an 34 o 34 indicates the valve is open and an 34 x 34 for by pass valve 144 indicates that valve 144 may be opened or closed for additional fine tuning temperature control by loading and unloading compressor 12 internal unloading of compressor 12 ie a reduction in displacement such as with a slide valve slot valve or a lift valve may also be used to obtain fine temperature control as is well known in the art it will be assumed that the temperature of conditioned space 106 is in the stage of initial pull down and thus refrigeration system 10 will be in full or maximum cool when prime mover 14 is an internal combustion engine the engine speed is usually controlled by electrical control 38 between two speeds called high speed and low speed with temperature pull down being initiated with a high speed cool mode 200 to obtain maximum cooling pilot solenoid valve 28 will be closed causing three way valve 30 to select the cooling circuit 40 liquid line valve 62 will be open to enable evaporator coil 102 to function in a cooling mode engine coolant valve 88 will be closed preventing heat from being applied to economizer heat exchanger 60 and economizer by pass valve 144 will be closed thus liquid high pressure refrigerant will be subcooled in heat exchanger coil 68 by the expanding flooded evaporating state of the refrigerant in the second refrigerant flow path defined by shell 70 refrigerant returns to compressor 12 via both the suction port s and the intermediate pressure port ip when the control error drops to a point indicated at 202 engine 14 is switched to the lower of its two standard operating speeds without change in the controllable valve open closed pattern entering a.