Engineering World Health: Cold BoxJosh Arenth, Cynthia Bien, Graham Gipson, Elise Springer, Brittany Wall

Department of Biomedical Engineering, Vanderbilt University, Nashville, TNAdvisors: Dr. Adam List, Department of Chemistry, Vanderbilt UniversityDr. Robert Malkin, Director of Engineering World Health, Duke University

Project Importance and Description Engineering World Health was created in 2001 by Dr. Robert Malkin to use the resources of collegiate engineering programs for the benefit of hospital conditions in developing nations. EWH is now a leader in helping to improve and positively impact the quality of healthcare in disadvantaged areas around the world.

Vaccines, blood, and other chemicals are kept in refrigerators mostly concentrated at the blood bank. However, these chemicals are sometimes needed on short notice and often in remote areas. In developing countries, they sometimes do not have a refrigeration area so they often keep a temporary supply of the most needed chemicals in the room. Due to heat or exposure to air, the chemicals tend to lose potency and prove to be no use to physicians. Our project will focus on designing a refrigerator of very small volume that is sufficient to store one day’s worth of supplies without the need for electricity or any outside fuel. The refrigerator needs to be portable and be maintained at about 10 degrees Celsius (50° F) for 12 hours. In addition, 500 units must be able to be manufactured for less than $100.

Constraints

Economic Analysis

Acknowledgements•Dr. Adam List, Department of Chemistry, Vanderbilt University•Dr. Robert Malkin, Director of Engineering World Health, Duke University•Dr. Joel Tellinghuisen, Department of Chemistry, Vanderbilt University•Dr. Todd Giorgio, Department of Biomedical Engineering, Vanderbilt University•Dr. Robert Roselli, Department of Biomedical Engineering, Vanderbilt University•Dr. Paul King, Department of Biomedical Engineering, Vanderbilt University

While the final prototype did not reach our desired goal of 12 hours below 10° C, testing did reveal that we were successful in the design and construction of a working heat sink, an inner compartment which protects its contents while allowing for outward heat transfer, and effective UV shielding. The theoretical model and final laboratory test suggest that the component of our design in need of further improvement is the outer insulatory layer. Our future work will consist of continuing to increase this layer in size, and searching for a new, better insulating material that is as durable, light, and cheap as EPS. We will also be working to include radiative heating and our UV shielding into our theoretical model.

Theoretical Model

Final Results

Materials

The main purpose of the project is to develop a cold box with the following specifications:

• Keeps medical supplies (especially vaccines and blood) maintained at 10° C for 12 hours.• Is as cost-effective as possible, so a manufacturer can produce 500 units for less than $100.• Does not require electricity or outside fuel.• Does not require highly skilled labor to assemble.

storage cavity

heat efflux

heat sink

heat-conductive inner wall

insulating outer wall

a + b + Δ → c

Choosing Our Design

Figure 5: In the cold box, an endothermic chemical reaction consumes thermal energy, thus drawing heat out of the inner cavity. This heat is trapped in the heat sink because of the outer insulating boundary

Figure 6: Prototype B altered slightly from Prototype A since three foam cups were used instead of the single paper-foam composite cup. Insulating tape was also used to improve the seal between the cups and the lid.

Prototype A Prototype B

Prototypes C & D

Component Materials Considered Final Choice Desired Properties

Outer casing Expanded polystyrene (EPS)CeramicVacuum

Multilayered EPS (foam) sandwich

InsulativeCost-effectiveAvailable

Heat sink Non-toxic, non-abrasive chemical compoundsHeat-absorbing liquidsEndothermic reaction

Water High thermal capacityCost-effectiveSafeAvailable

Inner casing GlassNon-reactive, corrosion-resistant metals

Aluminum LightweightCost-effective

Figure 7: Prototype C differed from those previous as it included an even thicker wall of nested foam cups. Prototype D differed from C by not using any adhesive materials.

Prototype E

Prototype A Prototype B Prototype C Prototype D Prototype E

Material Quantity Cost Quantity Cost Quantity Cost Quantity Cost Quantity Cost

Cardboard coffee cup 1 $0.04   $0.00   $0.00   $0.00   $0.00

Aluminum can 1 $0.08 1 $0.08 1 $0.08 1 $0.08 1 $0.08

EPS cup - 8 oz.   $0.00   $0.00 1 $0.02 1 $0.02   $0.00

EPS cup - 20 oz.   $0.00 2 $0.07 5 $0.19 3 $0.11 6 $0.22

EPS cup - 44 oz.   $0.00   $0.00   $0.00 1 $0.09 1 $0.09

EPS - sheet 30.25 $0.37 30.25 $0.37 30.25 $0.37 30.25 $0.37 30.25 $0.37

Reflective duct tape   $0.00 74 $0.16   $0.00   $0.00 325.25 $0.67

Gorilla glue   $0.00 0.1 $0.07   $0.00   $0.00   $0.00

Hot glue   $0.00   $0.00 1 $0.26   $0.00 1 $0.26

Aluminum Foil   $0.00   $0.00   $0.00   $0.00 405 $0.19

Bubble wrap   $0.00   $0.00   $0.00   $0.00 144 $0.13

Total Cost per unit   $0.49   $0.75   $0.91   $0.67   $2.02

Figure 8: Prototype E utilized the nested foam cups used in previous designs, but also included bubble wrap and insulating tape to prevent radiant heat from entering the device.

Lid

Insulating Tape

Heat Efflux

Bubble Wrap

Nested EPS Cups

Conclusions

Figure 3: Results from the test when the thermometer was placed inside the vial. Prototype C (green) performed the best, sustaining 10oC for 4.48 hours.

Figure 4: Testing constraints were changed in order to get these results, the thermometer was placed inside the inner chamber and measured the air temperature. Prototype E performed the best with ice, maintaining less than 10oC for 10.1 hours.

Time (hrs)

Te

mp

era

ture

(oC

)

Time (hrs)

Te

mp

era

ture

(oC

)

Ammonium Nitrate was chosen as the cooling agent. Salvation of solid ammonium nitrate in water is an endothermic process with an associated temperature drop. Below is the stoichiometric equation.

Viewing the interior of the device as an essentially isolated system, equation 1.2 predicts approximately how much ammonium nitrate a specific temperature drop requires.

(1.1)

(1.2)

It is assumed that the aqueous reaction system inside the device experiences a step change in temperature upon addition of ammonium nitrate. Though the aqueous reaction system may have an ambient temperature, but when ammonium nitrate is added, the system temperature instantaneously drops (equation 1.3).

(1.3)

Figure 1: Dark bold arrows represent thermal energy flows from the ambient environment (inf), aqueous reaction mixture (1), storage compartment (2), and biosample temperatures (3).

It was assumed that heat accumulates only within the fluid in each compartment’s interior, and not in the walls. The end result of thermal-energy balances was a system of three coupled linear ordinary differential equations in 1.4.

(1.4)

Figure 2: Particular solution curves of equation 1.4 given the set of initial conditions displayed in 1.5 and parameter set for prototype C. The inset displays a zoomed-in view of curves during the first 30 minutes. Large circles/light gray line corresponds to the aqueous reaction mixture temperature T1 ; small diamonds/medium gray line to storage container air temperature T2 ; small triangles/dark gray line to bio-sample temperature T3 .

(1.5)