refrigeration The term refrigeration refers to the process of removing heat from an enclosed space or substance for the purpose of lowering the temperature.International Dictionary of Refrigeration, http://dictionary.iifiir.org/search.phpASHRAE Terminology, ht ...

, flash-gas is refrigerant in gas form produced spontaneously when the condensed liquid is subjected to

boiling Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmosphere. Th ...

.Engineering Toolbox, Terminology. Flash-Gas
/ref> The presence of flash-gas in the liquid lines reduces the

efficiency Efficiency is the often measurable ability to avoid wasting materials, energy, efforts, money, and time in doing something or in producing a desired result. In a more general sense, it is the ability to do things well, successfully, and without ...

of the refrigeration cycle.Refrigeration and Air Conditioning Technology, William Whitman. 2005 New York, Page 37. . It can also lead several expansion systems to work improperly, and increase

superheating In thermodynamics, superheating (sometimes referred to as boiling retardation, or boiling delay) is the phenomenon in which a liquid is heated to a temperature higher than its boiling point, without boiling. This is a so-called ''metastable state ...

at the

evaporator An evaporator is a device used to turn the liquid form of a chemical substance, such as water, into a vapor. Uses Air conditioning and refrigeration Some air conditioners and refrigerators use a compressed liquid with a low boiling point, ...

.HVAC Information - Troubleshooting. Flash-Gas
/ref> This is normally perceived as an unwanted condition caused by dissociation between the volume of the system, and the pressures and temperatures that allow the refrigerant to be liquid. Flash-gas must not be confused with lack of

condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...

, but special gear such as receivers, internal heat exchangers, insulation, and refrigeration cycle optimizers may improve condensation and avoid gas in the liquid lines.

Common Causes

Heat absorption and pressure loss

The most common causes driving the fluid to change its phase when in the liquid line are excessive length of the pipeline, small diameter of the pipelines, and a lack of

subcooling The term subcooling (also called undercooling) refers to a liquid existing at a temperature below its normal boiling point. For example, water boils at 373 K; at room temperature (293 K) the water is termed "subcooled". A subcooled liqui ...

. These create low pressure loci that may cause the fluid to begin changing its phase, especially if the pipeline is exposed to heat, through, for example, gaps in insulation.Application Note 123, ACRP Remote Air-Cooled Condenser, Application and Placement, Ozan Tutunoglu. 2007, Page 1. If there is inadequate

, the refrigerant remains in thermodynamic conditions close to

saturation Saturation, saturated, unsaturation or unsaturated may refer to: Chemistry * Saturation, a property of organic compounds referring to carbon-carbon bonds **Saturated and unsaturated compounds ** Degree of unsaturation **Saturated fat or fatty aci ...

, promoting flash-gas formation. Heat may be absorbed not only from external sources, but from internal energy sources such as friction in the pipeline. On the other hand, pressure in the liquid line may also be upset by artefacts and conditions including excessive vertical rise in the liquid line, gear too small for the size of the piping, devices pumping refrigerant through the line, and any obstructions, all of which cause differences in pressure throughout the liquid line. Eventually, heat absorption and pressure losses in the liquid line modify the saturation conditions of the refrigerant to such an extent that the refrigerant boils and produces flash-gas.

Quality and amount of refrigerant

Refrigerant may also explain flash-gas occurrence. When a system lacks refrigerant, or has a leakage, it may exhibit flash-gas as the volume in the piping exceeds the capacity of the refrigerant to fill it as liquid. This may force the refrigerant into a

phase change In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic State of ...

. On the other hand, lack of refrigerant can sometimes also produce the opposite effect: an overall increase on the

(and

) which will depend on the size and design of the system and its piping. If the system's refrigerant is degenerate, flash-gas may also be produced, as physical properties of the fluid change. This happens because the piping is designed for a specific refrigerant mixture that allows liquid in the liquid line, given certain thermodynamic conditions. If the refrigerant mixture changes its composition considerably, the original designs will not be adequate. Degenerated refrigerant produced by leakage, chemical decomposition or loading with gas when using a glided refrigerant, will most likely make the system perform very poorly, alter the lubricating oil's circulation or composition, and may eventually render the equipment inoperative.

Gas production after expansion

It is also common to find that refrigerant begins to evaporate immediately after the expansion valve, before arriving to the

. This may also be considered as flash-gas but normally doesn't produce complications in the refrigeration cycle. Many refrigeration systems have the expansion valve set up inside the room being cooled, consequently generating productive refrigeration if absorbing heat from the room, to produce this kind of flash-gas between the expansion and the

. Besides, the expansion valve deregulates its operation if the fluid arriving to it is boiling. In this case, the boiling occurs after the expansion.

Detection

Flash-gas may be detected in the system by the observation of gas, bubbles or a foamy appearance of the liquid in the viewing glass in the liquid line. Depending on the location of the glass, this may also indicate an overwhelmed condenser, and the lack of these indicators at the glass does not definitely rule out flash-gas formation in the liquid line. Considering the saturation table for the refrigerant, if it is possible to confirm that a certain amount of condenser

is being produced, and the glass still exhibits gas in the liquid line, one may identify this with flash-gas being produced between the condenser and the glass. The flash-gas phenomena may create a deceiving temperature drop on the liquid line that can be misinterpreted as

. This is due to the fact that the refrigerant may use part of the heat obtained from lowering its temperature, to finish

vaporizing Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phenomenon, whereas boiling is a bulk phenomenon. ...

itself to be able to occupy the volume of the pipes at those pressures.

Prevention

Efforts to prevent flash-gas in the liquid line include a cautious design of the cooling system and its piping, but also the incorporation of gear that might help solve this type of difficulty. The inclusion of a refrigerant receiver is a common, cheap and simple way of decreasing the gas ratio in the liquid line. The incorporation of a

stage after the receiver reduces even more the chances to observe flash gas.Holdover Plates, Transport Refrigeration Systems. 2009 Hercules Manufacturing Company, Henderson, KY, Page 12

/ref> This

may be done in a reserved portion of the main condenser, or separately with a heat exchanger. Some receivers may incorporate an internal heat exchanger that draws heat form the subcooled liquid to superheat the gas compressors suction. There are also many kinds of independent

displays and applications such as refrigeration cycle optimizers; these help avoiding flash-gas in the liquid line by lowering the temperature away from the refrigerant's Saturation vapor curve, saturation curves. Some systems deal with flash-gas by separating it from the refrigerant that goes to the evaporator, as that portion of the refrigerant already evaporated and will only increase

.SWEP, Refrigeration Systems. Two Stages
/ref> One key feature when preventing flash-gas is the diameter of the piping. If the pipes are too thin and long, loss of pressure and friction tend to occur. If the evaporator is too high above the receiver, the rising pipes produce a small amount of vacuum at the topmost portion, making the fluid to undergo

ebullition Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmosphere. Th ...

and produce flash-gas. On the contrary, a refrigerant column that creates weight and pressure may reduce the chances of finding flash-gas. If the

is a plate exchanger placed below the level of the receiver, the pressure will not allow the refrigerant to boil easily. Insulating the liquid line can be helpful if one can determine that heat is being absorbed throughout the piping. On the contrary, if the liquid in the liquid line is hotter than the air outside, insulating the pipes on that section might increase flash-gas as natural subcooling is partially decreased.

Effects on the rest of the system

When preventing flash gas, one must take into account all the other features of the system and how they are affected. Increasing the pipeline diameter may have incidence on the oil circulation and affect the

gas compressor A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor. Compressors are similar to pumps: both increase the pressure on a fluid and both can transp ...

. A column of refrigerant creating weight pressure in the liquid line can be a solution for flash-gas, but may have incidence in the

expansion Expansion may refer to: Arts, entertainment and media * ''L'Expansion'', a French monthly business magazine * ''Expansion'' (album), by American jazz pianist Dave Burrell, released in 2004 * ''Expansions'' (McCoy Tyner album), 1970 * ''Expansio ...

and

evaporation Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...

. If the

floods and liquid arrives to the

, serious complications will show up, most likely destroying the

compressor A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor. Compressors are similar to pumps: both increase the pressure on a fluid and both can transp ...

(especially if the compressor is below the level of the receiver).

Effects of Flash-Gas

The main effect of the presence flash-gas in the liquid piping is the net loss of refrigeration capacity. In general terms this is produced in two ways. First, the expansion valve usually doesn't work properly if injecting a gas-liquid mixture such as refrigerant with flash-gas presence. Second, a portion of the heat required for the refrigerant to flash into gaseous form is drawn from mechanical work such as pressure imbued by the

that moves the refrigerant. Both, lack of

, and untimed transformations of

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...

from work to heat: increase the

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

s and

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

s along the pipes; decrease the

thermodynamic reversibility In thermodynamics, a reversible process is a process, involving a system and its surroundings, whose direction can be reversed by infinitesimal changes in some properties of the surroundings, such as pressure or temperature. Throughout an enti ...

of the process, and raise the overall magnitude of

entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynam ...

produced throughout the whole cycle. This means the refrigeration efficiency of the cycle worsens as it moves away from Carnot's theoretical ideal efficiency. A loss of

performance A performance is an act of staging or presenting a play, concert, or other form of entertainment. It is also defined as the action or process of carrying out or accomplishing an action, task, or function. Management science In the work place ...

implies that the system uses more energy to produce less refrigeration. Besides, these kinds of efficiency losses not only underutilize gear, but may end up lowering the operative life of the main components, especially the expansion valve and the

References

{{DEFAULTSORT:Subcooling Thermodynamic cycles Heat pumps