Boilers and Chillers: The Big Picture
Lesson 4.1
Why Central Plants Exist
Every AHU, fan coil, and VAV box you have worked on so far needed two things: heat and cold. It got them from somewhere. That somewhere is the central plant.
A central plant concentrates the generation of heat and cooling into a small number of high-capacity machines:
Hot Water Plant: One or more boilers heat water to 140-180 degrees F. That water circulates through the building, giving up heat at coils in AHUs and terminal units, then returning to be reheated.
Chilled Water Plant: One or more chillers cool water to 44-48 degrees F. That water circulates through the building, absorbing heat at coils, then returning to be rechilled.
The Boiler Plant
Condensing vs. non-condensing: Modern condensing boilers can achieve 90-98% efficiency by recovering heat from flue gases. Non-condensing boilers cap out around 80-85% efficiency.
OAT reset: the key sequence: Hot water supply temperature setpoint resets based on outdoor air temperature. Cold outside = higher supply temp. Mild outside = lower supply temp. A typical reset schedule: HWST-SP at 180 degrees F when OAT is 20 degrees F, down to 140 degrees F when OAT reaches 60 degrees F.
Lead/lag and rotation: One boiler handles the load normally; the lag boiler starts when the lead cannot maintain setpoint. Rotation balances wear.
The hot water loop: Pumps circulate water. Differential pressure across the loop tells the controller how hard the pumps are working. VFDs throttle pump speed to maintain the DP setpoint.
The Chilled Water Plant
Chiller types: Centrifugal (large buildings), screw (medium buildings), scroll (light commercial). All connect the same way from a BAS perspective.
Chilled water setpoint reset: Delivering 48 degrees F instead of 44 degrees F when loads are lower allows the chiller to operate more efficiently.
Cooling towers and condenser water: Cooling tower fans blow air across warm condenser water, cooling the rest through evaporation.
The delta-T problem: A chiller is designed around a specific temperature differential. Low delta-T means the chiller has to circulate far more water. This is one of the most common central plant problems.