Faculty of Engineering - Energy saving at home: Tumble dryer

FACULTY OF ENGINEERING

Practical Guidelines on how to save energy at home

Tumble dryers

The discussion of tumble dryers starts with a literature study which explains the operation of tumble dryers and more.  After the literature study there is a consumer profile that provides home owners with the most critical information about a few tumble dryers with EU ratings.  The third and second to last subsections provides the results of the practically measured tumble dryers and the financial model respectively.  Last but not least, to summarize all the information and the results, the reader is provided with a conclusion.

 

Literature study

The literature study has three subsections, they are the structure of tumble dryers, how tumble dryers work and the EU labels of the three different types of tumble dryers.  This project focusses on two of the three tumble dryers, they are the air vented and condenser tumble dryers seeing as both of them consumes electrical energy to perform the necessary work.

 

Structure

The basic components of tumble dryers are described in this section of the report.  Currently there are three main types of tumble dryers, they are air-vented, condenser and gas-fired tumble dryers.  The differences between these tumble dryers are as follows:

  • Air-vented tumble dryer – In a vented tumble dryer the hot, damp air is extracted out of the drum and pumped through a vent to the air outside the tumble dryer.  A vented kit can also be installed so that the hot, damp air can be pumped to a location away from the tumble dryer.
  • Condenser tumble dryer – In a condenser tumble dryer the hot, damp air is condensed into water which is collected in a tank inside the tumble dryer.  The condenser tumble dryer is equipped with a light that indicates when the tank is full and needs to be emptied.
  • Gas-fired tumble dryer – The gas-fired tumble dryer uses gas to heat the element instead of electrical energy.

 

The figure shows the basic components of an air vented tumble dryer.  The discussion of air vented, condenser and gas fired tumble dryers will be centered on this figure, seeing that there are only small differences between the three different tumble dryers.

 

The main components of a tumble dryer is as follows:

  • Power supply – The power supply supplies the electrical energy that is needed to power the controls, monitor, motor, blower and the element for air-vented and condenser tumble dryers.  For gas-fired tumble dryers the element is heated by means of gas, thus the tumble dryer should also have a gas supply.
  • Controls and monitors – The controls are used to set the specific drying cycle etc. and the monitors serve as an interface.  Normally tumble dryers are also equipped with a timer so that the drying time can be set manually.
  • Blower – The blower is used to suck in cool, dry air and blow out the hot, damp air.  The blower controls the entire air flow process in the tumble dryer, but the blower is actually the last step in the process.
  • Drum – The wet laundry is situated in the drum where it comes into direct contact with hot, dry air while the drum is spun around, this process dries the laundry.
  • Drum support rollers – The drum support rollers keeps the drum in its place as the laundry tumbles inside it.
  • Motor, idler pulley, spring and drum belt – The motor is used to drive the drum belt which drives the drum.  The drum belt is held in place with a set of idler pulleys.
  • Heating duct, element and thermostat – The heating duct guides the cool, dry air that is sucked into the tumble dryer to the element, where the air is heated.  The thermostats are used to control the heating element to ensure that the temperature stays in the correct range.
  • Lint trap, exhaust duct and vent – The lint trap removes dust and bits of fluff from the hot, damp air that leaves the drum, the exhaust duct guides this air to the blower which blows the air out of the tumble dryer through the vent.
  • Door gasket and catch – The door gasket and catch ensures that the drum is air tight when the tumble dryer is in operation.

 

How tumble dryers work

The tumble dryer goes through the following processes to dry laundry:

  • The blower sucks in cold air through an air intake; which is usually situated at the front of the tumble dryer to stop it from getting dusty; this air is then sucked up to the heating element through the heating duct.  As the cool air passes over the heating element, it is warmed and turned into hot, dry air.
  • The hot, dry air enters the drum through small holes at the back of the drum and rises to the top of the drum.  The drum is rotated by the drum belt that is driven by the motor, the motor also drives the blower.  As the drum rotates the paddles lifts the laundry to the top of the drum, when the laundry reaches the top of the drum, gravity pulls down the laundry and it falls through the hot, dry air that absorbs the moisture in the laundry.
  • The hot, damp air is sucked out of the drum by the blower, this air is then passed through the lint filter where dust and bits of fluff is removed from the air, and the clean air is then guided through the exhaust duct to the blower which blows the air out through the vent.  In the model as shown in the figure above, the air is sucked out at the back, but in some models the air passes through the drum and leaves the drum through a vent that is situated in the door.

 

EU energy efficiency label

The EU energy efficiency labels for air-vented, condenser and gas-fired tumble dryers are shown in the figure below.  The relative rating of the tumble dryers varies from “A+++” to “D”.  The other important fields that are included are the weighted annual electrical energy consumption for 160 drying cycles, the cycle time of the standard cotton programme at full load in minutes, the capacity in kilogrammes for the standard cotton programme at full load, the noise emissions in decibels and there is also a pictogram to indicate the type of tumble dryer.

 

The energy efficiency class is based on the energy efficiency index.  It takes the weighted annual energy consumption for the standard cotton programme at full and partial load, the standard annual energy consumption and the duration and power used in the left-on mode and off-mode into account.

 

Consumer profile

The tumble dryer consumer profile is shown in the table below.  The first column is the criteria column, the second column is the unit’s column and columns three and four are the tumble dryer columns.

Tumble Dryer Consumer Profile
Criteria Unit Tumble dryers
Energy efficiency class   B B
Weighted annual electrical energy consumption kWh 251 262
Type of tumble dryer   Condenser dryer Condenser dryer
Cycle time of standard cotton program at full load min 90 120
Capacity for standard cotton program at full load kg 7 8
Noise emission dB 66 63
       
Cost of electrical energy per unit R 1.51 1.51
Weighted annual cost of electrical energy R 379.01 395.62
Cost of tumble dryer R 5999 6399
Typical lifecycle y 14 14
Total lifecycle cost R 11305.14 11937.68

 

Tumble dryers aren’t very efficient, thus there aren’t many tumble dryers with ratings higher than “A” or “B”, although the rating system goes all the way to “A+++”.  The tumble dryers that have been included in the tumble dryer consumer profile has energy efficiency class ratings of “B” and both are condenser dryers, seeing that they are more common than air vented dryers.  As can be seen from the tumble dryer consumer profile, the cost of electrical energy over the total lifecycle of the tumble dryers is less than the cost of the tumble dryers.  This is not uncommon amongst EU rated products, seeing that the manufacturers are allowed to increase their product prices if they meet the EU energy efficiency standards.

 

Practical measurements

The practically measured tumble dryer is a fairly new model, but it does not have an EU rating.  It has a capacity of 8.2 kg and a power rating of 3.68 kW.  The tumble dryer has a dedicated 230 V 16 A earthed wall socket, thus the measurements were taken at the distribution board.  For the practical measurements the tumble dryer was filled to its maximum capacity and set to an automatic drying cycle, which shuts of the tumble dryer when the clothes are dry.  The measurements were taken over a full month.  The practically measured values are shown in in the table below.

Tumble Dryer Practical Measurements
Criteria Unit Air vented tumble dryer
Total electrical energy consumption kWh 78.2
Number of cycles N 23
Electrical energy consumption per cycle kWh 3.4
Average cycle time h 1.25
Annual electrical energy consumption (160 Cycles) kWh 544

 

As can be seen from the practically measured results, the tumble dryer was used a total of 23 times in the month that the measurements were taken and the total electrical energy consumed in that month was 78.2 kWh, giving an average of 3.4 kWh per drying cycle.  The annual electrical energy consumption is calculated to be 544 kWh.  The meter that was used is not able to measure the left-on-mode and left-of-mode power consumptions, thus the annual electrical energy consumption should rise slightly if it is taken into account.

 

Financial model

The tumble dryer financial model is shown in the table below.  The first column is the criteria column, the second column is the unit’s column and column three is the calculations column.  By replacing the practically measured tumble dryer with the 8 kg, “B” rated tumble dryer shown in the consumer profile, the household; in which the practically measured tumble dryer is used; can save R425.82 per year.  Although the running costs will be cut in half if the old tumble dryer is replaced, the payback period is still longer than 14 years, which is the average useful life of a tumble dryer, thus it would not be a wise investment.

Tumble Dryer Financial Model
Criteria Unit Calculations
Annual electrical energy consumption (Old tumble dryer) kWh 544
Cost of electrical energy per unit R 1.51
Cost of annual electrical energy consumption R 821.44
     
Weighted annual electrical energy consumption (New tumble dryer) kWh 262
Cost of weighted annual electrical energy consumption R 395.62
Cost of new tumble dryer R 6399
     
Savings per year R 425.82
Payback period y 15.03

 

Conclusion

Even though the new EU rated tumble dryers consume less than half of the electrical energy than their older counterparts, they still aren’t very efficient.  For this reason it is recommended that households dry their laundry on laundry racks and laundry lines, which saves large quantities of electrical energy and large amounts of money while saving the planet.  Also, the lifecycle of the laundry can be increased if it is not dried in a tumble dryer.

 

By replacing older model tumble dryers with “B” or higher rated tumble dryers, the electrical energy usage of tumble dryers in homes can be reduced to 0.5%, saving 0.5% on tumble dryers alone.  Even more savings can be achieved if tumble dryers aren’t used at all, this can be done by hanging laundry on a laundry rack or line.

 

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To receive additional supporting information or to share your own electrical energy saving tips contact: Karlé le Roux