The function of evaporator is to absorb heat from surrounding location or medium which is to be cooled by means of refrigerant. The refrigerant either boils as it flows through a pipe, tube or other type of space so that liquid is continuously wetting all the inside surface or it boils in a shell around submerged tubes through which the fluid to be cooling is flowing. Various methods are used for evaporators, depending upon the refrigerant to be used and evaporator application, but iron, steel and copper predominate. Read more
Basically, compressor in refrigeration system have following main function
1. Remove low temperature and low pressure refrigerant vapors from cooling coil through suction line.
2. Compress refrigerant vapors by increasing the pressure and temperature resulting in an increase of boiling point of refrigerant.
3. Discharge the refrigerant vapors of high temperature and pressure to condenser through discharge line.
Compression of refrigerant gas may be accomplished by any one of several different means such as positive displacement (reciprocating, rotary, gear types) and centrifugal (centrifugal compressor). The common type of compressors in use are
1. Reciprocating compressor
2. Rotary compressor
3. Centrifugal compressor
4. Screw compressor
5. Scroll compressor Read more
The compressor in vapor absorption system consumes a lot of energy. To avoid this, vapor absorption refrigeration system has been developed. The absorption system uses heat energy supplied by a flame or some other source of heat.
Michel Faraday experiment on liquefaction of gas:
vapor absorption refrigeration system is based on the principle of experiment developed by Michel Faraday. He sealed in a bent test tube a compound of silver chloride (a white powder) and dry ammonia gas which have been absorbed by the powder. He heated the end of tube containing the powder and at the same time, cooled the opposite end of the tube with water. Ammonia vapor was released during the application of heat and was condensed to cool end of the tube. When the flame under the powder was extinguished, vapor that had not been liquefied was reabsorbed by the powder reducing the pressure on liquid ammonia. The liquid ammonia begins to boil, change back to vapor and was reabsorbed by the powder. The end of tube containing boiling liquid was intensely cold. The evaporating ammonia having drawn heat from the nearest substance, the test tube itself. Read more
Refrigeration can be defined as the process of transferring heat from low temperature region to high temperature region. In other words, it is the process of cooling the substance. This can be achieved only if heat is removed from that substance.
Principle of refrigeration: Principle of refrigeration is based on second law of thermodynamics. It states that heat doesn’t flow from low temperature body to high temperature body without the help of external work. In refrigeration process, since heat has to be transferred from low temperature body to high temperature body some external work has to be done according to second law of thermodynamics as shown below (fig 2.) Here, the external work is done by compressor, condenser etc. The machine which works under this principle and serves the purpose of refrigeration is called refrigerator. Read more
Drying is a complex operation involving heat and mass transfer along with physical and chemical changes. Physical changes include shrinkage, puffing, crystallization. Chemical and biochemical reaction includes changes in color, texture, odor or properties of solid product. Storage of high moisture grain in long term causes insect infestation, mold growth, development of off odor, lowering nutritive value and germination of grain. This causes quality deterioration of grain and in long term, make unfit for human consumption. Therefore, drying is the most crucial part for grain preservation. Not only to increase shelf life of grain and maintain quality, drying of grain facilitate handling and processing. Read more
Grading is defined as segregation of heterogeneous material into series of grades reflecting different quality characteristics of significance to the users . Grain right after produced and harvested in the field, is heterogeneous mixture of Grains are graded on different basis to make good money in the market. Because it affect the price that the end users or consumers are always willing to pay. Grading system simplify trade in market by meeting the requirement of buyers and at the same time motivates producers to produce desired quality in reasonable amount. The requirement of users widely depends on end use of the grain. For instance, low protein wheat flour is required for making cakes and muffins whereas high protein wheat flour is required for crusty bread and pizzas.
Heterogeneity of grain in a harvested lot may be due to environment conditions. For instance, the grain may be ready to harvest in some part of farm whereas in may still be immature in other part of farm. Besides, moisture content, fungal infection, discoloration, pest and insect damage mars the quality of sound grain.
Generally there are two type of grading system; fair and average quality system and numeric system.
Fair Average Quality System: In this system, samples are graded in such a way that it represents average quality of the grain. The consignment are distributed to purchasers with the quality near the average.
Numeric system: In this system of grading, grains are separated into divisions of quality that are defined by grading factors. Each division is identified by grade name or number. Grains are traded on the basis of these grades. This system allows buyers to obtain specific quality as desired by selecting the grade. Such grading system is practiced in United States, Canada and Australia.
Grading factors are those factors, which on failure to meet the requisite quality level will result in lower grade. There are some of the common grading factors that are applicable for cereal crops. However not all grading factors are applicable for all types of cereal grains. Examining the whole consignment for grading is impossible thing. Therefore, adequate sample is withdrawn from the parcel and grading test of grain is perform in laboratory. The collected sample is passed through seed grader to obtain homogeneous sample and to scale down our sample quantity for ease of grading. Below are some of the commonly used grading factors .
1. Test weight: This is the bulk density of grain expressed in kilogram per hectoliter (Kg/hL). It is determined by specific procedures using specified equipment.
2. Varietal purity: It is the percentage of purity of variety of grains. To determine varietal purity, percentage of inferior quality grain is measured. Grain with high varietal purity is considered high quality grain without contamination of any other unintended varieties or objectionable varieties of the same grain.
3. Vitreousness: Vitreousness is the glossy or shiny appearance that indicates hardness. More vitreous the kernel, more is the protein content. The kernels that are broken, damaged, severely bleached or shows sign of starchiness fall in the category of non-vitreous kernel.
4. Soundness: Sound kernel refers to the well-developed, mature and physically undamaged grain. Soundness of kernel is determined by evaluating the extent of overall physical damage of the grain.
5. Maximum limit of foreign material: Foreign material refers to that material other than specified grain after dockage has been removed. Example of foreign material includes other cereal grains, inseparable seeds, thistle heads, heated kernels, pieces of stems, moldy grain, insect wings, rodent droppings etc. Foreign matter can be further classified as organic foreign material (OFM) and inorganic foreign material (IFM).
Factors affecting grading:
Besides the grading factors, more other things need to be considered during grading. Proper grading also requires to take account on following factors that affect grading. Sometimes they these factors are also included in grading factors as well.
1. Dockage: It is any material other than kernel of grain of standard quality that is mixed with parcel grain. Dockage must be and can be separated from the parcel of grain before that grade can be traded.
2. Moisture content: The moisture content of cleaned grain must lie within specified limit to quality to fall under certain grade. Moisture content of damp grain can be reduced either by mixing with dry grain or by drying grain again. Moisture content is important factor in grain trading as, even a small increment in moisture increases the likelihood of grain being infected by mold, insects, pests and favor germination.
3. Protein content: Globally there is high value for quality protein. Therefore, cereal grains (especially wheat) having high protein content is much valued. However, it is also matter of fact that grain with different protein content are required for different end purpose. Example; hard wheat (high protein content) is required for making bread whereas soft wheat (low protein content) is required for making biscuits and cakes.
Advantage of grading system:
1. Producer is assure for reasonable grain price relative to its quality. This also motivates farmers to produce, harvest and store grain of high quality.
2. It is easier for costumer to select their product according to their intended use.
3. Grading helps to get maximum net return from the grain for the buyers.
4. Grading facilitates trading. For globalization of trade, trade without barriers and to comply with trade agreement and treaties, grading is a must do thing.
Disadvantage of grading system:
1. The number of grades provided for each type of grain is not agreeable throughout the world.
2. Grading reduces the throughput capacity of handling and transporting system.
3. Grading requires physical separation, binning mechanism, separate storage and containment that represents overall cost to the system.
4. Sometimes, grading requires additional handling and operating cost. For instance; grain that has become damp during storage needs to be properly dried to reduce moisture content to specified level before shipping.
1. Canada Grains Council (CGC). 1982. Philosophy of grading. In: Grain Grading for Efficiency and
Profit. Winnipeg, MB: Canada Grain Council, 6 p.
2. Chakraverty, A., Mujumdar, A. S., & Ramaswamy, H. S. (2003). Handbook of postharvest technology: cereals, fruits, vegetables, tea, and spices (Vol. 93). CRC press.
Name : Pratiksha Shrestha
Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.
Understanding the physical properties of grain are important for equipment design for handling, storage aeration and processing. Even the simple technique of drying and storage requires knowledge on heat and moisture transport phenomenon. Important properties of cereal grains are grain size, shape, volume, density, porosity, angle of repose, heat capacity, thermal diffusivity and many others. However, these properties of cereal grains largely depends on moisture content, temperature and density of grain itself. Read more