There are no problems; only opportunities. Right? Well, energy prices seem to be on the rise again. While it’s hard to spin that fact into an opportunity for your facilities, growing energy costs make it more important than ever to seize upon EMOs. What are EMOs? They’re opportunities: Energy Management Opportunities, to be more precise.
No matter where your facility is located, your cooling system consumes a large amount of energy each year. Preventive maintenance measures and the right operating techniques can help reduce energy consumption and reduce operating costs, while also extending the life of your cooling system.
Many EMOs for cooling systems can be easily implemented and do not require major equipment retrofits.
Check Thermostat Calibration
If a thermostat is out of calibration, substantial amounts of energy get wasted, and maintaining comfort becomes difficult. Use the following steps to ensure that the thermostat is operating at the designated temperature setting. (If you are unable to conduct a thorough inspection, call the thermostat manufacturer or a local service organization for assistance.)
Check each controlled device. Verify that devices are installed and connected according to the approved shop drawings. Ring out or trace each electrical connection. Pressure test each pneumatic line. Be sure a zone thermostat is connected to the correct damper or valve. Installation mistakes where the zone thermostat is connected to the wrong damper or valve are common.
Adjust setpoints. Also, reset schedules and throttling ranges according to approved shop drawings.
Check temperatures and humidities. While air systems are running, check temperatures and humidities at all control points, including building zones. Compare these readings with setpoints, and observe system response to abnormal conditions. To determine whether variables are under control, change setpoints temporarily and observe the system response.
Check interlocks. Check all interlocks to fan and pump motors and to primary heating and cooling equipment. Even though an electrician may do the electrical work, the building engineer must guarantee that the work is done properly and the equipment operates as designed.
Report. Create a report of all tests, indicating conditions observed, final setpoints, reset schedule, and throttle ranges. All reports should be logged and brought to the attention of the property manager, chief engineer, or building owner on a quarterly basis.
Adjust Thermostat Settings
Adjusting the thermostat setting is a simple way to reduce energy consumption. By maintaining an indoor temperature of 74°F instead of 70°F, you will not seriously affect comfort conditions and can save approximately 5 percent on energy costs. Higher humidity levels permit additional savings, but you should keep humidity at 50 percent or less. Make the temperature changes gradually (1°F every 24 hours) so that occupants can adjust to the changes gradually.
Inspect Cooling Distribution System
Utility companies estimate that up to 30 percent of conditioned air is lost through leaky ducts. Inspect the cooling system’s ductwork, and repair leaks and remove blockages as soon as possible. Check the insulation around ducts and, if necessary, install new insulation.
Clean and Purge to Maintain Minimum Condensing Temperature
Heat transfer capacities are reduced when scale or dust fouls the heat transfer surfaces on evaporators and condensers. Water circulated in evaporative-cooled condensers and cooling towers should be filtered and chemically treated to reduce the formation of scale, algae, and chalky deposits. Dust on air-cooled condensers should be periodically cleaned.
To keep chiller efficiencies high, water-side condenser tubing must be kept clean and refrigerant levels must be maintained. Unfortunately, it is difficult to predict the extent to which fouling will occur during chiller operations. For systems in which little fouling occurs, it is common to clean condenser tubes each winter and chiller tubes once every few years. Where heavier fouling is anticipated, more frequent water treatment and a midseason tube cleaning may be necessary.
Or, Automate the Process
Automatic tube cleaning is another option. One method for automatic tube cleaning uses a cylindrical brush that is periodically forced from one end of the condenser tube bundle to the other by reversing the direction of water flow. The flow is temporarily reversed one or more times per day in order to move the brushes through the tubes. Another method of automatic tube cleaning uses sponge balls that are injected into the intake water piping and collected from the discharge water piping. In this method, distribution of the balls throughout the system is random but continuous, resulting in clean tubes and low fouling factors.
Retrofit Cooling Equipment and Change Operating Conditions
The cooling load is reduced when all applicable means of reducing thermal loads and cooling losses are applied to your building. Window shading, glazing, and insulation are three common load reducers. Reduced cooling loads may permit you to retrofit compressors and motors on chilled water equipment so that they can be smaller and more efficient at the new operating loads. Lowering condenser temperature, reducing head pressure and/or coolant temperature, and keeping discharge water temperature high are additional ways to modify operating conditions to keep energy costs down.
Lower Condenser Temperature
Whenever possible, lower your condenser temperature. The chiller’s efficiency is increased by about 6 percent for every 5 degrees the condenser temperature is lowered. Before you make any changes to your system, however, consult the manufacturer to ensure that your warranty will not become void.
Reduce Head Pressure and/or Coolant Temperature
Lowering the condensing pressure/temperature (in vapor-compression refrigerant cycles) or coolant temperature (in absorption cycles) as temperatures and humidity fall can help you save energy and money. Exact savings amounts will depend on your climate and geographic location. Be sure to check with the manufacturer before making any adjustments.
Keep Discharge Water Temperature High
Unnecessarily low chilled-water temperatures waste energy. Raising the chilled-water discharge temperature raises operating temperatures and reduces the chiller workload. About 1 to 1.25 percent of a chiller’s power requirements can be saved for every degree that chilled-water temperatures are raised. Chilled-water temperatures should be set to meet the requirements of the hottest zone. Before any changes are made to the chiller, however, consult the manufacturer. Keep in mind that raising chilled-water supply temperatures may affect the operating costs of large VAV air handling systems. Increases in supply air volumes result from the raising of chilled-water supply temperatures.
A number of techniques may be applied to control the discharge water temperature. In order to take full advantage of sensible heat load reductions, reset the discharge water temperature automatically based on outdoor air temperatures. It is advisable, though, to equip the system with a manual, built-in override capability that adjusts the discharge water temperature to meet space comfort requirements.
Scheduling can originate from a sensor that monitors a system’s part-load condition. For example, the position of a cooling coil’s valve can be used to determine the reduction in coil load. Also, the leaving water temperature from the coil can be used to determine the reduction in coil load. In both cases, the reset of the chiller leaving water temperature may be overridden by the high limit of the room’s humidistat.
Shut Down Cooling Systems during Unoccupied Periods
Only a small portion of a building actually requires 24-hour operation. Substantial energy savings can be obtained by turning off or reducing the operating levels of cooling systems during nights, weekends, and other periods of low occupancy. Determine low-occupancy periods in your building and develop a shutdown schedule to stop unnecessary fans and motors. Remember, however, that long periods of shutdown can cause high energy consumption at start-up.
Add an Economizer Cycle
Incorporate an economizer cycle into air handling units that use an outdoor air intake. The economizer cycle provides cooling by circulating outdoor air when outdoor air temperature and humidity are favorable for free cooling.
Add Winter Cooling Capabilities
For buildings in which it is impractical or impossible to use an economizer cycle, other cooling techniques may provide a viable energy management alternative.
A patented technique called Thermocycle has been used for years. Condenser water is circulated between the cooling tower and the chiller. Refrigerant in the chiller is pumped between the condenser and the evaporator without operating the chiller’s compressor. This allows about 10 to 20 percent of the chiller’s rated capacity to be available for cooling chilled water—an amount usually sufficient for winter cooling requirements.
Another technique, Strainercycle, is used for winter, spring, and fall cooling. When the ambient temperature is cold enough, the cooling tower produces condenser water at 45 to 50°F. The condenser water is treated and strained before being pumped directly into the chilled water system. The long-term effects of clogging and corrosion are not known, but the energy cost savings are substantial enough to more than cover any repairs the chilled water system might need.
A simple plate and frame heat exchanger operated as a water-side free-cooler can achieve results similar to those achieved by the Thermocycle and Strainercycle devices. This heat exchanger completely isolates the condenser water from the chilled water side and eliminates the issues associated with fouling of the chiller tubes. All water is diverted around the chiller through isolation valves. From an EMO perspective, the free-cooler option produces the best economy without increasing maintenance requirements.
Operate Multiple Chillers Efficiently
Chillers operate most efficiently between 70 and 90 percent of their full load. It is more efficient to operate one chiller between 70 and 90 percent of its full load than to operate two or more chillers below 70 percent. Although chiller loading can be performed manually, optimization can only be achieved through a computerized control application.
More Energy Management Opportunities
Applying these EMOs to your facility should help increase the performance of your HVAC systems and help you realize a greater return on investment in your equipment. Basic maintenance and common sense go a long way in the proper management of energy. By no means, however, are these your only opportunities. Facility managers need to be resourceful and creative in order to maximize building performance. By applying these traits, you will be able to identify unique opportunities for your buildings.
More Information
This article is based on content from BOMI International’s Energy Management & Controls course. This course is now available as an online, instructor-led course using the BOMI eCampus. Please visit www.bomi.org for details, or go to http://www.bomi.org/CourseOffering.aspx?id=6718 to enroll now.