Sustainable condenser water system strategies
The condenser water system has a big impact on efficiency, long-term maintainability and total cost of ownership for the chilled and condenser water system
- Obtain an overview of basic condenser water design considerations.
- Learn the opportunities to improve water quality and conserve water through system design.
- Understand the relationship between various strategies and components and the importance of a fully integrated system approach.
In a condenser water system, there are several components that impact energy consumption of the overall chilled/condenser water system:
- Chiller compressors.
- Cooling tower fans.
- Condenser water pumps.
- Water quality.
It is appropriate to briefly discuss the total system efficiency considerations that are typical in any condenser water system selection and design.
Condenser water supply temperature control
Fundamental to efficiency of a chilled water system is compressor lift. Lift, or head pressure, refers to the difference in refrigerant pressure between the condenser and the evaporator. The higher the lift, the more work the compressor is required to do.
The chilled and condenser water temperatures both affect the lift. Reducing the entering condenser water temperature and/or increasing the leaving chilled water temperature will reduce the amount of work required by the compressor. While one strategy to decrease lift may be to reset chilled water supply temperature, there will be upper limits in any application.
In some applications such as health care where low supply air dewpoint control is required, special care must be taken in resetting chilled water supply temperature. With constant chilled water temperatures, colder condenser water temperature still results in significant reduction in compressor energy of the chiller. A good rule of thumb is 2% kw/ton reduction per 1°F reduction in condenser water supply temperature.
Assuming a constant 70% load or 350 tons, let’s compare the efficiency and power consumption with condenser water supply temperatures of 85°F and 65°F.
Power consumption at these two points is:
@85°F CWST: 350 tons x 0.5494 kw/ton = 192.29 kW
@65°F CWST: 350 tons x 0.3276 kw/ton = 114.66 kW
The reduction is power consumption at the lower CWST is 192.29 – 114.66 kW = 77.63 kW, which equates to a 40% reduction. This is in line with and helps to validate the rule of thumb mentioned above (approximately 2% increase in efficiency per degree of CWST reduction).
At any given ambient wet bulb temperature, producing colder condenser water requires additional cooling tower capacity resulting in increased tower fill and/or tower fan horsepower. Careful coordination with cooling tower manufacturer is recommended to optimize the tower size versus fan horsepower. Cooling tower fans operate via on/off controls or modulate via variable frequency drives to maintain a condenser water supply temperature.
VFDs provide the most energy efficient method of controlling the fans. While this cannot be ignored in optimizing the design of the system and controls, cooling tower fan energy is significantly less per ton than chiller compressor energy. In general, it is fair to say that the goal for an efficient system should maintain the lowest approach temperature possible.
Cooling tower approach = CWST – ambient wet bulb temp
Said another way, to optimize system efficiency, variable speed cooling tower fan control should, in general, aim to control the CWST to the lowest temperature possible given the ambient wet bulb.
Optimizing condenser water temperatures
With a constant temperature differential, reducing condenser water flow rate increases the efficiency of the pumping and cooling tower system, while reducing the efficiency of the chiller. While the most efficient flow rate (gallons per minute (GPM)/ton) cannot be generalized without a full evaluation of the system and cooling load profile, the once standard 3 gpm/ton (85°F/95°F condenser water entering/leaving temperature) is no longer the best design flow for all systems.
A full system model and accounting for piping savings and reduced pumping energy may show 2 or even 1.5 gpm per ton to be optimal. Further, with improvements in chiller efficiencies and the cost effectiveness of VFDs, variable condenser water flow is increasing in viability for consideration as part of overall plant optimization.
Other energy conservation strategies impact the proper condenser water system selection and design. Strategies like waterside economizer, airside economizer and heat recovery chillers cannot be individually discussed in sufficient detail here. The key point is that these types of systems affect the overall load and energy profile of the cooling towers, pumps and compressors and must be evaluated in concert to see the full picture of efficiency.
To summarize, the components and fundamental strategies discussed thus far are all interrelated. Each of these must be evaluated and modeled as a complete system using the building cooling load profile and weather data and actual equipment efficiency based on specific selections. The “right answer” is also dependent on many other factors including equipment and piping costs, utility rates, expertise of plant maintenance staff, availability of service, etc.
Water quality management
In addition to design selections and strategies, operations and maintenance programs have a significant impact on overall water system efficiency, environmental quality, water usage and longevity of equipment and components.
Water quality management is often viewed as maintenance, more than design and has a large impact on the lifecycle cost of the overall system. While most operators would agree that controlling scale, deposits and biological fouling is important to limit obstruction and corrosion of tubes, pipes and components, the magnitude of the impact of a small amount of scale on efficiency is often underestimated.
As water is evaporated in a cooling tower, the concentration of dissolved solids becomes greater. When the solubility of a mineral is exceeded that mineral is deposited on a heat transfer surface or pipe as scale.
The thermal conductivity of minerals in the condenser water is less than that of copper condenser tubes in a chiller. Calcium carbonate, the most common condenser water scale, has a conductivity of about 0.24 times that of copper. A scale layer of 0.02 inches on the condenser tube walls can reduce the thermal efficiency by 15% resulting in significant energy costs.
The concentration of minerals and contaminants in the recycled condenser water can be limited by intentionally wasting some of the water to drain, referred to as blowdown, or BD. When this water is replaced with fresh makeup (identified as MU) water, the concentration is reduced until evaporation (in this case E) occurs in subsequent cycles of the water through the system.
Cycles of concentration can be defined as the number of times that the minerals in the makeup water are concentrated in the condenser water by evaporation. COC can be calculated by using a conductivity meter to measure the conductivity of the condenser water and dividing by the measured conductivity of the makeup water. Another simple way to measure cycles with proper meters installed, is to divide the volume of makeup water by blowdown.
COC = MU/BD
The equation for this simplified method of measuring cycles best illustrates what the cycles is really telling us and the benefit of cycling. A higher COC indicates that a larger percentage of the total makeup water was to replace evaporated water and less was to replace blowdown.
MU = E + BD
Less blowdown conserves water. In addition to reducing the environmental impact, less makeup water results in a savings of water and sewer charges, as well as lower chemical treatment costs as less of any scale and corrosion inhibitors used are dumped to drain.
In general, between three and six cycles is found to be an economical range for condenser water systems. The optimum number of cycles is highly dependent on the makeup water quality, water and sewer rates and chemical costs. The optimum COC (maximum economic cycles) should be evaluated in a cooperative effort involving the operator, local water treatment vendor and design engineer.
Probably the most common method of controlling cycles is conductivity control. With the method, the conductivity of the condenser water is constantly measured. When it reaches a preset level, the automatic blowdown valve opens, dumping high mineral content water to drain. As this water is replaced with makeup water, the conductivity drops. When it is below setpoint, the blowdown valve closes.
One potential problem with conductivity-controlled blowdown can occur with hard makeup water and is often overlooked. When scale forms, calcium carbonate is removed from the water and the conductivity is reduced. As a result, the conductivity controller does not open the blowdown valve, the water is overcycled and additional scale is generated. This potential must be considered when looking at the overall COC strategy and method of control.
The second method, proportional blowdown, involves measuring makeup and proportionally controlling blowdown to reach a predetermined COC number. As most facilities will already have metering in place on makeup and blowdown to obtain evaporation credits on their water bills, this method is often the simplest and most economical. The potential downfalls with this method are that, unlike conductivity control, it cannot compensate for changes in makeup water quality or for small leaks in the system.
Each system has a unique set of parameters involving cycles, inhibitor chemistry and dosage, makeup and blowdown that results in the lowest total operating cost.
The chemistry and quality of makeup is a key consideration that is often left to be part of the equation as the water treatment vendor develops a recommended chemical treatment program for the installed system. Water hardness is one property that has a significant impact on necessary water treatment and deserves a careful evaluation of options during design.
Soft water for condenser water makeup
Calcium hardness, alkalinity, pH and water temperature determine the solubility of calcium carbonate. Most cooling towers use raw municipal water for makeup, sources of which vary in quality. This is most commonly addressed with limiting COC, injection of chemical scale inhibitors or injection of mineral acids. Using ion exchange softening for cooling tower makeup is less common but can help reduce scale while also controlling corrosion and conserving water by allowing higher COC.
There are often claims made that soft water is more corrosive than hard water, based on the theory that a thin layer of calcium carbonate acts as a buffer to protect metals. While it is true that a very thin layer of calcium carbonate can help inhibit corrosion, it is not true that soft water universally and directly attacks metal surfaces. Corrosion is dependent on many variables including pH, alkalinity, dissolved solids, oxygen and temperature.
With the classical use of sulfuric acid for alkalinity and pH control, very tight control of the acid concentration is required and the pH is often controlled to levels below the pH range for corrosion control of copper and steel. In addition, corrosion inhibitors are then required due to the aggressive nature of acids.
When evaluating the use of soft water for cooling tower makeup, softener regeneration wastewater and the cost of salt should also be considered. As with all strategies, the complete system must be evaluated. Overall, the use of soft water for cooling tower makeup generally reduces total operating costs, extends the life of equipment and helps to protect the environment and promote water conservation.
Cooling towers make great “air scrubbers.” With a large amount of air moving through a cooling tower, significant amounts of airborne dust and debris are introduced into the condenser water. Suspended solids contribute to scaling, biological fouling and corrosion throughout the condenser water system. With a large amount of dirt settling out into the basin, solids are also a large contributor to cold water basin corrosion.
Sweeper piping and sand or tangential solids separators are an effective method of removing dirt from the system at the source (the cooling tower basin). Alternatively, side stream separators remove solids from within the condenser water piping — constantly filtering a percentage of condenser water flow. Side-stream filtration is cost effective but does not prevent solids from settling in the basin.
Implementing effective solid filtration is critical to extending the overall life and efficiency of the system and its components.
In most areas of the United States, water is a relatively low cost and a very effective heat transfer medium, making evaporative cooling towers the most effective means for rejecting heat into the atmosphere. Nonetheless, the cost of consuming, discharging and treating condenser water represents a significant operating expense and creative ways to reduce makeup water should be carefully evaluated on every project.
Normal air conditioning results in large quantities of cooling coil condensate, especially in humid climates. Depending on the building layout and location of air handling units and cooling towers, this condensate is often able to be returned to the plant by gravity. Because the need for cooling tower makeup and generation of cooling coil condensate are simultaneous, all condensate recovered can be used for cooling tower makeup without any need for storage.
Cooling coil condensate does not contain dissolved minerals and is nearly pure. Therefore, recovering this water for use in cooling tower makeup can greatly reduce blowdown (increase COC) and reduce chemical treatment cost. It also can reduce or eliminate the need for softening when it can be shown that cooling coil condensate will always be available.
In evaluating the return on investment of condensate recovery, the overall system must be evaluated and modeled with the airside systems, load profile and weather bin data. First cost of the condensate recovery system must also take corrosion-resistant materials into consideration for piping and storage tanks because of the corrosive nature of such pure water.
Various design strategies and components of a condenser water system significantly impact the overall life cycle cost, efficiency and longevity of the complete chilled/condenser water system. Proper evaluation requires that all aspects and strategies be analyzed together as a complete system and that all stakeholders are involved in the evaluation and decision-making process. When this occurs, enhancements with relatively short payback can also have a positive impact on the environment and simplify maintenance for the life of the building.
Kevin Miller, PE, LEED AP, is a principal at Certus Consulting Engineers. With 24 years of experience, specializing in health care buildings, Miller is a founding principal and continues to bring technical expertise and provide creative solutions to the industry.
Davis Clum is a mechanical designer at Certus Consulting Engineers. Clum specializes in health care buildings. Clum has a passion for sustainable design and brings a creative and fresh perspective, working with industry partners toward innovative solutions.