Continuous maintenance for college campus.
Through much of the 150-year-old University of Maryland's growth, factors other than operations service and maintenance drove design consideration. Chiller plant systems were started up, handed over to the owner, accepted and turned over to the operations staff. Most of the time, commissioning was not performed. An adequate operations training program also was
not part of the validation process, so as the university grew, systems degraded over time from the original efficiency and/or capacity and equipment failed.
This article discusses the general processes of planning, design, installation and ownership transfer, including important points about maintenance and application.
During the initial phase of development, sustainability must be considered, including environmental impacts, energy consumption, carbon emissions, O&M requirements and the intended life expectancy. These are all factors that need to be evaluated to form the project budget for design and overall life cycle.
After the supporting infrastructure is defined, the project moves into design. At the completion of the planning phase, the owner is presented with the conceptual program narrative; a life-cycle cost analysis with recommended best value; and an estimated budget. The author recommends using the 2008 ASHRAE
Handbook--HVAC Systems and Equipment, Chapters 1 and 3, for developing design selection criteria during the initial design development.
[FIGURE 1 OMITTED]
In the design phase an architect's team of engineers begins to develop the concept. The engineer needs to know what the owner wants for a chiller plant (Figure 1 shows a primary variable flow system). Does the owner prefer a central plant with primary variable flow or primary-secondary distribution? Each system could have first cost advantages or life-cycle advantages. Control systems can be designed as pneumatic, direct digital control (DDC) or a combination. Engineers need to know if the owner desires remote monitoring through a DDC overlay, wireless access monitoring and controlling energy consumption. Ease of maintenance is another consideration that needs owner involvement.
Photo 1 shows an example of how having owner input reduced the cost of building a central plant and provided adequate space for chiller tube removal. Regardless of whether the owner intends to provide in-house staff for maintenance or outsource maintenance, the equipment must be serviceable.
At the completion of the design phase, the A&E team and owner should have reached agreement on a ready-to-bid/install set of specifications and contract drawings and a project budget. Within the specification, a commissioning (Cx) plan is developed with the A&E Cx's firm, or the owner's Cx representative. The building path from installation to turnover shows the contractor(s) the project expectations. This is the time to address any questions regarding the ability of the project to meet the needs of the building program and provide for sustainable, extended performance to meet the owner's needs.
Contractors installing equipment are required to follow through on completion of the design documents and specifications. However, the means and methods used to install what makes up the contract documents are not dictated by the engineer. The contractor must meet the intent of the design, but engineers routinely note that the contract documents are diagrammatic only. Final field coordination is left to the builder.
Installing for ease of service and maintenance is partly influenced by how the contractor works with the engineer's design to install the equipment. For instance, service isolation valves can be installed in a manner where they cannot be accessed. Other common examples are chiller end covers installed without regard for removal, and pumps installed next to equipment control panels. O&M staff needs access to clean chiller condenser and evaporator tubes so as to maintain heat transfer efficiency. Also, service diagnostics and controls maintenance need to be performed without compromising National Electric Code and with reasonable access to panels. Contractors may install components exactly as the engineer laid out the equipment, even if the design is incorrect. Or, contractors sometimes can make field decisions to change the layout to allow for service and maintenance.
During the installation phase the commissioning agent works with the contractor and A&E on behalf of the owner to interpret drawings and specifications to ensure compliance with design intent, especially where installation means and methods are left to interpretation. During this installation phase, the Cx authority develops functional operation tests (FPT) based on the design intent with the approved submissions of equipment and control systems. This is the time to develop a training program of the owner's operations staff. As the project approaches completion, these functional performance start-up plans (previously agreed to by the complete project team) are implemented, providing a smooth transition from installation to acceptance/ownership. Owner/operator training is presented to the O&M staff, and O&M maintenance manuals for all systems are presented in deliverable format with documents identifying warranty contacts and information.
Where the concept began is where it ends, owners initially had the responsibility to convey design criteria to the A&E team, validate these criteria during the design, and accepts the project as built from the installing contractor when this criterion has been met. Once accepted, to ensure the building operates sustainably, the owner must keep the equipment operating efficiently by performing operations and maintenance as instructed. Performance of required maintenance is as much a part of sustainability as the design. Next, we'll focus on maintaining a chiller plant in a sustainable manner.
Maintaining the Chiller
The chiller is possibly the single largest energy user within the plant, performing daily, monthly and annual maintenance is crucial to ensure dependable and efficient operation. Operators should regularly log system pressures of water and refrigerant and oil. Temperatures should be recorded including refrigerant and water. Motor operation by measurement of voltage and amperage can provide a gauge to the system capacity. With this information an operator can develop a trend of performance.
For instance, measurement of the leaving condenser water temperature subtracted from the condensed liquid temperature shows the small temperature difference or heat transfer across the condenser. Generally, following initial operation, a difference in measurement of 2[degrees]F to 3[degrees]F (1[degrees]C to 1.6[degrees]C) is normal for fouling of the condenser tubes. If this temperature difference increases, less heat from the chiller is transferred to the condenser water rejected to the cooling tower. The greater the difference, the less efficient the chiller operates.
From an operator's standpoint, a 4[degrees]F to 6[degrees]F (2[degrees]C to 3[degrees]C) difference means the chiller is probably surging as load increases. This surging is caused by condenser pressure building up to a point where the pressure backs up against the centrifugal compressor, causing the centrifugal wheel to slip or go toward a zero equalized pressure then again allow suction gas to enter the suction compress repeatedly. As a general rule, at greater than 6[degrees]F (3[degrees]C) the chiller will not load to 100% without faulting on high condenser pressure. During these periods of elevated condenser pressure, at minimum, energy consumption is increased and performance is reduced, costing more and not producing the desired chilled water. The solution is to clean the condenser tubes (Photo 2).
Art of "Walking Around to Manage" the Chiller Plant
"Manage by Walking Around" applies to sustainable central plant management as well as managing people. Operators logging oil pressure and temperature can evaluate the compressor mechanically. Oil pressure, when initially started with new oil, is 35 psig (343 kPa) (for example). If it is found to start falling slowly and the drop continues to fall under the same operating conditions, replacing the oil filter or calling a trained service technician may be warranted to avoid an unexpected failure. Evaluation of the oil temperature can also be a clue to the stability of the oil. Should the chiller be operated continually at low capacity with high condenser water temperature, the oil will tend to heat up with possible viscosity loss causing the oil pressure to fall from its normal pressure.
Logged readings can provide a trend for the owner to spot potential problems. I use a chiller log (Figure 2) for monitoring and also measuring chiller performance. The log inventories chiller parameters. With today's chiller control panels, data can easily be incorporated into a proactive performance report.
Pump Maintenance. This can directly affect the chiller and building performance. The owner needs to consider log reports, at a minimum, and also measurement of the pressure drops across the suction inlet to the pump and discharge of the pump. I minimize inaccuracies in readings by creating design/ installation criteria for a manifold across the pump that uses a single gauge for all readings (Photo 3).
Incorporating annual cleaning of the inlet strainers helps ensure a clean system without obstruction of debris within the condenser or chilled water system(s). Based on the routine checking of pressure drop across the inlet strainer, using the manifold gauge configuration may indicate the need for more frequent preventive maintenance based on seasonal issues (pollen in the spring and leaves in the fall getting drawn into the condenser water system). Greasing pump bearings and verifying seal lubrication annually or biannually ensures longevity of the equipment.
Cooling Towers. The cooling tower is responsible for rejecting the building's heat as well as the heat generated by the chiller. Ensuring a clean tower by using proper water treatment methods and annual cleaning maintains the efficiency of heat rejection (Photo 4). Operating the chiller with reduced entering condenser water temperature can improve the efficiency and possibly the capacity of a chiller by reducing the lift required to compress the refrigerant through the compression cycle.
Generally, reducing the entering water by 0.4[degrees]F (0.22[degrees]C) for each 1% capacity reduction maintains adequate control of the entering water temperature through the range of chilled water plant performance. (1) For instance, if the chiller is operating at 70% capacity with the 100% design water temperature at 85[degrees]F (29[degrees]c), operating at nominally 73[degrees]F (23[degrees]C) maintains more efficient chiller performance. Operating below this value could increase performance.
I use variable speed drive (VSD) technology for cooling towers, as well as for chillers and pumps to further enhance energy conservation. A VSD also provides less wear on the cooling tower fan motor, bearings and drive system and increases energy efficiency savings.
Towers create other maintenance concerns; they draw or blow air through the assembly. Cooling towers act as air washers as they filter airborne dirt, grass, leaves and general trash. Sunlight creates an atmosphere where algae can grow. If not treated, the combination can foul tower heat transfer and, ultimately, send solids and bacteria through the condenser loop to the chiller where fouling can occur along with corrosion, resulting in a loss in chiller plant performance.
Central Plant Automation. Automatic control systems allow for total integration through the use of DDC including using Btu/h (cooling capacity) and kW (energy) measurements. ASHRAE Guideline 22-2008, Instrumentation for Monitoring Central Chilled-Water Plant Efficiency, covers COP measurement of chilled water plants for total measurement of chilled water plants. Control algorithms allow for accurate, efficient control for independent operation. Through BACnet[R], Modbus and other methods of electronic data transfer, integration of systems can act as one central plant management operation without direct operator intervention and manual adjustment.
Pneumatic control, though not prevalent in modern control systems, does have merit. I think where fast two-position control is needed, such as with isolation valve actuation medium pressure (30 to 60 psig [308 to 515 kPa]), pneumatic control is preferred over DDC electric actuation for speed, power and torque.
In all cases with controls, periodically providing maintenance calibration check of sensors and actuators is necessary. It is the owner's responsibility to develop a maintenance plan for recalibration and periodic recommissioning of plant automation systems to ensure long-term functionality and successful operation and control of energy.
Pneumatic controls require more maintenance than DDC systems. The operator should consider manufacturers' requirements for specific calibration checks to actuators, pneumatic control piping controls and air compressor and after dryer maintenance, including compressor maintenance oil, filter and belt maintenance dependent on the type of compressor and air dryer usually comprised of a small refrigeration system. Refrigerated air dryers remove moisture from the pneumatic control air that can, if not controlled, corrode and block pneumatic controls, resulting in device and possibly system failure.
For maintenance, the operators also need to consider ancillary plant maintenance for such items as refrigerant monitoring (see ANSI/ASHRAE Standard 15-2007, Safety Standard for Refrigeration Systems, for specific requirements for refrigeration machinery room safety). Refrigerant monitors require annual inspection and calibration of either refrigerant specific TLV-TWA (threshold limit value-time weighted average) or oxygen sensor values. Refrigerant monitors use filters for sensor pickup tubes that constantly pass mechanical room air, stopping up the filters, which will require re placement. Additionally, mechanical equipment rooms provide for air exchanges. Therefore, mechanical room fans require inspection and maintenance, fresh air inlet dampers require periodic maintenance adjustment and cleaning to ensure operation.
Today's HVAC systems may be energy efficient in design. However, if they are not installed for maintenance serviceability, they will not be maintained by the owner to the best possible standards. If the owner is not provided training or does not commit to providing for necessary proactive maintenance, mechanical equipment will not achieve long-term efficiency and life expectancy. Maintenance planning is critical to the success of a sustainable efficient chiller plant, whether a decentralized plant for a single facility, a large central plant for distribution to a campus environment or a design consisting of other chiller technologies not identified within the scope of this article.
(1.) AHRI 550/590-2003, Performance Rating of Water-Chilling Packages Using the Vapor Compression Cycle, Table 3 m
John Vucci is associate director HVAC Systems at the University of Maryland in College Park, Md. He is a member of several ASHRAE technical committees and chair of the committee for proposed ASHRAE standard Method of Test for Field Test of Liquid-Chilling Packages. He is also a committee member of Standard 1 5, Safety Standard for Refrigeration Systems.
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|Title Annotation:||TECHNICAL FEATURE|
|Date:||Jan 1, 2011|
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