Yesterday, I commissioned another aerated windrow system for composting. These aerated windrows were 25 ft wide, 9 ft high and 200 ft long. Because its a high rainfall climate, we covered the windrow with Compostex fabric. There is one 6″ diameter HDPE pipe running down the length of the pile, aerated with a centrifugal blower and a timer. Four datalogging “buttons” continually measure temperature. After 4 weeks of primary composting, the pipe will be pulled out, and the pile reformed on another pipe for a further 4 weeks of composting.
We were using HDPE piping and it made me think of a discussion that we had at the US Composting Council. What about thermal expansion? What happens when we bury this pipe, or encase it in concrete? What happens when we use negative aeration with this pipe? Using an unrestrained expansion coefficient of 120 x 10-6 m/m/oC, and a temperature variation from -20 C to 70 C, this pipe can expand or contract up to 2.17 ft! If the pipe was PVC, the thermal expansion coefficient is 50.4 x 10-6 m/m/oC, so not quite as much as with HDPE piping!
What about if the pipe is restrained? We learned about this in 2004 when we provided the aeration piping for four aerated windrows at S&W Rock Products in Lynden, WA. The client preferred using HDPE piping, but was worried about differences in thermal expansion characteristics. The aeration extrusion was PVC. We provided HDPE piping for two windrows, and PVC piping for another two windrows. We encased the piping completely in concrete, which was the first application for this type of aeration floor.
After almost 10 years, the aerated floor is still functioning well. There was no concrete cracking due to differential thermal expansion.
What happens if we were to use negative aeration where the temperature differential may be much greater? HDPE has a higher temperature tolerance than PVC, therefore is the better option for negative aeration. I understand that some facilities have used HDPE pipe under the concrete for negative aerated facilities. I would be inclined to encase the piping in concrete to ensure optimal restraint from thermal expansion. We have now worked with a client to design a polyethylene aeration spigot called the Foundation Air spigot that will allow us the option of working with either PVC or HDPE piping, will allow us to recess the spigot below the level of the concrete, and also give us the option for negative aeration.