Study Confirms Coil Cleaning Save $$$$
By Ross D. Montgomery, P.E., and Robert Baker, ASHRAE members
Although it’s known theoretically that cleaning a coil can result in energy savings, little actual testing data and research exist to prove the point. As a result, building managers often ignore or reduce resources devoted to air-handler maintenance when faced with budget constraints. If proper maintenance is an important consideration in overall energy costs, conserving in that budget area can be self defeating.
Through our privately funded testing, monitoring and analysis, we believe we found a methodology and regimen that proves maintaining air-handler components in a clean condition can save energy dollars and improve other building parameter changes and efficiencies such as improved dehumidification and comfort, along with less mold and bacteria. Thus, we are encouraging IAQ environmental parameter improvements, better tenant satisfaction, and increased worker effectiveness.
It is difficult to find a building where such a study can be held. Fortunately, the owner and managers of a landmark 34-floor building on Times Square in New York City wanted to see what impact a dramatic change in coil cleaning nature and frequency might have. This building has only four large air handlers (SF-6, SF-7, SF-8, SF-9; 250 [880 kW], 123 [433 kW], 121 [425 kW] and 81 tons [285 kW], respectively) to service its 1.2 million ft2 (111 500 m2) of air-conditioned and heated space throughout the year.
The test project was performed at the building July through September 2005 to monitor and analyze the HVAC energy
use before and after restoration of two air handlers, SF-8 (121 tons [425 kW]) and SF-9 (81 tons [285 kW]), which are similar in their constant volume operation to the other two air handlers
in the building, and are located on the 34th floor mechanical room. This total of four air handlers interact by providing heating and cooling to the tenants of the 34 floors of the building. Periodic future data readings will measure and document an ongoing O&M program designed to maintain the enhanced level of performance.
No direct way of measuring energy use or demand savings exists because instruments cannot measure the absence of energy or demand. However, the absence of energy use or demand can be calculated by comparing measurements of energy use and/or demand before and after an energy conservation measure (ECM) (see ASHRAE Guideline 14-2002, Measurement of Energy and Demand Savings for details and more specific testing criteria and methods).
The ECM data collection was started on approximately Aug. 21, 2005. The ECM cleaning of the coils occurred on Aug. 27 and 28. During the study, specific operational parameters on SF-8 and 9 were monitored with energy balance and temperature/humidity data points being recorded for one week prior to the ECM. The recording was resumed for an additional week following the ECM. Several critical data points such as coil differential pressure, air and water temperatures before and after the coil, condensate temperature, supply air velocities, outside air temperatures, humidity’s before and after the coil, were monitored on SF-8 and 9 and both units were properly and completely cleaned.
To add reliability in our instrumentation calibration and accuracy, a certified and independent testing, adjusting and balancing (TAB) firm was used to test and calibrate the instrumentation that logged pressure, temperature, humidity, air velocity and volumetric flow rates, voltage and amps, during the course of our study period. In all, some 54 data points were continuously logged throughout the study period.
The daily variation in outside air temperature was nearly the same in the time span of this ECM (Figure 1). (As can be observed in the various charts, the building HVAC systems are operated in this building only during 6 a.m. to 6 p.m. Monday through Friday.)
The study has yielded the following overall results and conclusions:
· Restoration of the one air handler resulted in improvements that will lead to energy savings of up to $40,000 this year, in accordance with the results and assumptions of this study. (The coil is 30 years old, and its last cleaning was one year ago, so the coil was in a dirty state.)
· Restoring the air handler resulted in a decrease in the pressure drop across the coil, of approximately 14%. This has resulted in a corresponding increase in airflow. The result is that the fan is producing that much more work in the form of cooling.
· Restoring the air handler resulted in an increase of 19 tons to 22 tons (67 kW to 77 kW) of cooling added to SF-9. We estimate that 100 tons (352 kW) of cooling capacity will be added to the building once all four air handlers in the building are restored in a similar manner. (Building has a total of 1,800 [6330 kW] tons available capacity.)
· Restoring the air handler increased the thermal efficiency of the cooling coil 25% with respect to its ability to transfer its energy to its sensible loads.
· Restoring the air handler increased the thermal efficiency of the cooling coil 10% with respect to its ability to transfer its energy to its latent loads. (This is especially significant as it helps to cure the only IAQ-related complaint from building occupants, which was elevated humidity levels in certain interior locations.)
· Restoring the air handler will continue to save energy by decreasing the load on the chiller plant, and making the heat transfer of this loading more efficient. It reduces the time of multiple chiller operation and its associated pumps, cooling towers, chemical costs, wear and tear, etc. It also increases the awareness and practice of scheduling of plant operations and optimization techniques.
Bibliography
Steiskal, P. 1993. “Kahoe Test and Balance Field Manual.” Cleveland: Kahoe
ARTI-21 CR/611/40050-01. 2000. “Executive Summary—The Role of Filtration in Maintaining Clean HX Coils.”
Proctor Engineering Group. 1999. “Statewide Measurement Performance Study #3A—An Assessment of Relative Technical Degradation Rates.”
Siegel, J. 2002. “Particulate fouling of HVAC heat exchangers.” www.ce.utexas.edu/prof/siegel/thesis/siegel_dissertation.pdf.
Crowther, H. 2000. “Installing absorption chillers.” ASHRAE Journal 42(7):41–42.