A tunnel freezer is cooled by selectively supplying liquid nitrogen to banks of spray nozzles. The banks are switched on and off as required so that at any given time the pressure upstream of each operational spray nozzle will be substantially constant at a level which closely approximates the maximum heat transfer coefficient associated with the spray nozzle or the maximum effectiveness of the spray nozzle or a balance therebetween. The invention is in contrast to the prior art where the flow of liquid nitrogen to the spray nozzles is continuously varied in response to the thermal load.