A refrigeration based air handling system design process for significant energy and cost savings in cleanroom and other applications requiring large air change rates is presented. The process utilizes a by pass around the air conditioning system, the ratio of bypass to air conditioning flow being such that minimal or no reheat of the air is required for applications having relative humidity (RH) control requirements and with RH control being achieved via cooling. If dehumidification is achieved by adsorptive processes, then the by pass ratio is varied so as to minimize cooling of the heated dry air. In other non relative humidity control applications the bypass is varied to minimize the air conditioning flow, thereby decreasing cost, but by using optimum cooling coil velocities in a manner such that system energy for airflow is minimized. The energy and cost savings achieved by this process vary between 65% to 15% depending on the Class of the cleanroom and/or on the number of air changes per hour required.