In the evaporator there is a process of transition of refrigerant from a liquid phase state into a gaseous with one and the same pressure, the pressure within the evaporator is the same everywhere. In the process of transition of a substance from liquid into gaseous (its bumping) in the evaporator - the evaporator absorbs heat as opposed to the condenser, which highlights heat into the environment. so Through two heat exchangers, the heat exchange process is occurred between two substances: a cooled substance that is around the evaporator and the outer air, which is around the condenser.

Liquid Freon Movement Scheme

The solenoid valve - overlaps or opens the refrigerant feed to the evaporator, always either completely open or completely closed (maybe absent in the system)

Thermostatic valve (TRV) is accurate deviceregulating the refrigerant supply to the evaporator depending on the intensity of the refrigerant boiling in the evaporator. It prevents liquid refrigerant to enter the compressor.

The liquid freon comes on TRV, the refrigerant throttle occurs through the membrane to TRV (Freon is sprayed) and begins to boil due to the pressure drop, gradually drops are converted to gas, on the entire site of the evaporator pipeline. Starting with a throttling device of the TRV, the pressure remains constant. Freon continues to boil and on a certain segment of the evaporator completely turns into gas and further, passing by the evaporator of the gas, begins to heat the air, which is in the chamber.

If, for example, the boiling point of freon -10 ° C, the temperature in the chamber is +2 ° C, the freon turns into the gas in the evaporator begins to heat up and at the output of the evaporator, its temperature should be -3, -4 ° C, thus Δt ( The difference between the boiling temperature of the refrigerant and the gas temperature at the output of the evaporator) must be \u003d 7-8, this is normal operation mode. With this Δt, we will know that at the exit from the evaporator there will be no particles of non-swollen freon (they should not be) if boiling will occur in the pipe, then it means not all power is used to cool the substance. The pipe is thermally insulated so that the freon does not heat up to the ambient temperature, because The refrigerant gas is cooled by a compressor stator. If there is still a liquid freon in the pipe, it means that the dose of feeding it into the system is too big, or the evaporator is supplied weak (short).

If Δt is less than 7, the evaporator is filled with freon, it does not have time to throw out and the system works incorrectly, the compressor is also poured with liquid freon and fails. In the biggest side, the overheating is not as dangerous than overheating in a smaller side, when Δt ˃ 7 may occur the stator of the compressor, but a small excess of overheating may not be felt in any way the compressor and when working it is preferable.

With the help of fans that are in the air cooler, a cold is removed from the evaporator. If it did not happen, the tubes were covered with ice and at the same time the refrigerant would achieve the temperature of his saturation at which it ceases to boil, and further, even regardless of the pressure drop in the evaporator, Freon would have fallen liquid without evaporated, pouring a compressor.

Group of Companies "MEL" - Wholesale provider of air conditioning systems Mitsubishi Heavy Industries.

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Compressor capacitor blocks (CKB) for cooling ventilation are increasingly distributed when designing central cooling systems of buildings. The benefits of their obvious:

First, this is the price of one kW cold. Compared to chiller systems, the cooling air cooling with the KKB does not contain an intermediate coolant, i.e. Water or non-freezing solutions, so it costs cheaper.

Secondly, the convenience of regulation. One compressor condenser unit operates on one aircraft installation, so the control logic is united and is implemented using standard controllers of the supply settings.

Thirdly, ease of installation of the KKB for cooling the ventilation system. No additional air ducts, fans, etc. Only the evaporator heat exchanger is embedded and that's it. Even additional insulation of air ducts is often not required.

Fig. 1. CKB LENNOX and a circuit of its connection to the supply unit.

Against the background of such wonderful advantages in practice, we face a variety of examples of air conditioning system of ventilation, in which the KKB is either not working at all, or in the process of work very quickly fail. Analysis of these facts shows that often the reason in the improper selection of the KKB and the evaporator for cooling the supply air. Therefore, we consider the standard method of selection of compressor condenser aggregates and try to show the errors that are allowed.

Incorrect, but most common, methods for selecting the KKB and evaporator for direct flow of supply units

1. As the source data, we need to know air flow supply installation. Set for example 4500 m3 / hour.
2. Impact installation direct-flow, i.e. Without recycling, it works for 100% outdoor air.
3. We define the construction area - for example, Moscow. The calculated parameters of the outer air for Moscow + 28c and 45% humidity. These parameters accept for the initial air parameters at the entrance to the evaporator supply system. Sometimes air parameters take "with a margin" and set + 30s or even + 32c.
4. Set the necessary air parameters at the output from the supply system, i.e. At the entrance to the room. Often, these parameters are set to 5-10 ° C lower than the required temperature of the supply air indoor. For example, + 15c or even + 10c. We will focus on the average value of + 13c.
5. Further S. i-D help Charts (Fig. 2) We build the air cooling process in the ventilation cooling system. Determine the required cold consumption in specified conditions. In our embodiment, the required cost of cold is 33.4 kW.
6. We select KKB at the required cost of cold 33.4 kW. There is a large and nearest smaller model in the KKB lineup. For example, for the manufacturer Lennox, these are models: TSA090 / 380-3 by 28 kW cold and TSA120 / 380-3 by 35.3 kW 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 on the facility, when joint work Supply installation and chosen by us by the CKB according to the described method.

The first problem is the overestimated CKB performance.

Air conditioner ventilation is selected to the parameters of the outer air + 28c and 45% humidity. But the customer plans to exploit it not only when on the street + 28c, in the premises it is often hot due to internal insoles from + 15 ° C on the street. Therefore, the controller establishes the temperature of the supply air in best case + 20С, and at worst even lower. The KKB issues either 100% of performance, or 0% (with rare exceptions for smooth control using external VRF units in the form of KKB). KKB When decreasing the temperature of the outer (elevated) air, its performance does not reduce its performance (and in fact even slightly increases due to greater overcooling in the condenser). Therefore, with a decrease in the air temperature at the entrance to the evaporator, the KKB will strive to produce and less air temperature at the outlet of the evaporator. At our calculations, the air temperature is obtained at the output of + 3c. But this can not be, because Freon boiling point in evaporator + 5C.

Consequently, a decrease in the air temperature at the inlet to the evaporator to + 22c and below, in our case leads to an overestimated Performance of the CKB. Next, there is nohaparation of freon in the evaporator, the return of the liquid refrigerant to absorb the compressor and, as a result, the output of the compressor is due to mechanical damage.

But on this, our problems, oddly enough, do not end.

The problem of the second is an understated evaporator.

Let's look carefully at the selection of the evaporator. When selecting the supply installation, specific parameters of the evaporator are specified. In our case, this air temperature at the inlet + 28c and humidity is 45% and at the output + 13c. So? The evaporator is selected precisely on these parameters. But what will happen when the air temperature at the entrance to the evaporator will be, for example, not + 28c, and + 25c? It is easy enough to answer if you look at the heat transfer formula of any surfaces: q \u003d k * f * (TB-TF). K * F - The heat transfer coefficient and the heat exchange area will not change, these values \u200b\u200bare constant. TF - the boiling point of Freon will not change, because It is also supported by constant + 5C (in normal operation). But TV - average temperature The air has become less than three degrees. Consequently, the amount of transmitted heat will become less proportional to the temperature difference. But the KKB "doesn't know about it" and continues to give out 100% performance. Liquid freon returns to the absorption of the compressor and leads to the problems described above. Those. The estimated evaporator temperature is the minimum working temperature of the KKB.

Here you can argue - "But what about the work of Offs Split systems?" Calculated temperature in Split + 27c indoors, and in fact they can work up to + 18c. The fact is that in split systems, the surface area of \u200b\u200bthe evaporator is selected with a very large margin, at least 30%, just to compensate for the heat transfer reduction when the temperature is reduced indoors or reduce the velocity of the inner block fan. And finally,

The problem is the third - the selection of the KKB "With the stock" ...

Reserve for performance during the selection of the KKB is extremely harmful, because The stock is liquid freon at the compressor suction. And in the final we have a crack compressor. In general, the maximum performance of the evaporator should always be greater than the performance of the compressor.

We will try to answer the question - how to properly pick up the CKB for the supply systems?

First, it is necessary to understand the fact that the source of the cold in the form of a compressor-capacitor unit cannot be the only one in the building. Air conditioning system of ventilation can only remove part of the peak load entering the room with ventilation air. And to support a certain temperature indoors in any case falls on local closers (VRF internal blocks or fanoxes). Therefore, the CKB should not maintain a certain temperature when cooling the ventilation (this is not possible due to the control of the regulation), and reduce the heat gain into the room when a certain outdoor temperature is exceeded.

An example of air conditioning ventilation system:

Initial data: Moscow city with calculated parameters for air conditioning + 28c and 45% humidity. Consumption of supply air 4500 m3 / hour. Heat inside of the room from computers, people, solar radiation, etc. Make up 50 kW. Calculated temperature in premises + 22c.

Air conditioning should be seen in such a way that it is enough for worst conditions (maximum temperatures). But also ventilation conditioners should work without any problems and with some intermediate versions. Moreover, most of the time of the ventilation air conditioning system work just when loading 60-80%.

• We specify the calculated outdoor air temperature and the calculated temperature of the internal. Those. The main task of the KKB is the cooling of the supply air to the room temperature. When the outdoor temperature is less than the desired air temperature in the room - the CKB does not turn on. For Moscow from + 28C to the desired temperature indoor + 22c, we obtain the difference in temperatures 6C. In principle, the temperature difference on the evaporator should not be more than 10s, because The temperature of the supply air 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 calculated temperature + 28c to + 22c. It turned out 13.3 kW cold (I-D diagram).

• We select at the required performance of 13.3 KKB from the line of the popular Lennox manufacturer. We select the nearest smaller KKB TSA.036/380-3C. capacity of 12.2 kW.

• We select the supplement evaporator from the worst parameters for it. This is the outdoor temperature equal to the desired temperature in the room - in our case + 22c. The performance of the evaporator on cold is equal to the performance of the CKB, i.e. 12.2 kW. Plus reserve in terms of performance 10-20% in case of contamination of the evaporator, etc.

• Determine the temperature of the supply air at an outdoor temperature + 22c. We get 15c. Above the boiling point of Freon + 5C and above the temperature of the dew point + 10c, it means that the insulation of the air ducts can not be made (theoretically).

• Determine the remaining inside of the premises. It turns out 50 kW of internal insoles plus a small part of the air supply air 13.3-12.2 \u003d 1.1 kW. Total 51.1 kW - calculated performance for local regulatory systems.

Conclusions: The main idea to which I would like to pay attention is the need for calculating the compressor condenser block not at the maximum temperature of the outer air, but to the minimum in the range of operation of the air conditioner ventilation. The calculation of the KKB and the evaporator, carried out at the maximum temperature of the supply air leads to the fact that normal operation will be only with the range of outer temperatures from the calculated and higher. And if the temperature outside is below the calculated - there will be incomplete boiling of freon in the evaporator and the refund of the liquid refrigerant to absorb the compressor.

One of the most important elements for parokompassing machine is an . He performs the main process refrigeration cycle - Selection from the cooled environment. Other elements of the refrigeration circuit, such as a condenser, expansion device, compressor, etc., only ensure the reliable operation of the evaporator, so it is precisely the choice of the latter to pay due attention.

It follows from this that by selecting the equipment for the refrigeration unit, it is necessary to start with the evaporator. Many beginner repairmen often admit typical mistake And the installation is started with the compressor.

In fig. 1 shows the scheme of the most ordinary parokompression refrigeration machine. Its cycle specified in the coordinates: Pressure R and i.. In fig. 1B point 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 (status parameter functions).

Fig. 1 - diagram and in coordinates of the usual parokompression machine: RU expansion device RK - condensation pressure, RO - Pressure boiling.

Graphic image Fig. 1B displays the condition and functions of the refrigerant, which varies depending on the pressure and enthalpy. Section AU On the curve Fig. 1b characterizes the refrigerant in a saturated pair state. Its temperature corresponds to a boiling start temperature. The fraction of the refrigerant in the refrigerant is 100%, and overheating is close to zero. On the right side of the curve AU The refrigerant has a condition (refrigerant temperature more temperature boiling).

Point IN is critical for this refrigerant, since it corresponds to the temperature at which the substance cannot go to liquid state, regardless of how high the pressure will be high. On the segment of the aircraft, the refrigerant has a saturated fluid condition, and in the left side - a supercooled fluid (the refrigerant temperature is less than the boiling point).

Inside Krivoy ABC The refrigerant is in a state of the pair of chosen mixture (the fraction of the pair in the unit of 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 a state of a boiling chicken mixture. In this case, the fraction of steam depends on a certain refrigeration cycle and is 10-30%.

At the exit from the evaporator, the boiling process may not be completed and the point 1 may not coincide with the point 7 . If the refrigerant temperature at the exit from the evaporator is greater than the boiling point, we get an overheating evaporator. His value ΔTpergrev It is the difference in refrigerant temperature at the outlet of the evaporator (point 1) and its temperature on the saturation line AV (point 7):

ΔTpergrev \u003d T1 - T7

If point 1 and 7 coincide, the refrigerant temperature is equal to the boiling point, and overheating ΔTpergrev It will be zero. Thus, we get a flooded evaporator. Therefore, when choosing an evaporator, it is first necessary to make a choice between the flooded evaporator and the evaporator with overheating.

Note that under equal conditions, the flooded evaporator is more profitable for the intensity of the heat selection 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 a saturated steam, and it is impossible to supply a wet medium to the compressor. IN otherwise There is a high probability of the appearance of hydrowards, which will be accompanied by the mechanical destruction of the components of the compressor. It turns out that if you choose a flooded evaporator, then it is necessary to provide for additional protection of the compressor from the saturated pair.

If you give preference to the evaporator with overheating, then you do not need to take care of the protection of the compressor and getting into it a saturated steam. The likelihood of hydraulic beats will occur only in case of deviation from the desired indicator of overheating. In normal conditions of operation of the refrigeration unit overheating ΔTpergrev Must be within 4-7 K.

When a decrease in overheating ΔTpergrev, The intensity of the selection of environmental heat is rises. But with excessively low values ΔTpergrev (less than 3k) there is a possibility of falling into a compressor of a wet steam, which may cause the appearance of hydraulic impact and, consequently, damage to the mechanical components of the compressor.

In the opposite case, with high reading ΔTpergrev (More than 10 K), this suggests that an insufficient refrigerant is incorporated into the evaporator. The intensity of the selection of heat from the cooled medium is dramatically reduced and the thermal mode of the compressor is worse.

When choosing an evaporator, another question arises due to the size of the boiling temperature of the refrigerant in the evaporator. To solve it first, it is necessary to determine which temperature of the cooled environment should be ensured for normal operation of the refrigeration unit. If air is used as a cooled medium, then, in addition to the temperature at the outlet of the evaporator, it is necessary to consider and humidity at the outlet of the evaporator. Now consider the behavior of the temperatures of the cooled medium around the evaporator during the operation of the usual refrigeration unit (Fig. 1a).

Not to delve into this topic We will neglect the pressure losses on the evaporator. We will also assume that the occurring heat exchange between the refrigerant and environmental Exercised by direct flow scheme.

In practice, such a scheme is used not often, since the effectiveness of heat exchange it is inferior to the countercurrent scheme. But if one of the coolants has a permanent temperature, and the testimony of overheating is small, then the forward flow and countertocks will be equivalent. It is known that the average temperature of the temperature pressure does not depend on the flow diagram of the flow. Consideration of the forward-flow scheme will provide us with a more visual idea of \u200b\u200bheat exchange, which occurs between the refrigerant and the cooled medium.

To begin with, we introduce a virtual value L.equal to the length of the heat exchange device (condenser or evaporator). Its value can be determined from the following expression: L \u003d W / Swhere W. - corresponds to the inner volume of the heat exchange device, in which the refrigerant circulation occurs, m3; S. - Survey of the surface of the heat exchange M2.

If we are talking about a refrigerator, then the equivalent length of the evaporator is almost equal to the length of the tube in which the process occurs 6-1 . Therefore, its outer surface is washed by the cooled medium.

Initially, pay attention to the evaporator, which acts as an air cooler. In it, the process of selection of heat from air occurs as a result of natural convection or with the help of a compulsory infringement of the evaporator. Note that in modern refrigeration plants, the first method is practically not used, since air cooling by natural convection is ineffective.

Thus, we assume that the air cooler is equipped with a fan, which provides forced by blowing the evaporator with air and is a tubular-ribbed heat exchange unit (Fig. 2). Its schematic image is shown in Fig. 2b. Consider the main values \u200b\u200bthat characterize the process of blowing.

Temperature difference

The temperature difference on the evaporator is calculated as follows:

ΔТ \u003d ta1-te2,

where ΔТ. Its ranging from 2 to 8 to (for tubular ribbed evaporators with forced blowing).

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

Fig. 2 - diagram and temperature parameters of 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 evaporator length;
• That - Boiling temperature of refrigerant in the evaporator.

Maximum temperature pressure

The maximum temperature of the air at the entrance to the evaporator is determined as follows:

Dtmasks \u003d TA1 -

This indicator is used in the selection of air coolers, since foreign manufacturers of refrigeration equipment provide evaporator cooling capacity values Qcisp depending on the magnitude Dtmasks. Consider the method of selection of the refrigeration air cooler and determine the calculated values Dtmasks. To do this, we present in the example the generally accepted recommendations for the selection of value Dtmasks:

• for freezing cameras Dtmasks Located within 4-6 K;
• for chambers of storage of unpacked products - 7-9 K;
• for chambers of storing hermetically packaged products - 10-14 K;
• for air conditioning installations - 18-22 K.

The degree of overheating steam at the exit of the evaporator

To determine the degree of overheating steam at output from the evaporator, use the following form:

F \u003d ΔTpergro / Dtmasks \u003d (T1-T0) / (TA1-T0),

where T1. - Couple temperature of refrigerant at the exit of the evaporator.

This indicator is practically not used for us, but in foreign directories it is planned that the readings of the cooling capacity of air coolers Qcisp corresponds to the value f \u003d 0.65.

During operation value F. It is customary to take from 0 to 1. Suppose that F \u003d 0., then ΔTergre \u003d 0., and the refrigerant at the exit from the evaporator will have a saturated pair. For this model of the air cooler, the actual cooling capacity will be 10-15% more than the indicator given in the directory.

If a F\u003e 0,65., then the cooling capacity for this model of the air cooler must be less than the value given in the directory. Suppose that F\u003e 0.8., then actual performance for this model will be 25-30% more valuesshown in the catalog.

If a F-\u003e 1.then the cooling capacity of the evaporator Qusp-\u003e 0. (Fig.3).

Fig.3 - Dependence of the evaporator cooling capacity Qcisp from overheating F.

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

• medium-gradatic temperature pressure DTCR \u003d TASR-T0;
• the average air temperature that passes through the evaporator TASR \u003d (TA1 + TA2) / 2;
• minimum temperature pressure Dtimin \u003d Ta2.

Fig. 4 - diagram and temperature parameters displaying the process of cooling water on the evaporator:

where Th1 and Te2. water temperature at the inputs and outlet of the evaporator;

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

The evaporators in which the cooling medium acts as a liquid, have the same temperature parameters as for air coolers. The digital values \u200b\u200bof the temperature of the cooled fluid, which are necessary for the normal operation of the refrigeration unit, will be different than the corresponding parameters for air coolers.

If the temperature difference is water ΔTe \u003d Te1-Te2then for casing-tube evaporators ΔT. It should be maintained in the range of 5 ± 1 K, and for lamellar evaporators the indicator ΔT. will be within 5 ± 1.5 K.

Unlike air coolers in liquid coolers, it is necessary to maintain not maximum, but the minimum temperature pressure Dtim \u003d Te2 - 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 casing-tube evaporators Minimum temperature pressure Dtim \u003d Te2 It should be maintained within 4-6 k, and for lamellar evaporators - 3-5 K.

The specified range (the difference between the temperature of the cooled medium at the exit of the evaporator and the boiling point of the refrigerant in the evaporator) must be maintained for the following reasons: with an increase in the difference, the cooling intensity begins to decline, and with a decrease, the risk of freezing of the cooled liquid in the evaporator increases, which can cause its mechanical destruction.

Design solutions evaporators

Regardless of the method of using various and refrigerants, heat exchange processes occurring in the evaporator are subject to the main technological cycle of the cooling production, according to which refrigeration plants are created and heat exchangers. Thus, to solve the problem of optimizing the heat exchange process, it is necessary to consider the conditions for the rational organization of the technological cycle of the cooling production.

As you know, cooling a certain environment is possible with the help of a heat exchanger. His constructive solution It should be chosen according to the technological requirements that are presented to these devices. Especially an important point is the correspondence of the device to the technological process of thermal processing of the medium, which is possible under the following conditions:

• maintaining a given working process temperature and control (regulation) above the temperature regime;
• selection of device material, according to chemical properties environments;
• control over the duration of the environment in the device;
• compliance of operating speeds and pressure.

Another factor on which the economic rationality of the apparatus depends is performance. First of all, it is influenced by the intensity of heat exchange and compliance with the hydraulic resistance of the device. The implementation of these conditions is possible under the following circumstances:

• providing the necessary velocity of workers' media for the implementation of the turbulent regime;
• creating the most suitable conditions to remove condensate, scale, inlet, etc.;
• creature favorable conditions for the movement of workers' media;
• prevent possible device contaminants.

Other important requirements There are also low weight, compactness, simplicity of design, as well as the convenience of mounting and repairing the device. To comply with these rules, such factors should be taken into account as: the configuration of the heating surface, the presence and type of partitions, the method of placement and fastening of the tubes in pipe lattices, dimensions, device of cameras, bottoms, etc.

The convenience of operation and reliability of the device are influenced by such factors as strength and tightness of detachable compounds, compensation of temperature deformations, convenience for maintenance and repair of the device. These requirements are based on the design and selection of the heat exchanger unit. The main role in this is the provision of the required technological process in cold-help production.

In order to choose the correct design of the evaporator, you must be guided by the following rules. 1) The cooling of the liquids is best carried out using a tubular heat exchanger of a rigid design or compact plate heat exchanger; 2) the use of tubular-ribbed devices is due in the following conditions: The heat transfer between the working media and the wall on both sides of the heating surface is significantly different. At the same time, fins must be installed on the side of the smallest heat transfer coefficient.

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

• ensuring appropriate conditions for the condensate on the air coolers;
• reducing the thickness of the hydrodynamic boundary layer by increasing the speed of operation of the working bodies (installation of inter-tubular partitions and breakdown of the beam of tubes on the moves);
• improving the flow of heat exchange surface by working bodies (the entire surface should actively participate in the heat exchange process);
• compliance with the main temperature indicators, thermal resistances, etc.

Analyzing individual thermal resistance can be chosen optimal method Increase the intensity of heat exchange (depending on the type of heat exchanger and the nature of the working bodies). In the liquid heat exchanger, the transverse partitions are rationally installed only with several strokes in the pipe space. When heat exchange (gas with gas, liquid with liquid), the amount of fluid flowing through the inter-tube space can be larger, and, as a result, the speed indicator will reach the 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 to improve heat exchange equipment Refrigerators. In this regard, research in the field of energy and chemical equipment is carried out. This is a study of the regime characteristics of the flow, the turbulization of the flow by creating artificial roughness. In addition, the development of new heat exchange surfaces is underway, thanks to which the heat exchangers will become more compact.

Choose a rational approach to calculating the evaporator

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

To produce thermal calculation of the surface heat exchanger, it is necessary to solve the equation and thermal Balance, taking into account certain working conditions of the device (the structural dimensions of heat transfer surfaces, the limits of temperature changes and circuits, relative to the movement of the cooling and cooled medium). To find a solution to this task, you need to apply rules that will make results from the source data. But because of numerous factors, find common decision For various heat exchangers is impossible. Together with this there are many methods of approximate calculation, which is easy to produce in manual or machine version.

Modern technologies allow you to choose an evaporator using special programs. Basically, they are provided by heat exchange equipment manufacturers and allow you to quickly choose the desired model. When using such programs, it is necessary to consider what they suggest the work of the evaporator under standard conditions. If the actual conditions differ from the standard, then the performance of the evaporator will be different. Thus, it is desirable to always conduct verification calculations of the evaporator's design chosen by you, relative to the actual conditions of its work.

Many repairmen often ask us the next question: "Why in your schemes food Er to the evaporator is always supplied from above, is it mandatory requirement When connected by evaporators? "This section makes clarity in this question.
A) a little story
We know that when the temperature in the cooled volume decreases, the boiling pressure at the same time falls, since the total temperature drop remains almost constant (see section 7. "The effect of cooled temperature").

A few years ago, this property was often used in the refrigeration facility in the chambers with a positive temperature to stop compressors when the temperature of the refrigeration chamber reached the required value.
Such property technology:
had two pre-
ND regulator
Pressure regulation
Fig. 45.1.
First, it allowed to do without a master thermostat, since the ND relay performed a double function - a specifying and safety relay.
Secondly, to ensure the defrosting of the evaporator, it was enough to configure the system so that the compressor starts at a pressure corresponding to the temperature above 0 ° C, and thus save on the defrost system!
However, when the compressor stopped, in order for the boiling pressure to exactly corresponding to the temperature in refrigerated chamberThe constant presence of fluid in the evaporator was required. That is why at that time the evaporators were powered very often from the bottom and all the time were half filled with a liquid refrigerant (see Fig. 45.1).
Nowadays, pressure regulation is rarely used, as it has the following negative moments:
If the capacitor has air cooling (the most frequent case), the condensation pressure changes strongly (see Section 2.1. "Condensers with air cooled. Normal work "). These condensation pressure changes necessarily lead to changes in the boiling pressure and, therefore, changes in the total temperature difference on the evaporator. Thus, the temperature in the refrigeration chamber cannot be maintained stable and will be exposed to great changes. Therefore, it is necessary to either use water capacitors. cooling, or apply an effective system for stabilizing condensation pressure.
If at least small anomalies occur in the installation of the installation (by boiling or condensation pressure), leading to a change in the total temperature difference on the evaporator, even minor, the temperature in the refrigeration chamber cannot be more supported at the specified limits.

If the compressor injection valve is not sealing enough, then when the compressor stops the boiling pressure is growing rapidly and there is a danger of increasing the frequency of the "Start stop" cycles of the compressor.

That is why today the temperature sensor in the cooled volume is most often used to turn off the compressor, and the ND relay performs only the protection functions (see Fig. 45.2).

Note that in this case the method of in-peat of the evaporator (from below or above) almost does not have a noticeable influence on the quality of regulation.

B) the design of modern evaporators

With an increase in the cooling capacity of evaporators, their size, in particular the length of the tubes used for their manufacture, also increase.
So, in the example in fig. 45.3, Designer for obtaining performance in 1 kW should consistently connect two sections of 0.5 kW each.
But such technology has limited use. Indeed, when doubling the length of pipe loss pipelines is also doubled. That is, pressure losses in large evaporators are becoming too large.
Therefore, with an increase in power, the manufacturer no longer has separate sections sequentially, and connects them in parallel in order to keep pressure loss as low as possible.
However, it is necessary that each evaporator is strictly driven by the same amount of fluid, and therefore the manufacturer installs a fluid dispenser at the input to the evaporator.

3 sections of the evaporator connected in parallel
Fig. 45.3.
For such evaporators, the question of, from below or on top of them, is no longer worth it, because they are powered only through a special fluid distributor.
Now consider ways to resemind pipelines to various types evaporators.

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

The refrigerant enters the input of the evaporator E and then descends on the first section (bend 1, 2, 3). Further, it rises in the second section (bend 4, 5, 6 and 7) and before leaving the evaporator at the outlet of it S, again falls on the third section (bends 8, 9, 10 and 11). Note that the refrigerant is lowered, rises, then again lowers, and moves towards the direction of the cooled air movement.
We now consider an example of a more powerful evaporator, which has significant sizes and isted using a liquid distributor.

Each share of full consumption of the refrigerant enters the entry of its section E, rises in the first row, then it falls in the second row and leaves the section through its output S (see Fig. 45.5).
In other words, the refrigerant rises, then lowers in the pipes, always moving against the direction of the cooling air movement. So, whatever the type of evaporator, the refrigerant is alternately descended, it rises.
Consequently, the concepts of evaporator read from above or below does not exist, especially for the most common occasion when the evaporator is powered through a fluid distributor.

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

Pos. 1: This evaporator isted through the TRV, which is configured to ensure overheating 7k. To ensure such overheating of vapors leaving the evaporator, serves a certain area of \u200b\u200bthe exterior pipeline length, blown by warm air.
Pos. 2: We are talking about the same site, but with the direction of air movement coinciding with the direction of the refrigerant movement. It can be stated that in this case the length of the area of \u200b\u200bthe pipeline that provides overheating of the vapor increases, since it is blown by colder air than in the previous case. This means that the evaporator contains less fluid, therefore, TRV in more than It is blocked, that is, the boiling pressure is lower and the cooling capacity is lower (see also section 8.4. "Thermore-regulating valve. Exercise").
Pos. 3 and 4: Although the evaporator is powered below, and not on top, as on the pos. 1 and 2, there are the same phenomena.
Thus, although in most examples of evaporators with a direct expansion cycle considered in this manual, they are powered from above, this is done exclusively for simplifying and for the purposes of a more understandable material presentation. In practice, the refrigerator installer really almost never make an error in connecting the fluid dispenser to the evaporator.
In the event that you have doubts if the direction of air pass through the evaporator is not very clearly designated to choose the method of connecting pipelines to the evaporator, strictly follow the developer's prescriptions in order to achieve cold-performance declared in the evaporator documentation.