Central utility plants
While Carl C. Schultz, PE, LEED AP, makes a number of very good and valid points in his two-part Engineering Green Buildings column (“Rethinking Central Utility Plants,” August and September 2009), he fails to mention a number of items counter to his arguments.
First, with respect to district heating, central plants allow many fuels that would be impractical on a building scale — not just the high-carbon-footprint example of coal Mr. Schultz mentions — to be used. Two examples are biomass and refuse-derived fuels. Regardless of central-plant or distribution-system losses, a district heating plant using biomass will have a very low carbon footprint (zero, by some definitions), a result that simply cannot be achieved with, say, decentralized condensing natural-gas boilers. Also, it is important to point out that not all distribution systems are inefficient. Low-temperature hot-water systems operate at very low loss levels — less than 5 percent typically — which easily can be offset by the improved efficiency of a central plant that modulates to loads much better than, and avoids essentially all of the cycling losses of, an individual in-building heating plant. Low-temperature hot-water district heating systems can operate at temperatures equivalent to those supplied by condensing gas boilers.
With respect to district cooling, Mr. Schultz bases his argument against it largely on the improved efficiencies of small-scale chillers. Many of the advances he references also are available with larger-scale machines. No single chiller can match the part-load efficiency of a central plant, especially one operated by a staff that can dispatch not only chillers, but cooling towers and other ancillaries, optimally. Of course, in reality, the improved chiller efficiencies that Mr. Schultz uses to justify his decentralization argument will be available only to larger buildings, and where they are available, economics often result in inherently less-efficient air-cooled equipment being installed. Additionally, with the changing mix of refrigerants, dealing with inventory or changeover is much easier with a central plant than it is with many distributed building systems.
With both district heating and district cooling, maintenance must be considered. A central plant will be much easier to maintain than many smaller in-building plants of equivalent technology and age, and the regular maintenance a central plant is much more likely to receive will help maintain a higher level of efficiency. Also, fuels can be substituted with much less effort in a central plant than they can with decentralized equipment. For these reasons and others, central plants are likely to have much longer usable lifetimes and consume less embodied energy than decentralized equipment and have lesser cradle-to-grave impacts on the environment.
Yes, heating and cooling technologies are evolving, but central-plant-based systems are part of that evolution, offering options that are either not available or not cost-effective/practical with decentralized approaches.
(In the interest of full disclosure, I have spent most of my 34 years of professional experience researching and developing both central and decentralized systems. While I am the chair of American Society of Heating, Refrigerating and Air-Conditioning Engineers [ASHRAE] Technical Committee [TC] 6.2, District Energy, I twice have served as chair of ASHRAE TC 6.8, Geothermal Energy Utilization, which represents the distributed technology of geothermal heat pumps.)
Gary Phetteplace, PhD, PE
GWA Research LLC
While most of the major points made in the article “Multiparameter Demand-Controlled Ventilation” (August 2009) are pretty accurate, some of the assumptions, indications, and characterizations are not.
Let's be clear: ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality, does not regulate carbon dioxide (CO2) — neither interior-space levels nor differentials. It sets design and operating minimums for dilution ventilation rates required for comfort — essentially, the minimum quantity sufficient to satisfy 80 percent of occupants in terms of body-odor perception. References to differential CO2 are contained in an appendix showing the basic derivation of the ventilation rates used in tables.
Many misunderstand demand-controlled ventilation (DCV), automatically associating it with CO2. DCV is any method by which ventilation is adjusted based on the number of people in an intermittently and/or unpredictably occupied space. CO2 control is not recommended for spaces with stable populations or low densities. Thus, it is not suitable for total ventilation control in most buildings. Mechanical and electronic counters, time-of-day schedules, binary sensors, security systems, ticket sales, and other methods and devices used to estimate space population are more reliable in calculating required minimum rates and resetting intake set points than CO2 alone.
Differential CO2 is useful only in estimating per-person ventilation rates. It does not help dilute building-generated pollutants or directly satisfy the building component (per floor area) of Standard 62.1.
Since 2003, the ventilation tables have had more than “cfm/person” rates. As a result, the relationship between occupancy and differential CO2 no longer is linear, and spaces may require a unique algorithm for CO2-set-point control. The biggest problem with using CO2 alone is that you are measuring CO2 alone. It is not a valid indicator of ventilation rates and, as the author points out, can lead to some extremely large control errors on either side of minimum requirements. For more information on the sources of error in differential-CO2 ventilation control, see “CO2-Based Demand Control Ventilation: Do Risks Outweigh Potential Rewards?” in the October 2004 issue of ASHRAE Journal.
More reliability than CO2 sensors alone can provide is needed, if you intend to satisfy both indoor-air-quality and energy imperatives without the risks involved with indirect control methods.
Leonard A. Damiano
In terms of sustainability and ventilation (“Sustainable Ventilation in High-Rise Office Buildings,” August 2009), I recommend accurate diagnostic monitoring of both carbon-dioxide concentration and dew-point temperature be performed with frequent data review so that ventilation and moisture-management performance can be achieved and maintained.
Considering the data-accuracy problems with individual monitoring sensors, I have found greater sustainability and accuracy can be achieved with a centralized, shared-sensor approach, by which air samples are drawn to an equipment box and analyzed, with the results stored for later review.
David W. Bearg, PE, CIH
Life Energy Associates
In July, you published a “war story” (“Finding the Right Equipment”) in which the author described the problem of a large water-cooled packaged air conditioner's main supply fan overloading upon startup. He attributed the problem to the fan being forward-curved and told how he instructed the contractor to partially close all of the main dampers to increase the amount of resistance in the ductwork. That is similar to driving a car with the gas pedal to the floor and controlling the car's speed with the brakes.
I would have advised slowing down the fan before adding unnecessary resistance with balance dampers. This is common practice in the testing-adjusting-balancing field. Airflow should be choked with dampers only to keep a unit running until a speed change is made with sheaves or a variable-frequency drive.
The first step is to open all of the balance dampers in a system. Next, start the fan, and check the total airflow with a duct traverse. Then, adjust fan speed until 110 to 115 percent of design flow is achieved, confirming with a traverse. A reduction in speed will result in a proportional drop in airflow (Fan Law 1), as well as a drop in the horsepower required by the cube of the speed change (Fan Law 3). After that, a starter is not likely to trip, and balancing can begin. Once balancing is completed, speed can be adjusted to obtain the correct total flow.
To say there was not enough resistance in the ductwork (big ductwork means less friction and less energy needed) and that the problem was the result of the fan being forward-curved is not accurate. Unnecessarily choking airflow with balance dampers causes a fan to consume more power than if it had been slowed down and the system properly balanced. During balancing, only the amount of resistance needed to proportionally balance outlets should be added. Also, at least one complete path from the fan to the worst-case outlet should be left wide open, meaning no more resistance than is necessary is added.
Regenerative Dual Duct
Regarding the article “Regenerative Dual Duct: A Case Study” (January 2009), how does a direct evaporative cooler dehumidify?
Michael McGraw, PE, LEED AP
Summit MEP Consultants
Contrary to popular belief, direct-evaporative-cooling equipment can be used to dehumidify. It can be used to eliminate the parasitic fan losses cooling coils impose on a system over the course of a year and reduce system noise levels.
The strategy goes back at least as far as Willis Carrier. His paper “The Contact-Mixture Analogy Applied to Heat Transfer With Mixtures of Air and Water Vapor,” published in ASME Journal in 1937 (Volume 59, pages 49-53), led to the development of cooling coils and defines the mathematics of both cooling and evaporation processes. All of Carrier's research was performed with evaporative-cooling equipment.
Evaporation and dehumidification are the same process, driven by the difference in vapor pressure between air and water. If the water being put over evaporative media is below the dew-point temperature of the air, moisture will migrate to the water from the air, where it will condense and convey to the water the vapor's heat. This is accomplished by chilling the water delivered over direct-evaporative media.
To apply this technique, one must observe the necessary mass-energy balance and not overirrigate the media. Overirrigation can cause media to disintegrate. Multiple tiers of media and media depths of 3 ft or more may be required. The temperature of the water leaving media is a direct indication of dew-point temperature. Evaporative-media manufacturers always should be consulted.
This technique offers major advantages over cooling coils. Thermodynamically, the conditioned air cannot be supersaturated. Furthermore, precise humidity control can be provided using waste-heat energy during heating season. The air passing through the system is scrubbed of both particulate and gaseous contaminants, improving indoor-air quality. In independent testing performed on 8-in.-deep media at RWTH Aachen University in Aachen, Germany, this technique not only removed about 75 percent of bio-organisms from air, it did not reintroduce the bio-organisms — even when sump water was significantly contaminated. Greater media depth increases contaminant-removal effectiveness.
Both high schools in Wausau School District in Wausau, Wis., have Regenerative Dual Duct systems. Wausau West High School (313,000 sq ft) still has a couple of coils, but those mostly are direct-expansion coils serving only three systems. Wausau East High School (334,000 sq ft) is a new facility. There, the air-handling systems were designed with neither cooling coils nor heating coils. Both facilities use 100-percent outdoor air and are fully air-conditioned.
Mark S. Lentz, PE
Lentz Engineering Associates Inc.
Sheboygan Falls, Wis.
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