Concept
This is a water cooled vest; it is designed to cool down firefighters and lower their core body temperature. Water is pumped from an ice chest, through cooling packs installed in the vest (absorbing heat from one’s body), and back into the ice chest.
Background
Firefighters fight fires in crews which cycle in and out of the fire; they cannot stay in a fire for too long because it is very physically challenging. While they are cycled out, they go to ‘rehab’; before they go back into a fire, they must bring their vital signs (like heart rate, blood pressure, etc) back to healthy levels. One vital sign they struggle to control during rehab is their core body temperature – doing lots of strenuous physical activity in a burning building raises your body temperature by quite a lot.
Currently, firefighters only place a wet rag over their necks to cool down. The Cooling Vest is designed to absorb heat from the body during firefighters’ rehab period, helping them recover faster. This solution has the advantage of providing adjustable cooling, as specified by the user, as well as keeping them dry.
Requirements:
Must absorb at least 50W of heat
Must work for at least 3 hours
Run off of 12V or 120V
Durable
Cheap
Adjustable cooling
Must work with the ice chest they already use
Must be portable
Testing/Analysis
In order to determine the heat transferred out of the user’s body, a very simple test was done using the cooling bags; A bag was connected to running water, and held in place on a test subject’s torso. Temperature of outlet water was measured before the bag was held on the subject’s body (to be used as Tin), and then monitored while being held against a user’s body and recorded after stabilizing. Additionally, the flow rate of the water was recorded. This test was done at two different flow rates:
This resulted in the following cooling for each flow rate:
The heat absorbed in this test both met the cooling requirement, and demonstrated adjustability in cooling for different flow rates. The completed vest was designed with 3 of these bags, resulting in a maximum cooling of ~160W - well beyond the cooling required.
Design Challenges
Design Process
The goal of this system was to efficiently cover a person with a cold surface (ice water), without getting the user wet. The system circulates cold water through a series of channels that traverse the vest. These channels are melted/sealed into two layers of HDPE plastic sheet (Cooling Pack shown at left).
To reliably pump water through those channels there are bulkhead fittings installed in each cooling pack to attach tubing between each cooling pack as well as the input and output for the vest. To regulate how much cool water is flowing through the vest there is a simple ¼” valve; this allows the user to easily change how quickly the warm water is replaced with fresh cold water.
Each cooling pack was made by sealing channels into HDPE with a hand sealer. This leaves a number of exposed seal edges throughout the bag (circled at left). These edges are very weak compared to the rest of the seal, and is often the source of leaks in the bag. To avoid bags tearing at the edges, the process/equipment used to seal was modified such that the bag was not stressed while sealing.
There was also an issue with connecting in/out flow to each bag; in the original design, clear 1/2” tubing was simply stuffed into a channel on the in/out ports of each bag with sealant. No sealant was found to be effective enough on these particular materials (that would fill the gaps needed).
Instead of this method, bulkhead fittings were used. The bulkhead fittings were modified to not puncture or tear the bag, and also not block the flow of water.
Takeaways
While it is certainly a useful skill to be able to make a suboptimal design work, coming up with a better design is much more useful. There were lots of problems with getting cooling packs to stop leaking, and lots of re-making of leaking cooling packs. After some initial testing, a decision should have been made to use a simpler, more reliable design (particularly because one of our main requirements was durability). While it may have been challenging, it would have been a better idea to realize the shortcomings of the design and change directions, rather than trying to make it work.
Additionally, this project was required managing a group; it required giving everyone a direction to work on. Many of the members on this project were happy to help out, but were often unsure how. Making sure that everyone was always on the same page, and giving everyone a way to contribute, was a very helpful way to effectively manage the team’s time.