Notes from Windward: #68


Hanging the Bat

the first of our hybrid compressors

  March 30:

     The conversion of biomass into methanol involves the compression of a wide range of gases; some like hydrogen are explosive, some like carbon monoxide are poisonous, and some like oxygen are problematic in that lubrication oil bursts into flame in the presence of pure oxygen.

     To add to the fun, we'll have to be able to compress this range of gases to four different pressure ranges: 1 to 4 atmospheres, 4 to 16 atm, 16 to 64 atm, and finally 64 to 200 atm (around 3,000 psi).

     The common air compressor uses reciprocating pistons. They move back and forth in a cylinder, and have a set of rings fitted into grooves cut into the side of the piston to seal the gap between the piston and the cylinder wall. That works well, for the most part, but that's always some "blow by" which is air that seeps past the piston rings and gets into the crankcase. Which is why cars have Positive Crankcase Valves which the vacuum caused by the engine to suck that blow-by gas back into the carbureator.

     Blow-by's no big deal when all you're compressing is air, but when the gas you're compressing is explosive or poisonous, blow by isn't acceptable. There are sealed, industrial compressors designed to compress these sorts of gases, but they're not small and they're definitely not cheap.

A typical piston driven air compressor

     And so, we'll be developing a series of hybrid compressors that will use either pneumatic cylinders or hydraulic cylincers depending on the stage of compression involved. For example, pneumatic cylinders are rated up to 250 psi (around 16 atm), whereas hydraulic cylinders are able to withstand in excess of 3,000 psi (around 200 at).

     Long term readers may recall the four nifty pneumatic cylinders that we were able to pick up at The Bins for scrap value. Their cylinder tubes are made from high-strength aluminum, and are good for pressures up to 250 psi.

Alison gives scale to the pneumatic cylinders

     As with the steam engine, we're content to do our research and development work using compressed air since that way when we encounter opportunities to learn (more commonly referred to as "mistakes") won't pose a threat to our health.

     In order to work the cyclinder's piston in and out, the cylinder had to be securely mounted to something. In this case we decided to use the wall of the PowerLab to mount the cylinder since that would allow us to use gravity to draw down the piston thereby refilling the cylinder with air in preparation for the next compression cycle.

the bat compressor mounted on the wall of the PowerLab

     For lack of a better name for this concept, we've been referring to it as a bat compressor since it hangs on the wall like a sleeping bat.

     The sequence will be that a set of PV panels on the roof of the PowerLab will charge up a battery that's voltage is being monitored by one of the Butterfly control computers. When the battery's voltage exceeds 25.4 volts, the Butterfly will know that it's fully changed, and will initiate a compression cycle by turning on the winch. This will spool the cable that runs from the wench, through a pully at the end of the piston, and then back up to the top of the bat.

the cable pulling the piston upwards to compress the gas

     We had to do a bit of rummaging through the back bins at Red's Trading Post--our all time favorite store for finding all sorts of unusal parts. In this case we were able to turn up a pulley with a 1/2" bearing in the center. A bit of shop time later, and we had a set of custom sized bushings that matched up the pully with the yoke, the presumption being that we're going to need to attach weight to the end of the shaft in order to keep a downward pressure sufficient to ensure that the cable remains taunt as the motor reverses and unwinds the cable in preparation for the next stroke.

the end-of-the-shaft assembly

  April 2:

     This first edition of the "Bat" compressor is operated via a 12-volt wench--since the powerlab's main system operates at 24 volts, the Bat had to have its own battery. The energy to operate the compressor will come from a 12 volt photovoltaic panel and like the grid-tie inverter, it will only operate when there's electricity being generated. When the panels have charged the battery to its fully charged voltage (25.4 volts), the Butterfly controler will initiate a compression cycle, and then wait for the voltage to build up again to the fully charged state before initiating another compression cycle. But for now, we're just using the manual over-ride to get some practice cycles in.

Opalyn puts the Bat through its paces

  April 6:

     Now that we have the Bat working mechanically, it's time to hook up some of the pneumatic components, particularly the check valves that control the flow of air into and out of the Bat. We experimented with various types of check valves, before settling on a low-pressure brass type designed for water lines.

pressure testing the Bat

     The check valves are the two cylinders just to each side to the tee that connects to the pneumatic cylinder. The valve on the right allows air into the Bat as the piston is allowed to fall, and the valve on the left allows the compressed air to exit the cylinder as the piston is drawn upwards.

     There are four pressure ranges that we'll be working with, each a multiple of four times the level before it. The first two stages will involve pneumatic cylinders, and the third and fourth stages will us hydraulic cylinders.

     In practice, the Bat we're working with here will serve as the second stage of the compression system (60 psi to 240 psi), but this cylinder was the easiest to work with so we went with it first in order to make sure that the concept actually worked. As you can see in the pic, the Bat had no problem doing a four-to-one compression, and the 12 VDC winch didn't slow down a bit--indeed, if we didn't stop the Bat in mid-stroke, it would have blown out the low-range pressure gauge that we'd used to evaluate the various styles of check-valves available through local hardware stores, the goal being to use as many off-the-shelf components as possible.

Notes From Windward - Index - Vol. 68