Heating, ventilation, and air conditioning (HVAC) systems consume vast amounts of water and energy. However, if your facility is looking for ways to minimize energy and water consumption, a variety of treatment and conservation methods are available to help you meet your goals through reduction, reuse, and recycling.
Water and Energy Consumption Overview
In general, boiler systems provide heat for HVAC and other processes, consuming quite a bit of energy and potentially large amounts of water.
Closed Loop Systems
Closed loop systems are filled with a set volume of water, and they do not consume much water unless experiencing a leak. However, they do consume energy by pumping water around a facility, generally for heating and cooling purposes.
Evaporative cooling towers consume a great deal of water and are one of the most efficient methods for ejecting heat from inside a building or facility to the atmosphere.
Air-cooled chillers require approximately 1.5 kilowatts per ton of cooling, whereas water-cooled chillers use approximately 0.85 kilowatts per ton. A ton of cooling is equivalent to 12,000 Btu’s.
Air-cooled chillers have a much lower daily capacity than water-cooled chillers because they require more surface area. Their capacity is typically limited to 7.5–500 tons of cooling, whereas water-cooled chillers can be quite large, with a capacity of up to 4,000 tons.
|Air-Cooled Chillers||Water-Cooled Chillers|
|Efficiency||1.5 kW/ton of cooling||0.85 kW/ton of cooling|
|Capacity||7.5–500 tons||10–4,000 tons|
The purpose of an evaporative cooling tower is to maximize the surface area of the water flow to transfer heat to the atmosphere through evaporation of water. Air is pulled up through water falling through or spraying down through the cooling tower, and heat is drawn out. This process can waste a great deal of water if not optimized with proper treatment and control; therefore, cooling towers should be one of the first pieces of equipment you evaluate when looking to reduce water usage.
This graph depicts the relationship between the makeup water added to a cooling tower, discharged blowdown, evaporation, and cycles of concentration. The amount of evaporation depends on the amount of heat added to the cooling water by system processes. This heat is removed and ejected to the atmosphere outside. This is relatively constant as dictated by the heat expelled by the chiller to the water. Every pound of water evaporated through the tower removes nearly 1,000 Btu’s of heat.
The amount of makeup and blowdown water depends on the cycles of concentration, or how many times each volume of water is evaporated. In a once-through system, water passing through the tower is sent directly to the drain. These evaporative cooling towers consume the most amount of water. Without controlling blowdown, 1.25–1.50 cycles is the typical range. With some blowdown control, at 2 cycles of concentration, the makeup water requirement drops from 3,000 to 2,000 gpm, a reduction of over 30%. Savings further increase from 2 to 3 cycles with a 25% reduction; however, as the level approaches the constant volume of evaporated water (indicated by the blue line in the graph above), returns begin to diminish.
Increasing cycles can contribute to a higher mineral concentration as water evaporates, which can lead to scaling and fouling. These issues need to be controlled to maintain system efficiency.
HVAC Sustainability: Maintaining Equipment
At installation, new equipment runs at maximum efficiency. The goal of a good water treatment program is to maintain equipment in like-new, as-manufactured condition by keeping surfaces clean and mitigating corrosion and microbiological blooms. Fouling from dissolved minerals in makeup water can insulate heat transfer surfaces if not managed properly. Corrosion deposits on heat exchanger surfaces pose another risk, as does direct exchanger surface corrosion caused by heat flux. Corrosion products can insulate heat exchanger surfaces, impairing equipment performance while requiring greater energy to achieve the same degree of cooling.
Use of high-quality makeup water helps preserve equipment’s as-manufactured condition. Additionally, fouling potential must be balanced with the maximum degree of evaporation.
When seeking to improve sustainability, alternative sources of makeup water should be investigated to supplement and reduce the primary makeup source. A trained water treatment team can audit your water systems and come up with suggested sources of reuse water.
Why are Chemical Inhibitors Necessary?
Chemical inhibitors are used to mitigate the interrelated processes of corrosion deposition and biofouling from bacteria and algae or plant life. Corrosion can lead to deposition, and corrosion and deposition can both lead to biofouling. These issues will negatively impact system efficiency.
Conserve Energy by Maintaining Cleanliness
If you’re looking to conserve energy at your facility, keep the following in mind:
- Dirty systems are not as efficient as clean systems. They require more pumping water to achieve the same degree of cooling because the surfaces are not being cooled effectively. Fouling can also divert flow through the tower fill, insulate heat transfer, and put greater strain on system pumps.
- Mineral scale from makeup water and microbiological fouling may cause more energy to be consumed and prevent heat transfer and cooling.
- If fouled, heat exchangers can become large consumers of wasted energy.
- 1/16 inch of scale on heat exchanger surfaces in a 500-ton chiller could easily cause consumption of over $100,000 worth of energy compared to $80,000 if it were clean. This consumption is directly related to greenhouse gas emissions, which would increase proportionally.
|500-Ton Chiller||Clean Condenser||Scaled Condenser|
|Water Use||7,200,000 gallons/year||7,200,000 gallons/year|
|CO2 Generation||2,500,000 pounds/year||3,310,000 pounds/year|
Water Resource Management
Recycling water entails treating streams of otherwise unsuitable water by investing in treatment and equipment to obtain high-quality water.
The first step in reducing water usage is to ensure systems are operating at maximum efficiency. Strategies may include:
- Reviewing work practices to eliminate waste, immediately repairing any leaks
- Optimizing water source usages
- Improving equipment efficiency by increasing cycles of concentration for evaporative cooling
- Conserving water through education
Educating yourself on the operations and needs of your system is a vital step for effective water and energy reduction. This can consist of:
- Knowing the optimum setpoints for your cooling tower to prevent waste when the system is not operating at the maximum cycles of concentration based on the makeup water chemistry
- Maximizing boiler system cycles
- Cleaning fouled surfaces immediately to prevent energy impairment
- Mitigating equipment corrosion to prevent additional CO2 consumption associated with the manufacture and replacement of equipment and piping
- Maintaining system performance to minimize the need for remediation
- Although cleaning a fouled system with acid can be effective, it can also cause corrosion and impair equipment, wasting energy and water. It is best practice to maintain system cleanliness instead of waiting for fouling to occur before resolving system issues.
Examine the quality of any water you may want to add to makeup streams. If the quality of the water is sufficient for reusing in other processes, you can install equipment to capture that water and plumb it to a usable location.
For example, boiler blowdown may replace city water in closed loops, eliminating boiler additions to the discharge stream and reducing costs.
Cooling tower water is not reusable if a constituent in the water is at its maximum stable concentration, limiting additional cycling. A cooling tower operating efficiently will maximize the concentration of the minerals in the water. The resulting concentrated water can contribute to scaling when reused if not properly pretreated to remove dissolved solids.
Recycling saturated cooling tower water involves capturing effluent and installing equipment such as reverse osmosis (RO) systems or softeners to remove calcium and magnesium, the two primary components of scale formation caused by over-cycling.
Municipal Gray Water
Many plants underutilize gray water generated at their local municipal facilities. Gray water is different from water containing sewage; it typically comes from washing applications (showering, washing machines, etc.).
Advantages of Using Gray Water
- Costs less than city water
- Reduces effluent discharge to the receiving stream
- Has fewer quality issues (such as salinity and brackishness) than alternate secondary water sources such as surface waters
- Contains some treatment chemicals such as chlorine or a scale or corrosion inhibitor
Challenges of Using Gray Water
- Quality varies as industrial processes and treatment change
- High levels of organics increase the risk of microbiological blooms and chlorine demand
- High phosphate concentrations may contribute to deposition and add nutrients for bacteria and algae
- May contain or produce airborne pathogens such as Legionella
- The presence of ammonia may increase chlorine demand and nitrifying bacteria and cause copper alloy corrosion
Audits & Reports
The ChemTreat team can evaluate your existing treatment program and historical system data to gain a deep understanding of how water is used at your facility. During their tour of your plant, our representatives can create a sketch of the water systems, which our Visual Design team can turn into a detailed schematic for your reference.
These illustrations are useful to help understand how water is balanced through a facility, where water is flowing, and where it is discharged.
Our representatives will note areas in your systems where energy and/or water are being wasted and work with our subject matter experts to put together a report outlining recommendations for system improvements.
Computer programs like ChemTreat’s CTVista®+ intelligent water management software provide additional insight to augment in-person audits. We use both in-house and third-party water modeling programs to come up with recommendations for optimizing water usage.
These programs can advise on key system parameters such as pH and saturation of minerals such as calcium, magnesium, and silica. The Langelier Saturation Index can be used to determine the optimum range for operation without wasting water. A good operating window includes avoiding:
- Evaporating too much water and over-concentrating minerals until they become unmanageable and precipitate on heat exchanger surfaces
- Under-cycling and wasting good quality water
Automation is recommended for optimizing cooling systems and helping prevent waste and fouling. Even small fouling levels are cumulative and can eventually impair efficiency, so correcting issues as soon as possible is recommended.
Automation offers superior on-line control and monitoring of water treatment chemistries. The data collected by automation technology also improves KPI monitoring to ensure results are being achieved and sustained.
Traditional methods for monitoring and controlling water systems involve grab samples and bench testing to analyze inert tracers present in chemical treatment. These traced chemicals can present increased treatment costs.
Newer methods use ion-selective electrodes to directly read inhibitor levels, eliminating the need for sampling and water conditioning. These low-maintenance sensors can output information to data logging software. Specific parameters such as water flow, temperature, pH etc. can be tracked to alert operators when water quality is outside the specified limits.
Software such as CTVista+ can also compile collected data to generate reports that can be sent to operators on a regular basis to increase visibility of system trends and offer insight into where adjustments may be needed.
Data Analytics and Predictive Technology
Modeling software such as ChemTreat’s Condenser Performance Monitoring Program can gather plant data and analyze it to aid in pinpointing specific system issues as well as provide troubleshooting guidance. These tools can also help determine when a system cleaning is needed to maintain efficiency.
Advancements in Water Treatment Chemical Development
In a holistic water treatment program, all connected systems in a facility or campus are evaluated to ensure downstream performance is not negatively impacted by upstream treatment. Chemicals such as phosphates, for instance, should not be added if downstream systems are having bacteria issues.
Additionally, some inhibitors are limited by the EPA, making it difficult to apply enough product to achieve the desired results/protection.
It is not unusual for a cooling system program to contain phosphate, potassium, or nitrogen, which act as macronutrients and make it difficult to mitigate algae and bacterial blooms. This is a big reason the EPA limits these items in discharge.
Applying cost-optimized inhibitors can increase sustainability gains. For instance, ChemTreat’s unique FlexPro® inhibitor offers a non-fouling treatment option.
Phosphate reacts with calcium to form calcium phosphate scale on heat exchanger surfaces. The following charts illustrate the relationship between water or tube wall temperature and heat transfer resistance. This example is at a Gulf Coast site using phosphate treatment during a very hot summer. The high temperature increased saturation and caused phosphate fouling, which impaired performance. The operators performed periodic acid cleanings throughout the summer to restore heat transfer and remove calcium phosphate scale.
ChemTreat removed phosphate from the program to achieve excellent corrosion protection without fouling the heat exchangers. Heat transfer was maintained even on hot summer days, allowing the facility to stay on-line longer while reducing the frequency of acid cleanings. This improved the site’s environmental outlook, as acid cleanings can expel contaminants to wastewater plants and the environment while also elevating greenhouse gas emissions by increasing the quantity of solids and phosphates in the discharge water.
Results are examples only. They are not guaranteed. Actual results may vary.
To enhance the sustainability of your water systems and treatment programs, it is important to keep the following steps in mind:
Step 1: Reduce Water and Energy Consumption
- Ensure equipment is operating at maximum efficiency
- Use historical data of water temperatures and energy consumption from when the equipment was new to benchmark performance
- Operate at maximum efficiency and cycles of concentration while minimizing water consumption
Step 2: Reuse Water
Work with your water treatment provider to find ways to reuse water at your facility. Water sources such as air handler condensate and rainwater tend to contain few contaminants and will not require much pretreatment for reuse.
Step 3: Recycle Water with Advanced Technology Offerings
Even seawater or water with high levels of dissolved solids can be recycled in plant processes if the proper equipment is in place. Investing in filtration and/or RO systems can help your facility recycle more water.
As with all other technologies, due diligence is necessary to determine the feasibility for utilizing these methods. It is always important to consult your equipment manuals and guides and seek guidance from your local water treatment representative to address your specific needs.