Analysis of the applicability of air heat exchangers

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Analysis of the Applicability of Air Heat Exchangers Wang Huiyong, Lu Yuanwei (Beijing University of Technology) Abstract: To analyze the energy-saving effects and applicability of air heat exchangers, performance tests were conducted on existing sensible and total heat exchangers, while simultaneously measuring the temperature and humidity of outdoor air in order to calculate the energy consumption of fresh air during air conditioning operation. The analysis results show that the ability of a total heat air heat exchanger to recover waste heat from exhaust air is approximately 4 times that of a sensible heat air heat exchanger. Keywords: Air heat exchanger; Sensible heat; Latent heat; Total heat Analysis of the applicability of air heat exchanger Wang Huiyong, Lu Yuanwei (Beijing University of Technology) ABSTRACT: In order to analyze the applicability and energy-saving efficiency of air heat exchangers, the parameters related to sensible heat and total heat exchange were studied; at the same time, the outdoor temperature and humidity were measured in order to determine the amount of fresh air required. The capacity of the total heat exchangers to recover exhaust heat is 4 times that of the sensible heat exchangers. KEYWORDS: Air heat exchanger; Sensible heat; Latent heat; Total heat At present, most air-conditioned buildings do not recover the energy contained in the exhaust air, but instead discharge it directly outside. Meanwhile, fresh air from the outside is processed directly by the air-conditioning units to meet the requirements set by the “Indoor Air Quality Standards,” which results in energy waste. The \"Design Code for Heating, Ventilation and Air Conditioning\" and other relevant building design and energy-saving standards recommend the use of exhaust air heat exchangers whenever possible; the \"Energy-Saving Design Code for Public Buildings\" states that the energy contained in the exhaust air from air-conditioned areas (or rooms) is considerable, and its recovery can yield significant energy-saving and environmental benefits. An air heat exchanger is a device that can recover some of the waste heat from air conditioning exhaust air, thereby reducing the energy consumption required for fresh air. It can be divided into sensible heat exchangers and total heat exchangers. To analyze the energy-saving efficiency and applicability of air heat exchangers, while testing the performance of the existing sensible and total heat exchangers in the laboratory, the temperature and humidity of outdoor air were also measured in order to calculate the energy consumption of fresh air during air conditioning operation. 1 　Testing of the thermal performance of sensible and total heat air exchangers: The laboratory already has a disc-shaped sensible heat exchanger and a membrane-type total heat exchanger, as shown in Figure 1. The disc-shaped sensible heat exchanger is made by stamping copper sheets with a diameter of 200 mm and a thickness of 0.3 mm into a disc shape, with openings made on the sides to serve as air inlets; the membrane-type total heat exchanger features a rectangular channel structure made of a heat and mass exchange material (organic ultrafiltration membrane), with each channel having a length of 200 mm, a spacing of 5 mm between channels, and a height of 225 mm. Using the existing ventilation waste heat recovery test bench in the laboratory, as shown in Figure 2, the thermal performance of disc-type sensible heat exchangers and membrane-type total heat exchangers was evaluated. Outdoor hot and humid air and indoor dry and cold air flowed across the air exchanger to exchange heat (or moisture). Temperature and humidity measurement points 1, 2, 3, and 4 are installed at the inlet and outlet of the new exhaust air; their values are measured using a WS 301A temperature and humidity meter. Pressure measurement points 5 and 6 are placed on the pipes at the exhaust air inlet and outlet, and the air resistance loss as the new exhaust air passes through the heat exchanger is determined using a WQ21151 capacitive differential pressure transmitter. Wind speed measurement points 7 and 8 are installed at the inlet of the new exhaust air; their values are measured using a Testo 425 precision anemometer. This allows the system’s wind speed to be adjusted via the fan speed control switch, thereby ensuring that the volume of exhaust air is equal. To ensure that the test is conducted under stable conditions, the measurements are taken after the system has been operating continuously and stably for 30 minutes. The actual testing conditions adopted were those specified in the standard \"Air 2 – Air Energy Recovery Ventilation Systems\" (draft for comment), namely an outdoor dry-bulb temperature of 35 °C and a wet-bulb temperature of 28 °C; an indoor dry-bulb temperature of 27 °C and a wet-bulb temperature of 19.5 °C. The results are shown in Figures 3 and 4. Figure 1 depicts the existing air heat exchanger in the laboratory, while Figure 2 shows a schematic diagram of the testing system. The maximum sensible heat transfer efficiency of the disc-shaped sensible heat exchanger was measured to be 71.3%; the maximum sensible heat transfer efficiency of the membrane-type total heat exchanger was 70%, its latent heat transfer efficiency was 7016%, and its total heat transfer efficiency was 70.3%. Figure 3: Heat transfer efficiency of the disc-shaped sensible heat exchanger under standard test conditions. 2: Measurement results of outdoor temperature and humidity. The measurement period was from 2007206206 to 2007206215, from 8:30 to 22:30 every day; the total measurement time was 150 hours. A WS 301A temperature and humidity transmitter was used for data collection, with one set of data recorded every hour. The curves showing the actual changes in temperature and humidity during the measurement period are shown in Figure 5. During the measurement period, the outdoor temperature ranged from approximately 25 to 37 °C, with a maximum of 37.2 °C and an average of 30.7 °C; the outdoor humidity ranged from approximately 10 to 21 g/kg of dry air, with a maximum of 21.5 g/kg of dry air and an average of 15.8 g/kg of dry air. 3 　Calculation and analysis of the energy consumption for fresh air in air conditioning. Due to the significant climate differences between the north and south of China, the energy consumption per unit mass of fresh air required for air conditioning, as well as the ratio of sensible heat to latent heat energy in that fresh air consumption, vary in different regions with varying climate conditions, and even across different seasons within the same region. The formula for calculating the energy consumption of fresh air in air conditioning systems, based on mass flow rate (kg/s), is as follows: QS = Cp (te – ti) (1) QL = L (de – di) (2) Q = QS + QL (3) In Issue 4, Wang Huiyong et al.: Analysis of the Applicability of Air Heat Exchangers ·2·3· Here, Cp represents the specific heat capacity of air at constant pressure, with a value of 1.005 kJ/(kg·K); L is the latent heat of vaporization of water, equal to 2.540 kJ/g (at normal temperature); ti and te denote the indoor and outdoor air temperatures respectively (in °C); di and de represent the indoor and outdoor air humidities respectively (in g/kg of dry air); QS, QL, and Q represent the sensible heat, latent heat, and total heat energy consumption respectively (in kJ/kg). Figure 6 Composition of fresh air energy consumption during measurement period 4 Analysis of the suitability of air heat exchangers When the volume of fresh air and exhaust air is the same, based on the maximum heat exchange efficiency of sensible heat and total heat exchangers, as well as the composition of energy consumption related to fresh air in air conditioning systems, it is possible to estimate the amount of energy that can be recovered from the exhaust air when using either a sensible heat exchanger or a total heat exchanger, as well as the fresh air energy consumption of the air conditioning system before and after using such exchangers. The calculation formulas are: Q′ S = QS ·εS (4) Q′ L = QL ·εL (5) Q′= Q′ S + Q′ L (6) Where Q′ S, Q′ L, and Q′ represent the amount of sensible heat recovered, the amount of latent heat recovered, and the total amount of heat recovered, respectively (in kJ/kg); εS and εL represent the sensible heat efficiency and the latent heat efficiency, respectively. The energy recovery performance of the disc-type sensible heat exchanger and the membrane-type total heat exchanger is shown in Figures 7 and 8. As can be seen from Figure 6, during the measurement period, the maximum value of the sensible heat energy consumption related to fresh air handling in the air conditioning system was 10.3 kW, while the average value was 3.9 kW. The heat transfer efficiency of the disc-type sensible heat exchanger was 71.3%; therefore, the maximum amount of sensible heat energy that could be recovered was 7134 kW, with an average energy consumption of 2.78 kW. During the measurement period, the maximum full-heat energy consumption of the air conditioning system for fresh air was 33.5 kW, while the average value was 15.3 kW. The full-heat exchange efficiency of the membrane-type full-heat exchanger was 70.3%; therefore, the maximum amount of fresh air energy that could be recovered was 23.55 kW, with an average fresh air energy consumption of 10.76 kW. It can be seen that the heat recovery capacity of membrane total heat exchangers is approximately 4 times that of disc-type sensible heat exchangers; therefore, using membrane total heat exchangers enables more effective recovery of the residual heat in air conditioning exhaust air compared to using disc-type sensible heat exchangers. 5 　Conclusion 1) When selecting air heat exchangers, the latent heat energy consumption in the air conditioning fresh air energy consumption (which accounts for a large proportion) cannot be ignored. 2) The author tested the thermal performance of the self-designed disc-shaped sensible heat exchanger and membrane total heat exchanger, and analyzed their energy recovery capabilities. The results show that the membrane total heat exchanger’s ability to recover the waste heat from exhaust air is approximately 4 times that of the disc-type sensible heat exchanger; thus, the membrane total heat exchanger can recover the waste heat in air-conditioning exhaust air more effectively than the disc-type sensible heat exchanger. References: GB/T 18883-2002 Standard for indoor air quality; GB 50019-2003 Code for design of heating, ventilation and air conditioning systems; GB 50189-2005 Energy efficiency design standard for public buildings. Cao Yang. Air2 – Air energy recovery ventilation system (draft for comments). 2006.