Innovations in Comfort, Efficiency, and Safety Solutions.
Systems & Sequences
Part 2 of 2
Steven R. Calabrese
With the holidays approaching, as everyone else I find myself pressed
for time as I write this column. As promised last month, I present part
two in a two-part series on systems and sequences…maybe part two being
a little bit shorter than part one. ( Part 1 of 2 ) But that’s the price I pay for
waiting till the last minute to get my shopping done!
So in part one, I covered airside systems, which included packaged
rooftop units, fan coil units, VAV air handling units, and terminal
units (VAV & fan-powered boxes). For each topic, I described the
system in terms of sequence of operation and specification
requirements, followed by a listing of do’s and don’ts regarding
control system design and operation. In this part of the series, I
follow the same format, and discuss waterside systems (pumps, boilers,
There are multitudes of various pumping systems to consider, so we’ll just consider the popular two-pump system, with each pump sized at full capacity, meaning that either pump can handle the full GPM requirements of the system that its serving, with the other pump standing by in the event of failure of the primary operating pump. I case you missed it, this is referred to as “primary/standby”, or “primary/backup” operation. The pumping system can be constant or variable volume, where in the case of variable volume, the pumps would be equipped with variable frequency drives (VFDs), and the drives would be controlled to vary the motor speed, and hence the GPM rate, of the pumping system. This is typically done by monitoring the differential pressure in the supply and return mains, establishing a suitable pressure setpoint, and ramping the VFD up and down as required to maintain setpoint. For either type system (constant or variable volume), there is typically a “seasonal” changeover function, meaning that, at some point in the calendar year, or perhaps based upon the outside air temperature, the pumping system is disabled. For hot water pumping systems, this would happen in the springtime, and for chilled water pumping systems, it would happen in the fall.
Do’s & Don’ts
For this type of pumping system, do specify automatic alternation, meaning that the pumps alternate from primary to backup (and vice versa) based on a schedule or based on run-time hours. This gives each pump equal service, and helps to ensure even wear on the pumps. Don’t specify a single, common flow switch or differential pressure switch to indicate flow status. These days it’s more common anyway to fit each pump motor with a current sensing switch. No, you’re not directly monitoring the presence or absence of flow, but it’s still a pretty reliable indicator of such, and has become the industry standard for monitoring pump operation.
Boiler systems function to maintain hot water temperature setpoint (whatever that may be). For systems with multiple boilers, the boilers are sequenced to meet demand, meaning that if one boiler can’t maintain system setpoint, the next boiler is pressed into operation. The boilers themselves either operate from their own controls, or are staged/modulated via external control, by either a manufactured boiler sequencer panel, or by Direct Digital Control (DDC).
Do’s & Don’ts
For energy conservation and control optimization purposes, do specify outdoor air reset control. This dictates that as the outside air temperature increases, the hot water temperature setpoint is decreased, or “reset” downward. And for boilers that are not direct vented, do prove combustion air before allowing boiler operation. This typically entails commanding open a motorized combustion air damper, and monitoring the end switch on the motor to verify that the motor has indeed stroked fully. Don’t specify unoccupied mode shutdown for hot water boilers. And don’t turn off the pumps until the boilers are allowed to cool down. In other words, incorporate a delay whereupon shutdown of the boiler system would allow the pumps to continue to run for a short period of time, to dissipate the residual heat within the boiler plant.
Chiller systems function to maintain chilled water temperature setpoint. As with boiler systems, if there are multiple machines, then the chillers are sequenced to meet chilled water demand. Chillers operate from their own controls; chiller manufacturers equip their chillers with specialized control systems that meet the requirements of the mechanical equipment being controlled. In this sense, chillers are packaged equipment, saying that all required controls components, with the exception of a few “loose controls” that have to be field-installed, come standard as “part of the package”.
Do’s & Don’ts
As discussed above, chillers are basically packaged equipment, however there is one feature that chiller manufacturers have built in to their control systems, and that is the ability to accept an external setpoint signal. So do specify a setpoint signal from the Building Automation System (BAS), such that the BAS can determine and set the proper chilled water temperature setpoint at any given time under any given load or operating condition. Don’t vary the chilled (or condenser) water flow through the chiller, below the manufacturer’s recommended minimum. These days there is a push to equip all motors with VFDs, and that’s fine, however certain applications merit special attention, and chilled water pumping is one of them. Again, identify the manufacturer’s requirement for the minimum allowable GPM rate, and adhere to that requirement.
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