Match properties of heat transfer and coupled heat and mass transfer processes in air-conditioning system
Highlights
► Investigates match properties of heat or mass transfer processes in HVAC system. ► Losses are caused by limited transfer ability, flow and parameter mismatching. ► Condition of flow matching is the same heat capacity of the fluids. ► Parameter matching is only reached along the saturation line in air–water system. ► Analytical solutions of heat and mass transfer resistance are derived.
Introduction
The purpose of air-conditioning systems is to provide a suitable indoor environment with respect to temperature, humidity, fresh air, etc. There are many kinds of heat transfer devices in air-conditioning systems, including evaporators, condensers, and sensible heat recovery devices. Also, there are many kinds of coupled heat and mass transfer devices, such as cooling towers and humidifiers in air–water contact devices and dehumidifiers and regenerators in air–liquid desiccant contact devices. Many researchers have studied the performance of heat or coupled heat and mass transfer processes in air-conditioning systems both theoretically [1], [2], [3], [4] and experimentally [5], [6], [7], [8], [9], [10], [11]. The optimization of these heat and mass transfer processes involves making structural improvements to handling processes, distributing the heat transfer area differently in various devices, adjusting flow rates, etc. With the help of optimization, inefficient structures can be avoided, heat and mass transfer areas can be placed in the most effective positions, and the energy consumption of the air-conditioning system will be reduced. In analyzing these heat and mass transfer processes, certain principles are expected to guide the optimization.
Pinch technology, which is widely utilized in certain industrial processes [12], [13], [14], is used to optimize the heat transfer network from the perspective of the whole system and determine the match properties of simultaneous heating and cooling processes, thus minimizing the required additional input of heat or cold as much as possible. However, in air-conditioning systems, buildings require cooling in summer and heating in winter, so simultaneous heating and cooling requirements are rare. Entransy is a thermological parameter that was introduced to analyze the heat transfer process [15]. Equivalent heat resistance on the basis of entransy loss is used to optimize the heat transfer process, in which achieving the lowest equivalent heat resistance under certain constraints is the most important goal [16], [17].
Existing research has mainly focused on the transfer characteristics of a single heat or mass transfer process, but many issues regarding performance optimization remain unaddressed, such as how to achieve better performance at limited heat and mass transfer areas, how to design and arrange the air handling process, and what special principles or rules apply.
This paper focuses on the match properties of heat and mass transfer processes. The conditions of flow matching and parameter matching are analyzed, and the unmatched coefficients are derived through the entransy loss method. It is hoped that the results will benefit the optimization of sensible heat or coupled heat and mass transfer processes in air-conditioning systems.
Section snippets
Match properties of the sensible heat transfer process
According to the fluids’ temperature variation through the heat transfer process, the heat exchangers in air-conditioning systems can be divided into two types. In one type, the temperature of each fluid changes during the process, such as in air–water heat exchangers, air–air heat exchangers, and water–water heat exchangers. In the other type, the temperature of one side changes while the reverse side maintains a constant temperature, such as in evaporators and condensers. The heat transfer
Match properties of coupled heat and mass transfer processes
As shown in Fig. 10, the heat transfer process and mass transfer process are not independent but coupled with each other during the air handling process; this is true for air washers, cooling towers, evaporative coolers, dehumidifiers, and regenerators. Taking the heat and mass transfer process between air and water as an example, temperature difference ΔT and vapor pressure difference Δp (or Δω) between the two fluids are the driving forces behind heat transfer and mass transfer, respectively.
Application of match properties in coupled heat and mass transfer process
A humidifier is the main heat and mass transfer device used in air-conditioning systems. A conventional air handling process is shown in Fig. 17a, in which the inlet air is heated first and then hot air comes into a packing module and makes contact with the circulated water, thus achieving air humidification. Does this handling process achieve the best results? In this section, two processes are compared: the popular system (Type I) shown in Fig. 17a, and a new system (Type II) shown in Fig. 17
Conclusion
The match properties of the heat transfer process and the coupled heat and mass transfer process of air-conditioning systems are the subject of this paper. The main conclusions can be summarized as follows:
- (1)
Loss in the heat transfer process in the heat exchanger is caused by a limited heat transfer ability and unmatched flow rates of the two fluids. Limited heat and mass transfer ability, flow mismatching, and parameter mismatching lead to loss in the coupled heat and mass transfer process.
- (2)
The
Acknowledgements
This research was supported by National Natural Science Foundation of China (No. 51138005) and the Foundation for the Author of National Excellent Doctoral Dissertation of China.
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