Working programm, guidelines and
test assignments
for 2nd year students (abbr.) of specialties:
27.12 – product technology Catering
35.11 - merchandising and examination of goods
(correspondence forms y of training - Faculty of "Technological Management"
and evening forms of education - Faculty "Evening")
www.msta.ru
Moscow - 2002
1. Purpose and objectives of the discipline
Introduce students to by physical methods obtaining low temperatures; refrigeration; cycle; main refrigerants and coolants, designs of refrigeration machines, types of refrigerators.
To familiarize yourself with the methods of refrigeration processing of food raw materials and products with the basics of heat and mass transfer during various types refrigeration processing; with processes taking place in products of plant and animal origin when their temperature decreases, as well as during storage.
Teach students to determine the dimensions of refrigeration chambers, calculate heat inflows, build a cycle and select a refrigeration machine for various technological conditions.
The student must acquire skills in calculating the duration of refrigeration treatment and the final temperature of the product, be able to select an appropriate method of refrigeration treatment, and an effective refrigeration machine
| Speciality | Well | Form of training | General | Audi. | Total | Lek. | Lab. | Prak. | Self slave. | Zach. | Ex. | Counter. slave. | Well. slave. |
| 06.16 | 2abbr | in absentia | - | - | - | - | |||||||
| 35.11.00 | 2abbr | Evening | - | - | - | - | - |
Hours according to the academic schedule
3. Thematic plan of lectures
| No. | Name of lectures | Correspondence abbreviation (hours) | Evening (hours) |
| Thermodynamic principles of obtaining low temperatures, refrigeration cycles. Refrigerants. | |||
| Types of refrigerators. Types of heat flow into chambers. Technological refrigeration equipment. | |||
| Basic properties of food products. Methods for preserving raw materials and food products. | |||
| Cooling, freezing, freezing of food products. | |||
| Cold storage. Heating and defrosting. |
4. Thematic plan practical classes(lab. worker)
Basic literature
Additional literature
Working programm
Refrigeration technology
7.1.1. Refrigeration machines
Methods for obtaining low temperatures: phase transitions, throttling, adiabatic expansion, vortex effect, thermoelectric cooling. Second law of thermodynamics. Thermodynamic diagrams T-S And I - lgp. Carnot cycle. Representation of a reverse circular process in thermodynamic diagrams.
Refrigerants, coolants and their properties. Areas of use. Cycle of a single-stage refrigeration machine. Determination of the main characteristics of the cycle. Coefficient of performance
The main elements of refrigeration machines: compressors, condensers, evaporators, throttling devices. Their purpose, classification and selection principles.
The influence of operating modes of a refrigeration machine on its cooling capacity, power and coefficient of performance. Assembly of refrigeration machines
7.1.2. Refrigeration units
Types of refrigerators. Layout of the refrigerator. Calculation required area refrigeration chambers according to the required capacity and type of cargo.
Enclosing structures of refrigeration plants. Heat and waterproofing materials. Calculation of the thickness of thermal insulation of the enclosing structures of the refrigeration chamber. Modern Constructive decisions in the field of construction of refrigeration enterprises.
Types of heat inflows into a refrigerated room. Their calculation.
Methods of cooling refrigeration chambers: direct, using coolant. Scheme refrigeration units: pumpless and pump-circulation. Advantages and disadvantages. Schemes of refrigeration units operating on freons.
Principles of automation of refrigeration machines and installations.
Basics of operating refrigeration units. Optimal operating mode, basic requirements and maintenance conditions.
Cold storage
Conditions for storing food in refrigerators. Changes in products during storage. Shrinkage of products and measures to reduce it. Formation and role of protective shells. Packaging products and placing them in the refrigerator compartments. Methods for cooling storage chambers and placing cooling devices in them.
Deadlines cold storage food products. Features of food storage technology. Storage of products in a controlled gas environment.
7.2.7. Heating and defrosting
Warming refrigerated foods is a technique for this process. Defrosting food and the significance of this process. Distribution of moisture in the product during defrosting. Methods of defrosting in air (slow and fast), in a steam-air environment, in a liquid environment (water and brine), with industrial frequency currents. Comparative assessment in various ways defrosting. Defrosting modes.
Test
When studying the course, the student must complete a test consisting of two tasks:
1. "Construction and calculation refrigeration cycle» according to specified conditions.
2. "Calculation of temperature in the thermal center e refrigerated product" of a given type.
Selecting an option to perform test work based on the student's number indicated in the grade book. If the cipher is four-digit, then the first digit is not taken into account. If the cipher is two or one digit, then zeros are added before the digit to get a three-digit digit.
Using table Appendix 1, data for task No. 1 is selected. Using table Appendix 2, data for task No. 2 is selected.
For example: for cipher 057, the data selection will be:
Task #1: tkam= -10°C; tvd1= 20°C; Qo= 80 kW; refrigerant – R717;
Task No. 2: product - pork; physical model - cylinder; characteristic size -- 2R= 0.03 m; cooling duration -- τ =50min.; product initial temperature -- t n=14°C; coolant temperature -- ts= 1°C; type of cooling medium - air.
When completing the test you must:
Write the text carefully, without abbreviations;
All calculations should be carried out in the SI system;
All pages must be numbered, at the end of the text indicate the date of completion and sign the work;
Do not rewrite text from methodological instructions and literary sources;
Provide a list of references used.
Table of main parameters of characteristic points of the cycle
According to the table data, the following are determined:
1. Specific mass refrigeration capacity:
q0 = i1" - i4 , kJ/kg.
2. Specific work of compression of the refrigerant in the compressor:
l= i2 - i1", kJ/kg.
3. Specific heat, removed from the refrigerant in the condenser:
qк = i2 + i3", kJ/kg.
4. Heat balance equation:
qк = q0+l , kJ/kg.
5. Theoretical cycle coefficient of performance:
e = qо / l, kg/s
6. Mass performance of the compressor, that is, the mass of the refrigerant circulated by the compressor in 1 second:
Ma = Q0 / q0, kg/s.
7. Specific volumetric cooling capacity of the compressor:
q v = q0 / v1" , kJ/m³.
8. The actual volumetric capacity of the compressor, that is, the volume of vapor taken by the compressor from the evaporator:
V d = M A V1"=Q 0 /q v, m³/s.
9. Volume described by compressor pistons:
Vh= V d/ λ, kg/s,
Where λ – compressor supply coefficient (volume losses in the compressor), depends on the operating mode, type of refrigerant, compressor design and is calculated:
λ = λi λw.
Here λi– volumetric indicator coefficient, taking into account volumetric losses in the compressor due to the presence of dead space and resistance in the valves:
λi = 1 –with (P To /P 0 – 1),
Where With - relative dead space in the compressor:
For ammonia c = 0,04…0,05;
For freon c = 0,03…0,04.
λw– heating coefficient, taking into account volumetric losses from heating the refrigerant in the compressor cylinder.
λw = T 0 /T k = ( 273 +t 0)/ ( 273 + t To ).
10. Theoretical power expended by the compressor for adiabatic compression of the refrigerant:
N T =M A l, kW.
11. Indicated power expended in the actual working process to compress the refrigerant in the compressor cylinder:
N i = N T / ηi, kW ,
where ηi is the indicator efficiency, taking into account energy losses from heat exchange in the cylinder and from resistance in the valves during suction and discharge:
ηi = λw+ b · t O,
For ammonia b = 0,001;
For freon b = 0,0025.
12. Effective power - power on the compressor shaft taking into account mechanical losses (friction, etc.):
Ne = Ni / ηmech , kW,
Where η mech = 0.7…0.9 – mechanical efficiency.
13. Motor shaft power:
Nel = Ne / ηel , kW,
Where ηel= 0.8…0.9 - coefficient of performance (efficiency) of the electric motor.
Table for selecting initial data for task No. 1.
Appendix 2
Initial data for task No. 2
| Cipher numbers | Cipher digit | ||||||
| Last | Second | First | |||||
| Product * | Cooling duration, τ, min | Product initial temperature, tн, °C | Ambient temperature, ts, °C | Type of cooling medium | |||
| View | Physical model | Characteristic size ** 2R, m | |||||
| Beef | Plate | 0,04 | Air | ||||
| Fish | Cylinder | 0,05 | Air | ||||
| Apple | Sphere | 0,06 | Air | ||||
| Pork | Plate | 0,05 | Water | ||||
| Tomato | Sphere | 0,06 | CaCl2 solution | ||||
| Strawberry | Sphere | 0,03 | Air | ||||
| Carrot | Cylinder | 0,04 | Air | ||||
| Pork | Cylinder | 0,03 | CaCl2 solution | ||||
| Potato | Plate | 0,04 | Water | ||||
| Bird | Plate | 0,04 | CaCl2 solution |
Notes:* - it is assumed that the product does not have packaging, regardless of the properties (type) of the cooling medium;
** - characteristic size value ( 2R) corresponds to its full thickness for a plate, and to its diameter for a cylinder and sphere.
Application 3
REFRIGERATION AND TECHNOLOGY
Ministry of Education and Science of Ukraine
ODESSA NATIONAL ACADEMY OF FOOD TECHNOLOGIES
Department of Heat and Cooling Engineering
Lecture notes
"Refrigeration equipment"
for students of professional direction 7.090221
full-time and part-time forms of education
Approved
specialty council
7.090221
Odessa ONAPT 2008
Lecture notes on the course “Refrigeration Equipment” for bachelors of specialty 7.090221 full-time and part-time courses / Compiled by S.F. Gorykin, A.S. Titlov. – Odessa: ONAPT, 2008. – 188 p.
Compiled by S.F. Gorykin, Ph.D. tech. Sciences, Associate Professor
A.S. Titlov, Ph.D. tech. Sciences, Associate Professor
Reviewer: Professor, Department of Ecology, Odessa National Academy of Food Technologies, Doctor of Engineering. Sciences Geller V.Z.
Responsible for the release S.F. Gorykin, Ph.D. tech. Sciences, Associate Professor
Introduction
Refrigeration equipment is a set of interconnected technical means designed for the creation, distribution and use of artificial cold. In this case, it is necessary to distinguish between refrigeration systems and refrigeration technological equipment.
The first of them is a complex of refrigeration equipment (one or more compressors, condensers, various types of evaporators, receivers, etc.), in which a refrigerant circulates, directly producing artificial cold. Such complexes are called refrigeration machines. Of several fundamentally different refrigeration machines, the food industry uses exclusively steam compression refrigeration machines.
The second is intended for cooling, freezing and refrigerated storage of perishable food products (PPF). It is called refrigeration technological equipment.
Based on the nature of the impact on food safety products, a distinction is made between refrigeration equipment for cooling and for freezing products. Cooling (a decrease in temperature not lower than cryoscopic temperature) is usually carried out in cooling chambers (except for liquid SPP). Freezing (lowering the temperature significantly below cryoscopic) can be carried out either in freezing chambers (chamber freezers) or in special devices– quick-freezers.
Commercial refrigeration equipment and household refrigeration equipment are subject to separate consideration.
These lectures should under no circumstances be considered by students as the sole source of information. In it, the authors only systematized the material from various textbooks and tried, if possible, to bring it closer to the specifics of our Odessa National Academy of Food Technologies (ONAPT).
The Appendix, in addition to the reference materials necessary for the calculation and selection of refrigeration equipment, includes a list of topics for independent work and questions used for testing.
At the end there is a list of literature available in the ONAPT library, which students may need when studying the course, completing a calculation and graphic task (CGT), tasks for independent work and successful completion modules.
Lecture 1. Applications and physical principles of obtaining low temperatures
1.1. Areas of application of artificial refrigeration
Artificial (machine) cold is widely used in the national economy. With its help, it turned out to be possible to quite simply and effectively regulate the speed of various chemical processes and promote their most favorable course.
IN Food Industry Artificial cold is primarily used as an excellent preservative for SPP. What is the basis for the effect of cold on SPP? On two factors.
Firstly, under conditions of low temperatures, the rate of chemical reactions of degradation of valuable nutrients in SPP slows down and thereby their “biochemical spoilage” slows down.
Secondly, low temperatures slow down (and sometimes even stop) the vital activity of microorganisms, i.e. prevent “microbial” spoilage of SPP.
Ever since humanity realized that cold storage of SPP is the most effective way preserving their high nutritional qualities (and this is the end of the last - the beginning of this century), in all industrialized countries, intensive construction of specialized enterprises began - refrigerators, designed for the accumulation on a large scale and long-term storage of SPP reserves.
Products in the chambers of such refrigerators can be stored chilled or frozen. Cooling of SPP is a decrease in temperature not lower than cryoscopic (usually to 0...4 С). Freezing is a more significant decrease in temperature, significantly lower than cryoscopic (currently minus 18...minus 25С).
However, it is wrong to think that artificial cold in the food industry is used only to increase the shelf life of food products. Currently, cold is a powerful factor of technological impact on SPP. It is known that with the help of artificial cold it is possible, for example, to successfully “clarify” juices and wines, carry out high-quality “ripening” of meat and cheeses, dry grain, peel buckwheat kernels, etc.
A very large consumer of artificial cold is chemical industry. At various stages of technological processes for the production of nitric acid, ammonia synthesis, ethylene, rubber production, chemical fibers Artificial cold is widely used. In many chemical reactors, speed control chemical reaction carried out using artificial cold. IN oil And gas In industry, cold is used for purification, separation and liquefaction of various components and fractions. There are specialized production facilities for purifying lubricating oils from paraffins, separating xylenes, liquefying and purifying gases. IN metallurgy And mechanical engineering artificial cold is used for low-temperature hardening and aging of metals and alloys, ultra-precision metal processing, pipe bending; construction technology - to combat groundwater, improve the structure of concrete; medicine– for storing blood and creating an organ bank for transplantation. IN last years is developing rapidly cryosurgery. The Odessa State Academy of Refrigeration (OSAKh) has created unique cryoinstruments, including those for microsurgery of the eye and brain. The undoubted advantage of cryosurgery is a more successful fight against internal bleeding and ruptures.
Special mention should be made of air conditioning. Comfort systems designed to create comfortable conditions for people in residential and public buildings. Tens of millions of refrigeration machines operate in such installations - autonomous and centralized - especially in countries with hot climates. However, nowadays, increasingly, in the construction of residential and public buildings in industrialized countries, year-round air conditioning systems are used, when the same refrigeration machine is used in the summer to cool the indoor air, and in the winter to heat it (in heat pump mode).
In addition to comfort, there is technological air conditioning. Such systems provide optimal climatic conditions for a particular technological process. Until recently, all computer centers were equipped with powerful air conditioners, because... Computers, especially tube ones, could not operate without intensive heat removal from the room. At the Odessa Precision Engineering Plant, huge workshops have long been equipped with air conditioners that maintain a temperature of 190.5C throughout the entire workshop. This is done to eliminate the influence of fluctuations in ambient temperature on the accuracy of parts processing.
We also note that, as a rule, all cultural and sports facilities, passenger and freight transport, large-capacity vehicles, and cranes are equipped with air conditioning systems.
There are other uses of artificial cold.
Course of lectures on theoretical foundations refrigeration technology
Lecture 1
TNT for food supply
Continuous cold chain (CCC) ensures reduction of losses and preservation of product quality with:
collection (production)
processing
transportation
storage and sale.
Refrigeration technology in the chemical, petrochemical, gas, and metallurgical industries.
Production of cryoproducts O 2 , N 2 , He, Ar, Kr.
Comfort and technological air conditioning systems (SAC)
nuclear SCR – apartments, cottages
centralized VCS – public and industrial buildings
transport hard currency - cars, trains, airplanes, ships.
heat pumps
liquefaction plants
cooling systems for superconducting materials
cryosystems
cryoinstruments
low-temperature blood preservation units
cryobanks
cryogranulators
purification of gas streams
air cleaning
trapping
extraction
Cleaning of drains
disposal
thermal matting of missile system elements
production of liquefied oxygen and hydrogen
rocket refueling
Sections
General information
Physical Basics TNT (physical processes of 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 diagrams of gas refrigeration machines.
Low temperatures are temperatures below the temperature environment.
Environment – atmospheric air, water bodies, soil.
Temperature is assigned to the Celsius degree scale (o C) and the Kelvin scale (K)
The temperature of absolute zero on the Celsius scale is (-273.16 o C)
The entire Kelvin scale is based on individual reference 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.
Instruments that measure temperature are calibrated using these reference points.
TNT is conventionally divided into:
cryogenic technology (deep cold)
refrigeration (moderate cold)
The main task of deep cold is to liquefy gases; separation of liquefied gases to produce cryogenic products (oxygen, nitrogen, etc.); technologies for using cryoproducts.
Air consists of:
Normal boiling point is the boiling point at atmospheric pressure.
| gas | normal boiling point |
|
| TO | 0 C |
|
| O2 | 90,36 | -182,8 |
| N 2 | 77,36 | -195,8 |
| air | 81,16 | -192,0 |
| H 2 | 20,46 | -252,7 |
| He | 4,26 | -268,9 |
Practical use cryoproducts obtained as a result of air separation:
Oxygen-O 2 . Used in welding metals, for purging blast furnaces and Martynov furnaces (metallurgy). In chemistry, to produce synthetic gasoline. In the rocket and space complex, as an oxidizer in rocket engines. In respiratory medicine (mostly).
Nitrogen-N 2. Energy carrier (refrigerant for freezing and storing food and biological materials). In mechanical engineering, as a neutral medium during welding. In chemistry, as raw materials for production mineral fertilizers ammonia based. In medicine, for cooling cryoinstruments.
Hydrogen-H2. 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 is also used to produce heavy water, which is used in nuclear technology.
Natural gas is a mixture of:
| Methane CH4, | t s =-161 o C |
| Ethane C2H6 | t s =-9 o C |
| Propane C 3 H 8 | t s =-42 o C |
| Butane C4H10 | t s =-12 o C |
When separating gas, heavy fractions are separated, starting from propane and above, which can condense at atmospheric pressure.
Light fractions are used in chemical industry, and are also burned. The main way to obtain cryogenic temperatures, including for the separation of gas mixtures, is by expanding a gas pre-compressed to the required pressure level in chokes or expansion machines (expanders).
Air and gas separation plants are complex systems, including compressors, expanders, and regenerative heat exchangers.
Industrial cryoequipment in single small-scale production.
^ The main method of obtaining temperatures of moderate cold.
A system that carries out a closed thermodynamic cycle is called a refrigeration machine.
Refrigeration machine(ХМ) is a machine designed to transfer heat from a medium with a low temperature, for the purpose of cooling it, to a medium with a higher temperature due to the supply of energy from external source.
The thermodynamic cycle of CM consists of the following sequential processes:
Evaporation (boiling) or heating of a refrigerant at low temperature and low pressure.
Increasing the pressure (compression) of a vapor or gaseous refrigerant.
Condensation or cooling of the refrigerant at more than high temperature, the higher the pressure.
Reduced pressure (expansion) of the refrigerant.
According to the area of application, CM is usually divided into:
industrial
trading
household
Cooling capacity XM
It is designated Q 0 and is measured in kW.
Industrial HMs are produced with refrigeration capacity
Q 0 =100…15000 kW
Trading XM
Q 0 =1.0…500 kW
Household cold
Q 0 =0.1…5.0 kW
Quantitative production is characterized by the fact that small chemical substances are produced in millions of pieces per year (household chemical substances in the world produce 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.
| Q 0 , kW | pcs/year | Main Application |
| 0,1 | 4∙10 6 | Household cold |
| 1,0 | 4∙10 5 | Trade cold |
| 10,0 | 4∙10 4 | |
| 100,0 | 4∙10 3 | |
| 1000,0 | 4∙10 2 | Industrial cold |
| 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 whose temperature level is lower than the ambient temperature.
Natural cooling is the use of ambient temperature to cool various processes if the temperature is low enough. This includes:
Using cold atmospheric air V winter time of the year
Using the cold of water ice accumulated in winter, etc.
General classification of refrigeration machines.
heat-using
^ Heat sources
For any heat engine (HM, in which a reverse thermodynamic cycle is carried out, or an energy cycle, in which a direct thermodynamic cycle is carried out), two heat sources are required: a source of low-temperature heat (LHT) and a source of high-temperature heat (HHT). Each of these sources can transfer heat to the system or receive (receive) heat from the system, i.e. be a heat sink. The environment (OS) can play the role of INT and IVT. It can be a heat source and a heat sink.
A thermodynamic system is a set of bodies interacting with each other and the environment. It, or part of it, is separated from the environment by a control surface with a given permeability.
CM is a thermodynamic system that interacts with the environment; the characteristic forms of interaction are thermal and mechanical.
Thermodynamic processes and chemical cycles are carried out using a working substance - a refrigerant (RA).
The state of a thermodynamic system is characterized by the parameters of the state of the working substance.
State parameters are physical quantities:
"" – thermal parameters of the state.
“enthalpy, J; internal energy U, J; entropy S» – caloric state parameters.
The most widely used: , ; u, ; s, .
A thermodynamic process is a process in which at least one of the state parameters changes.
A thermodynamic cycle is a set of sequential 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 technical - this is the work spent on compression and movement of the working substance.
Q 1-2 = (2 - 1)-
Isoentropic Q = 0,
Isobaric p = const, Q 1-2 = 2 - 1
^ Second law of thermodynamics
Heat cannot spontaneously transfer from a system with less to a system with 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 differential of entropy 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 a “=” sign. For irreversible there will be a ">" sign. For a circular process it would be:
For cyclic operation of a chemical machine, it is necessary that there be not only a heat supply, but also a heat removal, and therefore a heat source and a heat sink are needed.
This is the most important conclusion from the second law of thermodynamics.
Substituting TdS into equations 1 and 2, we get
Obtaining low temperatures using phase transformations of working substances.
Phase transformations are: boiling, evaporation, melting and sublimation.
I – boiling line;
II – melting line;
III – sublimation line.
Kr - critical depression, in which the state of liquid and vapor is not distinguishable.
t.A is 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 – vapor. As the temperature increases, 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 | tcr, o C | P cr, MPa | t f , o C | Рf, 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 occurring with heat supply at constant pressure and at constant temperature (for monosubstances).
The required temperature level is ensured by the corresponding pressure level (see figure above). If the boiling point is below ambient temperature, cooling can be carried out using this process.
The cooling effect is determined by the heat of vaporization, denoted Latin letter"r".
x – degree of dryness.
1st term: - internal warmth vaporization, spent on imparting the necessary energy to the molecules of a substance during the transition from liquid to vapor.
2nd term: - external heat of vaporization, spent to overcome external pressure (difference in specific volumes).
As the pressure increases, the boiling point increases, and the heat of vaporization decreases and at the critical point (at Tcr) r = 0.
The boiling vaporization process 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 ensured by the sufficiently low pressure at which the process occurs.
Evaporation is a process of vaporization that occurs on the free surface of a liquid at a temperature below normal temperature boiling of a substance. 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 in conditions of low relative humidity at 0 degrees is 2500
Sublimation. In the region below the triple point (see figure), the substance is either solid or in 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 changing from a solid to a gaseous state is called sublimation.
The sublimation process is very effective because... the heat of sublimation is equal to the sum of the heats of fusion and vaporization (boiling).
In practice, sublimation of carbon dioxide CO 2 (dry ice), the triple point of which is higher than atmospheric pressure (see table) P f = 0.528, is widely used; P atm = 0.1 MPa.
At atmospheric pressure and temperature -77.7 o C (indicated on the tablet), the heat of sublimation is 573.
The sublimation process is used for freeze drying. If a frozen product containing water is placed in a vacuum below the pressure of the triple point of water (0.00061 MPa), then when heat is supplied, the water will sublimate - leave the product and the product will be dehydrated.
Melting– the process of transition of a working substance from a solid to a liquid state, which occurs 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 temperature decreases, but at the same time the heat of fusion decreases compared to water ice. So, for example, a 30% solution table salt allows us to obtain a temperature of -21.2 o C and a heat of fusion of 192.55. A solution of calcium chloride in water allows us to obtain -55 o C. The achieved melting temperature in aqueous solutions of salts is characterized by a concentration - temperature (T-x) diagram.
At point E, the solution is saturated with both components simultaneously. 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 liquid state, under the curves - in the solid.
Thus, line 1 and 2 are the melting or crystallization line. Concentration x E is called eutectic, and temperature T E is called eutectic temperature. For a given pair of substances, a lower temperature cannot be achieved.
The reference book contains a huge amount of data on the parameters of eutectic solutions from different components. In practice, this effect is used in everyday life (bag - refrigerator, 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 the CM, which pumps coolant through it.
Lecture 3 .
Diagram of the state of working substances.
Currently, 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 ones: , ; S.
Using the equation of state, phase diagrams are constructed for engineering calculations. 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 the saturation line x=0 and x=1 (x is the degree of dryness of the substance in the area saturation). In the saturation region, lines x=const are drawn (line constant degree dryness of the substance).
The S – T diagram is used for the analysis of processes and cycles, and the P diagram is used for chemical engineering calculations.
For the diagram – P, T=const, =const, S=const, x=const, x=0, x=1 are plotted.
A general diagram of the state of the working substance in S – T coordinates, which reflects all possible states of the working substance:
A) Solid matter;
B) Two-phase state solid - liquid;
B) Liquid substance.
D) Two-phase state liquid - vapor;
D) Vaporous substance;
E) Gaseous substance in the temperature range above Tcr.
Regions: I – liquid-vapor;
II – dry superheated steam;
III – supercooled liquid;
IV – solid-vapor;
V – solid-liquid;
VI – solid supercooled body.
Processes: 1-2 – liquid boiling (P=const);
1-3 – throttling of liquid with ↓ pressure in the wet area
4-5 – melting;
6-7 – sublimation;
8-9 – throttling in the area of superheated steam (gas);
8-10 – isentropic expansion of steam (gas);
11-12 – gas throttling outside the inversion line;
5-1 – heating the liquid to a state of saturation according to P=const.
4-5-1-2-8 – isobar.
^ Cooling by expansion of gases.
This means that the gas is pre-compressed to a pressure p 1 and then expanded to more low pressure, for example, to atmospheric. The cooling achieved depends on the expansion method.
(2) external forces. and returns to first place
Cooling by throttling.
Gas throttling is the process of a drop in pressure of the working substance as it flows through constrictions in the channel. Characteristic properties throttling:
a) The gas flow does not perform external work;
b) The pressure drops quickly without heat exchange with the environment;
c) The process takes place along the line, and the internal energy U and volumetric energy PV change.
When throttling, energy is expended on pushing the gas through a narrow cross-section, while the kinetic energy (speed) increases sharply and the temperature decreases. After a narrow cross-section, the gas velocity decreases sharply and irreversible losses associated with gas pushing again heat the flow.
The process by =const (ℐ=const) is executed only on endpoints.
The law of conservation of energy is observed
U 1 + P 1 V 1 = U 2 + P 2 V 2
Temperature T 2<Т 1 , если U 2 P 1 V 1
In principle, depending on which region of the state diagram throttling occurs, cooling (T 2<Т 1) и охлаждение (Т 1 <Т 2).
To assess 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 diagram are connected to each other, then this will be an inversion line.
In accordance with the differential equations of thermodynamics
For an ideal gas whose isenthalps and isotherms coincide, the effect of cooling or heating the gas is a property of the real gas.
The integral throttling effect is a finite change in temperature for a finite change in pressure.
In practice, the differential effect corresponding to a pressure change of 1 bar (0.1 MPa) is used, then
For air about C
^ Isothermal throttling effect.
This is the cooling capacity that can be obtained by heating steam from T2 to T1.
The throttling process is irreversible, occurs with increasing entropy, it is ineffective and is not used in refrigeration technology, 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 when gases expand to produce work.
Pre-compressed gas can be expanded to lower pressure in expansion machines called expanders. Turbo expanders and, in some cases, piston expanders are used.
The work removed from the expander shaft can be used to compress gas and generate electrical energy.
When the gases of pre-compressed gas expand 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 occurs without losses and without heat exchange with the environment, then it will take place 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 the infinitesimal change in temperature to the infinitesimal change in pressure.
According to 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 conditions: P 1 = 1 MPa (10 bar), T 1 = 300 K. Expansion to atmospheric pressure,
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 standard for assessing the efficiency of cooling by gas expansion.
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. Ranque effect.
Pre-compressed gas is supplied to 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 to a temperature. The cooled air, a fraction of which, is discharged through the diaphragm; the heated part of the air, a fraction of which, is removed from the tube through the valve. Heated air has a temperature.
By changing the position of the valve along the axis of the tube, you can change the ratio of the flow of cold and hot gas. In this case, the temperatures Tg and Tx will also change. The expansion process in a vortex tube is obviously irreversible, just like throttling (it occurs with entropy). It is known that if, after expansion, hot and cold flows are mixed with each other, then the temperature will be equal to T dr.
Characteristics of the process in a vortex tube.
The graph shows the dependence of the achieved temperature decrease in the pipe on the fraction of air cooling. Maximum cooling is achieved at a cooling air fraction of .
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 rotation speed multiplied by the number of rotor nozzles. In the receptor, the gas periodically contracts and expands.
As a result of this 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 entrance to the receptor varies from close to p 2 (for example, 0.1 MPa) to a higher pressure, but slightly less than p 1.
From the hot end of the receptor, heat is transferred to the environment, i.e. part of the energy of the compressed gas is released.
The pulsation 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 an expander.
*Only for share
^ 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 used to measure temperature are built on this principle.
Opened in 1812 Seebeck. In 1834 Peltier discovered the opposite effect, i.e. heating and cooling of opposite junctions.
Semiconductor element structure:
Two dissimilar semiconductors 1 and 2 are connected to each other by a junction, the other end is connected by a hot junction, connected to a direct current source. As a result of the passage of current, according to the Peltier effect, one of the junctions is cooled and heat Q 0 from the cooled object can be supplied to it. The second junction heats up and heat Q g is removed to the environment. The cooling effect depends primarily on the properties of the semiconductor material, namely on their thermal emf, denoted by the letter, . Transferred by the Peltier effect is equal to
the difference between the thermal emf of semiconductors, multiplied by the current strength and the absolute temperature of the cold junction.
The materials of semiconductors 1 and 2 are selected in such a way that the Peltier coefficient for them is equal in value and opposite in sign.
Then the cooling capacity according to the Peltier effect will be equal to Q = 2T x.
The full implementation of the Peltier effect is hindered by two physical factors: 1) thermal conductivity of semiconductors, as a result of which heat flows back from the hot junction to the cold junction; 2) heating of semiconductors from the Joule heat generated by the conductor when current passes through it.
Cooling capacity of semiconductor element:
(1)
Introduction
Refrigeration machine
Operating principle of compression refrigeration machines
Operating principle of absorption refrigeration machines
Operating principle of steam ejector refrigeration machines
The operating principle of refrigeration machines using vortex coolers
Operating principle of thermoelectric refrigeration machines
Introduction
Refrigeration is a scientific discipline and branch of technology that covers methods for obtaining and using low temperatures (cold) for the production and storage of food products.
The use of refrigeration technology makes it possible to preserve the properties of food products, as well as obtain food products with new properties.
Without refrigeration technology, it is impossible to supply a growing population with quality food. In the process of production and increasing the volume of sales of food products, the role of refrigeration equipment is important, which allows:
create stocks of perishable food products in a wide range;
increase the shelf life of frozen food products;
sell seasonally produced food products evenly throughout the year;
reduce commodity losses during storage and transportation of food products;
introduce progressive methods of providing services to the population by trade and public catering enterprises.
Refrigeration machine
A refrigeration machine is a device used to remove heat from a cooled body at a temperature lower than the ambient temperature. The processes occurring in refrigeration machines are a special case of thermodynamic processes, i.e. those in which there is a consistent change in the state parameters of the working substance: temperature, pressure, specific volume, enthalpy. Refrigeration machines operate on the principle of a heat pump - they take heat from the body being cooled and, using energy (mechanical, thermal, etc.), transfer it to a cooling medium (usually water or ambient air), which has a higher temperature than the body being cooled. Refrigeration machines are used to obtain temperatures from 10°C to -150°C. The region of lower temperatures refers to cryogenic technology. The operation of a refrigeration machine is characterized by its cooling capacity.
The first refrigeration machines appeared in the middle of the 19th century. One of the oldest refrigeration machines is absorption. Its invention and design are associated with the names of J. Leslie (Great Britain, 1810), F. Carré (France, 1850) and F. Windhausen (Germany, 1878). The first vapor compression machine powered by ether was built by J. Perkins (Great Britain, 1834). Later, similar machines were created using methyl ether and sulfur dioxide as a coolant. In 1874, K. Linde (Germany) built an ammonia vapor-compression refrigeration machine, which marked the beginning of refrigeration engineering.
The operation of refrigerators is based on the refrigeration cycle. A simple steam cycle of a mechanical refrigeration machine is implemented using four elements that form a closed refrigeration circuit - a compressor, a condenser, a throttle valve and an evaporator or cooler (Fig. 1). The steam from the evaporator enters the compressor and is compressed, causing its temperature to increase. After leaving the compressor, the high temperature and pressure steam enters the condenser, where it is cooled and condensed. Some capacitors use a subcooling mode, i.e. further cooling the condensed liquid below its boiling point. From the condenser, the liquid passes through the throttle valve. Since the boiling (saturation) temperature for a given pressure is lower than the temperature of the liquid, its intensive boiling begins; in this case, part of the liquid evaporates, and the temperature of the remaining part drops to the equilibrium saturation temperature (the heat of the liquid is spent on its transformation into steam). The throttling process is sometimes called internal cooling or self-cooling because the process reduces the temperature of the liquid refrigerant to the desired level. Thus, saturated liquid and saturated steam exit the throttle valve. Saturated steam cannot effectively remove heat, so it is bypassed by the evaporator and supplied directly to the compressor inlet. A separator is installed between the throttle and the evaporator, in which steam and liquid are separated.
Rice. 1. Diagram of the refrigeration cycle.
Operating principle of compression refrigeration machines
Compression refrigerators are the most common and versatile. The main components of such a refrigerator are:
a compressor that receives energy from the electrical network;
condenser located outside the refrigerator;
evaporator located inside the refrigerator;
thermostatic expansion valve, TRV, which is a throttling device;
refrigerant, a substance circulating in the system with certain physical characteristics.
All elements of the refrigeration machine are required to have high tightness. Depending on the type of refrigeration compressor, compression machines are divided into piston, turbocompressor, rotary and screw.
The refrigerant under pressure enters the evaporator through a throttling hole (capillary or expansion valve), where, due to a sharp decrease in pressure, the liquid evaporates and turns into steam. In this case, the refrigerant takes away heat from the inner walls of the evaporator, due to which the interior of the refrigerator is cooled.
The compressor draws refrigerant from the evaporator in the form of steam, compresses it, due to which the temperature of the refrigerant rises and pushes it into the condenser.
In the condenser, the refrigerant heated as a result of compression cools, giving off heat to the external environment, and condenses, that is, turns into liquid. The process is repeated again.
Thus, in the condenser, under the influence of high pressure, the refrigerant condenses and turns into a liquid state, releasing heat, and in the evaporator, under the influence of low pressure, it boils and turns into a gaseous state, absorbing heat.
A thermostatic expansion valve (TEV) is necessary to create the required pressure difference between the condenser and the evaporator at which the heat transfer cycle occurs. It allows you to correctly (most completely) fill the internal volume of the evaporator with boiled refrigerant. The flow area of the expansion valve changes as the thermal load on the evaporator decreases; as the temperature in the chamber decreases, the amount of circulating refrigerant decreases. A capillary is an analogue of a expansion valve. It does not change its cross-section, but throttles a certain amount of refrigerant, depending on the pressure at the inlet and outlet of the capillary, its diameter and the type of refrigerant.
Usually there is also a heat exchanger that equalizes the temperature at the outlet of the condenser and the evaporator. As a result, already cooled refrigerant enters the throttle, which is then further cooled in the evaporator, while the refrigerant coming from the condenser is heated before entering the compressor and condenser. This allows you to increase the efficiency of the refrigerator.
When the required temperature is reached, the temperature sensor opens the electrical circuit and the compressor stops. When the temperature rises (due to external factors), the sensor turns on the compressor again.
To increase the economic efficiency of a refrigeration machine (reduce energy costs per unit of heat removed from the body being cooled), the steam sucked in by the compressor is sometimes overheated and the liquid is supercooled before throttling. For the same reason, multi-stage or cascade refrigeration machines are used to obtain temperatures below -30°C.
In multi-stage refrigeration machines, steam is compressed sequentially in several stages with cooling between individual stages. In this case, in two-stage refrigeration machines the boiling point of the refrigerant reaches -80 °C.
In cascade refrigeration machines, which are several refrigeration machines connected in series, which operate on different refrigerants that are most suitable in terms of their thermodynamic properties for given temperature conditions, a boiling point of up to -150 ° C is obtained.
Operating principle of absorption refrigeration machines
The working substance in absorption refrigerators are solutions of two components with different boiling points at the same pressure. The component that boils at a lower temperature acts as a refrigerant; the second serves as an absorbent. In the temperature range from 0 to -45°C, machines are used where the working substance is an aqueous solution of ammonia (refrigerant - ammonia). At cooling temperatures above 0°C, absorption machines operating on an aqueous solution of lithium bromide (coolant - water) are predominantly used.
Absorption systems retain the condenser, throttle valve and evaporator, but instead of the compressor, four other elements are used: absorber, pump, steam generator (reboiler) and pressure reducing valve. Steam from the evaporator enters the absorber. There it comes into contact with an absorbent liquid, which absorbs the refrigerant in the vapor phase; the pressure in the absorber decreases, which ensures a continuous supply of steam from the evaporator. During the absorption process, heat is released, therefore, the absorber must be cooled, for example, by circulating water. The cold mixture of absorbent liquid and refrigerant enters the pump, where its pressure increases. Since an increase in fluid pressure is accompanied by only a slight change in its volume, the work required for this is small. After leaving the pump, the cold high-pressure liquid enters the reboiler, where heat is supplied to it and most of the refrigerant evaporates. This moderately superheated high pressure steam passes through the condenser and goes through the normal refrigeration cycle, while the absorbent is cooled and returned to the absorber (via a pressure reducing valve) to repeat the cycle. The actual absorption cycle differs from the ideal one in that part of the absorbent evaporates in the boiler and is carried away along with the refrigerant vapor. If it is not separated from the refrigerant before entering the evaporator, it will cause the temperature in the evaporator to rise, or in practice the pressure in the evaporator will be significantly less than the saturation pressure at the temperature that the evaporator should be. The separation of the absorbent from the refrigerant partly occurs in a separator, which is located between the condenser and the boiler and serves to condense the absorbent and return it to the boiler along with a small amount of associated refrigerant. The mechanical work of absorption refrigeration units is much less than that of compression refrigeration units, but the overall energy costs are significantly higher. The energy supplied to the boiler is much greater than that removed from the absorber by cooling water. Where electricity is expensive and heat and cooling water are cheap, absorption plants are more cost-effective than compression plants. The use of absorption machines is very beneficial in enterprises where secondary energy resources are available (waste steam, hot water, waste gases from industrial furnaces, etc.).
Operating principle of steam ejector refrigeration machines
A method of producing cold without performing mechanical work is to eject steam from the evaporator. In such an installation, the refrigerant is water, so the temperature in the refrigeration chamber cannot be below 0 ° C.
A steam ejector refrigerator consists of an ejector, an evaporator, a condenser, a pump and a expansion valve. Water serves as a refrigerant; steam at a pressure of 0.3-1 MN/m2 is used as an energy source, which enters the ejector nozzle, where it expands. As a result, a reduced pressure is created in the ejector and, as a consequence, in the evaporator of the machine, which corresponds to a boiling point of water slightly above 0°C (usually about 5°C). In the evaporator, due to partial evaporation, the cold water supplied to the consumer is cooled. The steam sucked from the evaporator, as well as the working steam of the ejector, enters the condenser, where it turns into a liquid state, giving off heat to the cooling medium. Part of the water from the condenser is supplied to the evaporator to replenish the loss of cooled water.
Steam ejector units are used in industry, where there is high and medium pressure steam and cheap water for cooling. These units are also used on ships because the small number of moving parts makes them easy to maintain and repair.
The operating principle of refrigeration machines using vortex coolers
Cooling is carried out due to the expansion of air pre-compressed by a compressor in blocks of special vortex coolers.
It has not become widespread due to its high noise level, the need to supply compressed air (up to 10-20 atm), its very high consumption, and low efficiency. Advantages - greater safety of use, since no electricity is used and there are no moving mechanical parts or dangerous chemical compounds in the design; durability, reliability.
Air-expansion refrigeration machines belong to the class of gas refrigeration machines. The refrigerant is air. At temperatures down to approximately -80°C, the economic efficiency of air machines is lower than that of vapor compression machines. More economical are regenerative air refrigeration machines, in which the air is cooled before expansion either in a counterflow heat exchanger or in a regenerator heat exchanger. Depending on the pressure of the compressed air used, air refrigeration machines are divided into high and low pressure machines. There are air machines operating in a closed and open cycle.
Operating principle of thermoelectric refrigeration machines
The thermoelectric refrigerator is built on Peltier elements, is silent, but is not widely used due to the high cost of cooling thermoelectric elements. However, cooler bags, small car refrigerators, and drinking water coolers are often made with Peltier cooling.
A thermoelectric refrigerator operates based on the Peltier effect, which involves the release or absorption of heat when an electric current passes through a thermocouple junction. In Fig. Figure 2 schematically shows the cross-section of such a refrigerator with a volume of 65 dm3, capable of maintaining the temperature of the refrigerating chamber 10 ° C below the ambient temperature. At the top are 72 thermocouples that provide cooling, consuming most of the 135 watts of electricity needed to run the refrigerator. There are special ribs in the air blowing channel for better heat removal, and plates are installed in the chamber to increase the heat transfer surface. Such refrigerators on ships are designed to store six tons of frozen or chilled food. The industry also produces other types of thermal refrigerators, in particular thermostats for laboratory needs.
Rice. 2. Thermoelectric refrigerator (can be made portable). 1 – cooling fins; 2 – fan; 3 – blinds; 4 – thermoelements; 5 – thermal insulation; 6 – cold plates.
Content
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Introduction. Physical principles of obtaining low temperatures…………………………………………………………………………………... | |
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Schemes and cycles of refrigeration machines…………………………… | |
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Refrigeration compressors……………………………... | |
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Refrigerants…………………………………………………………………… | |
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Schemes and cycles of two-stage compression……………………. | |
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Heat exchangers of refrigeration machines…………….. | |
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Absorption refrigeration machines………………………... | |
LECTURE 1
Introduction.
Physical principles of obtaining low temperatures.
1. Brief history of the development of refrigeration technology.
2. Ice and ice-salted cooling.
3. Cooling during phase transitions.
4. Throttling.
5. Adiabatic expansion.
6. Vortex effect.
7. Thermoelectric cooling.
Methods of accumulating and using natural cold have been known for many centuries. These include: accumulation of ice and snow in special glaciers, storage of food in deep pits (using low average ground temperature), cooling of water as it evaporates.
The world's first refrigeration machine was designed in 1834 in London and ran on ethyl ether, but was not widely used. In 1872, the Englishman Boyle invented the ammonia refrigeration machine, which marked the beginning of the industrial use of refrigeration equipment.
Initially, artificial refrigeration began to be used on a large scale in the preparation and transportation of food products. The first installation for freezing meat was built in Sydney in 1861. In the same year (also in Australia), a refrigeration machine was installed at an oil refinery to separate paraffin from crude oil, which marked the beginning of the introduction of artificial refrigeration in the chemical industry. By the end of the 70s and beginning of the 80s. The last century dates back to the first attempts to transport meat from South America and Australia to France and England on refrigerated ships with air and absorption refrigeration machines. Transportation of food in ice-cooled railroad cars began in 1858 in the United States. The first large refrigerator was built in Boston (USA) in 1881. In the same year, a refrigerator was built in London, and in 1882 - in Berlin.
In Russia, refrigeration began to take shape later and developed slowly. The first refrigeration machines appeared in 1888 in the fisheries in Astrakhan. In 1889, two refrigeration plants were built at the breweries. Since 1892, small ice factories began to appear in the Caucasus, Central Asia, and Crimea. The first refrigerator with a capacity of 250 tons was built in 1895 in Belgorod. The first railway transportation in ice-cooled cars began in Russia at the same time as abroad, namely in 1860. Until 1914, only 29 refrigerators with a total capacity of 45,600 tons were built. At this time, the capacity of refrigerators in the United States was approaching 2 million tons. In all branches of industry in Russia there were 296 refrigeration units. In total, in 1917 there were 58 refrigerators with a total capacity of 57,300 tons. Refrigerated transport was also insufficiently developed: in 1917 in Russia there were only 650 two-axle railway cars with ice-salt cooling and one refrigeration (refrigerated) ship.
Any heated body can be cooled naturally to the temperature of its surroundings. It is possible to cool the body to a temperature lower than the ambient temperature only artificially.
Heat can only be removed from a body by another body whose temperature is lower than the temperature of the body being cooled. The amount of heat that a cooling body removes from the body or medium being cooled determines its refrigeration effect, or cooling capacity.
The cooled medium can be the air of a chamber with perishable products, water when obtaining ice, earthen soil when sinking mines, etc.
Bodies that perform physical processes that occur at low temperatures with significant heat absorption are used as coolers. These include processes of changes in the physical state of the body, expansion processes, thermoelectric processes, etc.
Cooling using processes of changing the physical state of bodies. Processes of changes in the state of aggregation occur without a change in body temperature, since the heat absorbed (or released) by the body in these processes is spent on overcoming (or increasing) the adhesion forces between molecules. For cooling, processes of changes in the state of aggregation that occur with heat absorption are used:
melting - the transition of solids into a liquid state;
sublimation - the transition of solids directly into a vapor state;
boiling is the transition of liquid bodies into a vapor state.
Bodies with possibly low temperatures of melting, sublimation, boiling and with high heat of melting of sublimation and boiling are used in refrigeration technology as coolants.
The most accessible cooling body is water ice with a melting point of 0° C. The cooling capacity of 1 kg of ice corresponds to its heat of fusion g = 335 kJ/kg. Eutectic ice, which is a frozen solution of water and salt, as well as a mixture of crushed ice or snow with salt, has a lower melting point. The drop in the melting point of these bodies below 0°C is explained by the fact that in them, in addition to melting, the process of dissolving salt in water also occurs, accompanied by a decrease in the melting point of the mixture and a decrease in the heat of fusion. The temperature and heat of fusion of the mixture depend on the type of salt and its content in the mixture.
The most common mixtures are: sodium chloride with ice (melting point up to -21.2 ° C) and calcium chloride with ice (melting point up to - 55° C).
A body that has a low temperature and high heat of sublimation is solid carbon dioxide (carbon dioxide CO2), which is called dry ice. Under atmospheric conditions, such ice passes from the solid state directly into the gaseous state (bypassing the liquid phase) at a temperature of -78.9 ° C, and each kilogram of it absorbs about 575 kJ of heat.
In some cases, liquids with a very low boiling point are used for artificial cooling, for example liquid air (boiling point -192 C), liquid oxygen (-183 ° C), liquid nitrogen (-196 ° C).
Using a change in the state of aggregation (melting ice, boiling liquid air, sublimation of solid carbon dioxide) for the purpose of cooling has a number of disadvantages. In particular, cooling bodies, perceiving heat from the cooled medium and changing their state of aggregation, lose their cooling ability. Therefore, continuous cooling is possible only with an infinitely large supply of cooling fluid. So, for continuous cooling of the food storage chamber, you can use ice, but as it melts, it must be replaced with new one.
Continuous cooling can be achieved with the same amount of coolant if, after obtaining the refrigerating effect, it is returned to its original state. This is done in refrigeration machines. To maintain a constant low temperature of the working fluid in the machine, the principle of boiling liquid bodies is most often used. Considering that the boiling point of a liquid depends on pressure, it is possible to achieve the required boiling temperature by maintaining a certain pressure in a closed apparatus corresponding to the physical properties of the boiling liquid. As the pressure decreases, the boiling point decreases. For example, water at atmospheric pressure boils at 100°C, but if you place water in a closed vessel and reduce the pressure to 0.0009 MPa, the water will boil at 5°C. Ammonia at a pressure of 0.1 MPa boils at -33.6°C; when the pressure decreases to 0.05 MPa, the boiling point drops to -46.5°C.
If a closed apparatus with a saturated liquid is placed in a cooled environment, the temperature of which is slightly higher than the boiling point of the liquid at the pressure created in the apparatus, then the liquid will boil, and the heat necessary for vaporization will be removed from the cooled environment. To maintain constant pressure in the apparatus and a constant low boiling point of the liquid, the resulting vapors should be continuously removed.
Cooling by expansion of gases. When a compressed gas expands and performs external work at the expense of internal energy, the temperature of the gas decreases. The greatest cooling of air can be achieved with adiabatic expansion, which occurs without heat exchange with the environment at constant entropy. In this process, the work of expansion is performed only due to the internal energy of the gas. If air, compressed to 9 MPa at ambient temperature, is adiabatically expanded to 0.1 MPa, its temperature will drop to -190 ° C.
Cooling due to throttling. Throttling is the process of reducing the pressure of a liquid or gas without changing enthalpy. In practice, it is carried out when a liquid or gas passes through a narrowed section (valve, tap, etc.) from a cavity of high pressure to a cavity of lower pressure. This process is also a kind of expansion process with a decrease in the internal energy of the body. However, no useful work is created during the throttling process. Internal energy is spent on overcoming friction when a liquid or gas passes through the narrowed section of a valve or tap.
