Background

The focus of project was to improve the technical diving experience for scientific and commercial divers who uses the closed-circuit rebreather (CCR). A CCR is a type of technical scuba gear that circuits the gasses used within the cylinder tank. This type of gear allows the diver to have a more prolonged duration than regular scuba gear. Due to increasing pressure with increasing depth under water, CCR needs to dynamically manage the proportion of oxygen and nitrogen and helium to avoid oxygen toxicity and nitrogen narcosis due to increasing partial pressure of the gas. The addition of helium is to replace the nitrogen avoiding nitrogen narcosis, while introducing issues with high thermal conductivity, leading to potential risks of hypothermia.

Therefore, a heat exchanger is preferred to heat up the gas mix underwater compensating loss of heat for divers.

Condition Simulation

To design an effective heat exchanger for CCR applications, it is critical to model the gas properties underwater. The location of heat exchanger is determined to be placed ahead of the mouthpiece at the 2nd stage low-pressure hose.

Through phone calls, on-site interviews, and in-depth research, the CCR system and external environment to the system has been thoroughly studied to model the underwater characteristic flow. With research into gas blending (correlation between ppO2 and underwater depth, proportion of nitrogen and the effect of nitrogen narcosis, volume of lung size and the density of gas, and initial pressure of the gas inside the cylinder), gas properties (the density and thermodynamic properties of gas at different pressure and temperature), and heat transfer properties (Joule-Thomson effect during gas expansion), the temperature of the gas mixture at 70m underwater (the target depth of technical diving) is calculated to be 11.7℃ with 15.17% Oxygen, 47.33% nitrogen and 37.50% Helium at the pressure of 801.76 kPa.

The gas mixture will be flowing at speed between u_min=1.34 m/s to u_max=2.15 m/s at 70m underwater. With the same mass flow rate (m ̇) maintained with mixture switched to air and pressure scaled to 1 atm environment in the lab, the air will be flowing between u_min=6.46 m/s to u_max=10.34 m/s. To heat up the air mixture from T_i=11.7 ℃ to Target T_f=20℃, the amount of heat will be Q ̇_avg=7.47W. To achieve the same amount of heat transfer, the target temperature difference of air ΔT_air=22.43℃.

Based on the calculation, the design criteria for the heat exchanger are set for heating air flow of u=10 m/s at initial temperature T_i~10℃ at 1 atm to achieve 25℃ temperature increase at ambient temperature of T_ambient=10℃.

Experiment Setup

The experimental setup was devised from a need to simulate the ocean environment in an indoor setting at a low cost. It is difficult to recreate pressure at varying depths in the available lab setting. Using ice to cool the water to a desired temperature allows the water to maintain a steady temperature found at those varying cold ocean depths. The experiment procedure is as follows: air is injected into the first tank that is filled with ice to cool the air, simulating the temperature that comes out of the tank in the closed-circuit rebreather. Next, a thermocouple sensor detects the temperature of the air, T_i and a pitot tube measures the air flow speed, v_i, before it enters the second tank. The second tank is filled with ice and water and contains the heat exchanger. An external power source feeds energy into the heat exchanger without disrupting the process. After the cold air passes through the heat exchanger, it is measured at the exit by another pair of thermocouples and pitot tubes to measure the temperature, T_e, and flow speed, v_e, respectively, of the exiting air. The data recorded by this process gives the values needed to find the rate of heat transfer, Q ̇, from the heat exchanger to the air. Q ̇ will then be compared to the power of the system, P , to find the efficiency of the heat exchanger.

A corrugated pipe is installed outside the heat exchanger to simulate the CCRs underwater environment, with a metal pipe being used in Tank #1 so the air flowing through will get cold faster than if it were a rubber-insulated high-pressure hose that exists in actual CCR systems.

Based on the known design criteria, several design concepts of heat exchanger were proposed and evaluated through design decision matrix. Key evaluation factors included mass flow constriction, heating efficiency, manufacturability, and possibilities of failure. Based on the insights from the interview with divers, a design with minimum modification to existing system, non-invasive to the breathing channel, and easy to be replaced and installed will be favored by divers. Based on the evaluations, the heating wrap or the heat wire design solution is selected to proceed to actual testing. It offers minimal invasion to the current closed-circuit breathing channel while it is easy to be manufactured and reduces potential risks of breaking down. The testing will be conducted to evaluate the different parameters of the heat wire design solution.

Data Collection

I designed the LabView data collection program to compile all data from the sensors. The data were used to evaluate the heat exchanger design based on different design parameter such as configuration and power input. The heating effect of the heat exchanger are compared by configuration to derive the final design suggestions.

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