The invention resides in improvements to refrigeration systems which rely on circulation of refrigerant gas through compression and expansion phases, and thereby discharging heat from a fluid to be cooled. The invention includes a subcooler (38) in the refrigerant loop, downstream of the refrigerant condenser (34) and a gas trap (36) between the condenser (34) and the subcooler (38), that assures temperature drop in the subcooler (38). The invention also comprehends a shut-off valve (44) between the compressor and the heat source heat exchanger (28). The invention further includes a high capacity-to-volume oil to air heat exchanger (48), for cooling the lubricating oil in the oil loop (26). Preferred refrigerant is ammonia. Incorporating the above improvements into refrigeration systems enables an overall reduction in system sizing. Such systems, having heat exchange capacity of at least 200,000 Btu/hr., up to at least 500,000 Btu/hr., can be mounted in a frame (14) whereby the overall refrigeration unit (10) comprising refrigeration system (13) and frame (14) can fit a standard 80,000 pound capacity truck. Preferred embodiments do not require cooling water; the only required utilities being a motive power source, used primarily to power the compressor (30). The shut-off valve (44) between the compressor and the heat source heat exchanger (28) is used to trap refrigerant in the heat source heat exchanger (28) when the refrigeration system (13) is shut down. This enables maintaining a sufficient amount of refrigerant in the heat source heat exchanger (28) to facilitate an adequate rate of pressure build-up at the compressor (30) when the system is re-started, even under intervening temperatures at least as cold as 30 degrees F., or less.