abstract:

At present, the centralized evaporative cooling air conditioning system has been more and more widely used in western China, but its disadvantages are large air duct and poor flexibility, and it is impossible to regulate and control multiple rooms separately. Aiming at the shortcomings of centralized system, a semi-centralized evaporative cooling air conditioning system different from the traditional fan-coil plus fresh air system is proposed, and its feasibility is analyzed theoretically.

Key words:

Environmental protection and energy saving of evaporative cooling semi-centralized air conditioning system

1. Present situation of evaporative cooling technology

The evaporative cooling process uses water as refrigerant. Because chlorofluorocarbons are not used, there is no pollution to the atmospheric environment, and fresh air can be directly used, which greatly improves the indoor air quality. Like the usual mechanical refrigeration principle, refrigeration is provided by evaporation of refrigerant. But for evaporative cooling, energy is obtained by evaporation of water. It does not compress and condense the evaporated water vapor back to liquid water before evaporation. Generally, water can be directly replenished to maintain the evaporation process.

According to the application analysis of evaporative cooling air conditioning in Urumqi, Xi, Harbin and Beijing, it can be known that its operating energy consumption is about 1/5 of that of conventional air conditioning equipment (the installed power of mechanical refrigeration system is about 50w/m2, and the installed power of evaporative cooling system is 10 w/m2, saving electricity by 80%). In terms of initial investment, it is about12 of that of conventional air-conditioning equipment (the cost of mechanical refrigeration is about 400 yuan /m2, and the cost of evaporative cooling system is about 250 yuan /m2, saving investment by 30-50%), and it has humidification function; In terms of indoor air quality, evaporative cooling system is obviously superior to conventional air conditioning system, because it runs with fresh air of 100%, and it uses water as refrigerant, without CFCS, and has no pollution to the atmospheric environment.

This technology was introduced into China in the mid-1980s, and has been studied and applied in arid areas in western China (especially in Xinjiang). Because of the large temperature difference between day and night in northwest China, the air is dry, and the wet bulb temperature of outdoor air conditioners is relatively low in summer (generally lower than 22 degrees). The temperature difference between day and night is large, and the temperature (dry bulb temperature) between morning and evening and noon every day is quite different; In winter, the outdoor dry bulb temperature is low, mostly in dry and cold climate (if only indoor heating is provided, the relative humidity of indoor air is generally lower than 20%). These unique meteorological conditions provide a natural application place for evaporative cooling technology, because evaporative cooling is a cooling technology suitable for dry areas, which uses moisture evaporation to absorb heat to reduce the air supply temperature, thus reducing the room temperature. It is precisely because of the special climatic conditions in the west that it is possible for evaporative cooling air conditioning system to replace conventional air conditioning system. At present, evaporative cooling air conditioning system has been widely used in hotels, office buildings, restaurants, entertainment, gymnasiums, theaters and other public and civil buildings and some industrial buildings in Xinjiang. Only Urumqi Green Messenger Central Air Conditioning Co., Ltd. has completed more than 70 projects in Xinjiang [1].

2. Problems existing in evaporative cooling air conditioning

At present, mechanical refrigeration and air conditioning systems are still widely used in many high-rise buildings and public buildings in western China. Although these systems provide a comfortable working and living environment, compared with evaporative cooling air conditioning units, they have huge one-time investment, expensive operation cost, complicated maintenance and conservation, and will cause "sick building syndrome" and environmental pollution. Especially after the outbreak of SARS, the safety of air conditioning system has been paid more attention by people in HVAC industry and the Ministry of Health. Indoor air quality has been paid more and more attention. The evaporative cooling system runs with fresh air of 100% without using CFCS, which has no pollution to the atmospheric environment and is obviously superior to the conventional air conditioning system. At present, centralized evaporative cooling system is widely used in western China, which has the advantages of long service time and convenient maintenance. The whole system only has air duct laying, and there is no waterway layout that needs air conditioning, so the design is simple and the cost is low. Because there is no need to set water pipes in the ceiling, the problem of condensed water leakage is completely eliminated. In addition, the system mostly uses fresh air, which greatly improves the indoor air quality, and at the same time, it can save energy by using fresh air in the transitional season.

The centralized evaporative cooling system also has some shortcomings: firstly, the application of the unit direct evaporative cooling air conditioner may lead to high indoor humidity (the indoor humidity is about 75% through the field test of the completed system in Urumqi). Secondly, due to the use of cold air to cool the room, the specific heat of air is small and the air volume of the system is large, which leads to the fact that the air duct of the system occupies more space than the general semi-centralized air conditioning system, resulting in poor flexibility. Thirdly, considering the cost, there is no cheap terminal product to realize the separate control and adjustment of multiple rooms. However, from the point of view of design and economy, it is feasible to use comfortable air conditioners with low temperature and humidity control accuracy, especially in large entertainment places, restaurants, shopping malls, stadiums, conference centers, various activity centers and other public places. This is also an important reason why the centralized evaporative cooling air conditioning system has been widely used in Xinjiang in recent years [2].

3. Proposed semi-centralized evaporative cooling air conditioning system.

Because of the shortcomings of centralized system, such as large air duct, poor flexibility and inability to control multiple rooms separately. Therefore, the application of central air conditioning system is limited in some occasions. Because the traditional semi-centralized air conditioning system can independently adjust the temperature of each room, it is suitable for hotel rooms, writing rooms and other places where it is difficult to arrange air ducts and floors are low. Therefore, aiming at the shortcomings of centralized system, this paper puts forward a semi-centralized evaporative cooling air conditioning system which is different from the traditional fan-coil unit plus fresh air system, and makes a feasibility analysis in theory.

3. 1 semi-centralized evaporative cooling air conditioning system

This system is slightly different from the traditional fan coil plus fresh air system. The refrigerant used in the traditional fan-coil unit plus fresh air system is cold water provided by the chiller, so the chiller is the core. The core of semi-centralized evaporative cooling system is the evaporative cooling part, which uses the evaporation of water to obtain energy. It does not compress and condense the evaporated water vapor back to liquid water before evaporation, but directly supplements water to maintain the evaporation process. The fresh air in the system is treated by evaporative cooling fresh air unit, and single-stage or multi-stage evaporative cooling can be selected according to outdoor design parameters and load characteristics. See Figure 3- 1 for the specific schematic diagram.

Traditional semi-centralized system evaporative cooling semi-centralized system

Figure 3- 1 Comparison between traditional system and evaporative cooling system

In the process of direct evaporative cooling, fresh air is humidified with equal enthalpy, and the temperature of circulating water is approximately equal to the wet bulb temperature of inlet air. Taking Urumqi as an example in summer, the calculated wet bulb temperature of outdoor air conditioner is about 18℃. When air is directly evaporated and cooled, the circulating water temperature can theoretically reach 18℃. If the indirect-direct evaporative cooling process is adopted, the fresh air will be cooled with the same humidity first, and then humidified with the same enthalpy, so that the treated circulating water temperature will be further reduced to 13 ~ 16℃. Although the water temperature treated by the above two methods is 7 ~ 12℃ higher than the chilled water temperature of the water chiller, it can still bear part of the load after entering cooling coils as long as the water volume is increased. Therefore, the main difference between the semi-centralized evaporative cooling system and the traditional system is that its load is entirely borne by the evaporative cooling process, and there is no need for water chillers and cooling water systems, so its initial investment is greatly reduced, and the comprehensive cost of one-time investment is only 40% ~ 80% of that of traditional refrigeration and air conditioning methods.

3.2 Feasibility analysis

In order to explore the feasibility of semi-centralized evaporative cooling air conditioning system in northwest China, taking Urumqi climate as an example, the design schemes are discussed and compared. See Figure 3-2 for indoor and outdoor status points and parameters in Urumqi.

Figure 3-2 Indoor and Outdoor Status Points

Location: Urumqi in summer

Season: summer

Tgw: Outdoor dry bulb temperature is 34. 1℃

Tsw: outdoor wet bulb temperature 18℃

Tgn: The indoor design temperature is 27℃

Relative humidity 60%

Atmospheric pressure 906.7 millibar

3.2. 1 traditional fan coil unit+fresh air system

As can be seen from Figure 3-2, the moisture content dw of outdoor air in summer is less than the moisture content dn of indoor air, that is, outdoor air needs humidification. In order to achieve this goal, in the traditional fan-coil plus fresh air system, a steam humidification system is usually installed in front of the fan to humidify the treated air at an isothermal temperature. See Figure 3-3.

Air treatment process (W outdoor air state point, N indoor air state point, KL fresh fan temperature rise)

Figure 3-3 Change Diagram of Air State of Traditional Fan Coil Unit plus Fresh Air System

3.2.2 Semi-centralized evaporative cooling system [fan coil unit+direct evaporative cooling fresh air unit] [3]

For the semi-centralized system with fan-coil unit and direct evaporative cooling fresh air unit, the ventilation process is shown in Figure 3-4.

Figure 3-4 Fan Coil Unit+Direct Evaporative Cooling Fresh Air Unit

Direct evaporative cooling of fresh air units, the maximum efficiency of direct evaporative cooling ηDEC can reach 90%, calculated as ηDEC=90%:

(3- 1)

Note: tws outdoor air wet bulb temperature

Direct evaporative cooling using circulating water treatment is an isenthalpic humidification process, so the state of point L can be determined. The circulating water temperature is finally fixed at the machine dew point L close to the outdoor wet bulb temperature. According to formula (3- 1):

tsh=tL=tw-(tw-tws)×90%

=34. 1-(34. 1- 18)×90%= 19.6℃

Note: tsh direct evaporative cooling circulating water temperature

When circulating water is introduced into the fan coil, because the water temperature of circulating water is slightly higher than the dew point temperature of indoor air 18.4℃, the indoor return air can only be cooled by equal humidity.

3.2.3 Semi-centralized evaporative cooling system [fan coil unit+(indirect+direct) evaporative cooling fresh air unit]

For the semi-centralized system of fan coil+(indirect+direct) evaporative cooling fresh air unit, the ventilation process is shown in Figure 3-5.

Figure 3-5 Fan Coil Unit+(Indirect+Direct) Evaporative Cooling Fresh Air Unit

Indirect+direct evaporative cooling fresh air unit. The efficiency ηIEC of plate-fin indirect evaporative cooler produced by Green Messenger Central Air Conditioning Co., Ltd. can reach 60 ~ 75% at the highest. If ηIEC=60%:

(3-2)

Note: tws outdoor air wet bulb temperature

Indirect evaporative cooling is an equal humidity cooling process, and the state of point P can be determined according to formula (3-2).

tP=tw-(tw-tws)×60%

=34. 1-(34. 1- 18)×60%

=24.4℃

According to tp=24.4℃, the wet bulb temperature tps= 14.8℃, and the inlet temperature of direct evaporative cooling is 24.4℃. According to the formula (3- 1), tsh=tL=tp-(tp-tps)×90%.

=24.4-(24.4- 14.8)×90%

= 15.76℃

Note: tsh direct evaporative cooling circulating water temperature

Circulating water is introduced into the fan coil unit. Because the water temperature of circulating water is lower than the dew point temperature of indoor air 18.4℃, indoor return air can be dehumidified and cooled.

3.2.4 Semi-centralized evaporative cooling system [fan coil unit+(indirect 1+ indirect 2+ direct) evaporative cooling fresh air unit]

Fan coil unit+(indirect 1+ indirect 2+ direct) evaporative cooling fresh air unit, and the ventilation process is shown in Figure 3-6.

Figure 3-6 Indirect 1+ Indirect 2+ Direct Evaporative Cooling Semi-centralized System

A three-stage evaporative cooling fresh air unit with surface cooler section is adopted (the first stage indirect evaporative cooling section is cooled by cooling tower), and the surface cooler section uses cooling water of cooling tower to cool fresh air. This cooling tower, which introduces cooling water into the surface cooler, realizes the same wet cooling treatment as indirect evaporative cooling. Therefore, this indirect+direct evaporative cooling unit with cooling tower is also called three-stage evaporative cooling unit (two-stage indirect evaporative cooling+direct evaporative cooling). If the cooling water of cooling tower is used, the cooling efficiency can reach about η cooling tower = 40 ~ 50%, and the final temperature of air ≈ the initial wet bulb temperature of air w+6 ~ 8℃. According to the calculation of cooling tower η= 50%, there are:

(3-3)

First, the state of point P can be determined according to formula (3-3).

tP=tw-(tw-tws)×50%

=34. 1-(34. 1- 18)×50%

=26℃

According to the enthalpy-humidity diagram, the inlet dry-bulb temperature of indirect evaporative cooling is 26℃, and the wet-bulb temperature tps is 15.3℃. According to formula (3-2), the state of Q point can be determined.

tQ=tp-(tP-tPs)×60%

=26-(26- 15.3)×60%

= 19.6℃

According to the enthalpy-humidity diagram, the inlet dry-bulb temperature of direct evaporative cooling is 19.6℃, and the wet-bulb temperature tQS is 13.5℃. Then the state of the L point can be determined according to the formula (3- 1).

tL=tQ-(tQ-tQS)×90%

= 19.6-( 19.6- 13.5)×90%

= 14. 1℃

Circulating water is introduced into the fan coil unit. Because the water temperature of circulating water is lower than the dew point temperature of indoor air 18.4℃, indoor return air can be dehumidified and cooled.

4. Concluding remarks

Semi-centralized evaporative cooling system uses water as refrigerant and has no water chiller. Direct system and (indirect+direct) system have no cooling water system, so their initial investment is lower than that of traditional semi-centralized system, and their operating costs are less.

Because the water supply temperature of semi-centralized evaporative cooling system is high, the water supply is large. Among them, the cooling water in the direct evaporative cooling section will directly affect the refrigeration capacity of the unit. When the cooling water required by the load is large, it is necessary to consider water replenishment and water replenishment. , need further discussion.

refer to

1. Xiang,, et al. Study on the Application of Evaporative Cooling Technology in Northwest China, Proceedings of the National HVAC Technology Information Network Conference 4 1 19 ~ 423.

2. Liu Ming, Engineering Application of Evaporative Cooling Air Conditioning Technology, 2002 Joint Academic Annual Meeting of Heating, Ventilation, Air Conditioning, Refrigeration and Thermodynamics in Five Northwest Provinces, 84~87.

3. Chen Peilin, Application of evaporative cooling in air conditioning, Xi 'an Refrigeration, 1999, 1: 1 ~ 7.

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