Pressurization

Keeping a room pressurized with conditioned air can eliminate the problems associate with un-controlled air infiltration. A properly designed system can provide the correct amount of air, conditioned to the requirements of the environment to keep out the undesired particles and reduce the moisture coming into the space.

How much air is needed? For keeping a room pressurized to 0.005” water gauge, any opening in the room will need to have an airflow flow of about 33 feet per minute. This velocity will keep the airborne particles from entering the space. A measurement of all the free openings in a room times 33 feet per minute will give the amount of air needed to pressurize a space.

Many times, measuring all the cracks and openings is not very practical. In these cases, a rule of thumb of 0.33 to 0.50 cfm per square foot of floor area will give a fast calculation.

Conditioning

Total conditioning of the air to the proper conditions is also important. Conditioning implies the total condition of the air, for the proper temperature, humidity and filtration.

The system used to bring in conditioned fresh air should have adequate capacity to cool the air from design summer conditions to the desired room temperature. If additional cooling is desired, the system will need to discharge below the room temperature.

If the system is located in where the winter design temperature is below freezing, the unit must have heat or be shut down during the times the temperature is below freezing to prevent possible damage to water lines, etc in the space.

Filtration

Filtration is a big part of total air conditioning. Since the air supplied from the pressurization unit is bringing air into the critical space from outside or a non-critical area, it will need to be filtered to a level that ensures product quality.

Most units will have commercial grade 30% filters as the first level of filtration in the unit. These filters remove the larger particles and help extend the life of the more costly final filters. They are designed to be changed monthly.

The secondary filters in units will be either 95% ASHRAE type or HEPA filtration. The 95% filters are 95% efficient on a 1 micron particle, removing essentially 100% of yeasts molds and bacteria.

HEPA filters are 99.97% efficient on a 0.3 micron particle. Each filter is tested for efficiency. HEPA filters will provide essentially sterile air.

Since the unit is supplying air to a sanitary, hygienic area, the unit must be of sanitary design. Double wall construction, drain pans throughout and access to the unit interior are a few of the features that ensure the unit can be washed and sanitized.