Peffley Chilled Water Plant
I enjoyed the article “Improving Efficiency With Variable-Primary Flow” (by James J. Nonnenmann, PE, and Daniel J. Flynn, PE, April 2010, http://bit.ly/9LklBK), but would like to know more about the controls. How are the chillers staged on and off?
Gil Avery, PE, FASHRAE
The Roy E. Peffley Chilled Water Plant includes a plant-level distributed control system (DCS) that provides control and monitoring of the plant auxiliary systems and supervisory control and monitoring of the chillers. Each chiller includes a microprocessor-based control panel with human-machine interface that performs startup, normal shutdown, emergency shutdown, capacity control, and safety protection.
An operating chiller includes three separate control strategies:
Chiller-load limiting, by which chiller load is limited to the maximum allowed by modulating a flow-control valve at the unit.
Chiller-flow limiting, by which chiller flow is limited to the maximum allowed by modulating a flow-control valve at the unit (6,800 gpm). The rate of change of chilled-water flow is controlled through the same flow-control valve.
Minimum chiller flow. The DCS monitors chilled-water-flow rate and modulates the minimum-flow bypass valve to maintain the flow rate above the chiller's minimum flow (3,000 gpm).
A chiller will continue to operate at maximum load (rated capacity plus 10 percent or maximum amp draw at the motor) until the chilled-water supply temperature increases to above set point. At that time, an indication is provided to the operators through the DCS. The operators then select which chiller, cooling tower, and pump(s) to operate next and input a permissive into the DCS to begin the startup sequence.
James J. Nonnenmann, PE
I read Jerry Pindus' “Managing Your Facilities” column (“Balancing Heat for Savings and Comfort,” http://bit.ly/9oaTOQ) in the April 2010 issue and wished that things were as simple and easy as he describes.
He cites the example of a six-story, 120-family apartment house in Queens, N.Y., that had a one-pipe (steam, I assume) heating system. I would like to know what cooperative or condominium has the money to install accurate temperature sensors in each of its apartments at the same time. Then, he discusses something that a study by a New York State agency determined is almost impossible: balancing a one-pipe steam heating system. Adjusting a gate valve, he says, resolved the imbalance. After 47 years in the consulting-engineering business, I think gate valves are good at shutting off the flow of a fluid in a piping system, but they are quite useless in balancing. Did he mean a globe valve?
I would love to see some clarifications.
John H. Szalkay, PE
Forest Hills, N.Y.
Our clients utilize a number of wireless sensors in each of their buildings. By moving a few of those sensors to new locations for feedback, our clients use the recorded information to analyze how their building spaces are being heated and to make adjustments, thus, improving the heat balance of their buildings. And you are correct: Globe valves, not gate valves, were adjusted to reduce the steam to the lines of the apartments.
As I noted in my column, you also can use different sized air valves, install additional heating radiators and reflectors, add wall insulation to prevent drafts, and identify bad steam traps and forgotten air valves in the basement. One important step is to educate tenants and make sure they properly communicate with management if their unit is overheated, rather than open windows, during winter.
Proactively working to better balance a building is an essential part of facility management. An energy-management system (EMS) can pay for itself within two years and continue saving on fuel costs going forward. Using the superintendent for minor repairs and adjustments is inexpensive. Clearly, this is something every building can do. With the cost of fuel, it's essential that every building take these steps and make the investment to start saving. Furthermore, in New York City, benchmarking, an important component of balancing a building, now is mandated.
The beauty is that by monitoring, collecting and saving information, and benchmarking, you can continually tweak a system over time as your budget allows. Our customers are getting better at balancing their buildings all of the time. With an EMS, improving balance is not difficult and ties into other larger goals, including improving overall HVAC-system efficiency, while keeping tenants' apartments at a comfortable temperature.
U.S. Energy Group
Fresh Meadows, N.Y.
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