Compact vacuum pipe solar water heater: 1) Non pressurized. 2) Vacuum pipe 24pcs,1. ... . New design compact solar water heater thermo-siphon system work principle:Vacuum tubes collector absorbs the heat energy to ... heat the water circulating within tubes & tank,when using the hot water,the high ... water tank. Model No. SL-1B24pcs vacuum tubes,length:1.8m,diameter:58mm.Capacity:200L.