A NEW DESIGN IN HOUSEHOLD CLOTHES TUMBLER DRYER
Abstract:
This is a project done by us in R.N ENGINEERING in banglore guided by Mr.PRADEEP BANSAL(Design engineer.).This paper explores the feasibility of a novel design of household clothes tumbler dryer, where the elec- trical elements of a standard tumbler clothes Dryer A (which heat the ambient air entering the drum) were replaced by a water-to-air finned tube heat exchanger (Dryer B). To achieve the objective of the study, experimental performances of the two dryers (e.g., Dryers A and B) were evaluated for the same operating conditions. During the experiments, Dryer B was found to have shorter drying times, lower moisture extraction rates for the same power input and hence significantly more efficient than Dryer A. This type of dryer may be adopted by the industry in the future for its relatively low cost, higher energy efficiency and environmental benefits
Introduction :
Improved energy efficiency in both the domestic and commer- cial environment is an engineering challenge for all industries worldwide. All around the world, large scale Combined Heat and Power Plants (CHP) are being built. These plants not only generate electricity but also simultaneously produce process heat (in terms of hot water) that could be distributed to local areas or large resi- dential apartments and buildings. The overall efficiency of these plants can be as high as 90%, due to the utilization of waste heat in the form of hot water from the plant. These power plants can be viable only if process heat is required continuously throughout the year. To make the optimum use of this hot water supply, many household appliances can be adapted accordingly, such as a house- hold clothes tumbler dryer, where the heating element to heat the air in the dryer can be replaced with a water-to-air heat exchanger. Electricity is high grade and high quality energy that results in a loss of its energy potential when used for low purpose work such as heating applications. In addition to significant environmental benefits, hot water would be much cheaper than electricity.
This paper evaluates the feasibility of a novel design of house- hold clothes tumbler dryer, where the electrical elements of a stan- dard tumbler clothes dryer (which heat the ambient air entering the drum) were replaced by a water-to-air finned tube heat exchanger using hot water as a medium. The performance compar- ison of this unit with the identical conventional tumbler dryer has been carried out. The 1.6 kW electrical heater that heats the
ambient air entering the drum of the standard Dryer A was re- placed by a water-to-air, wavy finned tube heat exchanger (Dryer B). Operating parameters that affect the performance of the adapted dryer such as mass flow and temperature of water flowing through the heat exchanger were investigated.
2. Literature review – drying process and performance
A literature search on the topic suggests that the dryer put for- ward is new and has not been previously researched. However the performance and efficiency of traditional tumbler dryers have been investigated both experimentally and by computer models [1–8].
Lambert et al. [1] developed a model of the drying process with- in a domestic household dryer based on the laws of conservation of mass and energy. Characteristics of the drying process were exam- ined and these included the moisture content of the clothes and the rate of evaporation over time. Bassily and Colver [2] examined the affect on drying times and energy consumption of a dryer when various operating parameters are varied. They found that by increasing the drum load (the weight of clothes), air flow decreases and the area for mass transfer of water increases. These changes in- crease the rate of evaporation. Therefore, the energy consumption per kg of clothes decreases with increasing load. The increase of heater power was found to increase the evaporation rate, hence decreasing drying time, while fan speed showed to have little influ- ence on evaporation rate. Conde [3], like Lambert et al. [1] devel- oped a simple model of the drying process, in which it was found that lowering the partial pressure of water in the drying air in- creases the speed of the drying process. The partial pressure of water in the drying air can be lowered by either decreasing the
:
Nomenclature
Cp specific heat of a fluid (kJ kg—1 °C—1)
m mass (kg)
m_ mass flow rate (kg s—1)
MER moisture extraction rate (kW h/kg)
t time (s)
T temperature (°C)
Q_ rate of heat transfer (kW)
Subscripts
in inlet
out outlet
w associated with the water
air humidity content or increasing the temperature. High temper- ature can degrade the quality of clothes; therefore priority falls on keeping the humidity content to a minimum. Deans [4] also devel- oped a simple model of a dryer, from which it was noted that the factors that influence the drying time are the mass of clothes and the power input into the dryer. Bansal et al. [5] simulated the per- formance of conventional air vented tumbler clothes dryers with two improved versions using heat recovery heat exchangers. They found that the open-cycle condensing dryer and the closed-cycle condensing dryer with heat recovery were significantly were respectively 14% and 7% more efficient than the air vented dryer. Braun et al. [6] found the performance of an air heat pump cycle for tumbler clothes dryers was up to 40% more energy efficient than the conventional electric dryer as against 14% projected improvement by open-cycle and closed dryers using heat recovery heat exchangers.
Cooper [7] analysed the performance of the Fisher and Paykel ED56 tumbler dryer and found that decreasing the power input of the dryer to 75% at latter stages of the drying cycle increased en- ergy consumption, however decreasing the power input to 50% showed marginal improvement. This is because decreasing power input increased drying time and hence had little effect on the mois- ture extraction rate. An increase of air mass flow was found not to increase energy efficiency. This is because the temperature that the air is entering the drum is lower, as the air is exposed to the heat- ing elements for a shorter time.
Williamson and Bansal [8] studied large commercial tumbler dryers in which a model was developed of a dryer incorporating heat recovery via an air to air heat exchanger. This showed an en- ergy saving of 8%. However, having a heat exchanger within the dryer system has some disadvantages. Firstly, lint may cause foul- ing of the heat exchanger, degrading its performance and secondly, a pressure drop across the heat exchanger increases fan input power.
3. Description of the dryers
Experimentation was carried out on two similar household tumbler dryers: a standard Dryer A (Fig. 1) and a modified Dryer B model. For both machines, their dry load capacities were 4.5 kg and efficiency star ratings were 1 out of 6. Both also incorporated reverse tumbling in which the drum and fan reverse their direction of rotation. This occurred every eight minutes for Dryer A and every six and a half minutes for Dryer B. Dryer B was modified by replacing its electrical heating element, located at the back, with a heat exchanger as shown in Fig. 2.
3.1. Drying process
3.1.1. Dryer A
A schematic of the drying process for the Dryer A is shown in Fig. 3. The inlet fan draws ambient air into the cabinet (1–2). This pressurises the cabinet (2–3) causing air to flow over the heating elements, which are placed at the back of the drum (3–4). Heated
Fig. 1. Dryer A: conventional household tumbler dryer.
Fig. 2. Back of the modified Dryer B – with a heat exchanger installed.
air then enters the drum and extracts moisture from wet clothes placed in the drum (4–5). The air flows from the drum, through internal ducting, and is then exhausted into the room through the rear of the cabinet (5–6).
3.1.2. Dryer B
The drying process of the modified Dryer B is the same as for Dryer A except that ambient air is heated by a heat exchanger be- tween 3 and 4 (see Fig. 4), gaining heat from a hot water supply.
3.2. Description of the Heat Exchanger
The hot water heat exchanger installed in Dryer B consists of copper tubing and aluminium wavy fins, as shown in Fig. 5. The hot water passes through the tubes while air flows over the tubes and through the fins perpendicular to the direction of water flow. These extended fin surfaces effectively improve the overall heat transfer performance of the heat exchanger [9].
4. Measure of performance
The performance of a dryer can be characterised in two main ways: moisture extraction rate (MER) and the drying time [7,8].
Fig. 5. Close up of the heat exchanger within the modified dryer.
dryer is better performing if it has a lower MER and a shorter dry- ing time.
MER is defined as the total power consumed multiplied by the dry- ing time in hours, divided by the mass of water evaporated from the clothes (Eq. (1)). The total power consumed by Dryer A was that expended by the heating elements and the motor which drives
MER ¼
Q_ x t
mWater Removed
rkW h1
kg
ð1Þ
both the fan and the drum. For the modified Dryer B, the power consumed was that expended by the electrical motor and the rate of heat supplied to the air from the water. Drying time is simply the time taken to complete sufficient drying of the clothes. The
5. Experimental approach
A matrix outlining the conducted experiments is shown in Table
1. This shows that two runs were performed on Dryer A under
Fig. 3. Drying process of Dryer A (a) 2D schematic. (b) 3D schematic – adapted from [4].
Fig. 4. Drying process of the modified Dryer B.
Table 1
Details of experimental plan.
the power consumed by the heating elements from the power con- sumed by the dryer
Type of dryer
Drying variable Test number
Test description
Q_ water-to-air ¼ m_ w Cp ðT w;in — T w;out Þ ð3Þ
Dryer A Control test 1 Performed under standard running conditions
2 Performed under standard running conditions
Power to heaters 3 Power input of 1.92 kW
4 Power input of 1.59 kW
5 Power input of 1.43 kW
For Dryer B, a power transducer was used to measure the power drawn by the electrical motor. The rate of heat transfer from the water to the air in the heat exchanger was calculated from the measurements of temperatures taken from thermocouples on the tubes of the heat exchanger and the mass flow of water. The water mass flow can be given by
Dryer B Inlet water temperature
Water mass flow rate
6 890 °C inlet water temp, water flow rate 0.1 kg/s
7 880 °C inlet water temp, water flow rate 0.1 kg/s
8 870 °C inlet water temp, water flow rate 0.1 kg/s
9 880 °C inlet water temp, water flow rate 0.18 kg/s
10 880 °C inlet water temp, water flow rate 0.07 kg/s
Q_ water-to-air ¼ m_ w Cp ðT w;in — T w;out Þ ð3Þ
The total power input for the modified dryer is therefore de- scribed by
Q_ total ¼ Q_ water-to-air þ ðQ_ fan þ Q_ drum Þ ð4Þ
A Sartorius platform scale having a resolution of 5 g was used to measure the mass of the clothes during a drying test cycle without the removal of clothes from the drum. The pressure was been mea- sured in two locations of the dryer: the drum and the cabinet. Two
normal running conditions (as it would run in a household envi- ronment). The test parameters included ambient air temperature of 20–22 °C, inlet air humidity of 55–65% and a power input of
1.73–1.75 kW. These tests (1 and 2 in Table 1) were referred to as the control tests and were repeated to insure consistency in data. Following this, further tests were conducted with different power inputs whilst keeping the other parameters fixed. The MER and drying time values gained from the experiments on Dryer A formed the baseline data for comparison with Dryer B.
6. Experimental setup
6.1. Testing method
The two dryers were tested as per the AS/NZS 2442.1-1996 standard [10]. This outlines how tests should be carried out on a dryer to determine its performance. A standard 4.5 kg load consist- ing of cotton of specific size and colour was used for every test. The bone dry mass of this load is that of the material when it has no moisture. The standard specifies that the test load should weigh
190% of its bone dry mass value.
6.2. Sensors
Twelve thermocouples were employed in Dryer A to measure temperatures at different locations, including inlet and exit air, the heating element temperatures and the air temperature at the back of the drum. The thermocouples were ‘radiation’ shielded to improve their accuracy. For Dryer B, a further 4 thermocouples were used to measure the surface temperatures of the tubes of the heat exchanger. These were used to estimate the temperature of water flowing through the tubes. Thermo-paste was placed be- tween the tube and thermocouples. The thermocouples were se- cured with tape and wire and then insulated, ensuring no air gap between the couples and pipe and hence no temperature gradient between them. The accuracy of the thermocouples was found to be
±0.08 °C.
Two Humirel HM 1500LF humidity transducers, having an accu- racy of 0.1% RH were employed to measure relative humidity (RH) at the inlet and outlet of the dryer. The power input to the dryers was measured with two Carrel Precision transducers. One to mea- sure the power input for the heating elements while the other was for the whole appliance. The consumption of power by the electri- cal motor that runs the drum and fan was calculated by subtracting
pressure tappings were installed, one at the drum door and the other at the rear of the cabinet. Air hoses connected these to two manometers, with a resolution of ±10 Pa. Air volume flow was measured using the ‘bag method’ [11]. This involved the construc- tion of a large plastic bag of known volume and minimal resistance. The bag was then filled with exhaust air over a measured time (Fig. 6), enabling the calculation of the volume flow. Water, at con- stant temperature and flow, was supplied to the heat exchanger in Dryer B.
7. Experimental results and discussion
7.1. Analysis of the drying process – Dryer A
Variations in load mass and evaporation rate for Dryer A over time is shown in Fig. 7. The evaporation rate is zero at time zero, as the clothes have just been put into the dryer. The evaporation rate is also zero at the end of the cycle, since the dryer is turned off at this point. The drying process can be divided into three suc- cessive phases: transient, constant and falling. The profiles of mass and evaporation rate were consistent between the two tests.
Corresponding exit humidity and exit temperatures are shown in Fig. 8. In the constant rate phase both relative humidity and temperature were 30 °C and 85% RH, respectively. In the falling rate phase, air temperature increases and relative humidity de- creases due to reducing moisture in the clothes at this stage.
Fig. 6. Plastic bag used to measure air flow rate.
Fig. 7. Clothes mass (in drum) and evaporation rate vs. time in control tests for Dryer A (see Table 1).
Fig. 8. Relative humidity and temperature of exhaust air vs. time in control tests for Dryer A (see Table 1).
The power input to the heating elements and the total power input to the dryer is shown in Fig. 9. Power input is constant throughout, except for vertical lines every 8 min when the drum’s rotation was reversed.
7.2. Performance of Dryer A with different ratings
The MER and drying time for different power inputs for Dryer A are shown in Fig. 10. As expected the drying time decreases with the increasing power input to the system. This is because the aver- age temperatures entering the drum are higher, increasing the driving force for evaporation (Fig. 11). The exception to the general trends for the drying time and MER was the test with the total power input of 1.59 kW. The MER rate was 1.14 kW h/kg compared
to the average MER of the control tests of 1.17 kW h/kg. This equates to 2.6% increase in efficiency.
7.3. Analysis of the drying process – modified Dryer B
Thermodynamic analysis of modified Dryer B is similar to Dryer A. Exhaust humidity, temperature, mass and evaporation rate over time are the same for both dryers except the power input into the dryer here is due to hot water, leading to different air temperature entering the drum.
7.3.1. Effect of heating water inlet temperature
The MER values and drying times for tests done on modified dryer, for a range of inlet water temperatures, are given in Fig. 12.
2.0
1.8
1.6
1.4
1.2
1.0
0.80.6
0.4
0.2
| |
Heating Elements
0.0
0 20 40 60 80 100 120 140
Time (min)
Fig. 9. Total power to dryer and power input to heating element vs. time (see Eq. (2)).
Fig. 10. MER and drying time for different total power inputs for Dryer A.
Fig. 11. Average air temperature entering the drum with different power input vs. time for Dryer A.
Fig. 12. MER and drying time for different water inlet temperatures for Dryer B.
| |
Fig. 13. Rate of heat transfer vs. water inlet temperatures for Dryer B.
Fig. 15. Rate of heat transfer vs. water mass flow rates for Dryer B.
It is shown that increasing the inlet water temperature decreases the drying time. Increasing the water temperature from 66 °C to 86 °C yielded a decrease of drying time of 41 min (from 170 to 129 min). At an inlet water temperature of 66 °C the clothes took 171 min
to dry, however the MER for this test was lower, 1.017 kW h/kg, compared to other tests. This was 3.7% more efficient compared to the MER from 79 °C inlet water temperature. This test yielded the highest MER of 1.06 kW h/kg (as can be seen in Fig. 13).
Fig. 14. MER and drying time for different water mass flow rates for Dryer B.
Fig. 16. MER and drying time for different power inputs for both the dryers.
7.3.2. Effect of mass flow rate of water
The MER values and drying times for tests with different flows are given in Fig. 14. Water mass flow does not seem to affect the drying time as much as changing the inlet water temperature. Increasing the mass flow from 0.066 kg/s to 0.18 kg/s decreases the drying time by 7 min (from 145 to 138 min). This is because changing the mass flow of water does not change the rate of heat transfer appreciably (as can be seen in Fig. 15). However the MER values are within 1.3% of each other.
7.3.3.Other considerations
A close evaluation of the HX surface after a few experiments indicates some signs of lint formation, which may need attention before commercialization of the product.
7.4. Performance comparison of the two dryers
The MER values and the drying times for different power inputs for all tests are plotted in Fig. 14. The modified Dryer B dries the load 15–18 min quicker than Dryer A for the same power input, suggest- ing the modified dryer is more efficient. For the same power input, the modified dryer has MER approximately 0.1–0.12 kW h/kg lower than Dryer A- that is 75% lower MER. Indeed this modification of dryer, to use hot water, is not only feasible but is also more efficient than the existing dryer with electrical elements. A literature search has shown that it is currently the only household appliance success- fully running on hot water.
Possible reasons for Dryer B being more efficient than Dryer A is that Dryer A suffers from loss of heat, where heating elements radi- ate heat out of the cabinet, which is then conducted through back plate and out into the surroundings. It is probably less than the heat radiated by copper tubes containing hot water. This view is supported by the fact that the temperatures entering the drum of Dryer B were higher for same power input. Higher temperatures increase evaporation rate, decrease drying time and increase effi- ciency (as can be seen in Fig. 16).
Another reason why the modified dryer is more efficient is be- cause its air mass flow is slightly higher: 0.032 kg/s compared to
0.030 kg/s. This means that the air residence time within the drum is lower resulting in higher driving potential for evaporation. This is however contradictory to the findings of Cooper [7] who found that increasing flow rate by 5% would have little effect on dryer efficiency.
8. Conclusions
In this study, a new design of a household clothes tumbler dryer was successfully modified by replacing its electrical elements with a heat exchanger to utilise hot water to dry clothes. The modified Dryer B for the same total power input had drying times 15–
18 min shorter MER value (i.e., 0.1–0.12kW h/kg lower) meaning that it is significantly more efficient (i.e., about 11%) than the con- ventional dryer. Increasing the inlet water temperature to the heat exchanger of the modified Dryer B decreased the drying time sig- nificantly but increased the MER value and hence less efficient.
Decreasing the power input into the heaters of Dryer A, in- creases the drying time as the temperatures entering the drum are lower, leading to only subtle effects on the MER value as any decrease in power input is usually offset by the increase in drying time. However the drying run on Dryer A with total power input of
1.59 kW is 2.6% more efficient than the dryer under standard oper- ating conditions (average of 1.74 kW total power). A power input of a lower of higher value than 1.59 kW into the dryer yielded a slightly higher MER value. It is recommended that further research and development should be undertaken to assess if lint fouling on the fins of the heat exchanger may have any long term perfor- mance issues.
Referens
[1] A.J.D. Lambert, F.P.M. Spruit, J. Claus, Modelling as a tool for evaluating the effects of energy-saving measures. Case study: a tumbler drier, Applied Energy
38 (1991) 33–47.
[2] A.M. Bassily, G.M. Colver, Performance analysis of an electric clothes dryer, Drying Technology 21 (3) (2003) 500–524.
[3] M.R. Conde, Energy conservation with tumbler drying in laundries, Applied
Thermal Engineering 17 (12) (1997) 1163–1172.
[4] N. Williamson, P.K. Bansal, An experimental investigation on the energy efficiency of gas commercial laundry dryers
Engineering 218 (2004) 143–152.
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