The central plant of a large facility is where the chiller, boiler, cooling tower, and pumps are located. The total load of all the buildings conditioned by the equipment should be used to size the capacity of the central plant. The total load is rarely used for the sizing, however, because each air handler or fan coil unit is sized for the local peak load. The peak load will never occur at the same time at all zones of a large facility simultaneously. Spaces with different exposures, space loads, and roof or floor exposure will peak at different times of the day, and usually on different days of the year as well.
Also, consider that an air handler rarely operates at peak design conditions. This is when all the lights are on, all the equipment in the space is operating at maximum capacity, and the maximum number of people are occupying the space. Further, all of this will not occur that one afternoon in late summer when the solar insolation is maximum.
These factors combine to reduce the maximum loading on the chiller and boiler. This effect is accounted for by the diversity factor in the HVAC calculations. The total load should be multiplied by a factor from 0.6 to 0.9 to generate the heating and cooling load used for sizing the central plant equipment.
Every type of occupancy will have different factors to consider, in selecting a diversity factor.
A facility with little exterior exposure and high interior loads, such as a favorite movie house in a mall, will have a high diversity factor, because of the high people load. Schools and colleges also operate closer to the peak load. A tall glass-faced office building, on the other hand, has exterior zones that account for a good portion of the peak load. Different sides of the building will peak at different times of the day, and this peak may occur after the close of business when the relatively constant interior load drops. The diversity factor for such a facility will be lower than for the theater-school category.
There are other important considerations to be made in determining the capacity of the central plant equipment. If the equipment serves many remote sites, such as separate buildings on a college campus, there will be more transmission losses—heat transferred to or from the distribution piping—than for a single building served from a basement-level facility. The same principle applies to secondary systems, for example, at each node on the large college campus loop. These are constant losses that should not be included in the diversity factor calculation.
The designer must also make considerations for the long term. The HVAC system will remain in operation for perhaps 20 years. During that time, air-handler motors will wear, heat-transfer surfaces will suffer diminished capacity, pipes will experience scale buildup, and the cooling-tower fill will become clogged. Controls will edge out of adjustment, damper louvers will seal less effectively, boiler burners will become clogged, and door and window seals will become worn.
All of these things will make extra demands on the system, and the central station equipment (which will itself suffer reduced capacity) must have the capability to meet that demand.
Another consideration is the expected professional oversight of the system. A large college campus may have a competent, vigilant staff to maintain the equipment; a small office building may do little more than answer trouble calls. A system with full EMS controls will guide the management team to potential trouble and keep them on an optimum maintenance schedule. It will stray out of adjustment rarely.
A final consideration for the designer is the future use of the building served by the central plant. If the owner anticipates future expansion, this may be built into the central plant design, such as allowing extra space for equipment and installing fittings to incorporate an additional boiler or chiller. In addition, if the facility is beginning a program to install computers in many offices or classrooms, it would be prudent to reserve some capacity for this; although, if the program changes are in the distant future, reserving capacity will cause the equipment to run at inefficient part-load conditions, in which case it would be prudent to provide space and fittings for an additional chiller or boiler. Such decisions as these must be made in close cooperation with the building administrators and engineering staff.
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Excerpt from Energy Conservation Projects.