The key component of air conditioning is air distribution _ as the air picks up the heat to keep the area /space cool by circulating the air within the room after passing over the evaporator coil _ where it gets cooled. Fresh air from outside is added to the system. We will see each component of the air distribution that affects energy consumption.
Purpose of fresh air
The fresh air needs to be regularly added in to the room or space through the air handling unit or indoor unit in small systems. The purpose of the fresh air is to dilute the carbon dioxide build up within the room / space. This aspect is very important for comfort of people in the air conditioned zone.
The carbon dioxide level within the room /space has to be less than 1000 ppm. Desirable level is at less than 800 ppm of carbon dioxide or 500 ppm plus what is in fresh air. In most of the green buildings designed, they have demand controlled ventilation i.e., taking fresh air through filter based on sensing through carbon dioxide sensors _ and the damper will open the moment the carbon dioxide level is more than 500 ppm. By having demand controlled ventilation, the fresh air is taken only based on demand. Thus, energy is saved as frequent cooling of fresh air to the room (inside) temperature needs a good amount of energy.
There is no provision of fresh air in the modern day window air conditioners or split air conditioners, and hence lots of people complain of headache, which is due to build-up of carbon dioxide level. It may not be out of place to bring it to your notice _ that the research carried out at Singapore University says that two adults and a child gets in to the bedroom at 10 pm with 500 ppm carbon dioxide level and the level of carbon dioxide at 6 am morning is 3000 ppm. The above experiment is with a window air conditioner.
Air handling unit room space
The air handling unit /indoor unit have to be a free space all around for maintenance and connecting the refrigerant /chilled water piping with valves and also for cleaning the pre filters at regular intervals.
The air haling unit room /indoor unit is part of the air conditioned space and has to be kept clean. We have seen at many places, the air handling unit rooms are dumped with all sorts of materials blocking the filters. Even cleaning the filters become almost impossible.
Filter maintenance
Filters in the indoor units of smaller units are of synthetic materials, and they are mostly used for preventing the cooling coil getting choked.
When the filters are choked, the air quantity across the cooling coil is less _ consequently consuming more energy. Hence, the filters need to be removed once a month and cleaned with detergent or water jet.
In air handling units of central plants, the filter can be cleaned regularly (say) every fifteen days, and when the pressure drop across the filter is high, it requires replacement.
If replacement is not done, the pressure across the filter is more _ consequently increasing energy consumed as the static pressure increases and the fan will consume more energy.
Evaporator cooling coil /cooling coil in indoor /air handling units
As already discussed above, the filters protect the cooling coil, and sometimes for lack of manpower the cleaning is subcontracted. Obviously, if it is not done properly, then the fine dust particles pass through filters and get stuck to the cooling coil _ as there is moisture on the coil. Due to the above process, the air handling units are blocked consuming more energy.
To overcome the above problem, the indoor units or smaller units need to be completely cleaned with water jet annually _ preferably before the summer sets in.
For the air handling units, it is suggested that the cooling coil cleaning is done at least twice annually or more times depending on the cooling coil’s condition.
Fans
Fans shall be cleaned for both indoor unit and air handling unit at least once in a year. As the fans are struck with dusts, they will be making noise both in indoor unit of splits as well as the duct cable units and also in air handling units. Power across the fan motor should be measured, when it is new and kept in record. If you can compare it with power measured before cleaning the filters, you will find the amount of extra energy being consumed.
Air distribution system
Air from fan outlets is taken into the room or space to be air conditioned _ and taken back to the air handling room for the process of cooling and dehumidifying the air. The entire air is distributed to the space through ducts with ceiling or wall outlets depending on the type of arrangement. We have observed in many five star hotel lobbies _ they have white air distribution terminal _ (say) continuous grill completely coated with black soot. This is mainly because of not cleaning the filters. Air distribution is important for any AC system through the ceiling /wall terminal outlets – as it is the only visible component to the occupants /visitors. The occupants only will tell if it is not cooling enough or over cooling. It may not be possible to satisfy all the occupants. Hence, ASHRAE standard says as long as 80% of occupants are satisfied the comfort level is good.
Duct cleaning frequency
The frequency of duct cleaning shall be at least once in five years for hospitals, hotels etc., elsewhere once the dust accumulates or once in every 10 years. Nowadays, lots of modern gadgets are available for duct cleaning. The dusts in the ducts is carried to conditioned space, hence, we have pre filters in the air handling units as well as indoor units although they are very much parts of the air conditioned area /space.
