Engineering World Health: COLD BOX

Transcription

1 Engineering World Health: COLD BOX Cynthia Bien Josh Arenth Graham Thomas Gipson Brittany Wall Elise Springer Group 19 Engineering World Health organization background Founded in 2001 by Dr. Robert Malkin at Duke University Charitable organization that collaborates with collegiate engineering programs Improves conditions of hospitals in developing nation Employs a multi-step process: (1) Assesses hospitals (2) Ships container of refurbished medical equipment (3) Installs equipment and trains at location (4) Returns to location to reinforce training 1

2 Reasons we work with EWH Desire to impact and improve healthcare in developing nations Use engineering skills to develop simple, affordable devices Give others an opportunity that was given to us Problem statement How do we build a portable, reusable device that: Keeps a pre-cooled 5-mL fluid volume at 10 C/50 F (assuming an ambient temperature of 20 C/68 F) for up to 12 h Operates without electricity or outside fuel (including ice) Cost of manufacture does not exceed $100 for 500 units ($0.20 per unit) Assembly may be done before shipping and not require highly skilled labor 2

3 Our approach Determine a unique and efficient way to sustain 10 C/50 F for up to 12 h Classify possible materials as good conductors or good insulators Ultimately choose materials that are both sufficiently cheap and perform well Initial design: chemical heat sink-driven, cup-like device Initial design for prototyping Outer layer / casing Outer portion of container must be a good insulator (i.e., be an intrinsically poor conductor) in order to block environmental heat influx by conduction, convection, and radiation. Ideal materials Expanded polystyrene (EPS, Styrofoam) Ceramic Gas/vacuum sandwiched d between two layers Materials chosen EPS beverage cup 3

4 Initial design for prototyping Heat sink Cold box must have a component to remove and trap heat from box contents Ideal materials Non-toxic, non-abrasive chemical compounds Heat-absorbing material with large heat capacity Endothermic chemical reaction Materials chosen Ice, water, and sodium chloride Ammonium nitrate Sodium bicarbonate and acetic acid binary mixture Initial design for prototyping Inner casing Cold Box must have an inner layer to separate the contents of the box from heat-sink materials, yet still allow for efficient heat transfer (i.e., have high thermal conductivity). Ideal materials Non-reactive metal (corrosion-resistant) Glass Materials chosen Aluminum soda can 4

5 Chemical heat sink Heat-conductive inner wall (aluminum) Insulating outer wall (EPS) Storage cavity Heat efflux a + b + Δ c Heat from surroundings kept from being conducted or convected in. Schematic description In the cold box, an endothermic chemical reaction (generalized here) 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. Prototype A Outer layer / casing: paper-plastic p p composite (mostly paper) p Inner-chamber layer: aluminum Heat sink / cooling technique: water (267 ml) + NaCl(s) (10 g) + ice Measurement technique: LabWorks thermistor-based temperature t probe (in Dr. Joel Tellinghuisen s i physical chemistry lab) 5

6 3/26/2008 Prototype A As seen in the physical chemistry lab before (left) and during a test (above) (above). Temperature probes were inserted through the lid to measure both the air temperature inside the can and the temperature of a small test vial filled with water. Prototype A data Duration where temperature stayed below 10 C/50 F: 22 min for air, 24 min for vial Problem: Temperature does not stay below threshold for 12 h 6

7 Prototype B Outer layer / casing: polystyrene-air-polystyrene y p y y sandwich Sealants: Gorilla Glue and reflective duct tape Inner-chamber layer: aluminum Heat sink / cooling technique: salt-ice bath, dilution: [NaCl] = 0.48 M Measurement technique: LabWorks thermistor-based temperature probe (in Dr. Joel Tellinghuisen s physical chemistry lab) Lid Insulating tape Inner chamber Heat efflux Heat from surroundings kept from being conducted or convected in. Nested foam cups Chemical heat sink a + b + Δ c Trapped air Trapped air Prototype B As seen in a cut-away diagram. Hopefully more layers of EPS and the introduction of sandwiched air layers will help prevent heat conduction. 7

8 Prototype B data Duration where temperature stayed below 10 C/50 F: h for air ( % from previous) h for vial (+991.7% from previous) Problem: Even with better insulation, temperature exceeds threshold before 12 h Prototype B cost analysis Material Cost Cost unit Quantity Cost Styrofoam $0.016 /cup 1 $0.02 Styrofoam $0.043 /lid 1 $0.04 Aluminum can /can 1 $0.00 Insulating tape $0.227 /yd 0.26 $0.06 Hot glue $0.722 /oz 0.5 $0.36 Total cost for Prototype B: $0.48 Challenge: Amount is over double what one unit should cost. Solution: Develop theoretical model to help maximize efficiency while minimizing necessary materials (and thus cost) 8

9 Prototype C* Outer layer / casing: multi-layered EPS sandwich Sealants: Hot glue and duct tape Inner-chamber layer: aluminum Heat sink / cooling technique: ice and water Measurement technique: Vernier Go!Temp USB-based temperature probe interfaced to Logger Lite software (purchased by group; $39.00, vernier.com/go/gotemp.html) * Only used to test insulative abilities of device, not the efficacy of a given chemical heat sink. Prototype C As seen in after completion. The design is fundamentally the same with the addition of thicker insulating walls. 9

10 Inner chamber Heat efflux Prototype C As seen in a cut-away diagram. If seven layers of EPS (shown in gold) doesn t keep heat out, then what will? Chemical heat sink a + b + Δ c Heat from surroundings kept from being conducted or convected in. Trapped air Prototype C data Duration where air temperature (within can) stayed below 10 C / 50 F: t = h (+117.6% increase from Prototype B) Assume that temperature instantaneously drops below threshold, even though it really takes about 2 3 min. 10

11 Basic heat transfer model aluminum tyrene insulation tyrene insulation q 1 q 2 q 3 polyst polyst T in = 10 C T 1 T 2 T ambient = 20 C Steady-state assumption: all heat fluxes (q s) between layers are equal. Model is applied to walls (cylindrical), base and lid of cold box. Advanced heat transfer model Schematics of different compartments our cold box. From left to right: polystyrene insulating container, aluminum cooling compartment, glass/plastic sample vial. 11

12 Consultation with Dr. Roselli has led to development of a thorough theoretical model for our device. Three coupled differential equations (above) must be solved numerically in MATLAB to obtain expressions for temperature as a function of time. 12

13 Past work Finalized NCIIA proposal p and began regular meeting with our advisor Dr. Adam List (Chemistry). Agreed on overall design approach: chemical reaction for cooling encased in a thermos-like vessel. Developed lab protocols and obtained necessary equipment. Designed and fabricated three prototypes (A, B and C) and collected data on their efficiency at cooling and maintaining temperature for given period. Compiled cost, MSDS, and thermochemical h data on materials Generated a theoretical model for heat transfer. Current / future work Application of theoretical model to aid in design of new prototypes. Exploration of new options for thermochemical heat sinks, specifically based on cost. Meeting with Dr. Roselli (BME) to get his expert opinion of the design. 13

14 Acknowledgments Dr. Adam List (Chemistry) Primary advisor Dr. Robert Malkin (BME, Duke) Primary advisor Dr. Tellinghuisen (Chemistry) Director of physical chemistry lab, allowed us to use facilities Dr. Giorgio (BME) Contributed to development of heat transport model Dr. Roselli (BME) Contributed to development of heat transport model Aaron Cofey (BME graduate student) Donated a hot-wire foam cutter for prototyping Ladies of the BME office Ordered and received temperature probe 14