Abstract: A system for making extruded cement-based articles. The process can include forming a cement-based feed mixture including water, aggregate and binder and directing the feed mixture to a vacuum zone, wherein sufficient vacuum is maintained in the vacuum zone to remove entrained air and, preferably, excess water, from the feed mixture. The feed mixture is held in the vacuum zone at a negative pressure for a time sufficient to form a vacuum-treated or densified mixture. The vacuum-treated or densified mixture is passed through an extruder at an elevated pressure to provide an extruded cement-based article.

Abstract: A heat exchanger comprising a refrigerant tube in coil form leading from the output side of a compressor and a water tube in coil form tapped from a source of water to be heated, the tubes being coiled together so that each coil of the water tube is interposed between a coil of said refrigerant tube, and vice-versa. Inner and outer cylindrical sleeves are disposed within and around the coiled tubes thereby defining an annulus which enhances the heat transfer from said refrigerant tube to the water tube. A housing surrounds said coiled tubes and said sleeves, and sealed end caps are provided at each end of the exchanger.