In order to increase the safety of operation of the refrigeration unit, it is recommended that condensers, linear receivers and oil separators (devices high pressure) with a large amount of refrigerant should be placed outside the engine room.
This equipment, as well as receivers for storing refrigerant reserves, must be surrounded by a metal barrier with a lockable entrance. Receivers must be protected from sunlight and precipitation by a canopy. Apparatuses and vessels installed indoors can be located in a compressor shop or a special equipment room if it has a separate exit to the outside. Passage between smooth wall and the device must be at least 0.8 m, but it is allowed to install devices near walls without passages. The distance between the protruding parts of the devices must be at least 1.0 m, and if this passage is the main one - 1.5 m.
When mounting vessels and apparatus on brackets or cantilever beams, the latter must be embedded in the main wall to a depth of at least 250 mm.
Installation of devices on columns using clamps is allowed. It is prohibited to punch holes in columns to secure equipment.
For installation of devices and further maintenance of condensers and circulation receivers, metal platforms with fencing and stairs are installed. If the length of the platform is more than 6 m, there should be two stairs.
Platforms and stairs must have handrails and edges. The height of the handrails is 1 m, the edge is at least 0.15 m. The distance between the handrail posts is no more than 2 m.
Tests of apparatus, vessels and pipeline systems for strength and density are carried out upon completion installation work and within the time limits provided for by the “Rules for the Design and safe operation ammonia refrigeration units".

Horizontal cylindrical devices. Shell-and-tube evaporators, horizontal shell-and-tube condensers and horizontal receivers are installed on concrete foundations in the form of separate pedestals strictly horizontally with a permissible slope of 0.5 mm per 1 m linear length towards the oil sump.
The devices rest on antiseptic wooden beams at least 200 mm wide with a recess in the shape of the body (Fig. 10 and 11) and are attached to the foundation with steel belts with rubber gaskets.

Low-temperature devices are installed on beams with a thickness no less than the thickness of the thermal insulation, and under
Wooden blocks with a length of 50-100 mm and a height equal to the thickness of the insulation are placed in belts at a distance of 250-300 mm from each other around the circumference (Fig. 11).
To clean condenser and evaporator pipes from contamination, the distance between their end caps and walls should be 0.8 m on one side and 1.5-2.0 m on the other. When installing devices in a room to replace pipes of condensers and evaporators, a “false window” is installed (in the wall opposite the cover of the device). To do this, an opening is left in the building's masonry, which is filled thermal insulation material, sewed up with boards and plastered. When repairing devices, the “false window” is opened and restored upon completion of the repair. Upon completion of work on placing the devices, automation and control devices are installed on them, shut-off valves, safety valves.
The cavity of the apparatus for the refrigerant is purged with compressed air, and strength and density tests are carried out with the covers removed. When installing a condenser-receiver unit, a horizontal shell-and-tube condenser is installed on the platform above the linear receiver. The size of the site must ensure all-round maintenance of the device.

Vertical shell and tube condensers. The devices are installed outdoors on a massive foundation with a pit for draining water. When making the foundation, the bolts for securing the lower flange of the apparatus are placed in concrete. The condenser is installed with a crane on packs of pads and wedges. By tamping wedges, the apparatus is positioned strictly vertically using plumb lines located in two mutually perpendicular planes. In order to prevent the plumb lines from swinging by the wind, their weights are lowered into a container with water or oil. The vertical position of the apparatus is caused by the helical flow of water through its tubes. Even with a slight tilt of the device, water will not normally wash the surface of the pipes. Upon completion of the alignment of the apparatus, the linings and wedges are welded into bags and the foundation is poured.

Evaporative condensers. They are supplied assembled for installation and installed on a platform whose dimensions allow for all-round maintenance of these devices. ‘The height of the platform is taken into account the placement of linear receivers under it. For ease of maintenance, the platform is equipped with a ladder, and when top position For fans, it is additionally installed between the platform and the upper plane of the device.
After installing the evaporative condenser, connect it to circulation pump and pipelines.

The most widely used are evaporative condensers of the TVKA and Evako types produced by VNR. The drop-repellent layer of these devices is made of plastic, so welding and other work with open flames should be prohibited in the area where the devices are installed. Fan motors are grounded. When installing the device on a hill (for example, on the roof of a building), lightning protection must be used.

Panel evaporators. They are supplied as separate units and are assembled during installation work.

The evaporator tank is tested for leaks by pouring water and installed on concrete slab 300-400 mm thick (Fig. 12), the height of the underground part of which is 100-150 mm. Antiseptic wooden beams or railway sleepers and thermal insulation are laid between the foundation and the tank. Panel sections are installed in the tank strictly horizontally, level. Side surfaces The tank is insulated and plastered, and the mixer is adjusted.

Chamber devices. Wall and ceiling batteries are assembled from standardized sections (Fig. 13) at the installation site.

For ammonia batteries, sections of pipes with a diameter of 38X2.5 mm are used, for coolant - with a diameter of 38X3 mm. The pipes are finned with spirally wound fins made of 1X45 mm steel tape with fin spacing of 20 and 30 mm. The characteristics of the sections are presented in table. 6.

Total length of battery hoses in pumping schemes should not exceed 100-200 m. The battery is installed in the chamber using embedded parts fixed in the ceiling during the construction of the building (Fig. 14).

Battery hoses are placed strictly horizontally and level.

Ceiling air coolers are supplied assembled for installation. Bearing structures devices (channels) are connected to the channels of embedded parts. The horizontal installation of the devices is checked using the hydrostatic level.

Batteries and air coolers are lifted to the installation site by forklifts or other lifting devices. The permissible slope of the hoses should not exceed 0.5 mm per 1 m linear length.

To remove melt water during defrosting, they are installed drain pipes, on which heating elements of the ENGL-180 type are fixed. The heating element is a glass fiber tape based on metal heating cores made of an alloy with high resistivity. Heating elements they are wound onto the pipeline spirally or laid linearly, secured to the pipeline with glass tape (for example, tape LES-0.2X20). On the vertical section of the drain pipeline, heaters are installed only in a spiral manner. When laying linearly, the heaters are secured to the pipeline with glass tape in increments of no more than 0.5 m. After the heaters are secured, the pipeline is insulated with non-flammable insulation and sheathed with a protective metal sheath. In places where the heater has significant bends (for example, on flanges), an aluminum tape with a thickness of 0.2-1.0 mm and a width of 40-80 mm should be placed under it to avoid local overheating.

Upon completion of installation, all devices are tested for strength and density.

The MEL group of companies is a wholesale supplier of air conditioning systems to Mitsubishi Heavy Industries.

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Compressor-condensing units (CCU) for ventilation cooling are becoming increasingly common in the design of central cooling systems for buildings. Their advantages are obvious:

Firstly, this is the price of one kW of cold. Compared to chiller systems, cooling supply air with the help of KKB does not contain an intermediate coolant, i.e. water or non-freezing solutions, therefore it is cheaper.

Secondly, ease of regulation. One compressor-condenser unit operates for one air-conditioning unit, so the control logic is uniform and is implemented using standard air-conditioning unit control controllers.

Thirdly, simplicity installation of KKB for cooling the ventilation system. No additional air ducts, fans, etc. are needed. Only the evaporator heat exchanger is built in and that’s it. Even additional insulation of supply air ducts is often not required.

Rice. 1. KKB LENNOX and diagram of its connection to the air handling unit.

Against the backdrop of such remarkable advantages, in practice we come across many examples of air conditioning ventilation systems in which the air conditioning units either do not work at all or very quickly fail during operation. Analysis of these facts shows that the reason is often the incorrect selection of the air conditioning unit and the evaporator for cooling the supply air. Therefore, we will consider the standard methodology for selecting compressor-condenser units and try to show the mistakes that are made in this case.

INCORRECT, but the most common, method for selecting a KKB and evaporator for direct-flow air handling units

1. As initial data, we need to know the air flow air handling unit. Let's set 4500 m3/hour as an example.
2. The supply unit is direct-flow, i.e. no recirculation, operates on 100% outside air.
3. Let's determine the construction area - for example, Moscow. Calculated parameters of outdoor air for Moscow are +28C and 45% humidity. We take these parameters as initial parameters air at the inlet to the evaporator of the supply system. Sometimes the air parameters are taken “with a reserve” and set at +30C or even +32C.
4. Let us set the necessary air parameters at the outlet of the supply system, i.e. at the entrance to the room. Often these parameters are set 5-10C lower than the required supply air temperature in the room. For example, +15C or even +10C. We will focus on the average value of +13C.
5. Further using i-d charts(Fig. 2) we build the air cooling process in the ventilation cooling system. We determine the required cooling flow under given conditions. In our version, the required cooling flow is 33.4 kW.
6. We select the KKB according to the required cooling flow of 33.4 kW. There is a nearby large and a nearby smaller model in the KKB line. For example, for the manufacturer LENNOX these are models: TSA090/380-3 for 28 kW of cold and TSA120/380-3 for 35.3 kW of cold.

We accept a model with a reserve of 35.3 kW, i.e. TSA120/380-3.

And now we will tell you what will happen at the site when the air handling unit and the air handling unit we selected work together according to the method described above.

The first problem is the overestimated productivity of KKB.

The ventilation air conditioner is selected for outdoor air parameters of +28C and 45% humidity. But the customer plans to operate it not only when it’s +28C outside; the rooms are often already hot due to internal heat excess starting from +15C outside. Therefore, the controller sets the supply air temperature at best to +20C, and at worst even lower. KKB produces either 100% performance or 0% (with rare exceptions of smooth control when using VRF outdoor units in the form of KKB). When the outside (intake) air temperature decreases, the KKB does not reduce its performance (and in fact even slightly increases due to greater subcooling in the condenser). Therefore, when the air temperature at the inlet to the evaporator decreases, the KKB will tend to produce a lower air temperature at the outlet of the evaporator. Using our calculation data, the output air temperature is +3C. But this cannot be, because... The boiling point of freon in the evaporator is +5C.

Consequently, lowering the air temperature at the evaporator inlet to +22C and below, in our case, leads to an overestimated performance of the KKB. Next, the freon does not boil enough in the evaporator, the liquid refrigerant returns to the compressor suction and, as a result, the compressor fails due to mechanical damage.

But our problems, oddly enough, do not end there.

The second problem is a LOWERED EVAPORATOR.

Let's take a closer look at the selection of the evaporator. When selecting an air handling unit, specific parameters for the operation of the evaporator are set. In our case, this is the air temperature at the inlet +28C and humidity 45% and at the outlet +13C. Means? the evaporator is selected EXACTLY for these parameters. But what will happen when the air temperature at the evaporator inlet is, for example, not +28C, but +25C? The answer is quite simple if you look at the formula for heat transfer of any surfaces: Q=k*F*(Tv-Tf). k*F – heat transfer coefficient and heat exchange area will not change, these values ​​are constant. Tf - the boiling point of freon will not change, because it is also maintained at a constant +5C (in normal operation). But TV - the average air temperature has dropped by three degrees. Consequently, the amount of heat transferred will become less in proportion to the temperature difference. But KKB “does not know about this” and continues to provide the required 100% productivity. Liquid freon returns to the compressor suction again and leads to the problems described above. Those. the calculated evaporator temperature is MINIMUM operating temperature KKB.

Here you can object: “But what about the work of on-off split systems?” The design temperature in the splits is +27C in the room, but in fact they can operate up to +18C. The fact is that in split systems the surface area of ​​the evaporator is selected with a very large margin, at least 30%, just to compensate for the decrease in heat transfer when the temperature in the room drops or the fan speed of the indoor unit decreases. And finally,

Problem three – selection of KKB “With RESERVE”...

The productivity reserve when selecting a KKB is extremely harmful, because The reserve is liquid freon at the compressor suction. And in the end we have a jammed compressor. In general, the maximum evaporator capacity should always be greater than the compressor capacity.

We will try to answer the question - how to choose the CORRECT KKB for supply systems?

Firstly, it is necessary to understand that the source of cold in the form of a compressor-condensing unit cannot be the only one in the building. Conditioning the ventilation system can only remove part of the peak load entering the room from ventilation air. And in any case, maintaining a certain temperature inside the room falls on local closers ( indoor units VRF or fan coil units). Therefore, the KKB should not maintain a certain temperature when cooling the ventilation (this is impossible due to on-off regulation), but should reduce heat input into the premises when a certain outside temperature is exceeded.

Example of a ventilation and air conditioning system:

Initial data: Moscow city with design parameters for air conditioning +28C and 45% humidity. Supply air flow 4500 m3/hour. Excess heat in the room from computers, people, solar radiation, etc. are 50 kW. Estimated room temperature +22C.

The air conditioning capacity must be selected in such a way that it is sufficient under the worst conditions (maximum temperatures). But ventilation air conditioners should also work without problems even with some intermediate options. Moreover, most of the time, ventilation air conditioning systems operate just at 60-80% load.

• We set the calculated temperature of the external air and the calculated temperature of the internal air. Those. the main task KKB – cooling of supply air to room temperature. When the outside air temperature is less than the required indoor air temperature, the KKB DOES NOT TURN ON. For Moscow, from +28C to the required room temperature of +22C, we get a temperature difference of 6C. In principle, the temperature difference across the evaporator should not be more than 10C, because the supply air temperature cannot be less than the boiling point of freon.

• We determine the required performance of the KKB based on the conditions for cooling the supply air from the design temperature of +28C to +22C. The result was 13.3 kW of cold (i-d diagram).

• We select 13.3 KKB from the line of the popular manufacturer LENNOX according to the required performance. We select the nearest SMALLER KKB TSA036/380-3с with a productivity of 12.2 kW.

• We select the supply evaporator from the worst parameters for it. This is the outside air temperature equal to the required indoor temperature - in our case +22C. The cold productivity of the evaporator is equal to the productivity of the KKB, i.e. 12.2 kW. Plus a performance reserve of 10-20% in case of contamination of the evaporator, etc.

• We determine the temperature of the supply air at an outside temperature of +22C. we get 15C. Above the boiling point of freon +5C and above the dew point temperature +10C, this means that insulation of the supply air ducts does not need to be done (theoretically).

• We determine the remaining excess heat in the premises. It turns out 50 kW of internal heat excess plus a small part from the supply air 13.3-12.2 = 1.1 kW. Total 51.1 kW – calculated performance for local control systems.

Conclusions: The main idea that I would like to draw attention to is the need to calculate the compressor-condenser unit not on maximum temperature outside air, and to the minimum within the operating range of the ventilation air conditioner. Calculation of the KKB and evaporator carried out for the maximum supply air temperature leads to the fact that normal operation will only occur in the range of external temperatures from the design temperature and above. And if the outside temperature is lower than the calculated one, there will be incomplete boiling of freon in the evaporator and the return of liquid refrigerant to the compressor suction.

One of the most important elements For vapor compression machine is . It performs the main process of the refrigeration cycle - selection from the cooled environment. Other elements of the refrigeration circuit, such as the condenser, expansion device, compressor, etc., only ensure reliable operation of the evaporator, so it is the choice of the latter that must be given due attention.

It follows from this that when selecting equipment for a refrigeration unit, it is necessary to start with the evaporator. Many novice repairmen often make a typical mistake and start completing the installation with a compressor.

In Fig. Figure 1 shows a diagram of the most common vapor compression refrigeration machine. Its cycle, specified in coordinates: pressure R And i. In Fig. 1b points 1-7 of the refrigeration cycle is an indicator of the state of the refrigerant (pressure, temperature, specific volume) and coincides with the same in Fig. 1a (functions of state parameters).

Rice. 1 – Diagram and in coordinates of a conventional vapor compression machine: RU expansion device, Pk– condensation pressure, Ro– boiling pressure.

Graphic representation fig. 1b shows the state and functions of the refrigerant, which vary depending on pressure and enthalpy. Line segment AB on the curve in Fig. 1b characterizes the refrigerant in the state of saturated vapor. Its temperature corresponds to the starting point of boiling. The refrigerant vapor fraction is 100%, and superheat is close to zero. To the right of the curve AB the refrigerant has a state (the temperature of the refrigerant is greater than the boiling point).

Dot IN is critical for a given refrigerant, since it corresponds to the temperature at which the substance cannot go into a liquid state, no matter how high the pressure is. On the section BC, the refrigerant has the state of a saturated liquid, and on the left side - a supercooled liquid (the refrigerant temperature is less than the boiling point).

Inside the Curve ABC the refrigerant is in the state of a vapor-liquid mixture (the proportion of vapor per unit volume is variable). The process occurring in the evaporator (Fig. 1b) corresponds to the segment 6-1 . The refrigerant enters the evaporator (point 6) in the state of a boiling vapor-liquid mixture. In this case, the share of steam depends on the specific refrigeration cycle and is 10-30%.

At the exit from the evaporator, the boiling process may not be completed, period 1 may not coincide with the point 7 . If the temperature of the refrigerant at the outlet of the evaporator is higher than the boiling point, then we get an overheated evaporator. Its size ΔToverheat represents the difference between the temperature of the refrigerant at the outlet of the evaporator (point 1) and its temperature at the saturation line AB (point 7):

ΔToverheat=T1 – T7

If points 1 and 7 coincide, then the refrigerant temperature is equal to the boiling point, and the superheat ΔToverheat will be equal to zero. Thus, we get a flooded evaporator. Therefore, when choosing an evaporator, you first need to make a choice between a flooded evaporator and an overheated evaporator.

Note that, under equal conditions, a flooded evaporator is more advantageous in terms of the intensity of the heat extraction process than with overheating. But it should be taken into account that at the outlet of the flooded evaporator the refrigerant is in a state of saturated vapor, and it is impossible to supply a humid environment to the compressor. Otherwise, there is a high probability of water hammer occurring, which will be accompanied by mechanical destruction of compressor parts. It turns out that if you choose a flooded evaporator, then it is necessary to provide additional protection for the compressor from saturated steam entering it.

If you give preference to an evaporator with overheating, then you do not need to worry about protecting the compressor and getting saturated steam into it. The likelihood of water hammer occurring will only occur if the superheat value deviates from the required value. Under normal operating conditions of a refrigeration unit, the amount of superheat ΔToverheat should be within 4-7 K.

When the superheat indicator decreases ΔToverheat, the intensity of heat extraction from the environment increases. But at extremely low values ΔToverheat(less than 3K) there is a possibility of wet steam entering the compressor, which can cause water hammer and, consequently, damage to the mechanical components of the compressor.

Otherwise, with a high reading ΔToverheat(more than 10 K), this indicates that insufficient refrigerant is entering the evaporator. The intensity of heat extraction from the cooled medium sharply decreases and the thermal conditions of the compressor worsen.

When choosing an evaporator, another question arises related to the boiling point of the refrigerant in the evaporator. To solve this, it is first necessary to determine what temperature of the cooled medium should be ensured for normal operation of the refrigeration unit. If air is used as the cooled medium, then in addition to the temperature at the outlet of the evaporator, it is also necessary to take into account the humidity at the outlet of the evaporator. Now let us consider the behavior of the temperatures of the cooled medium around the evaporator during operation of a conventional refrigeration unit (Fig. 1a).

In order not to delve into this topic, we will neglect the pressure losses on the evaporator. We will also assume that the heat exchange occurring between the refrigerant and environment carried out according to a direct-flow scheme.

In practice, such a scheme is not often used, since in terms of heat transfer efficiency it is inferior to a counterflow scheme. But if one of the coolants has a constant temperature, and the overheating readings are small, then forward flow and counter flow will be equivalent. It is known that the average temperature difference does not depend on the flow pattern. Consideration direct-flow circuit will provide us with a more visual understanding of the heat exchange that occurs between the refrigerant and the cooled environment.

First, let's introduce the virtual quantity L, equal to the length heat exchange device (condenser or evaporator). Its value can be determined from the following expression: L=W/S, Where W– corresponds to the internal volume of the heat exchange device in which the refrigerant circulates, m3; S– heat exchange surface area m2.

If we are talking about a refrigeration machine, then the equivalent length of the evaporator is almost equal to the length of the tube in which the process takes place 6-1 . Therefore her outside surface washed by a cooled environment.

First, let's pay attention to the evaporator, which acts as an air cooler. In it, the process of removing heat from the air occurs as a result of natural convection or with the help of forced blowing of the evaporator. Note that in modern refrigeration units the first method is practically not used, since air cooling by natural convection is ineffective.

Thus, we will assume that the air cooler is equipped with a fan, which provides forced air flow to the evaporator and is a tubular-fin heat exchanger (Fig. 2). Its schematic representation is shown in Fig. 2b. Let's consider the main quantities that characterize the blowing process.

Temperature difference

The temperature difference across the evaporator is calculated as follows:

ΔT=Ta1-Ta2,

Where ΔTa is in the range from 2 to 8 K (for tubular-fin evaporators with forced air flow).

In other words, during normal operation of the refrigeration unit, the air passing through the evaporator must be cooled not lower than 2 K and not higher than 8 K.

Rice. 2 – Scheme and temperature parameters air cooling on the air cooler:

Ta1 And Ta2– air temperature at the inlet and outlet of the air cooler;

• FF– refrigerant temperature;
• L– equivalent length of the evaporator;
• That– boiling point of the refrigerant in the evaporator.

Maximum temperature difference

The maximum temperature pressure of air at the evaporator inlet is determined as follows:

DTmax=Ta1 – To

This indicator is used when selecting air coolers, since foreign manufacturers refrigeration equipment provides evaporator cooling capacity values Qsp depending on size DTmax. Let's consider the method for selecting an air cooler for a refrigeration unit and determine the calculated values DTmax. To do this, let us give as an example generally accepted recommendations for selecting the value DTmax:

• For freezers DTmax is within 4-6 K;
• for storage rooms for unpackaged products – 7-9 K;
• for storage rooms for hermetically packaged products – 10-14 K;
• for air conditioning units – 18-22 K.

Degree of steam superheat at the evaporator outlet

To determine the degree of steam superheat at the outlet of the evaporator, use the following form:

F=ΔToverload/DTmax=(T1-T0)/(Ta1-T0),

Where T1– temperature of the refrigerant vapor at the outlet of the evaporator.

This indicator is practically not used in our country, but in foreign catalogs it is stipulated that the cooling capacity readings of air coolers Qsp corresponds to the value F=0.65.

During operation the value F It is customary to take from 0 to 1. Let us assume that F=0, Then ΔТoverload=0, and the refrigerant leaving the evaporator will be in the state of saturated vapor. For this air cooler model, the actual cooling capacity will be 10-15% greater than the figure given in the catalog.

If F>0.65, then the cooling capacity for a given model of air cooler must be less than the value given in the catalog. Let's assume that F>0.8, then the actual performance for this model will be 25-30% greater value given in the catalogue.

If F->1, then the evaporator cooling capacity Quse->0(Fig. 3).

Fig. 3 – dependence of the evaporator cooling capacity Qsp from overheating F

The process depicted in Fig. 2b is also characterized by other parameters:

• arithmetic mean temperature difference DTsr=Tasr-T0;
• average temperature of the air that passes through the evaporator Tasp=(Ta1+Ta2)/2;
• minimum temperature difference DTmin=Ta2-To.

Rice. 4 – Diagram and temperature parameters showing the process of water cooling on the evaporator:

Where Te1 And Te2 water temperature at the evaporator inlets and outlets;

• FF – coolant temperature;
• L – equivalent length of the evaporator;
• T is the boiling point of the refrigerant in the evaporator.

Evaporators in which the cooling medium is liquid have the same temperature parameters as for air coolers. The numerical values ​​of the cooled liquid temperatures that are necessary for the normal operation of the refrigeration unit will be different than the corresponding parameters for air coolers.

If the temperature difference across the water ΔTe=Te1-Te2, then for shell-and-tube evaporators ΔTe should be maintained in the range of 5±1 K, and for plate evaporators the indicator ΔTe will be within 5±1.5 K.

Unlike air coolers, in liquid coolers it is necessary to maintain not a maximum, but a minimum temperature pressure DTmin=Te2-To– the difference between the temperature of the cooled medium at the outlet of the evaporator and the boiling point of the refrigerant in the evaporator.

For shell-and-tube evaporators, the minimum temperature difference is DTmin=Te2-To should be maintained within 4-6 K, and for plate evaporators - 3-5 K.

The specified range (the difference between the temperature of the cooled medium at the outlet of the evaporator and the boiling point of the refrigerant in the evaporator) must be maintained for the following reasons: as the difference increases, the cooling intensity begins to decrease, and as it decreases, the risk of freezing of the cooled liquid in the evaporator increases, which can cause its mechanical failure. destruction.

Evaporator design solutions

Regardless of the method of use of various refrigerants, the heat exchange processes occurring in the evaporator are subject to the main technological cycle refrigeration-consuming production, according to which they are created refrigeration units and heat exchangers. Thus, in order to solve the problem of optimizing the heat exchange process, it is necessary to take into account the conditions rational organization technological cycle of refrigeration-consuming production.

As is known, cooling of a certain environment is possible using a heat exchanger. His constructive solution should be selected according to the technological requirements that apply to these devices. Especially important point is the compliance of the device with the technological process of thermal treatment of the environment, which is possible under the following conditions:

• maintaining a given temperature of the working process and control (regulation) over temperature conditions;
• selection of device material, according to chemical properties environment;
• control over the length of time the medium remains in the device;
• correspondence of operating speeds and pressure.

Another factor on which the economic rationality of the device depends is productivity. First of all, it is influenced by the intensity of heat exchange and compliance with the hydraulic resistance of the device. These conditions may be met under the following circumstances:

• ensuring the necessary speed of working media to implement turbulent conditions;
• creating the most suitable conditions for removing condensate, scale, frost, etc.;
• Creation favorable conditions for the movement of working media;
• preventing possible contamination of the device.

Other important requirements are also light weight, compactness, simplicity of design, as well as ease of installation and repair of the device. To comply with these rules, factors such as the configuration of the heating surface, the presence and type of partitions, the method of placing and fastening the tubes in the tube sheets should be taken into account, dimensions, arrangement of chambers, bottoms, etc.

The ease of use and reliability of the device is influenced by factors such as the strength and tightness of detachable connections, compensation for temperature deformations, and ease of maintenance and repair of the device. These requirements form the basis for the design and selection of a heat exchange unit. The main role in this is to ensure the required technological process in refrigeration production.

In order to choose the right design solution for the evaporator, you must be guided by the following rules. 1) cooling of liquids is best done using a rigid tubular heat exchanger or a compact one plate heat exchanger; 2) the use of tubular-fin devices is due to the following conditions: the heat transfer between the working media and the wall on both sides of the heating surface is significantly different. In this case, the fins must be installed on the side with the lowest heat transfer coefficient.

To increase the intensity of heat exchange in heat exchangers, it is necessary to adhere to the following rules:

• ensuring proper conditions for condensate removal in air coolers;
• reducing the thickness of the hydrodynamic boundary layer by increasing the speed of movement of the working fluids (installation of inter-tube partitions and dividing the tube bundle into passages);
• improving the flow of working fluids around the heat exchange surface (the entire surface should actively participate in the heat exchange process);
• compliance with basic temperature indicators, thermal resistances, etc.

By analyzing individual thermal resistances, you can choose the most the best way increase the intensity of heat exchange (depending on the type of heat exchanger and the nature of the working fluids). In a liquid heat exchanger, it is rational to install transverse partitions only with several strokes in the pipe space. During heat exchange (gas with gas, liquid with liquid), the amount of liquid flowing through the inter-tube space can be extremely large, and, as a result, the speed indicator will reach the same limits as inside the tubes, which is why the installation of partitions will be irrational.

Improving heat exchange processes is one of the main processes for improving heat exchange equipment refrigeration machines. In this regard, research is being carried out in the fields of energy and chemical engineering. This is the study of the regime characteristics of the flow, turbulization of the flow by creating artificial roughness. In addition, new heat exchange surfaces are being developed, which will make heat exchangers more compact.

Choosing a rational approach for calculating the evaporator

When designing an evaporator, structural, hydraulic, strength, thermal and technical and economic calculations should be carried out. They are performed in several versions, the choice of which depends on performance indicators: technical and economic indicators, efficiency, etc.

To perform a thermal calculation of a surface heat exchanger, it is necessary to solve the equation and heat balance, taking into account certain operating conditions of the device ( design dimensions heat transfer surfaces, temperature change limits and patterns regarding the movement of the cooling and cooled medium). To find a solution to this problem, you need to apply rules that will allow you to obtain results from the original data. But due to numerous factors, it is impossible to find a general solution for different heat exchangers. At the same time, there are many methods for approximate calculations that are easy to perform manually or by machine.

Modern technologies allow you to select an evaporator using special programs. They are mainly provided by manufacturers of heat exchange equipment and allow you to quickly select the required model. When using such programs, it is necessary to take into account that they assume the operation of the evaporator under standard conditions. If actual conditions differ from standard conditions, the evaporator performance will be different. Thus, it is advisable to always carry out verification calculations of the evaporator design you have chosen, relative to its actual operating conditions.

Many repairmen often ask us the following question: “Why in your circuits is power supply Eg always supplied to the evaporator from above; is this a mandatory requirement when connecting evaporators?” This section brings clarity to this issue.
A) A little history
We know that when the temperature in the cooled volume decreases, the boiling pressure drops at the same time, since the overall temperature difference remains almost constant (see section 7. “Influence of the temperature of the cooled air”).

Several years ago, this property was often used in refrigeration trade equipment in chambers with a positive temperature to stop compressors when the temperature of the refrigeration chamber has reached the required value.
This property technology:
had two pre-
LP regulator
Pressure regulation
Rice. 45.1.
Firstly, it made it possible to do without a master thermostat, since the LP relay performed a dual function - a master and safety relay.
Secondly, to ensure defrosting of the evaporator during each cycle, it was enough to configure the system so that the compressor starts at a pressure corresponding to a temperature above 0 ° C, and thus save on the defrost system!
However, when the compressor stopped, in order for the boiling pressure to exactly correspond to the temperature in the refrigerator compartment, a constant presence of liquid in the evaporator was required. That is why at that time evaporators were often fed from below and were always half filled with liquid refrigerant (see Fig. 45.1).
Nowadays, pressure regulation is used quite rarely, since it has the following negative points:
If the condenser is air cooled (most common case), condensation pressure varies greatly throughout the year (see section 2.1. "Condensers with air cooled. Normal operation"). These changes in condensing pressure necessarily lead to changes in evaporation pressure and therefore changes in the total temperature drop across the evaporator. Thus, the temperature in the refrigerator compartment cannot be maintained stable and will be subject to large changes. Therefore, it is necessary either to use water-cooled condensers cooling, or use effective system stabilization of condensation pressure.
If even small anomalies occur in the operation of the installation (in terms of boiling or condensation pressures), leading to a change in the total temperature difference across the evaporator, even a slight one, the temperature in the refrigeration chamber can no longer be maintained within the specified limits.

If the compressor discharge valve is not tight enough, then when the compressor stops, the boiling pressure increases rapidly and there is a danger of increasing the frequency of the compressor start-stop cycles.

This is why the temperature sensor in the refrigerated volume is most often used today to shut down the compressor, and the LP relay performs only protection functions (see Fig. 45.2).

Note that in this case, the method of feeding the evaporator (from below or from above) has almost no noticeable effect on the quality of regulation.

B) Design of modern evaporators

As the cooling capacity of evaporators increases, their dimensions, in particular the length of the tubes used for their manufacture, also increase.
So, in the example in Fig. 45.3, the designer, to obtain a performance of 1 kW, must connect two sections of 0.5 kW each in series.
But such technology has limited applications. Indeed, when the length of the pipelines doubles, the pressure loss also doubles. That is, the pressure losses in large evaporators quickly become too large.
Therefore, as power increases, the manufacturer no longer arranges the individual sections in series, but connects them in parallel in order to keep pressure losses as low as possible.
However, this requires that each evaporator be supplied with strictly the same amount of liquid, and therefore the manufacturer installs a liquid distributor at the inlet to the evaporator.

3 evaporator sections connected in parallel
Rice. 45.3.
For such evaporators, the question of whether to power them from below or from above is no longer worth it, since they are powered only through a special liquid distributor.
Now let's look at ways to customize pipelines for various types of evaporators.

To begin with, as an example, let's take a small evaporator, the low performance of which does not require the use of a liquid distributor (see Fig. 45.4).

The refrigerant enters the evaporator inlet E and then descends through the first section (bends 1, 2, 3). It then rises in the second section (bends 4, 5, 6 and 7) and, before leaving the evaporator at its outlet S, descends again through the third section (bends 8, 9, 10 and 11). Note that the refrigerant falls, rises, then falls again, and moves towards the direction of movement of the cooled air.
Let us now consider an example of a more powerful evaporator, which is of considerable size and is powered by a liquid distributor.

Each fraction of the total refrigerant flow enters the inlet of its section E, rises in the first row, then falls in the second row and leaves the section through its outlet S (see Fig. 45.5).
In other words, the refrigerant rises and then falls in the pipes, always moving against the direction of the cooling air. So, whatever the type of evaporator, the refrigerant alternates between falling and rising.
Consequently, the concept of an evaporator being fed from above or from below does not exist, especially for the most common case, when the evaporator is fed through a liquid distributor.

On the other hand, in both cases we saw that air and refrigerant move according to the countercurrent principle, that is, towards each other. It is useful to recall the reasons for choosing such a principle (see Fig. 45.6).

Pos. 1: This evaporator is powered by an expansion valve, which is configured to provide 7K superheat. To ensure such superheating of the vapor leaving the evaporator, it serves specific area length of the evaporator pipeline, blown with warm air.
Pos. 2: It's about about the same area, but with the direction of air movement coinciding with the direction of movement of the refrigerant. It can be stated that in this case, the length of the pipeline section providing superheating of the vapor increases, since it is blown with colder air than in the previous case. This means that the evaporator contains less liquid, therefore the expansion valve is more closed, that is, the boiling pressure is lower and the cooling capacity is lower (see also section 8.4. “Thermostatic expansion valve. Exercise”).
Pos. 3 and 4: Although the evaporator is powered from below, and not from above, as in pos. 1 and 2, the same phenomena are observed.
Thus, although most examples of direct expansion evaporators discussed in this manual are top-fed, this is done solely for the sake of simplicity and clarity of presentation. In practice, the refrigeration installer will almost never make a mistake in connecting the liquid distributor to the evaporator.
In the event that you have doubts, if the direction of air flow through the evaporator is not very clearly indicated, in choosing the method of connecting the piping to the evaporator, strictly follow the manufacturer's instructions in order to achieve the cooling performance declared in the evaporator documentation.