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EVAPORATORS

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EVAPORATORS

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EVAPORATORS
Evaporators are heat exchangers in which evaporation of
the refrigerating fluid takes place, subtracting heat from the space that we want to cool. They are constituted by a bundle of pipes in which the refrigerating fluid is evaporated by extracting heat from the surroundings, which is essentially the phenomenon of cold production, or cooling power, that is desired to be achieved.
The evaporator consists of a metal container, (housing), to which the refrigerant comes from the throttling valve, partially vaporized, producing in it the boiling of the liquefied part, at low pressure, which causes the heat extraction of the environment around it, that is, the cold room. At the end of the vaporization process of the last drops of liquid, dry saturated steam is produced.

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The temperature of this steam can increase, producing a reheating at the expense of the
heat extracted to the external environment, and even of the own friction of the steam as it passes through the evaporator

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The evaporators can be of different types, since the cold industrial technique covers a
very wide range of applications, while they must be adapted to different working conditions that depend, above all on the temperatures and the degree of humidity, (title) , from the steam to the evaporator inlet. Depending on the different requirements of different applications, the evaporators are manufactured with a wide variety of criteria, types, shapes, sizes and materials, being able to. classified in the following form, according to:

- The type of operation
- The type of construction
- The phase to be cooled

Classification of evaporators according to the operation:
a) Dry expansion
evaporators b) Flooded evaporators

a) Dry Expansion Evaporators.- In dry expansion evaporators, the cooling
fluid is supplied by an expansion device in such a way that the liquid evaporates completely along the evaporator from which it usually exits with a slight overheating ; the refrigerant is in the strictly necessary proportion, to form a dry saturated vapor, which will provide a good performance in the compressor.
In dry expansion evaporators, the rolling valve controls the rate of admission of
the refrigerant in the evaporator so that all the liquid evaporates along the length
of the evaporator, from which it can emerge as dry saturated vapor , or as is generally the case in
form of superheated steam.

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In the dry expansion evaporator the amount of liquid refrigerant fluid varies with the thermal load; when the charge is small, the amount of liquid in the evaporator will be small; as the charge increases, the liquid in the evaporator will increase to be able to absorb with its evaporation the higher thermal load. In the dry expansion evaporator the heat transfer efficiency is maximum when the load is higher as this situation corresponds to a greater internal surface wetted by the liquid refrigerant fluid.
The fundamental advantage of dry expansion evaporators is the greater drag
of the oil that reaches it. However, they need larger areas of exchange and loss
of load when circulating for them greater amount of refrigerating fluid is more important than in the flooded ones.
With this type of evaporator, in the case of refrigerants not miscible with the oil, or
not miscible under the conditions of the evaporator, separators and other accessories must be used to
ensure the return of oil to the compressor.

b) Flooded evaporators.- In flooded evaporators, their interior is always completely filled with liquid refrigerant, the supply is regulated by means
of a float valve, which keeps the liquid level in the evaporator constant; are a variant
of the previous model, but with a faster and more uniform expansion of the refrigerant, by means of
tubes connected in a bypass to distribution manifolds.

The flooded evaporators consist of a series of tubes connected by one end
to another tube of larger diameter, through which the cooling liquid enters, and at the other end they are joined to
another tube, also of a larger diameter than the previous one, through from which the suction of the vaporized refrigerant fluid is homogeneously effected by the compressor. In order for the distribution of refrigerant liquid to be uniform, it is necessary that the evaporators of this type be installed well leveled; cooling fins are often provided to improve operating conditions.

The liquid refrigerant fluid filling the evaporator, as a result of the heating from the outside, evaporates and the vapor in the form of bubbles ascends the tubes to the feed tank.

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The object of this reservoir is to separate the liquid from the vapor by returning the liquid to the evaporator and sending the dry saturated vapor to the compressor; in addition, the feeder tank also separates
the steam produced in the lamination valve in such a way that this vapor does not flow through the
evaporator directly to the compressor.

The main advantages of flooded evaporators are:
- High rate of heat transfer
- Circulation of lower flow of refrigerant through the evaporator
- Aspiration by the compressor of saturated steam

In effect, the internal surface of the evaporator is always in contact with the liquid refrigerant fluid which produces an improvement in the heat transfer. In addition, as the steam produced in the lamination does not flow through the evaporator, the flow of circulating refrigerant fluid is smaller, reducing frictional losses. On the other hand with this type of evaporator it is assured that the compressor always sucks saturated steam.

The main disadvantages are:
- Charging of high refrigerant in the circuit
- Difficulty of returning oil to the compressor
- Bulky supply element

As the entire evaporator is filled with liquid and a full feeder tank, also of liquid is needed, the amount of refrigerant required in the circuit increases. Due to the constitution
of the flooded evaporator the lubricating oil that reaches it will have great difficulty in leaving due to
the low speed of the refrigerating fluid even in the case of refrigerants miscible with the
oil. When this happens, the oil forms a film inside the evaporator that reduces the heat transfer capacity and can result in a decrease in the amount of oil in the compressor crankcase below that needed for proper lubrication. To avoid such problems, an oil separator is usually installed in the discharge line between the compressor and the condenser.


CLASSIFICATION OF EVAPORATORS ACCORDING TO THEIR CONSTRUCTION:
a) Of smooth tube
b) Of plate
c) Of fins

a) Smooth tube evaporators.- Smooth tube evaporators consist of a tube,
which is given the most practical form (open or closed zigzag, oblong shape, etc.), and are placed both in the as well as on the walls, where cleanliness is essential, easy defrosting and cooling of liquids below 1 ° C because in these cases the capacity of the evaporator is not affected by the formation of frost on its surface.
The most commonly used materials for the manufacture of smooth or naked tube evaporators are
steel and copper; the steel tube is used in the case of large evaporators for economic reasons and for evaporators that are used with ammonia as refrigerating fluid. Copper pipes are used in small evaporators and never with ammonia for attacking this refrigerant to copper in the presence of moisture.

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b) Evaporators of plates.- They are of two types:
- Of printed plates
- Of tube between plates

The evaporator of circular tubes between stamped plates is formed by a flat pipe, usually in zigzag, embedded between two metal sheets welded together at the ends.
If the space between the pipe and the plates is filled with a eutectic solution allows a
reserve capacity . This type of evaporator, with eutectic solution, is used in
momentarily large load applications , since the reserve of capacity stored in the solution allows the use of equipment of lower capacity
with an investment saving and operating cost.
Sheet plate evaporators are a variant of the tube plate type, and consist of
two corrugated metal plates, welded together, to which the most convenient form is given
so that by the same the cooling fluid circulates, thus forming an internal structure of
tubes in which the vaporization of the refrigerating liquid takes place.
This system is widely used in domestic refrigerators, forming the freezer; it is also
used the one formed by plates of greater size in conservative cabinets or in industrial exhibitors,
forming the own cabinet of storage.

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c) Dry system evaporators of tubes
and fins. - Dry system evaporators of tubes and fins, are systems generally used in the cooling of cabinets, chambers and furniture.
The tubes are made of copper and are shaped like a fork or zigzag, with fins, generally aluminum, of very varied shapes and sizes, providing a good thermal conductivity between them and depending on the conditions in which they are going to to work; the fins must have adequate spacing to facilitate air circulation between them and prevent the formation of frost, which would act as a thermal insulation.

 The fins evaporators are used in cases where there is little space. In that
case the addition of fins to the bare tube increases the overall effective surface of the evaporator which results
in an increase in heat transmitted per unit length of the tube. The fins on the outside of
the tubes increase the heat exchange in gas-cooling evaporators, so that the
evaporators with fins are frequent in air-cooling.
In case of liquid cooling the use of evaporators with
external fins is not suitable . Only in cases where the liquid velocity is very large, on the outside of the
evaporator it is desirable to use fins inside the tubes that conduct the refrigerant, mainly in the case of operating with halogenated refrigerants of low film coefficient.
The fin evaporators are very sensitive to the dirt that accumulates in the form of dust and
lint of the air adhering to the wet external surfaces which produces a rapid decrease of the speed of heat transmission.
Air convectors in forced convection are widely used, and have great advantages
due to its small size, compact form, easy installation, uniform temperature and a regulation of the degree of relative humidity, which is achieved by the inclination of the fan blades, placement of adjustable shutters at the exit of the air, or varying the speed of the motor that drives the compressor.

CLASSIFICATION OF EVAPORATORS ACCORDING TO THE PHASE IN WHICH
THE MATERIAL IS TO BE COOLED

a) For cooling solids
b) For cooling gases
c) For cooling liquids

a) Evaporators for cooling solids .- In solids cooling only
plate evaporators are used that take the heat of the material to be cooled by conduction. The reason
for using these types of evaporators is that they lend themselves well to shape their shape, in accordance with
that of the outer surface of the object to be cooled, maximizing the contact between both and avoiding, as far as possible, the formation of separate surface areas which decrease the heat exchange between the material to be cooled and the refrigerating fluid.

b) Evaporators for cooling of gases.- The most important application of evaporators
of this type is the cooling of the air in cold rooms and in air conditioning.
There are two fundamental subtypes:
- Natural convection
- Forced convection

Natural.- convection evaporators used in household refrigerators, cabinets
exposure of refrigerated, large storage rooms products, and generally in those
applications where low air speeds and a minimum product dehydration are desired
. The air circulation in the case of these evaporators is greatly influenced by the size and location of the evaporator. In small evaporators baffles are frequently used to ensure good air circulation. The insulation is placed to avoid condensations
that drip on the stored product.
- Forced Convection Evaporators.- They are formed by flap tubes, inside a chamber
metal and with one or more fans to achieve forced air circulation. Frost formation reduces the capacity of a forced convection evaporator whereby, when air is cooled below 1 ° C an automatic defrost system must be used.
c) Evaporators for cooling liquids.- There are a number of evaporators for cooling liquids, such as:
- Double tube
- Baudelot
- Tank
- Casing and coil
- Casing and tubes
- Atomizer
- Evaporator of double tube.- Consists of two concentric tubes; the refrigerant flows through the space between the two tubes, and the liquid to be cooled by the inner tube in countercurrent. This type of evaporator, which can be flooded or of dry expansion, allows a high heat transfer and is used in the wine and brewery industries, in compact commercial water chillers of small capacity, etc.
- Chiller Baudelot.- It can work flooded or in dry expansion; consists of a series of
horizontal tubes placed one underneath another, through which the refrigerating fluid circulates and, on the
outside, the liquid to be cooled circulates by gravity forming a thin film, falling from the
surface of the top tube to the bottom where it is collected in a collecting channel. This type of cooler is ideal for applications where the aeration of the liquid to be cooled is an important factor, and has the advantage of obtaining temperatures close to the freezing point, without danger of damaging the evaporator in the event of unforeseen freezing. It is used for the cooling of milk, wine, beer broth, and carbonated water in bottling plants.
- Tank cooler. - The tank cooler consists of a refrigerant fluid coil of
tube, installed inside a large tank containing the liquid to be cooled. This cooler is used in cases where sanitation is not an important factor, in applications of large and frequent fluctuations of the load, given its great inertia, and in applications where the liquid enters the cooler at relatively high temperatures . Much is used for cooling water, brine and other secondary coolant liquids.
- Cooler of casing and coil.- It is formed by one or more coils enclosed in
a steel casing, it can operate in two ways:
a) The refrigerating fluid passes in dry expansion through the coil and the liquid to be cooled circulates in the casing, this system having the advantage of its thermal inertia
b) It is used with the refrigerating fluid in the casing, in flooded regime, Fig II.33, and the liquid to be
cooled in the coil; this type is called an instant liquid cooler; it can not be used in applications where cooling below 3.5 ° C is required, given the danger of serious deterioration in the case of freezing.

Evaporator for cooling of liquid of carcasa and tubes.- It is the type that is more used, by:

Having a high efficiency
Requires little space
Its ease of maintenance
Its adaptability to almost all applications of liquid cooling

It consists of a cylindrical steel housing in which parallel horizontal straight tubes are arranged in place at the ends by perforated plates.

 - Atomizing evaporator.- It has a similar construction to the one of casing and tubes, with the difference
of existing on the upper tubes atomizing nozzles that send liquid refrigerant
on the tubes, through which interior the refrigerating fluid to cool, of which the fraction not evaporated is collected in the lower part of the cooler, from where it is pumped back to the nozzles.
The advantages of this type of cooler are:
Its high efficiency
The low load of necessary refrigerating fluid

Its disadvantages are:
the need for a recirculating liquid refrigerant pump and its high cost

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