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HEPA Filters

Filter Change Criteria

There are 3 basic reasons to replace HEPA filters:

  1. The filter is too clogged to meet the required air flow for the system.
  2. The filter fails a required filtration test.
  3. To reduce operating costs.

The first reason is a fairly extreme example and the filter would have to be extremely dirty or the system designed on the edge of its capacity for this to be the reason to change a filter. Not just neglect but changing requirements in an area can trigger a filter change.

The second reason is fairly self-explanatory but there are some misconceptions about filtration. The filters actually get better at trapping particles as they get dirty. This means that filtration efficiency is not generally a consideration in replacing a filter. However, it can still fail to filter sufficiently if the media or seal fail due to damage.

The third reason is by far the most common and can be approached several ways but you need to consider more than just electricity and filter costs.

Determining Thresholds

The first two reasons are determined by the facility needs so these are normally dictated. We will concentrate on minimizing operating costs since it is the criteria we have the most control over.

A common rule of thumb is once filters are 150% of initial pressure drop it is cost effective to change them, but the reality is that it is very much influenced by system design and fan efficiency. It is a trade-off between capital expenditures to improve initial efficiency and recurring costs of power, down-time, and filters. If you are not fortunate enough to be able to influence the initial design and budget then you have to spend your time on the back end optimizing filter changes.

The ideal is to monitor power consumption/cost and replace at the breaking point where filter replacement gets you sufficient power savings to justify the cost. Many clean rooms the major cost is availability so it is common to have scheduled filter changes every 6 months to a year and then they are all changed together to minimize down time. Even in these situations you may be able to calculate less arbitrary intervals and be able to realize significant savings.

Some facilities and applications are more flexible where monitoring power and replacing the filters at the tipping point is the more cost-effective method. Many systems will not allow you to monitor power directly so criteria such as fan speed can be substituted but it will take a little more effort to find the threshold.

Considerations During System Design

If you are fortunate enough to have influence at the design stages specifying you can create a major reduction in operating costs. Several avenues are by specifying thicker media packs in filters, more (or larger) filter units, and higher efficiency motors. AJ Manufacturing’s Lighted units can allow more filter area in the same ceiling space by eliminating the need for separate lighting. The Criticlean Ultra line of fan filter units has some of the highest efficiency motors available to reduce power consumption.

Reducing Operating Costs of Existing Systems

Using deeper filter media will also extend the time between filter changes and reduce power consumption. We have a “High Flow” filter option for our units that can accommodate more dirt and can be specified to reduce the down time and power consumption of a fan filter unit or HEPA diffuser. They are backward compatible with most AJ Manufacturing HEPA products.

Rebalance the system with minimum damping is another way to reduce operating costs. If the system is dampened at every duct then the supply system is over-worked. Ideally the system should be balanced so that at least one damper is full open and the supply fan run at the minimum power to provide the required CFM. Depending on the variability of the supply fan this is not always possible. Upgrading to an EC motor that can be variably controlled will improve efficiency.

Similar with smaller fan powered devices such as fan filter units. Upgrading to an EC motor unit like the Criticlean series may reduce operating costs sufficiently to justify their expense.

ASHREA has many resources available to help guide engineers through system design and maintenance scheduling as well as provide specific formulas and calculations to use.