An installation with several chillers or boilers can benefit from staging the equipment on-line, matching the load to the capacity of the equipment. A manual operation can use a small boiler, for example, to handle the minimal summer water-heating load on a boiler, instead of operating a large unit at an inefficient low-load status.
Computerized controls are available to automate this process, and to otherwise optimize the use of the equipment for most effcient operation, and a longer useful lifetime of the chillers and/or boilers. A full-scale energy-management system is not required, as all the functions necessary to control four or more units are available in a small, compact cabinet. The heart of this modulation system is the automatic sequencing of unit stages, for boilers or chillers. Capacity is added or subtracted one stage at a time. The system has the capability to control the setpoints, starting the next stage before the present one reaches full capacity. An elaboration on this modulation is a microprocessor that keeps track of the rate of change of temperature or pressure. This enables it to anticipate the system, staging units on or off as required to match the building load as it changes. This minimizes energy wasted from over- or under-shooting the setpoint, which can be significant in a climate with large daily temperature swings or in buildings with an extreme fluctuation in daily loads. This smooth modulation of loading on the boilers or chillers also minimizes short cycling, which is trying on the equipment and wasteful of energy.
Boiler control systems have other features peculiar to the heating mode. Outside temperature is measured, and the heating system is prevented from operating unless the temperature drops below the setpoint. This avoids a common problem with space-heating systems, which often continue on standby—using energy all the while—when the temperature is warm. This is not always just a seasonal variation, but a daily one too in the heat of summer. Only sensitive automated controls can take advantage of these regular daily temperature variations.
A final characteristic is useful in saving energy of both chiller and boiler systems. A number of space-temperature settings are keyed into the memory, to vary temperature setpoint according to a 7-day, 24-hour schedule. For example, early shutdown begins the “night” temperature an hour or more before the close of business, permitting the building mass to maintain comfortable conditions until the building is vacated. If the outside temperature is mild, the controller can shut down the system even earlier, or establish a more moderate space temperature for further energy savings.
This system can perform the same function for early morning warmup, or cooldown as the case may be. The system varies the starting time and cycle duration according to the outside temperature and the inside setpoint temperature for the optimum energy usage to attain comfortable conditions during the regular business hours.
__________
Excerpt from Energy Conservation Projects.