Engr 103,Section 088, Group 01

ENGR 103 - Spring 2013

Freshman Engineering Design Lab

Heat pipe and Its Application in Cooking Project Design Proposal

Date Submitted: May 17, 2013

Submitted to: John Speidel, [email protected]

Group Members: Maria Tabbut, [email protected]

Luis Castro, [email protected]

David Williams, [email protected]

Frank Kivuyo, [email protected]

Abstract:

A heat pipe is a device designed to transfer heat from hot sources to cold sources or from

cold sources to hot sources. The main goal is to implement heat pipe application in a cooking pot

in order to increase its efficiency by saving time and energy. During the design of the cooking

pot it will be necessary to know the basic thermodynamics processes as well as the performance

of the material that will be used in the project. The exterior hollow pipe will be made of copper,

and the inner wick will be made of copper-mesh. This will be the fundamental design prototype.

During the time of the design, the cooking pot will be built and tested in order to improve the

flaws in the prototype. Alterations will be made in the design prototype if suitable, for instance

change in the outer hollow pipe from copper to silver as silver is safer for the consumer and will

enhance the efficiency of the cooking pot. If this prototype is successful, a remodeled design of

this cooking pot can be introduced to the consumer world.

ENGR-103 Freshman Design Proposal Section 088, Group 01

1

1. Introduction

With the knowledge of heat pipes and thermodynamics a heat pipe integrated with a cooking

pot will be designed. The goals of the cooking pot will be to reduce the amount of heat lost, as

well as the amount of time spent cooking. The materials used for the heat pipe will be from

careful calculations and analysis over the course of the design process. Constraints will also be

an important decision in designing. There will be mechanical and electrical design costs. The

heat pipe will also need to fit the dimensions of a cooking pot and still be able to operate

correctly. The objectives are to build a heat pipe that is successfully integrated with a cooking

pot which improves heat loss and time spent cooking.

Heat pipes use basic thermodynamics principles. The main principle of a heat pipe is the

transfer of thermal energy from a high temperature source to a lower temperature region called a

sink. There are two ends - hot and cold - in a heat pipe. By providing a heat source to the hot

end, the fluid in the saturated liquid phase will evaporate, taking heat in, and the vapor will

transfer the heat to the cold end. At the cold end the fluid in the saturated vapor phase will

condense, releasing the heat, and return to the hot end by capillary action in the saturated liquid

phase to continue the cycle [1]. The design of a heat pipe is broken down into three parts. The

first is a vessel that seals the working fluid and capillary wicking structure. This vessel can be

made from any pipe, such as steel or copper. Next, inside the vessel is the wick structure. The

wick structure enables the working fluid to move from the condensed section back to evaporated

section. Finally, the last necessary part in a heat pipe is the working fluid. This fluid is what will

be condensed and evaporated inside the heat pipe. This fluid helps make the heat pipe work [2].

Using these key parts of a heat pipe, these parts will be integrated into a cooking pot.

This heat pipe will be integrated into a cooking pot design that will reduce the amount of

heat loss and time spent in cooking. Heat pipes are typically not used for cooking and this design

will be a test for the applicability of if heat pipes can change the way people cook. The cooking

pot will be designed by applying the concepts and theory of heat pipes. The heat pipe will be

positioned at the center of the cooking pot. An electrical source will also be included. This

electrical source will convert electrical energy into heat. This heat will heat the hot end of the

heat pipe. The cooking pot will also include a lid that will seal the heat in.


2

2. Technical Activities

The major task is the construction of the heat pipe that is centered on the cooking pot. First,

will be the designing and testing of the heat pipe. With an understanding of the best materials for

the heat pipe, then the cooking pot can be integrated and tested. Once the most suitable heat pipe

is created, it will be integrated into a cooking pot. The heat pipe should be molded into the center

of the pot. Underneath the heat pipe will be an electrical tape which will generate heat from

electricity, then it will be convert the electrical energy into thermal energy. This generated heat

will heat the heat pipe and allow for the heat transfer process to begin, which then allows for heat

to be released into the thermal cooking pot. Figure 1 below shows the initial design proposal in

detail on how the cooking pot will be designed.

Figure 1: Schematics of the cooking pot

2.1 Cooking Pot Design

Once the heat pipe is built it will be integrated into a cooking pot. This cooking pot must

be sealed tight to insulate the heat. The material the pot will be made of is stainless steel. This

material was chosen for its safety in cooking to protect the consumers buying the product.

Stainless steel is nonreactive, meaning it will not break down into the food or liquid being

cooked with it. The dimensions of the pot will be roughly 14 to 16 inches high for it to be able to

fit the heat pipe inside. The width will also be about 14 to 16 inches long and 9 inches wide.

These dimensions were chosen, so it could fit all types of food including meat such as beef or

turkey. The heat pipe will also be fitted into the bottom of the cooking pot, so there will be a

sealed hole at the bottom for the heat pipe to fit in.


3

2.2 Heat Pipe Design

I. Working Fluid

Within the heat pipe, an important factor is the application of the heat pipe is the

working fluid. The latent heat of vaporization must be considered with the working

fluid as well.

The latent heat of vaporization is the energy required to transform an amount of

something from a liquid to a gas at a certain pressure. This process often will boil

the working fluid at a point below the atomic pressure boiling point. The idea of

water was considered as a working fluid. For water as a working fluid, it will

typically boil at a low temperature of around 0 C [3]. This is because the heat pipe

contains a vacuum allowing the fluid to boil below the atomic pressure. With the

ability to boil at lower temperatures, water can be a more attractive working fluid

because the transition from liquid to gas can happen quicker as oppose to other

fluids [3].

A wide range of working fluids can be used for a heat pipe, but for this project

pure water was chosen as the most suitable working fluid. For food to cook, the pot

needs to reach temperatures up to 100 degrees Celsius. Water has a temperature

range from 5 to 230 Celsius [4]. This is in the perfect range for what is needed to

cook food in the cooking pot. When compared to other working fluids water

seemed to be the best fit. Liquid ammonia was considered however its temperature

range was from -70 to 60 Celsius [4]. Its highest temperature was not hot enough to

meet cooking standards. Also, it is extremely crucial to use non-toxic working

fluids due to the fact that the heat pipe will be used in cooking applications. So,

water again proves to be the best of the working fluids for this project because of

its excellent thermal properties.

II. Wick (Copper-mesh)

Metal copper mesh will be the wick used in the heat pipe. Inside the hollow copper

pipe, the mesh will be lined and it will transfer the fluid in the liquid phase using

capillary action to heat the hot end of the pipe in order to start a new cycle. Mesh is

a very porous material and will be very useful for capillary action. However, the


4

pores must be small in the mesh to allow an increase in flow resistance and high

capillary pressure. To get the maximum capillary pressure the following equation

can be used [5]:

Pcap = 2

In the above equation, the capillary pressure is equal to two times the surface

tension, sigma (), divided by the radius of curvature in the tube (r). For the mesh in

the heat pipe, 20 x 20 meshes rolled up and layered with cloth will be used. A

picture of the mesh is below in Figure 2.

III. Vessel(Copper Pipe)

A copper pipe will be used as the outside vessel. Copper will be used because it is

able to heat up and release heat more efficiently than some other metals. One factor

that is important to keep in mind is the thermal conductivity of copper. It is

important that the heat pipe is cycling the heat. With the use of Fouriers Law, in

the equation below, the appropriate length of the copper can be determined [6].

Figure 2: Heat Pipe Materials


5

q = (k A dT ) / s

In the equation above, the conductive heat transfer is equal to the thermal

conductivity (k), the heat transfer area (A), and the temperature difference (dT)

divided by the material thickness (s). This conductive heat transfer will then be

expressed in units of watts. The size dimensions for the heat pipe must be

considered as well. The heat pipe must be small enough to fit inside the cooking

pot which is 14 inches; therefore it can only be around half a foot long. The

diameter of the pipe will be half an inch. Figure 3 below shows the picture of the

final design of the heat pipe.

2.3 Electrical Design

With the heat pipe now created, it will need to be heated up. The best option chosen for

the cooking pot will be electrical heat tape. This heat tape will be wrapped around the bottom of

the heat pipe. This tape is heated by electricity. The tape can generate the heat necessary to heat

the heat pipe. This temperature desired is 200 degrees Celsius at the bottom electrical plug will

Figure 3: Principle of a heat pipe


6

be used to heat up the heat pipe. Three tests were performed to obtain maximum satisfactory

working condition of the heat pipe. Fouriers Law was used to determine the heat transfer on a

cylindrical surface i.e. the heat pipe. The surface area (A) for transferring the heat is directly

proportional to the radius of the pipe (r) and the pipe length (l). This gives us the following

equation [7]:

Because the exact radius of the heat pipe was known and also knowing that as the radius of the

inner pipe and the outer pipe increase so does the heat transfer area, the Fouriers Law equation

was used to determine the heat transfer [6].

In the above equation, the heat transfer (Q) is equal to the conductivity constant (k) times the

surface area (A), determined above, by the change in temperature divided by the change in pipe

radius.

Figure 4: Heat tape being wound on a heat pipe


7

2.4 Cooking Pot Design

The final design can be shown in Figure 7. When the cooking pot and heat pipe come

together, it will look like the figure below. The cooking pot is made of stainless steel and the heat

pipe is centered in the middle of the pot. The cooking pot dimensions are 6in radius and 12 in

height. The pot can hold about 4 gallons of water by solving for the volume of the pot and then

changing the units. The equation for the volume of a cylinder is in the equation below, where r

equals the radius and h equals the height:

Volume =

To hold the cooking pot and the heat pipe together, holes were screwed into the pot and

the section to hold the electrical heat tape container. Then, the heat pipe was put into those holes.

To cover up the holes and seal the stainless steel and heat pipe together, epoxy was used. Epoxy

can withstand temperatures of up to 550 F, so it would not melt in the application.

Figure 5: Cooking Pot Design

3. Results

To make sure that a desirable working condition for the heat pipe were met; multiple tests

were performed and each produced different results. The setup of the heat pipe testing can be

shown below in figure 8. The heat pipe was held vertically on a stand. On the top part of the

heat pipe a sensor was attached to measure the heat coming out. This sensor was connected to a

block calibrator that converted the values of the sensor to temperatures. The dryer was the heat


8

source, getting as hot as 200 C plus to heat the pipe. Aluminum foil was also added towards the

bottom of the heat pipe to concentrate the heat.

On the first test and second test, a leakage was found on the copper cap due to the

inadequate soldering. The third test produced satisfactory results and the following data and

result were obtained. A 100mL graduated cylinder was used to measure the amount of water the

heat pipe can hold. The volume of the tested heat pipe measured 22mL of water. This means that

the amount of water used in the heat pipe will be one third of the pipes volume, 11mL. The

copper was heated for duration of 5 minutes. A record of the temperature was taken at 30 second

intervals with a starting temperature of 24.7C. The test was stopped once the temperature had

reached the desired temperature of 100C at the top of the pipe, which is a temperature hot

enough to boil water. Water from the copper pipe was measured again after the test to ensure no

leakage took place. The measurement showed no loss of water.

An excel sheet was used to graph the temperature range. Figure 9 below shows the data

recorded every 30 seconds. Figure 10 below shows a steady linear increase of temperature as

predicted.

Figure 6: A detailed image of testing the heat pipe


9

Figure 7: Temperature Data

The cooking pot was also tested to see if the goals would be met. The setup for testing

began with plugging in the electrical heat tape, to act as the heating source, as well as fill the

cooking pot with water to the top. Water was used to see if it would boil. Placed in the water was

a sensor that sensed the temperature of the water. With the setup complete, testing could begin.

The sensor reading began with an initial temperature of 27.7 C. After 25 minutes of testing, the

sensor had reached 33.8 C. The graph in figure 11 below shows the data tested. The temperature difference after 25 minutes was 6.1 C. For the water to reach a desired temperature of 100 C, it

would take about 8.5 hours. This time proved to be undesirable to the goals that were trying to be

reached, such as decreased cooking time. 8.5 hours will be too long to boil water. Even though

the cooking pot failed to give the desired results, the heat pipe inside was still reaching the

desired temperatures. The amount of water to boil was just too much.

Figure 9: Cooking Pot Testing Data

Figure 8: Graph of Temperature vs. Time


10

4. Future Work

After a number of tests, it was observed that the heat pipe was not able to efficiently

heat the water in comparison to commonly used conventional methods such as the heating the

water on a stove top or heating it by fire. The main drawbacks of this prototype were, firstly, the

heated part of the copper pipe did not have considerable surface area in contact with the bottom

half of the container as it was encapsulated by the steel cup which did reduce its efficiency.

Secondly, choosing a large control volume such as the 12 quartz container seemed to also have

reduced a hypothetical amount of efficiency in comparison to its smaller surface area heat pipe.

Thirdly, the lack of insulation around the heat tape which was wound around the heat pipe could

have also reduced the efficiency as some amount of heat was dissipated during the transfer

process of the heat from the heat tape to the heat pipe.

Fig 10: Condensation on the lid Fig 11: Slow condensation on the walls

Future improvements to the project can be made by tackling the above identified

drawbacks. To fix the issues of insufficient surface area of contact and ineffective heating of the

lower half of water in the container, a longer pipe bend to occupy the base within the pot in a

circular design can be utilized. This would make the heating process more efficient by heating

the cooler layer at the bottom, as well as providing enough area of contact with the water.

Reducing the volume of water heated by choosing a smaller container would enable quicker

results. If the amount of heat transferred by the heat pipe can heat up a certain volume of water in


11

a reasonable amount of time, the volume of the container should be close enough to that certain

volume. Otherwise, a bigger heat source and pipe will have to be used to make up for the

additional volume of water. Lastly, addition of insulation to the exposed side of the heat tape

would aid in the better utilization of the heat produced. Since the heat tape is double sided, the

insulation would prevent the loss of the other half of energy produced on the exposed side to a

significant degree, and redirect it to the heat pipe making a more efficient connection between

the heat tape and the heat pipe. Thus, a higher temperature can be obtained from the heat pipe.


12

References

[1] Heat Pipes, Cooler Master,[online]

2013, http://odm.coolermaster.com/manufacture.php?page_id=8 (Accessed: April 10, 2013).

[2] P. Kew and D. Reay, Heat Pipes: Theory, Design and Applications, 5th

Ed. Burlington:

Buttenworth-Heinemann, 2006.

[3] Heat Pipe Technology, Thermacore, [online] 2013, http://www.thermacore.com/thermal-

basics/heat-pipe-technology.aspx (Accessed: June 4, 2013)

[4] Typical Operating Characteristics of Heat Pipes, Enteron, [online] 2006,

http://www.enertron-inc.com/enertron-products/heat-pipe-selection.php (Accessed: May 2013)

[5] Conductivity Heat Transfer, EngineeringToolBox, [online] 2013, http//www/engineeringtoolbox.com

(Accessed: April 30, 2013)

[6] Heat Transfer Engineering, Engineers Edge, [online] 2013, http/www.engineeringedge.com

Accessed: May 4, 2013)

Engr 103,Section 088, Group 01

Documents

Transcript of Engr 103,Section 088, Group 01