1. Heat Operated vapor Compression Refrigeration Cycle
Refrigeration Technology
Chapter5： Vapor and Gas Refrigeration Cycles
Heat Operated vapor Compression Refrigeration Cycle
------Vapor Jet Refrigeration
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2. Refrigeration Technology Chapter5： Vapor and Gas Refrigeration Cycles
Principles of vapor jet refrigeration systems
Vapor jet refrigeration is other type of vapor compression refrigeration cycle driven by heat source of high temperature.
As the same as the mechanical vapor compression refrigeration cycle, the vapor jet refrigeration cycle can accomplish the removal of heat through the evaporation of a refrigerant at a low pressure and the rejection of heat through the condensation of the refrigerant at a higher pressure.
The pressure difference in the system is created by a high pressure vapor or steam ejector, as shown in Fig.5-15.
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3. Refrigeration Technology Chapter5： Vapor and Gas Refrigeration Cycles
The refrigerant in the system shown in Fig.5-15 is water.
A supply of high-pressure vapor, usually steam, passes through a nozzle in which it acquires a high velocity and some vacuum whilst expanding down to evaporator pressure.
Fig.5-15, refrigeration system using a steam ejector or thermo-compressor
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4. Refrigeration Technology Chapter5： Vapor and Gas Refrigeration Cycles
This steam, known as the motive steam, shares its momentum with vapor from the evaporator so that the resulting mixture has sufficient velocity to move against the pressure gradient up to the condenser pressure in a diffuser, in which the mixture is decelerated and compressed.
Both the motive vapor and the vapor drawn from the evaporator are condensed, and the condensate is then divided into two flows, one to pass an expansion valve and feed the evaporator and the other to supply the boiler of motive steam through a feed pump.
Fig.5-15, refrigeration system using a steam ejector or thermo-compressor
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5. Refrigeration Technology Chapter5： Vapor and Gas Refrigeration Cycles
2. An analysis for a basic steam jet refrigeration system
The basic steam jet refrigeration system, seen in Fig.5-16, consists of the following components:
an evaporator;
a refrigerated object;
a boiler;
a nozzle, a mixing chamber;
a diffuser;
a condenser;
an expansion valve;
pumps.
Fig.5-16, a scheme of a steam jet efrigeration system and T-s and h-s diagrams of its cycle
(B –boiler; E –evaporator; NZ –nozzle; MC –mixing chamber; Df – diffuser; Co –condenser; RV –expansion valve; P –pump; RO –refrigeration object )
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6. Refrigeration Technology Chapter5： Vapor and Gas Refrigeration Cycles
In the steam jet system the vapor power cycle ( '-4-1) and the refrigeration cycle ( ) are realized simultaneously.
This can be explained by the progressive, independent observation that the state of the motive steam and refrigerant vapor changes.
Motive steam is expanded in the nozzle from point 1 to point 5, but after mixing with refrigerant vapor it is compressed from 5 to 2, or in the mixture from a to b.
Fig.5-16, a scheme of a steam jet efrigeration system and T-s and h-s diagrams of its cycle
(B –boiler; E –evaporator; NZ –nozzle; MC –mixing chamber; Df – diffuser; Co –condenser; RV –expansion valve; P –pump; RO –refrigeration object )
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8. Refrigeration Technology
Chapter5： Vapor and Gas Refrigeration Cycles
Due to the critical point of CO2 (73.77 bar and 30.85℃) the refrigeration cycle has to be operated transcritically when the ambient temperature is near or higher than the critical temperature.
Fig.5-21 shows the subcritical and transcritical operation of CO2.
In this case the evaporation takes place at subcritical pressure and temperature and the heat rejection at supercritical state.
Fig.5-21, cycles of subcritical and transcritical operation of CO2
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9. Refrigeration Technology Chapter5： Vapor and Gas Refrigeration Cycles
As seen in Fig.5-22, the main components of a CO2 refrigeration cycle are compressor, gas cooler (instead of a condenser because of the supercritical heat rejection, which occurs sometimes), internal heat exchanger, expansion valve, evaporator and low-pressure receiver.
At the transcritical cycle the compressor sucks refrigerant as superheated vapor and compresses to high pressure.
Fig.5-22, Schematic diagram of a CO2 refrigeration cycle (heat pump cycle)
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10. Refrigeration Technology Chapter5： Vapor and Gas Refrigeration Cycles
At the supercritical pressure, the CO2 is cooled in the gas cooler by transferring a heat flux to the ambient climate.
The CO2 is cooled down near to the ambient temperature.
In the internal heat exchanger the high-pressure CO2 is cooled and the low-pressure CO2, as saturated vapor from the receiver, is superheated. Then the refrigerant is throttled to low pressure.
Fig.5-22, Schematic diagram of a CO2 refrigeration cycle (heat pump cycle)
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11. Refrigeration Technology Chapter5： Vapor and Gas Refrigeration Cycles
REFERENCES
1. Gosney M.W., Principles of Refrigeration, Cambridge University Press, 1982
2. International Institute of Refrigeration, A brief history of refrigeration (
3. Pennington, W., How to find accurate vapor pressure of LiBr water solution. Refrig. Eng. 1955, 63, 57-61
4. ASHRAE, ASHRAE Handbook, New York, ASHRAE Publisher, 2005
5. Löwer H., Thermodynamische und physikalische Eingenschaften der wässrigen Lithiumbromid Lösung. Dissertation, Technische Hochschule Karlsruhe, 1960
6. Vliet G.C., Lawson M. B., Lithgow R. A., Water-lithium bromide double-effect absorption cooling analysis, Final Report Texas Univ., Austin. Center for Energy Studies, 12, 1980
7. US Patent , 1998
Lazzarin R.M., Longo G.A. Gasparella A., Ammonia-water absorption machines for refrigeration: theoretical and real performances, Int J Refrigeration, 1996,
9. Dossat R.J., principles of refrigeration, Prentice-Hall, Inc, 1991
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