Air-conditioning Systems

What Air-conditioning Means. ..

Air-conditioning means to modify the temperature and humidity of a room in order to achieve a more comfortable living condition. An air-conditioning system has the capacity to take the air of a room, treat and deliver it back in a cool and dehumidified state. An air-conditioner also normally has the capacity to heat the room when it is equipped with either the reverse cycle system or with an electrical heater. An air-conditioner is also supplied with a room temperature controller (thermostat), an on/off switch and a fan-speed controller with multiple speeds.

If the air-conditioner also has the heating function, then the control panel also gives the choice between cooling and heating. This choice can also be automatically made, in such a way that once you set the desired temperature, the air-conditioner control will automatically choose the functioning mode to reach and maintain the set temperature. In this case the air-conditioner becomes an environmental control system.

All types of thermostats are found in air conditioning systems - pneumatic and electrical. By themselves, they are all satisfactory instruments, but the results they achieve are dependent on the correct installation of their sensing elements. Even the site for a direct acting thermostat to control one single-berth cabin must be chosen with care. If it is masked behind curtains, or too far away from the air inlet, control will be too sluggish. The correct location for a thermostat to control a block of cabins is more difficult to find. One can pick on a "typical" cabin - but if the occupant opens his porthole he can upset the whole block. Another possibility is to site the thermostat in the alleyway of the block of cabins. This position may be affected more by an open door or draught in the alleyway than by the temperature of the cabins.

Yet another possibility is to site the thermostat in the re-circulation air trunk, carrying air back from the accommodation to the unit. If the re-circulation grille is close to an outside door, this position too can be affected by outside air temperature when the door is open, rather than by cabin temperature. A large public room, say a lounge, may be impossible to control satisfactorily by one thermostat. If one thermostat is positioned awkwardly, it may sense a temperature higher than the average in the room and cause air to be delivered, which will be too cold for the comfort of those sitting around the edge of the room.

Similarly, a thermostat sited at the edge of the room may leave too high a temperature in the central area. The only satisfactory arrangement for such rooms is to have different controls for different parts of the room. Accurate temperature records for cargo spaces are essential to guide the operating engineer and also as evidence that correct temperatures were maintained in the event of any cargo arriving in less than a perfect condition.

Remote reading thermometers are usually platinum resistance thermometers, with a resistance of about 100Q at 0°C (known as the PT-100 type), whose bulbs are rubber-covered and vulcanised to the cable to ensure complete water-tightness.

The calibration of this type of thermometer can also be checked in melting ice. The earliest installations of this type were connected via a selector switch in the engine room, to an indicator in which manual adjustment of a Wheatstone bridge circuit was made to ensure zero deflection on a sensitive galvanometer. Great care was taken to prevent accidental circuit resistance, all terminal boxes had joints screwed and soldered and the overall accuracy was +0.1°F. This manual balancing has now been replaced in modern installations with self-balancing electronic indicators, strip chart recorders, or data loggers.

The recorders and data loggers used, have sometimes been general-purpose instruments and not engineered with the accuracy required for refrigerated cargo in mind. Tolerances have increased to + 0.1°C and even + 0.2°C. Yearly checks on thermometers in melting ice are advisable, and if the thermometer bulb or probe is not fully watertight, a rubber or plastic covering before testing should protect it.

The refrigerated cargoes that demand the most precise control of temperature are those carried at about 0°C, and great precision is required to be sure that the cargo is not accidentally frozen. Accordingly, testing thermometers in melting ice check the calibration at the point (in the range) where accuracy is most required.

Thermistors in place of resistance thermometers are gaining acceptance as reliable thermometers. The sensor has a resistance of thousands of ohms and stray circuit resistances become relatively less important since they are swamped out.

Types of air-conditioners

There are three types of air-conditioners:

1) Independent direct expansion units as self-contained or two-part units which are used to air-condition one or two rooms close to one another

The air-conditioner treats the room air and delivers it back directly to the room, through air ducting with sizes from 75 to 175 mm which avoids making complicated and long distribution systems. An interesting version of the independent unit is the split model which is built in two parts: a compressor assembly and a separate evaporator/fan assembly which can be installed several metres apart from the compressor, saving cabin space and permitting the air-conditioning installation in boats where there is no space for both components in one piece. The temperature control is made by stopping and running the compressor and also by controlling the fan speed.

2) Central units with direct expansion circuit to several evaporators (fan coils)

These are very common units used in land installations where they can be called 'multi-split'. In marine applications there are some installations made with this configuration where one (large) compressor supplies several evaporators. Unfortunately this simplified configuration makes the system rigid and it tends to become unbalanced, particularly when the thermal load is reduced at night and with the large compressor balanced for the high load of the day, it becomes far in excess of the reduced night load. This situation can cause an increase in the fan coil noise into the cabins. In addition to this, the piping for the refrigerant connection to each fan coil could become a weak point if it is not correctly designed and installed, as any leak will stop the entire system and the repair could be a real hassle. With this type of system it is not possible to connect several compressors in parallel on the same circuit; each compressor must have its independent circuit connected to its evaporators. The temperature of each room is controlled by stopping the fan of that room or by stopping the refrigerant flow to that fan coil. In either case again the system becomes unbalanced if not properly designed as the compressor capacity is still the same while the fan coil load is reduced.

3) Central systems with chilled (or heated) water distribution to several fan coils each installed in the room to be air-conditioned.

In this case the central system, which can be made with one or more compressors, cools (or heats) the water of a closed water circuit which is pumped to each fan coil. This type of unit has several advantages:

The distribution system of the chilled (or heated) water has the same characteristics as a heating circuit but instead of a boiler there is one or more chiller compressors and at the place of the radiator in each room, there are the fan coils. Each fan coil is completely independent of the central unit, which is set to keep the fresh water circuit temperature at a preset value (normally +12°C in summer mode and +40°C in winter mode); all the fan coils are connected in parallel to the fresh water circuit and the room temperature.