Vapor-compression refrigeration
Vapour-compression refrigeration or vapor-compression refrigeration system (VCRS),
Refrigeration may be defined as lowering the temperature of an enclosed space by removing heat from that space and transferring it elsewhere. A device that performs this function may also be called an
Description
Vapor-compression uses a circulating liquid
The superheated vapor then passes through the condenser. This is where heat is transferred from the circulating refrigerant to an external medium, allowing the gaseous refrigerant to cool and condense into a liquid. The rejected heat is carried away by either the water or the air, depending on the type of condenser.
The condensed liquid refrigerant, in the thermodynamic state known as a saturated liquid, is next routed through an expansion valve where it undergoes an abrupt reduction in pressure. That pressure reduction results in the adiabatic flash evaporation of a part of the liquid refrigerant. The auto-refrigeration effect of the adiabatic flash evaporation lowers the temperature of the liquid and vapor refrigerant mixture to where it is colder than the temperature of the enclosed space to be refrigerated.
The cold refrigerant liquid and vapor mixture is then routed through the coil or tubes in the evaporator. Air in the enclosed space circulates across the coil or tubes due to either thermal
To complete the
Refrigerants
The selection of working fluid has a significant impact on the performance of the refrigeration cycles and as such it plays a key role when it comes to designing or simply choosing an ideal machine for a certain task. One of the most widespread refrigerants is "Freon". Freon is a trade name for a family of haloalkane refrigerants manufactured by DuPont and other companies. These refrigerants were commonly used due to their superior stability and safety properties: they were not flammable at room temperature and atmospheric pressure, nor obviously toxic as were the fluids they replaced, such as sulfur dioxide. Haloalkanes are also an order(s) of magnitude more expensive than petroleum-derived flammable alkanes of similar or better cooling performance.
Unfortunately, chlorine- and fluorine-bearing refrigerants reach the upper atmosphere when they escape. In the
Newer refrigerants that have reduced
More benign refrigerants are currently the subject of research, such as
Thermodynamic analysis of the system
The
From point 2 to point 3, the vapor travels through part of the condenser which removes the superheat by cooling the vapor. Between point 3 and point 4, the vapor travels through the remainder of the condenser and is condensed into a high-temperature, high-pressure subcooled liquid. Subcool is the amount of sensible heat removed from the liquid below its maximum saturation. The condensation process occurs at essentially constant pressure.
Between points 4 and 5, the subcooled liquid refrigerant passes through the expansion valve and undergoes an abrupt decrease of pressure. That process results in the adiabatic flash evaporation and auto-refrigeration of a portion of the liquid (typically, less than half of the liquid flashes). The adiabatic flash evaporation process is
Between points 5 and 1, the cold and partially vaporized refrigerant travels through the coil or tubes in the evaporator where it is totally vaporized by the warm air (from the space being refrigerated) that a fan circulates across the coil or tubes in the evaporator. The evaporator operates at essentially constant temperature and boils off all available liquid after adding pressure to the refrigerant in order to make sure the liquid has evaporated completely. This is a safeguard for the compressor, as it cannot pump liquid.
The resulting superheated vapor returns to the compressor inlet at point 1 to complete the thermodynamic cycle.
The above discussion is based on the ideal vapor-compression refrigeration cycle which does not take into account real world items like frictional pressure drop in the system, slight internal irreversibility during the compression of the refrigerant vapor, or non-ideal gas behavior (if any).
Types of gas compressors
The most common compressors used in refrigeration are
- Hermetic motor, hermetic compressor
- Hermetic motor, semi-hermetic compressor
- Open motor (belt driven or close coupled), hermetic compressor
- Open motor (belt driven or close coupled), semi-hermetic compressor
Typically in hermetic, and most semi-hermetic compressors (sometimes known as accessible hermetic compressors), the compressor and motor driving the compressor are integrated, and operate within the refrigerant system. The motor is hermetic and is designed to operate, and be cooled by, the refrigerant being compressed. The obvious disadvantage of hermetic motor compressors is that the motor drive cannot be maintained in situ, and the entire compressor must be removed if a motor fails. A further disadvantage is that burnt out windings can contaminate whole refrigeration systems requiring the system to be entirely pumped down, and the refrigerant replaced.
An open compressor has a motor drive which is outside of the refrigeration system, and provides drive to the compressor by means of an input shaft with suitable gland seals. Open compressor motors are typically air-cooled and can be fairly easily exchanged or repaired without degassing of the refrigeration system. The disadvantage of this type of compressor is a failure of the shaft seals, leading to loss of refrigerant.
Open motor compressors are generally easier to cool (using ambient air) and therefore tend to be simpler in design and more reliable, especially in high pressure applications where compressed gas temperatures can be very high. However the use of liquid injection for additional cooling can generally overcome this issue in most hermetic motor compressors.
Reciprocating compressors
Reciprocating compressors are piston-style, positive displacement compressors.
Rotary screw compressors
Rotary screw compressors are also positive displacement compressors. Two meshing screw-rotors rotate in opposite directions, trapping refrigerant vapor, and reducing the volume of the refrigerant along the rotors to the discharge point.
Small units are not practical due to back-leakage but large units have very high efficiency and flow capacity.
Centrifugal compressors
Centrifugal compressors are dynamic compressors. These compressors raise the pressure of the refrigerant by imparting velocity or dynamic energy, using a rotating impeller, and converting it to pressure energy.
Centrifugal Compressor Surge
Chillers with centrifugal compressors have a 'Centrifugal Compressor Map' that shows the "surge line" and the "choke line." For the same capacity ratings, across a wider span of operating conditions, chillers with the larger diameter lower-speed compressor have a wider 'Centrifugal Compressor Map' and experience surge conditions less than those with the smaller diameter, less expensive, higher-speed compressors. The smaller diameter, higher-speed compressors have a flatter curve.,[4][5][6]
As the refrigerant flow rate decreases, some compressors change the gap between the impeller and the volute to maintain the correct velocity to avoid surge conditions.[7]
Scroll compressors
Scroll compressors are also positive displacement compressors. The refrigerant is compressed when one spiral orbits around a second stationary spiral, creating smaller and smaller pockets and higher pressures. By the time the refrigerant is discharged, it is fully pressurized.
Others
-
Diaphragm pump
-
Axial-flow compressor of a jet engine
-
Liquid ring
-
Roots blower
Compressor lubrication
In order to lubricate the moving parts of the compressor, oil is added to the refrigerant during installation or commissioning. The type of oil may be mineral or synthetic to suit the compressor type, and also chosen so as not to react with the refrigerant type and other components in the system. In small refrigeration systems the oil is allowed to circulate throughout the whole circuit, but care must be taken to design the pipework and components such that oil can drain back under gravity to the compressor. In larger more distributed systems, especially in retail refrigeration, the oil is normally captured at an oil separator immediately after the compressor, and is in turn re-delivered, by an oil level management system, back to the compressor(s). Oil separators are not 100% efficient so system pipework must still be designed so that oil can drain back by gravity to the oil separator or compressor.
Some newer compressor technologies use magnetic bearings or air bearings and require no lubrication, for example the Danfoss Turbocor range of centrifugal compressors. Avoiding the need for oil lubrication and the design requirements and ancillaries associated with it, simplifies the design of the refrigerant system, increases the heat transfer coefficient in evaporators and condensers, eliminates the risk of refrigerant being contaminated with oil, and reduces maintenance requirements.[8]
Control
In simple commercial refrigeration systems the compressor is normally controlled by a simple pressure switch, with the expansion performed by a capillary tube or thermal expansion valve. In more complex systems, including multiple compressor installations, the use of electronic controls is typical, with adjustable set points to control the pressure at which compressors cut in and cut out, and temperature control by the use of electronic expansion valves.
In addition to the operational controls, separate high-pressure and low-pressure switches are normally utilised to provide secondary protection to the compressors and other components of the system from operating outside of safe parameters.
In more advanced electronic control systems the use of floating head pressure, and proactive suction pressure, control routines allow the compressor operation to be adjusted to accurately meet differing cooling demands while reducing energy consumption.
Other features and facts of interest
The schematic diagram of a single-stage refrigeration system shown in Figure 1 does not include other equipment items that would be provided in a large commercial or industrial vapor compression refrigeration system, such as:
- A horizontal or vertical pressure vessel, equipped internally with a demister, between the evaporator and the compressor inlet to capture and remove any residual, entrained liquid in the refrigerant vapor because liquid may damage the compressor. Such vapor–liquid separators are most often referred to as "suction line accumulators". (In other industrial processes, they are called "compressor suction drums" or "knockout pots".)
- Large commercial or industrial refrigeration systems may have multiple expansion valves and multiple evaporators in order to refrigerate multiple enclosed spaces or rooms. In such systems, the condensed liquid refrigerant may be routed into a pressure vessel, called a receiver, from which liquid refrigerant is withdrawn and routed through multiple pipelines to the multiple expansion valves and evaporators.
- Filter Dryers, installed before the compressors to catch any moisture or contaminants in the system and thus protect the compressors from internal damage
- Some refrigeration units may have multiple stages which requires the use of multiple compressors in various arrangements.[9]
In most of the world, the
Applications
Refrigeration application | Short descriptions | Typical refrigerants used |
---|---|---|
Domestic refrigeration | Appliances used for keeping food in dwelling units | R-600a, R-134a, R-22, |
Commercial refrigeration | Holding and displaying frozen and fresh food in retail outlets | R-134a, R-404A, R-507 |
Food processing and cold storage | Equipment to preserve, process, and store food from its source to the wholesale distribution point | R-123, R-134a, R-407C, R-410A, R-507 |
Industrial refrigeration | Large equipment, typically 25 kW to 30 MW, used for chemical processing, cold storage, food processing, building, and district heating and cooling | R-123, R-134a, R-404A, R-407C, R-507, R-717 |
Transport refrigeration | Equipment to preserve and store goods, primarily foodstuffs, during transport by road, rail, air, and sea | R-134a, R-407C, R-410A |
Electronic cooling | Low-temperature cooling of CMOS circuitry and other components in large computers and servers[10] | R-134a, R-404A, R-507 |
Medical refrigeration | R-134a, R-404A, R-507 | |
Cryogenic refrigeration | Ethylene, propane, nitrogen, helium |
Economic analysis
Advantages
- Very mature technology.
- Relatively inexpensive.
- Can be driven directly using mechanical energy (water, car or truck motor) or with electrical energy.
- Efficient up to 60% of Carnot's theoretical limit (as evaluated in ASHRAE testing conditions: evaporation temperature of −23.3 °C, condensing temperature of 54.4 °C, and ambient temperature of 32 °C)[citation needed] based on some of the best commercially available compressors, as produced by manufacturers Danfoss, Matsushita, Copeland, Embraco, Bristol, and Tecumseh. However, many refrigeration systems use compressors that have lower efficiencies of between 40 and 55%, since the 60% efficient ones cost almost twice as much as the lower efficiency ones.
Disadvantages
Many systems still use
With the ultimate phasing out of HCFCs already a certainty, alternative non-haloalkane refrigerants are gaining popularity. In particular, once-abandoned refrigerants such as hydrocarbons (butane for example) and CO2 are coming back into more extensive use. For example, Coca-Cola's vending machines at the 2006 FIFA World Cup in Germany used refrigeration utilizing CO2.[11] Ammonia (NH3) is one of the oldest refrigerants, with excellent performance and essentially no pollution problems. However, ammonia has two disadvantages: it is toxic and it is incompatible with copper tubing.[12]
History
In 1805, the American inventor
In 1834, an American expatriate to Great Britain, Jacob Perkins, built the first working vapor-compression refrigeration system in the world.[14] It was a closed-cycle that could operate continuously, as he described in his patent:
- I am enabled to use volatile fluids for the purpose of producing the cooling or freezing of fluids, and yet at the same time constantly condensing such volatile fluids, and bringing them again into operation without waste.
His prototype system worked although it did not succeed commercially.[15]
A similar attempt was made in 1842, by American physician,
The first practical vapor compression refrigeration system was built by
The first
See also
- Absorption refrigerator
- Air conditioning
- Flash evaporation
- Heat pump
- Heating, ventilation, and air conditioning (HVAC)
- Magnetic refrigeration
- Refrigerant
- Refrigeration
- Refrigeration cycle
- Working fluid
References
- ISBN 978-81-7371-461-0.
- saturation pressure. A superheated vapor is at a temperature higher than the saturation temperature corresponding to its pressure.
- ^ r744.com – Everything R744 Archived 2017-07-24 at the Wayback Machine, The Natural Refrigerant R744 (CO)2, 2006–2012
- ^ Fundamentals of Centrifugal Chillers | Johnson Controls
- ^ Chilled Water Plant Design Guide | Taylor Engineering | Pages 281
- ^ Chiller Surge
- ^ Centrifugal Chiller - Fundamentals | McQuay
- ^ "The Different Types of Cooling Compressors". Retrieved 2024-01-13.
- ^ Vapor-compression refrigeration cycles, Schematic diagrams of multi-stage units, Southern Illinois University Carbondale, 1998-11-30
- .
- ^ 2006 Environmental Performance, the Coca-Cola Company Archived 2011-11-10 at the Wayback Machine (scroll down to pdf page 6 of 9 pdf pages).
- ^ Ammonia Refrigeration – Properties of Ammonia, osha.gov, 2011
- ISBN 1-57958-464-0.
- ISBN 978-0-12-374996-3.
- ISBN 0-262-52001-X.
- ^ "Patent Images". pdfpiw.uspto.gov.
- ^ "What's on". Scienceworks. 16 September 2023.
Further reading
- Yunus A. Cengel and Michael A. Boles (2008). Thermodynamics: An Engineering Approach (6th ed.). McGraw-Hill. ISBN 978-0-07-352921-9.