Choosing a chilled water cooling system
HVAC systems consume 25% to 30% of the energy in a typical commercial building. Industrial facilities with controlled environments and large process loads can consume more than this. Generally speaking, 50% of the HVAC energy is used for the source cooling systems.
A variety of cooling systems will meet the cooling requirements of a facility, ranging from incremental or modular type configurations to large central plant designs. The final design solution is typically based on application load requirements, operating costs, and first cost.
Current trends suggest a design philosophy that incorporates smaller, modular stand-alone systems. This may be driven by economic conditions and the concept that smaller is better, more efficient, and avoids higher maintenance and operating costs. In some circumstances, it is simpler to replace a smaller modular component that fails rather than live through the catastrophic failure of a large machine. While this level of thinking certainly has merit, in some applications there may be lost opportunities to maximize the overall system efficiency of a central chilled water (CHW) plant.
Central CHW plants are generally designed to provide CHW for multiple locations from a central distribution plant. The ASHRAE 2008 HVAC Systems and Equipment handbook suggests that these types of systems “represent approximately 25% of HVAC applications.” Such systems are typically used in large commercial facilities, health care facilities, university campuses, industrial buildings, and other similar facilities that have multiple diversified cooling loads. In some circumstances, chiller plants will be specifically designed for process cooling in industries such as dairy, pharmaceutical, printing, and others requiring process fluid temperature control.
To begin, several factors must be evaluated. First, is there enough of a diversified load to consider a central plant? Second, is there any benefit to have the system consolidated in one location? Third, will the end user have staff capable of operating and maintaining the plant? The last and most important consideration is the cost and return on investment compared to other options. Once these issues have been verified, the design of the plant can move forward.
There are a number of machines which can produce CHW. Air- or water-cooled are the two general categories. Within these are a variety of compressor types such as reciprocating, scroll, screw, and centrifugal. The most common in large applications is a centrifugal chiller. Newer technologies have allowed these machines to maximize efficiency by using variable speed compressors to better match the part load requirements. Other types of machines commonly used are absorption machines, which can use steam, natural gas, or hot water. There are even absorption machines that use hot water generated by thermal solar panels. These are used on a smaller scale and more successful in the Southwest, but the technology has potential.
The success and efficiency of the plant is dependant in “right sizing.” In other words, the plant size should correctly match the design load and be capable of maintaining a high level of efficiency in response to the variation in cooling requirements based on operating conditions.
This is accomplished by matching all of the equipment, including towers, pumps, distribution, and control strategies. The final result should be the lowest plant kW/ton consumption, which includes all of the devices necessary to meet the facility cooling requirements.
The analysis begins by identifying the system diversity. An example is a large office campus type configuration where each building will have a load profile based on its location, construction, and use. It would be extremely unusual for all of the buildings to require 100% cooling at the same time. A computer modeling program is the most effective method to simultaneously analyze all of these variables. Each building will have a design load and time that this load occurs. A load calculation of all combined buildings will indicate a maximum load time for the system. Similar to the design of a variable air volume system, all zones or buildings do not peak at the same time. The block load is the maximum load imposed on the building, which is less than the sum of the peak loads. This holds true for the design of a CHW plant and is commonly known as diversity. This exercise helps to establish the overall plant capacity and is critical to sizing the chiller plant.
To begin this process, consideration should be given to redundancy requirements and opportunities such as peak shaving with ice storage, or other sustainable technologies that could be incorporated into the design. Other considerations include using engine-driven machines, absorption machines, or some combination. Traditionally, absorption machines are favored if waste heat is available, there are significant operational cost savings based on utility rates, or there are electrical demand limitations.
The process begins by evaluating the number, type, and efficiencies of the machines required, including standby capacity. To illustrate a selection process, if a 3,200-ton cooling load is required, options may be four 1,000-ton machines, two 1,500-ton machines, and a 500 ton machine, or one 2,000-ton machine and a 1,500-ton machine. The combinations and operation inefficiencies can be mind-boggling. Seasonal loads are also something to consider. Cooling loads during spring and fall can be substantially less than the summer loads. Given the number of hours in the off-peak cycle and the performance of a chiller, it may be more efficient to select a smaller machine for this time frame rather than a large machine operating at part load with poor part-load performance. Of course, outdoor temperature is not always the guide for some geographic locations, as high humidity levels must be considered. The designer must include all features.
Setting the standard
Several standards are used to identify a chiller’s efficiency. Electric-driven chillers use ratings that identify energy consumption (kW/ton) at full load and part load. A chiller plant will rarely operate at 100% continuously, with the exception of cooling requirements for process loads. Due to the limited time a chiller operates at full load, it becomes important to understand the energy consumption at part load and the number of operating hours in part-load conditions. These conditions can be attributed to occupancy schedules, weather conditions, and changes in the internal loads.
The Air-Conditioning and Refrigeration Institute (ARI) standard ARI 550/590 is used to define efficiency standards for centrifugal, screw, and reciprocating chillers. The standard uses a formula to establish average chiller efficiency and is known as the integrated part load value (IPLV). It is based on four operating conditions that use the “percent of design load” (cooling load) and “head,” which refers to the condenser pressure and is a function of outdoor dry bulb (DB) temperatures for air-cooled machines, and DB/WB (wet bulb) temperature, which affects entering condenser water temperatures for water-cooled machines. The formula assumes that a percentage of operating hours is at 100% load with 85 F entering chilled water temperature (ECWT), 42% of the hours are at 75% load and 75 F ECWT, 45% of the hours are at 50% load and 65 F ECWT, and 12% of the operating hours are at 25% load and 65 F ECWT. Other design parameters used are 44 F leaving CHW temperature, 85 F ECWT at 95 F outdoor dry bulb, 3 gpm per ton condenser water flow, and 2.4 gpm per ton CHW flow. Changing any of the above conditions will modify the kW/ton load profiles. To accommodate these conditions, ARI also recognizes the nonstandard part load value (NPLV) rating. There are technical reports that indicate that the driving force with respect to chiller efficiency is the ”head,” which is a function of weather conditions and will affect multiple chillers equally. The net result is that the curves are similar in nature and both are acceptable methods to evaluate part load performance.
Several factors that impact the evaluation are considered. First, internal cooling loads tend to be approximately 35% to 45% of the load; this can be established by the load calculations. Second, because the internal loads can be relatively stable with respect to loading conditions, weather conditions will have a greater impact on the performance of the chiller operation. For this reason, as the load drops to the (IPLV/NPLV) increments of 25%, 50%, 75%, and 100%, the appropriate performance curves are used to determine the power consumed. The spreadsheet provides a simple method of comparing equipment efficiencies and the effects of staging on/off points for multiple chillers.
Campus building example
For this example, there are four 260,000-sq-ft, five-story buildings in a campus configuration in Milwaukee. The buildings are used for general office and training facilities for a large electronics equipment manufacturer. The buildings are occupied from 6 a.m. to 8 p.m. six days a week. Mechanical cooling is available April through October. The base cooling load is 2,400 tons. Redundancy is a consideration; however, the cost of a 100% standby machine is prohibitive. Figure 3 represents a selection of three 1,000-ton machines. The spreadsheets also evaluate the use of variable speed compressors versus a constant speed compressor. Although there is an additional premium for variable speed compressors, the data provided can help determine the most cost-effective selection.
Based on the results, the variable frequency drives (VFDs) will save 268,363 kWh of energy per year. The added cost of the drives is $20,000 per machine, or $60,000 total. Using an average cost of electricity of $0.12/kWh, the annual savings will be about $32,203 per year or a simple payback of 1.87 years, which could easily justify the premium cost. Another fact to consider is that there are times when all three machines are operating, the VFD allows the efficiency to remain as high as possible, which also provides justification for the added standby capacity.
The chiller selection can be the most tedious part of the design, due to compressor types, efficiencies, refrigerants, quantity, and first cost. Spreadsheets can provide a quick and reasonable method to get the “big picture” comparisons without running a computer model for every little change.
After the chillers are selected, the remaining support components such as cooling towers, CHW pumps, and condenser water pumps must be selected in a manner that will maximize the overall plant efficiency.
Cooling tower efficiency can be easily overlooked in design. There are two basic types of towers: a direct contact or open cooling tower, which exposes the water directly to the air, and closed-circuit, which involves indirect contact between the water and air. Tower selection is based on the approach temperature. For an open tower, this is the difference between the outdoor wet bulb temperature and the required entering condenser water temperature, typically 85 F. To achieve 85 F water temperature, the maximum wet bulb temperature cannot be greater than 78 F using a 7 F approach temperature. Closed-circuit towers have the advantage of maintaining a closed condenser water loop, which minimizes evaporative losses and chemical costs. The performance of these units is limited to the dry bulb temperature. In some cases, water within a tower sump is sprayed over the coil to provide some evaporative cooling relief. This provides slightly colder condenser water; there is still the limiting factor of material (coil tubes) that separates the air from the condenser water.
The tower most commonly selected is the open cooling tower. This type generally has a cross-flow configuration that consists of a fan on the top, inlet louvers along the sides, sump in the bottom, and a corrugated type fill from top to bottom. Water is delivered to trays on the top and allowed to flow down opposed to air stream, which is delivered below the water tray.
When selecting a tower, the word “gracious” comes to mind; the tower(s) shouldn’t be oversized, and there are some simple low- and no-cost items that will be financially beneficial. Like many large pieces of equipment, there are standard casing or frame sizes. Towers are selected on approach temperature and outdoor wet bulb temperature. The amount of fill material determines the suspension time of the water in the tower casing and size of the fans required to enhance the evaporative cooling effect. By raising the wet bulb temperature or increasing the load slightly, there may be an opportunity to add more fill within a standard casing, thereby increasing the surface area. By maximizing the surface area, the convective airflow through the unit will delay the fan operation, which reduces energy consumption. Other opportunities to maximize efficiency include adding VFDs to the fans. There are several strategies that include VFDs either for all fans or for only the last/first stage. These are dependent on cost and local weather conditions. Control of fans is usually based on the tower leaving water temperature, which can be reset by the chiller loading; general accepted practice is to produce the coldest water possible, thereby maximizing chiller efficiency.
If cold weather operation is anticipated, an indoor sump should be considered. This basically allows all the tower water to drain into the building, leaving the tower and tower sump dry, which avoids potential icing problems. Another option is to have sump heaters in the tower basin or to have the tower sumps drained anytime a freeze condition occurs. However, this can be costly with water and chemical charges.
Several strategies exist for selecting and operating condenser water pumps. The most basic design incorporates an individual pump for each chiller assigned to an individual tower. Assuming the CHW pumps are in the same configuration produces multiple individual or isolated circuits per chiller. This is somewhat old-school theory based on keeping separate circuits, and thereby avoiding a total system failure and equal operating hours on all the equipment. While this still has merit in some applications, newer designs allow more flexible equipment combinations.
Pumps and valves
In some applications, variable speed drives are used to reduce pumping costs. Control of the pumps is accomplished by monitoring head pressure or water temperature. This is where it becomes important to have an in-depth analysis of the effects on chiller efficiency. Remember, the IPLV/NPLV ratings are based on standard ARI conditions of 3 gpm/ton at 85 F, 75 F, 65 F, and 55 F ECWT. As flow changes, so do the temperature of the water and the head pressure. Savings in pump energy have to be balanced against chiller performance, which will require an in-depth analysis with the chiller manufacturer and accurate computer modeling.
Be aware that some manufacturers will not honor warranties or performance if minimum flows are not maintained or ensured. A more common method of condenser water system design is to have one pump per chiller and, possibly, one additional pump for standby. These can be headered together and cycled with the startup of a chiller; the tower cells are headered together with automatic valves per cell for temperature control. As the water temperature rises, each cell can be engaged, allowing more cooling surface area. As the leaving water rises, the fans are energized to maintain the required setpoint. As with any open system, pump selection is critical with respect to net positive suction head (NPSH). Simply stated, this is the amount of water pressure on the inlet side of the pump to prevent air entering the system and creating cavitations that will destroy the pump impeller.
CHW pumps are selected in much the same way as condenser pumps are. Modern pumping strategies engage several pumping configurations such as primary-secondary (Figure 3), variable primary flow (Figure 4), and distributive systems (Figure 5).
A primary-secondary system incorporates a constant speed primary pump that matches the flow requirements of each chiller. The secondary pumps can be configured as individual full flow/standby or parallel operating configuration to vary the system water flow in response to the system CHW demands. A crossover is used to compensate for the imbalanced flow conditions that occur in part load conditions when the secondary flow does not match the primary flow. Leaving CHW temperature is modulated in response to a BAS reset schedule. Historically, it seems beneficial to maintain the coldest leaving water temperature and highest temperature difference to minimize pump energy.
Variable primary flow is a hybrid of the primary-secondary system. This configuration does not use constant speed primary pumps and allows the CHW flow to be varied at each chiller. A bypass pipe and valve is used to maintain the minimum flow requirements of the chillers. Consideration must be given to chiller ratings in response to variations in flow.
Distributive pumping systems are designed to circulate CHW within the central plant. Each building has a pumping system that is sized to overcome the pressure drop from the plant to the point of use. This is advantageous in large multibuilding configurations with long distribution piping systems. If the plant pumping system is designed to overcome the pressure drop of the entire distribution system, there is a high degree of probability that the buildings closest to the plant will experience overpressurization and temperature control issues.
Another point to consider is the system static head; this would apply to all large multiple-building pumping systems. If there are high-rise buildings, consideration should be given to the total static head on the system. This will have a direct effect on the pressure classifications of valve, pipe, flanges, etc., that can add cost. A more appropriate solution is to use a heat exchanger at the point of use. The plant is the production side and subject to more traditional pressures; the load side of the heat exchanger is now the only component subject to the higher pressure that is an effective method to control equipment and material costs.
The successful design of a CHW plant requires an in-depth analysis of many factors in addition to those discussed here. This article touches on the basic concepts and strategies used in the decision/analysis process. For each strategy, there will be more than just one or two possibilities. Designers should be mindful that the efficiency of one component will not define the overall system efficiency. Every change in equipment and system configuration must be evaluated based on the whole system. This ensures maximum system performance with favorable operating costs. Of course, successful design also requires careful consideration of the control sequences.
Zak is a principal with Graef-USA Inc., headquartered in Milwaukee, where he manages the MEP group. He is a member of ASHRAE and NCEES, and is on the editorial advisory board of Consulting-Specifying Engineer. He was an adjunct assistant professor at the Milwaukee School of Engineering for 20 years and is a registered professional engineer in 24 states.
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