The Heat Exchanger is based on a 2 gallon Rubbermaid cooler. It holds approximately 1.5 gallons of water and is heated by a 1500 watt 120 volt water heater element. The heat exchange is accomplished with 10 feet of 1/2" copper tubing.
The Mash Tun is based on a 10 gallon Rubbermaid cooler. The lid is drilled and fitted with a grommet through which the return manifold is fitted. The return manifold is used for circulating the mash and for delivering the sparge water. It's contructed of 1/2" copper pipe and I've cut slots into the end of the T section in order to distribute the liquid without disturbing the grain bed.
Controller turns on heating elements in Hot Liquor Tank and Heat Exchanger and heats water to preset temperatures. The water in the HLT will be transferred to the MLT and used for the mash in. Since the HEX is only 1.5 gallons and will not take very long to heat, it will probably be turned on after the strike water reached the target temp.
2. Mash >>
The mash will be circulated by the pump. It will travel from MT to Pump to HX and back to MT. The HX will be used to maintain mash temperature for single infusion mashes and to increase temperature for step mashes and mash out. Sparge water will be heating in the HLT during the mashing process.
During the sparge, hot water is pumped through the HX to the MT and the sweet runnings are collected from the MT in the BK. I may end up bypassing the HX, it just depends on whether I run into any problems with the HLT temps. 
