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Guide to Inspections of Low Acid Canned Food 17

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fill the sensing tube depending on the temperature range to be controlled. The system temperature would be adjusted by adjusting the spring tension on the steam control valve. This type of temperature control is not normally found on retort systems.

One of the most common types of retort temperature control systems found in the U.S. is the pneumatic (air to open) steam control valve (Attachment 2). This valve is opened by supplying air to a diaphragm which is connected to the valve stem. The valve is closed by spring pressure on the valve stem. The valves are air-to-open for safety reasons. If the power or air supply fails, the loss of air to the system results in the steam valve closing and the retort pressure/temperature falling. These valves are commonly connected to a mechanical recorder/controller which controls the supply of air to the valve (Attachment 2) When using the mechanical recorder/controller a temperature bulb in the retort is connected through a thermal tube to a Bourdon tube located in the control panel. The temperature changes in the retort cause the Bourdon tube to expand and contract. Expansion/contraction of the Bourdon tube causes the recording arm to move which also controls the amount of air to the control valve. As the temperature in the retort rises, the supply of air to the valve diaphragm would be reduced, the spring would force the valve partly closed and less steam would be supplied to the retort. As the temperature in the retort starts to fall, more air is supplied to the valve diaphragm and the valve is forced open, with more steam being supplied to the retort.

Pneumatic air controlled valves are also common in many advanced computer controlled retort systems. In these systems the air supply to the steam control valve is usually controlled through an electronic solenoid.

With any retort using air for control of the system it is very important that the air supply to the instruments and control valves be kept clean and free of oil, water, rust and other contaminants. It is ideal for the air supply to the instruments to be separate from the air supply to the rest of the plant. This air supply should be equipped with devices for keeping the air clean, such as air filters, condensate traps, water drains and oil traps. Maintenance of the air supply system on a routine basis is essential to the continued accurate operation of pneumatic air controls.

A simple mechanical/electrical control system for an electronic solenoid operated steam control valve may consist of a temperature dial to set the selected retort temperature, a temperature sensing element in the retort which is connected to a Bourdon tube or some other type of expanding gas or liquid sensor, and a steam supply valve which is controlled by air, an electric solenoid or rotary motor. These systems act much the same way that a thermostat in your house would be used to control the temperature. As the set temperature in the retort is reached the expansion of the liquid/gases in the temperature sensing element causes the Bourdon tube to expand, expansion of the tube causes electrical contacts to open or close, depending upon the design of the control mechanism. As the electrical contacts close an electrical source is supplied to an electronic solenoid or motor to operate the valve. This type of system may supply steam in an on/off fashion instead of proportionally.

A similar type system would replace the Bourdon Tube with a solid state electronic temperature controller using a thermocouple or resistance temperature devices (RTD) to sense the temperature in the retort. The solid state electronic circuits would then be set to control the retort at the selected operating temperature through the use of an electronic solenoid, solenoid/air or electric motor operated steam control valve. The solid state circuits may provide for on/off or proportional operation of the steam valve depending upon the control system and manufacturer.

In systems using RTDs or thermocouples a low level electrical signal is sent to a control mechanism. It is important not only for a firm to properly calibrate the RTD/thermocouple but to insure that the electrical signal sent to the sensor is not interfered with. The circuits (wires) used to transmit the RTD signal to the signal convertor or sensor must be designed and installed to prevent electrical interference on the circuit. Electrical interference on the circuit can cause false readings to be received from the RTD/thermocouple. The firm should have in place a system for routine calibration and maintenance of RTD's and thermocouples including the complete control circuit.


Addition of a manually set electrical/mechanical clock to the pneumatic air/electrically controlled valve systems mentioned above provides for timing of the retort thermal process. An additional mechanical/electrical clock for the timing of the vent cycle on steam retorts further automates the system. These systems are designed so that both a time and temperature must be reached for venting; the process timing does not start before the process temperature is reached, and the process ends after the set process time has been reached. These systems still require the observation and manual recording of MIG thermometer readings and

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