We are building a large semiconductor manufacturing plant and attached central utility plant. Semiconductor manufacturing facilities have specific process requirements that affect the HVAC energy modeling. The chiller plant has a set of 40F chillers for dehumidification and a set of 54F chillers for sensible cooling. The 54F CHW chillers are all heat recovery chillers. For sensible cooling of the clean room manufacturing space, the facility utilizes re-circulating fan filter units (FFUs) with sensible coils in the air stream. The coils supply air at 65F-70F and use CHW at 54F. The recirculation is required to continually move air through the HEPA filters for room cleanliness. Outside air, to replace process exhaust, is about 25% of the building recirculating air volume. OA is supplied through a pre-conditioning system. Because of limitations on mixed air temperature, and requirements for the manufacturing, the outside air must be dehumidified to a 47F dewpoint, and then reheated to 65F-70F, before mixing with the fan-coil return air. When humidification is required, a high pressure spray atomizing grid is used, avoiding the need for steam. Two cooling coils are used; the upstream one supplied with 54F CHW, and the downstream one supplied with 40F CHW. During warm conditions, some of the reheat can be provided by a "run-around" coil set with a complementary pre-cooling coil. The ASHRAE ECB method is difficult to apply for this building. A strict reading would indicate the OA pre-conditioning unit is a component not defined by section 11.6, to be modeled the same in the budget model as in the proposed model. Also the requirement for 20F differential between the supply and zone temperature would require the fan-coil airflow be reduced by 65%-75%, and would not produce a practical system for this application. We propose the following approach: 1. The budget heating system will not include chiller heat recovery to the HW loop. 2. The budget CHW loop would either be a single loop at 40F, as is typical in these applications, or the dual loops as designed. A CHW loop with the budget defined 44F supply CHW would not be able to dehumidify the OA, would not then maintain the same space conditions between the models, and would unfairly penalize the proposed design. 3. We would propose modeling the budget fan coils with the same airflow as in the proposed design, as a system with a design temperature of 55F would not provide proper space conditions. 4. The addition of the run-around coil to the OA-preconditioning system is non-standard and an additional cost so we would take that out of the budget building model. The savings in reheat and cooling from this system would be calculated with an exceptional calculation, as this component is not included in the DOE2.2 simulation program. Please let us know if this approach is acceptable for our energy model.
As noted in your narrative, some of the energy efficiency measures incorporated into this type of project are not easily modeled with the ECB modeling protocol of ASHRAE 90.1-1999. Your proposed alternative modeling methodology is a reasonable approach in dealing with these measures, and thus acceptable. You might also consider separating the comfort conditioning systems and the process control systems. The narrative states that the dehumidification by cooling and reheat is a specific process requirement. If it is possible for the team to discretely model these two systems separately, the process energy can then be deducted from the savings calculations. Applicable internationally.
Related Addenda (Corrections & Interpretations)