Can someone give me some guidance and resources on how to design a positive pressure ventilation system (not airconditioning) for an electrical room.  Things I'm looking for:

1.  How much pressure is required to successfully keep out dust?
2.  Types of filters available and allowable velocities through the filters.
3.  Are inlet and outlet fans needed or can a fan blowing into the room alone be sufficient?

Thanks.

Your design parameters are covered in ASHRAE Standard 62-2001, "Ventilation for Acceptable Indoor Air Quality."
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Ventilation Rate Procedure (Section 6.1) presents "...one way to achieve acceptable indoor air quality", namely, supplying each occupied space with ventilation air of specific quality and quantity. Outdoor air is used to dilute anticipated contaminants to acceptable concentration levels.

In contrast, the Indoor Air Quality Procedure (6.2) presents "...an alternative performance method...for achieving acceptable air quality" by quantitatively describing acceptable indoor air quality. It sets limits on the concentration of known and specifiable contaminants in an effort to achieve acceptable indoor air quality in the occupied zone in a more direct way that the Ventilation Rate Procedure. More specifically, the Indoor Air Quality Procedure sets concentration limits for 10 contaminants, prescribes subjective analysis to determine acceptable odor levels, and describes the use of treated recirculated air to reduce the minimum outdoor airflow rates presented in the standard

Your internal pressure must over come external wind pressure.

Inlet fans can be used for pressurized rooms

Also refer too NFPA 90A
http://hfpa.otsg.amedd.army.mil/refs/1191/MH1191_20020702_08_HVAC.pdf