Fundamentals of food refrigeration technology. Fundamentals of refrigeration
3.1 History of the development of refrigeration technology
The use of cold to preserve food has long been known. To do this, first ice and snow were used, and then mixtures of ice and salt, which made it possible to obtain temperatures below 0°C. In the 19th century industrial refrigeration machines. The first refrigerator was invented in 1834 by the Englishman Perkinson. Ethylene ether was used as a refrigerant. Later, in 1871, the Frenchman Thénier created a machine that ran on methyl ether, and in 1872, the Englishman Boyle invented a refrigeration machine that used ammonia.
In our country, refrigeration machines on an industrial scale were first used in 1888 in the fisheries in Astrakhan. In 1889, two refrigeration plants were built at the breweries. The first industrial refrigerator with a capacity of 250 tons was built in 1895 in Belgorod. Transportation of products in ice-cooled railway cars began in 1860. However, in pre-revolutionary Russia, refrigeration technology was poorly developed. In 1917, there were only 58 refrigerators in the country with a total capacity of 57 thousand tons and a cooling capacity of about 24 thousand kW. The refrigeration transport consisted of 6,500 two-axle ice-cooled railway cars and one refrigerated vessel with a carrying capacity of 185 tons.
The use of artificial cold on a large scale in our country began after the Great October Socialist Revolution. During the years of Soviet power, large refrigerators were built in the meat, fish, dairy and other branches of the food industry, as well as in transport. Already in 1941, the capacity of refrigerators in our country was 370,000 tons.
Along with the growth of the refrigeration capacity, refrigeration engineering and instrumentation are constantly developing. Chillers are produced mainly in the form of automated units. Much attention is paid to the design and manufacture of small automated refrigeration machines.
Small refrigerators are widely used in trade and public catering (refrigerators, cameras, counters, showcases, refrigerated vending machines), in everyday life (refrigerators, air conditioners), in transport, in agriculture, medicine and other industries National economy. In trade and public catering of the country, the total number of small refrigeration units exceeds 2 million units. Tens of millions of refrigerators are used in everyday life.
Refrigeration transport has been widely developed. Railway refrigerated transport has been noticeably replenished with trains, sections and separate autonomous cars with machine cooling. The number of refrigerator ships equipped with modern refrigeration equipment has increased. The automobile refrigerating transport was created anew.
In order to preserve and process an ever-increasing amount of food products, it is necessary to increase the volume and speed up the construction of refrigerators and refrigeration equipment, as well as technically improve existing refrigeration enterprises. In the coming years, it is planned to significantly increase the capacity of refrigerators in the food, meat and dairy industries. Refrigeration capacities will also increase in the trade system and in agriculture. They will be equipped with the latest refrigeration equipment with a greater degree of factory readiness, automation and mechanization of production processes.
3.2 Cold storage of food
3.2.1 Food refrigeration
Cooling- the process of lowering the temperature of the product not below cryoscopic.
Cryoscopic temperature It is customary to consider the temperature at which the solid phase (crystals) begins to precipitate from the tissue fluid of the product.
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Meaning |
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Beef |
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Mutton |
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Canned meat |
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Hard cheeses |
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Processed cheeses |
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Grape |
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The cooling process is represented in a rectangular coordinate system (Figure 8)
Vertically - the temperature of the cooled product, horizontally - the duration of the cooling process.
Cooling starts at a temperature Tnach, i.e. the temperature of the product before placing it in the cooling chamber. As a rule, the cooling process is a curve approaching the cryoscopic temperature, but never reaching the value Tcr.
Shock chilling- used for semi-finished products and finished products with an estimated shelf life of 4-5 days. During blast chilling, the core temperature of the product must drop to 3°C in no more than 90 minutes. During this process, the temperature inside the blast chiller at the initial stage is -15…-25 C and is maintained at this level until the temperature on the surface of the product reaches minus 10 C, after which the temperature rises to 0 C.
Freezing food
Freezing- the process of partial or complete transformation of the tissue fluid of the frozen product into ice. The presence of a phase transition is a distinctive feature of the freezing process.
Perishable food products are frozen to increase the shelf life (meat, fish, poultry) or to obtain products with new taste qualities (ice cream, fruits and berries)
The shelf life of perishable foods with a high moisture content is much longer when frozen than when refrigerated. The transformation of product moisture from a liquid state into a crystalline one leads to a significant inhibition of the vital activity of microorganisms, as well as a significant decrease in the rate of biochemical and chemical reactions. During freezing, changes occur that do not allow the product to be completely restored to its original form. Technological irreversibility is not a disadvantage, if food and taste indicators and presentation do not deteriorate.
Technologically, the process of freezing is preceded by the process of cooling the product to a cryoscopic temperature, after which ice formation begins - the phase transition of a liquid medium into a solid state.
hypothermia- this is a decrease in the temperature of an object below cryoscopic without crystallization of the water contained in it.
Upon reaching the cryoscopic temperature tcr, ice crystals in the tissue fluid are not formed due to the residual thermal motion of molecules. For the formation of stable ice crystals, a certain decrease in temperature relative to cryoscopic is required. Such a decrease in temperature is commonly called supercooling tpo. For each type of food products, the maximum supercooling tpo has specific values: 5 ° С - for meat, poultry, fish; 6°C - for milk; 11°C - for eggs.
The presence of artificial crystallization stimulators (mechanical impurities, pollution) leads to the formation of stable crystal nuclei at lower supercooling values.
The formation of stable nuclei of crystals and the subsequent growth of crystals of liquid tissues are accompanied by the release of heat of the phase transition of the liquid into ice and an increase in temperature to cryoscopic. Moreover, the temperature increase occurs very quickly, as a result of which this phenomenon is called a temperature jump.
Shock freezing - lowering the temperature in the thickness of the product to -18C in a time not exceeding 240 minutes. It is used for freezing semi-finished products and ready meals, the expected shelf life of which will not exceed 60 days. In the initial stage, the temperature inside the chamber is reduced to -40C and maintained at this level until the temperature inside the product reaches -18C. At the end of the freezing process, the chamber enters the standard cooling mode. Shock freezing allows you to preserve the original structure of the product, its taste and aroma.
With any method of freezing, heat is removed from the surface of the product, and the deep layers have a higher temperature. If, after the freezing process, the product is placed in a storage chamber, then after a long time the temperature will equalize. This temperature is called final freezing temperature.
When freezing, not all tissue fluid turns into ice. Tissue fluid that has turned into ice is called frozen. The relative amount of this moisture is usually estimated as a fraction of frozen moisture:
The ratio of the mass of ice to the mass of tissue fluid.
3.2.2 Food refrigeration and storage practices
For rational refrigeration storage of chilled and frozen products, the following conditions must be met:
store high-quality products that have undergone regulated technological processing before storage
Maintain temperature storage
maintain a certain humidity during storage
Comply with sanitary and hygienic storage conditions
use additional products that increase the stability of stored products
Use rational packaging for food storage
Refrigerated perishable food products are stored in chambers at temperatures close to cryoscopic, and the relative humidity of the air is maintained at 80-85%.
The temperature regime for storing frozen products is selected based on the duration of storage. with a short period of storage, the temperature in the chamber is -8 ... -12C. Frozen products intended for more long-term storage stored at a temperature not higher than -18C. Lower temperatures are used to ensure the safety of products containing unsaturated fatty acids.
The shelf life of a food product is considered to be such, after which foreign odors that are not inherent in the product begin to appear in the stored product, the product acquires an aftertaste, its color and appearance change.
During storage of animal fats, microorganisms and enzymes cause the breakdown of unsaturated fatty acids, which leads to the formation of ketones, which contribute to the appearance of a bitter aftertaste of the product.
When storing fish and seafood, protein molecules break down to form amino acids, in particular trimethylamine, the presence of which indicates the beginning of spoilage.
Storage of chilled foodstuffs at low positive temperatures (0...2C) ensures the safety of meat, fish, poultry for 1-2 weeks. Perishable products of plant origin, with the rational organization of the storage regime, can be stored practically until the new harvest.
3.2.3 Warming and thawing food
Warming - the process of raising the temperature of a chilled food product at a rate that prevents condensation from forming on the surface of the product. Warming is especially important for products such as eggs, fruits, vegetables, some dairy and gastronomic products, canned food, etc.
Some products do not need refrigeration. These include, for example, butter, sour cream, cottage cheese, cheeses, salted fish products, etc. Moisture condensing on the surface does not harm them. Such products are released from refrigerators without warming. Almost very rarely resort to warming and chilled meat products.
Produce warming in special chambers with enhanced circulation of conditioned air. Such chambers are called defrosters. Products in the chambers are stacked so that they are freely washed by air. The temperature of the product during warming is increased gradually. Accordingly, the air temperature in the defroster is increased, adjusting it in such a way that it is always 2-3°C higher than the product temperature. The relative humidity of the supplied air is also regulated. To prevent the product from being moistened, it is maintained at a level of 80%. Too dry air is also undesirable due to shrinkage of products.
The final temperature to which the products must be heated depends on the temperature and humidity of the outside air. If the air is very humid, the final temperature of the thawed products should be 2-3°C lower than its temperature, and if it is sufficiently dry (with a relative humidity of 40-45%), then this difference should be 4-5°C. Warming practically lasts about 30-40 hours.
Defrosting is performed to return the product to its original state, which it had before freezing. At the same time, the ice crystals in the product turn into a liquid, which is distributed over the tissue, as it was before freezing. In practice, it is not possible to achieve complete reversibility of the freezing process. The ability of cells and fibers to retain moisture is significantly reduced due to injury to them by ice crystals and a decrease in the ability of proteins to hydrate. Therefore, part of the juice flows out of the product and is lost.
Defrosting methods used in practice:
slow defrosting in air at a temperature in the chamber of 0 ... 4 C
fast defrosting in the air at a temperature in the chamber of 15 ... 20C
fast defrosting in a steam-air environment at a temperature of 25 ... 40C
defrosting in liquid heat carriers at a temperature of 4 ... 20C
Slow defrost provides best recovery product, uniform distribution of cellular fluid. But low defrosting rates lead to the development of biochemical and microbiological processes, the activity of which can lead to product spoilage.
Fast defrosting is used mainly for food products used in industrial processing (production of sausages, canned food).
3.2.4 Food refrigeration technology
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Product |
Cooling Options |
Cooling time |
Cooling method |
Technology features |
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Meat and meat products |
In the storage room |
Carcasses, half carcasses, quarters are cooled to a temperature in the thickness of the thigh 2-4C |
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In the cooling chamber |
Temperature in the thickness of the thigh 3-4C |
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On air |
The process can be accelerated by lowering the temperature to -2C and increasing the air speed by 4 m/s. Time is reduced to 4-6 hours. |
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In ice water |
The skin of the carcass is bleached, spots from bruises disappear. The disadvantage is the possibility of cross m / b seeding. |
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Artificial ice, 2% salt solution |
The possibility of m / b seeding is minimal |
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1-2C below cryoscopic |
Luggage storage |
Cells with eggs must be staggered |
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Milk and dairy products |
Luggage storage |
3.2.5. Food freezing technology
Freezing in air. It is carried out in freezers. Air in the chamber with a temperature of -30 ... -40C at a speed of 1-2 m / s. The disadvantage is the duration of the process and the loss of moisture from the surface layer of the frozen product (shrinkage).
Irrigation freezing. Liquid, cooled heat carrier (aqueous salt solution) is fed through nozzles to irrigate the product.
Freezing by immersion. More efficient process with uniform product immersion from all sides.
The disadvantage of these methods is the need to use sealed packaging to prevent contact of the product with the freezing medium (non-contact freezing)
Freezing in cryogenic liquids(liquid air or liquid nitrogen) has not found wide application due to its high cost.
Freezing of certain groups of goods
Meat and meat products frozen in freezers at a temperature of -30..-35C, speed of forced air movement 1..3 m/s. The duration of freezing is 19..27 hours. Meat is considered frozen, the average final temperature of which is 10C lower than the cryoscopic one.
bird frozen in freezers with natural or artificial air circulation. The temperature in the chamber air is -18C. Duration 48-72h. When the temperature drops to -23C and the air speed increases to 3-4 m/s, the duration is reduced to 36-24 hours. When freezing poultry in liquid heat carriers (propylene glycol, an aqueous solution of calcium chloride), the carcasses are placed in sealed packages and immersed in a solution cooled to -28C. Most often, immersion is used at the initial stage of freezing, and the final one takes place in the freezer. In this case, the carcasses are immersed for 20-40 minutes, and then transferred to a storage chamber with an air temperature of -23-30C.
For freezing fish a high quality product is used that does not have external defects and signs of damage. When freezing in air, the temperature in freezers is maintained at -30C and below. Forced air circulation is used to intensify the process. Fatty fish (herring) are frozen at lower temperatures.
Fish of valuable species, especially large in size, is recommended to be glazed with ice after freezing. To do this, fish are immersed several times in fresh water. cold water and removed to form a thin crust of ice on the surface. The formed crust of ice protects the fish from shrinkage, and the lack of contact of fat with atmospheric oxygen prevents oxidation.
Freezing egg products(proteins, yolks, melange) are produced in freezers with a temperature of -20 ... -25C and an air speed of 3-4m / s. The egg mass is considered frozen if the temperature in the center of the package is -6..-10C. Whole eggs in the shell are not frozen, because. in this case, mechanical damage to the shell occurs due to the expansion of the contents.
Cottage cheese frozen in freezers with a temperature of -28..-30C., the temperature in the storage room should be -18C.
Butter freeze to -18C. The temperature in the freezer is -25C for 2 days.
3.2.6 Keeping food refrigerated
Chilled products are stored in a refrigerator at a temperature of 2..3C. The humidity regime of storage chambers is maintained in accordance with the technological requirements for each type of product.
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Product |
Temperature storage |
Storage Options |
Storage time |
Storage Features |
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In the form of carcasses or half carcasses in limbo |
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In paper, plastic bags, shrink film (10 days) |
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Candling should be done once every 2 months |
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Fresh fish |
On crushed ice |
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Caviar grainy |
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Salty fish |
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Smoked fish |
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forced air circulation |
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Fat-free cottage cheese is better preserved. |
Freezing of meat, poultry and fish, i.e. freezing the surface layer helps to extend the shelf life. Frozen meat at a temperature of -1..-2C is stored in a suspended state for up to 17 days.
Poultry freezing occurs up to a temperature of 0..-1C in the skinny pectoral muscle and up to 4C at a depth of 5 mm. For freezing in the air, the bird is placed in freezers with a temperature of -23C and an air speed of 3-4 m/s. Duration 2-3h.
Freezing of fish occurs in air or in liquid heat carriers, which increases the shelf life of fish up to 25 days. Freezing in air at a temperature of -20C.
Fruit storage. One of the main conditions for the storage of fruits is the creation optimal mode, at which all biochemical processes occurring in the fruits after their removal are slowed down. In addition, the mode of storage should ensure the complete and long-term preservation of the properties inherent in fruits and their resistance to microbiological deterioration.
The storage temperature is set for apples -0.5÷0.5°С, for pears, peaches, apricots, cherries, sweet cherries 0°С, oranges and lemons -0.5÷4°С, tangerines 0.3-2°С . At the same time, relative air humidity is recommended for apples, pears and grapes 85-90%, stone fruits 80-85%, citrus fruits 78-83%.
Fresh bananas and pineapples received for storage are released from packaging material and sorted according to the degree of maturity and quality. When storing green bananas in the chamber, a temperature of 12-14°C and a relative humidity of 85-90% are maintained under artificial air circulation. Bananas in the process of storage gradually ripen and reach consumer maturity. Ripe bananas are stored at 12°C, and mature pineapples at 7.5-8°C and 85-90% relative humidity. The duration of storage under these conditions is 10-12 days, depending on the general condition of the fruit. Green pineapples are stored at a temperature of 15-16°C and a relative humidity of 85-90%. Under these conditions, fruits ripen within 5-6 days.
It has been established that the shelf life of all types of fruits is significantly increased in a controlled gas environment, in which an increased content of carbon dioxide and low oxygen. For this purpose, the fruits are placed in special hermetically sealed chambers or containers.
The duration of refrigeration storage of various fruits, depending on the gas composition of the environment
Berries (strawberries, raspberries, currants, gooseberries, cranberries, lingonberries, etc.), various fruits (plums, apricots, peaches, citrus fruits, etc.) are stored frozen. The storage temperature of frozen fruits is -18°C, the shelf life is up to 12 months.
Place food in the refrigerators of catering establishments and grocery stores as follows:
Meat (chilled and frozen) - hang;
Carcasses of chilled poultry - on racks in one row;
Frozen poultry and game - in standard boxes on pedestals (stacked on the floor) or on racks;
Partial fish (steam and frozen) - in standard boxes, baskets on undercarriages or on shelves of racks;
Sturgeon fish - on racks or hang;
Salted fish - in standard barrels on the bottoms;
Milk - in standard flasks on pedestals or racks;
Curdled milk, kefir, sour cream, cottage cheese - in standard containers on undercarriages or racks;
Ghee butter - in wooden barrels on pallets;
Cheese - on the shelves;
Eggs - in standard boxes in stacks on the bottoms;
Frozen culinary products - in cardboard boxes on pallets or racks;
Greens - in boxes on pedestals or racks;
Cucumbers - in baskets, boxes or sacks on pallets;
Tomatoes - in boxes on pedestals or racks;
Fruits - in boxes on pedestals or racks;
Berries - in sieves in stacks on pallets or racks;
Fermentations, pickles, marinades - in standard containers on pallets or racks;
Wines - in bottles in the supine position;
Vodka products, beer, water - in bottles, installed in boxes or baskets, which are placed on pedestals; beer is also stored in barrels on the bottoms.
Frozen food storage
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Product |
Storage Options |
Storage time |
Storage Features |
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The temperature in the thickness of the thigh is not higher than -8C. |
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For fatty breeds -30C |
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Butter. |
Packing in plastic bags |
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3.3 The concept of a continuous cold chain
Continuous cold chain- a set of technical means (refrigerators) that provide continuous cooling of perishable products from the producer to the consumer.
Diagram of a continuous cold chain
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Sphere of production |
1 - production (preparation) refrigerator
2,4,6 - refrigerated transport
3 base cooler
5 - distribution refrigerator
7- refrigerators POP
To ensure the best preservation of products, the temperature regimes of chilled or frozen products must be maintained unchanged in the process of moving from link to link.
Industrial refrigerators- are located within the production of p / f. are intended for primary refrigeration treatment (cooling or freezing). Differ in high performance with relatively small spaces for storage of processed products.
Basic refrigerators- designed for long-term storage of food products and the creation of stocks. They are characterized by a large area and capacity, as well as increased requirements for temperature and humidity conditions.
Distribution refrigerators– provide a uniform supply of wholesale bases with seasonal products throughout the year. They are supplied with railway sidings and loading and unloading areas for automobile refrigeration transport.
Stationary refrigerators POP- serve for short-term storage of products. The shelf life does not exceed a few days, so the requirements for temperature and humidity are less stringent.
Commercial refrigeration equipment- intended for short-term storage, demonstration, display and sale of products at the POP. Such equipment includes: refrigerated display cases, counters, refrigerated cabinets.
3.4 Refrigerated transport
Connects the links of a continuous refrigeration chain, ensuring that the temperature of the products remains unchanged from moving from one link to another.
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Refrigerators low temperature |
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Refrigerators high temperature |
3.5. The simplest steam compression machine
Cooling is called the removal of heat from the body, accompanied by a decrease in its temperature. The cooled and cooling body - the working substance - participate in the cooling process. Cooling, in which the temperature of the cooled body can only be lowered to the ambient temperature is called natural. Cooling a body below ambient temperature is called artificial.
In catering, the most common type of refrigeration machines are steam compression machines. The working substance is low-boiling liquids, which, when performing a refrigeration cycle, change their state of aggregation, turning from a liquid into a vapor and vice versa.
1 - compressor
2 - evaporator
3- throttle device
4 -capacitor
5 - refrigerator
Compressor- compresses steam to high pressure, i.e. liquid refrigerant is converted to vapor
Evaporator- an apparatus in which a liquid refrigerant boils due to the heat removed from the product. The temperature of the boiling refrigerant is 10-12C lower than the temperature of the cooled object.
Throttle device– Provides pressure reduction of the refrigerant.
Capacitor- provides cooling of the superheated refrigerant vapors to the condensation temperature and the transformation of the vapor into a liquid.
3.6 Refrigerants and coolants
The working substance circulating in the refrigeration machine, with the help of which the reverse circular process or cycle is performed, is called refrigerant (refrigerant).
In modern practice, there are two types of main refrigerants: ammonia and freons (freons)
Ammonia (R717) has a boiling point of -33.4C, a high heat of vaporization, and poor solubility in oil. It is used in piston compressor units. The disadvantages include high toxicity, flammability and explosiveness. Ammonia has a pungent odor and is highly irritating to the mucous membranes of the respiratory tract and eyes. When working with ammonia refrigeration units, strict adherence to safety regulations is required.
Freons (freons) are halogen derivatives of saturated hydrocarbons. They are chemically inert, non-explosive. Marking R11, R12, R113, R502, R22. Reaching the atmosphere, freons release chlorine, which is involved in the destruction of the ozone layer, especially R22. Therefore, most European countries have banned their production and use. R22 is approved for use in Russia until 2020.
Refrigerants (refrigerants) used to transport cold from the source of production (evaporator) to the cooled object (chamber). As refrigerants, aqueous solutions of salts are used - brines and one-component substances that freeze at low temperatures (ethylene glycol). Salt solutions: sodium, calcium, magnesium chlorides.
The textbook contains information about the purpose and design of refrigeration equipment, physical principles obtaining low temperatures, types and cycles of refrigeration machines. The main and auxiliary elements of refrigeration units are given. Theoretical foundations of refrigeration technology, methods for calculating the processes of refrigeration preservation, processing and storage of raw materials and foodstuffs are given. The problems of changes occurring during the processing, storage, defrosting, transportation and sale of chilled and frozen food products are considered. For students of higher educational institutions. It may be useful for students of advanced training institutes, trade and food industry specialists.
Physical processes for obtaining low temperatures.
Cooling is the process of lowering body temperature. For cooling, you need to have two bodies: cooled and cooling - a source of low temperature. Cooling continues as long as heat exchange takes place between the bodies. The source of low temperature must function continuously, since cooling must be carried out continuously. This is possible with a sufficiently large supply of coolant or if its original state is constantly restored. The latter is widely used in refrigeration engineering using various refrigeration machines.
Distinguish natural and artificial cooling. In natural cooling, heat from a hotter body passes to a less heated one (environment). Artificial cooling involves getting the temperature of the cooled medium below the ambient temperature. Low temperatures are obtained by physical processes during which heat is absorbed from the outside without raising the body temperature. The main physical processes accompanied by the absorption of heat include phase transitions of a substance: melting or melting when a body changes from a solid state to a liquid state; evaporation or boiling when a body changes from a liquid state to a vapor state; sublimation or sublimation during the transition of a body from a solid state directly to a gaseous state.
TABLE OF CONTENTS
Introduction
SECTION I. REFRIGERATION EQUIPMENT
Chapter 1. Physical essence and methods of obtaining artificial cold
1.1.Physical processes for obtaining low temperatures
1.2 Cooling methods
Chapter 2
2.1. Thermodynamic cycle of refrigeration machines
2.2. Calculation of the cycle of refrigeration machines
2.3. The principle of operation of steam compression refrigeration machines
2.4 Cooling system of the refrigeration unit
2.5.Refrigerants and coolants
Chapter 3. Types of refrigeration machines
3.1 Gas and vortex refrigeration machines
3.2 Compression steam chillers
3.3 Absorption and sorption refrigeration machines
3.4. Steam jet chillers
Chapter 4. Compressors of refrigeration machines
4.1 Piston compressors
4.2 Rotary compressors
4.3 Screw compressors
4.4 Turbochargers
Chapter 5
5.1 Capacitors
5.2 Evaporators
5.3 Cooling appliances
Chapter 6 Auxiliary equipment refrigeration machines and installations
Chapter 7. Automation, automatic control and units of refrigeration machines and installations
7.1 Automation of refrigeration units
7.2 Automatic regulation and control
7.3. Units of refrigeration machines and installations
Chapter 8. Refrigerated structures and refrigeration equipment
8.1 Classification of food refrigerators
8.2 Cooling media, their properties and parameters
8.3. Devices for measuring and controlling the parameters of cooling media and products
8.4.Designs of refrigerators
8.5. Mechanization of loading and unloading operations and transport and storage operations
8.6.Heat balance of refrigerated premises, cooling systems cold rooms, methods of heat removal from the cold consumer
8.7 Refrigeration process equipment
8.8. Refrigeration trade equipment
8.9. Methods and equipment for machineless cooling
SECTION II. REFRIGERATING FOOD TECHNOLOGY
Chapter 9
9.1 Principles of food preservation
9.2. Effect of low temperatures on the growth and reproduction of microorganisms
9.3. Impact of low temperatures on cells, tissues and organisms
9.4 Auxiliaries used in refrigeration and storage
Chapter 10
10.1. Cooling
10.2 Freezing
10.3 Freezing
Chapter 11. Thermophysical parameters of food products and their changes during refrigeration
11.1. Thermophysical parameters of food products
11.2.Change of thermophysical parameters of food products and temperature graphs
Chapter 12. Heat and mass transfer processes in refrigeration technology
12.1. Thermal calculation of the cooling process.
12.2. Thermal calculation of the freezing process
12.3 Heat and mass transfer during cold storage
12.4 Heat and mass transfer during defrosting
Chapter 13
13.1 Refrigeration of vegetable products
13.2 Refrigeration of products of animal origin
13.3. Industrial methods of cooling products of animal origin
Chapter 14
14.1 Freezing plant products
14.2 Freezing of products of animal origin
14.3 Quick frozen products
14.4 Freeze drying of products
Chapter 15
15.1 Characteristics of cold storage
15.2. Storage conditions for perishable products
15.3 General product changes during storage
15.4. Changes in the composition and properties of fruits and vegetables
15.5. Storage conditions for products of animal origin
15.6 Changes in animal products during refrigerated storage
15.7. Refrigerated storage of food products at the consumer
Chapter 16
16.1 Heating and defrosting technology
16.2. Classification and analysis of methods for defrosting food products
16.3. Devices for defrosting raw materials and food
16.4 Changes that occur in food during defrosting
16.5. Methods for calculating the parameters of the defrosting process of certain types of products
Chapter 17. Transportation of chilled and frozen foodstuffs
17.1 Classification and a brief description of refrigeration transport
17.2. Container shipping
17.3. Conditions, terms and features of transportation of various food products
17.4. Rules for the acceptance of transported products
Applications
Bibliography.
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General information about refrigeration equipment.
Cold is an excellent preservative that slows down the development of microorganisms. Therefore, in catering establishments, cold is used to store products at low temperatures in chambers, cabinets, counters and showcases. At the same time, the taste of the products and their appearance remain almost unchanged. The concept of cold means a low content of heat in the body. Refrigeration is the removal of heat from food products, accompanied by a decrease in their temperature. There are artificial and natural cooling. With natural cooling, the temperature of the food can be reduced to ambient temperature. And with artificial - lower temperatures are obtained. At catering establishments, several methods of artificial cold are used, which are based on the processes of changing the aggregate state of a substance - melting, evaporation and sublimation.
Melting is the process of changing a substance from a solid to a liquid state.
Boiling is the transition of a substance from a liquid state to a gaseous state.
Sublimation is the process of transition of a substance from a solid state to a gaseous state without passing through the liquid phase.
The most widely used process is the use of the latent heat of vaporization of liquids boiling at low temperatures. Such liquids are called refrigeration units. Heat transfer is carried out in a special device called a refrigeration machine.
Under the auspices of the UN, two important international documents have been developed and signed - the Vienna Convention for the Protection of the Ozone Layer (1985). The Montreal Protocol on Substances that Deplete the Ozone Layer and its Addendum (London, June 1990), which set a timetable for the reduction and elimination of CFCs (chlorinated carbons).
In November 1992, in Copenhagen, at the fourth meeting of the parties to the Montreal Protocol, new amendments to the protocol were adopted, tightening the schedule for reducing the production and consumption of ozone-hazardous compounds for the CFC group and providing for a 100% cessation of their production and consumption by 1996 and a 70% reduction in 1994 .
One of the cardinal solutions for protecting the ozone layer is to reduce or stop the production of CFCs with the transition to halogen-carbons that do not affect the ozone. Such compounds include freons - 22, 23, 32, 125, and others that are inert to ozone or have an insignificant ozone-depleting ability due to the fact that they either contain hydrogen nitrogen and therefore decompose in the lower atmosphere, or do not contain chlorine or bromine.
Currently, the use of Chlalok-12 has been banned in Europe since 1995, and in some states since 1994.
Leading CFC-producing countries have developed and agreed by UNEP alternative substitutes for all areas of application of ozone-hazardous substances with properties that meet the requirements of the respective industries.
Cooling methods
Ice cooling. Ice cooling is the most in a simple way cooling of food products, the physical basis of which is the process of melting ice and snow. Depending on the method of production, ice can be natural or artificial.
Ice cooling is used in structures called glaciers, they can have a different arrangement of ice in relation to the cooled chambers with products. However wide application received glaciers with lateral ice placement. Ice is laid in such quantity that it is enough for a certain time, and the volume of ice should be 4-5 times the volume of the chambers with products. With the ice method, you can lower the temperature to 6-8 degrees C and humidity 90-95%.
Ice-salt cooling. The cold source is a mixture of ice and table salt. The more salt, the lower the temperature of the mixture. The decrease in temperature occurs up to a certain limit. The most low temperature ice with table salt is -21.20 "C. The salted mixture allows you to create lower temperatures in a chilled environment compared to ice cooling.
Cooling with dry ice. This method is based on the sublimation of solid carbon dioxide. Dry ice - solid carbon dioxide, which in appearance is a piece of a substance similar to chalk, but very cold to rapidly evaporating at ordinary temperatures. Under normal conditions, it turns from a solid state directly into a vapor. At the same time, the temperature drops to -78.90*C. The cooling capacity of dry ice is 1.9 times that of water ice. Dry ice is very convenient for cooling food, as it does not release moisture, does not contaminate food, and has a low temperature. However, its use is limited due to the relatively high temperature.
Refrigeration machines
A refrigerating machine is a set of devices necessary for the continuous removal of heat from the cooled medium at low temperature and its transfer to the environment at high temperature.
Existing refrigeration machines are divided into two groups: compressor: operating with the expenditure of mechanical energy and adsorption - operating with the expenditure of thermal energy. Compressor refrigeration machines are most widely used in all sectors of the national economy.
Characteristics of refrigerants. The refrigerant is a chemical substance designed to remove heat from the cooled medium. To do this, use special easy-boiling liquids that have a low boiling point at atmospheric pressure. Currently, ammonia and freon-22 refrigerants are widely used.
Ammonia is a colorless gas with a pungent odor that is irritating to mucous membranes. Therefore, when it leaks through leaks, it can be detected by smell. Ammonia also has a high mutual solubility in water. It is used in refrigerating machines of average and big productivity. The use of ammonia as a refrigerant in machines low power limited, as it has disadvantages (toxicity, explosiveness, flammability).
Freon-22 is a colorless gas with a slight specific smell, so its leakage from the system is difficult to detect. It becomes noticeable only when its content in the air is more than 20%. It easily penetrates through leaks, is neutral to metals, explosive, but not flammable. At atmospheric pressure, its boiling point is 400*C. The advantage of freon-22 is its harmlessness, only when its content in the air is more than 30%, there are signs of poisoning of the body due to lack of oxygen.
Compressor chillers These machines consist of the following main parts: evaporator, condenser, compressor and expansion valve.
The evaporator is a device that had the form of a coiled finned-tube battery, in which the refrigerant boils at low temperatures due to the heat absorbed from the environment. The evaporator is installed inside the refrigerating cabinet, in its upper part.
A condenser is a device designed to cool freon vapor and turn it into a liquid. To speed up the cooling of freon, air is blown through the condenser with a special fan.
The compressor is a device that sucks refrigerant vapor from the evaporator and sends it to the condenser in a compressed state. The compressor consists of a cylinder, a piston and an electric motor.
Regulating valve - a device that regulates the amount of liquid freon supplied to the evaporator. In addition, the control valve reduces the freon pressure to ensure a low-temperature boiling condition.
Thus, the weight of the main parts of the refrigeration machine are interconnected by a closed system of pipelines, in which the same amount of freon and its vapors continuously circulate.
To improve the operating mode, the circuit of the refrigeration machine includes a number of additional devices: a receiver, automation devices, etc.
Freon automatic compressor machine. These machines are currently used for cooling showcases, cabinets, chambers, counters, the evaporators of which are installed inside the refrigerated object. For ease of operation and repair, some devices are combined into one unit and called a unit. At present, the plants produce FAK-1.5MZ open-type units. The evaporator and control valve are installed in the cooling chamber, and the rest of the machine is installed on a stamped plate and forms a unit. The unit is installed next to the cooling chamber and connected to the evaporator by tubes through which the refrigerant (freon) circulates.
The principle of operation of the machine is as follows: the refrigerant, once in the evaporator, boils, turns into a liquid state into a gaseous one. At the same time, it actively absorbs heat from the tubes and fins of the evaporator. Vapors in the evaporator are sucked off using a compressor, which sends them in a compressed state (6-8 atm.) To the condenser. In the condenser, with the help of cooled air, the refrigerant, having a high pressure, passes into a liquid state. The liquid refrigerant enters the evaporator through a control valve, which reduces the pressure and regulates its supply. Thus, in a closed system, the same amount of freon and its vapors continuously circulate.
Refrigeration hermetic units. The industry produces more advanced refrigeration machines with hermetic compressors of the FPS brands. Its main advantage is that the electric motor and compressor are in one hermetic casing and form a single unit. This unit can work for a long time, since it does not have oil seals that prevent freon leakage.
FGC is much smaller in size and weight. This is achieved by reducing the size of the engine, the absence of a transmission mechanism and better cooling it with freon vapor.
FGK works almost silently, not giving vibrations to the foundation.
Refrigeration unit VS. These units differ from FGK units only in a narrower operating temperature range, lower weight and dimensions of the condenser. Shielded hermetic unit FG-1.1 is structurally designed as follows. that only the rotor of the electric motor is in the sealed cavity. Removing the stator from the sealed cavity simplifies its assembly and allows quick replacement during repairs. Hermetic compressors will become the main units of refrigeration machines used in catering, as they have less weight, dimensions and consume less energy.
The absence of oil seals in the design of the unit eliminates refrigerant leakage and significantly increases the reliability of operation.
Brief information on thermal insulation materials. Heat-insulating materials are used to insulate cabinets, counters and showcases, or to minimize the heat gain e refrigerated equipment.
The following requirements are imposed on thermal insulation materials: strength, durability, stability, low cost, low coefficient of thermal conductivity and heat capacity, harmlessness, biostability, low hygroscopicity. In the manufacture of refrigeration equipment in the industry, heat-insulating materials are used: foam glass-porous glass mass, alfol - corrugated aluminum sheets, mineral cork, foam plastics, asbestos, roofing material and bitumen.
Kalnin Igor MartynovichCourse of lectures on theoretical foundations refrigeration technology
Lecture 1
TNT for food supply
Continuous cold chain (CCC) reduces losses and preserves the quality of products with:
collection (production)
processing
transportation
storage and sale.
Refrigeration technologies in the chemical, petrochemical, gas, metallurgical industries.
Production of cryoproducts O 2 , N 2 , He, Ar, Kr.
comfort and technological air conditioning systems (ACS)
nuclear SCR - apartments, cottages
centralized VCS - public and industrial buildings
transport hard currency - cars, trains, aircraft, ships.
heat pumps
liquefaction plants
cooling systems for superconducting materials
cryosystems
cryoinstruments
low-temperature units for blood preservation
cryobanks
cryogranulators
purification of gas streams
air cleaning
capturing
extraction
Cleaning of drains
disposal
thermal matting of rocket complex elements
production of liquefied oxygen and hydrogen
refueling rockets
Sections
General information
Physical foundations of TNT (physical processes for obtaining low temperatures)
Thermodynamic fundamentals of refrigeration machines (methods for analyzing the efficiency of processes and cycles)
Working substances of refrigeration machines
Cycles and diagrams of steam refrigeration machines
Cycles and schemes of gas refrigeration machines.
Low temperatures are temperatures below ambient temperature.
Environment - atmospheric air, water bodies, soil.
The temperature is set to a scale of degrees Celsius (o C) and a scale of Kelvin (K)
The temperature of absolute zero on the Celsius scale is (-273.16 o C)
The entire Kelvin scale is based on individual fixed points: 273K is the temperature of the triple point of water; 373K is the boiling point of water; from 0 to 273 - also has reference points, which are characterized by phase transformations of various substances.
These reference points are used to calibrate temperature measuring instruments.
TNT is conditionally divided into:
cryogenic technology (deep cold)
refrigeration technology (moderate cold)
The main task of deep cold is the liquefaction of gases; separation of liquefied gases to obtain cryoproducts (oxygen, nitrogen, etc.); technologies for the use of cryoproducts.
Air is made up of:
The normal boiling point is the boiling point at atmospheric pressure.
| gas | normal boiling point |
|
| To | 0 С |
|
| O2 | 90,36 | -182,8 |
| N 2 | 77,36 | -195,8 |
| air | 81,16 | -192,0 |
| H2 | 20,46 | -252,7 |
| He | 4,26 | -268,9 |
Practical use cryoproducts obtained as a result of air separation:
Oxygen-O 2 . Used for welding metals, for blowing blast furnaces and martynov (metallurgy) furnaces. In chemistry, to produce synthetic gasoline. In the rocket and space complex, as an oxidizing agent in rocket engines. In medicine for breathing (mostly).
Nitrogen-N 2 . Energy carrier (refrigerant for freezing and storage of products and biological materials). In mechanical engineering, as a neutral medium in welding. In chemistry, as a raw material for the production of mineral fertilizers based on ammonia. In medicine, for cooling cryoinstruments.
Hydrogen- H 2 . Its production from water or from hydrocarbons (methane-CH 4) is a non-cryogenic process. Liquefied hydrogen is used as an environmentally friendly motor fuel. It also produces heavy water, which is used in nuclear technology.
Natural gas is a mixture of:
| Methane CH 4 , | t s \u003d -161 o C |
| Ethane C 2 H 6 | t s \u003d -9 o C |
| Propane C 3 H 8 | t s \u003d -42 o C |
| Butane C 4 H 10 | t s \u003d -12 o C |
During gas separation, heavy fractions are separated, starting from propane and above, which can condense at atmospheric pressure.
Light fractions are used in the chemical industry and are also burned. The main method for obtaining cryogenic temperatures, including for the separation of gas mixtures, is the expansion of gas pre-compressed to the required pressure level in throttles or expansion machines (expanders).
Air and gas separation plants are complex systems that include compressors, expanders, and regenerative heat exchangers.
Production cryoequipment in a single small-scale production.
^ The main way to obtain temperatures of moderate cold.
A system that implements a closed thermodynamic cycle is called a refrigerating machine.
A chiller (XM) is a machine designed to transfer heat from an environment with a low temperature, in order to cool it, to an environment with a higher temperature due to the supply of energy from an external source.
The thermodynamic cycle of HM consists of the following successive processes:
Evaporation (boiling) or heating of a refrigerant at low temperature and low pressure.
Pressurizing (compressing) a vapor or gaseous refrigerant.
Condensation or cooling of a refrigerant at a higher temperature, the higher pressure.
Reducing the pressure (expansion) of the refrigerant.
According to the field of application, HM is usually divided into:
industrial
trading
household
Cooling capacity XM
Denoted Q 0 , and measured in kW.
Industrial HM are produced with a cooling capacity
Q 0 \u003d 100 ... 15000 kW
Trading XM
Q 0 \u003d 1.0 ... 500 kW
household cold
Q 0 \u003d 0.1 ... 5.0 kW
Quantitative production is characterized by the fact that small CMs are produced in millions of pieces per year (domestic CMs in the world release 90,000,000 pieces/year). Large machines from 1000 kW and above are produced in quantities of several hundred.
Approximate demand in Russia, various refrigeration capacities and purposes.
| Q0, kW | pcs/year | Main scope |
| 0,1 | 4∙10 6 | household cold |
| 1,0 | 4∙10 5 | Trading cold |
| 10,0 | 4∙10 4 | |
| 100,0 | 4∙10 3 | |
| 1000,0 | 4∙10 2 | industrial refrigeration |
| 10000,0 | 40 |
II Physical foundations of low temperature technology
Definition
Artificial cooling - lowering the temperature of an object below the ambient temperature.
Artificial cold is heat, the temperature level of which is lower than the ambient temperature.
Natural cold - using the ambient temperature for cooling various processes if the temperature is low enough. This includes:
The use of cold air in the winter season
Use of cold water ice accumulated in winter, etc.
General classification of refrigeration machines.
heat-using
^ Heat sources
For any heat engine (HM, in which the reverse thermodynamic cycle is carried out, or the energy cycle, in which the direct thermodynamic cycle is carried out), two heat sources are needed: a low-temperature heat source (LHT) and a high-temperature heat source (HHT). Each of these sources can give heat to the system or receive (take away) heat from the system, i.e. be a heat sink. The environment (OS) can play the role of INT and IWT. It can be both a heat source and a heat sink.
A thermodynamic system is a collection of bodies interacting with each other and with the environment. It, or part of it, is separated from the environment by a control surface with a given permeability.
HM is a thermodynamic system that is in interaction with the environment, the characteristic forms of interaction are thermal and mechanical.
Thermodynamic processes and CM cycles are carried out with the help of a working substance - a refrigerant (CA).
The state of the thermodynamic system is characterized by the parameters of the state of the working substance.
The state parameters are physical quantities:
«» – thermal parameters of the state.
"enthalpy, J; internal energy U, J; entropy S » – caloric parameters of the state.
The most widely used are: , ; u, ; s, .
A thermodynamic process is a process in which at least one of the state parameters changes.
The thermodynamic cycle is a set of successive thermodynamic processes, as a result of which the system returns to its original state in all respects.
The basic equations for the calculation and analysis of thermodynamic processes and cycles follow from the first and second laws of thermodynamics.
^ First law of thermodynamics
The amount of heat supplied to the system through the control surface is equal to the change in internal energy and the work done by the system against external forces.
(1), PdV = L abs
D = dU + d(PV) (2)
D 
(PV) = PdV + VdP
; VdP = L tech.
L tech. is the work expended on compressing and moving the working substance.
Q 1-2 \u003d (2 - 1) -
Isentropic Q = 0,
Isobaric p = const, Q 1-2 = 2 - 1
^ Second law of thermodynamics
Heat cannot spontaneously transfer from a system with a lower temperature to a system with a higher temperature. To carry out such a process, it is necessary to expend energy. The direction of heat supply or removal is characterized by the state parameter - entropy.
The total entropy differential will be the change in the amount of heat per temperature.
Entropy is called reduced heat.
Q is the heat involved in the process.
T is the temperature at which the process occurs.
Entropy is a measure of the reversibility of a process
For the final process, equal to the integral from the initial to the final, the amount of heat per temperature here will be equal or greater.
For a reversible process, there will be an "=" sign. For irreversible there will be a ">" sign. For a circular process would be:
For the cyclic operation of the HM, it is necessary that there is not only a heat supply, but also a heat removal, and therefore a heat source and a heat sink are needed.
Hence the most important conclusion from the second law of thermodynamics.
Substituting in equations 1 and 2 instead of TdS, we get
Obtaining low temperatures by means of phase transformations of working substances.
Phase transformations are: boiling, evaporation, melting and sublimation.
I - boiling line;
II - melting line;
III - sublimation line.
Kr - critical melancholy, in which the state of liquid and vapor are indistinguishable.
t.A - the triple point of equilibrium of three phases: liquid, solid and gaseous.
On lines I, II, III, the following are in equilibrium, respectively: liquid - vapor, solid- liquid, solid - steam. As the temperature rises, the working substance changes phase states.
On these lines, temperature and pressure are unambiguously related: the higher the pressure, the higher the temperature, and vice versa. These lines are called saturation lines.
For each working substance there is a boiling point at atmospheric pressure, which is called the normal boiling point: T s , K; t s , o C - is an important characteristic of this working substance.
| t s , o C | t cr, o C | R cr, MPa | t f , o C | Р f , MPa |
|
| Water H 2 O | 100 | 374,5 | 22,56 | 0 | 0,00061 |
| Ammonia NH 3 | -33,35 | 132,4 | 11,52 | -77,7 | 0,6 |
| Carbon dioxide CO 2 | -78,5 | 31,0 | 7,38 | -56,6 | 0,554 |
| Air | -192 | -140 | 3,76 | -208 | 0,01 |
Boiling- a process that occurs with the supply of heat at a constant pressure and at a constant temperature (for monosubstances).
The required temperature level is provided by the corresponding pressure level (see figure above). If the boiling point is lower than the ambient temperature, then cooling can be carried out using this process.
The cooling effect is determined by the heat of vaporization, denoted by the Latin letter "r".
x is the degree of dryness.
1st term: - the internal heat of vaporization spent on imparting the necessary energy to the molecules of the substance during the transition from liquid to vapor.
2nd term: - external heat of vaporization spent to overcome external pressure (difference in specific volumes).
With an increase in pressure, the boiling point increases, and the heat of vaporization decreases at the critical point (at T cr) r = 0.
Boiling vaporization is used in vapor-liquid refrigeration machines. In laboratory practice and in some technological processes, the effect of vaporization is used: liquid air, nitrogen and other liquefied gases.
The required boiling point is provided by a sufficiently low pressure at which the process takes place.
Evaporation- this is the process of vaporization occurring on the free surface of the liquid, at a temperature below normal temperature boiling matter. This process is associated with the nonequilibrium state of the vapor phase above the liquid and the liquid itself.
The evaporation effect of water evaporating under conditions of low relative humidity at 0 degrees is 2500
Sublimation. In the area below the triple point (see figure), the substance is either in a solid or in a gaseous state. The points of curve III are determined by the temperatures and pressures at which the solid and gaseous phases are in equilibrium. The process of transition from a solid to a gaseous state is called sublimation.
The sublimation process gives a great effect, because. the heat of sublimation is equal to the sum of the heats of melting and vaporization (boiling).
In practice, the sublimation of carbon dioxide CO 2 (dry ice) is widely used, the triple point of which is above atmospheric pressure (see table) Р f = 0.528; R atm = 0.1 MPa.
At atmospheric pressure and a temperature of -77.7 o C (available on the plate), the heat of sublimation is 573.
The sublimation process is applied to freeze drying. If a frozen product containing water is placed under vacuum below the pressure of the triple point of water (0.00061 MPa), then when heat is applied, water will sublimate - it will come out of the product and the product will be dehydrated.
Melting- the process of transition of the working substance from a solid to a liquid state, occurring with the absorption of heat, while the heat of fusion is absorbed. For water ice, the heat of fusion is 334.88
To obtain low temperatures using the melting effect, solutions of (aqueous) salts and acids are used. In this case, the melting point decreases, but at the same time, the heat of fusion also decreases compared to water ice. So, for example, a 30% solution of common salt allows you to get a temperature of -21.2 o C and a heat of fusion of 192.55 A solution of calcium chloride in water allows you to get -55 o C. The melting point achieved in aqueous salt solutions is characterized by a concentration-temperature diagram (T- X).
At point E, the solution is saturated simultaneously with both components. Below point E, there are two solid phases, saturated, respectively, with a predominance of components A and B. Above the curves, the solution is in a liquid state, below the curves, in a solid state.
Thus, lines 1 and 2 are the line of melting or crystallization. The concentration x E is called the eutectic temperature, and the temperature T E is called the eutectic temperature. For a given pair of substances, a lower temperature cannot be reached.
The help desk has great amount data on the parameters of eutectic solutions from different components. In practice, this effect is used in everyday life (a bag - a refrigerator, in road transport).
In road transport: an insulated truck body, the walls of which consist of panels filled with a eutectic solution and a tubular heat exchanger is built into it, is connected to an HM, which pumps a coolant through it.
Lecture 3 .
Diagram of the state of working substances.
At present, for any working substance (refrigerant) used, equations of state have been developed that describe the relationship in the thermal parameters of the state: P, MPa; t (T), o C (K); υ, m 3 / kg - and two caloric:, ; S.
Using the equation of state for engineering calculations, state diagrams are built. Two types of diagrams are used: S - T, - P. For the S - T diagram, isolines P = const, = const, = const are plotted on the field, as well as a saturation line x = 0 and x = 1 (x is the degree of dryness of the substance in the area saturation). Lines x=const are plotted in the saturation region (the line of the constant degree of dryness of the substance).
The S-T diagram is used for process and cycle analysis, and the P-diagram for HM engineering calculations.
For a diagram - P, T=const, =const, S=const, x=const, x=0, x=1 are plotted.
A generalizing diagram of the state of the working substance in the coordinates S - T, which reflects all possible states of the working substance:
A) a solid
B) Two-phase state solid - liquid;
B) liquid substance.
D) Two-phase state liquid - vapor;
D) a vaporous substance;
E) A gaseous substance in the temperature range above T cr.
Regions: I - liquid-steam;
II - dry superheated steam;
III - supercooled liquid;
IV - solid body-steam;
V - solid-liquid;
VI - solid supercooled body.
Processes: 1-2 – liquid boiling (Р=const);
1-3 - liquid throttling with ↓ pressure in the wet area
4-5 - melting;
6-7 - sublimation;
8-9 - throttling in the area of superheated steam (gas);
8-10 - expansion of steam (gas) along the isentrope;
11-12 - gas throttling outside the inversion line;
5-1 - heating of the liquid to the state of saturation according to Р=const.
4-5-1-2-8 - isobar.
^ Cooling by expanding gases.
This means that the gas is pre-compressed to a pressure p 1 and then it is expanded to a lower pressure, for example, to atmospheric pressure. The cooling achieved depends on the expansion method.
(2) external forces. and returns to the original
Cooling by throttling.
Gas throttling is the process of pressure drop of the working substance when flowing through constrictions in the channel. Characteristic properties of throttling:
a) The gas flow does no external work;
b) The pressure drops quickly without heat exchange with the environment;
c) The process goes along the line , while changing the internal energy U and volume energy PV.
When throttling, energy is expended on pushing the gas through a narrow section, while the kinetic energy (velocity) increases sharply and the temperature decreases. After a narrow section, the gas velocity decreases sharply and the irreversible losses associated with pushing the gas heat the flow again.
The process for =const (ℐ=const) is only performed on endpoints.
The law of conservation of energy is observed
U 1 + P 1 V 1 \u003d U 2 + P 2 V 2
Temperature T 2<Т 1 , если U 2 P 1 V 1
In principle, depending on in which area of the state diagram throttling occurs, cooling can also be obtained as a result (T 2<Т 1) и охлаждение (Т 1 <Т 2).
To estimate the expected result, the differential Joule-Thompson effect is used.
It is the ratio of an infinitesimal change in temperature to an infinitesimal change in pressure.
If , then there will be cooling;
If , then there will be cooling;
If at the inflection points of the line =const in the state diagram. If these points in the state dmagram are connected to each other, then this will be an inversion line.
In accordance with the differential equations of thermodynamics
For an ideal gas, in which the isenthalps and isotherms coincide, the effect of cooling or heating the gas is an accessory of the real gas.
The integral throttling effect is a finite change in temperature for a finite change in pressure.
In practice, a differential effect is used corresponding to a pressure change of 1 bar (0.1 MPa), then
For air about C
^ Isothermal throttling effect.
This is the cooling capacity that can be obtained by heating steam from T 2 to T 1 .
The throttling process is irreversible, takes place with an increase in entropy, it is ineffective and is not used in refrigeration engineering, but it is used in cryogenic technology in liquefaction and gas separation plants along with other cooling processes, for example, in the Linde cycle
^ Cooling during the expansion of gases with the receipt of work.
The pre-compressed gas can be expanded to a lower pressure in expansion machines - expanders. Turbo expanders are used, and in some cases piston expanders.
The work removed from the expander shaft can be used to compress gas and generate electrical energy.
When expanding gases of pre-compressed gas from pressure P 1 to P 2 in the expansion machine, with the return of work, the gas temperature in all cases decreases.
Work is done by changing the enthalpy of the expanding gas. If the process is carried out without losses and without heat exchange with the environment, then it will pass along the line S=const and, therefore, will be reversible. The cooling effect in a reversible isentropic expansion process is characterized by the ratio of an infinitesimal change in temperature to an infinitesimal change in pressure.
In accordance with the differential equation of thermodynamics
 | ; |
 |
integral effect
For air 
Isothermal effect
To calculate this effect, you can use the approximate equation:
, where k is the adiabatic exponent
Under the conditions: P 1 \u003d 1 MPa (10 bar), T 1 \u003d 300 K. Expansion to atmospheric pressure,
- this is the temperature when the gas expands to pressure P 2 along the isentrope. This is the maximum possible low temperature that can be obtained at given T, P 1 , P 2 . Therefore, the temperature difference is used as a reference for evaluating the efficiency of gas expansion cooling.
In reality, this temperature difference cannot be achieved, because the expansion process occurs with losses, with an increase in entropy, and the actual temperature to which the expanded gas has cooled will be higher, i.e. and 
The temperature efficiency of the process is determined by:
^ Cooling by gas expansion in a vortex tube. Rank effect.
The pre-compressed gas is fed into the pipe through a nozzle directed tangentially to the pipe. In the pipe, the gas swirls in the space between the diaphragm and the valve. When the flow swirls, its central part gives off energy to the peripheral layers and cools down to a temperature. The cooled air, the proportion of which is removed through the diaphragm; the heated part of the air, the proportion of which is removed from the tube through the valve. The heated air has a temperature.
By changing the position of the valve along the axis of the tube, it is possible to change the ratio of the flow of cold and hot gas. In this case, the temperatures Tg and Tx will also change. The process of expansion in a vortex tube is obviously irreversible, as well as throttling (occurs from entropy). It is known that if, after expansion, hot and cold flows are mixed together, the temperature will be equal to T etc.
Characterization of the process in a vortex tube.
The graph shows the dependence of the achieved temperature drop in the pipe on the proportion of air cooling. Maximum cooling is achieved with the air cooling fraction .
Thermal and material
Gas through the nozzles in the distributor rotor is periodically supplied to the receptor tubes with a frequency equal to the rotor speed multiplied by the number of rotor nozzles. In the receptor, the gas periodically contracts and expands.
As a result of such a pulsating process, a constant temperature distribution is established in it from (0.7 ... 0.9) T 1, at the beginning of the receptor, to (1.7 ... 2.0) T 1, at the end of the receptor.
The pressure at the inlet to the receptor varies from close to p 2 (for example, 0.1 MPa) to a higher pressure, but somewhat less than p 1 .
Heat is removed from the hot end of the receptor to the environment; part of the energy of the compressed gas is given off.
The pulsating process is likened to the process of gas expansion with energy removal (in principle, the heat removed can be usefully used).
In this regard, the temperature efficiency of this process is quite high and can approach the efficiency of gas expansion in the expander.
*Share only
^ Cooling using electrical and magnetic effects.
Thermoelectric effect (semiconductor coolers)
The thermoelectric effect is based on the phenomenon of the occurrence of EMF in a circuit of two dissimilar conductors, if the junctions of these conductors have different temperatures. Thermocouples are built on this principle, which are used to measure temperature.
Opened in 1812 Seebeck. In 1834 Peltier discovered the opposite effect, i.e. heating and cooling opposite junctions.
Semiconductor device:
Two dissimilar semiconductors 1 and 2 are interconnected by a junction, the other end is connected by a hot junction, connected to a DC source. As a result of the passage of current, according to the Peltier effect, one of the junctions is cooled and heat Q 0 can be supplied to it from the cooled object. The second junction is heated and the heat Q g is removed to the environment. The cooling effect depends primarily on the properties of the semiconductor material, namely on their thermo EMF, denoted by the letter,. Transferred by the Peltier effect is equal to
the difference in thermal EMF of semiconductors, multiplied by the current strength and the absolute temperature of the cold junction.
Semiconductor materials 1 and 2 are selected in such a way that the Peltier coefficient for them is equal in magnitude and opposite in sign.
Then the cooling capacity according to the Peltier effect will be equal to Q = 2T x .
Full implementation of the Peltier effect is prevented by two physical factors: 1) thermal conductivity of semiconductors, as a result of which heat flows back from the hot junction to the cold one; 2) heating of semiconductors from the Joule heat released by the conductor during the passage of current through it.
Cooling capacity of semiconductor element:
(1)
