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The Design of a Vacuum Dryer

December 11999

 

Abstract

This report has been written to evaluate the feasibility of a residential vacuum dryer.  Research obtained on conventional residential dryers and industrial vacuum dryers was used to compile the specifications of a residential vacuum dryer.  The basic energy flows required by a vacuum dryer, including work and heat transfer was calculated.  The results obtained from the calculations proved that the residential vacuum dryer was not feasible under the outlined constraints.

1.0 Introduction

This report is created on the request made by the Innovation Centre of Waterloo Ontario[1]. The objective of this report is to evaluate the feasibility of a residential vacuum dryer. The vacuum has existed in the market for many years but they have been designed for commercially and industrial use. Commercial and industrial vacuum dryers are large, costly and consume a lot of energy [2]. However, the advantage of using an industrial vacuum dryer is that it provides rapid and dust-free drying using minimal amounts of heat [2]. These beneficial characteristics are desired in a residential dryer.
 
The vacuum dryer works on the principle that water travels from an area of high vapour pressure to an area of low vapour pressure. Using this principle, a vacuum dryer will be able to dry clothes by decreasing the vapour pressure of water at a low temperature, and then, withdraw all of the moisture content, embedded in the clothing, with the suction of air [2].

This report is based on theoretical research, and no prototyping, detailed calculations, and detailed drawings were done.

1.1 Approach to Design

There are many factors to consider when devising a possible design for a vacuum dryer.  These factors include the time taken to dry a specific load, the energy consumption, the noise produced, the cost to produce, the life expectancy, and the maintainability of the vacuum dryer.

The first design possibility was an “oven rack” idea.  In this scenario the laundry load would be place on racks. This design eliminates the need to have a drum that spins.  The simplicity of this design is one of its major advantages.  However this design was decide against because of the need to lay out the clothes on the racks.

 The second possible design was of a dryer with a spherical drum.  The spherical drum was considered because it maximized the load capacity.  However, the idea was considered unfeasible because the cost associated with building the spherical drum would be high when compared with that of a cylindrical drum.

The design that was decided to be implemented was of a vacuum dryer which had a system of drums.  The system of drums contained an inner drum enclosed by an outer drum.  The outer drum sealed the vacuum while the inner drum held the laundry load.  The inner drum was perforated to prevent the clothes from being clogged, and to evenly distribute the vacuum.  In this design scenario the inner drum spun while the outer drum remained stationary.  This design was decide to be the most practical design.

Freehand sketches and AutoCAD drawings were drawn to determine the looks and the placement of the internal components of the design that was decided to be implemented.

 

2.1 Current Status of Dryers

To facilitate the creation of the vacuum dryer it is necessary to study the current status of conventional household dryers.    A feature common to most household dryers is a drum that tumbles the clothes.  The tumbling of clothes speeds up the drying process since it exposes more clothing surface to the warm air.  Most conventional dryers have similar dimension.  A good representation of the average size of a dryer is one of dimensions of 69 cm wide by 71 cm long by 109 cm high [3].   Maintaining similar dimensions for the vacuum dryer will ensure easy installation.  In conventional dryers the dimensions for the humid air exhaust outlet 10 cm, and the tumbling drum has a volume of 190 cubic cm [3].  Maintain these dimensions in the vacuum dryer will make it more compatible.   The price of conventional dryers ranges between $750 to $1000[4].  Maintaining a similar price for vacuum dyers will ensure it acceptability in the consumer market.   Conventional dryers, in general take about 45 minutes to dry a load of six towels weighing 2.3-kg [3].  Maintaining a similar time limit in the design of the vacuum dryer is a crucial for its success.

2.2 Theory

 

A heat driven dryer uses a lot of energy to increases the partial pressure of water by forcing the water into the air from the clothing.  This statement is described by the van’t Hoff equation [5]

ln =

Where

T1- is the room temperature in Kelvin

T2- is the operating temperature in Kelvin

P1- is the partial pressure of water in kilopascals at T1

P2- is the partial pressure of water in kilopascals at T1

R- is the universal gas constant

DH- is 40.66 kJ·mol-1 for water

 

As water is evaporated from clothes the humidity or the water vapour in the dryer builds up.  To prevent the negative effect of this build up the humid air is pumped out.  In a heat-driven dryer this pumping rate is approximately 6.2±0.1 m3·min-1 (1). The air that is pumped out is replace by the air in the cabinet through the perforations in the drum. 

 

Convection is a process by which heat is transferred by the movement of a heated fluid such as air.  In conventional dryers the surrounding air is heated by convection [5].

 

The work done to heat he water is the energy transferred into the water, given by the following equation

W = DE = VIt

Where

W - is the work done in joules

V - is the potentional difference in volts

I -  is the current in amperes

t - is the elapsed time is seconds

 

3.0 Experimental Data

 

Table 1 : Experimental Data

 

Light Load

Medium Load

Heavy Load

Volume of Dry Clothes (m3)

2.83 x 10-2

7.08 x 10-2

1.13 x 10-1

Mass of Dry Clothes (kg)

1.6928

4.2320

6.7712

Mass of Wet Clothes (kg)

5.622

14.055

22.488

Volume of Water (m3)

3.9292 x 10-3

9.8230 x 10-3

1.5717 x 10-2

Mass of Water (kg)

3.9292

9.8230

1.5717 x 10-1

 

3.1 Experimental Techniques

This experiment was conducted to determine the load requirements for a vacuum dryer.  The data from this experiment will be used to determine the volume of water absorbed by clothing in a light, medium, and heavy load.

 

1.)    Three pieces of dry clothing were selected.  Two times the volume of the clothing constitutes a light load, five times a medium load, and eight times a heavy load.

2.)    Using an electronic scale the dry clothes were weighed.

3.)    The clothes were soaked in water.

4.)    The clothes were wrung out, to represent the wetness of clothes coming out of a spin cycle in a conventional washer.

5.)    A 4.5-gallon container was weighed.

6.)    The wet clothes were placed in the container.

7.)    The container and the wet clothes were weighed together.

8.)    The wet clothes were placed flat into a graduated container.

9.)    The volume of the wet clothes was recorded from the graduated container.

10.)            The mass of the water absorbed by the clothing was calculated, using the following equation:

Dm = M – m

 

Where, Dm is the mass of water absorbed, M is the mass of the wet clothes, and m is the mass of the dry clothes, all measured in kilograms.

11.)            The volume of the water absorbed by the clothes was calculated using the equation below:

Vwater = Dm / r

Where Dm is the mass of the water absorbed in kilograms, and r is the density of the water in kilograms per cubic metre (assumed to be 1000kg/m3).

12.)      The data was recorded in tabular format.

 

3.2 Analysis of Experimental Data

 

Data from the experiment was collected and compiled into graphs of the number of groups versus the light, medium, and heavy loads.  Data beyond the 24-kg mark in table 1 was considered extreme and was eliminated.  This data was excluded from the overall analysis because it was not consistent with the groping of the other data, and was therefore deemed unscientific.

 

Figure 1 shows the number of groups versus the mass of water absorbed in a light laundry load.  The spread of the data in figure 1 is an important trend.  The spread shows that most of the group considered a light load to be between one to four kilograms. This trend suggests that every group used similar experiment techniques to since most of the data is concentrated in one area.

 

Figure 2 shows the number of groups versus the mass of water absorbed in kilograms in a medium load.  In this graph the data is more spread than figure 1, but it has two peaks.  The peak in the graph shows that many groups used a common factor to obtain the mass of water.  The greater spread in this graph is due to the fact that a variety of factors was used and applied to a rage of masses from figure 1.  The irregularities of certain data points in figure 2 occur because certain groups selected the mass of the medium load to be their starting basis.

 

Figure 3 shows the number of groups versus the mass of water absorbed in kilograms in a heavy load.   The major trend observed on this graph is the distribution of the data.  The data in this graph is spread across the entire horizontal axis, with two areas where there is a peak in the data.  The peaks suggest that groups used a common factor to obtain the data for the heavy load.  The sperad in this graph is more distributed because a variety of factors was applied to a wide rage of masses from figure 2.

 

Figure 4 shows a line graph of the mass of water versus the type of load Analysis of this graph reveals an interesting trend.  As the load size increases from light, to medium, to heavy the spread between the points gradually increases.  From this observation it is possible to conclude that predicating smaller quantities is simpler than predicting larger quantities, since there is increasing error in the data as the load size increases.

 

4.0 Vacuum Dryer Thermodynamics

 

Vacuum dryers work on the premise of lowering the atmospheric pressure of the dryer, thereby lowering the boiling point of the water.  To achieve a vacuum, work is done by the pump to expand the volume of the dryer.  The action of the pump simulates the drawing of an air-tight seal along a tube of infinite length.  Work done by the atmosphere on the pump-seal is

Watm = Po(V2-V1)

 

where Po is the atmospheric pressure of 101.325 kPa, V2 is the final volume of the dryer (here taken to be 425.66 m3), and V1 is the initial volume of the dryer [6].  V1 » 0.17±0.03 m3 » 6.0±0.1 ft3 (1).  Work done by the dryer air on the pump is given by [6]

 

Wvac = PoV1·ln

 

Therefore total work to be done by the pump to evacuate the air from the chamber is given by [6]

Wtotal = Watm – Wvac

 = Po(V2-V1) - PoV1·ln

Similarly, the work done by the pump to evacuate the water must be calculated.  Work done by the pump on the water is given by [6]

 

W = (Po - P2)(V3 – V1)

 

where W is work in joules, Po is the atmospheric pressure in pascals, P2 is the operating pressure of the vacuum dryer in pascals (chosen to be 1228 Pa), V3 is the volume occupied by the water vapour at 15°C (77.93 m3/kg), and V1 is the volume of the dryer in m3.

So, total work done by the pump is [6]

 

Wtotal = Po(V2 - V1) - PoV1·ln+ (Po - P2)(V3 – V1)

Heat transfer during air removal is given by [6]

Q = PoV1·ln

where Q is the heat transferred in joules, Po is atmospheric pressure in pascals, V1 is the initial volume of the dryer in m3, and V2 is the simulated final volume of the dryer after pumping [6].

Heat transfer during water removal is given by [6]

 

Q = MH20(ug – uf) + P2(V3 – V1)

 

where Q is the heat transferred in kJ, M is the mass of the water in kg, ug is the energy per mass of gaseous water (in kJ/kg), and uf is the energy per mass of liquid water (in kJ/kg).  P2 is the operating pressure of the dryer (in Pa), V3 is the potential volume of the water at the given operating temperature (in m3), and V1 is the initial volume of the dryer (in m3).

 

4.1 Calculations

 

Work done by the pump for one light load:      

 

   

 

Heat transferred to the dryer for one light load: 

 

 

 

Amount of energy consumed by a heat driven dryer in one year is (1)

934 kWh/a

where

1 kWh = 3.6 x 103 kJ

 

Thus the energy required is 3.36 x 106 kJ for one year of operation.

 

Then the energy required for one year of operation of a heat-driven dryer is 11 times greater than the energy required for one small load of laundry in a vacuum dryer.

 

Assuming 500 small loads of laundry per year, for an average 4 person family, results in the following amount of energy consumed by a vacuum dryer:

1.10 x 108 kJ per annum.

Therefore, energy consumed by vacuum dryer is 33 times greater than the energy consumed by a heat-driven dryer.

 

4.2 Error Analysis

Calculations contained error because of various assumptions made.  The first assumption that causes error was assuming that no water evaporates during the vacuum creation.  Another assumption made was that air behaves as an Ideal gas.  Moreover, work transferred to the dryer only comes from the pump when in fact other sources are present.  These errors were unavoidable in the report and therefore caused a  ± 10% error in the calculations.

5.0 Preliminary Design

The methodology of arriving at the preliminary design was to observe the current status of dryers.  The criteria was arrived at by noting the many disadvantages of conventional dryers.  The constraints, such as the dimensions and the time taken to dry clothes, were arrived at by noting the standard created by conventional dryers.

5.1 Design Criteria

The design of the vacuum dryer will have many characteristics that conventional dryers lack.  The vacuum dryer will be designed to minimize the floor space it occupies and maximize its load capacity.  The vacuum dryer will be designed to take less time in drying clothes than conventional dryers do.  The design will incorporate a layer of insulation to minimize the loss of thermal energy and minimize the noise levels. The dyer will be energy efficient to reduce the overall cost per load. Conventional dryers have an operational life of around 20 to 25 years[4], and require minimal maintenance, thus the vacuum dryer must also be durable.  To ensure durability and minimal maintenance the dryer would have very few moving parts and must be made of durable metal such as steel or aluminium. 

5.2 Design Constraints

In order for the vacuum dryer to be practical it has to at least match or outperform the conventional dryers.  First the vacuum dryer should not be significantly higher priced than conventional dryers. Conventional dyers retails for about $750 to $1000 therefore vacuum dyers should not retail significantly higher than this price [4].  A large dryer occupies no more than 6400 cm2 of floor space, therefore the vacuum dryer should not exceed this limit[3].  The electrical supply entering a house is 220 volts therefore a vacuum dryer should not consume anymore than this limit [3]. The time factor is one of the most important design constraints. The vacuum dyer will be completely unfeasible if it takes significantly longer to dry clothes than conventional dryers. A load of 6 bath towels (weighing 5 lbs.) will dry in a conventional dyer in about 40-50 minuets, therefore the length of time needed to dry the same load in a vacuum dryer should not exceed that limit[3].

5.3 Preliminary Design

 

The methodology that the design team used to arrive at the preliminary design was done in three steps. First many freehand sketches of the vacuum dryer were done.  These drawings were of many different views.  These views helped the other team members get an idea of the  overall design and dimension.  It also helped them get an idea of the placement and the orientations of the various internal components.  The next step was to analyse the freehand sketches and create an AutoCAD drawing of the vacuum dryer.  The AutoCAD drawing allowed the team to use exact dimensions to create the overall appearance.  It also helped the team decide on the placement of the internal components in order to minimize space and complexity.   The AutoCAD drawing allows different views of the dryer  to be seen, without redrawing each view.

  

The dimension of the vacuum dyer is sixt- nine centimetres wide, seventy-two centimetres long, and 110 cm tall.  This dimensions represents the average dimensions of conventional dryers.  Maintaining these dimensions are important in order for the vacuum dryer to fit conveniently in a household.  The vacuum dryer consists of a system of drums which includes an internal drum and an external drum.  The internal drum is perforated to allow the vacuum to be evenly distributed around the clothes.   This perforation prevents the clothes from being clogged by the suction of the air.  In the overall design the internal drum is the only component that spins.  Spinning the drum is important because it exposes more clothing surface to the vacuum, which accelerates the drying time.  The external drum encloses the inner drum by a few centimetres.  The internal drum stays afloat by using ball bearings and tracks.  The purpose of the external drums is to contain the vacuum. The placement of the motor is very important in the design of the vacuum dryer.  Placing the motor outside the outer drum will cause the vacuum to leak at the location where the axle meets the outer drum.  To counter this effect the motor casement is part of the outer drum.  In this design the vacuum encloses the motor and no leak of the vacuum is possible.  To make the inner drum to spin a belt is attached from the motor to the inner drum. The vacuum pump is attached to the back of the outer drum so the gases can be taken out of the dryer to lower the pressure. To counter the effect of the decrease in temperature when the pressure is being lowered, a heating element is added to the outer drum.  The inner and the outer drums are made out of stainless steel, because of its strength.  The vacuum dryer has a door that has a locking mechanism that ensure that the door remains shut whenever the vacuum dryer is in operation.

 

6.0 Conclusions

 

This report concludes that a residential vacuum dryer is not feasible.  This conclusion was made by calculating the energy requirements of a vacuum dryer and comparing it to the energy requirements of the heat driven dryer.  The numerical calculations show that the vacuum dryer requires eleven times as much energy than a heat driven dryer.  Therefore this design is not feasible.

 

7.0 Recommendations

 

It is recommended that this report be further analyzed and research by individuals who have a greater understanding of thermodynamics so that the vacuum pump can be made more efficient.

 

8.0 References

 

1) Fraser, Roydon, Design a Vacuum Dryer Memorandum, October 99.

2) Atomic Vacuum Co, “Vacuum Dryer”, http://www.bicserve.com/htm/atomic/ind.htm, Date accessed Nov 25 1999

3) Maytag, “Dryers”, http://www.maytag.com/maytag/appliances/dryers.asp, Date accessed November 29 1999

4) K:\week11, “Changes to report- vacuum dryer”, accessed November 29 1999

5)      Oxtoby, David W, University of Chicago, “Principles of Modern Chemistry”, Saunders College Publishing, Toronto. 1986

6)      Prof R.A. Fraser, “ME100”, University of Waterloo, November 10, 1998