Refrigeration unit if 56 characteristics. Determination of the characteristics of the refrigeration plant

Ministry of Education and Science of the Russian Federation

NOVOSIBIRSK STATE TECHNICAL UNIVERSITY

_____________________________________________________________

SPECIFICATION
REFRIGERATION UNIT

Guidelines

for FES students of all forms of education

Novosibirsk
2010

UDC 621.565(07)

Compiled by: Cand. tech. Sciences, Assoc. ,

Reviewer: Dr. tech. sciences, prof.

The work was prepared at the Department of Thermal Power Plants

© Novosibirsk State

technical university, 2010

PURPOSE OF THE LABORATORY WORK

1. Practical consolidation of knowledge on the second law of thermodynamics, cycles, refrigeration units.

2. Familiarization with the IF-56 refrigeration unit and its technical characteristics.

3. Study and construction of cycles of refrigeration units.

4. Determination of the main characteristics of the refrigeration unit.

1. THEORETICAL BASIS OF THE WORK

REFRIGERATION UNIT

1.1. Reverse Carnot cycle

The refrigeration unit is designed to transfer heat from a cold source to a hot one. According to Clausius's formulation of the second law of thermodynamics, heat cannot by itself pass from a cold body to a hot one. In a refrigeration plant, such heat transfer does not occur by itself, but due to the mechanical energy of the compressor expended on compressing the refrigerant vapor.

The main characteristic of the refrigeration plant is the coefficient of performance, the expression of which is obtained from the equation of the first law of thermodynamics, written for the reverse cycle of the refrigeration plant, taking into account the fact that for any cycle, the change in the internal energy of the working fluid D u= 0, namely:

q= q 1 – q 2 = l, (1.1)

where q 1 – heat given to the hot spring; q 2 - heat taken from the cold source; l– mechanical operation of the compressor.

From (1.1) it follows that heat is transferred to the hot source

q 1 = q 2 + l, (1.2)

a coefficient of performance is the proportion of heat q 2 transferred from cold source to hot source per unit of compressor work expended

(1.3)

The maximum value of the coefficient of performance for a given temperature range between T mountains of hot and T the cold of cold heat sources has a reverse Carnot cycle (Fig. 1.1),

Rice. 1.1. Reverse Carnot cycle

for which the heat supplied at t 2 = const from the cold source to the working fluid:

q 2 = T 2 ( s 1 – s 4) = T 2 Ds (1.4)

and the heat given off t 1 = const from the working fluid to the cold source:

q 1 = T one · ( s 2 – s 3) = T 1 Ds, (1.5)

In the reverse Carnot cycle: 1-2 - adiabatic compression of the working fluid, as a result of which the temperature of the working fluid T 2 gets hotter T hot spring mountains; 2-3 - isothermal heat removal q 1 from the working fluid to the hot spring; 3-4 - adiabatic expansion of the working fluid; 4-1 - isothermal heat supply q 2 from the cold source to the working fluid. Taking into account relations (1.4) and (1.5), equation (1.3) for the coefficient of performance of the reverse Carnot cycle can be represented as:

The higher the e value, the more efficient the refrigeration cycle and the less work l needed to transfer heat q 2 from cold source to hot.

1.2. Vapour-compression refrigeration cycle

Isothermal heat supply and removal in a refrigeration unit can be carried out if the refrigerant is a low-boiling liquid, the boiling point of which at atmospheric pressure is t 0 £ 0 oC, and at negative boiling temperatures, the boiling pressure p 0 must be greater than atmospheric to prevent air from entering the evaporator. low compression pressures make it possible to make the compressor and other elements of the refrigeration unit lightweight. With a significant latent heat of vaporization r low specific volumes desirable v, which allows to reduce the dimensions of the compressor.

Ammonia NH3 is a good refrigerant (boiling point t k = 20 °C, saturation pressure p k = 8.57 bar and at t 0 \u003d -34 ° C, p 0 = 0.98 bar). Its latent heat of vaporization is higher than that of other refrigerants, but its disadvantages are toxicity and corrosiveness with respect to non-ferrous metals, therefore ammonia is not used in domestic refrigeration units. Good refrigerants are methyl chloride (CH3CL) and ethane (C2H6); Sulfur dioxide (SO2) is not used due to its high toxicity.

Freons, fluorochlorine derivatives of the simplest hydrocarbons (mainly methane), are widely used as refrigerants. The distinctive properties of freons are their chemical resistance, non-toxicity, lack of interaction with structural materials when t < 200 оС. В прошлом веке наиболее широкое распространение получил R12, или фреон – 12 (CF2CL2 – дифтордихлорметан), который имеет следующие теплофизические характеристики: молекулярная масса m = 120,92; температура кипения при атмосферном давлении p 0 = 1 bar; t 0 = -30.3 oC; critical parameters R12: p cr = 41.32 bar; t cr = 111.8 °C; v cr = 1.78×10-3 m3/kg; adiabatic exponent k = 1,14.

The production of freon-12, as a substance that destroys the ozone layer, was banned in Russia in 2000, only the use of already produced R12 or extracted from equipment is allowed.

2. operation of the IF-56 refrigeration unit

2.1. refrigeration unit

The IF-56 unit is designed to cool the air in the refrigerating chamber 9 (Fig. 2.1).

Fan" href="/text/category/ventilyator/" rel="bookmark">fan; 4 - receiver; 5 -capacitor;

6 - filter-drier; 7 - throttle; 8 - evaporator; 9 - refrigerator

Rice. 2.2. Refrigeration cycle

In the process of throttling liquid freon in throttle 7 (process 4-5 in ph-diagram), it partially evaporates, while the main evaporation of freon occurs in the evaporator 8 due to the heat taken from the air in the refrigerator chamber (isobaric-isothermal process 5-6 at p 0 = const and t 0 = const). Superheated steam with a temperature enters compressor 1, where it is compressed from pressure p 0 to pressure p K (polytropic, real compression 1-2d). On fig. 2.2 also shows the theoretical, adiabatic compression 1-2A at s 1 = const..gif" width="16" height="25"> (process 4*-4). Liquid freon flows into the receiver 5, from where it flows through the filter-drier 6 to the throttle 7.

Technical details

Evaporator 8 consists of finned batteries - convectors. The batteries are equipped with a throttle 7 with a thermostatic valve. Capacitor 4 with forced air-cooled, fan performance V B = 0.61 m3/s.

On fig. 2.3 shows the actual cycle of a vapor-compression refrigeration plant built according to the results of its tests: 1-2a - adiabatic (theoretical) compression of the refrigerant vapor; 1-2d - actual compression in the compressor; 2e-3 - isobaric cooling of vapors up to
condensing temperature t TO; 3-4* - isobaric-isothermal condensation of refrigerant vapor in the condenser; 4*-4 – condensate supercooling;
4-5 - throttling ( h 5 = h 4), as a result of which the liquid refrigerant partially evaporates; 5-6 - isobaric-isothermal evaporation in the evaporator refrigerator compartment; 6-1 - isobaric superheating of dry saturated steam (point 6, X= 1) up to temperature t 1.

Rice. 2.3. Refrigeration cycle in ph-diagram

2.2. performance characteristics

Main operational characteristics refrigeration unit are cooling capacity Q, power consumption N, refrigerant consumption G and specific cooling capacity q. Cooling capacity is determined by the formula, kW:

Q = Gq = G(h 1 – h 4), (2.1)

where G– refrigerant consumption, kg/s; h 1 – steam enthalpy at the evaporator outlet, kJ/kg; h 4 - enthalpy of the liquid refrigerant in front of the throttle, kJ/kg; q = h 1 – h 4 – specific cooling capacity, kJ/kg.

The specific volumetric cooling capacity, kJ/m3:

q v= q/ v 1 = (h 1 – h 4)/v 1. (2.2)

Here v 1 – specific volume of steam at the evaporator outlet, m3/kg.

The flow rate of the refrigerant is found by the formula, kg/s:

G = Q TO/( h 2D - h 4), (2.3)

Q = c’pmV AT( t IN 2 - t IN 1). (2.4)

Here V B \u003d 0.61 m3 / s - the performance of the fan that cools the condenser; t IN 1, t B2 - air temperature at the inlet and outlet of the condenser, ºС; c’pm is the average volumetric isobaric heat capacity of air, kJ/(m3 K):

c’pm = (μ cpm)/(μ v 0), (2.5)

where (μ v 0) \u003d 22.4 m3 / kmol - the volume of a kilo mole of air at normal physical conditions; (μ cpm) is the average isobaric molar heat capacity of air, which is determined by the empirical formula, kJ/(kmol K):

(μ cpm) = 29.1 + 5.6 10-4( t B1+ t IN 2). (2.6)

Theoretical power of adiabatic compression of refrigerant vapors in the process 1-2A, kW:

N A = G/(h 2A - h 1), (2.7)

Relative adiabatic and actual cooling capacities:

k A = Q/N BUT; (2.8)

k = Q/N, (2.9)

representing the heat transferred from a cold source to a hot one, per unit of theoretical power (adiabatic) and actual (electrical power of the compressor drive). The coefficient of performance has the same physical meaning and is determined by the formula:

ε = ( h 1 – h 4)/(h 2D - h 1). (2.10)

3. Refrigeration test

After starting the refrigeration unit, it is necessary to wait until the stationary mode is established ( t 1 = const t 2D = const), then measure all instrument readings and enter them in the measurement table 3.1, based on the results of which build a refrigeration plant cycle in ph- and ts-coordinates using the steam diagram for freon-12 shown in fig. 2.2. The calculation of the main characteristics of the refrigeration unit is performed in Table. 3.2. Evaporation temperatures t 0 and condensation t K is found depending on the pressure p 0 and p K according to the table. 3.3. Absolute pressures p 0 and p K is determined by the formulas, bar:

p 0 = B/750 + 0,981p 0M, (3.1)

p K = B/750 + 0,981p KM, (3.2)

where AT- barometric pressure, mm. rt. Art.; p 0M - excess pressure of evaporation according to the manometer, atm; p KM - excess condensation pressure according to the manometer, atm.

Table 3.1

Measurement results

Table 3.2

Calculation of the main characteristics of the refrigeration plant

Table 3.3

Freon-12 saturation pressure (CF2 Cl2 – difluorodichloromethane)

1. Scheme and description of the refrigeration unit.

2. Tables of measurements and calculations.

3. Completed task.

Exercise

1. Build a refrigeration cycle in ph-diagram (Fig. P.1).

2. Make a table. 3.4 using ph-diagram.

Table 3.4

Initial data for building a refrigeration plant cycle ints - coordinates

2. Build a refrigeration cycle in ts-diagram (Fig. P.2).

3. Determine the value of the coefficient of performance of the reverse Carnot cycle according to the formula (1.6) for T 1 = T K and T 2 = T 0 and compare it with the COP of the actual installation.

LITERATURE

1. Sharov, Yu. I. Comparison of cycles of refrigeration units using alternative refrigerants / // Energy and thermal power engineering. - Novosibirsk: NSTU. - 2003. - Issue. 7, - S. 194-198.

2. Kirillin, V. A. Technical thermodynamics / , . – M.: Energy, 1974. – 447 p.

3. Vargaftik, N. B. Reference book on thermophysical properties of gases and liquids / . - M.: science, 1972. - 720 p.

4. Andryushchenko, A. I. Fundamentals of technical thermodynamics of real processes / . – M.: graduate School, 1975.

The IF-56 unit is designed to cool the air in the refrigerating chamber 9 (Fig. 2.1). the main elements are: a freon piston compressor 1, an air-cooled condenser 4, a throttle 7, evaporative batteries 8, a filter-drier 6 filled with a desiccant - silica gel, a receiver 5 for collecting condensate, a fan 3 and an electric motor 2.

Rice. 2.1. Scheme of the IF-56 refrigeration unit:

Technical details

Evaporator 8 consists of two finned batteries - convectors. batteries are equipped with a throttle 7 with a thermostatic valve. Forced air-cooled condenser 4, fan performance

VB = 0.61 m3/s.

On fig. Figures 2.2 and 2.3 show the actual cycle of a vapor-compression refrigeration plant built according to the results of its tests: 1 - 2a - adiabatic (theoretical) compression of the refrigerant vapor; 1 - 2d - actual compression in the compressor; 2d - 3 - isobaric cooling of vapors up to

condensation temperature tk; 3 - 4* - isobaric-isothermal condensation of refrigerant vapor in the condenser; 4* - 4 - condensate subcooling;

4 - 5 - throttling (h5 = h4), as a result of which the liquid refrigerant partially evaporates; 5 - 6 - isobaric-isothermal evaporation in the evaporator of the refrigeration chamber; 6 – 1 – isobaric superheating of dry saturated steam (point 6, х = 1) up to temperature t1.

All produced in our country are small refrigeration machines are freon. They are not mass-produced for operation on other refrigerants.

Fig.99. Scheme of the IF-49M refrigerating machine:

1 - compressor, 2 - condenser, 3 - expansion valves, 4 - evaporators, 5 - heat exchanger, 6 - sensitive cartridges, 7 - pressure switch, 8 - water control valve, 9 - dryer, 10 - filter, 11 - electric motor, 12 - magnetic switch.

Small refrigeration machines are based on the above-mentioned freon compressor-condensing units of the corresponding capacity. The industry produces small refrigerators mainly with units with a capacity of 3.5 to 11 kW. These include machines IF-49 (Fig. 99), IF-56 (Fig. 100), KhM1-6 (Fig. 101); XMV1-6, XM1-9 (Fig. 102); HMV1-9 (Fig. 103); machines without special brands with AKFV-4M units (Fig. 104); AKFV-6 (Fig. 105).

Fig.104. Scheme of a refrigeration machine with an AKFV-4M unit;

1 - condenser KTR-4M, 2 - heat exchanger TF-20M; 3 - water control valve VR-15, 4 - pressure switch RD-1, 5 - compressor FV-6, 6 - electric motor, 7 - filter-drier OFF-10a, 8 - evaporators IRSN-12.5M, 9 - thermostatic valves TRV -2M, 10 - sensitive cartridges.

Machines with VS-2.8, FAK-0.7E, FAK-1.1E and FAK-1.5M units are also produced in significant numbers.

All these machines are intended for direct cooling of stationary cold rooms and various commercial refrigeration equipment of enterprises. Catering and grocery stores.

Wall-mounted ribbed coil batteries IRSN-10 or IRSN-12.5 are used as evaporators.

All machines are fully automated and equipped with thermostatic valves, pressure switches and water control valves (if the machine is equipped with a water-cooled condenser). The relatively large of these machines - XM1-6, XMB1-6, XM1-9 and XMB1-9 - are also equipped with solenoid valves and chamber temperature switches, one common solenoid valve is installed on the valve board in front of the liquid collector, with which you can turn off the supply of freon to all evaporators at once, and chamber solenoid valves - on pipelines supplying liquid freon to the cooling devices of the chambers. If the chambers are equipped with several cooling devices and freon is supplied to them through two pipelines (see diagrams), then a solenoid valve is placed on one of them so that not all cooling devices of the chamber are turned off through this valve, but only those that it feeds.

Refrigeration unit

The IF-56 unit is designed to cool the air in the refrigerating chamber 9 (Fig. 2.1).

Rice. 2.1. Refrigeration unit IF-56

1 - compressor; 2 - electric motor; 3 – fan; 4 - receiver; 5 -capacitor;

6 - filter-drier; 7 - throttle; 8 - evaporator; 9 - refrigerator

Rice. 2.2. Refrigeration cycle

In the process of throttling liquid freon in throttle 7 (process 4-5 in ph-diagram), it partially evaporates, while the main evaporation of freon occurs in the evaporator 8 due to the heat taken from the air in the refrigerator chamber (isobaric-isothermal process 5-6 at p 0 = const and t 0 = const). Superheated steam with a temperature enters compressor 1, where it is compressed from pressure p 0 to pressure p K (polytropic, real compression 1-2d). On fig. 2.2 also shows a theoretical, adiabatic compression of 1-2 A at s 1 = const. In the condenser 4, freon vapors are cooled to the condensation temperature (process 2e-3), then condense (isobaric-isothermal process 3-4 * at p K = const and t K = const. In this case, liquid freon is supercooled to a temperature (process 4*-4). Liquid freon flows into the receiver 5, from where it flows through the filter-drier 6 to the throttle 7.

Technical details

Evaporator 8 consists of finned batteries - convectors. The batteries are equipped with a choke 7 with a thermostatic valve. Forced air-cooled condenser 4, fan performance V B \u003d 0.61 m 3 / s.

On fig. 2.3 shows the actual cycle of a vapor-compression refrigeration plant built according to the results of its tests: 1-2a - adiabatic (theoretical) compression of the refrigerant vapor; 1-2d - actual compression in the compressor; 2e-3 - isobaric cooling of vapors up to
condensing temperature t TO; 3-4 * - isobaric-isothermal condensation of refrigerant vapor in the condenser; 4 * -4 - condensate subcooling;
4-5 - throttling ( h 5 = h 4), as a result of which the liquid refrigerant partially evaporates; 5-6 - isobaric-isothermal evaporation in the evaporator of the refrigeration chamber; 6-1 - isobaric superheating of dry saturated steam (point 6, X= 1) up to temperature t 1 .

Rice. 2.3. Refrigeration cycle in ph-diagram

Performance characteristics

The main operational characteristics of the refrigeration unit are the cooling capacity Q, power consumption N, refrigerant consumption G and specific cooling capacity q. Cooling capacity is determined by the formula, kW:

Q=Gq=G(h 1 – h 4), (2.1)

where G– refrigerant consumption, kg/s; h 1 – steam enthalpy at the evaporator outlet, kJ/kg; h 4 - enthalpy of the liquid refrigerant in front of the throttle, kJ/kg; q = h 1 – h 4 – specific cooling capacity, kJ/kg.

The specific volumetric cooling capacity, kJ / m 3:

q v= q/v 1 = (h 1 – h 4)/v 1 . (2.2)

Here v 1 is the specific volume of steam at the outlet of the evaporator, m 3 /kg.

The flow rate of the refrigerant is found by the formula, kg/s:

G = Q TO /( h 2D - h 4), (2.3)

Q = c’pm V AT ( t IN 2 - t IN 1). (2.4)

Here V B \u003d 0.61 m 3 / s - the performance of the fan that cools the condenser; t IN 1 , t B2 - air temperature at the inlet and outlet of the condenser, ºС; c’pm- the average volumetric isobaric heat capacity of air, kJ / (m 3 K):

c’pm = (μ from pm)/(μ v 0), (2.5)

where (μ v 0) \u003d 22.4 m 3 / kmol - the volume of a kilo mole of air under normal physical conditions; (μ from pm) is the average isobaric molar heat capacity of air, which is determined by the empirical formula, kJ/(kmol K):

(μ from pm) = 29.1 + 5.6 10 -4 ( t B1+ t IN 2). (2.6)

Theoretical power of adiabatic compression of refrigerant vapors in the process 1-2 A, kW:

N A = G/(h 2A - h 1), (2.7)

Relative adiabatic and actual cooling capacities:

k A = Q/N BUT; (2.8)

k = Q/N, (2.9)

representing the heat transferred from a cold source to a hot one, per unit of theoretical power (adiabatic) and actual (electrical power of the compressor drive). The coefficient of performance has the same physical meaning and is determined by the formula.