Air conditioning systems may be divided into two main classes, the central unit type in which the air is distributed to a group of spaces through ducting, and the self contained type, installed in the space it is to serve.
The central unit type is most widely used, in one or other of a number of alternative systems, characterized by the means provided o meet the varying requirements of each of the spaces being conditioned. The systems in general use are as follows:
- Zone Control System
- Double Duct System
- Reheat System
Zone Control System
This is the most popular because of its simplicity. The accommodation is divided into zones, having different heating requirements. Separate air heaters for each zone are provided at the central unit as shown in the figure below.
- One, Two, or Three zone heaters as required
- Pre-insulated pipes delivering air to zones
- Sound attenuating air terminal, with volume control
- Automatic steam valves, one per zone heater
- Steam trap, one per zone heater
- Multi step cooling thermostat
- Automatic Capacity Control valves
- Thermostatic expansion valve
- Sea water pump
- Fan starter
- Compressor starter
- Sea water pump starter
The regulation of air temperature by individual air quantity control in this system can give rise to difficulties unless special arrangements are made. For instance, a concerted move to reduce the air volume in a number of cabins would cause increased air pressure in the ducts, with a consequent increase in the air flow and possibly in noise level at the other outlets. This can be avoided but the economic factors usually put a limit on this. Some degree of control is possible through maintaining a constant pressure at the central unit, but since most of the variation in pressure drop takes place at the ducts, the effect is very limited. A pressure sensing device some way along each branch duct, controlling a valve at the entry to the branch, strikes a reasonable mean, and is fairly widely applied.
The temperature of the air leaving the heater varies in accordance with the outside temperature prevailing. This can be effectively performed by a self actuating regulator controlled by two thermostat sensors, one in the air leaving the heater, the other outside. Air quantity control in each room served gives individual refinement. In summer, air temperature is controlled by a multi step thermostat in the re-circulating air stream, which governs the automatic capacity control of the refrigerating plant.
“Marine Auxiliary Machinery” by H.D. McGeorge