- Email: [email protected]
Utilization of waste heat for energy conservation in domestic dryers
Accepted Manuscript Utilization of waste heat for energy conservation in domestic dryers
Su-Sheng Ma, Ching-Yi Tseng, You-Ren Jian, Tai-Her Yang, Sih-Li Chen PII:
S0360-5442(18)31519-6
DOI:
10.1016/j.energy.2018.08.011
Reference:
EGY 13487
To appear in:
Energy
Received Date:
05 March 2018
Accepted Date:
01 August 2018
Please cite this article as: Su-Sheng Ma, Ching-Yi Tseng, You-Ren Jian, Tai-Her Yang, Sih-Li Chen, Utilization of waste heat for energy conservation in domestic dryers, Energy (2018), doi: 10.1016/j.energy.2018.08.011
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Utilization of waste heat for energy conservation in domestic dryers Su-Sheng Maa, Ching-Yi Tsengb, You-Ren Jianc, Tai-Her Yangd, Sih-Li Chene,* aDepartment
of Mechanical Engineering, National Taiwan University, No. 1, Sec. 4,
Roosevelt Road, Taipei 10617, Taiwan bDepartment
of Mechanical Engineering, National Taiwan University, No. 1, Sec. 4,
Roosevelt Road, Taipei 10617, Taiwan cDepartment
of Mechanical Engineering, National Taiwan University, No. 1, Sec. 4,
Roosevelt Road, Taipei 10617, Taiwan dGiant
Lion Know-How Co., Ltd., 6F-5, No. 250, Sec. 4, Zhongxiao E. Road, Taipei 10692,
Taiwan eDepartment
of Mechanical Engineering, National Taiwan University, No. 1, Sec. 4,
Roosevelt Road, Taipei 10617, Taiwan * Corresponding author. Tel.: 886 2 33662726; Fax: 886 2 23631808. E-mail addresses: [email protected] (S.-S. Ma), [email protected] (C.-Y. Tseng), [email protected] (Y.-R. Jian), [email protected] (T.-H. Yang), [email protected] (S.-L. Chen).
1
ACCEPTED MANUSCRIPT ABSTRACT A physical means of dehumidification was used in this study to recover and reuse waste heat emitted from tumble dryers to increase drying efficiency and decrease power consumption. An experiment was conducted; waste heat from a dryer was dehumidified in a heat exchanger, and some heat was reclaimed to preheat external air. After the optimal mixture ratio of external and re-circulated air had been determined, that optimal air mixture was introduced into the dryer through the recirculation air ducts designed in this study. Experimental results showed that the improved dryer, operated at a 60% recirculation ratio produced the highest energy efficiency, resulting in 18% less power consumption than a standard dryer. In addition, this configuration produced the lowest specific moisture extraction rate (1.099) and the highest drying efficiency (60.6%). This efficiency was higher than that of the dryer (50%) and the proposed heat exchange system was thus more energy efficient than was the dryer. The theoretical mathematical model derived in this study matched experimental results. The average error values between the theoretical and experimental values for the drying efficiency and the specific moisture extraction rate were 0.7% and 0.6% ; the largest error values were only 1.5% and 1.2%. Keywords: Tumble dryer; cross-flow plate heat exchanger; heat recovery; re-circulation factor; drying efficiency; specific moisture extraction rate (SMER).
2
ACCEPTED MANUSCRIPT 1. Introduction With the continuous and rapid development of technology, global energy consumption has increased greatly. This has led to heightened environmental awareness, which has turned reducing energy usage and carbon emissions into a critical issue worldwide. In addition to developing new sources of energy, enhancing energy efficiency is vital to mitigating the problem. Therefore, the objectives of the present study were to retain the existing advantages of current dryers and to improve upon their disadvantages—low drying efficiency and high energy consumption—so that users can concurrently save energy and maintain convenient and comfortable lives. A tumble dryer operates by drawing in outside air, passing the outside air through a heater to increase its temperature and decrease its relative humidity, and transporting the dry air into a dryer drum powered by a motor and drive mechanism. The dryer drum rotates continuously to mix wet clothes, which have a higher humidity, with the dry air, which has a lower relative humidity. Because of the pressure difference between the water in wet clothes and water vapor, the water is desorbed into the dry air as the dryer rotates. The air is then expelled out of the dryer. After a specific amount of time, the wet clothes become dry. The power consumption of a dryer is dependent on its drying performance, which is affected by factors such as external temperature, relative humidity, the mass flow rate of air, the moisture content of the load, and the dryer’s heater. In household dryers, the heater is the component that consumes the most energy. The drying efficiency of most household dryers is approximately 50%. In other words, approximately 50% of the power consumed is used for drying clothes. The remaining 50% of the power is turned into waste heat and expelled into the atmosphere, which is extremely wasteful. Dryers are used extensively in modern society. The large quantities of waste heat emissions represent wasted energy. To achieve energy 3
ACCEPTED MANUSCRIPT conservation goals, researchers have proposed various methods of improving the energy efficiency of dryers through increasing drying efficiency or reducing energy consumption. Hekmat and Fisk [1] proposed several ways to improve the efficiency of a dryer, including 1) decreasing the mass flow rate of air, which reduces energy consumption by approximately 8%, and 2) increasing the temperature of intake air to reduce power consumed by the heater, which reduces power consumption by approximately 15%. Recovering and reusing waste heat can increase the temperature of intake air and thus conserve energy. However, recovering all of the waste heat may not necessarily increase drying efficiency; experiments must be performed to determine the optimal recirculation ratio of the waste heat emitted by the dryer. Therefore, Lambert et al. [2] created a model to simulate the energy savings of a dryer. They found that 75% recirculation produced the optimal drying efficiency (approximately 62%). This efficiency is approximately 15% higher than the 47% efficiency of the original factory model. Conde [3] added an interleaved heat exchanger to the dryer exhaust. This configuration uses the sensible heat of exhaust air to increase the temperature of the intake air. Conde also examined various sizes of the heat exchanger to determine which produced optimal benefits. Bansal et al. [4] improved upon dryer designs by installing a water-to-air heat exchanger behind the dryer drum. This replaces the traditional method of using a heater to preheat external air. Woo [5] developed a closed-loop dryer system integrated with a heat exchanger and heater. Results showed that increasing the power of the heater and the temperature of the intake air reduced drying times. Deans [6] proposed using sensitivity analysis to assess differences in drying times and specific energy consumption (SEC) in various operating conditions. Findings showed that SEC values were affected by ambient temperature and relative humidity. As room temperature increased, SEC decreased; in other words, dryer performance improved. Bassily and Colver [7] studied changes to dryer operating conditions by simulating the effect of various parameters, including load weight, 4
ACCEPTED MANUSCRIPT power consumption of the heater, mass flow rate of air, and moisture content of the clothes, on dryer performance. Yadav and Moon [8] performed a qualitative analysis of three internationally recognized standards [9–11] under various operating conditions using theories and experimental methods. Stawreberg and Nilsson [12] studied airflow and leakage in a condensing dryer. They found that high internal airflow and low external airflow produced the optimal specific moisture extraction rate (SMER) but also the most substantial leakage. The leakage aspect was improved in a subsequent study [13]. According to the aforementioned studies, introducing heated air into a dryer can improve drying efficiency. Air can be heated by a variety of sources; hot water, electric heaters, and heat pump systems are all viable sources. Recently, scholars such as Ambarita [14] constructed a drying cabinet that uses the heat waste from a split-type air conditioner to dry clothes. This represents another effective method of energy conservation. Incorporating this result into dryers in the future would create an energy-saving system of household appliances and would be a green research study. Summarizing the literature review, all proposed methods, such as controlling the recirculation ratio, dehumidifying the air through physical means or chemical means [15,16], or controlling the external air environment, advance the goal of introducing high-temperature, low-humidity air into the dryer. This allows the heater in the dryer to heat internal air to a temperature necessary for drying clothes in the shortest amount of time, thereby conserving energy as well as reducing drying time [17]. Unlike commercially available dryers, the dryer developed in the present study employs an innovative method of recycling waste heat by combining a heat exchanger and a novel recirculation control system. After evaluating the drying efficiency and SMER of the improved dryer, we determined that the new dryer is more energy efficient and environmentally friendly than currently available dryers. 5
ACCEPTED MANUSCRIPT 2. Experimental investigation 2.1. Physical dehumidifying system In this study, a heat exchanger and a recirculation control device were integrated into a dryer to address the disadvantages of a dryer. The drying efficiencies of the dryer and the improved dryer were then compared. The research process flowchart is shown in Fig. 1. First, energy-saving ideas were proposed (including heat recycling methods, heat exchanger choices, and designs for the recirculation control valve). Next, literature was reviewed to compare applications of relevant technologies. Subsequently, the design of the new dryer and new mathematical models were developed. Finally, the performance of the new dryer was tested and the resulting numerical data were analyzed and compared. Fig. 2 illustrates the setup of an experiment on a tumble dryer combined with a heat exchanger and a recirculation control system. Figs. 3 and 4 present the physical setup of that experiment. Fig. 5 shows the piping arrangement of the recirculation control device. A pipe connected the rear exhaust orifice of the dryer to a heat exchanger enveloped by heat insulation cotton (Fig. 3). A pipe connected the heat exchanger with a recirculation control system on the floor. The air reached an exhaust valve and a recirculation valve inside the dryer (Fig. 4); the air that entered the recirculation valve delivered reclaimed heat to the dryer through a black tube.
The added heat exchanger allows the exchange of external air and recirculated air to increase the temperature of intake air and reduce the temperature of recirculated air. This has the effect of reducing the absolute humidity of the recirculated air. Recirculation control valves control the recirculation ratio. The experiment primarily measured the dryers’ power consumption, drying efficiency, and rate of water evaporation at various recirculation flows 6
ACCEPTED MANUSCRIPT (which ranged from 0% to 100%) to determine the optimal recirculation ratio.
Fig. 1. Flowchart of dryer energy-saving research process.
7
ACCEPTED MANUSCRIPT
Fig. 2. Illustration of the setup of an experiment with a dryer combined with a heat exchanger and recirculation control system.
8
ACCEPTED MANUSCRIPT
Fig. 3. Physical setup of an experiment on a dryer combined with a heat exchanger and recirculation control device (1).
9
ACCEPTED MANUSCRIPT
Fig. 4. Physical setup of an experiment on a dryer combined with a heat exchanger and recirculation control device (2).
10
ACCEPTED MANUSCRIPT
Fig. 5. Piping arrangement of a recirculation control device.
11
ACCEPTED MANUSCRIPT Paired with the aforementioned system, the following experimental parameters were defined: 1. Recirculation ratio (x) A recirculation control device was used in this study to regulate the recirculation ratio, which enabled the examination of dryer performance at various recirculation ratios. This device is illustrated in Fig. 6, where 𝑚1 is the mass flow rate of exhaust air passing through a two-way valve and 𝑚2 is the mass flow rate of recirculated air entering the mixing tank. After obtaining flow rate readings in the experiment, the recirculation ratio can be defined as: 𝑥=𝑚
𝑚2
(1)
1 + 𝑚2
Because recirculation control is primarily determined by the opening angle of the recirculation control valve, the size of the opening angle affects the mass flow rate of air. For a given opening angle of the recirculation control valve, the opening angle and flow rate of the valve are linearly correlated [18]. In the cited work, the correlation between the opening angle and flow rate improves linearly when the opening angle is 30%–80%. Accordingly, when the control valve is opened, the flow rate increases steadily after the opening angle reaches 30%; when the opening angle reaches 80%, the flow rate peaks and remains constant thereafter. In the present study, however, the correlation between the opening angle and flow rate of the recirculation control valve showed slight linear growth when the recirculation ratio was 20%–60%. Specifically, after the recirculation ratio reached 20%, the flow rate increased steadily and drying efficiency improved. When the recirculation ratio was 60%, drying efficiency reached its optimal level; efficiency gradually declined after the ratio increased beyond 60% because air introduced excessive water vapor when the ratio was too high. Therefore, theoretical calculations of the recirculation ratio were based on the size of the 12
ACCEPTED MANUSCRIPT opening angle. The theoretical definition is: 𝑎𝑛𝑔𝑙𝑒 𝑜𝑓 𝑟𝑒𝑐𝑖𝑟𝑐𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑣𝑎𝑙𝑣𝑒(0~90 𝑑𝑒𝑔𝑟𝑒𝑒)
𝑥 = 𝑎𝑛𝑔𝑙𝑒 𝑜𝑓 𝑟𝑒𝑐𝑖𝑟𝑐𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑣𝑎𝑙𝑣𝑒(0~90 𝑑𝑒𝑔𝑟𝑒𝑒) + 𝑎𝑛𝑔𝑙𝑒 𝑜𝑓 𝑒𝑥ℎ𝑎𝑢𝑠𝑡 𝑣𝑎𝑙𝑣𝑒(0~90 𝑑𝑒𝑔𝑟𝑒𝑒)
(2)
In Eq. (2), an opening angle of 0° means the flow control valve is completely closed. An opening angle of 90° means the flow control valve is completely open. Thus, when the opening angle of the recirculation valve is 0° and the opening angle of the exhaust valve is 90°, the recirculation ratio is 0. When the opening angle of the recirculation valve is 90° and the opening angle of the exhaust valve is 0°, the recirculation ratio is 1. The circulation ratio ranges between 0 and 1 and can be calculated using Eq. (2).
13
ACCEPTED MANUSCRIPT
Fig. 6. Illustration of recirculation control device. 2. Amount of evaporated water (𝑀𝑤𝑟) The amount of evaporated water was calculated by measuring the difference in the weight of the dryer load before and after drying. (3)
𝑀𝑤𝑟 = 𝑀𝑤𝑓 ‒ 𝑀𝑤𝑖 (𝑘𝑔) 3. Rate of water evaporation (𝑚𝑤)
14
ACCEPTED MANUSCRIPT The rate of water evaporation was calculated in this study by weighing the wet load and the dry load and dividing the difference by the drying time. 𝑀𝑤𝑓(𝑘𝑔)
(4)
𝑚𝑤 = 𝑇𝑖𝑚𝑒(ℎ)
4. Total power consumed by the dryer (𝑃𝑡𝑜𝑡) A watt-hour meter was used to record the total power consumed by the dryer. 5. Drying efficiency (𝜂) Drying efficiency was defined in [2] as: 𝜂=
𝑟𝑎𝑡𝑒 𝑜𝑓 𝑒𝑛𝑒𝑟𝑔𝑦 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑓𝑜𝑟 𝑒𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑖𝑜𝑛 (𝑘𝑊) 𝑡𝑜𝑡𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑖𝑛𝑝𝑢𝑡 (𝑘𝑊)
=
=
𝑄𝑒𝑣𝑎 (𝑘𝑊) 𝑃𝑡𝑜𝑡 (𝑘𝑊) 𝑚𝑤ℎ𝑓𝑔𝑇𝑎𝑣𝑔 (𝑘𝑊)
(5)
𝑃𝑡𝑜𝑡 (𝑘𝑊)
where 𝑄𝑒𝑣𝑎 is the rate of heat transfer absorbed by water evaporation, 𝑇𝑎𝑣𝑔 is the average temperature of the load, 𝑚𝑤 is the rate of water evaporation, ℎ𝑓𝑔 is the latent heat of vaporization, and 𝑃𝑡𝑜𝑡 is the total power input of the dryer. The optimal recirculation ratio was determined by examining drying efficiency. 6. SMER SMER is defined as the ratio of total energy consumed to amount of evaporated water. 𝑆𝑀𝐸𝑅 =
𝑡𝑜𝑡𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑐𝑜𝑛𝑠𝑢𝑚𝑒𝑑(𝑘𝑊ℎ) 𝑀𝑤𝑟(𝑘𝑔)
(6)
Lower SMER values represent higher performance in dryers. 7. Energy saving percentage The energy saving percentage is defined as:
15
ACCEPTED MANUSCRIPT 𝐸𝑛𝑒𝑟𝑔𝑦 𝑠𝑎𝑣𝑖𝑛𝑔 𝑝𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 =
𝐸𝑟𝑒𝑐𝑖𝑟𝑐𝑢𝑙𝑎𝑡𝑒 ‒ 𝐸𝑏𝑎𝑠𝑒𝑙𝑖𝑛𝑒 𝐸𝑏𝑎𝑠𝑒𝑙𝑖𝑛𝑒
× 100%
(7)
The total energy used by a dryer without a heat exchanger was set as the baseline value and Eq. (7) was used to calculate the energy saving percentage at various recirculation ratios. 2.2. Experimental conditions The dryer load comprised 100% cotton towels. The moisture content was held constant at approximately 70% of the weight of the dry towels. The external environment of the dryer was held constant at a temperature of 25 °C and a relatively humidity of 55%. The mass flow rate of the dryer exhaust was approximately 0.02 kg/s. For the recirculation pipes of the dryer, benchmarks for recirculation ratio were calculated using (1) and (2) and the ratio of recirculated and exhaust air was adjusted accordingly. The recirculation ratio was also used to determine the drying time, which ranged from approximately 80–120 min. A watt-hour meter was used to record the total power consumed by the dryer. After drying, the dryer load was removed and weighed to determine the rate of water evaporation for each recirculation ratio. 2.3. Experimental and measuring apparatus The experimental and measuring apparatus used in this study are as follows: (1) Tumble dryer The tumble dryer (QD7561NA, TECO, Taiwan) used in the experiment has a capacity of 7 kg, an operation time of up to 180 min, and a dissipated power of 800 W. (2) Vane anemometer The vane anemometer (Testo 416, Testo, Australia) was used to measure the speed of both exhaust and recirculation. This anemometer has an estimated error range of ±0.2 m/s ± 1.5% and a measurement range of 0.6–40 m/s. (3) Humidity meter Five humidity meters (TRH-303, TECPEL, Taiwan) were used to measure temperature 16
ACCEPTED MANUSCRIPT and relative humidity. These humidity meters have a dry-bulb temperature error range of ± 0.3 °C, a measurement range of 0–100 °C, and a reaction time of < 15 sec. (4) Thermocouple A T-type thermocouple was used to measure the air inlet and outlet temperature of the high-and low-temperature sides of the dryer. The thermocouple uses the voltage differences due to temperature differences to measure temperature and has a measurement range of –200 to 400 °C. Before the experiment, the thermocouple underwent a temperature adjustment and achieved a measurement error of ±0.5 °C. (5) Data recorder All temperature data were transmitted to a data recorder (MV200, Yokogawa Electric Corporation, Japan). (6) Electronic weighing scale An electronic weighing scale (GX-6100, A&D, Japan) with a measurement range of 0– 6100 g and a precision of 0.01 g was used to estimate changes in load weights, calculate the amount of water vaporized, and determine the weights of the wet loads and the post-drying weights. Before use, the scale was set to zero. (7) Watt-hour meter A watt-hour meter was used to measure the total power consumption of the dryer. (8) Avometer A digital alternating-current HILA-9250 Avometer was used to measure changes in the power consumption of the dryer. The Avometer multiplied the operating voltage of the dryer and its operating current to determine the power consumption of the device. The Avometer has a voltage measurement range of 0–400 V (with a precision of ±0.1V) and a current measurement range of ±0.1A. (9) Crossflow plate heat exchanger 17
ACCEPTED MANUSCRIPT A crossflow plate heat exchanger (PWT 10/200/200-2.0, Klingenburg) that measures 20 × 20 × 16 cm in size and weighs 2 kg in weight was used to recover heat energy and reduce the temperature of recirculation air. 3. Theoretical investigation In the mathematical model constructed for the present study, input parameters such as external temperature, absolute humidity, the sensible heat effectiveness of the heat exchanger, the latent heat effectiveness of the heat exchanger, and recirculation ratio were used to predict the dryer’s exhaust conditions, rate of water evaporation, power consumption, and drying efficiency. The objective was to verify that results from the derived mathematical model were consistent with experimental results. The structure of the mathematical model and the flowchart for calculations in the mathematical model are shown in Figs. 7 and 8, respectively. The basic assumptions were as follows: 1. Tin is a constant value set at approximately 51 °C. 2. The heat exchanger has two effectiveness measurements: sensible heat effectiveness (𝜀𝑠𝑒𝑛 ) and latent heat effectiveness (𝜀𝑙𝑎𝑡). 3. Effects from gravity, pressure drop, and velocity loss are not considered. 4. The thermal and physical properties of air and of the dryer load are constants. 5. The heat transfer coefficient of the load is an average value. 6. The energy stored within the dryer is not considered.
18
ACCEPTED MANUSCRIPT
Fig. 7. Schematic of the mathematical model.
19
ACCEPTED MANUSCRIPT
Fig. 8. Flowchart of calculations in the mathematical model. The mass flow rates at the dryer’s inlet and outlet are held constant and not affected by air recirculation. Conservation of mass in the dryer drum (Fig. 8) produces the following equation: (8)
d𝑚 = 0 Expanding Eq. (8) produces the following formula: 𝑚𝑖𝑛(1 + 𝑊𝑖𝑛) + 𝑚𝑤 = 𝑚𝑒𝑥(1 + 𝑊𝑒𝑥) + 𝑚𝑙𝑒𝑎(1 + 𝑊𝑒𝑥)
(9)
As stated in the third assumption, velocity loss was not considered. This results in (10)
𝑚𝑖𝑛 = 𝑚𝑚𝑖𝑥 + 𝑚𝑝𝑒𝑛≅𝑚𝑙𝑒𝑎 + 𝑚𝑒𝑥 Combining Eqs. (9) and (10) produces: 20
ACCEPTED MANUSCRIPT (11)
𝑚𝑤 = (𝑚𝑒𝑥 + 𝑚𝑙𝑒𝑎)𝑊𝑒𝑥 ‒ 𝑚𝑖𝑛𝑊𝑖𝑛 Based on [6], the rate of water evaporation of the load is rewritten as: 𝑚𝑤 = 𝐻𝑑𝐴(𝑎𝑊𝑀 ‒
𝑊𝑖𝑛 + 𝑊𝑒𝑥 2
(12)
)
where A is the effective contact area of the load (measured to be 3.2 m2 in this experiment) and WM is the saturation humidity ratio of the air on the surface of the load (in kg/kg). WM can be determined from looking up meteorological tables or calculated using the Hyland– Wexler equation. The activity coefficient of water, a, is defined as follows: 𝑎=1‒
𝛿 + 𝛽𝑀𝑐 𝛾𝑀
1+𝛿
(13)
𝑐
δ, β, and γ are constants associated with the material of the load. For the present experiment, the material of the load was 100% cotton. According to [6], δ = 2, β = 18, and γ = 30. The humidity ratio of the mixed air can be calculated by the status of the mixed air in the mixing tank: '
(14)
𝑊𝑚𝑖𝑥 = 𝑥𝑊𝑒𝑥 + (1 ‒ 𝑥)𝑊𝑎𝑚𝑏
The latent heat effectiveness of the heat exchanger 𝜀𝑙𝑎𝑡 can be obtained experimentally. From '
this, 𝑊𝑒𝑥 can be calculated by: '
(15)
𝑊𝑒𝑥 = 𝑊𝑒𝑥 ‒ 𝜀𝑙𝑎𝑡(𝑊𝑒𝑥 ‒ 𝑊𝑎𝑚𝑏) where εlat is the latent heat effectiveness, which is defined as:
21
ACCEPTED MANUSCRIPT
l at
ex
' ex
ex amb
(16)
where ωamb is the humidity ratio of the functional fluids at the inlet of the heat exchanger '
(kg/kg); ωex is the humidity ratio of hot air entering the heat exchanger (kg/kg); and ωex is the humidity ratio of air cooled by the heat exchanger (kg/kg).
Combining Eqs. (14) and (15) produces: 𝑊𝑚𝑖𝑥 = 𝑥[𝑊𝑒𝑥 ‒ 𝜀𝑙𝑎𝑡(𝑊𝑒𝑥 ‒ 𝑊𝑎𝑚𝑏)] + (1 ‒ 𝑥)𝑊𝑎𝑚𝑏 = 𝑥(1 ‒ 𝜀𝑙𝑎𝑡)𝑊𝑒𝑥 + 𝑥𝜀𝑙𝑎𝑡𝑊𝑎𝑚𝑏 + (1 ‒ 𝑥)𝑊𝑎𝑚𝑏
(17)
After air passes through the heater, the temperature of the air increases but the absolute humidity ratio remains unchanged. Win is primarily determined by the status of mixing in the mixing tank. Simplifying the equation produces: 𝑊𝑖𝑛 =
𝑚𝑚𝑖𝑥 𝑚𝑖𝑛
𝑊𝑚𝑖𝑥 +
𝑚𝑙𝑒𝑎 𝑚𝑖𝑛
(18)
𝑊𝑎𝑚𝑏
Combining Eqs. (11), (12), (17), and (18) produces: 𝑊𝑒𝑥 =
𝐴 ∙ 𝑊𝑎𝑚𝑏 + 𝐵 ∙ 𝑊𝑀
(
where 𝐴 = 𝑚𝑖𝑛 ‒ 𝑚𝑒 𝐻𝑑𝐴
+𝑚
(
𝑖𝑛
(19)
𝐶
2
)[
𝐻𝑑𝐴 𝑚𝑒𝑥 2
𝑚𝑖𝑛
𝑚
]
(𝑥 ∙ 𝜀𝑙𝑎𝑡 + 1 ‒ 𝑥) + 𝑚𝑙𝑒𝑎 , 𝐵 = 𝐻𝑑 ∙ 𝐴 ∙ 𝑎, and 𝐶 = 𝑚𝑙𝑒𝑎 + 𝑚𝑒𝑥 + 𝑖𝑛
𝐻𝑑𝐴 2
)
‒ 𝑚𝑖𝑛 (1 ‒ 𝜀𝑙𝑎𝑡)𝑥.
From Eq. (18), the humidity ratio of the saturated air on the surface of the load 𝑊𝑀 is calculated as [189]:
22
ACCEPTED MANUSCRIPT 𝑝𝑤𝑠
(20)
𝑊𝑀 = 0.6298𝑝 ‒ 𝑝
𝑤𝑠
𝐶1
ln (𝑝𝑤𝑠) = 𝑇
𝑎𝑣𝑔
2
3
(21)
+ 𝐶2 + 𝐶3𝑇𝑎𝑣𝑔 + 𝐶4𝑇𝑎𝑣𝑔 + 𝐶5𝑇𝑎𝑣𝑔 + 𝐶6ln (𝑇𝑎𝑣𝑔)
0 ≤ 𝑇𝑎𝑣𝑔 ≤ 100℃ 3
0
where 𝐶1 =‒ 5.800 × 10 , 𝐶2 = 1.391 × 10 , 𝐶3 =‒ 4.864 × 10 =‒ 1.445 × 10
‒8
‒2
, 𝐶4 = 4.176 × 10
‒5
, 𝐶5
0
, and 𝐶6 = 6.546 × 10 .
𝑇𝑎𝑣𝑔 is the average temperature of the load, or the average of the intake and exhaust temperatures of the dryer: 𝑇𝑎𝑣𝑔 =
𝑇𝑖𝑛 + 𝑇𝑒𝑥 2
=
51 + 𝑇𝑒𝑥
(22)
2
The exhaust temperature 𝑇𝑒𝑥 is unknown. An iterative method was used to solve for 𝑇𝑒𝑥 in this study. First, an initial value was assumed for 𝑇𝑒𝑥 and substituted into (18) to solve for the humidity ratio of the exhaust (𝑊𝑒𝑥). Next, conservation of energy was used to calculate 𝑇𝑒𝑥; this value was compared with the initial 𝑇𝑒𝑥. If the two values were similar, the exhaust temperature of the dryer (𝑇𝑒𝑥) and the corresponding humidity ratio of the exhaust (𝑊𝑒𝑥) were established. After calculating the humidity ratio of the exhaust, the rate of water evaporation (𝑚𝑤) was calculated. The rate of water evaporation was an important indicator in this experiment. It was used primarily to determine the drying time, and also to identify improvement to the dryer system according to the recirculation ratio. Next, conservation of energy must be observed in the dryer drum, as in Eq. (23): (23)
𝑑𝐸 = 0
From the mathematical model constructed for the present study (Fig. 7), Eq. (14) can be 23
ACCEPTED MANUSCRIPT derived: (23)
𝑚𝑖𝑛ℎ𝑖𝑛 + 𝑚𝑤ℎ𝑓𝑔 = (𝑚𝑒𝑥 + 𝑚𝑙𝑒𝑎)ℎ
𝑒𝑥
The enthalpy of the humid air can be expressed as [19]: ℎ𝑖𝑛 = 𝑐𝑝𝑎𝑇𝑖𝑛 + 𝑊𝑖𝑛(2501 + 𝑐𝑝𝑤,𝑔𝑇𝑖𝑛)
(24)
ℎ𝑒𝑥 = 𝑐𝑝𝑎𝑇𝑒𝑥 + 𝑊𝑒𝑥(2501 + 𝑐𝑝𝑤,𝑔𝑇𝑒𝑥)
(25)
Combining Eqs. (23)–(25) produces:
𝑇𝑒𝑥 =
𝑚𝑤ℎ𝑓𝑔 + 𝑚𝑖𝑛[𝑐𝑝𝑎𝑇𝑖𝑛 + 𝑊𝑖𝑛(2501 + 𝑐𝑝𝑤,𝑔𝑇𝑖𝑛)] ‒ 2501(𝑚𝑒𝑥 + 𝑚𝑙𝑒𝑎)𝑊𝑒𝑥 (𝑚𝑒𝑥 + 𝑚𝑙𝑒𝑎)(𝑐𝑝𝑎 + 𝑊𝑒𝑥𝑐𝑝𝑤,𝑔)
(26)
The 𝑇𝑒𝑥 calculated from (26) was compared with the 𝑇𝑒𝑥 calculated from conservation of mass. If the two values were similar, the exhaust temperature of the dryer (𝑇𝑒𝑥) and the corresponding humidity ratio of the exhaust (𝑊𝑒𝑥) could be established. If the values were not similar, a different value for 𝑇𝑒𝑥 was chosen, and the computations for conservation of mass and conservation of energy were recalculated. The flowchart of calculations is shown in Fig. 8. 4. Results and discussion 4.1. Effectiveness testing of the heat exchanger A dryer combined with a heat exchanger and recirculation control device were examined in the present study. Recirculation control valves were used to adjust the recirculation ratio. In addition, the sensible heat and latent heat of the heat exchanger were used to increase the intake temperature as well as to decrease the humidity ratio. This ensured that the air entering the dryer was of high-temperature and low-humidity, which serves to improve the dryer’s performance. Because the mathematical analysis of a moist air-to-air heat exchanger is 24
ACCEPTED MANUSCRIPT complicated, the values of the sensible heat effectiveness and latent heat effectiveness of the heat exchanger used in the mathematical model were obtained through experimental readings. Readings were taken at the inlet and outlet of the heat exchanger (Fig. 9). The definitions of sensible heat effectiveness and latent heat effectiveness are provided below. The definition of sensible heat is: '
(𝑇𝑒𝑥 ‒ 𝑇𝑒𝑥)
𝜀𝑠𝑒𝑛 = (𝑇
(27)
𝑒𝑥 ‒ 𝑇𝑎𝑚𝑏)
'
where 𝑇𝑒𝑥 is the temperature of recirculated air before it enters the heat exchanger; 𝑇𝑒𝑥 is the temperature of recirculated air after it leaves the heat exchanger; and 𝑇𝑎𝑚𝑏 is the ambient room temperature. The definition of latent heat 𝜀𝑙𝑎𝑡 is shown on equation (16).
25
ACCEPTED MANUSCRIPT
Fig. 9. Locations of temperature and humidity readings. '
𝑊𝑒𝑥 is the humidity ratio of the recirculated air before it enters the heat exchanger; 𝑊𝑒𝑥 is the humidity ratio of recirculated air after it leaves the heat exchanger, and 𝑊𝑎𝑚𝑏 is the humidity ratio of the room.
'
Higher sensible heat effectiveness means that 𝑇𝑒𝑥 is closer to 26
ACCEPTED MANUSCRIPT '
𝑇𝑎𝑚𝑏. Higher latent heat effetiveness means that 𝑊𝑒𝑥 is closer to 𝑊𝑎𝑚𝑏. Intake air entering the dryer with a higher temperature and a lower humidity ratio can significantly improve drying efficiency. Experimental readings of temperature and humidity were taken at the heat exchanger inlet and outlet. The temperature of the recirculated air at the inlet and outlet were 35.1 °C ( '
𝑇𝑒𝑥) and 31.8 °C (𝑇𝑒𝑥), respectively. The humidity ratio of the recirculated air at the inlet and '
outlet were 31.08 g/kg (𝑊𝑒𝑥) and 28.32 g/kg (𝑊𝑒𝑥), respectively. Regarding the cold air at the inlet, the temperature was 27.1 °C (𝑇𝑎𝑚𝑏) and the humidity ratio was 13 g/kg (𝑊𝑒𝑥). Using (27) and (28), the sensible heat effectiveness (𝜀𝑠𝑒𝑛) was calculated to be approximately 0.41 and the latent heat effectiveness (𝜀𝑙𝑎𝑡) to be approximately 0.15. 4.2. Performance comparison between a dryer and a dryer with a heat exchanger In this study, a heat exchanger was used to pre-heat external air to increase the temperature of the air at the dryer inlet. Drying efficiency, SMER, and total power consumption were obtained and compared with the results from a dryer that does not include a heat exchanger. Finally, using the total power consumed by a dryer as the baseline, the percentage of power saved by the dryer with a heat exchanger was calculated. Two dry loads of the same weight (1.35 kg) were saturated with water to produce two wet loads of the same weight (2.18 kg). The amount of time required to return the wet loads to the dry weight, rate of water evaporation (kg/h), total power consumed, and total energy consumed were measured to calculate the SMER and drying efficiency. These were then used to compare dryer performances; results are listed in Table 2. The drying efficiency of the dryer with a heat exchanger was 60%, which was approximately 8.6% higher than the drying efficiency of the dryer without a heat exchanger. In addition, the SMER of the former (1.113) 27
ACCEPTED MANUSCRIPT was lower than the SMER of the latter (1.304), which indicated that the improved dryer consumed less energy per kilogram of water evaporated. The energy savings was approximately 16.1%. Therefore, using a heat exchanger to decrease the moisture in exhaust air was found to also lower the humidity ratio of the mixed air and raise the temperature of the mixed air. These all improved the performance of the dryer. Table 2 Comparison of experimental values and performances of a dryer and a dryer with a heat exchanger. Time Required to Power Percentage Return Total Power Drying Consumption SMER of Energy Wet Consumption Efficiency (kWh/kg) Rate Saved (kWh) (%) Load to (kW) (%) Dry Weight (min) Dryer
84
0.773
1.100
1.304
51.3
-
Improved dryer with heat exchanger
82
0.677
0.925
1.113
60.0
16.1%
4.3. Performance of a dryer with a heat exchanger at various recirculation ratios Table 2 shows that integrating a heat exchanger with the dryer can decrease the total power consumption of the dryer. Thus, subsequent testing examined the SMER, drying efficiency, rate of water evaporation, and total power consumption of a dryer with a heat exchanger at various recirculation ratios. Using the total power consumed by a dryer as the baseline, the percentage of power saved by various recirculation ratios was calculated. First, the rate of water evaporation was analyzed for various recirculation ratios. The rate of water evaporation was found to decrease as recirculation ratio increased. The reason for this is that 28
ACCEPTED MANUSCRIPT the rate of water evaporation was affected by the humidity ratio of the air at the dryer inlet. Therefore, increasing the recirculation ratio also increases the moisture content of the inlet air, which then decreased the dryer’s rate of water evaporation. The total power consumption was affected by the temperature of the air at the dryer inlet. Because mixing the high temperature intake air with preheated external air increased the temperature of the air at the dryer inlet, the heater consumed less power. However, increasing the recirculation ratio also increased the moisture content of the air at the dryer inlet and thus the drying time. This lead to higher power consumption. Because the recirculation ratio’s effect on power consumption is dominated by the increases or decreases of two factors, the optimal recirculation ratio must be determined through experimental findings. The experimental findings are shown in Table 3. All recirculation ratios presented power savings over the dryer. However, a recirculation ratio of 60% produced the highest drying efficiency (60.6%) and the optimal power savings percentage (18%), as shown in Fig. 10. Furthermore, a recirculation ratio of 100%, i.e., when no external air is introduced, was still more energy efficient than a dryer. However, at recirculation ratios higher than 70%, drying time increased because the humidity ratio of the air mixture was excessively high, which decreased the rate of water evaporation. At a recirculation ratio of 70%, the drying time was 32% longer than the drying time of a dryer. At a recirculation ratio of 100%, the drying time was 45% longer. Therefore, these ratios were less suitable for practical applications. Table 3 Performance of a dryer with a heat exchanger at various recirculation ratios. Recirculation
Rate of water
SMER
Drying
Total power
Energy
ratio
evaporation
(kWh/kg)
efficiency
consumption
saving
29
ACCEPTED MANUSCRIPT (%)
(kg/h)
(%)
(kWh)
percentage (%)
0
0.609
1.113
59.9
0.925
16.1
20
0.589
1.109
60.1
0.931
15.4
40
0.578
1.107
60.2
0.984
10.5
60
0.547
1.099
60.6
0.902
18.0
70
0.498
1.128
59.1
0.993
9.7
80
0.488
1.132
58.9
0.951
13.5
90
0.449
1.156
57.7
1.036
5.8
100
0.425/
1.237
53.9
1.052/
4.4
Fig. 11 shows a comparison of the theoretical and experimental values of drying efficiency and SMER. Consistent trends between the experimental and theoretical values were identified. The average error between theoretical and experimental values for drying efficiency was 0.7%; the largest error was 1.5%. The average error between theoretical and experimental values of SMER was 0.6%; the largest error was 1.2%. This indicated that the mathematical model established in this study produced values that approximated experimental values and the model can aid in choosing the optimal recirculation ratio for dryers. Fig. 11 also shows that for both experimental and theoretical values, a recirculation ratio of approximately 60% produced optimal drying efficiencies. In addition, a recirculation ratio of 60% represented a turning point after which the drying efficiency began to decrease. This was related to the humidity ratio of the recirculated air in the dryer. Overly high humidity ratio causes an overly long drying time and excessive large power consumption, which then decreases drying efficiency.
30
ACCEPTED MANUSCRIPT
Fig. 10. Percentage of power saved in a dryer with a heat exchanger at various recirculation ratios.
Fig. 11. Comparison of theoretical and experimental values for (a) drying efficiency and (b) SMER.
4.4. A perspective on the energy efficiency of dryers In the United States, a dryer consumes over 1000 kWh annually on average [20]; this represents a very high proportion of the total energy consumed by all home appliances. The 31
ACCEPTED MANUSCRIPT American climate is more arid than the Taiwanese climate, and has high temperatures during the summer. This arid and hot climate is very suitable for dryer operations. In contrast, Taiwan has an island climate. The hot and humid environment has a substantial impact on drying efficiency. As a result, dryer usage in Taiwan is relatively low, and air conditioning use represents the majority of energy consumption among home appliances. However, dryer usage has begun to increase in recent years as more and more dryers that feature energy efficiency have appeared on the market. Dryers awarded with Taiwan’s Energy Label certification must display the product’s energy factor on the product. According to the Ministry of Economic Affairs’ Bureau of Energy regulations, the measured energy factor of dryers must be higher than 1.7 kg/kWh. For example, a popular dryer model in Taiwan (model TCD-7.0RJ) has a capacity of 7 kg and its energy consumption is 1.3 kWh. If the dryer is used for two hours per day, its annual energy consumption is 949 kWh. This amount approximates the annual energy consumption of a US dryer. However, drying clothes completely in Taiwan’s humid climate likely requires more than two hours. Therefore, dryers consume a substantial amount of energy. However, a critical issue is that currently available dryers do not recycle waste heat. Waste heat generated while drying clothes is directly exhausted to the outside and represents wasted energy. Therefore, a dryer that integrates a heat exchanger and a recirculation control device was designed in this study. The heat exchanger addresses the effect of a humid climate on drying efficiency. The recirculation control addresses energy waste by effectively reintroducing waste heat into the dryer. For both in the United States, where dryer usage is already high, and in Taiwan, where dryer usage is increasing, the dryer designed in this study can achieve the desired effects of reducing energy consumption and greater energy efficiency. 4.5. Analysis of errors in experimental data (experimental vs. theoretical values) The performance of the dryer was assessed in relation to different recirculation ratios, 32
ACCEPTED MANUSCRIPT temperatures, relative humidity levels, water evaporation rates, drying efficiency levels, and power consumption levels. All experimental data were obtained through experimental apparatus. Considering the errors between experimental and theoretical values that inevitably arise from manually operated experimental apparatus of limited precision, a mathematical model (Fig.8) was constructed to produce theoretical values. The theoretical values were compared with experimental values to ascertain whether they were consistent and to estimate errors between them. Table 4 compares the experimental and theoretical performance of a dryer with a heat exchanger. Fig.12 respectively depict the water evaporation rate, the total power consumption rate of the dryer, the drying efficiency of the dryer, and the SMER of the dryer. These four figures include experimental values (marked with blue dots) and theoretical ones (marked with red lines), allowing a comparison between experimental and theoretical results, and the mean relative errors between the results. The relative mean error was 5.0% between the experimental and theoretical rates of water evaporation (Fig. 12a); 4.7% between the experimental and theoretical total power consumption rates of the dryer (Fig. 12b); 0.7% between the experimental and theoretical drying efficiency of the dryer (Fig. 12c); and 0.6% between the experimental and theoretical SMER values of the dryer (Fig. 12d). All these errors were within acceptable ranges; changes in experimental and theoretical values in the figures also suggested acceptable relative mean errors.
33
ACCEPTED MANUSCRIPT Table 4 Performance of a dryer combined with a heat exchanger (expressed as experimental/theoretical values) Recirculation
Rate of water
ratio
SMER
Drying
Total power
evaporation (kg/h) (kWh/kg)
efficiency (%)
consumption (kW)
0
0.609/0.597
1.113/1.126
59.9/59.37
0.677/0.672
20
0.589/0.589
1.109/1.114
60.1/60.0
0.653/0/656
40
0.578/0.577
1.107/1.106
60.2/60.4
0.639/0.638
60
0.547/0.569
1.099/1.105
60.6/60.5
0.602/0.628
70
0.498/0.544
1.128/1.114
59.1/60.0
0.652/0.607
80
0.488/0.525
1.132/1.129
58.9/59.2
0.553/0.593
90
0.449/0.539
1.156/1.144
57.7/58.4
0.518/0.617
100
0.425/0.452
1.237/1.224
53.9/54.6
0.526/0.553
(%)
34
ACCEPTED MANUSCRIPT
(a)
(b)
(c)
(d)
Fig. 12. Comparison of Experimental and theoretical chart. (a) Water evaporation rates (b) Total power consumption rates (c) Drying efficiency values (d) SMER values.
35
ACCEPTED MANUSCRIPT 5. Conclusion 1.
Using the same drying time of a dryer, the drying efficiency of the dryer with a heat exchanger was 60%, which represents an increase of approximately 8.6% over the drying efficiency of the dryer. The percentage of power saved is approximately 16%. Therefore, an integrated heat exchanger helped improve drying efficiency.
2.
For the physical dehumidification system, which comprised a dryer with a heat exchanger, a recirculation ratio of 60% produced an optimal drying efficiency of 60.6% and optimal SMER value of 1.099. Compared with a dryer without a heat exchanger, the improved dryer used approximately 18% less power.
3.
Using the heat exchanger to recover and reuse heat (i.e., 0% recirculated air) reduced the total power consumed by the dryer by 16.1% compared to a dryer. Furthermore, using the heat exchanger to decrease the humidity ratio of the recirculated air (i.e., 100% recirculated air) reduced the total power consumed by the dryer by 4.4% compared with a dryer.
4.
Results from the theoretical mathematical model derived in this study matched the experimental results. The largest error between theoretical and experimental drying efficiencies was only 1.5%. The mathematical model constructed in this study can be used for calculations in future designs of the recirculation control device for a dryer with a heat exchanger to configure a dryer with increased performance.
5.
The mean relative errors were 5.0% between the experimental and theoretical rates of water evaporation, 4.7% between the experimental and theoretical total power consumption rates of the dryer, 0.7% between the experimental and theoretical drying efficiency of the dryer, and 0.6% between the experimental and theoretical SMER of the dryer. These mean relative errors indicated that the theoretical results from the mathematical model formalized in this study were consistent with the experimental 36
ACCEPTED MANUSCRIPT results. This finding is expected to serve as a reference for future research. Glossary A
Effective contact area of the load
m2
a
Activity coefficient of water
-
𝑐𝑝𝑎
Specific heat of air at a constant pressure
kJ/kgK
𝑐𝑝𝑤,𝑔
Specific heat of water vapor at constant pressure
kJ/kgK
𝐻𝑑
Mass transfer coefficient between the dryer load and air
kg/m2s
ℎ𝑎𝑚𝑏
Enthalpy of ambient air
kJ/kg
ℎ𝑒𝑥
Enthalpy of air exiting the dryer drum
kJ/kg
ℎ𝑖𝑛
Enthalpy of air entering the dryer drum
kJ/kg
ℎ𝑓𝑔
Latent heat of evaporation
kJ/kg
𝑀𝑤𝑓
Weight of dryer load after drying
kg
𝑀𝑤𝑖
Weight of dryer load before drying
kg
𝑀𝑤𝑟
Amount of evaporated water
kg
𝑚𝑒𝑥
Mass flow rate of air exiting the dryer drum
kg/h
𝑚𝑖𝑛
Mass flow rate of air entering the dryer drum
kg/h
𝑚𝑙𝑒𝑎
Mass flow rate of air leaked from the dryer drum
kg/h
𝑚𝑚𝑖𝑥
Mass flow rate of air after mixing
kg/h
𝑚𝑝𝑒𝑛
Mass flow rate of external air that penetrates the heater
kg/h
𝑚𝑤
Rate of water evaporation
kg/h
𝑃𝑡𝑜𝑡
Total energy consumed by the dryer
kW
𝑝
Total pressure of all gases
kPa
37
ACCEPTED MANUSCRIPT 𝑝𝑤𝑠
Saturation vapor pressure of water
kPa
𝑄𝑒𝑣𝑎
Rate of heat transfer absorbed by water evaporation
kW
SMER specific moisture extraction rate
kWh/kg
𝑇𝑎𝑣𝑔
Average temperature of the dryer load
K
𝑇𝑎𝑚𝑏
Ambient room temperature
K
𝑇𝑒𝑥
Temperature of air exiting the dryer drum
K
Tin
Temperature of air entering the dryer drum
K
𝑇𝑒𝑥
Temperature of air at the outlet after passing through the heat exchanger
K
𝑉𝑒𝑥
Flow rate of air at the outlet after passing through recirculation control m/s
'
valves 𝑉𝑟𝑒
Flow rate of air entering the mixing tank after passing through the m/s recirculation control valves
𝑊𝑎𝑚𝑏
Absolute humidity ratio of the ambient air
kg/kg
𝑊𝑒𝑥
Absolute humidity ratio of air exiting the dryer drum
kg/kg
𝑊𝑖𝑛
Absolute humidity ratio of air entering the dryer drum
kg/kg
𝑊𝑀
Humidity ratio of the saturated air on the surface of the load
kg/kg
𝑊𝑚𝑖𝑥
Absolute humidity ratio of air after mixing
kg/kg
'
𝑊𝑒𝑥
Absolute humidity ratio of air at the outlet after passing through the heat kg/kg exchanger
x
Recirculation ratio
-
Greek letters β
Constant
-
38
ACCEPTED MANUSCRIPT γ
Constant
-
δ
Constant
-
𝜀𝑠𝑒𝑛
Sensible heat effectiveness of the heat exchanger
-
𝜀𝑙𝑎𝑡
Latent heat effectiveness of the heat exchanger
-
𝜂
Drying efficiency
%
Acknowledgements This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Declarations of interest None.
39
ACCEPTED MANUSCRIPT References [1] Hekmat D, Fisk WJ. Improving the energy efficiency of residential clothes dryers [dissertation]. Berkeley (CA): University of California, 1983. [2] Lambert AJD, Spruit FPM, Claus J. Modelling as a tool for evaluating the effects of energy saving measures. Appl Energy 1991;38:33–47. [3] Conde MR. Energy conservation with tumbler drying in laundries. Appl Therm Eng 1997;17:1163–72. [4] Bansal P, Sharma K, Islam S. Thermal analysis of a new concept in a household clothes tumbler dryer. Appl Energy 2010;87:1562–71. [5] Woo S. An experimental study on the performance of a condensing tumbler dryer with an air-to-air heat exchanger. Korean J Chem Eng 2013;30:119–200. [6] Deans J. The modeling of a domestic tumbler dryer. Appl Therm Eng 2001;21:977–90. [7] Bassily AM, Colver GM. Performance analysis of an electric clothes dryer. Dry Technol 2003;21:499–524. [8] Yadav V, Moon CG. Fabric-drying process in domestic dryers. Appl Energy 2008;85:143–58. [9] Australia/New Zealand Standard. Performance of household electrical appliances— Rotary clothes dryers, Part 1: Energy consumption and performance. AS/NZS 2442.1:1996, 1996-2003. [10] ANSI/AHAM. Household tumble type clothes dryers. AHAM HLD-1-1992, 1992. [11] International Electrotechnical Commission. Tumble dryers for household use – methods for measuring the performance. IEC 61121, Ed.3.0, 1992-2005. [12] Stawreberg L, Nilsson L. Modelling of specific moisture extraction rate and leakage ratio in a condensing tumble dryer. Appl Therm Eng 2010;30:2173–9. [13] Stawreberg L, Berghel J, Renström R. Energy losses by air leakage in condensing 40
ACCEPTED MANUSCRIPT tumble dryers. Appl Therm Eng 2012;37:373–9. [14] Ambarita H. Performance of a clothes drying cabinet by utilizing waste heat from a split-type residential air conditioner. Case Stud Therm Eng 2016;8:105–14. [15] Yang CM. Energy-efficient air conditioning system with combination of radiant-cooling and periodic total heat exchanger. Energy 2013;59:467–77. [16] Jia CX, Dai YJ, Wu JY, Wang RZ. Experimental comparison of two honeycombed desiccant wheels fabricated with silica gel and composite desiccant material. Energy Convers Manag 2006;47:2523–34. [17] Minea V. Review: drying heat pumps–Part I: system integration. Int J Refrig 2013;36:643–58. [18] Baoling C, Zhe L, Zuchao Z, Huijie W, Guangfei M. Influence of opening and closing process of ball valve on external performance and internal flow characteristics. Experimental Thermal and Fluid Science 2017;80:193-202. [19] Parsons R. ASHRAE Handbook–Fundamentals. Atlanta, GA: American Society of Heating Refrigerating and Air-Conditioning. 2001, p. 6.2–10. [20] Lin SC. The Residential Sector Electricity Use Research in Taiwan. Journal of Taiwan Energy 2017;4: 285-302.
41
ACCEPTED MANUSCRIPT Highlights
Recover and reuse waste heat of tumble dryers through physical dehumidification
Heat exchanger with a 60% recirculation ratio has highest energy efficiency
Its power consumption is 18% lower than that of traditional dryers
It has lowest specific moisture extraction rate and highest drying efficiency
Drying efficiency was 50% higher than that of the traditional dryer
Su-Sheng Ma, Ching-Yi Tseng, You-Ren Jian, Tai-Her Yang, Sih-Li Chen PII:
S0360-5442(18)31519-6
DOI:
10.1016/j.energy.2018.08.011
Reference:
EGY 13487
To appear in:
Energy
Received Date:
05 March 2018
Accepted Date:
01 August 2018
Please cite this article as: Su-Sheng Ma, Ching-Yi Tseng, You-Ren Jian, Tai-Her Yang, Sih-Li Chen, Utilization of waste heat for energy conservation in domestic dryers, Energy (2018), doi: 10.1016/j.energy.2018.08.011
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Utilization of waste heat for energy conservation in domestic dryers Su-Sheng Maa, Ching-Yi Tsengb, You-Ren Jianc, Tai-Her Yangd, Sih-Li Chene,* aDepartment
of Mechanical Engineering, National Taiwan University, No. 1, Sec. 4,
Roosevelt Road, Taipei 10617, Taiwan bDepartment
of Mechanical Engineering, National Taiwan University, No. 1, Sec. 4,
Roosevelt Road, Taipei 10617, Taiwan cDepartment
of Mechanical Engineering, National Taiwan University, No. 1, Sec. 4,
Roosevelt Road, Taipei 10617, Taiwan dGiant
Lion Know-How Co., Ltd., 6F-5, No. 250, Sec. 4, Zhongxiao E. Road, Taipei 10692,
Taiwan eDepartment
of Mechanical Engineering, National Taiwan University, No. 1, Sec. 4,
Roosevelt Road, Taipei 10617, Taiwan * Corresponding author. Tel.: 886 2 33662726; Fax: 886 2 23631808. E-mail addresses: [email protected] (S.-S. Ma), [email protected] (C.-Y. Tseng), [email protected] (Y.-R. Jian), [email protected] (T.-H. Yang), [email protected] (S.-L. Chen).
1
ACCEPTED MANUSCRIPT ABSTRACT A physical means of dehumidification was used in this study to recover and reuse waste heat emitted from tumble dryers to increase drying efficiency and decrease power consumption. An experiment was conducted; waste heat from a dryer was dehumidified in a heat exchanger, and some heat was reclaimed to preheat external air. After the optimal mixture ratio of external and re-circulated air had been determined, that optimal air mixture was introduced into the dryer through the recirculation air ducts designed in this study. Experimental results showed that the improved dryer, operated at a 60% recirculation ratio produced the highest energy efficiency, resulting in 18% less power consumption than a standard dryer. In addition, this configuration produced the lowest specific moisture extraction rate (1.099) and the highest drying efficiency (60.6%). This efficiency was higher than that of the dryer (50%) and the proposed heat exchange system was thus more energy efficient than was the dryer. The theoretical mathematical model derived in this study matched experimental results. The average error values between the theoretical and experimental values for the drying efficiency and the specific moisture extraction rate were 0.7% and 0.6% ; the largest error values were only 1.5% and 1.2%. Keywords: Tumble dryer; cross-flow plate heat exchanger; heat recovery; re-circulation factor; drying efficiency; specific moisture extraction rate (SMER).
2
ACCEPTED MANUSCRIPT 1. Introduction With the continuous and rapid development of technology, global energy consumption has increased greatly. This has led to heightened environmental awareness, which has turned reducing energy usage and carbon emissions into a critical issue worldwide. In addition to developing new sources of energy, enhancing energy efficiency is vital to mitigating the problem. Therefore, the objectives of the present study were to retain the existing advantages of current dryers and to improve upon their disadvantages—low drying efficiency and high energy consumption—so that users can concurrently save energy and maintain convenient and comfortable lives. A tumble dryer operates by drawing in outside air, passing the outside air through a heater to increase its temperature and decrease its relative humidity, and transporting the dry air into a dryer drum powered by a motor and drive mechanism. The dryer drum rotates continuously to mix wet clothes, which have a higher humidity, with the dry air, which has a lower relative humidity. Because of the pressure difference between the water in wet clothes and water vapor, the water is desorbed into the dry air as the dryer rotates. The air is then expelled out of the dryer. After a specific amount of time, the wet clothes become dry. The power consumption of a dryer is dependent on its drying performance, which is affected by factors such as external temperature, relative humidity, the mass flow rate of air, the moisture content of the load, and the dryer’s heater. In household dryers, the heater is the component that consumes the most energy. The drying efficiency of most household dryers is approximately 50%. In other words, approximately 50% of the power consumed is used for drying clothes. The remaining 50% of the power is turned into waste heat and expelled into the atmosphere, which is extremely wasteful. Dryers are used extensively in modern society. The large quantities of waste heat emissions represent wasted energy. To achieve energy 3
ACCEPTED MANUSCRIPT conservation goals, researchers have proposed various methods of improving the energy efficiency of dryers through increasing drying efficiency or reducing energy consumption. Hekmat and Fisk [1] proposed several ways to improve the efficiency of a dryer, including 1) decreasing the mass flow rate of air, which reduces energy consumption by approximately 8%, and 2) increasing the temperature of intake air to reduce power consumed by the heater, which reduces power consumption by approximately 15%. Recovering and reusing waste heat can increase the temperature of intake air and thus conserve energy. However, recovering all of the waste heat may not necessarily increase drying efficiency; experiments must be performed to determine the optimal recirculation ratio of the waste heat emitted by the dryer. Therefore, Lambert et al. [2] created a model to simulate the energy savings of a dryer. They found that 75% recirculation produced the optimal drying efficiency (approximately 62%). This efficiency is approximately 15% higher than the 47% efficiency of the original factory model. Conde [3] added an interleaved heat exchanger to the dryer exhaust. This configuration uses the sensible heat of exhaust air to increase the temperature of the intake air. Conde also examined various sizes of the heat exchanger to determine which produced optimal benefits. Bansal et al. [4] improved upon dryer designs by installing a water-to-air heat exchanger behind the dryer drum. This replaces the traditional method of using a heater to preheat external air. Woo [5] developed a closed-loop dryer system integrated with a heat exchanger and heater. Results showed that increasing the power of the heater and the temperature of the intake air reduced drying times. Deans [6] proposed using sensitivity analysis to assess differences in drying times and specific energy consumption (SEC) in various operating conditions. Findings showed that SEC values were affected by ambient temperature and relative humidity. As room temperature increased, SEC decreased; in other words, dryer performance improved. Bassily and Colver [7] studied changes to dryer operating conditions by simulating the effect of various parameters, including load weight, 4
ACCEPTED MANUSCRIPT power consumption of the heater, mass flow rate of air, and moisture content of the clothes, on dryer performance. Yadav and Moon [8] performed a qualitative analysis of three internationally recognized standards [9–11] under various operating conditions using theories and experimental methods. Stawreberg and Nilsson [12] studied airflow and leakage in a condensing dryer. They found that high internal airflow and low external airflow produced the optimal specific moisture extraction rate (SMER) but also the most substantial leakage. The leakage aspect was improved in a subsequent study [13]. According to the aforementioned studies, introducing heated air into a dryer can improve drying efficiency. Air can be heated by a variety of sources; hot water, electric heaters, and heat pump systems are all viable sources. Recently, scholars such as Ambarita [14] constructed a drying cabinet that uses the heat waste from a split-type air conditioner to dry clothes. This represents another effective method of energy conservation. Incorporating this result into dryers in the future would create an energy-saving system of household appliances and would be a green research study. Summarizing the literature review, all proposed methods, such as controlling the recirculation ratio, dehumidifying the air through physical means or chemical means [15,16], or controlling the external air environment, advance the goal of introducing high-temperature, low-humidity air into the dryer. This allows the heater in the dryer to heat internal air to a temperature necessary for drying clothes in the shortest amount of time, thereby conserving energy as well as reducing drying time [17]. Unlike commercially available dryers, the dryer developed in the present study employs an innovative method of recycling waste heat by combining a heat exchanger and a novel recirculation control system. After evaluating the drying efficiency and SMER of the improved dryer, we determined that the new dryer is more energy efficient and environmentally friendly than currently available dryers. 5
ACCEPTED MANUSCRIPT 2. Experimental investigation 2.1. Physical dehumidifying system In this study, a heat exchanger and a recirculation control device were integrated into a dryer to address the disadvantages of a dryer. The drying efficiencies of the dryer and the improved dryer were then compared. The research process flowchart is shown in Fig. 1. First, energy-saving ideas were proposed (including heat recycling methods, heat exchanger choices, and designs for the recirculation control valve). Next, literature was reviewed to compare applications of relevant technologies. Subsequently, the design of the new dryer and new mathematical models were developed. Finally, the performance of the new dryer was tested and the resulting numerical data were analyzed and compared. Fig. 2 illustrates the setup of an experiment on a tumble dryer combined with a heat exchanger and a recirculation control system. Figs. 3 and 4 present the physical setup of that experiment. Fig. 5 shows the piping arrangement of the recirculation control device. A pipe connected the rear exhaust orifice of the dryer to a heat exchanger enveloped by heat insulation cotton (Fig. 3). A pipe connected the heat exchanger with a recirculation control system on the floor. The air reached an exhaust valve and a recirculation valve inside the dryer (Fig. 4); the air that entered the recirculation valve delivered reclaimed heat to the dryer through a black tube.
The added heat exchanger allows the exchange of external air and recirculated air to increase the temperature of intake air and reduce the temperature of recirculated air. This has the effect of reducing the absolute humidity of the recirculated air. Recirculation control valves control the recirculation ratio. The experiment primarily measured the dryers’ power consumption, drying efficiency, and rate of water evaporation at various recirculation flows 6
ACCEPTED MANUSCRIPT (which ranged from 0% to 100%) to determine the optimal recirculation ratio.
Fig. 1. Flowchart of dryer energy-saving research process.
7
ACCEPTED MANUSCRIPT
Fig. 2. Illustration of the setup of an experiment with a dryer combined with a heat exchanger and recirculation control system.
8
ACCEPTED MANUSCRIPT
Fig. 3. Physical setup of an experiment on a dryer combined with a heat exchanger and recirculation control device (1).
9
ACCEPTED MANUSCRIPT
Fig. 4. Physical setup of an experiment on a dryer combined with a heat exchanger and recirculation control device (2).
10
ACCEPTED MANUSCRIPT
Fig. 5. Piping arrangement of a recirculation control device.
11
ACCEPTED MANUSCRIPT Paired with the aforementioned system, the following experimental parameters were defined: 1. Recirculation ratio (x) A recirculation control device was used in this study to regulate the recirculation ratio, which enabled the examination of dryer performance at various recirculation ratios. This device is illustrated in Fig. 6, where 𝑚1 is the mass flow rate of exhaust air passing through a two-way valve and 𝑚2 is the mass flow rate of recirculated air entering the mixing tank. After obtaining flow rate readings in the experiment, the recirculation ratio can be defined as: 𝑥=𝑚
𝑚2
(1)
1 + 𝑚2
Because recirculation control is primarily determined by the opening angle of the recirculation control valve, the size of the opening angle affects the mass flow rate of air. For a given opening angle of the recirculation control valve, the opening angle and flow rate of the valve are linearly correlated [18]. In the cited work, the correlation between the opening angle and flow rate improves linearly when the opening angle is 30%–80%. Accordingly, when the control valve is opened, the flow rate increases steadily after the opening angle reaches 30%; when the opening angle reaches 80%, the flow rate peaks and remains constant thereafter. In the present study, however, the correlation between the opening angle and flow rate of the recirculation control valve showed slight linear growth when the recirculation ratio was 20%–60%. Specifically, after the recirculation ratio reached 20%, the flow rate increased steadily and drying efficiency improved. When the recirculation ratio was 60%, drying efficiency reached its optimal level; efficiency gradually declined after the ratio increased beyond 60% because air introduced excessive water vapor when the ratio was too high. Therefore, theoretical calculations of the recirculation ratio were based on the size of the 12
ACCEPTED MANUSCRIPT opening angle. The theoretical definition is: 𝑎𝑛𝑔𝑙𝑒 𝑜𝑓 𝑟𝑒𝑐𝑖𝑟𝑐𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑣𝑎𝑙𝑣𝑒(0~90 𝑑𝑒𝑔𝑟𝑒𝑒)
𝑥 = 𝑎𝑛𝑔𝑙𝑒 𝑜𝑓 𝑟𝑒𝑐𝑖𝑟𝑐𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑣𝑎𝑙𝑣𝑒(0~90 𝑑𝑒𝑔𝑟𝑒𝑒) + 𝑎𝑛𝑔𝑙𝑒 𝑜𝑓 𝑒𝑥ℎ𝑎𝑢𝑠𝑡 𝑣𝑎𝑙𝑣𝑒(0~90 𝑑𝑒𝑔𝑟𝑒𝑒)
(2)
In Eq. (2), an opening angle of 0° means the flow control valve is completely closed. An opening angle of 90° means the flow control valve is completely open. Thus, when the opening angle of the recirculation valve is 0° and the opening angle of the exhaust valve is 90°, the recirculation ratio is 0. When the opening angle of the recirculation valve is 90° and the opening angle of the exhaust valve is 0°, the recirculation ratio is 1. The circulation ratio ranges between 0 and 1 and can be calculated using Eq. (2).
13
ACCEPTED MANUSCRIPT
Fig. 6. Illustration of recirculation control device. 2. Amount of evaporated water (𝑀𝑤𝑟) The amount of evaporated water was calculated by measuring the difference in the weight of the dryer load before and after drying. (3)
𝑀𝑤𝑟 = 𝑀𝑤𝑓 ‒ 𝑀𝑤𝑖 (𝑘𝑔) 3. Rate of water evaporation (𝑚𝑤)
14
ACCEPTED MANUSCRIPT The rate of water evaporation was calculated in this study by weighing the wet load and the dry load and dividing the difference by the drying time. 𝑀𝑤𝑓(𝑘𝑔)
(4)
𝑚𝑤 = 𝑇𝑖𝑚𝑒(ℎ)
4. Total power consumed by the dryer (𝑃𝑡𝑜𝑡) A watt-hour meter was used to record the total power consumed by the dryer. 5. Drying efficiency (𝜂) Drying efficiency was defined in [2] as: 𝜂=
𝑟𝑎𝑡𝑒 𝑜𝑓 𝑒𝑛𝑒𝑟𝑔𝑦 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑓𝑜𝑟 𝑒𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑖𝑜𝑛 (𝑘𝑊) 𝑡𝑜𝑡𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑖𝑛𝑝𝑢𝑡 (𝑘𝑊)
=
=
𝑄𝑒𝑣𝑎 (𝑘𝑊) 𝑃𝑡𝑜𝑡 (𝑘𝑊) 𝑚𝑤ℎ𝑓𝑔𝑇𝑎𝑣𝑔 (𝑘𝑊)
(5)
𝑃𝑡𝑜𝑡 (𝑘𝑊)
where 𝑄𝑒𝑣𝑎 is the rate of heat transfer absorbed by water evaporation, 𝑇𝑎𝑣𝑔 is the average temperature of the load, 𝑚𝑤 is the rate of water evaporation, ℎ𝑓𝑔 is the latent heat of vaporization, and 𝑃𝑡𝑜𝑡 is the total power input of the dryer. The optimal recirculation ratio was determined by examining drying efficiency. 6. SMER SMER is defined as the ratio of total energy consumed to amount of evaporated water. 𝑆𝑀𝐸𝑅 =
𝑡𝑜𝑡𝑎𝑙 𝑒𝑛𝑒𝑟𝑔𝑦 𝑐𝑜𝑛𝑠𝑢𝑚𝑒𝑑(𝑘𝑊ℎ) 𝑀𝑤𝑟(𝑘𝑔)
(6)
Lower SMER values represent higher performance in dryers. 7. Energy saving percentage The energy saving percentage is defined as:
15
ACCEPTED MANUSCRIPT 𝐸𝑛𝑒𝑟𝑔𝑦 𝑠𝑎𝑣𝑖𝑛𝑔 𝑝𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 =
𝐸𝑟𝑒𝑐𝑖𝑟𝑐𝑢𝑙𝑎𝑡𝑒 ‒ 𝐸𝑏𝑎𝑠𝑒𝑙𝑖𝑛𝑒 𝐸𝑏𝑎𝑠𝑒𝑙𝑖𝑛𝑒
× 100%
(7)
The total energy used by a dryer without a heat exchanger was set as the baseline value and Eq. (7) was used to calculate the energy saving percentage at various recirculation ratios. 2.2. Experimental conditions The dryer load comprised 100% cotton towels. The moisture content was held constant at approximately 70% of the weight of the dry towels. The external environment of the dryer was held constant at a temperature of 25 °C and a relatively humidity of 55%. The mass flow rate of the dryer exhaust was approximately 0.02 kg/s. For the recirculation pipes of the dryer, benchmarks for recirculation ratio were calculated using (1) and (2) and the ratio of recirculated and exhaust air was adjusted accordingly. The recirculation ratio was also used to determine the drying time, which ranged from approximately 80–120 min. A watt-hour meter was used to record the total power consumed by the dryer. After drying, the dryer load was removed and weighed to determine the rate of water evaporation for each recirculation ratio. 2.3. Experimental and measuring apparatus The experimental and measuring apparatus used in this study are as follows: (1) Tumble dryer The tumble dryer (QD7561NA, TECO, Taiwan) used in the experiment has a capacity of 7 kg, an operation time of up to 180 min, and a dissipated power of 800 W. (2) Vane anemometer The vane anemometer (Testo 416, Testo, Australia) was used to measure the speed of both exhaust and recirculation. This anemometer has an estimated error range of ±0.2 m/s ± 1.5% and a measurement range of 0.6–40 m/s. (3) Humidity meter Five humidity meters (TRH-303, TECPEL, Taiwan) were used to measure temperature 16
ACCEPTED MANUSCRIPT and relative humidity. These humidity meters have a dry-bulb temperature error range of ± 0.3 °C, a measurement range of 0–100 °C, and a reaction time of < 15 sec. (4) Thermocouple A T-type thermocouple was used to measure the air inlet and outlet temperature of the high-and low-temperature sides of the dryer. The thermocouple uses the voltage differences due to temperature differences to measure temperature and has a measurement range of –200 to 400 °C. Before the experiment, the thermocouple underwent a temperature adjustment and achieved a measurement error of ±0.5 °C. (5) Data recorder All temperature data were transmitted to a data recorder (MV200, Yokogawa Electric Corporation, Japan). (6) Electronic weighing scale An electronic weighing scale (GX-6100, A&D, Japan) with a measurement range of 0– 6100 g and a precision of 0.01 g was used to estimate changes in load weights, calculate the amount of water vaporized, and determine the weights of the wet loads and the post-drying weights. Before use, the scale was set to zero. (7) Watt-hour meter A watt-hour meter was used to measure the total power consumption of the dryer. (8) Avometer A digital alternating-current HILA-9250 Avometer was used to measure changes in the power consumption of the dryer. The Avometer multiplied the operating voltage of the dryer and its operating current to determine the power consumption of the device. The Avometer has a voltage measurement range of 0–400 V (with a precision of ±0.1V) and a current measurement range of ±0.1A. (9) Crossflow plate heat exchanger 17
ACCEPTED MANUSCRIPT A crossflow plate heat exchanger (PWT 10/200/200-2.0, Klingenburg) that measures 20 × 20 × 16 cm in size and weighs 2 kg in weight was used to recover heat energy and reduce the temperature of recirculation air. 3. Theoretical investigation In the mathematical model constructed for the present study, input parameters such as external temperature, absolute humidity, the sensible heat effectiveness of the heat exchanger, the latent heat effectiveness of the heat exchanger, and recirculation ratio were used to predict the dryer’s exhaust conditions, rate of water evaporation, power consumption, and drying efficiency. The objective was to verify that results from the derived mathematical model were consistent with experimental results. The structure of the mathematical model and the flowchart for calculations in the mathematical model are shown in Figs. 7 and 8, respectively. The basic assumptions were as follows: 1. Tin is a constant value set at approximately 51 °C. 2. The heat exchanger has two effectiveness measurements: sensible heat effectiveness (𝜀𝑠𝑒𝑛 ) and latent heat effectiveness (𝜀𝑙𝑎𝑡). 3. Effects from gravity, pressure drop, and velocity loss are not considered. 4. The thermal and physical properties of air and of the dryer load are constants. 5. The heat transfer coefficient of the load is an average value. 6. The energy stored within the dryer is not considered.
18
ACCEPTED MANUSCRIPT
Fig. 7. Schematic of the mathematical model.
19
ACCEPTED MANUSCRIPT
Fig. 8. Flowchart of calculations in the mathematical model. The mass flow rates at the dryer’s inlet and outlet are held constant and not affected by air recirculation. Conservation of mass in the dryer drum (Fig. 8) produces the following equation: (8)
d𝑚 = 0 Expanding Eq. (8) produces the following formula: 𝑚𝑖𝑛(1 + 𝑊𝑖𝑛) + 𝑚𝑤 = 𝑚𝑒𝑥(1 + 𝑊𝑒𝑥) + 𝑚𝑙𝑒𝑎(1 + 𝑊𝑒𝑥)
(9)
As stated in the third assumption, velocity loss was not considered. This results in (10)
𝑚𝑖𝑛 = 𝑚𝑚𝑖𝑥 + 𝑚𝑝𝑒𝑛≅𝑚𝑙𝑒𝑎 + 𝑚𝑒𝑥 Combining Eqs. (9) and (10) produces: 20
ACCEPTED MANUSCRIPT (11)
𝑚𝑤 = (𝑚𝑒𝑥 + 𝑚𝑙𝑒𝑎)𝑊𝑒𝑥 ‒ 𝑚𝑖𝑛𝑊𝑖𝑛 Based on [6], the rate of water evaporation of the load is rewritten as: 𝑚𝑤 = 𝐻𝑑𝐴(𝑎𝑊𝑀 ‒
𝑊𝑖𝑛 + 𝑊𝑒𝑥 2
(12)
)
where A is the effective contact area of the load (measured to be 3.2 m2 in this experiment) and WM is the saturation humidity ratio of the air on the surface of the load (in kg/kg). WM can be determined from looking up meteorological tables or calculated using the Hyland– Wexler equation. The activity coefficient of water, a, is defined as follows: 𝑎=1‒
𝛿 + 𝛽𝑀𝑐 𝛾𝑀
1+𝛿
(13)
𝑐
δ, β, and γ are constants associated with the material of the load. For the present experiment, the material of the load was 100% cotton. According to [6], δ = 2, β = 18, and γ = 30. The humidity ratio of the mixed air can be calculated by the status of the mixed air in the mixing tank: '
(14)
𝑊𝑚𝑖𝑥 = 𝑥𝑊𝑒𝑥 + (1 ‒ 𝑥)𝑊𝑎𝑚𝑏
The latent heat effectiveness of the heat exchanger 𝜀𝑙𝑎𝑡 can be obtained experimentally. From '
this, 𝑊𝑒𝑥 can be calculated by: '
(15)
𝑊𝑒𝑥 = 𝑊𝑒𝑥 ‒ 𝜀𝑙𝑎𝑡(𝑊𝑒𝑥 ‒ 𝑊𝑎𝑚𝑏) where εlat is the latent heat effectiveness, which is defined as:
21
ACCEPTED MANUSCRIPT
l at
ex
' ex
ex amb
(16)
where ωamb is the humidity ratio of the functional fluids at the inlet of the heat exchanger '
(kg/kg); ωex is the humidity ratio of hot air entering the heat exchanger (kg/kg); and ωex is the humidity ratio of air cooled by the heat exchanger (kg/kg).
Combining Eqs. (14) and (15) produces: 𝑊𝑚𝑖𝑥 = 𝑥[𝑊𝑒𝑥 ‒ 𝜀𝑙𝑎𝑡(𝑊𝑒𝑥 ‒ 𝑊𝑎𝑚𝑏)] + (1 ‒ 𝑥)𝑊𝑎𝑚𝑏 = 𝑥(1 ‒ 𝜀𝑙𝑎𝑡)𝑊𝑒𝑥 + 𝑥𝜀𝑙𝑎𝑡𝑊𝑎𝑚𝑏 + (1 ‒ 𝑥)𝑊𝑎𝑚𝑏
(17)
After air passes through the heater, the temperature of the air increases but the absolute humidity ratio remains unchanged. Win is primarily determined by the status of mixing in the mixing tank. Simplifying the equation produces: 𝑊𝑖𝑛 =
𝑚𝑚𝑖𝑥 𝑚𝑖𝑛
𝑊𝑚𝑖𝑥 +
𝑚𝑙𝑒𝑎 𝑚𝑖𝑛
(18)
𝑊𝑎𝑚𝑏
Combining Eqs. (11), (12), (17), and (18) produces: 𝑊𝑒𝑥 =
𝐴 ∙ 𝑊𝑎𝑚𝑏 + 𝐵 ∙ 𝑊𝑀
(
where 𝐴 = 𝑚𝑖𝑛 ‒ 𝑚𝑒 𝐻𝑑𝐴
+𝑚
(
𝑖𝑛
(19)
𝐶
2
)[
𝐻𝑑𝐴 𝑚𝑒𝑥 2
𝑚𝑖𝑛
𝑚
]
(𝑥 ∙ 𝜀𝑙𝑎𝑡 + 1 ‒ 𝑥) + 𝑚𝑙𝑒𝑎 , 𝐵 = 𝐻𝑑 ∙ 𝐴 ∙ 𝑎, and 𝐶 = 𝑚𝑙𝑒𝑎 + 𝑚𝑒𝑥 + 𝑖𝑛
𝐻𝑑𝐴 2
)
‒ 𝑚𝑖𝑛 (1 ‒ 𝜀𝑙𝑎𝑡)𝑥.
From Eq. (18), the humidity ratio of the saturated air on the surface of the load 𝑊𝑀 is calculated as [189]:
22
ACCEPTED MANUSCRIPT 𝑝𝑤𝑠
(20)
𝑊𝑀 = 0.6298𝑝 ‒ 𝑝
𝑤𝑠
𝐶1
ln (𝑝𝑤𝑠) = 𝑇
𝑎𝑣𝑔
2
3
(21)
+ 𝐶2 + 𝐶3𝑇𝑎𝑣𝑔 + 𝐶4𝑇𝑎𝑣𝑔 + 𝐶5𝑇𝑎𝑣𝑔 + 𝐶6ln (𝑇𝑎𝑣𝑔)
0 ≤ 𝑇𝑎𝑣𝑔 ≤ 100℃ 3
0
where 𝐶1 =‒ 5.800 × 10 , 𝐶2 = 1.391 × 10 , 𝐶3 =‒ 4.864 × 10 =‒ 1.445 × 10
‒8
‒2
, 𝐶4 = 4.176 × 10
‒5
, 𝐶5
0
, and 𝐶6 = 6.546 × 10 .
𝑇𝑎𝑣𝑔 is the average temperature of the load, or the average of the intake and exhaust temperatures of the dryer: 𝑇𝑎𝑣𝑔 =
𝑇𝑖𝑛 + 𝑇𝑒𝑥 2
=
51 + 𝑇𝑒𝑥
(22)
2
The exhaust temperature 𝑇𝑒𝑥 is unknown. An iterative method was used to solve for 𝑇𝑒𝑥 in this study. First, an initial value was assumed for 𝑇𝑒𝑥 and substituted into (18) to solve for the humidity ratio of the exhaust (𝑊𝑒𝑥). Next, conservation of energy was used to calculate 𝑇𝑒𝑥; this value was compared with the initial 𝑇𝑒𝑥. If the two values were similar, the exhaust temperature of the dryer (𝑇𝑒𝑥) and the corresponding humidity ratio of the exhaust (𝑊𝑒𝑥) were established. After calculating the humidity ratio of the exhaust, the rate of water evaporation (𝑚𝑤) was calculated. The rate of water evaporation was an important indicator in this experiment. It was used primarily to determine the drying time, and also to identify improvement to the dryer system according to the recirculation ratio. Next, conservation of energy must be observed in the dryer drum, as in Eq. (23): (23)
𝑑𝐸 = 0
From the mathematical model constructed for the present study (Fig. 7), Eq. (14) can be 23
ACCEPTED MANUSCRIPT derived: (23)
𝑚𝑖𝑛ℎ𝑖𝑛 + 𝑚𝑤ℎ𝑓𝑔 = (𝑚𝑒𝑥 + 𝑚𝑙𝑒𝑎)ℎ
𝑒𝑥
The enthalpy of the humid air can be expressed as [19]: ℎ𝑖𝑛 = 𝑐𝑝𝑎𝑇𝑖𝑛 + 𝑊𝑖𝑛(2501 + 𝑐𝑝𝑤,𝑔𝑇𝑖𝑛)
(24)
ℎ𝑒𝑥 = 𝑐𝑝𝑎𝑇𝑒𝑥 + 𝑊𝑒𝑥(2501 + 𝑐𝑝𝑤,𝑔𝑇𝑒𝑥)
(25)
Combining Eqs. (23)–(25) produces:
𝑇𝑒𝑥 =
𝑚𝑤ℎ𝑓𝑔 + 𝑚𝑖𝑛[𝑐𝑝𝑎𝑇𝑖𝑛 + 𝑊𝑖𝑛(2501 + 𝑐𝑝𝑤,𝑔𝑇𝑖𝑛)] ‒ 2501(𝑚𝑒𝑥 + 𝑚𝑙𝑒𝑎)𝑊𝑒𝑥 (𝑚𝑒𝑥 + 𝑚𝑙𝑒𝑎)(𝑐𝑝𝑎 + 𝑊𝑒𝑥𝑐𝑝𝑤,𝑔)
(26)
The 𝑇𝑒𝑥 calculated from (26) was compared with the 𝑇𝑒𝑥 calculated from conservation of mass. If the two values were similar, the exhaust temperature of the dryer (𝑇𝑒𝑥) and the corresponding humidity ratio of the exhaust (𝑊𝑒𝑥) could be established. If the values were not similar, a different value for 𝑇𝑒𝑥 was chosen, and the computations for conservation of mass and conservation of energy were recalculated. The flowchart of calculations is shown in Fig. 8. 4. Results and discussion 4.1. Effectiveness testing of the heat exchanger A dryer combined with a heat exchanger and recirculation control device were examined in the present study. Recirculation control valves were used to adjust the recirculation ratio. In addition, the sensible heat and latent heat of the heat exchanger were used to increase the intake temperature as well as to decrease the humidity ratio. This ensured that the air entering the dryer was of high-temperature and low-humidity, which serves to improve the dryer’s performance. Because the mathematical analysis of a moist air-to-air heat exchanger is 24
ACCEPTED MANUSCRIPT complicated, the values of the sensible heat effectiveness and latent heat effectiveness of the heat exchanger used in the mathematical model were obtained through experimental readings. Readings were taken at the inlet and outlet of the heat exchanger (Fig. 9). The definitions of sensible heat effectiveness and latent heat effectiveness are provided below. The definition of sensible heat is: '
(𝑇𝑒𝑥 ‒ 𝑇𝑒𝑥)
𝜀𝑠𝑒𝑛 = (𝑇
(27)
𝑒𝑥 ‒ 𝑇𝑎𝑚𝑏)
'
where 𝑇𝑒𝑥 is the temperature of recirculated air before it enters the heat exchanger; 𝑇𝑒𝑥 is the temperature of recirculated air after it leaves the heat exchanger; and 𝑇𝑎𝑚𝑏 is the ambient room temperature. The definition of latent heat 𝜀𝑙𝑎𝑡 is shown on equation (16).
25
ACCEPTED MANUSCRIPT
Fig. 9. Locations of temperature and humidity readings. '
𝑊𝑒𝑥 is the humidity ratio of the recirculated air before it enters the heat exchanger; 𝑊𝑒𝑥 is the humidity ratio of recirculated air after it leaves the heat exchanger, and 𝑊𝑎𝑚𝑏 is the humidity ratio of the room.
'
Higher sensible heat effectiveness means that 𝑇𝑒𝑥 is closer to 26
ACCEPTED MANUSCRIPT '
𝑇𝑎𝑚𝑏. Higher latent heat effetiveness means that 𝑊𝑒𝑥 is closer to 𝑊𝑎𝑚𝑏. Intake air entering the dryer with a higher temperature and a lower humidity ratio can significantly improve drying efficiency. Experimental readings of temperature and humidity were taken at the heat exchanger inlet and outlet. The temperature of the recirculated air at the inlet and outlet were 35.1 °C ( '
𝑇𝑒𝑥) and 31.8 °C (𝑇𝑒𝑥), respectively. The humidity ratio of the recirculated air at the inlet and '
outlet were 31.08 g/kg (𝑊𝑒𝑥) and 28.32 g/kg (𝑊𝑒𝑥), respectively. Regarding the cold air at the inlet, the temperature was 27.1 °C (𝑇𝑎𝑚𝑏) and the humidity ratio was 13 g/kg (𝑊𝑒𝑥). Using (27) and (28), the sensible heat effectiveness (𝜀𝑠𝑒𝑛) was calculated to be approximately 0.41 and the latent heat effectiveness (𝜀𝑙𝑎𝑡) to be approximately 0.15. 4.2. Performance comparison between a dryer and a dryer with a heat exchanger In this study, a heat exchanger was used to pre-heat external air to increase the temperature of the air at the dryer inlet. Drying efficiency, SMER, and total power consumption were obtained and compared with the results from a dryer that does not include a heat exchanger. Finally, using the total power consumed by a dryer as the baseline, the percentage of power saved by the dryer with a heat exchanger was calculated. Two dry loads of the same weight (1.35 kg) were saturated with water to produce two wet loads of the same weight (2.18 kg). The amount of time required to return the wet loads to the dry weight, rate of water evaporation (kg/h), total power consumed, and total energy consumed were measured to calculate the SMER and drying efficiency. These were then used to compare dryer performances; results are listed in Table 2. The drying efficiency of the dryer with a heat exchanger was 60%, which was approximately 8.6% higher than the drying efficiency of the dryer without a heat exchanger. In addition, the SMER of the former (1.113) 27
ACCEPTED MANUSCRIPT was lower than the SMER of the latter (1.304), which indicated that the improved dryer consumed less energy per kilogram of water evaporated. The energy savings was approximately 16.1%. Therefore, using a heat exchanger to decrease the moisture in exhaust air was found to also lower the humidity ratio of the mixed air and raise the temperature of the mixed air. These all improved the performance of the dryer. Table 2 Comparison of experimental values and performances of a dryer and a dryer with a heat exchanger. Time Required to Power Percentage Return Total Power Drying Consumption SMER of Energy Wet Consumption Efficiency (kWh/kg) Rate Saved (kWh) (%) Load to (kW) (%) Dry Weight (min) Dryer
84
0.773
1.100
1.304
51.3
-
Improved dryer with heat exchanger
82
0.677
0.925
1.113
60.0
16.1%
4.3. Performance of a dryer with a heat exchanger at various recirculation ratios Table 2 shows that integrating a heat exchanger with the dryer can decrease the total power consumption of the dryer. Thus, subsequent testing examined the SMER, drying efficiency, rate of water evaporation, and total power consumption of a dryer with a heat exchanger at various recirculation ratios. Using the total power consumed by a dryer as the baseline, the percentage of power saved by various recirculation ratios was calculated. First, the rate of water evaporation was analyzed for various recirculation ratios. The rate of water evaporation was found to decrease as recirculation ratio increased. The reason for this is that 28
ACCEPTED MANUSCRIPT the rate of water evaporation was affected by the humidity ratio of the air at the dryer inlet. Therefore, increasing the recirculation ratio also increases the moisture content of the inlet air, which then decreased the dryer’s rate of water evaporation. The total power consumption was affected by the temperature of the air at the dryer inlet. Because mixing the high temperature intake air with preheated external air increased the temperature of the air at the dryer inlet, the heater consumed less power. However, increasing the recirculation ratio also increased the moisture content of the air at the dryer inlet and thus the drying time. This lead to higher power consumption. Because the recirculation ratio’s effect on power consumption is dominated by the increases or decreases of two factors, the optimal recirculation ratio must be determined through experimental findings. The experimental findings are shown in Table 3. All recirculation ratios presented power savings over the dryer. However, a recirculation ratio of 60% produced the highest drying efficiency (60.6%) and the optimal power savings percentage (18%), as shown in Fig. 10. Furthermore, a recirculation ratio of 100%, i.e., when no external air is introduced, was still more energy efficient than a dryer. However, at recirculation ratios higher than 70%, drying time increased because the humidity ratio of the air mixture was excessively high, which decreased the rate of water evaporation. At a recirculation ratio of 70%, the drying time was 32% longer than the drying time of a dryer. At a recirculation ratio of 100%, the drying time was 45% longer. Therefore, these ratios were less suitable for practical applications. Table 3 Performance of a dryer with a heat exchanger at various recirculation ratios. Recirculation
Rate of water
SMER
Drying
Total power
Energy
ratio
evaporation
(kWh/kg)
efficiency
consumption
saving
29
ACCEPTED MANUSCRIPT (%)
(kg/h)
(%)
(kWh)
percentage (%)
0
0.609
1.113
59.9
0.925
16.1
20
0.589
1.109
60.1
0.931
15.4
40
0.578
1.107
60.2
0.984
10.5
60
0.547
1.099
60.6
0.902
18.0
70
0.498
1.128
59.1
0.993
9.7
80
0.488
1.132
58.9
0.951
13.5
90
0.449
1.156
57.7
1.036
5.8
100
0.425/
1.237
53.9
1.052/
4.4
Fig. 11 shows a comparison of the theoretical and experimental values of drying efficiency and SMER. Consistent trends between the experimental and theoretical values were identified. The average error between theoretical and experimental values for drying efficiency was 0.7%; the largest error was 1.5%. The average error between theoretical and experimental values of SMER was 0.6%; the largest error was 1.2%. This indicated that the mathematical model established in this study produced values that approximated experimental values and the model can aid in choosing the optimal recirculation ratio for dryers. Fig. 11 also shows that for both experimental and theoretical values, a recirculation ratio of approximately 60% produced optimal drying efficiencies. In addition, a recirculation ratio of 60% represented a turning point after which the drying efficiency began to decrease. This was related to the humidity ratio of the recirculated air in the dryer. Overly high humidity ratio causes an overly long drying time and excessive large power consumption, which then decreases drying efficiency.
30
ACCEPTED MANUSCRIPT
Fig. 10. Percentage of power saved in a dryer with a heat exchanger at various recirculation ratios.
Fig. 11. Comparison of theoretical and experimental values for (a) drying efficiency and (b) SMER.
4.4. A perspective on the energy efficiency of dryers In the United States, a dryer consumes over 1000 kWh annually on average [20]; this represents a very high proportion of the total energy consumed by all home appliances. The 31
ACCEPTED MANUSCRIPT American climate is more arid than the Taiwanese climate, and has high temperatures during the summer. This arid and hot climate is very suitable for dryer operations. In contrast, Taiwan has an island climate. The hot and humid environment has a substantial impact on drying efficiency. As a result, dryer usage in Taiwan is relatively low, and air conditioning use represents the majority of energy consumption among home appliances. However, dryer usage has begun to increase in recent years as more and more dryers that feature energy efficiency have appeared on the market. Dryers awarded with Taiwan’s Energy Label certification must display the product’s energy factor on the product. According to the Ministry of Economic Affairs’ Bureau of Energy regulations, the measured energy factor of dryers must be higher than 1.7 kg/kWh. For example, a popular dryer model in Taiwan (model TCD-7.0RJ) has a capacity of 7 kg and its energy consumption is 1.3 kWh. If the dryer is used for two hours per day, its annual energy consumption is 949 kWh. This amount approximates the annual energy consumption of a US dryer. However, drying clothes completely in Taiwan’s humid climate likely requires more than two hours. Therefore, dryers consume a substantial amount of energy. However, a critical issue is that currently available dryers do not recycle waste heat. Waste heat generated while drying clothes is directly exhausted to the outside and represents wasted energy. Therefore, a dryer that integrates a heat exchanger and a recirculation control device was designed in this study. The heat exchanger addresses the effect of a humid climate on drying efficiency. The recirculation control addresses energy waste by effectively reintroducing waste heat into the dryer. For both in the United States, where dryer usage is already high, and in Taiwan, where dryer usage is increasing, the dryer designed in this study can achieve the desired effects of reducing energy consumption and greater energy efficiency. 4.5. Analysis of errors in experimental data (experimental vs. theoretical values) The performance of the dryer was assessed in relation to different recirculation ratios, 32
ACCEPTED MANUSCRIPT temperatures, relative humidity levels, water evaporation rates, drying efficiency levels, and power consumption levels. All experimental data were obtained through experimental apparatus. Considering the errors between experimental and theoretical values that inevitably arise from manually operated experimental apparatus of limited precision, a mathematical model (Fig.8) was constructed to produce theoretical values. The theoretical values were compared with experimental values to ascertain whether they were consistent and to estimate errors between them. Table 4 compares the experimental and theoretical performance of a dryer with a heat exchanger. Fig.12 respectively depict the water evaporation rate, the total power consumption rate of the dryer, the drying efficiency of the dryer, and the SMER of the dryer. These four figures include experimental values (marked with blue dots) and theoretical ones (marked with red lines), allowing a comparison between experimental and theoretical results, and the mean relative errors between the results. The relative mean error was 5.0% between the experimental and theoretical rates of water evaporation (Fig. 12a); 4.7% between the experimental and theoretical total power consumption rates of the dryer (Fig. 12b); 0.7% between the experimental and theoretical drying efficiency of the dryer (Fig. 12c); and 0.6% between the experimental and theoretical SMER values of the dryer (Fig. 12d). All these errors were within acceptable ranges; changes in experimental and theoretical values in the figures also suggested acceptable relative mean errors.
33
ACCEPTED MANUSCRIPT Table 4 Performance of a dryer combined with a heat exchanger (expressed as experimental/theoretical values) Recirculation
Rate of water
ratio
SMER
Drying
Total power
evaporation (kg/h) (kWh/kg)
efficiency (%)
consumption (kW)
0
0.609/0.597
1.113/1.126
59.9/59.37
0.677/0.672
20
0.589/0.589
1.109/1.114
60.1/60.0
0.653/0/656
40
0.578/0.577
1.107/1.106
60.2/60.4
0.639/0.638
60
0.547/0.569
1.099/1.105
60.6/60.5
0.602/0.628
70
0.498/0.544
1.128/1.114
59.1/60.0
0.652/0.607
80
0.488/0.525
1.132/1.129
58.9/59.2
0.553/0.593
90
0.449/0.539
1.156/1.144
57.7/58.4
0.518/0.617
100
0.425/0.452
1.237/1.224
53.9/54.6
0.526/0.553
(%)
34
ACCEPTED MANUSCRIPT
(a)
(b)
(c)
(d)
Fig. 12. Comparison of Experimental and theoretical chart. (a) Water evaporation rates (b) Total power consumption rates (c) Drying efficiency values (d) SMER values.
35
ACCEPTED MANUSCRIPT 5. Conclusion 1.
Using the same drying time of a dryer, the drying efficiency of the dryer with a heat exchanger was 60%, which represents an increase of approximately 8.6% over the drying efficiency of the dryer. The percentage of power saved is approximately 16%. Therefore, an integrated heat exchanger helped improve drying efficiency.
2.
For the physical dehumidification system, which comprised a dryer with a heat exchanger, a recirculation ratio of 60% produced an optimal drying efficiency of 60.6% and optimal SMER value of 1.099. Compared with a dryer without a heat exchanger, the improved dryer used approximately 18% less power.
3.
Using the heat exchanger to recover and reuse heat (i.e., 0% recirculated air) reduced the total power consumed by the dryer by 16.1% compared to a dryer. Furthermore, using the heat exchanger to decrease the humidity ratio of the recirculated air (i.e., 100% recirculated air) reduced the total power consumed by the dryer by 4.4% compared with a dryer.
4.
Results from the theoretical mathematical model derived in this study matched the experimental results. The largest error between theoretical and experimental drying efficiencies was only 1.5%. The mathematical model constructed in this study can be used for calculations in future designs of the recirculation control device for a dryer with a heat exchanger to configure a dryer with increased performance.
5.
The mean relative errors were 5.0% between the experimental and theoretical rates of water evaporation, 4.7% between the experimental and theoretical total power consumption rates of the dryer, 0.7% between the experimental and theoretical drying efficiency of the dryer, and 0.6% between the experimental and theoretical SMER of the dryer. These mean relative errors indicated that the theoretical results from the mathematical model formalized in this study were consistent with the experimental 36
ACCEPTED MANUSCRIPT results. This finding is expected to serve as a reference for future research. Glossary A
Effective contact area of the load
m2
a
Activity coefficient of water
-
𝑐𝑝𝑎
Specific heat of air at a constant pressure
kJ/kgK
𝑐𝑝𝑤,𝑔
Specific heat of water vapor at constant pressure
kJ/kgK
𝐻𝑑
Mass transfer coefficient between the dryer load and air
kg/m2s
ℎ𝑎𝑚𝑏
Enthalpy of ambient air
kJ/kg
ℎ𝑒𝑥
Enthalpy of air exiting the dryer drum
kJ/kg
ℎ𝑖𝑛
Enthalpy of air entering the dryer drum
kJ/kg
ℎ𝑓𝑔
Latent heat of evaporation
kJ/kg
𝑀𝑤𝑓
Weight of dryer load after drying
kg
𝑀𝑤𝑖
Weight of dryer load before drying
kg
𝑀𝑤𝑟
Amount of evaporated water
kg
𝑚𝑒𝑥
Mass flow rate of air exiting the dryer drum
kg/h
𝑚𝑖𝑛
Mass flow rate of air entering the dryer drum
kg/h
𝑚𝑙𝑒𝑎
Mass flow rate of air leaked from the dryer drum
kg/h
𝑚𝑚𝑖𝑥
Mass flow rate of air after mixing
kg/h
𝑚𝑝𝑒𝑛
Mass flow rate of external air that penetrates the heater
kg/h
𝑚𝑤
Rate of water evaporation
kg/h
𝑃𝑡𝑜𝑡
Total energy consumed by the dryer
kW
𝑝
Total pressure of all gases
kPa
37
ACCEPTED MANUSCRIPT 𝑝𝑤𝑠
Saturation vapor pressure of water
kPa
𝑄𝑒𝑣𝑎
Rate of heat transfer absorbed by water evaporation
kW
SMER specific moisture extraction rate
kWh/kg
𝑇𝑎𝑣𝑔
Average temperature of the dryer load
K
𝑇𝑎𝑚𝑏
Ambient room temperature
K
𝑇𝑒𝑥
Temperature of air exiting the dryer drum
K
Tin
Temperature of air entering the dryer drum
K
𝑇𝑒𝑥
Temperature of air at the outlet after passing through the heat exchanger
K
𝑉𝑒𝑥
Flow rate of air at the outlet after passing through recirculation control m/s
'
valves 𝑉𝑟𝑒
Flow rate of air entering the mixing tank after passing through the m/s recirculation control valves
𝑊𝑎𝑚𝑏
Absolute humidity ratio of the ambient air
kg/kg
𝑊𝑒𝑥
Absolute humidity ratio of air exiting the dryer drum
kg/kg
𝑊𝑖𝑛
Absolute humidity ratio of air entering the dryer drum
kg/kg
𝑊𝑀
Humidity ratio of the saturated air on the surface of the load
kg/kg
𝑊𝑚𝑖𝑥
Absolute humidity ratio of air after mixing
kg/kg
'
𝑊𝑒𝑥
Absolute humidity ratio of air at the outlet after passing through the heat kg/kg exchanger
x
Recirculation ratio
-
Greek letters β
Constant
-
38
ACCEPTED MANUSCRIPT γ
Constant
-
δ
Constant
-
𝜀𝑠𝑒𝑛
Sensible heat effectiveness of the heat exchanger
-
𝜀𝑙𝑎𝑡
Latent heat effectiveness of the heat exchanger
-
𝜂
Drying efficiency
%
Acknowledgements This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Declarations of interest None.
39
ACCEPTED MANUSCRIPT References [1] Hekmat D, Fisk WJ. Improving the energy efficiency of residential clothes dryers [dissertation]. Berkeley (CA): University of California, 1983. [2] Lambert AJD, Spruit FPM, Claus J. Modelling as a tool for evaluating the effects of energy saving measures. Appl Energy 1991;38:33–47. [3] Conde MR. Energy conservation with tumbler drying in laundries. Appl Therm Eng 1997;17:1163–72. [4] Bansal P, Sharma K, Islam S. Thermal analysis of a new concept in a household clothes tumbler dryer. Appl Energy 2010;87:1562–71. [5] Woo S. An experimental study on the performance of a condensing tumbler dryer with an air-to-air heat exchanger. Korean J Chem Eng 2013;30:119–200. [6] Deans J. The modeling of a domestic tumbler dryer. Appl Therm Eng 2001;21:977–90. [7] Bassily AM, Colver GM. Performance analysis of an electric clothes dryer. Dry Technol 2003;21:499–524. [8] Yadav V, Moon CG. Fabric-drying process in domestic dryers. Appl Energy 2008;85:143–58. [9] Australia/New Zealand Standard. Performance of household electrical appliances— Rotary clothes dryers, Part 1: Energy consumption and performance. AS/NZS 2442.1:1996, 1996-2003. [10] ANSI/AHAM. Household tumble type clothes dryers. AHAM HLD-1-1992, 1992. [11] International Electrotechnical Commission. Tumble dryers for household use – methods for measuring the performance. IEC 61121, Ed.3.0, 1992-2005. [12] Stawreberg L, Nilsson L. Modelling of specific moisture extraction rate and leakage ratio in a condensing tumble dryer. Appl Therm Eng 2010;30:2173–9. [13] Stawreberg L, Berghel J, Renström R. Energy losses by air leakage in condensing 40
ACCEPTED MANUSCRIPT tumble dryers. Appl Therm Eng 2012;37:373–9. [14] Ambarita H. Performance of a clothes drying cabinet by utilizing waste heat from a split-type residential air conditioner. Case Stud Therm Eng 2016;8:105–14. [15] Yang CM. Energy-efficient air conditioning system with combination of radiant-cooling and periodic total heat exchanger. Energy 2013;59:467–77. [16] Jia CX, Dai YJ, Wu JY, Wang RZ. Experimental comparison of two honeycombed desiccant wheels fabricated with silica gel and composite desiccant material. Energy Convers Manag 2006;47:2523–34. [17] Minea V. Review: drying heat pumps–Part I: system integration. Int J Refrig 2013;36:643–58. [18] Baoling C, Zhe L, Zuchao Z, Huijie W, Guangfei M. Influence of opening and closing process of ball valve on external performance and internal flow characteristics. Experimental Thermal and Fluid Science 2017;80:193-202. [19] Parsons R. ASHRAE Handbook–Fundamentals. Atlanta, GA: American Society of Heating Refrigerating and Air-Conditioning. 2001, p. 6.2–10. [20] Lin SC. The Residential Sector Electricity Use Research in Taiwan. Journal of Taiwan Energy 2017;4: 285-302.
41
ACCEPTED MANUSCRIPT Highlights
Recover and reuse waste heat of tumble dryers through physical dehumidification
Heat exchanger with a 60% recirculation ratio has highest energy efficiency
Its power consumption is 18% lower than that of traditional dryers
It has lowest specific moisture extraction rate and highest drying efficiency
Drying efficiency was 50% higher than that of the traditional dryer