Union of Soviets

Socialist

Republics

State Committee

USSR for Inventions and Discoveries (53) UDC 629. 113. .06.628.83 (088.8) (72) Authors of the invention

V. S. Maisotsenko, A. B. Tsimerman, M. G. and I. N. Pecherskaya

Odessa Civil Engineering Institute (71) Applicant (54) TWO-STAGE EVAPORATORY AIR CONDITIONER

COOLING FOR VEHICLE

The invention relates to the field of transport engineering and can be used for air conditioning in vehicles.

Air conditioners for vehicles are known that contain an air slot evaporator nozzle with air and water channels separated from each other by walls made of microporous plates, while the lower part of the nozzle is immersed in a tray with liquid (1)

The disadvantage of this air conditioner is the low efficiency of air cooling.

The closest technical solution The invention is a two-stage air conditioner evaporative cooling for a vehicle, containing a heat exchanger, a tray with liquid in which the nozzle is immersed, a chamber for cooling the liquid entering the heat exchanger with elements for additional cooling of the liquid, and a channel for supplying air to the chamber external environment, made tapering towards the inlet of the chamber (2

In this compressor, elements for additional air cooling are made in the form of nozzles.

However, the cooling efficiency in this compressor is also insufficient, since the limit of air cooling in this case is the wet bulb temperature of the auxiliary air flow in the pan.

10 In addition, the known air conditioner is structurally complex and contains duplicate components (two pumps, two tanks).

The purpose of the invention is to increase the degree of cooling efficiency and compactness of the device.

The goal is achieved by the fact that in the proposed air conditioner the elements for additional cooling are made in the form of a heat exchange partition located vertically and fixed to one of the chamber walls with the formation of a gap between it and the chamber wall opposite it, and

25, on the side of one of the surfaces of the partition, there is a reservoir with liquid flowing down the said surface of the partition, while the chamber and the tray are made in one piece.

The nozzle is made in the form of a block of capillary-porous material.

In fig. 1 shown circuit diagram air conditioner, Fig. 2 raeree A-A in Fig. 1.

The air conditioner consists of two stages of air cooling: the first stage is cooling the air in heat exchanger 1, the second stage is cooling it in nozzle 2, which is made in the form of a block of capillary-porous material.

A fan 3 is installed in front of the heat exchanger, driven so rotation by an electric motor 4 °. To circulate water in the heat exchanger, a water pump 5 is installed coaxially with the electric motor, supplying water through pipelines 6 and 7 from chamber 8 to reservoir 9 with liquid. Heat exchanger 1 is installed on a tray 10, which is made integral with the chamber

8. A channel is adjacent to the heat exchanger

11 for supplying air from the external environment, while the channel is made planally tapering in the direction towards the inlet 12 of the air cavity

13 chambers 8. Elements for additional air cooling are placed inside the chamber. They are made in the form of a heat exchange partition 14, located vertically and fixed to the wall 15 of the chamber, opposite the wall 16, relative to which the partition is located with a gap. The partition divides the chamber into two communicating cavities 17 and 18.

The chamber is provided with a window 19, in which a drip eliminator 20 is installed, and an opening 21 is made in the pan. When the air conditioner is operating, fan 3 drives the total air flow through heat exchanger 1. In this case, the total air flow L is cooled, and one part of it is the main flow L

Due to the execution of channel 11 tapering towards the inlet hole 12! cavity 13, the flow speed increases, and into the gap formed between the mentioned channel and the inlet hole, outside air, thereby increasing the mass of the auxiliary flow. This flow enters the cavity 17. Then this air flow, going around the partition 14, enters the chamber cavity 18, where it moves in the opposite direction to its movement in the cavity 17. In cavity 17 towards the movement air flow A film 22 of liquid flows down the partition - water from reservoir 9.

When the air flow and water come into contact, as a result of the evaporation effect, heat from the cavity 17 is transferred through the partition 14 to the water film 22, promoting its additional evaporation. After this, a flow of air with a lower temperature enters the cavity 18. This, in turn, leads to an even greater decrease in the temperature of the partition 14, which causes additional cooling of the air flow in the cavity 17. Consequently, the temperature of the air flow will decrease again after going around the partition and entering the cavity

18. Theoretically, the cooling process will continue until its driving force becomes zero. In this case driving force of the evaporative cooling process is the psychometric difference in the temperature of the air flow after it has been rotated relative to the partition and comes into contact with the film of water in cavity 18. Since the air flow is pre-cooled in cavity 17 with a constant moisture content, the psychrometric temperature difference of the air flow in cavity 18 tends to zero when approaching the dew point. Therefore, the limit of water cooling here is the dew point temperature of the outside air. Heat from the water enters the air flow in cavity 18, while the air is heated, humidified and released into the atmosphere through window 19 and drip eliminator 20.

Thus, in chamber 8, a counter-current movement of heat-exchanging media is organized, and the separating heat-exchange partition makes it possible to indirectly pre-cool the air flow supplied for cooling water due to the process of water evaporation. The cooled water flows along the partition to the bottom of the chamber, and since the latter is completed in one whole with the tray, then from there it is pumped into heat exchanger 1, and is also spent on wetting the nozzle due to intracapillary forces.

Thus, the main flow of air.L.„, having been pre-cooled without changes in moisture content in heat exchanger 1, is supplied for further cooling to nozzle 2. Here, due to the heat and mass exchange between the wetted surface of the nozzle and the main air flow, the latter is humidified and cooled without changing its heat content. Next, the main air flow through the opening in the pan

59 yes it cools, at the same time cooling the partition. Entering the cavity

17 of the chamber, the air flow flowing around the partition is also cooled, but there is no change in moisture content. Claim

1. A two-stage evaporative cooling air conditioner for a vehicle, containing a heat exchanger, a sub-tank with liquid in which the nozzle is immersed, a chamber for cooling the liquid entering the heat exchanger with elements for additional cooling of the liquid, and a channel for supplying air from the external environment into the chamber, made tapering in direction to the inlet of the chamber, i.e. in that, in order to increase the degree of cooling efficiency and compactness of the compressor, the elements for additional air cooling are made in the form of a heat exchange partition located vertically and mounted on one of the chamber walls with the formation of a gap between it and the chamber wall opposite it, and on the side of one of the On the surface of the partition, a reservoir is installed with liquid flowing down the said surface of the partition, while the chamber and the tray are made as one whole.

In modern climate control technology, much attention is paid to the energy efficiency of equipment. This explains the increased Lately interest in water evaporative cooling systems based on indirect evaporative heat exchangers (indirect evaporative cooling systems). Water evaporative cooling systems may be effective solution for many regions of our country, the climate of which is characterized by relatively low air humidity. Water as a refrigerant is unique - it has a high heat capacity and latent heat of vaporization, is harmless and accessible. In addition, water has been well studied, which makes it possible to fairly accurately predict its behavior in various technical systems.

Features of cooling systems with indirect evaporative heat exchangers

Main feature and the advantage of indirect evaporative systems is the ability to cool the air to a temperature below the wet bulb temperature. Thus, the technology of conventional evaporative cooling (in adiabatic humidifiers), when water is injected into the air flow, not only lowers the air temperature, but also increases its moisture content. In this case, the process line on the I d-diagram of moist air follows an adiabatic path, and the minimum possible temperature corresponds to point “2” (Fig. 1).

In indirect evaporative systems, the air can be cooled to point “3” (Fig. 1). The process in the diagram in this case goes vertically down along the line of constant moisture content. As a result, the resulting temperature is lower, and the moisture content of the air does not increase (remains constant).

In addition, water evaporation systems have the following positive qualities:

• Possibility of combined production of cooled air and cold water.
• Low power consumption. The main consumers of electricity are fans and water pumps.
• High reliability due to the absence of complex machines and the use of a non-aggressive working fluid - water.
• Ecological cleanliness: low level noise and vibration, non-aggressive working fluid, low environmental hazard industrial production systems due to low manufacturing complexity.
• Simplicity design and relatively low cost, associated with the absence of strict requirements for the tightness of the system and its individual components, the absence of complex and expensive cars (refrigeration compressors), small excess pressure in the cycle, low metal consumption and the possibility of widespread use of plastics.

Cooling systems that use the effect of heat absorption during water evaporation have been known for a very long time. However, on this moment Water evaporative cooling systems are not widespread enough. Almost the entire niche of industrial and domestic cooling systems in the region of moderate temperatures is filled with refrigerant vapor compression systems.

This situation is obviously associated with problems in the operation of water evaporation systems when negative temperatures and their unsuitability for operation at high relative humidity of outside air. It also affected that the main devices similar systems(cooling towers, heat exchangers) used previously had large dimensions, weight and other disadvantages associated with operation in high humidity conditions. In addition, they required a water treatment system.

However, today, thanks to technological progress, highly efficient and compact cooling towers have become widespread, capable of cooling water to temperatures that are only 0.8 ... 1.0 ° C different from the wet-bulb temperature of the air flow entering the cooling tower.

Here it is worth special mentioning the cooling towers of the companies Muntes and SRH-Lauer. Such a low temperature difference was achieved mainly due to original design cooling tower nozzles with unique properties— good wettability, manufacturability, compactness.

Description of the indirect evaporative cooling system

In an indirect evaporative cooling system atmospheric air from environment with parameters corresponding to point “0” (Fig. 4), is pumped into the system by a fan and cooled at constant moisture content in an indirect evaporative heat exchanger.

After the heat exchanger, the main air flow is divided into two: auxiliary and working, directed to the consumer.

The auxiliary flow simultaneously plays the role of both a cooler and a cooled flow - after the heat exchanger it is directed back towards the main flow (Fig. 2).

At the same time, water is supplied to the auxiliary flow channels. The point of supplying water is to “slow down” the rise in air temperature due to its parallel humidification: as is known, the same change in thermal energy can be achieved either by changing only the temperature or by changing temperature and humidity simultaneously. Therefore, when the auxiliary flow is humidified, the same heat exchange is achieved by a smaller temperature change.

In indirect evaporative heat exchangers of another type (Fig. 3), the auxiliary flow is directed not to the heat exchanger, but to the cooling tower, where it cools the water circulating through the indirect evaporative heat exchanger: the water is heated in it due to the main flow and cooled in the cooling tower due to the auxiliary one. Water moves along the circuit using a circulation pump.

Calculation of indirect evaporative heat exchanger

In order to calculate the cycle of an indirect evaporative cooling system with circulating water, the following initial data are required:

• φ ос — relative humidity of the ambient air, %;
• t ос — ambient air temperature, ° C;
• ∆t x - temperature difference at the cold end of the heat exchanger, ° C;
• ∆t m - temperature difference at the warm end of the heat exchanger, ° C;
• ∆t wgr - the difference between the temperature of the water leaving the cooling tower and the temperature of the air supplied to it according to the wet thermometer, ° C;
• ∆t min - minimum temperature difference (temperature difference) between the flows in the cooling tower (∆t min<∆t wгр), ° С;
• G r — mass air flow required by the consumer, kg/s;
• η in — fan efficiency;
• ∆P in - pressure loss in the devices and lines of the system (required fan pressure), Pa.

The calculation methodology is based on the following assumptions:

• Heat and mass transfer processes are assumed to be equilibrium,
• There are no external heat inflows in all areas of the system,
• The air pressure in the system is equal to atmospheric pressure (local changes in air pressure due to its injection by a fan or passing through aerodynamic resistance are negligible, which makes it possible to use the I d diagram of humid air for atmospheric pressure throughout the calculation of the system).

The procedure for engineering calculation of the system under consideration is as follows (Figure 4):

1. Using the I d diagram or using the program for calculating moist air, additional parameters of the ambient air are determined (point “0” in Fig. 4): specific enthalpy of air i 0, J/kg and moisture content d 0, kg/kg.
2. The increment in the specific enthalpy of air in the fan (J/kg) depends on the type of fan. If the fan motor is not blown (cooled) by the main air flow, then:

If the circuit uses a duct-type fan (when the electric motor is cooled by the main air flow), then:

Where:
η dv — electric motor efficiency;
ρ 0 — air density at the fan inlet, kg/m 3

Where:
B 0 — ambient barometric pressure, Pa;
R in is the gas constant of air, equal to 287 J/(kg.K).

3. Specific enthalpy of air after the fan (point “1”), J/kg.

i 1 = i 0 +∆i in; (3)

Since the “0-1” process occurs at a constant moisture content (d 1 =d 0 =const), then using the known φ 0, t 0, i 0, i 1 we determine the air temperature t1 after the fan (point “1”).

4. The dew point of the ambient air t dew, °C, is determined from the known φ 0, t 0.

5. Psychrometric temperature difference of the main flow air at the outlet of the heat exchanger (point “2”) ∆t 2-4, °C

∆t 2-4 =∆t x +∆t wgr; (4)

Where:
∆t x is assigned based on specific operating conditions in the range ~ (0.5…5.0), °C. It should be borne in mind that small values ​​of ∆t x will entail relatively large dimensions of the heat exchanger. To ensure small values ​​of ∆t x it is necessary to use highly efficient heat transfer surfaces;

∆t wgr is selected in the range (0.8…3.0), °C; Lower values ​​of ∆t wgr should be taken if it is necessary to obtain the minimum possible cold water temperature in the cooling tower.

6. We accept that the process of humidifying the auxiliary air flow in the cooling tower from state “2-4”, with sufficient accuracy for engineering calculations, proceeds along the line i 2 =i 4 =const.

In this case, knowing the value of ∆t 2-4, we determine the temperatures t 2 and t 4, points “2” and “4” respectively, °C. To do this, we will find a line i=const such that between point “2” and point “4” the temperature difference is the found ∆t 2-4. Point “2” is located at the intersection of the lines i 2 =i 4 =const and constant moisture content d 2 =d 1 =d OS. Point “4” is located at the intersection of the line i 2 =i 4 =const and the curve φ 4 = 100% relative humidity.

Thus, using the above diagrams, we determine the remaining parameters at points “2” and “4”.

7. Determine t 1w - the water temperature at the outlet of the cooling tower, at point “1w”, °C. In the calculations, we can neglect the heating of water in the pump, therefore, at the entrance to the heat exchanger (point “1w’”) the water will have the same temperature t 1w

t 1w =t 4 +.∆t wgr; (5)

8. t 2w - water temperature after the heat exchanger at the inlet to the cooling tower (point “2w”), °C

t 2w =t 1 -.∆t m; (6)

9. The temperature of the air discharged from the cooling tower into the environment (point “5”) t 5 is determined by the graphic-analytical method using an i d diagram (with great convenience, a set of Q t and i t diagrams can be used, but they are less common, therefore in this i d diagram was used in the calculations). The specified method is as follows (Fig. 5):

• point “1w”, characterizing the state of water at the inlet to the indirect evaporation heat exchanger, with the specific enthalpy value of point “4” is placed on the t 1w isotherm, separated from the t 4 isotherm at a distance ∆t wgr.
• From the point “1w” along the isenthalp we plot the segment “1w - p” so that t p = t 1w - ∆t min.
• Knowing that the process of heating the air in the cooling tower occurs at φ = const = 100%, we construct a tangent to φ pr = 1 from point “p” and obtain the tangent point “k”.
• From the point of tangency “k” along the isenthalpe (adiabatic, i=const) we plot the segment “k - n” so that t n = t k + ∆t min. Thus, a minimum temperature difference between the cooled water and the auxiliary air in the cooling tower is ensured (assigned). This temperature difference guarantees the operation of the cooling tower in the design mode.
• We draw a straight line from point “1w” through point “n” until it intersects with the straight line t=const= t 2w. We get point “2w”.
• From point “2w” we draw a straight line i=const until it intersects with φ pr =const=100%. We get point “5”, which characterizes the state of the air at the outlet of the cooling tower.
• Using the diagram, we determine the desired temperature t5 and other parameters of point “5”.

10. We compose a system of equations to find the unknown mass flow rates of air and water. Thermal load of the cooling tower by auxiliary air flow, W:

Q gr =G in (i 5 - i 2); (7)

Q wgr =G ow C pw (t 2w - t 1w); (8)

Where:
C pw is the specific heat capacity of water, J/(kg.K).

Thermal load of the heat exchanger along the main air flow, W:

Q mo =G o (i 1 - i 2); (9)

Thermal load of the heat exchanger by water flow, W:

Q wmo =G ow C pw (t 2w - t 1w) ; (10)

Material balance by air flow:

G o =G in +G p ; (11)

Heat balance for cooling tower:

Q gr =Q wgr; (12)

The heat balance of the heat exchanger as a whole (the amount of heat transferred by each flow is the same):

Q wmo =Q mo ; (13)

Combined thermal balance of the cooling tower and water heat exchanger:

Q wgr =Q wmo; (14)

11. Solving equations from (7) to (14) together, we obtain the following dependencies:
mass air flow along the auxiliary flow, kg/s:

mass air flow along the main air flow, kg/s:

G o = G p ; (16)

Mass flow of water through the cooling tower along the main flow, kg/s:

12. The amount of water required to recharge the water circuit of the cooling tower, kg/s:

G wn =(d 5 -d 2)G in; (18)

13. Power consumption in the cycle is determined by the power spent on the fan drive, W:

N in =G o ∆i in; (19)

Thus, all the parameters necessary for structural calculations of the elements of the indirect evaporative air cooling system have been found.

Note that the working flow of cooled air supplied to the consumer (point “2”) can be additionally cooled, for example, by adiabatic humidification or any other method. As an example in Fig. 4 indicates the point “3*”, corresponding to adiabatic humidification. In this case, points “3*” and “4” coincide (Fig. 4).

Practical aspects of indirect evaporative cooling systems

Based on the practice of calculating indirect evaporative cooling systems, it should be noted that, as a rule, the auxiliary flow rate is 30-70% of the main flow and depends on the potential cooling ability of the air supplied to the system.

If we compare cooling by adiabatic and indirect evaporative methods, then from the I d-diagram it can be seen that in the first case, air with a temperature of 28 ° C and a relative humidity of 45% can be cooled to 19.5 ° C, while in the second case - up to 15°C (Fig. 6).

"Pseudo-indirect" evaporation

As mentioned above, an indirect evaporative cooling system can achieve lower temperatures than a traditional adiabatic humidification system. It is also important to emphasize that the moisture content of the desired air does not change. Similar advantages compared to adiabatic humidification can be achieved through the introduction of an auxiliary air flow.

There are currently few practical applications of indirect evaporative cooling systems. However, devices of a similar, but slightly different operating principle have appeared: air-to-air heat exchangers with adiabatic humidification of the outside air (systems of “pseudo-indirect” evaporation, where the second flow in the heat exchanger is not some humidified part of the main flow, but another, completely independent circuit).

Such devices are used in systems with a large volume of recirculated air that needs cooling: in air conditioning systems for trains, auditoriums for various purposes, data processing centers and other facilities.

The purpose of their implementation is to reduce the operating time of energy-intensive compressor refrigeration equipment as much as possible. Instead, for outside temperatures up to 25°C (and sometimes higher), an air-to-air heat exchanger is used, in which the recirculated room air is cooled by the outside air.

For greater efficiency of the device, the outside air is pre-humidified. In more complex systems, humidification is also carried out during the heat exchange process (water injection into the heat exchanger channels), which further increases its efficiency.

Thanks to the use of such solutions, the current energy consumption of the air conditioning system is reduced by up to 80%. Annual energy consumption depends on the climatic region of operation of the system; on average, it is reduced by 30-60%.

Yuri Khomutsky, technical editor of Climate World magazine

The article uses the methodology of MSTU. N. E. Bauman for calculating the indirect evaporative cooling system.

Ecology of consumption. The history of the direct evaporative cooling air conditioner. Differences between direct and indirect cooling. Application options for evaporative air conditioners

Air cooling and humidification through evaporative cooling is a completely natural process that uses water as a cooling medium and heat is effectively dissipated into the atmosphere. Simple laws are used - when a liquid evaporates, heat is absorbed or cold is released. Evaporation efficiency increases with increasing air speed, which is ensured by forced circulation of the fan.

The temperature of dry air can be significantly reduced by the phase change of liquid water to vapor, and this process requires significantly less energy than compression cooling. In very dry climates, evaporative cooling also has the advantage of increasing the humidity of the air when conditioning it, making the occupants more comfortable. However, unlike vapor compression cooling, it requires a constant source of water, and constantly consumes it during operation.

History of development

Over the centuries, civilizations have found original methods to combat the heat in their territories. An early form of cooling system, the "windcatcher", was invented many thousands of years ago in Persia (Iran). It was a system of wind shafts on the roof that caught the wind, passed it through the water, and blew cooled air into the interior. It is noteworthy that many of these buildings also had courtyards with large reserves of water, so if there was no wind, then as a result of the natural process of evaporation of water, hot air rising upward evaporated the water in the courtyard, after which the already cooled air passed through the building. Nowadays, Iran has replaced wind catchers with evaporative coolers and uses them widely, and the market, due to the dry climate, reaches a turnover of 150,000 evaporators per year.

In the US, the evaporative cooler was the subject of numerous patents in the twentieth century. Many of whom, since 1906, proposed the use of wood shavings as a gasket that transports large amounts of water in contact with moving air and supports intense evaporation. The standard design, as shown in the 1945 patent, includes a water reservoir (usually equipped with a float valve to adjust the level), a pump to circulate water through the wood chip pads, and a fan to blow air through the pads into the living areas. This design and materials remain a staple of evaporative cooler technology in the southwestern United States. In this region they are additionally used to increase humidity.

Evaporative cooling was common in aircraft engines of the 1930s, such as the engine for the Beardmore Tornado airship. This system was used to reduce or completely eliminate the radiator, which would otherwise create significant aerodynamic drag. In these systems, the water in the engine was kept under pressure using pumps that allowed it to be heated to temperatures in excess of 100°C, since the actual boiling point depends on pressure. Superheated water was sprayed through a nozzle onto an open pipe, where it instantly evaporated, receiving its heat. These pipes could be located under the surface of the aircraft to create zero drag.

External evaporative cooling units were installed on some vehicles to cool the interior. They were often sold as additional accessories. The use of evaporative cooling devices in automobiles continued until vapor compression air conditioning became widespread.

Evaporative cooling is a different principle than vapor compression refrigeration units, although they also require evaporation (evaporation is part of the system). In the vapor compression cycle, after the refrigerant evaporates inside the evaporator coil, the refrigerant gas is compressed and cooled, condensing into a liquid state under pressure. Unlike this cycle, in an evaporative cooler the water evaporates only once. The evaporated water in the cooling device is discharged into a space with cooled air. In a cooling tower, the evaporated water is carried away by the air flow.

Evaporative Cooling Applications

There are direct, oblique, and two-stage evaporative air cooling (direct and indirect). Direct evaporative air cooling is based on the isenthalpic process and is used in air conditioners during the cold season; in warm weather, it is possible only in the absence or insignificant moisture release in the room and the low moisture content of the outside air. Bypassing the irrigation chamber somewhat expands the scope of its application.

Direct evaporative cooling of air is advisable in dry and hot climates in the supply ventilation system.

Indirect evaporative air cooling is carried out in surface air coolers. To cool the water circulating in the surface heat exchanger, an auxiliary contact device (cooling tower) is used. For indirect evaporative cooling of air, you can use devices of a combined type, in which the heat exchanger simultaneously performs both functions - heating and cooling. Such devices are similar to air recuperative heat exchangers.

Cooled air passes through one group of channels, the inner surface of the second group is irrigated with water flowing into the pan and then sprayed again. Upon contact with the exhaust air passing in the second group of channels, evaporative cooling of the water occurs, as a result of which the air in the first group of channels is cooled. Indirect evaporative air cooling makes it possible to reduce the performance of an air conditioning system compared to its performance with direct evaporative air cooling and expands the possibilities of using this principle, because the moisture content of the supply air in the second case is lower.

With two-stage evaporative cooling air conditioners use sequential indirect and direct evaporative cooling of the air in the air conditioner. In this case, the installation for indirect evaporative air cooling is supplemented with an irrigation nozzle chamber operating in direct evaporative cooling mode. Typical spray nozzle chambers are used in evaporative air cooling systems as cooling towers. In addition to single-stage indirect evaporative air cooling, multi-stage air cooling is possible, in which deeper air cooling is carried out - this is the so-called compressor-free air conditioning system.

Direct evaporative cooling (open cycle) is used to reduce the air temperature using the specific heat of evaporation, changing the liquid state of water to a gaseous state. In this process, the energy in the air does not change. Dry, warm air is replaced by cool and humid air. The heat from the outside air is used to evaporate water.

Indirect evaporative cooling (closed loop) is a process similar to direct evaporative cooling, but uses a specific type of heat exchanger. In this case, the moist, cooled air does not come into contact with the conditioned environment.

Two-stage evaporative cooling, or indirect/direct.

Traditional evaporative coolers use only a fraction of the energy required by vapor compression refrigeration units or adsorption air conditioning systems. Unfortunately, they increase air humidity to uncomfortable levels (except in very dry climates). Two-stage evaporative coolers do not increase humidity levels as much as standard single-stage evaporative coolers do.

In the first stage of a two-stage cooler, warm air is cooled indirectly without increasing humidity (by passing through a heat exchanger cooled by external evaporation). In the direct stage, pre-cooled air passes through a water-soaked pad, where it is further cooled and becomes more humid. Because the process includes a first, pre-cooling stage, the direct evaporation stage requires less humidity to achieve the required temperatures. As a result, according to manufacturers, the process cools air with a relative humidity ranging from 50 to 70%, depending on the climate. In comparison, traditional cooling systems increase air humidity to 70 - 80%.

Purpose

When designing a central supply ventilation system, it is possible to equip the air intake with an evaporation section and thus significantly reduce the cost of air cooling during the warm season.

In the cold and transitional periods of the year, when the air is heated by supply heaters of ventilation systems or indoor air by heating systems, the air heats up and its physical ability to assimilate (absorb) increases, and with increasing temperature - moisture. Or, the higher the air temperature, the more moisture it can assimilate. For example, when the outside air is heated by a heater by a ventilation system from a temperature of -22 0 C and a humidity of 86% (outdoor air parameter for HP in Kiev), to +20 0 C - the humidity drops below the boundary limits for biological organisms to an unacceptable 5-8% air humidity. Low air humidity negatively affects the skin and mucous membranes of humans, especially those with asthma or pulmonary diseases. Standardized air humidity for residential and administrative premises: from 30 to 60%.

Evaporative air cooling is accompanied by the release of moisture or an increase in air humidity, up to a high saturation of air humidity of 60-70%.

Advantages

The amount of evaporation - and therefore heat transfer - depends on the outside wet-bulb temperature which, especially in summer, is much lower than the equivalent dry-bulb temperature. For example, on hot summer days when the dry bulb temperature exceeds 40°C, evaporative cooling can cool the water to 25°C or cool the air.
Because evaporation removes much more heat than standard physical heat transfer, heat transfer uses four times less air flow than conventional air cooling methods, saving significant amounts of energy.

Evaporative cooling versus traditional air conditioning methods Unlike other types of air conditioning, evaporative air cooling (bio-cooling) does not use harmful gases (freon and others) as refrigerants, which are harmful to the environment. It also uses less electricity, thereby saving energy, natural resources and up to 80% in operating costs compared to other air conditioning systems.

Flaws

Low performance in humid climates.
An increase in air humidity, which in some cases is undesirable, results in two-stage evaporation, where the air does not contact and is not saturated with moisture.

Operating principle (option 1)

The cooling process is carried out due to the close contact of water and air, and the transfer of heat into the air by evaporation of a small amount of water. The heat is then dissipated through the warm and moisture-saturated air leaving the unit.

Operating principle (option 2) - installation on the air intake

Evaporative cooling units

There are different types of evaporative cooling units, but they all have:
- heat exchange or heat transfer section, constantly wetted with water by irrigation,
- a fan system for forced circulation of outside air through the heat exchange section,

2018-08-15

The use of air conditioning systems (ACS) with evaporative cooling as one of the energy-efficient solutions in the design of modern buildings and structures.

Today, the most common consumers of thermal and electrical energy in modern administrative and public buildings are ventilation and air conditioning systems. When designing modern public and administrative buildings to reduce energy consumption in ventilation and air conditioning systems, it makes sense to give special preference to reducing power at the stage of obtaining technical specifications and reducing operating costs. Reducing operating costs is most important for property owners or tenants. There are many ready-made methods and various measures to reduce energy costs in air conditioning systems, but in practice the choice of energy-efficient solutions is very difficult.

One of the many HVAC systems that can be considered energy efficient is the evaporative cooling air conditioning systems discussed in this article.

They are used in residential, public and industrial premises. The process of evaporative cooling in air conditioning systems is provided by nozzle chambers, film, nozzle and foam devices. The systems under consideration can have direct, indirect, or two-stage evaporative cooling.

Of the above options, the most economical air cooling equipment is direct cooling systems. For them, it is assumed that standard equipment will be used without the use of additional sources of artificial cold and refrigeration equipment.

A schematic diagram of an air conditioning system with direct evaporative cooling is shown in Fig. 1.

The advantages of such systems include minimal maintenance costs during operation, as well as reliability and design simplicity. Their main disadvantages are the inability to maintain supply air parameters, the exclusion of recirculation in the serviced premises and dependence on external climatic conditions.

Energy costs in such systems are reduced to the movement of air and recirculated water in adiabatic humidifiers installed in the central air conditioner. When using adiabatic humidification (cooling) in central air conditioners, it is necessary to use potable quality water. The use of such systems may be limited in climate zones with a predominantly dry climate.

Areas of application for air conditioning systems with evaporative cooling are objects that do not require precise maintenance of heat and humidity conditions. Usually they are run by enterprises in various industries, where a cheap way to cool internal air is needed in conditions of high heat intensity of the premises.

The next option for economical cooling of air in air conditioning systems is the use of indirect evaporative cooling.

A system with such cooling is most often used in cases where the internal air parameters cannot be obtained using direct evaporative cooling, which increases the moisture content of the supply air. In the "indirect" scheme, the supply air is cooled in a heat exchanger of the recuperative or regenerative type in contact with an auxiliary air stream cooled by evaporative cooling.

A variant diagram of an air conditioning system with indirect evaporative cooling and the use of a rotary heat exchanger is shown in Fig. 2. Scheme of SCR with indirect evaporative cooling and the use of recuperative heat exchangers is shown in Fig. 3.

Indirect evaporative cooling air conditioning systems are used when supply air is required without dehumidification. The required air parameters are supported by local closers installed in the room. The determination of the supply air flow is carried out according to sanitary standards, or according to the air balance in the room.

Indirect evaporative cooling air conditioning systems use either outside or exhaust air as auxiliary air. If local closers are available, the latter is preferred, as it increases the energy efficiency of the process. It should be noted that the use of exhaust air as auxiliary air is not allowed in the presence of toxic, explosive impurities, as well as a high content of suspended particles contaminating the heat exchange surface.

Outside air is used as an auxiliary flow in the case when the flow of exhaust air into the supply air through leaks in the heat exchanger (i.e. heat exchanger) is unacceptable.

The auxiliary air flow is cleaned in air filters before being supplied for humidification. An air conditioning system design with regenerative heat exchangers has greater energy efficiency and lower equipment costs.

When designing and selecting circuits for air conditioning systems with indirect evaporative cooling, it is necessary to take into account measures to regulate heat recovery processes during the cold season in order to prevent freezing of heat exchangers. It is necessary to provide for reheating the exhaust air in front of the heat exchanger, bypassing part of the supply air in a plate heat exchanger and regulating the rotation speed in the rotary heat exchanger.

Using these measures will prevent freezing of heat exchangers. Also in calculations when using exhaust air as an auxiliary flow, it is necessary to check the system for operability during the cold season.

Another energy-efficient air conditioning system is a two-stage evaporative cooling system. Air cooling in this scheme is provided in two stages: direct evaporative and indirect evaporative methods.

“Two-stage” systems provide for more precise adjustment of air parameters when leaving the central air conditioner. Such air conditioning systems are used in cases where greater cooling of the supply air is required compared to direct or indirect evaporative cooling.

Air cooling in two-stage systems is provided in regenerative, plate heat exchangers or in surface heat exchangers with an intermediate coolant using an auxiliary air flow - in the first stage. Air cooling in adiabatic humidifiers is in the second stage. The basic requirements for auxiliary air flow, as well as for checking the operation of SCR during the cold season, are similar to those applied to SCR circuits with indirect evaporative cooling.

The use of air conditioning systems with evaporative cooling allows you to achieve better results that cannot be obtained using refrigeration machines.

The use of SCR schemes with evaporative, indirect and two-stage evaporative cooling allows, in some cases, to abandon the use of refrigeration machines and artificial refrigeration, and also to significantly reduce the refrigeration load.

By using these three schemes, energy efficiency in air handling is often achieved, which is very important when designing modern buildings.

History of evaporative air cooling systems

Over the centuries, civilizations have found original methods to combat the heat in their territories. An early form of cooling system, the “windcatcher,” was invented many thousands of years ago in Persia (Iran). This was a system of wind shafts on the roof that caught the wind, passed it through the water and blew cooled air into the interior. It is noteworthy that many of these buildings also had courtyards with large reserves of water, so if there was no wind, then as a result of the natural process of evaporation of water, hot air rising upward evaporated the water in the courtyard, after which the already cooled air passed through the building. Nowadays, Iran has replaced “wind catchers” with evaporative coolers and uses them widely, and the Iranian market, due to the dry climate, reaches a turnover of 150 thousand evaporators per year.

In the US, the evaporative cooler was the subject of numerous patents in the 20th century. Many of them, dating back to 1906, proposed the use of wood shavings as a gasket, carrying large amounts of water in contact with moving air and maintaining intense evaporation. The standard design from the 1945 patent includes a water reservoir (usually equipped with a float valve to adjust the level), a pump to circulate water through the wood chip pads, and a fan to blow air through the pads into the living areas. This design and materials remain central to evaporative cooler technology in the southwestern United States. In this region they are additionally used to increase humidity.

Evaporative cooling was common in aircraft engines of the 1930s, such as the engine for the Beardmore Tornado airship. This system was used to reduce or completely eliminate the radiator, which would otherwise create significant aerodynamic drag. External evaporative cooling units were installed on some vehicles to cool the interior. They were often sold as additional accessories. The use of evaporative cooling devices in automobiles continued until vapor compression air conditioning became widespread.

Evaporative cooling is a different principle than vapor compression refrigeration units, although they also require evaporation (evaporation is part of the system). In the vapor compression cycle, after the refrigerant evaporates inside the evaporator coil, the cooling gas is compressed and cooled, condensing under pressure into a liquid state. Unlike this cycle, in an evaporative cooler the water evaporates only once. The evaporated water in the cooling device is discharged into a space with cooled air. In a cooling tower, the evaporated water is carried away by the air flow.

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When constructing processes on the i - d diagram and choosing a technological scheme for air treatment, it is necessary to strive for the rational use of energy, ensuring economical consumption of cold, heat, electricity, water, as well as saving the construction area occupied by equipment. To this end, it is necessary to analyze the possibility of saving artificial cold by using direct and indirect evaporative cooling of air, using a scheme with regeneration of heat from exhaust air and recycling heat from secondary sources, if necessary, using first and second air recirculation, a bypass scheme, as well as controlled processes in heat exchangers.

Recirculation is used in rooms with significant excess heat, when the supply air flow rate determined to remove excess heat is greater than the required outside air flow rate. In the warm season of the year, recirculation makes it possible to reduce cold costs compared to a direct-flow scheme of the same productivity, if the enthalpy of the outside air is higher than the enthalpy of the removed air, and also to eliminate the need for second heating. During the cold period, significantly reduce heat costs for heating the outside air. When using evaporative cooling, when the enthalpy of the outdoor air is lower than that of the indoor and exhaust air, recirculation is not practical. The movement of recirculation air through a network of air ducts is always associated with additional energy costs and requires a building volume to accommodate recirculation air ducts. Recirculation will be advisable if the costs of its design and operation are less than the resulting savings in heat and cold. Therefore, when determining the supply air flow rate, you should always strive to bring it closer to the minimum required value of outside air, adopting the appropriate air distribution scheme in the room and the type of air distributor and, accordingly, a direct-flow scheme. Recirculation is also not compatible with heat recovery from exhaust air. In order to reduce the heat consumption for heating the outside air in the cold season, it is necessary to analyze the possibility of using secondary heat from low-potential sources, namely: the heat of exhaust air, waste gases of heat generators and process equipment, the heat of condensation of refrigeration machines, the heat of lighting fixtures, the heat of waste water and etc. Heat exchangers for regenerating the heat of the removed air also make it possible to slightly reduce the consumption of cold during the warm season in areas with a hot climate.

To make the right choice, you need to know the possible air treatment schemes and their features. Let's consider the simplest processes of changing the state of air and their sequence in central air conditioners serving one large room.

Typically, the determining mode for choosing a processing flow chart and determining the performance of an air conditioning system is the warm period of the year. During the cold period of the year, they strive to maintain the supply air flow rate determined for the warm period of the year and the air treatment scheme.

Two-stage evaporative cooling

The wet bulb temperature of the main air flow after cooling in the indirect evaporative cooling surface heat exchanger has a lower value compared to the wet bulb temperature of the outdoor air, as a natural limit for evaporative cooling. Therefore, when subsequent processing of the main flow in a contact apparatus using the direct evaporative cooling method, lower air parameters can be obtained compared to the natural limit. This scheme of sequential air processing of the main air stream by indirect and direct evaporative cooling is called two-stage evaporative cooling. The layout of the central air conditioner equipment, corresponding to two-stage evaporative air cooling, is presented in Figure 5.7 a. It is also characterized by the presence of two air flows: main and auxiliary. Outdoor air, which has a lower wet-bulb temperature than the indoor air in the room being served, enters the main air conditioner. In the first air cooler, it is cooled using indirect evaporative cooling. Next, it enters the adiabatic humidification unit, where it is cooled and humidified. Evaporative cooling of water circulating through the surface air coolers of the main air conditioner is carried out when it is atomized in the adiabatic humidification unit in the auxiliary flow. The circulation pump takes water from the sump of the auxiliary flow adiabatic humidification unit and supplies it to the main flow air coolers and then to spraying in the auxiliary flow. The loss of water from evaporation in the main and auxiliary flows is replenished through float valves. After two stages of cooling, air is supplied to the room.