Ministry of Education and Science of the Russian Federation

NOVOSIBIRSK STATE TECHNICAL UNIVERSITY

_____________________________________________________________

DEFINITION OF CHARACTERISTICS
REFRIGERATION UNIT

Methodical instructions

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. Acquaintance with the refrigerating unit IF-56 and its technical characteristics.

3. Study and construction of refrigeration cycles.

4. Determination of the main characteristics, refrigeration unit.

1. THEORETICAL BASIS OF 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' formulation of the second law of thermodynamics, heat cannot by itself pass from a cold body to a hot one. In a refrigeration plant, this heat transfer does not occur by itself, but due to the mechanical energy of the compressor, spent on the compression of the refrigerant vapor.

The main characteristic of the refrigeration unit is the refrigeration coefficient, the expression of which is obtained from the equation of the first law of thermodynamics, written for the reverse cycle of the refrigeration unit, 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 removed from a cold source; l- mechanical operation of the compressor.

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

q 1 = q 2 + l, (1.2)

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

(1.3)

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

Rice. 1.1. Reverse Carnot cycle

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

q 2 = T 2 ( s 1 – s 4) = T 2 Ds (1.4)

and the heat given off at 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 higher temperature 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 a cold source to a working fluid. Taking into account relations (1.4) and (1.5), equation (1.3) for the refrigerating coefficient 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 required for heat transfer q 2 from cold source to hot.

1.2. Cycle of a vapor compression refrigeration unit

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

A good refrigerant is ammonia NH3 (at the boiling point tк = 20 оС, saturation pressure p k = 8.57 bar and at t 0 = -34 оС, p 0 = 0.98 bar). Its latent heat of vaporization is higher than that of other refrigerating agents, but its disadvantages are toxicity and corrosiveness towards non-ferrous metals, therefore, ammonia is not used in household refrigeration units. Methyl chloride (CH3CL) and ethane (C2H6) are good refrigerants; sulphurous anhydride (SO2) is not used due to its high toxicity.

Freons - fluorochlorine derivatives of the simplest hydrocarbons (mainly methane) - are widely used as refrigerants. Distinctive properties of freons are their chemical resistance, non-toxicity, lack of interaction with construction 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 depletes the ozone layer, was banned in Russia in 2000, only the use of R12 already produced or extracted from equipment is allowed.

2. operation of the IF-56 refrigeration unit

2.1. refrigeration unit

Unit IF-56 is designed to cool air in 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 - refrigerating chamber

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 refrigerating chamber (isobaric-isothermal process 5-6 at p 0 = const and t 0 = const). Superheated steam with temperature enters compressor 1, where it is compressed from pressure p 0 to pressure p K (polytropic, valid compression 1-2d). In 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 data

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

In fig. 2.3 shows the actual cycle of a vapor compression refrigeration unit, built according to the results of its tests: 1-2а - adiabatic (theoretical) compression of refrigerant vapors; 1-2d - actual compression in the compressor; 2d-3 - isobaric cooling of vapors to
condensing temperature t TO; 3-4 * - isobaric-isothermal condensation of refrigerant vapors in the condenser; 4 * -4 - condensate overcooling;
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 refrigerating chamber; 6-1 - isobaric superheat of dry saturated steam (point 6, NS= 1) to temperature t 1.

Rice. 2.3. Refrigeration cycle in ph-chart

2.2. performance characteristics

The main operational characteristics of the refrigeration unit are refrigeration capacity Q, power consumption N, refrigerant consumption G and specific refrigerating 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 - enthalpy of steam at the outlet from the evaporator, kJ / kg; h 4 - enthalpy of the liquid refrigerant before the choke, kJ / kg; q = h 1 – h 4 - specific refrigerating 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 outlet of the evaporator, m3 / kg.

Refrigerant consumption is found by the formula, kg / s:

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

Q = c’pmV IN( t AT 2 - t IN 1). (2.4)

Here VВ = 0.61 m3 / s - capacity of the fan cooling the condenser; t IN 1, tВ2 - air temperature at the inlet and outlet of the condenser, ºС; c’pm- average volumetric isobaric heat capacity of air, kJ / (m3 K):

c’pm = (μ cpm)/(μ v 0), (2.5)

where (μ v 0) = 22.4 m3 / kmol - the volume of a kilo mole of air under 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 AT 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 refrigerating 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 (electric power of the compressor drive). The coefficient of performance is the same physical meaning and is determined by the formula:

ε = ( h 1 – h 4)/(h 2D - h 1). (2.10)

3. Refrigeration testing

After starting the refrigeration unit, it is necessary to wait for the establishment of a stationary mode ( t 1 = const, t 2D = const), then measure all the readings of the instruments and enter them into the measurement table 3.1, based on the results of which, build the cycle of the refrigeration unit 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 carried out in table. 3.2. Evaporation temperature t 0 and condensation t K is found depending on the pressures 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 IN- atmospheric pressure on the barometer, mm. rt. Art .; p 0M - excess pressure of evaporation according to the manometer, ati; pКМ - overpressure of condensation according to the manometer, ati.

Table 3.1

Measurement results

	The quantity	Dimension	Meaning	Note
	Evaporation pressure, p 0M			by pressure gauge
	Condensing pressure, p KM			by pressure gauge
	The temperature in the refrigerator compartment, t HC			thermocouple 1
	Refrigerant vapor temperature in front of the compressor, t 1			thermocouple 3
	Refrigerant vapor temperature after the compressor, t 2D			thermocouple 4
	Condensate temperature after the condenser, t 4			thermocouple 5
	Air temperature after the condenser, t AT 2			thermocouple 6
	Air temperature in front of the condenser, t IN 1			thermocouple 7
	Compressor drive power, N			by wattmeter
	Evaporation pressure, p 0			by formula (3.1)
	Evaporation temperature, t 0			according to table (3.3)
	Condensing pressure, p TO			by formula (3.2)
	Condensing temperature, t TO			according to table 3.3
	Enthalpy of refrigerant vapor in front of the compressor, h 1 = f(p 0, t 1)			on ph-chart
	Enthalpy of refrigerant vapor after the compressor, h 2D = f(p TO, t 2D)			on ph-chart
	Enthalpy of refrigerant vapor after adiabatic compression, h 2A			on ph- diagram
	Enthalpy of the condensate after the condenser, h 4 = f(t 4)			on ph- diagram
	Specific volume of steam in front of the compressor, v 1=f(p 0, t 1)			on ph-chart
	Condenser air flow V IN			According to the passport fan

Table 3.2

Calculation of the main characteristics of the refrigeration unit

TO

	The quantity	Dimension		Meaning
	Average molar heat capacity of air, (m withpm)	kJ / (kmol × K)	29.1 + 5.6 × 10-4 ( t B1 + t AT 2)
	Volumetric heat capacity of air, with¢ pm	kJ / (m3 × K)	(m cp m) / 22.4
	c¢ p m V IN( t AT 2 - t IN 1)
	Refrigerant consumption G		Q TO / ( h 2D - h 4)
	Specific refrigerating capacity, q		h 1 – h 4
	Cooling capacity, Q		Gq
	Specific volumetric refrigerating capacity, qV		Q / v 1
	Adiabatic power, N a		G(h 2A - h 1)
	Relative adiabatic refrigerating capacity, TO BUT		Q / N BUT
	Relative real refrigeration capacity, TO		Q / N
	Cooling coefficient, e		q / (h 2D - h 1)

Table 3.3

Freon-12 saturation pressure (CF2 Cl2 - difluorodichloromethane)

40

1. Scheme and description of the refrigeration unit.

2. Tables of measurements and calculations.

3. Completed task.

The task

1. Build a refrigeration unit cycle in ph-chart (Fig. A.1).

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

Table 3.4

Initial data for constructing a refrigeration unit cycle ints -coordinates

2. Build a refrigeration unit cycle in ts-chart (Fig. A.2).

3. Determine the value of the coefficient of performance of the reverse Carnot cycle using the formula (1.6) for T 1 = T To and T 2 = T 0 and compare it with the coefficient of performance of a real installation.

LITERATURE

1. Sharov, Yu. I. Comparison of the cycles of refrigeration units on alternative refrigerants / // Energetika i teploenergetika. - Novosibirsk: NSTU. - 2003. - Issue. 7, - S. 194-198.

2. Kirillin, V.A. Technical thermodynamics /,. - M .: Energiya, 1974 .-- 447 p.

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

4. Andryushchenko, A. I. Fundamentals of technical thermodynamics of real processes /. - M .: Higher school, 1975.

Unit IF-56 is designed to cool air in refrigerating chamber 9 (Fig. 2.1). The main elements are: a freon reciprocating 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. Refrigerating unit diagram IF-56:

Technical data

Compressor brand
Number of cylinders
The volume described by the pistons, m3 / h
Refrigerant agent
Cooling capacity, kW
at t0 = -15 ° С: tк = 30 ° С
at t0 = +5 ° С tк = 35 ° С
Electric motor power, kW
Condenser outer surface, m2
The outer surface of the evaporator, m2

Evaporator 8 consists of two ribbed batteries - convectors. the batteries are equipped with a 7 throttle with a thermostatic valve. 4 forced air cooled condenser, fan capacity

VB = 0.61 m3 / s.

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

condensation temperature tк; 3 - 4 * - isobaric-isothermal condensation of refrigerant vapors in the condenser; 4 * - 4 - condensate overcooling;

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 refrigerating chamber; 6 - 1 - isobaric superheating of dry saturated steam (point 6, x = 1) to temperature t1.

All small refrigerating machines produced in our country are freon. They are not mass-produced to operate on other refrigerants.

Fig. 99. Refrigerating machine IF-49M:

1 - compressor, 2 - condenser, 3 - thermostatic valves, 4 - evaporators, 5 - heat exchanger, 6 - sensitive cartridges, 7 - pressure switch, 8 - water expansion 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 refrigeration machines mainly with units with a capacity of 3.5 to 11 kW. These include machines IF-49 (Fig. 99), IF-56 (Fig. 100), XM1-6 (Fig. 101); XMV1-6, XM1-9 (Fig. 102); XMV1-9 (Fig. 103); machines without special brands with AKFV-4M units (Fig. 104); AKFV-6 (Fig. 105).

Fig. 104. Refrigeration machine diagram with AKFV-4M unit;

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

A significant number of machines are also produced with units VS-2.8, FAK-0.7E, FAK-1.1E and FAK-1.5M.

All these machines are intended for direct cooling of stationary refrigerating chambers and various commercial refrigeration equipment catering establishments and grocery stores.

As evaporators, wall-mounted ribbed coil batteries IRSN-10 or IRSN-12.5 are used.

All machines are fully automated and equipped with thermostatic valves, pressure switches and water regulating valves (if the machine is equipped with a water-cooled condenser). Relatively large of these machines - ХМ1-6, ХМВ1-6, ХМ1-9 and ХМВ1-9 - are equipped, in addition, with solenoid valves and chamber temperature switches, one common solenoid valve is installed on the armature shield in front of the liquid manifold, with which you can turn off the supply of freon to all evaporators at once, and the chamber solenoid valves - on the 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 the cooling devices of the chamber are turned off by means of this valve, but only those that it supplies.