In HVAC systems, adiabatic evaporation is usually associated with air humidification, but recently the process has become increasingly popular in most different countries world and is increasingly used for “natural” air cooling.
WHAT IS EVAPORATIVE COOLING?
Evaporative cooling is the basis of one of the very first space cooling systems invented by man, where air is cooled due to the natural evaporation of water. This phenomenon is very common and occurs everywhere: one example would be the feeling of cold you experience when water evaporates from the surface of your body due to the influence of the wind. The same thing happens with the air in which water is atomized: since this process occurs without an external source of energy (this is what the word “adiabatic” means), the heat necessary to evaporate the water is taken from the air, which, accordingly, becomes colder.
Using this cooling method in modern systems air conditioning provides high cooling capacity with low power consumption, since in this case electricity is consumed only to support the process of evaporation of water. At the same time, as a coolant instead of chemical compositions ordinary water is used, which makes evaporative cooling more economically beneficial and does not harm the environment.
TYPES OF EVAPORATIVE COOLING
There are two main methods of evaporative cooling - direct and indirect.
Direct evaporative cooling
Direct evaporative cooling is the process of reducing the temperature of the air in a room by directly humidifying it. In other words, due to the evaporation of atomized water, the surrounding air is cooled. In this case, moisture is distributed either directly into the room using industrial humidifiers and nozzles, or by saturating the supply air with moisture and cooling it in a section of the ventilation unit.
It should be noted that in direct evaporative cooling conditions, a significant increase in the humidity of the indoor supply air is inevitable, therefore, to assess the applicability this method It is recommended to use the formula known as the “temperature and discomfort index” as a basis. The formula calculates the comfortable temperature in degrees Celsius, taking into account humidity and dry bulb temperature readings (Table 1). Looking ahead, we note that the direct evaporative cooling system is used only in cases where the outdoor air in the summer has high dry bulb temperatures and low absolute humidity levels.
Indirect evaporative cooling
To increase the efficiency of evaporative cooling when outdoor air humidity is high, it is recommended to combine evaporative cooling with heat recovery. This technology known as “indirect evaporative cooling” and is suitable for almost any country in the world, including countries with very humid climates.
General scheme The operation of a supply and ventilation system with recuperation is that hot supply air, passing through a special heat exchange cassette, is cooled by cool air removed from the room. The operating principle of indirect evaporative cooling is to install an adiabatic humidification system in exhaust duct supply and exhaust central air conditioners, with subsequent transfer of cold through the recuperator to the supply air.
As shown in the example, by using plate recuperator The outside air in the ventilation system is cooled by 6 °C. Application of evaporative cooling exhaust air will increase the temperature difference from 6°C to 10°C without increasing energy consumption and indoor humidity levels. The use of indirect evaporative cooling is effective for high heat fluxes, for example in office and shopping centers, data centers, production premises etc.
Indirect cooling system using the CAREL humiFog adiabatic humidifier:
Case: Estimating the costs of an indirect adiabatic cooling system compared to cooling using chillers.
Using the example of an office center with a permanent residence of 2000 people.
| Payment terms | |
| Outdoor temperature and humidity content: | +32ºС, 10.12 g/kg (indicators taken for Moscow) |
| Room temperature: | +20 ºС |
| Ventilation system: | 4 supply and exhaust units with a capacity of 30,000 m3/h (air supply according to sanitary standards) |
| Cooling system power including ventilation: | 2500 kW |
| Supply air temperature: | +20 ºС |
| Extract air temperature: | +23 ºС |
| Sensible heat recovery efficiency: | 65% |
| Centralized cooling system: | Chiller-fan coil system with water temperature 7/12ºС |
Calculation
- To make the calculation, we calculate the relative humidity of the exhaust air.
- At a temperature in the cooling system of 7/12 °C, the dew point of the exhaust air, taking into account internal moisture release, will be +8 °C.
- The relative humidity in the exhaust air will be 38%.
*It must be taken into account that the cost of installing a refrigeration system, taking into account all costs, is significantly higher compared to indirect cooling systems.
Capital costs
For analysis, we take the cost of equipment - chillers for the refrigeration system and a humidification system for indirect evaporative cooling.
- Capital cost of supply air cooling for an indirect cooling system.
The cost of one Optimist humidification rack manufactured by Carel (Italy) in an air handling unit is 7570 €.
- Capital costs for supply air cooling without an indirect cooling system.
The cost of a chiller with a cooling capacity of 62.3 kW is approximately 12,460 €, based on a cost of 200 € per 1 kW of cooling capacity. It must be taken into account that the cost of installing a refrigeration system, taking into account all costs, is significantly higher compared to indirect cooling systems.
Operating costs
For analysis we take the cost tap water 0.4 € per 1 m3 and the cost of electricity 0.09 € per 1 kW/h.
- Operating costs for supply air cooling for an indirect cooling system.
Water consumption for indirect cooling is 117 kg/h for one air handling unit, taking into account losses of 10%, we will take it as 130 kg/h.
The power consumption of the humidification system is 0.375 kW for one air handling unit.
The total cost per hour is 0.343 € per 1 hour of system operation.
- Operating costs for supply air cooling without an indirect cooling system.
We take the cooling coefficient equal to 3 (the ratio of cooling power to power consumption).
The total cost per hour is 7.48 € per 1 hour of operation.
Conclusion
Using indirect evaporative cooling allows you to:
Reduce capital costs for supply air cooling by 39%.
Reduce energy consumption for the building's air conditioning systems from 729 kW to 647 kW, or by 11.3%.
Reduce operating costs for building air conditioning systems from 65.61 €/hour to 58.47 €/hour, or by 10.9%.
Thus, although fresh air cooling represents approximately 10–20% of the total cooling demand for office and shopping centers, it is here that there are the greatest reserves for increasing the energy efficiency of a building without a significant increase in capital costs.
The article was prepared by TERMOKOM specialists for publication in ON magazine No. 6-7 (5) June-July 2014 (pp. 30-35)
For rooms with large excesses of sensible heat, where it is necessary to maintain high humidity in the internal air, air conditioning systems are used that use the principle of indirect evaporative cooling.
The circuit consists of a main air flow processing system and an evaporative cooling system (Fig. 3.3. Fig. 3.4). To cool water, irrigation chambers of air conditioners or other contact devices, spray pools, cooling towers and others can be used.
Water, cooled by evaporation in the air flow, with a temperature, enters the surface heat exchanger - the air cooler of the main air flow air conditioner, where the air changes its state from values to values (t.), the water temperature rises to. The heated water enters the contact apparatus, where it is cooled by evaporation to temperature and the cycle is repeated again. The air passing through the contact apparatus changes its state from parameters to parameters (i.e.). The supply air, assimilating heat and moisture, changes its parameters to the state t., and then to the state.
Fig.3.3. Indirect evaporative cooling circuit
1-heat exchanger-air cooler; 2-contact device
Fig.3.4. indirect evaporative cooling diagram
Line - direct evaporative cooling.
If there is excess heat in the room, then with indirect evaporative cooling the supply air flow will be
with direct evaporative cooling
Since >, then<.
<), что позволяет расширить область возможного использования принципа испарительного охлаждения воздуха.
A comparison of processes shows that with indirect evaporative cooling the SCR productivity is lower than with direct cooling. In addition, with indirect cooling, the moisture content of the supply air is lower (<), что позволяет расширить область возможного использования принципа испарительного охлаждения воздуха.
In contrast to the separate scheme of indirect evaporative cooling, devices of a combined type have been developed (Figure 3.5). The device includes two groups of alternating channels separated by walls. An auxiliary air flow passes through channel group 1. Water supplied through the water distribution device flows along the surface of the channel walls. A certain amount of water is supplied to the water distribution device. When water evaporates, the temperature of the auxiliary air flow decreases (with an increase in its moisture content), and the channel wall also cools.
To increase the cooling depth of the main air flow, multi-stage processing schemes for the main air flow have been developed, using which it is theoretically possible to achieve the dew point temperature (Fig. 3.7).
The installation consists of an air conditioner and a cooling tower. The air conditioner produces indirect and direct isenthalpy cooling of the air in the serviced premises.
In the cooling tower, evaporative cooling of the water that feeds the surface air cooler of the air conditioner occurs.
Rice. 3.5. Diagram of the design of a combined indirect evaporative cooling apparatus: 1,2 - group of channels; 3- water distribution device; 4- pallet
Rice. 3.6. Scheme of SCR two-stage evaporative cooling. 1-surface air cooler; 2-irrigation chamber; 3- cooling tower; 4-pump; 5-bypass with air valve; 6-fan
In order to standardize evaporative cooling equipment, the spray chambers of standard central air conditioners can be used instead of a cooling tower.
Outside air enters the air conditioner and is cooled at the first cooling stage (air cooler) with a constant moisture content. The second stage of cooling is the irrigation chamber, operating in isenthalpy cooling mode. Cooling of the water feeding the surfaces of the water cooler is carried out in a cooling tower. The water in this circuit circulates using a pump. Cooling tower is a device for cooling water with atmospheric air. Cooling occurs due to the evaporation of part of the water flowing down the sprinkler under the influence of gravity (evaporation of 1% of water lowers its temperature by about 6).
Rice. 3.7. diagram with two-stage evaporation mode
cooling
The air conditioner's irrigation chamber is equipped with a bypass channel with an air valve or has an adjustable process, which ensures regulation of the air directed into the room served by the fan.
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 treatment 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.
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 an 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 with a ventilation system from a temperature of -22 0 C and a humidity of 86% (outside 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 installation.
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,
