AIR CONDITIONING OF SMALL BUILDINGS
Air conditioning systems generally have common basic elements; however, they may differ dramatically in physical appearance and arrangement. Even with the same elements present, the manner in which systems are controlled and operated may also be quite different.
To cool a building, a refrigeration made must pump heat from the variant rooms of a building into a heat sink. The heat sink is usually the outdoor air but can also be a body of water or even the ground. Cooling systems vary mostly by the way heat is transferred from the rooms to the refrigeration machine and from there to the heat sink. The choice of the heat transfer methods depends on building type and size. The four major categories ( based on how heat is transferred from building spaces to refrigeration machine) are direct refrigerant, all-air, all-water and combination air-water.
Direct refrigerant systems
The direct refrigerant system is the simplest because it consists of little more than the basic refrigeration machine plus two fans. The indoor air is blown directly over the condenser coil. Direct refrigerant units are appropriate for cooling small to medium size spaces that require their own separate mechanical units.
In an all-air system, air is blown across the cold evaporator coil and then delivered by ducts to the rooms that require cooling. Air systems can effectively ventilate, filter and dehumidify air. The main disadvantage lies in the bulky duct-work that is required.
In an all-water system, the water is chilled by the evaporator coil and then delivered to fan-coil units in each space. Although the piping in the building takes up very little space, the fan-coil units in each room do require some space.
Combination Air-Water Systems
An air-water system is a combination of the above mentioned air and water systems. The bulk of the cooling is handled by the water and fan-coil units, while a small air system completes the cooling and also ventilates, dehumidifies and filters the air. Since most of the cooling is accomplished by the water system, the air ducts can be quite small.
AIR CONDITIONING OF SMALL BUILDINGS
In smaller buildings, the heat given off by a refrigeration machine is usually dumped into the atmosphere by blowing outdoor air over the condenser coil. Medium sized building often use a specialized piece of equipment called evaporative condenser to dump heat into the atmosphere by evaporating water. Since the refrigeration lines are limited in length, an evaporative condenser cannot be more than about 60ft from the compressor and evaporator coil. Thus, for large buildings, cooling towers are frequently a better choice.
ROOM AIR CONDITIONERS
For air conditioning single spaces like motel rooms, a window unit is often used. Each of these units essentially consists of a compressive refrigeration machine. The condenser coil, compressor and one fan are on the exterior side of an internal partition. The compressor is on the outside because it is the noisiest part of the equipment. On the interior side of the partition there is the evaporator coil and a fan to blow air over it. The evaporator typically operates below the dew-point temperature of the room air for the purpose of dehumidification, so condensate forms on the coil. It drains to a pan beneath the coil . The capacity of these units may range from about 4000Btu/h (1/3 ton) to 24,000Btu/h (2 tons).
Packaged units are pre-engineered self-contained units where most of the mechanical equipment is assembled at the factory. Consequently, they offer low installation, operating and maintenance costs. Usually small buildings are served by one package while larger single-storey buildings get several. Rooftop versions are the most common with each unit serving a separate zone. Packaged units are sometimes also used on the ground, for buildings with crawl spaces or above suspended ceiling when there is enough space below the roof.
Most homes and some other buildings find the split unit to be most appropriate. In the split unit, the compressor and condenser coils are outdoors while the air handling unit with the evaporator coil are indoors. As in all cooling systems, condensation from the evaporator coil must be drained away.