FIGURE 1: Chiller connection to bas.

In some cases, these control panels have allowed the chiller to share most of this information with the building automation system (bas). Many building operators will appreciate the variety of benefits provided by fully implementing such an integration, and understanding how to use the subsequently shared information.

What Can be Integrated?

For example, Table 1 lists the information available to a bas (from Johnson Controls) from a centrifugal chiller (from McQuay International). The table gives an indication of the amount and type of information available. The actual information available for a specific project depends on the brand of bas, the type and brand of chiller, options installed with the chiller, and the communication interface between the chiller and bas.

making introductions

Chiller and bas manufacturers must work as partners, providing components and sharing information to achieve successful integration of their products.

The chiller manufacturer provides the network to interconnect the chiller system and the chiller network interface, which provides addressing and converts messages on the chiller network to an EIA-232 or EIA-485 format. In addition, the chiller manufacturer releases all of the message formats (its protocols) to the bas company and participates in joint testing, certification, and documentation.

The bas manufacturer is responsible for providing a high-level connection point into the bas, and developing the software necessary to convert messages from the equipment into formats understood by both bas and chiller (Figure 1).

It is important for building operators to know what features to look for in order to achieve successful integration. They should obtain a list of bas companies and chiller manufacturers that have already integrated their products. It is also important to find out what information is exchanged through the integration.

Selecting a chiller and a bas with a proven, off-the-shelf integration already in place will simplify the installation work required. In fact, integrating the chiller and bas can be as easy as following the wiring instructions and parameter settings from a standard, printed manual available from the supplier.

Generally speaking, two standard protocols have emerged: ASHRAE's BACnet and Echelon's LonWorks. These message formats were created through industry agreements and defined by public documents; they have the potential to provide greater connectivity between chiller and bas.

Eventually, they may allow for the elimination of either the chiller or bas network interface, perhaps both. However, standard protocols are still in the earliest stages of development, and the full benefits of their implementation have not yet been realized. For now, it is still easier to select a chiller and bas that have been designed to interface with each other.

Why bother?

• Reduced bas installation cost;
• Enhanced energy management;
• The ability to apply bas management features;
• Building operator training; and
• Single-seat user interface.

Integrating the bas with the chiller plant can substantially reduce the cost of installing the bas because it eliminates duplication of components and wiring. The integration takes full advantage of all the sensors and system control accessories that are pretested and supplied with the chiller.

For example, chillers include factory-installed water temperature sensors, so there is no need for the bas to duplicate these; all the data can be shared through the communication interface. This logical sharing of sensors also eliminates control conflicts that can arise if data is input from two slightly different sensors reading the same temperature point.

Setpoints, Algorithms

For example, using the bas to reset the leaving chilled-water temperature setpoint can be a significant source of energy savings. Frequently, the changing of this setpoint is based on time of day or outside air temperature. However, it is possible, when the bas is integrated with the chiller, to change the chilled-water setpoint to satisfy the load requiring the most cooling. The bas can scan valve positions in the air-handling units and determine the optimal leaving chilled-water setpoint using a specific algorithm (Figure 2). An alert operator can monitor which of the air handlers is consistently calling for the most cooling and can save even more energy by fine-tuning the water balance.

Table 2 shows a comparison of various types of chillers. The COP rating is a rough indication of the full-load energy efficiency of the chillers at nominal operating conditions. To get a more accurate picture, the part-load characteristic of the chiller should also be considered. The second column gives an approximate indication of how much the energy consumed by the chiller will change as the leaving chilled-water temperature is changed by 1 degree C.

For example, a centrifugal chiller operating at 1,055 kW may consume 176.0 kW of power if the leaving chilled-water setpoint is 6 degree C. Consumption will be reduced by 3.3%, to 170.2 kW, if the leaving chilled-water setpoint is reset to 7 degree C while holding all other conditions constant.

Chiller sequencing is another energy management strategy commonly applied to chiller plants. There is a variety of sequencing algorithms used to determine the most energy-efficient combination of chillers to operate. These are based on factors such as:

• Chilled-water return header temperature;
• Plant load (flow x [chilled-water return header temperature - chilled-water supply header temperature]);
• A combination of chilled-water return header temperature and plant load; and
• Chiller amperage draw as a percentage of rated load amps (RLA).

Implementing a chiller sequencing algorithm for the first three methods requires the addition of sensors (flow and/or header temperature) which are not internal to the chiller. The last method can be achieved without additional sensors.

Finally, implementing a building-wide strategy to limit energy demand through the bas may be an effective means of controlling energy costs. This is because some utility companies bill according to demand, often based on average kW consumption over a 15-min interval. As the single-largest energy-consuming piece of equipment in the building, the chiller should be included in this strategy.

The bas has a predictive algorithm that anticipates when future demand will exceed the target maximum demand. It calculates the average consumption over the past 10 min, then predicts the future level of billed demand. If the bas anticipates that the future demand may exceed a predetermined target, it can take corrective action, such as sending a signal to one or more chillers to temporarily reset the amp limit, or turning off the equipment for a short period.

Table 1: Information provided from bas to chiller.

Be All Your BAS Can Be

• Alarm reporting:

The bas continuously compares readings with normal or typical conditions and reports exceptions. These exception reports can be displayed or saved at an operator workstation, or printed. The bas may also alert the operator through one of various paging methods.

When interfaced with the chiller, the alarm management features of the bas can alert the operator to any actual or even potential problems with the equipment. For example, the bas can indicate the possibility of a slow refrigerant leak by detecting a gradual increase in the liquid subcooling temperature. Or, the bas might alert the operator to the possibility of a blocked water strainer on the oil cooler if the temperature is trending upward.

• Historical data:

One of the main functions of the bas is to provide historical data for recordkeeping, and to assist the operator in troubleshooting mechanical system problems. Many bas's allow historical data to be graphed and stored in a format compatible with popular business software. These historical data recording and management features can be easily applied to information from a chiller that is integrated with the bas.

• Maintenance management:

The bas can run a preventive maintenance program that defines and schedules maintenance tasks to prolong the lifespan of major equipment, like chillers. Some of these tasks may be scheduled based simply on run time. Others may be scheduled because of an exception report.

For example, the bas can record the actual run time of the chiller and integrate this information into the building-wide preventive maintenance program. After a predetermined number of hours of operation, the bas will schedule specific routine maintenance operations. The bas may also order maintenance based on detecting an exception. Thus, it can track changes in the condenser approach temperature to alert maintenance personnel when condenser tube cleaning should be scheduled.

• Scheduling:

The bas is often used to control building equipment based on a time schedule, typically to save energy. Because of holidays, overtime, and variable temperature conditions, time schedules need to be constantly adjusted to accommodate the actual needs of the building.

Because of the variety of mechanical systems in a building, this scheduling can be complicated. A bas can provide a single point of entry for all time schedules, to ensure that they are intelligently coordinated. For example, the operating schedule of the chiller is tied to the air-handling units so, with integration, both can be controlled by the bas.

Management reports:

Facility managers use the bas to generate routine monthly operating reports and exception reports, which usually analyze a system failure. The bas can use data from an integrated chiller to provide detailed information on the cooling system, such as energy efficiency and operating schedules, to include with routine reports. For exception reporting involving a chiller shutdown, the bas can provide historical data that may indicate the cause of the failure.

• Building operator training:

While building operators today usually have little trouble using the bas to display information and issue simple commands, they may not completely understand all the building systems. Using bas features such as dynamic graphics, operators can become more familiar with aspects of the chiller and the overall operation of the chiller plant.

Dynamic graphics consist of a static background screen with real-time information or links to other graphics superimposed. The background screens are typically created using one of three methods - scanning a color photograph of the exterior of the building; importing CAD drawings, such as building floor plans; or using a drawing tool and symbol library to create a schematic, such as of an air-handling unit.

The bas may, for example, use the graphics capability to create a detailed diagram of the refrigeration cycle or other functions of an integrated chiller. Actual field values (such as pressure, temperature, and other variables available from the chiller control panel) can be superimposed on a drawing. In addition, the operator's office environment is more conducive to effective discussion and instruction than a noisy, hot plant room.

• Single-seat user interface:

Many brands of chillers offer a PC-based interface to the machine. These often have features similar to a bas, except that they are usually limited to monitoring only the chiller plant. In a building that is not equipped with a bas, this is useful and cost-effective. However, if a PC-based interface is used for the chiller in addition to a bas, the operator faces additional challenges.

Having a PC-based interface and a bas requires an additional PC, duplication of some software functions, and hard-wiring. Also, energy management strategies requiring access to building-wide data cannot be implemented as effectively if the bas is not integrated with the chiller.

Finally, the PC-based user interface employs management features, such as alarm reporting and historical data, in a different format from the bas. This requires the operator to learn and adjust to two different systems to do the same function.

Table 2: Nominal chiller ratings.

Conclusion

While most bas's have become more user-friendly over the past 10 years, the buildings where they are installed are becoming increasingly complex. The more systems within the building that can be integrated to a central bas, the more effectively the building operations can be maintained.

This is particularly true for the hvac system and especially for the chiller, because it is typically the most complex and dynamic system in the building. The chiller is also usually the largest energy-consuming element and the most common source of occupant complaints.

Chillers featuring microprocessor control panels offer a wealth of information which, when fully integrated with the bas, provide significant benefits to building operators. These benefits include enhanced energy savings; improved alarm reporting, maintenance scheduling, and breakdown analysis; enhanced operator training; and reduced installation costs.

Fortunately, these capabilities are not limited to new installations. An existing chiller equipped with a microprocessor control panel can be interfaced at any time. ES