambers.
In some old units cheaper material like rice husk was used as the thermal insulation.
Although the insulation itself is very cheap it necessitated very large wall
thicknesses and also caused many maintenance problems
For over three decades now the better designed cold stores have been insulated with materials like Expanded Polystyrene, Fiberglass or Polyurethane. Whereas the insulation on walls and ceilings is finished with cement sand plaster in conventional cold stores, the latest trend is to use sheet metal cladding. The cladding materials are aluminium sheet or pre-coated galvanised steel sheet.
A vapour barrier is provided I all cases to arrest moisture migration to the cold store. Barrier material (such as steel, aluminum, reinforced plastic sheets, metal foils, mastic type hot or cold application paints) is provided on the warmer side of the insulation.
The development of pre-insulated panels has brought in a revolution in cold storage construction the world over. Although, these panels have been in use for cold storage construction for over tow decades in the developed countries, in India, modern prefab panels have been introduced around 10 years back. Prefab panels, also called sandwich panels, are mainly available in two types:
Insulate DX panels are used for making cold stores, right from the small walk-ins
to very large cold stores. In fact the application of panels has gone beyond
the cold store sector and such panels are also used for the construction of
processing plants, prefabricated housed, warehoused, clean rooms, etc. Similar
construction is utilised for fabricating doors for the cold stores that ate
light and strong compared to the conventional wooden insulated doors.
The highlights of prefab panel construction are:
Although pre-insulated panels offer a tremendous opportunity in cold storage construction, their application in India has been limited to small cold rooms, walk-ins blast freezers and frozen food stores. The application of panels for medium temperature cold stores has not been widely accepted due to the high cost of the panel structure as compared to brick wall construction with conventional insulation.
The refrigeration system in a cold storage is usually a vapour compression system comprising the compressor, condenser, receiver, air cooling units and associate piping and controls.
In smaller cold rooms and walk-ins the practice is to use air cooled condensing units with seal, semi-sealed or open type compressors. In the light of the CFC phase out the trend now is to use HCFC-22, HFC-134a or other substitute refrigerants. In the medium and large sized units say 500 M.T. And higher capacities, the practice in India is to use a central plant with ammonia as the refrigerant. It is estimated that almost 95% of the large cold store units in India have ammonia refrigeration systems. Needless to say, that ammonia has proved itself as an economical land reliable refrigerant, especially, in the industrial refrigeration field including cold stores.
However, in some present day medium and large sized units with pre-fab insulated panel construction the trend is to use modular HCFC-22/HFC units which are compact, light weight and easy to maintain. However, the selection of the system and the refrigerant is a matter of designer's choice for a particular application.
Reciprocating compressors of slow speed type have been used on a large number of cold store units in the past. The medium speed reciprocating compressors with speeds ranging from 750 to 1450RPM, with a better energy efficiency and built-in capacity control have been installed in most of the recent units.
For very large cold stores and frozen food stores, screw compressors are being preferred. Screw compressors have very few moving parts and offer advantage of steeples capacity control in the range of 10 to 100%.
Absorption refrigeration systems, especially those based on Ammonia also offer an alternative to vapour compression systems. The absorption system has a basic advantage in that it can work on direct thermal heat available from any type of fuel oil or other agricultural waste. The saving in the operational cost would be considerable land the higher initial cost could be recovered through energy savings in 3 to 4 years. A proper study has to be made in each case before settling for the absorption system.
The halocarbon condensing units are mostly air cooled type although water cooled units with shell and tube condensers find application in a few of the large sized systems. In ammonia plants the types of condensers used are the atmospheric or the evaporative. The evaporative condensers consume less quantity of water for plant operation and are generally preferred on large cold sore units.
There are a number of designs of product coolers or air cooling units available in the market. The old cold sores in some states used for bulk storage have bunker type coils installed on the top floor with conventional ceiling fans used for air circulation over the coils. These units occupy large volumes in the cold store, carry large quantities of ammonia and have a very inefficient cooling performance. In cold stores constructed later, floor mounted air cooling units with prime surface coils and sheet metal air distribution ducting have been used.(Fig.6) Finned coils have also been used on these units in some cases.
However, the recent trend is to use ceiling mounted units with finned coils and axial flow fans with aluminium or stainless steel impellers and adjustable pitch. These units are compact and occupy less space.
Ammonia refrigeration plants mostly employ gravity fed systems with flooded coil evaporators, where a certain level of refrigerant is maintained in the evaporator coil. In case of larger cold store units the trend is to use liquid recirculation pumps for the circulation of low temperature and low pressure refrigerant. This system offers the advantage of a centralized control land an efficient performance of the evaporator coils due to force circulation of he liquid refrigerant.
Frost formulation on evaporator coils due to sub-zero evaporating temperature is an inevitable phenomenon in cold storage plant operation. Timely defrosting of coils is important as the frost build-up reduces heat transfer and affects the plant capacity. The most common defrosting methods are:
Water defrosting is the most practical and simple and can be done manually or through a timer. It is commonly employed on a majority of cold store units working on ammonia.
Hot gas defrosting involves la more complicated circuitry and controls and is usually eliminate employed on HCFC-22 / HFC plants which are normally designed for automatic operation. In case of ammonia systems it is common practice to provide hot gas defrosting on plants designed for liquid recirculation and having multiple evaporators rather than on plants with flooded evaporators.
Electric defrosting involves provision of electric heating elements fixed in the evaporator coil section. Although it is a convenient way of defrosting, additional power consumption and maintenance problems on heater are the factors which do not find favour with most users.
The control system in cold store plants incorporates:
Modern micro processor based control systems are now available for these applications in stand alone and integrated designs.
Digital temperature indicators cum controllers are also being used in recent installations replacing the conventional stem/dial type thermometers and thermostats.
Like any other industry, safety measures are important in the cold storage field also. Refrigeration systems have to be built as per proper specification to ensure that refrigerant leakage does not occur. It is essential to have proper emergency measures in case of any accidental leaks.
The building structure has to be designed with adequate safety factors and the thermal insulation has to be protected properly from any possible occurrence of fire. Emergency arms are provided in the cold store with switches in each chamber Battery operated alarm systems are now available which ensure the operation of any alarm system even during a power failure.
Energy cost constitutes a major part of the running cost of a cold store. Apart from the problems of the availability of electrical energy, he ever increasing rates of electrical energy seriously affect the economic viability of cold store units. The situation calls for serious attention of all those connected with the cold storage industry to consider all possible means to achieve the best possible economy in the energy consumption in cold store units.
Following are some of the measures adopted to achieve energy efficient operation.
|Table:1 Cold Storage Data for Some Fruits and Vegetables|
|Product||Storage Temp.°C||Product Life, Weeks||Storage Season|
||2-4||32-36||March to October
& October to January
|Cauliflower||0-1.5||8-10||February to April &
August to October
|Cabbage||0-1.5||10-12||February to May &
August to October
|Tomatoes||7||4-5||April to May &
September to October
|8||6||March to April & August to September|
|Ginger||2-4||14||February to April|
|Orange||7-8||8||February to April|
|6-8||8||April to July & Nov. to February|
|Apples||0-15||16-20||October to March|
|Eggs||1-5||20-24||May to December|
|Mango||8-10||4-6||May to June|
|Frozen Foods||-18 to -20||24 or more||Year round|