Ultrasonic Flowmeters Make Chiller Control Easier

Clamp-On Flowmeters Are Reliable and Easily Replaceable for Maximum Uptime

By Kevin H. Evans

For the Federal Aviation Administration (FAA), our most important product is the safety of the traveling public.  Every single thing we do is focused on this one goal. We operate facilities that allow modern levels of air transport to occur. One of our Air Route Traffic Control Center (ARTCC) in the southwestern United States handles approximately 5000+ commercial flights a day. Locally, our part of the team effort is to provide the environment that allows the rest of the team to not worry about anything outside of their responsibilities. Power, water, computer support, communications and air temperature are things that need to be present, but invisible to the personnel working the screens.

More specifically to my facility, thousands of different items from mainframe computers to multi-hundred-ton chill water refrigeration units must work in coordination with each other. Providing support to the rest of our team makes the facility's task easier.

This coordinated effort is made possible by control automation. For the control automation to work well, it needs to have information, and, in part, this information is provided by flowmeters.

We have flowmeters on the water coming into facility, flowmeters on the natural gas that fires our boilers, flowmeters on our electricity, flowmeters on the air delivered throughout the ducts of the buildings, and flowmeters on the hot and cold water loops that move throughout the facility. We even have flowmeters on the air vented from the buildings.

The heart of the ARTCC facility is the high-powered computers that manage the flight control data. These  aren't desktop PCs, but mainframes that generate considerable heat and must be kept cool. The HVAC air handling system is an essential part of the facilities I maintain.

Our main building depends on four chillers, each with a capacity of 350 tons of cooling. For the hot loop, we have three boilers which can transfer 3.5 million BTUs of heat into the water flowing through the hot-side piping. Another system is our humidifiers. Flowmeters are essential to avoid damage from a system running dry or water overflowing into other equipment.

The system design called for many Controlotron ultrasonic transit-time clamp-on flowmeters, now maufactured by Siemens (www.industry.usa.siemens.com) that report directly to a distributed control system. These meters work especially well for us because they do not change the flow in the pipe where they are making their measurements.

The whole system depends on its flowmeters. The chiller and boiler control system must know how much hot and cold water is being used to create the discharge temperature supplied to the chill loop and the condenser. Without this data, the machinery could have a sudden and catastrophic failure.

In all of these situations, the flowmeters provide the information needed to control the system. Even if we are forced into manual operation, the flowmeters are responsible for giving the human operators the information they need to make the system work.

Our facility runs 24 hours a day, seven days a week, all year long. For heating and cooling purposes, this means two chillers and two boilers in operation.

Following the operation cycle from the point where new chillers and boilers are rotated into the system, the process looks something like this. The first operation is to bring online a new chiller. When the start command is given, the chiller repeatedly checks the output of the flowmeters in the condenser and chilled water loops in order to make certain that proper operating conditions, water flow and valve positions exist.

The transit-time flowmeters provide the fail-safe information to the control processor in the chiller, allowing each step in the starting routine to proceed by verifying that the valves are in the correct position, and that water really is moving through the piping loops for the condenser and chill water sides of the refrigeration unit. Additionally, reports from the flowmeters are sent to the control automation network and regulate the pumps to move the water through the system.

When the oncoming chiller is fully operational and is providing chilled water to the system, a previously operating chiller is turned off and placed into reserve status. Again the flowmeters verify that the chiller is indeed off and the valves are closed. Proper optimization and careful programming can make the system a pushbutton operation, and significantly reduces the number of people needed to rotate fresh chillers into and out of operation. As the cycle continues, the second chiller is rotated into service, and the previously operating chiller is placed in reserve.

As all of the chiller rotations are happening, more transit-time flowmeters inside the chill water loop provide the information and feedback to ensure that the amount of water flowing to the air handlers' coils is the right amount for the building's heat load. Inside the air handlers, other flowmeters confirm that air really is moving to the vents located within the various rooms of the facility. Finally, the exhaust fans from the rooms have flowmeters that verify the air is being removed from the room, ensuring sufficient number of air changes per hour in the facility.

Similar to the chill water system, the heating system cycles boilers in and out of service and maintains proper temperature inside the hot water loop. Boilers can be tricky systems. Improper start-up and improper shutdown can severely damage such systems. Again the flowmeters are integral to the process, providing information for the boiler start-up and shutdown processes, and also verifying that water flow to the air handlers is correct.

Often heat and cooling are required at the same time in an air handler. In such situations flowmeters can balance the demands on the system and reduce overall energy requirements. One of the reasons the Siemens Controlotron flowmeters were selected was their ability to handle both hot and chill water in the air handlers.

In our operation, we have multiple redundant flowmeters so that we can depend on having them when we need them. With good control automation, when the power goes down, and you're on batteries, flowmeters can tell you when things have stopped. Sometimes they provide the critical bit of warning in order to ensure that things like electronic devices do not overheat from cooling loss, or that pipes do not freeze from lack of heat in the building.