Study on the heat and mass transfer taking place in a direct expansion (DX) air cooling and dehumidification coil

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Study on the heat and mass transfer taking place in a direct expansion (DX) air cooling and dehumidification coil

 

Author: Xia, Liang
Title: Study on the heat and mass transfer taking place in a direct expansion (DX) air cooling and dehumidification coil
Degree: Ph.D.
Year: 2010
Subject: Hong Kong Polytechnic University -- Dissertations
Air conditioning -- Equipment and supplies
Ventilation -- Equipment and supplies
Air quality management
Department: Dept. of Building Services Engineering
Pages: xxii, 187 leaves : ill. (some col.) ; 30 cm.
InnoPac Record: http://library.polyu.edu.hk/record=b2343021
URI: http://theses.lib.polyu.edu.hk/handle/200/5707
Abstract: Direct expansion (DX) Air Conditioning (A/C) units are commonly seen in small to medium sized buildings. In the evaporator of a DX A/C unit, or a DX air cooling coil, usually simultaneous heat and mass transfer, in the form of cooling and dehumidifying of the warm and humid air flowing through the cooling coil, takes place, when the surface temperature of the DX cooling coil is lower than the dew point temperature of the air stream. The use of a DX A/C unit having a single-speed compressor and air supply fan, normally relies on on-off control to only maintain indoor dry-bulb temperature, resulting in an uncontrolled equilibrium indoor humidity, and leading to a reduced level of thermal comfort, poor indoor air quality (IAQ) and low energy efficiency. Hence, DX A/C units having variable-speed compressor and air supply fan are increasingly used for pursuing a thermally comfortable indoor environment at a higher energy efficiency. Therefore, it is important to study the performance of the simultaneous heat and mass transfer taking place in the DX air cooling coil of a DX A/C unit having a variable-speed compressor and air supply fan, which has been inadequately investigated. This thesis begins with addressing the calculation of an important dimensionless parameter, i.e., steady state Equipment Sensible Heat Ratio (SHR) of a DX air cooling coil, which is defined as the ratio of the output sensible cooling capacity to the total output cooling capacity from the DX cooling coil. The determination of Equipment SHR is essential because a satisfactory indoor thermal environmental control in a space requires not only a match between the total output cooling capacity from the DX A/C unit serving the space and the total space cooling load, but also a match between unit's Equipment SHR and an Application SHR which is defined as a ratio of the space sensible cooling load to the total space cooling load. A Calculation Method for the Equipment SHR of a DX cooling coil has been developed and reported. The calculation method was validated through a comparison between its results and the experimental results from different operating conditions, i.e., different combinations of compressor speed and air supply fan speed, and different inlet air conditions to the DX cooling coil. With the Method developed, the effect of refrigerant evaporating temperature at fixed inlet air conditions on Equipment SHR was theoretically analyzed.
Secondly, the thesis presents an experimental study on estimating on the dehumidification effect on the airside of the superheated region (SPR) in a DX cooling coil. Previously when investigating the heat and mass transfer taking place in DX cooling coils, a dry airside in a SPR was normally assumed without validation. This assumption conflicted with the experimental observation from an operating DX cooling coil. Therefore, an experimental study has been carried out to examine the validity of such an assumption under different operating conditions. A lumped parameter calculation procedure was developed specifically for processing the experimental data. The experimental results suggested that the airside surface of the SPR in a DX air cooling coil was either fully or partially wet under all experimental conditions. Consequently assuming a dry airside in the SPR could lead to an underestimated total amount of water vapor condensed on the entire airside of the DX cooling coil. Thirdly, the thesis reports on the development of a modified Logarithmic Mean Enthalpy Difference (LMED) method for evaluating the total heat transfer rate in a wet air cooling coil operating under both unit and non-unit Lewis Factors (Le² /³) conditions. The development stemmed from the inaccurate assumption of Lewis Factor being unit in the existing LMED method, which has been extensively applied to modeling the combined heat and mass transfer taking place in both DX and chilled water cooling coils. The assumption resulted in calculation errors when non-unit Lewis Factors were encountered, which has been however observed by many researchers. A modified LMED (m-LMED) method has been therefore developed for calculating the total heat transfer rate under both unit and non-unit Lewis Factors. Although the m-LMED method was validated by comparing its prediction of the total heat transfer rate to that from numerically solving the fundamental governing equations of the heat and mass transfer in a wet chilled water cooling coil, the m-LMED method can also be applied to the DX cooling coils. Finally, the analytical solutions for evaluating the heat and mass transfer in both a wet DX cooling coil and a wet chilled water cooling coil, respectively, have been developed and are reported. The analytical solutions were validated by comparing their predictions with those from numerically solving the fundamental governing equations of the heat and mass transfer taking place in wet air cooling coils. With the analytical solutions, the distributions of air temperature and moisture content along the air flow direction in either a wet DX or a wet chilled water cooling coil can be evaluated. The analytical solutions can be a low-cost replacement to numerically solving the fundamental heat and mass transfer governing equations.

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