By Robert Schneider, M.E.
Tim Baker, M.E.
Steve Chase
TES Engineering
Proper operation of HVAC equipment is vital to the comfort and profitability of retail facilities. The approach to managing it, however, is frequently reactive and misses various opportunities to economize.
In a reactive scenario, complaints to management trigger a search for the poorly operating unit. Because of the risk of imminent failure and a six to ten week lead time for delivery, there is no time to investigate new, more energy–efficient alternatives, so the same unit or component is ordered. Likewise, contractors are hired quickly, with little or no scope definition and effectively without competitive bidding.
Most facilities' management is more vigilant, but the oversight usually is performed by generalists who lack HVAC expertise or by service/contracting companies who may have a potential conflict of interest in recommending whether a unit can be nursed along, needs major repair, or should be replaced. The tendency is still to make like-for-like replacements, and budgets typically are based on historical annual average repair costs or the proposal from the outside service firm.
Engineering guidance
In contrast, professional engineers are ethically and legally obligated to assess conditions and to provide technically appropriate recommendations. This provides a level of trust in the information presented that is not available from other sources.
The engineering recommendations can be tailored to the desire of the facility owner—including the desire to minimize expenditures. Even on a shoestring budget, looking at managing HVAC systems from a programmatic basis will enable operations directors to evaluate information and make wise decisions.
A five-year planned program that is executed by professional engineers assesses the facility's existing conditions and presents recommendations for the projected repairs or replacement of HVAC units.
Technical improvements
When replacing HVAC equipment that is 20 to 30 years old, technical advancements make a big difference in determining appropriate size. Not only are today's units more energy efficient, but engineered evaluations based on computer modeling are much more accurate than previous hand calculations or selection of size based on commonly accepted "rules of thumb."
An enclosed retail mall in the Northeast had 22 common area air cooled rooftop HVAC units and 16 tenant air cooled VAV rooftop units—all of which had been installed in 1992. A 5-year plan was developed to prioritize and generate design drawings for their replacement, as all were nearing the end of their useful life.
Field evaluation determined that the units were in decent condition, but several tenants had cooling complaints in various areas of the mall. Load calculations showed that most of the existing units were oversized in the common areas but undersized in the tenant spaces. Correcting the sizing of the units would be crucial not only for improved comfort, but also for proper operation of the units and extension of equipment life.
The analysis of replacement sequencing considered many factors, including deficiency of cooling, location, difficulty of replacement, size, installation cost, maintenance records, and personal experience of the mall maintenance personnel. The recommendation called for replacing the most undersized tenant units in the first year, followed by the rest of the units in years two through five as shown in Table 1 and Figure 1. By properly sizing the replacement units instead of performing like-for-like change-outs, estimated net energy savings for the mall totaled $73,020 annually.
Figure 1 Location and schedule of HVAC units to be replaced in the mall's 5-year plan.
Table 1 Five-year plan common and tenant RTU summary for a Northeastern mall.
Upgrades, budgeting, and dampers
A five-year plan for a Southern California mall made recommendations for required maintenance and efficiency upgrades. Priorities were based on addressing a food court odor issue and working within the mall's $100,000 annual budget. In addition, the field inspection and discussions with maintenance personnel revealed the need to replace chilled water piping throughout the system. An estimate was provided with the recommendation to perform the work in phases as budget allowed and to minimize disruption.
Table 2 lists the various projects and costs, as well as energy savings and payback periods where applicable. Aggregate annual energy savings were estimated at $70,900. Field investigations showed that outside air dampers and return air dampers did not function on 10 air handling units serving the mall common area and on all of the fan coils serving the food court seating and toilet areas. This was a major cause of the odor problems, as the food court exhaust fans were starved for make up air. Codes require a minimum CFM of outside air per person, which also made the bulk of the damper replacements or repairs a first-year priority.
The recommendation also called for upgrading the old pneumatic actuators to integrated economizers with humidistats and electronic controls. If the outside air were cooler and of less humidity than the mall return air, the economizers would modulate open, allowing lower- temperature air to enter the air handling unit. This would require less chilled water to meet the mall set point and reduce energy costs by $17,300 annually.
There is a common misconception that bringing outside air into a facility is an energy waste, and malfunctioning dampers are often just bolted shut when they falter. In fact, utilizing cool outside air—even if only during the morning and evening hours—offers the opportunity for energy-saving free cooling.
Tower replacements
Even in a must-replace situation, opting for an engineering solution vs. a like-for-like replacement pays off. A New England mall needed to replace its cooling towers due to the potential for failure. The load calculations and tower design optimization yielded savings of $52,000 on equipment cost avoidance and power and structural upgrade avoidance. Additional engineering improvements to the system produced $4,000 in equipment savings as well as annual savings of $30,900 in power and water costs over the existing operation.
Evaluate utility rebates
Working with utility companies to gain rebates for retail facilities is another aspect of a planned HVAC program. However, sometimes analysis shows the rebate isn't worth the investment.
Another New England mall needed to replace nine tenant VAV units, and the local utility offered a rebate to replace the air cooled units with evaporative cooled ones. The engineering analysis studied capacity needs and compared installation and operating costs of new high efficiency air cooled units vs. evaporative cooled ones. Even with the rebate, the owner's investment would be $1,089,900 for evaporative cooled units, compared to $1,053,000 for the alternative. Compared to existing equipment, the evaporative cooled units would save $71,700 annually through KWh and KWd reduction vs. $49,400 for the high efficiency air cooled units. However, the evaporative cooled units would incur an additional $23,600 in annual water and maintenance costs. Thus the high efficiency air cooled units would provide $1,300 more in annual operation savings. This information showed that the utility's rebate and proposal to replace the old air cooled units with evaporative cooled ones was not the right choice for the mall.
Consider skylights
A mall in the Southeast was planning to replace 16-year-old HVAC rooftop units, and had a long-term problem with cooling the large open food court area. Load calculations showed that the existing five 20-ton units were 35 tons short of the needed cooling capacity. The recommendation was to increase the size of the replacement units to 25 tons each, which was the maximum that could be supported by the existing electrical service, structure supports, and duct work. Tinting was added to a 2,820 sq. ft. portion of the skylights to reduce the cooling load in the food court by approximately 10 tons, thus avoiding costs of major upgrades to the system and higher annual costs of increased electrical service. Even though the new units were larger, their improved efficiency yielded annual power savings of $3,000. Redirecting air with adjustable diffusers and relocating the temperature sensor also enhanced comfort in the food court.
Bottom line benefits
Taking a planned approach to managing HVAC expenditures can reduce operating costs through energy efficiencies, extended equipment life, and utility rebates. And it can transform costs from being a hit to the bottom line to qualifying as capital expenditures.
The Internal Revenue Service views isolated repair costs—such as repairing a single compressor—as expenses in the year made, which reduce net profit. Planned capital expenditures are depreciated over time in much smaller annual amounts. Also, they are not included in the calculation of Earnings Before Interest, Taxes, Depreciation, and Amortization (EBITA), which is an indicator of profitability that is watched by investors. It is important to review such decisions with a professional financial counselor.
Additional areas of savings
As previously discussed, reducing the size of HVAC units is a fertile area—a 20% size reduction typically yields a 30% energy savings. But careful consideration of installation issues is fruitful as well. Planning replacement of units to minimize crane lifts can save tens of thousands of dollars and minimizes disruption at the facility. The use of helicopters has been restricted since 9/11, and some equipment weights preclude their use.
Competitive bidding on equipment and contractors produces significant savings. Installation can cost twice the amount spent on equipment, so it's important to have accurate design drawings and expert evaluation of the bids to select the properly qualified contractor with the best bid, not simply the lowest price.
Sometimes, custom units—which are more expensive—can minimize installation and total costs of the project, for example, to fit a large VAV unit on existing rooftop curbing. And occasionally, fundamental mistakes are uncovered, as in the case of a Southwestern mall that had tried to solve cooling problems for 15 years before a thorough engineering investigation discovered that design requirements provided to tenants to perform their own installations were mismatched to the base building HVAC system.
Many of these opportunities to capture savings can be materialized if a planned approach to HVAC is used. Planning provides the time element required for proper engineering evaluation.
By Steve Chase and Robert Schneider
TES Engineering
Cooling tower problems can present major threats to retail mall operations. When an engineering firm's site review reveals leaking, collapsing, or under-performing units, owners often ask about switching to air-cooled systems, but water-cooled systems have many comparative advantages.
Water-cooled systems on average use 25% less electrical energy. For a typical enclosed mall with an annual electric budget of $1.5 million, that saves $375,000 per year. Towers require just one-third of the rooftop heat rejection surface as air-cooled systems. Existing roof structures may not have adequate support for the heavier weight of air-cooled equipment and the electrical power requirements may need to be increased. And, depending on regional temperatures, water-cooled systems can provide the opportunity for free cooling in winter.
When these advantages are presented, the decision is usually to replace the mall's cooling towers, but to protect the owner's investment, it's important to find the real culprit to prevent another premature tower failure. It's also the time to build in operational efficiencies and reduced energy usage.
Water problems
Water, for all its benefits, also can cause problems such as corrosion and sediment or mineral build-up that damage equipment and prevent proper performance. System designs must consider water and sewer costs, maintenance costs, pumping and piping needs, the natural pH or hardness of the city water and possible changes in the water supply—for example, city water systems served by river water can experience shortages and changes in composition during droughts. Most cooling towers are open, exposing the water to contaminants and pollutants from outside air brought into the system. Effects can be chemical, such as increased alkalinity from exposure to lime from a nearby concrete plant, or microbial, since water supports growth of bacteria at 95˚F.
Because variations occur in pH, corrosion rates, evaporation that causes mineral build-up, and bacterial counts, active water treatment is needed. The answer has always been chemical treatment, but other methods are available as alternatives or adjuncts.
Chemical treatment
Traditionally, chemical water treatment systems have taken a broad-spectrum approach using either chlorine or bromine. The chemicals are an expensive maintenance item, so monitoring is important to insure they are being used properly and effectively. Biocides need to be rotated to keep bacteria from developing resistance. In addition, determining specific issues can allow the use of targeted chemicals, cutting overall cost—and can resolve major cooling system problems.
For example, a mall in El Paso, TX, sought help for old roof-mounted chilled water air handling units that were inefficient and leaked air and water. The engineering inspection also revealed that the cooling towers were white—instead of black—when viewed from a distance. Further investigation showed that the fill was mostly blocked with calcium deposits, which greatly reduced the amount of available air for evaporation. The mall was operating tower fans on full speed to cool the water, negating the value of the variable frequency drives (VFD) that had been installed to help reduce energy consumption. There were no back-up towers, so tower failure would have resulted in a loss of air conditioning for the property.
It turned out that the mall's water treatment system was not functioning and was hard to service; the previous vendor had been doing "drive-by" treatment and just sold chemicals instead of monitoring performance and troubleshooting. A new chemical water treatment system was designed to include proper injection of scale and corrosion inhibitor chemicals selected for the city water's calcium and pH. (The polymer chain of chemicals should be adjusted for each region's water.) The design for the replacement towers also included "sweeper" piping and filters to help keep the towers clean, along with maintenance service platforms to make access easier for cleaning crews.
Non-chemical treatment options
Some states and cities are banning the discharge of chemically treated tower water into sewer systems, which means that make-up water for cooling towers can no longer bleed off into the sanitary system. In addition, the push toward sustainability has the U.S. Environmental Protection Agency, the Department of Energy, and the American Society of Heating, Refrigeration and Air-conditioning Engineers looking at non-chemical alternatives for water treatment. Several technologies are beginning to move to the U.S. from Europe, with adoption highest on the coasts. These include:
- Filtration systems – Green sand, diatomaceous earth, and charcoal filters are used to remove impurities and minerals;
- Filters – Bag filters, side-stream filters, and sweeper piping are added at critical points in the cooling water system to remove debris, prevent mineral deposits, and eliminate or reduce blowdown;
- Magnets – Have performed well in closed-loop systems such as steam heating to remove minerals, but are not applicable to open systems like cooling towers;
- Electric pulse systems – While relatively expensive, they are being used effectively in small systems up to 150 tons by changing minerals into non-sticking powders (which also incorporate bacteria) that are easily filtered and removed during blowdown or that settle to the cooling tower basin for annual removal; and
- Water softening – Sodium chloride removes bacteria as effectively as chlorine or bromine with no odor or health and safety concerns (it is also used in swimming pool disinfectant systems). Care must be taken when softening, since it will create unstable ions that can cause corrosion.
It is important to correctly diagnose the problem and its location to choose the appropriate water treatment method or combined solutions. A mall in Boston, MA, had cooling towers that needed to be replaced due to massive corrosion and major water leaks after just nine years—less than half of their normal life. Investigation showed that the make-up water was from a city well for non-potable uses, which also fed the mall's toilets/urinals and irrigation system. Flush valves were a regular maintenance item, there were rust stains in the WCs, sidewalks were stained brown, and grass did not grow.
A third-party water consultant tested the well water and found it had a pH of 5.7, was full of manganese, and had a very high bacteria count. The mall's existing water softening system was inappropriate and had over-corrected the pH, so the resulting alkaline water degraded the towers by attacking the protective galvanizing.
After evaluating several water treatment options, a comprehensive water filtration system was developed that included green sand filters—that attract manganese—and potassium permanganate injections to control the pH. The old system added chemicals before the blowdown solenoid, which wasted around a third of the chemicals and rendered the system ineffective. The two 5,000–gallon storage tanks that held the non-potable water were cleaned and the softener system was eliminated for the make-up water. Piping changes included sweeper piping in the tower basins to keep debris from outside air from collecting inside the towers, and a non-loss side stream water cyclone filter and bag filter were added to collect solids from the tower water. This greatly reduced the chiller tube fouling, increasing chiller efficiency. The filtered water was returned to the tower. A water treatment company performed weekly testing with quarterly reviews by the water consultant, and on-site maintenance staff was trained.
The new cooling towers were designed as all stainless steel for extra corrosion insurance and used variable frequency fan drives (VFD) for improved efficiency. The integrated solutions produced annualized savings as follows:
| Reduced water & sewer charges |
$32,935 |
| Chemical savings |
1,000 |
| VFD energy savings |
2,000 |
| Chiller operation (side stream filtration) |
30,300 |
| Elimination: mechanical seals replacement |
1,000 |
|
|
|
| Total Annual Savings |
$67,235 |
In addition, solving the water issues allowed the mall to use the free cooling system from the building's original construction. Sediment, which had collected in the plate and frame, was flushed and the heat exchanger was put back into service.
Value of maintenance
Maintenance of water-cooled systems is an important management operational responsibility. Water systems should be designed to be maintainable, with all elements easily accessible.
Often, inoperable free cooling systems can be restored to efficient use by cleaning. Another Boston mall was planning to replace 25-year-old cooling towers, and noted that tenants on the upper floors always complained that it was too warm. The tower pump had just been rebuilt, and the mall was repairing one compressor per year on tenant area air-conditioning, with costs ranging from $9,000 to $19,000 depending on the tonnage. Inspection of the system showed that compressors in the water-cooled air conditioning units were operating, even though the tower water was cold enough for free cooling. None of the units could be maintained because there were no access doors to clean the free cooling coils or the D/X coils.
his partially cleaned CACU coil shows the degree of clogging that had been present,
rendering the free cooling system inoperable.
The main solution was to disassemble the air conditioning units, remove the coils, and clean them. Sheet metal spacers with access doors on both sides were added for future cleaning of all coils. The design changed the free cooling control valve on the air conditioning units to electronic two-way and added a refrigeration water regulating valve, which also enabled the addition of a VFD to the building pump for horsepower savings. It also added a tower bypass line to help regulate the water temperature during the winter, as well as ladder access to the towers for easier cleaning. The amount of air delivered to the tenants increased dramatically, which reduced the compressor run times and the fan horsepower. The free cooling coil increased comfort during the winter—at a fraction of the cost of running 60 HP compressors. Energy usage has dropped, and compressors will potentially have longer life.
Conclusion
Digging deeper to correctly identify problems and integrating water treatment, cleaning, and HVAC design can resolve underlying issues in water-cooled systems to insure optimum long-term performance. This approach also makes for happier tenants and saves energy.