Indirect Evaporative Air-Conditioner
Indirect evaporative cooler (with dual-mode operation) is promising technology for composite climate due to its low energy consumption and high efficiency in its range of applications…
- Dr Jahar Sarkar
Evaporative cooling is a method of utilizing natural cooling effect to cool the building. The evaporative coolers appeared around 2500 BC, when porous clay jars containing water were used by the ancient Egyptians for air cooling purpose. This evaporative cooling mechanism was applied to cool the ancient Egypt buildings and then spread across the hot region of Middle East. In early 1950s, the evaporative coolers of modern type were developed in USA and available in wide range of market places including Canada, USA, and Australia. Evaporative cooling has several benefits including local fabrication, energy and cost saving, no synthetics chemicals, reduced peak demand, improved air quality, environmental friendly, etc.
The evaporative cooling (EC) technology is based on heat and mass transfer between air and cooling water. It is in general two types: direct and indirect. Direct evaporative cooling (DEC) is based on mechanical and thermal contact between air and water, while indirect evaporative cooling (IEC) is based on heat and mass transfer between two streams of air, separated by a heat transfer surface with a dry side where only air is cooling and a wet side where both air and water are cooling. Both DEC and IEC are characterized by very high energy efficiency but also by significant water consumption rates. DEC functions well in dry climate as the air is direct contact with water (humidity increases); however, not suitable for wet climate. Furthermore, air quality is strongly dependent on water purity as well as cooling pad quality due to direct contact (dirty or bad quality water and/or pad may lead to Legionnaire’s disease). Hence, IEC is a better option as it works well for wet climate also and air quality is independent on water or pad quality. In this article, working principle, configuration, construction and commercial status of IEC are discussed in details.
Conventional Indirect Evaporative Cooler (IEC)
In the case of IEC technology, the primary (or product) air is cooling down in dry side and secondary (or working) air is cooling and humidified in the wet side. It is not as widely used as direct EC, but it is gaining popularity, because it cools air more than direct EC, and cools the air down from higher wet-bulb temperatures. As shown in Figure 1(a), the air temperature decreases by converting the sensible heat to the latent heat through water evaporation; however, the humidity of the air increases for DEC. In moist conditions, the relative humidity of the leaving air can reach as high as 80% and such a high humidity air is not suitable for direct supply to the space, because it may cause warping, rusting, and mildew of susceptible materials. Therefore, DEC is only suitable for dry and hot climates, or air conditioned spaces with simultaneous cooling and humidification demands. In view of this, indirect evaporative cooling (IEC), developed in 1903, is able to cool the air and avoids adding moisture to the air by separating water and air, which makes it more attractive in humid areas. IEC involves two air streams: primary air and secondary air as shown in Figure 1(b). The air directly cooled by water evaporation in wet channel is called the secondary air. The cool and moist secondary air is used to cool the primary air (the air to be supplied to air conditioned space) by a heat exchanger. At outlet, the primary air will have a lower temperature as at inlet, due to the transferred heat. The secondary (working) air is flowing inside the wet channels together with the water. The behavior of the air and water in the wet channel is similar with the DEC process. The water temperature is the wet bulb (WB) temperature of the secondary air. The heat transferred through the surface between the dry and wet channels is absorbed by the water as latent heat and a corresponding part of the water is evaporated being embedded by diffusion into the secondary air, increasing the moisture content of this air. If the secondary air arrives at the saturation state, after this stage forward the heat from the primary air is split as latent heat absorbed by the water and as sensible heat absorbed by the secondary air. Thus, the temperature of the secondary air at the outlet can be one of the following: (a) Lower than the WB temperature of the secondary air at the inlet (no saturation); (b) Equal with the WB temperature of the secondary air at the inlet (saturation is reached at the outlet); (c) Higher than the WB temperature of the secondary air at the inlet (saturation before the outlet).
Figure 1: (a) Direct evaporative cooler and (b) indirect evaporative cooler
The main advantage of the IEC is that primary air is cooled without modifying its moisture content. The main disadvantage of the IEC is that the cooling process of the primary air is limited by the WB temperature of the secondary air at the inlet. Because of this limitation, this type of equipment is also named wet bulb IEC.
According to the types of heat exchanger used in IEC, there are tubular type IEC, plate type IEC and heat pipe IEC, as shown in Figs. 2(a-c), respectively. In the plate and tubular type IEC, the first air and secondary air are separated by an air-to-air heat exchanger, while in the heat pipe IEC, the condenser section is used in the secondary air flow channel, and the evaporator section is used in primary air flow channel. In IEC, the primary air is cooled without direct contact with water, thus the absolute humidity of the first air remains unchanged, while its temperature decreases. The heat exchanger used in IEC avoids the direct contact of the primary air with water; however, it is at the cost of the decreased efficiency. Usually, the efficiency of IEC is 50–70%, which is lower than that of DEC. The efficiency of the IEC mainly depends on the efficiency of the heat exchanger, inlet air states and the air flow rate ratio of the first air to the secondary air. Compared with the plate type IEC, the flow channel of the tubular IEC is broader and hence not easy to be blocked. But its heat transfer efficiency is lower than that of the plate type IEC. Much research has been conducted to enhance the heat transfer of the tubular IEC. In the heat pipe IEC, the outlet air temperature of the heat pipe IEC can be 2.5 °C lower and the corresponding cooling efficiency is 5–9% higher as compared with heat pipe heat exchangers. Flow arrangement between primary air and secondary air in the conventional IEC may by parallel flow, cross flow or counter flow.
Regenerative Indirect Evaporative Cooler (R-IEC)
The regenerative indirect evaporative cooling was developed to decrease the primary air temperature at the outlet, below the WB temperature of the secondary air at the inlet. The regeneration consists in extracting a part of the primary air at its outlet and using it as secondary air. In this case, because the secondary air is already cooled, the corresponding WB temperature is sensible lower than the WB temperature of regular (outside) secondary air and the limit at which the primary air can be cooled became considerably lower. As shown in Figure 3, the warm primary air (1) is flowing inside the dry channels and transfers heat through the heat surface to the wet channels. At its outlet, the primary air (2) will have a lower temperature than at inlet. A part of the outlet primary air is used as secondary air (much cooler than that of conventional IEC) being introduced in the wet channels. The working process of the primary air (1-2) is realized at constant moisture content and the final dry bulb temperature of the primary air is considerable lower than in the case of conventional IEC and below the WB temperature of the primary air at inlet. At limit, the cooling process of the primary air could continue until reaching the WB temperature of the secondary air at the inlet. This type of equipment is also named sub wet bulb IEC. The main advantage of the R-IEC is that primary air is cooled at constant moisture content below the WB temperature of the primary air. The main disadvantage of the R-IEC is that the flow rate of the primary air is lower than in the case of basic IEC.
Figure 3: Schematics and psychrometric process of R-IEC
Dew Point Indirect Evaporative Cooler (D-IEC)
The dew point indirect evaporative cooler was developed to decrease the primary air temperature near the limit of the dew point (DP) temperature of the primary air at the inlet. The D-IEC consists in multiple stages of R-IEC equipment. The working principle of D-IEC equipment, with two stages of R-IEC is presented in Figure 4. The warm primary air (1) is flowing inside the dry channels and transfers heat through the heat surface to the wet channels. At outlet, the primary air (2a) will have a lower temperature. A part of the outlet primary air of the first stage is used as secondary air (always much cooler than that of basic IEC) of the first stage being introduced in the wet channels. The rest of the outlet primary air of the first stage is used as primary air of the second stage. The working process of the primary air (1-2a-2b) is realized at constant moisture and can approach the DP temperature of the primary air at the inlet on the first stage. The working process of the secondary air in all stages are (2a-3a), (2b-3b), etc., represented on the psychrometric chart. At limit, the cooling process of the primary air could continue near the DP temperature of the primary air at the inlet. This behavior of the primary air is justifying why these equipment are also named dry bulb IEC. The main advantage of the D-IEC is that primary air is cooled at constant moisture content almost near the DP temperature. The main disadvantage of the D-IEC is that flow rate of the primary air is decreasing with the number of stages.
Figure 4: Schematics and psychrometric process of D-IEC
Maisotsenko Indirect Evaporative Cooling (M-IEC)
The Maisotsenko indirect evaporative cooler, developed by Dr Maisotsenko in Russia, is representing an alternative possibility for cooling the primary air near the DP temperature of the inlet air. Figure 5 shows the schematics and psychrometric air handling process of an ideal M-IEC in which the working and product air have the same inlet condition. Similar to the structure of conventional IEC, there are two flow channels in the M-IEC: the dry flow channel designed for the product air stream and the working air stream channel. The main difference with conventional IEC is that, there are numerous holes distributed regularly in the air flow channels of the incoming working air. When the working air flows along the dry side of the working air channel, it is cooled and partially diverted to the wet side through the holes. The wet side of the working air channel is soaked in water, resulting in a change of stage from 1 to 3’, in which 3 is variable depending on the structure of the holes. The air in the wet side of the working air channel is able to absorb heat from its two adjacent sides, which results in the changes of stage gradually from 3’, 3’ to 3”. In the meantime, the product air flows over the other adjacent dry side (the product air dry channel) and is cooled from state 1 to 2. The driving force in the conventional IEC is the difference between the dry-bulb temperature of the primary air and the fixed wet-bulb temperature of the secondary air. However, in M-IEC, the difference between the dry-bulb temperature of the primary air and the decreasing web-bulb temperature or the dew-point temperature of the secondary air is the driving force. The lowest possible temperature of the primary air at the outlet of the M-IEC is the dew point temperature of the entering primary air. Therefore, the saturation efficiency of M-IEC based on the inlet wet-bulb temperature can be higher than 100%, and also higher than that of the conventional IEC. The main advantage of the M-IEC is that primary air is cooled without modifying the moisture content almost near the DP temperature. The main disadvantage of the M-IEC is the complex construction and flow scheme inside the equipment.
Figure 5: Schematics and psychrometric process of M-IEC
Application Opportunities in India
India possesses a large variation in climate and generally fall sunder five climatic zones i.e. hot-dry, warm-humid, composite, temperate and cold (Figure 6). Out of these, major areas undergo composite, hot-dry and warm-humid condition. In the warm-humid climatic zone, the direct evaporative cooler cannot be applied successfully because of the high relative humidity in the summer season of about 80–90%. Indirect evaporative cooler is also not so effective for this zone due to high specific humidity. Thermal comfort in this climate is possible only by using the air conditioner. In the hot-dry climatic zone, the direct evaporative cooling is more applied because of less relative humidity in the range i.e. 30–50%. The temperature and humidity can be easily controlled using direct evaporative cooling. For buildings and areas that do not have a central air conditioning system, direct air evaporative cooling can be a very economical and efficient way to reduce the temperature. In extremely dry climates, evaporative cooling of air has the added benefit of conditioning the air with more moisture for the comfort of building occupants. The major area of India undergoes composite climate zone. For this zone, DEC is best applied in dry season (March-June) and it is widely used; however, the IEC can be applicable in relatively humid season (June-September). It is interestingly to note that IEC can be operated in dual-mode (direct mode, i.e. outlet air of wet channel as product air in dry season and indirect mode, i.e. outlet air of dry channel as product air in wet season) and that IEC can be effectively and economically used for this climate zone. IEC can be effectively applied in coastal areas (humid-hot zone) also; however, the energy saving potential is much less than that in interior zone. The desiccant based IEC is another best option in the high humid zone, but not cost effective. In–overall, the indirect evaporative cooler has a high potential to use all climate zone (dual mode operation) and hence to save energy and to reduce green house gas emissions associated with cooling of buildings.
Figure 6: Various climate zones in India
Indirect evaporative cooler has many potential commercial applications including commercial kitchens, hotels and restaurants, hospitals, offices, other institutions, laundry and dry cleaning, industrial applications, agricultural applications, poultry sheds, transit buses and data centre. Several international manufacturers have developed the prototypes and some applications have been also reported. However, full-scale manufacturing as well as commercial applications has not stated yet may be due to the fact that IEC is immature technology, more complex and less cost effective compared to DEC. But, it is expected that this technology will become mature and available in common markets very soon.
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