induction motor thus it is necessary to limit the maximum speed of the exhaust blower to 60 hz to prevent overloading the motor this is accomplished with program code 1208 that limits the maximum speed to 80 of the speed range i e 60 hz the improved exhaust system has provision to operate the exhaust blower 45 controller 100 manually in the event of a malfunction this is accomplished by a manual speed potentiometer 106 and electrical contacts 108 that are operated by a maintained contact pushbutton a selector switch or a control relay the electrical contacts as shown in fig2 are in the position for manual operation and the exhaust blower speed is varied by turning potentiometer 106 the electrical contact 110 across terminals fwd and cm is closed to start the motor 47 of exhaust blower 45 frequency meters 74 and 112 are added to controllers 50 and 100 to inform the freezer operator of the gas flow fan motor speed and exhaust blower speed during operation the e trac 34 s 34 type ac inverter has a potentiometer ado that can be adjusted to calibrate the frequency meters the only purpose for the exhaust blower on a cryo quick freezer is to remove the nitrogen gas evolved within the freezer from the processing room this is necessary to prevent the accumulation of nitrogen within the processing room that could result in an oxygen deficient atmosphere however the nitrogen gas within the freezer must first be delivered to the freezer entrance by the gas flow fan the improved exhaust system solves the problem.