Using HVAC Systems to Save Water

03/03/2006 |

There's a way to reduce at least one of your utility consumptions by up to 30 percent

At Fulton County Health Center in Wauseon, OH, a water-saving initiative has been implemented that is expected to save 1 million gallons of water each year.

Air-conditioning has become an assumed expectation for all buildings. We have it in our homes, in our automobiles, in retail establishments, in most work environments - and it’s usually taken for granted until it fails to operate properly. Hardly anyone thinks about how much it costs to operate the system (except the people responsible for maintaining the equipment or those paying the utility bills). Maintenance professionals and facilities managers are keenly aware of the utility expenses involved in operating a large-scale air-conditioning plant. These managers are under constant pressure to reduce or minimize utility expenses without compromising comfort. If your building utilizes a cooling tower and large-scale air-handlers, there is now a way to reduce at least one of your utility consumptions by up to 30 percent.

Large-scale air-conditioning systems are usually made up of the following components:

  • High-volume air-handlers.
  • Centrifugal chillers.
  • Cooling towers.

When large air-handlers are used to cool a building, they are usually supplied with 45-degree F. water provided by a centrifugal chiller plant. The centrifugal chiller, in turn, transfers its heat from the air-handler to a cooling tower. The cooling tower is used to absorb the heat from the centrifugal chiller and then dissipate the heat by evaporation. They all work together to provide the comfort levels we enjoy in a modern facility. In the process of providing this comfort, these systems consume vast amounts of water. In the case of the Fulton County Health Center (FCHC) in Wauseon, OH, it’s not uncommon for such a cooling system to consume almost 3 million gallons of water per year.

FCHC operates a not-for-profit acute care hospital, a long-term care center, an independent living facility, a physical therapy rehabilitation building, and a medical office building. These services are housed in four buildings encompassing 281,500 square feet. The maintenance department at FCHC undertook an innovative project in 2005 that reduced the water consumption of its cooling system by 748,100 gallons in 2005 when compared to a “like-climate” previous year, and over 353,000 gallons per year when averaged over the 3 previous years. The project is only in its first stage and is expected to save 1 million gallons each year when fully implemented. Although the idea is simple, it has already been implemented into the design of the hospital’s new building project and will be incorporated into any future designs.

 

Project Results
Routine records of water consumption over the past 3 years were used in determining the success of the Fulton County Health Center (FCHC) project. Years 2002, 2003, and 2004 were compared with the values obtained in the 2005 season with the following results:

“Degree Days Cooling” data was collected from the National Weather Service website for Toledo, OH. In simple terms, these are hours per season above 65 degrees F. FCHC started its chilled-water systems at 50 degrees F. and above, so actual runtime hours would be higher. In this case, FCHC is using the “Degree Days Cooling” data as an objective benchmark to compare the cooling-demand load from season to season. The last column averages years 2002 through 2004.

The “Gallons Difference” section reveals how many more gallons were consumed each previous year when compared to 2005. Year 2002 was the closest in climate-load comparison to the current year (2005) due to the “Degree Days Cooling” data. The data showed that water consumption was 729,500 gallons less in 2005 than in 2002. This represents a $3,000 savings in water and sewer charges. The water treatment contract was also reduced for 2006 by $900. Years 2003 and 2004 were 60-percent cooler than 2005, but less water was consumed in 2005. Although there was not as much of a reduction when compared with 2003 and 2004, it is also important to note that the “Degree Days Cooling” data showed these years to be significantly cooler than 2005. However, 2005 was still less than any of the previous 3 years. When averaged over 2002 through 2004, there were 381,200 fewer gallons per year used after the project.

Condensate Water
Condensate water is a byproduct of the air-conditioning process in an HVAC system. Air passing through the air-handler is typically cooled by 45-degree F. water that is pumped through a cooling coil in the air-handler. As warm air passes through the coil, the air is cooled and humidity is condensed on the coil. This condensed water is then collected in a drip pan and sent down the sanitary drain. How much water is collected depends upon the temperature and humidity of the outside air.

At FCHC, this condensate water had the following characteristics:

  • It was cold (very close to 45 degrees F.).
  • It was clean (all the air is HEPA-filtered prior to the cooling coil).
  • It had very low solids (10 to 20um).
  • It had a pH level of 8.2.
  • It increased in volume as cooling demand increased.

For these reasons, this condensate water was considered a good candidate for a cooling-tower water application. This cold condensate water could help cool the water in the tower basin. Because it was clean, there was no need to filter the water. The low solids level offered a dilution benefit to the collected solids already in the tower basin. The pH level was compatible with the targeted level of the tower basin. The volume of condensate water generated would be directly proportional to the consumption of the tower system.

If water is left stagnate in a condensate drip pan, bacterial growth can be a concern. The air-handler drip pans were evaluated for proper drainage. Any bacteria contained in the condensate water weren’t a concern for the cooling tower; the cooling tower water system was already being treated for microbial and bacterial growth. Thus, the connection from the condensate collection pan to the cooling tower could be constructed via simple piping methods with no need for filtering.

Two air-handlers on the fourth-floor mechanical room were selected for the initial phase of the project. These units had a combined volume of 71,500 cubic feet per minute (CFM). The project was straightforward since these HVAC units were in close proximity to each other and the water could gravity-drain from the fourth-floor mechanical room to the cooling tower on the boiler room roof two stories below. No pumps would be needed, and the only expense was the cost of the piping and the labor of the maintenance workers. The project was accomplished over the winter months of 2004, when the cooling systems were normally shut down. The drain lines from these two air-handlers’ condensation drip pans were re-piped to send the water directly to the cooling tower. The air-conditioning season of 2005 marked the first use of the project.

Although this portion of the project only involved two of the seven air-handlers (35 percent of the total HVAC air volume), it showed immediate benefits. The volume of the water transferred from the air-handlers’ condensate measured as high as 2.75 gallons per minute. The most appealing benefit: All of the water is free. The initial material investment was $300, and the labor accomplished by the maintenance staff took less than 40 hours.

Benefits of the Project

  • Low Solids Level. Total dissolved solids level of the city water used for tower make-up is 460um. The solids level of the condensate water is between 10 and 20um, which is considered “high-quality” water. This dilutes the dissolved solids of the incoming water, resulting in a higher level of concentrations in the system.
  • Colder than City Water. The condensate water returns at a temperature of about 47 degrees F. Although this is not significantly lower than city water at 60 degrees F., it does achieve a slight cooling benefit for the tower water basin.
  • Reduced Bleed-Off. As water evaporates from the cooling tower, dissolved solids are concentrated in the remaining tower water. As the tower water reaches the saturation point in dissolved solids, water is automatically bled from the system and replaced by more dilute make-up water. This prevents the system from exceeding the saturation point of the water. The lower dissolved solids found in the “free water” allow for more evaporation before reaching the saturation point. The result is that bleed-off requirements are reduced.
  • Better pH Level. The pH level of the condensate water is 8.2 (compared to the city water level of 9.2). Although the difference is minimal, it is beneficial to the chemical treatment program.
  • Reduced Chemical Treatment. The water being added to the tower must be treated to control scale, fouling, corrosion, and biological growth. The use of “free water” results in a higher quality of water being added to the system and reduces the overall amount of water required. The result is a reduction in chemical and water costs.

Plans are under way to add the remaining air-handlers to the condensate collection system. This will almost triple the collection efforts, bringing an additional air-handler volume of 131,000 CFM to the system. Although it’s not anticipated that the water collection volume will triple, it is expected to have a dramatic increase.

The impact of this project on water and chemical consumption is enormous. The savings are felt not only economically, but also environmentally. These changes are extremely easy to accomplish, require very few technical considerations, and yet produce tremendous benefits.

Take notice of these results and consider these designs in future building applications. If these methods were implemented across the country, billions of gallons of water could be conserved and dramatic reductions in chemical-treatment applications could be achieved.

Mike Hurd is the maintenance director at Fulton County Health Center, located in Wauseon, OH.


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