China
Africa
Explore chapters and articles related to this topic
Energy Efficient Air Conditioning and Mechanical Ventilation
Published in Clive Beggs, Energy: Management, Supply and Conservation, 2010
On air-cooling applications where the evaporator may be operating below 0°C the fin spacing must allow for ice build-up. In order to maintain an adequate airflow through the evaporator it is necessary to defrost the coil periodically. Defrosting techniques involve either the use of an electric heating element built into the coil or periodically reversing the refrigeration cycle so that the evaporator effectively becomes a hot condenser. While essential for the correct operation of the system, the defrost process can be a potential source of energy wastage. It is therefore important that the defrost operation only be initiated when absolutely necessary and that the defrost heat be evenly distributed over the whole of the fin block. Prolonged defrosting is energy wasteful and therefore the defrost cycle should be stopped as soon as possible. If not controlled and monitored properly defrost systems can needlessly waste large amounts of energy.
Unburned Combustibles
Published in Charles E. Baukal, Industrial Combustion Pollution and Control, 2003
The basic principle behind this technique is to cool the gases sufficiently with some type of heat exchanger to condense out the VOCs, which are converted into a liquid that can be simply removed from the gas stream (see Fig. 7.32). Unfortunately, relatively cold temperatures are often required. In many cases, normal mechanical refrigeration may not get the stream sufficiently cold to condense out the VOCs efficiently. In fact, these temperatures may be low enough that normal materials must be replaced with materials capable of going to lower temperatures. For example, carbon steel may need to be replaced with copper or brass, which is often more expensive. Another significant problem in removing VOCs from exhaust gas streams is that the gases often contain large quantities of water that will freeze out well before the VOCs. This causes ice and frost to form on the cooling equipment, which reduces its efficiency and therefore frequent defrosting is often required. Another major obstacle of this technique for cleaning exhaus1 gas streams is that the
Introduction
Published in Hui Huang, Heat Pumps for Cold Climate Heating, 2020
Hui Huang, Xiangfei Liang, Bo Zheng
When the air source heat pump operates in a heating operation mode and meets the conditions of frosting, the heating capacity and COP decrease rapidly with the increase of the frost thickness. Therefore, it is necessary to defrost in time to make sure the system resumes to normal operating state. The main defrosting methods are reverse cycle defrosting, hot-gas bypass defrosting and thermal storage defrosting, etc.
Experimental investigation of the time–temperature difference (t–dT) defrosting control method in frost-free household refrigerators
Published in Science and Technology for the Built Environment, 2019
Chenxi Ni, Tiejun Wang, Bin Jiang, Fan Yang
Hot gas, reverse cycle, and electrical heaters are the most common defrosting techniques. The first two methods consume less energy but require some system modifications, which makes them unfeasible for most domestic refrigerators. Although an electrical heater increases the air-side pressure drop and presents corrosion problems, it is much cheaper. Therefore, electric heating is the most typical method for frost removal in evaporators of frost-free refrigerators (Kim et al. 2006; Melo et al. 2013). A key factor affecting the defrosting efficiency of electrical heater defrosting methods is the operation of the electric heater (Melo et al. 2013). Thus, the primary task is to accurately determine the starting and ending times of the defrosting process. If the starting time of the defrosting operation is too late, leading to incomplete removal of the remaining frost between the evaporator fins, ice blocks can form on the evaporator in the next period of frosting. In addition, the ice blocks would increasingly grow heavier, resulting in poor performance. This aggravated situation may lead to more energy waste or even refrigerator failure.
Review on the optimization studies of reverse cycle defrosting for air source heat pump units with multi-circuit outdoor coils
Published in International Journal of Green Energy, 2023
Mengjie Song, Yueyang Tian, Long Zhang, Chaobin Dang, Yanxin Hu, Yingjie Xu, Limei Shen, Tianzhuo Zhan
Since the essence of frost is ice, defrosting methods generally consist of mechanical and thermal defrosting. The former removes the frost layer on the surface of the fins by mechanical force and the latter by heating it. Mechanical defrosting methods include mechanical scraping, high-pressure air blowing, ultrasonic crushing, etc. In contrast, thermal defrosting methods include compressor shutdown defrosting, electric heating defrosting, hot water spray defrosting, hot gas bypass defrosting, RCD, etc. Among them, the RCD is to change the refrigerant flow direction by adjusting the four-way reversing valve so that the roles of the evaporator and the condenser are interchanged. Meanwhile, the indoor air fan is turned on, and the outdoor air fan is turned off. The role of the outdoor heat exchanger changes from an evaporator to a condenser, and the heat of the indoor air is used for rapidly melting frost and evaporating melted water. For RCD operation, defrosting energy comes from the electric energy input by the compressor and the indoor air fan, the heat stored in the metal of the indoor heat exchanger, and the forced convection between the indoor heat exchanger and the indoor air. The energy transferred to the outdoor heat exchanger through the refrigerant is used for not only melting frost and evaporating melted water but also heating the metal of the outdoor heat exchanger and its surrounding low-temperature air. Since the frost layer on the surface of the outdoor heat exchanger of an ASHP unit accumulates as time passes, the starting time and defrosting duration are also flexible. RCD has become the most widely used defrosting method for ASHP due to many advantages, such as simple system reconstruction, short defrosting duration, high defrosting efficiency, and no need for external auxiliary heat sources, etc.