Article by: Natthawut Ruengtrakoom, Ph.D.
Department of Mechanical Engineering, Faculty of Engineer, King Mongkut’s Institute of Technology Ladkrabang
If we consider the ratio of the energy used in a building, the large portion of used energy is the electrical energy consumed by air conditioning system (A/C system). This energy expense is used for air treatment process in order to improve the air quality for living space (conditioned space). To meet with the human comfort condition, the temperature, humidity and cleanness of the air supplied to living space must be treated.
If we consider the ratio of the energy used in a building, the large portion of used energy is the electrical energy consumed by air conditioning system (A/C system). This energy expense is used for air treatment process in order to improve the air quality for living space (conditioned space). To meet with the human comfort condition, the temperature, humidity and cleanness of the air supplied to living space must be treated.
Basically, the dehumidification process for large building can mainly be divided into two processes, including chemical dehumidification and mechanical dehumidification processes. The principle of chemical dehumidification process is the use the benefit of a desiccant which can be classified into two types: liquid and solid desiccants. The example of liquid desiccant is Li-Br solution, and activated alumina, silica gel and zeolites are the example of solid desiccant. The chemical dehumidification process for A/C system can help to increase the overall of the coefficient of performance (COP) of the system. Moreover, the system’s COP obtained from using chemical dehumidification process for A/C system is higher than that of using mechanical dehumidification process. However, if we consider about the expedience of installation and initial cost, the mechanical dehumidification process will become an attractive process. Therefore, the study and development of mechanical dehumidification process in order to obtain the higher system’s COP is still necessary.
Direct expansion (DX system) dehumidification is the one of interesting process of mechanical dehumidification. This process has continually been developed. Figure 1 shows the basic configuration of DX system. In basic system, the temperature and humidity of OA are treated by using only 1 evaporator. For dehumidification process, the moisture contained in OA has to be removed, so the temperature of OA must be reduced to dew point temperature. Basically, this temperature point is always colder than the desired point of supply air. Before used as supply air for conditioned space, very cold treated OA will be heated as it flows through reheating coil. Hot refrigerant vapor from compressor is used for hot fluid side. Figure 2 shows the configuration of modified DX system for better performance. The prototype was developed by Chinese researchers1.
According to the modified system, dehumidification process is achieved by 2 evaporators (two-stage direct expansion dehumidification). Moreover, exhaust air coil is installed to recovery the energy of return air before releasing to ambient as exhaust air. For OA treatment process, the operation of evaporator is divided into 2 sections which are high temperature section (HTS) and low temperature section (LTS). Firstly, the air is drawn from outside and passing through HTS. The temperature and humidity of the air will slightly be reduced. The deep humidification process is designed to happen at LTS. As same as basic DX system, the reheating coil is used to increase the sensible temperature of cold treated air leaving from LTS. After the temperature and humidity of OA are attained to desired point. Then, this treated air can be used as supply air for the refrigerated space.
According to the modified system, dehumidification process is achieved by 2 evaporators (two-stage direct expansion dehumidification). Moreover, exhaust air coil is installed to recovery the energy of return air before releasing to ambient as exhaust air. For OA treatment process, the operation of evaporator is divided into 2 sections which are high temperature section (HTS) and low temperature section (LTS). Firstly, the air is drawn from outside and passing through HTS. The temperature and humidity of the air will slightly be reduced. The deep humidification process is designed to happen at LTS. As same as basic DX system, the reheating coil is used to increase the sensible temperature of cold treated air leaving from LTS. After the temperature and humidity of OA are attained to desired point. Then, this treated air can be used as supply air for the refrigerated space.
