Automatic Temperature Controllers

The automatic control of space temperature in general becomes easier as the space becomes larger. This is because the thermal capacity of the contents provides a "flywheel" effect to overcome temperature variations caused by the functioning of the controllers. It is sometimes difficult to decide whether the best place for a controller's sensing element is in the space to be controlled or in the air delivery stream of the air cooler.

For cargo spaces, which may be used for fruit carriage, the air delivery temperature is the correct temperature to control in order to be sure that temperatures never fall low enough to injure fruit.

In systems with reversible fans, the sensing element is placed in a by-pass duct (between delivery and suction ducts) so that air delivery temperature is detected whichever way the fan is running.

An electric solenoid valve located before the expansion valve in the liquid line and incorporating a simple on/off thermostat usually controls a direct-expansion cooler. Alternatively, for small installations with only one cooler served by one compressor, the on/off thermostat may control stopping and starting of the compressor. Small installations, for example, integral refrigeration units on containers, can suffer from "short cycling", i.e. too frequent stopping and starting of the compressor, if the control is a simple on/off type and no other controls are fitted. This occurs under light-load conditions, i.e. high container temperature and low ambient conditions and there is little the operator can do to alleviate it, except for possibly restricting the condenser cooling to give the compressor more work to do.

The simplest controller used in brine (or chilled water) systems is the direct-acting type, where the temperature-sensing phial is liquid-filled and connected by capillary tubing to the head of a diaphragm valve in the brine delivery pipe. These controllers are found in air conditioning installations, but not used for cargo storage, as the alteration from one set-point to another cannot be carried out quickly and accurately.

A more sophisticated controller is the all-pneumatic controller with a compressed air operated valve in the brine pipe. The controller is mounted outside the battery room and receives a signal from a mercury-filled bulb in the air delivery stream, via a capillary tube and bellows.

The controller produces an output signal at varying pressures to maintain the brine valve in a "floating" or modulating, partially-open position. Slight adjustments to the set-point may be required to maintain the same air delivery temperature under ambient conditions as the capillary is affected by ambient temperature.

To overcome the limitations of capillary length, pneumatic controllers are available where the temperature-sensing bulb is mounted in the air stream, and integral with it is the pneumatic controller whose output is used to control the brine valve.

The pneumatic controller has its set-point adjusted by a pressure signal, which is supplied by a signal line from a central control station in the engine room. This system also provides modulating control.

Combined electronic-pneumatic systems are also used; here the sensing element is a resistance thermometer, and the primary controller is a Wheatstone bridge network arranged so that the output signal operates a solenoid valve (on/off) in a compressed air line. This compressed air line then controls a pneumatically-operated brine valve - but the mode of action is on/off and not modulating. It is essential that all the electric components of this system are completely watertight, as they are liable to soaking by condensation. Whatever type of controller is used, it is important that the brine valves are correctly set, and the spring tension is adjusted to facilitate fully-closed and fully-opened positions at the designed air pressures. If this is not so, there is a possibility of valves passing on brine when they are nominally closed. Brine valves are normally mounted within cooler rooms, where they are not subject to undue condensation.

However, if they are mounted outside cooler rooms, ice formation on the stems can be troublesome; the valve stem should be coated with low temperature grease. Engineers onboard the ship may be faced with a dilemma when the air delivery temperature as recorded on the recorder or data logger differs from that set on the controller. On such occasions it is not clear as to which is the true temperature.

It is recommended that recorders and data loggers should be calibrated regularly and they should be relied upon for accuracy. Where scale adjustment is provided on controllers, this should be used to calibrate the scale against the recorder or data logger.