Although we don't usually realize it, we can actually reduce the running cost of a building by removing the dust from the atmosphere. The furniture, walls and fittings are preserved longer. It also improves the health of the occupants.
Removing dust is usually done by installing filters in the incoming air stream or the circulating air stream. In air-conditioning systems, these filters are installed at the air handling units or AHU's.
How do we know when to replace the filters?
As more and more dust accumulates at the filters, the differential pressures between the inlet and the outlet will increase. By measuring the differential pressures, we will have a very good indication of how much the filter has been choked by dusts. Persons maintaining the air-conditioning plant should know at what differential a filter should be replaced. Some of the dust particles are so minute, that they cannot be seen by the naked eye. In all appearances, the filter looks as good as new.
These are some factors to consider when determining when to change the filter:
Mechanical strength of the filter
The final pressure of a filter is usually stated in the product. That is the strength of the filtration medium. However, other factors need to be considered - the strength of the filter walls, frames and holders if not adequately designed can result in the sagging of the filter housing that will result in leaks.
Changes in efficiency of the filter
This is particularly important in low efficiency filters. Their efficiencies can be radically impaired as a result of high final pressure losses. Particles that have been collected by the filter can become detached and accompany the air stream. Fibrous particles are less likely to become detached compared to granular particles. Because of this, higher final pressure drops are permissible for the former, although this should not exceed about 200 Pa in a pre-filter.
With granular particles, the final pressure loss should not be allowed to exceed 100 to 150 Pa.
Fine filters and HEPA filters are less sensitive to changes in quality. In principle, they can be allowed to have a final pressure loss up to the limit of their mechanical strength.
Permissible Air flow variations in the system
In this case, the flow pattern is more important than the particles carried along the air stream. The permissible final pressure loss here will depend on the total pressure and characteristics of the fan and any other control facilities.
The filter will affect the air flow as a result of dust accumulation. The filter has to be changed when the velocity of the air in a laminar flow installation cannot be maintained or has fallen below the required specifications.
Sometimes, the filters are replaced at regular intervals instead of pre-defined pressure loss indications. This is to prevent accumulation of bacteria and other microorganisms.
Suitable time window periods
Filters are often changed to coincide with pre-planned plant shutdown periods. In these cases, production disturbances are minimized. For example they can be arranged to be changed during holidays.
In terms of operating costs, there is an optimum final pressure point to balance. By having longer intervals between filter replacements, we do achieve lower filter replacement costs. On the other hand, this will increase the operating energy costs due to inefficiency.
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