Pneumatic actuation to create ram compaction force/compaction ...

[Pages:4]Title: Lessons learned to date from HMC development work and operations

Date: 7-26-2018

? Pneumatic actuation to create ram compaction force/compaction pressure: o The Gen-2 HMC is a "side loading" with extraction of the compacted product out of the top. One key derived requirement was to minimize crew interaction time. The Gen-2 uses a mechanical scissor-link compaction while the Gen-1 had been Pneumatic. o Pros: ? Can produce the same compaction force regardless of ram position ? Can produce very high compaction forces/pressures with a very small compressor ? Method of determining compaction force is simple remote from heated ram (use of pneumatic actuator chamber pressure and known compaction ram face area) ? Robust and simple system o Cons: ? The stored energy of the pneumatic system can pose challenges if there is sticking during operation. Needs to have a damper designed into system to prevent ram jump or lurch when becoming un-stuck. Mainly an issue when non-stick coatings are insufficient ? A liquid/oil damper is highly efficient but leaking in micro-gravity would be an issue. Would require use of an oil that is non-toxic to humans ? There are other damping options but they may require additional development ? If pneumatic actuator seals leaked then onsite service would be required to replace them (this did not prove to be an issue in the Gen 1 HMC which used pneumatic action)

? Mechanical actuation to create ram compaction force/compaction pressure (only tested a linkage style system because other mechanical options were not compact enough to allow an HMC system to fit into an ISS Express Rack): o Pros: ? Potentially avoids pressure vessel certification issues (nonetheless, waste processing section of chamber may still have to be treated as a pressure vessel and also require maintenance and replacement of seals). ? Has minimal ram jump or lurch when overcoming a stuck ram. o Cons: ? The mechanical linkage system generates very little force when the ram is in the retracted position. The retracted position is the position that the ram is in when placing trash into the system and also when you start the compaction process. ? Requires a load cell to monitor load/pressure on trash. Due to the nature of the design, it was required to place the load cell close to the heated portion of the hardware. If it fails, a backup means of monitoring the force would be extremely difficult to implement and require additional complexity and space. ? Cannot provide uniform or linear force/pressure on trash throughout stroke of ram. In the event that a larger volume of trash is entered into the system, the system would not be able to provide as much force on trash.

? Analysis of the Gen 2 linkage based mechanical actuation system revealed that to meet compaction force/compaction pressure requirements, a prohibitively large motor, ball screw shaft, and linkage mechanism would be needed to fit into the Gen2 allocated volume.

? The level of compaction and density of trash is critical to improving the heat transfer through the trash: o Insufficient compaction leaves void spaces in trash which severely affect the heat transfer through the trash. This is because void spaces may either be full of air or water vapor which are very highly thermally insulating. This was witnessed during Gen 1 HMC testing. o For example, we looked at processing trash with Brine contained in trash, and we were not getting good conduction. This mandated compacting out void spaces to improve conduction. Compaction helps spread liquid throughout trash improving the wastes' overall thermal conductivity

? Water removal from trash and sterilization: o Typically, 40 to 60% of the water was removed from trash during a low temperature drying step. The low temperature drying step did not require temperatures significantly higher than the vapor pressure of water under the applied process pressure. We investigated boil off at an absolute pressure of approx. 200 mBar or 3 PSIA and a process temperature between 65oC to 75oC during that, the low temperature, phase of water removal period. The boiling point at that temperature is approximately 60oC. This water was assumed to be loosely bound in trash. It was assumed that when the trash was compacted the loosely bound water would get squeezed into dry portions of trash and also directly against heated surfaces. o Ideally we want Vapor in the backend system, as liquid poses problems for sensors and contamination/fouling. o We had options but used a thermoelectric cooler because it covered psychrometric (dew point)_ requirements for the desired ISS Pressure/Temperature profile. Another option instead of a cooler is dilution but that option would not work under all ISS atmospheric conditions as specified in the SSP 52000 and I believe the same cabin air information is stated in the SSP 57000 document. o There are different levels of complexity associated with Cabin release vs. venting of gases. We are evaluating the ISS Vacuum Venting system (Ref. SSP 57000). The dewpoint limitations are the main constraint, because vaporization of water does not occur at a consistent rate and large amounts of saturated vapor and liquid can violate space vacuum venting system dewpoint limits. The dewpoint limits drive the process pressure for the HMC when venting to the VES. Precisely controlling the rate of vaporization of water in from trash can be difficult. The maximum possible rate can be controlled by controlling heater power but because of the larger thermal mass and thermal losses, thermal time constants are large and not necessarily constant. o To remove the water remaining in the trash after the initial lower temperature drying step, temperatures significantly higher than the saturation temperature at the applied process pressure was needed. This is assumed to be due to water being more tightly bound in trash, I.E., water in foods, water in cotton, etc.

o To effectively perform a dry heat sterilization process on the trash, all water must be removed otherwise the remaining water will constrain the temperatures at that location to the dewpoint which is not sufficiently high enough to meet dry heat sterilization standards. Moist heat sterilization is highly effective but cannot be relied on due to the varying amount of water in different trash loads therefore the dry heat sterilization protocol was followed.

? Cooling of processed trash: o To efficiently heat the HMC it requires insulation. The issue with this is that the same insulation significantly slows the cooling process o There are options that allow both high quality insulation and more rapid cooling of trash which did not get implemented in the Gen 2 HMC. o No forced air cooling was applied during the Gen 2 experiment period because of the time between each experiment in the lab was sufficient to appropriate cooling.

? Effect of food and sugary drinks on the process and hardware: o Sugary drinks present a significant challenge to an HMC system ? One of the main challenges in the waste model are sugary drinks (carbohydrates in general). We fount non-stick coating is necessary due to caramelization of these food items. ? Sugary drinks can get squeezed into HMC hardware beyond trash. When this occurs the water from the drinks is removed leaving a caramelized substance (simply termed "goo" that can clog water removal ports, fowl sensors, and negatively affect seals and seal surfaces. ? The "goo" does act as an effective binder of trash which helps the processed trash tiles to maintain their density achieved during compaction. The functions similarly to the molten plastic in the trash o Foods other than sugary drinks provide material that can become caramelized waste or "goo".

? Seals: o The Gen 1 HMC was originally designed to use spring-loaded PTFE seals in the pneumatic ram actuation section of the device. The reasons for choosing that type of seal were the very high temperature capability of PTFE and also for its low coefficient of friction. ? It was discovered that despite the PTFE seals being spring-loaded, it was difficult to get a good seal at lower pressures (the HMC actuator generally remained at relatively low pressures, less than 70 psi). The spring-loaded PTFE seals were very leaky (till about 5 psi) until getting significant pressure which was required to "energize the seal" which was the lingo used in the seal industry. ? The PTFE seal material marred easily rendering the seal un-functional. This was observed when the seals rode over hard caramelized "goo" and also surfaces that eventually became pitted. (talking about the ram seals which are dynamic but used at a very low frequency). o The spring-loaded PTFE seals were replaced with silicone o-ring seals. ? The silicone o-ring seals proved to be very tough and could ride over the goo and pits and maintain a seal. ? The issue with the o-rings was that increase in friction force caused by the increased coefficient of friction of the rubber compound and the force of the o-ring pressing

against the waste processing chamber sidewall. Regardless of the increase friction force issue, the use of the rubber seals proved to be the best solution. Maybe lower friction yet tough seals exist. High quality non-stick coatings on chamber wall should significantly reduce seal induced friction load on ram. o Seals "relax" or "creep" over time, especially at the elevated temperatures that occur in the heat melt compaction process ? It was observed that the flange seals that sealed different sections of the processing chamber together lost the ability to seal over time. This is due to the molecules in the seal material moving to their lowest energy state while compressed into the Oring groove. This process is significantly accelerated in the presence of elevated temperatures that occur in the heat melt compaction process ? The use of spring loaded rubber seals was considered but never tested. The springs in the seals would not have the issue with heat accelerated creep but the rubber portion of the seal would still likely be affected. Again no testing was performed to verify if that solution would be satisfactory or not. ? Miscellaneous: o Please reference publications for downstream effluent handling and recovery. The catalytic oxidizer hasn't yet been tested with the system. Also, in particular, pay attention to Mass balance ?it should close o It was observed that over time the fasteners that joined the different sections of the HMC waste processing chamber (Gen 1 unit because a significant number of tests were performed with that unit) loosened because of thermal cycling. This loosening occurred despite the use of split ring lock washers. o In house testing of non-stick coatings on coupons was performed. The testing indicated that caramelized "goo" had a very high adhesion to untreated aluminum and that the samples that had a non-stick coating were an order of magnitude less sticky. ? Previous results indicate that more research needed to be done on this using other compounds to create the "goo". ? Higher than expected adhesion occurred on the Gen 2 HMC unit that incorporated a non-stick coating. The coating visually appears non-uniform indicating a possible error in applying the coating. o Process temperatures: ? Early research done using the Gen 1 HMC showing a reduction in processed trash tile density when the waste was processed at a temperature of 180oC. The exact reason for this was not known. One theory was that any remaining water (even if only a very small quantity) was trapped in the cooled tile. The thought was that due to the lower viscosity of the plastic at higher temperature, the plastic more easily flowed through intersticial spaces within trash and trapped the water in small localized pockets. The vapor pressure at that temperature is approximately 145 psia and this created an observable push back against the piston.

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