Pre-cooling of outdoor air integral to facility operation
For 50 years, the publishing facility at the California Department of General Services, Office of State Publishing, in Sacramento pre-cooled outdoor air entering the facility with 57°F water pumped from a well and fed to chilled-water coils located in two penthouse mechanical rooms on the facility’s roof. The warm water leaving the coils was discharged into the American River.
This process had to be altered when it was discovered that the well water was polluted, and the Environmental Protection Agency (EPA) threatened to assess fines for polluting the American River. The plant required a quick solution. With summer approaching, plant employees would not stay on the job without proper pre-cooling of the outdoor air.
The state turned to Munters to develop a solution. Munters suggested its dry evaporative-cooling technology. Four indirect-evaporative-cooling (IEC) units were installed at the publishing facility, making it the largest IEC project in the state. The Munters Oasis IEC modules, with a total airflow capacity of 354,000 cfm and sensible-cooling production of more than 704 tons, provide preconditioning for the 100-percent outdoor air required for the plant’s production area.
Unlike direct-evaporative-cooling (DEC) systems, which cool outdoor air by the evaporation of water that enters the building, IEC, or dry-evaporative cooling, uses a horizontal polymer tube air-to-air heat exchanger. Scavenger outdoor air is drawn upward across the outside of the horizontal tubes while water is sprayed through the tube bundle. The outside surface of the tubes is chilled to the ambient wet-bulb (WB) temperature which, on a hot day in Sacramento, usually is 30°F lower than the ambient dry bulb (DB). Air entering the building is drawn through the inside of the horizontal polymer tubes and dry-cooled without the addition of moisture.
The cooling process is identical to that of refrigeration cooling in that the enthalpy of the dry-cooled air is reduced as it travels through the air-to-air heat exchanger. The hot, humid scavenger air is exhausted to the atmosphere off the top of the IEC module.
The efficiency of the system is measured by comparing the entering-air ambient DB temperature to the ambient WB temperature with full airflow through both sides of the air-to-air heat exchanger. Project specifications called for a 70-percent approach of the DB to the WB condition. For example, if ambient DB is 100°F and the WB is 70°F, the outdoor air should be cooled to 79°F DB within the horizontal wetted tubes.
Originally, personnel at the plant evaluated an air-cooled water chiller as a solution to solve the pre-cooling problem. This plan required the installation of a 700-ton chiller in the parking lot on grade and the chilled water to be piped up to the two penthouse mechanical rooms and connected to the pre-cooling coils.
The air-cooled chiller would have consumed 1.2 kw of electrical energy per ton, including the energy needed to pump water to the rooftop. The IEC modules, with an energy consumption of 0.2 kw per ton, not only were more energy efficient but would cost the state less to install. Estimated peak-electrical-demand reduction on the hottest days of summer would exceed 700 kw compared with the air-cooled chiller.
In addition to terminating well-water usage, the state was intent on minimizing the potable makeup water consumed by the evaporative-cooling systems.
A Dolphin non-chemical water-treatment system was specified for the water-recirculation sumps of both IEC systems and the existing air washers in the penthouse mechanical equipment rooms. The device encapsulates water-hardness minerals and particulate into a non-adherent powder that is harmlessly deposited in the bottom of the sump. The Dolphin unit controls scaling of the wetted heat-exchanger surfaces and reduces biological growth in the sump water.
In addition, during startup of the supply and scavenger fans, the polymer heat exchanger tubes flex slightly because of air-pressure differences from the wet to the dry side of each tube. The flexing causes the calcium deposits to drop harmlessly into the sump.
The combination of the Dolphin system and the self-cleaning feature of the polymer tubes allow the state to operate its IEC water sumps at higher cycles of concentration, reducing the amount of wasted water required for purging the minerals and particulate matter from the water-recirculation system.
In August 2011, a test was run on the IEC-3 module at the California Office of State Publishing. Ambient temperatures at 2 p.m. were 95.5°F DB and 68.5°F WB (Figure 1). The IEC unit was cooling the outdoor air, leaving the dry-side polymer tubes down to 74.1° DB and 61.5°F WB. This represents an approach to the ambient WB condition of 79.26 percent, 11 percent above the specified performance requirements with full flow through both sides of the air-to-air heat exchanger.
The dry-side flow through the heat exchanger was 98,200 cfm constant volume. The sensible cooling produced was calculated to be 192.6 tons. Assuming an 80-percent saturation efficiency for the air washers located downstream of the IEC module, the supply air to the building would be in the range 64°F DB, ignoring fan heat.
The sprays on the air washers are modulated on and off to maintain a room relative humidity of around 60 percent, which is optimal for the printing process. Leaving-air-temperature set point off the IEC coolers is controlled by varying the wet-side air-mass flow with variable-frequency drives on the scavenger fans.
Information and graph courtesy of Munters Corp.