When two evaporators with two different temperatures are used in a system, a two-temperature valve is installed at the outlet of the warmer evaporator to prevent the compressor suction power from bringing the pressure in the warmer evaporator lower than the desired pressure.
Look at figure 96 to see where the surge tank is installed. When a two-temperature valve is used, the compressor short-cycles, unless a surge tank is installed in the suction line. The reason for this is that immediately after the pressure in the suction lines drops to a predetermined cut-out setting, a switch in the low-pressure control opens, shutting off power to the compressor causing the two-temperature valve that controls the pressure inside the warmer evaporator to open, causing a rapid rise in the suction line. At this point, the switch in the low-pressure control (sensing the increase in pressures) closes its circuit and starts the compressor and causes the short-cycling. The surge tank prevents this by creating more low-pressure volume to absorb the sudden pressure rise that might affect the low-pressure control. It could be said that it is a type of shock absorber that smoothes out the fluctuations of the suction-line pressure changes.
